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Parachute recovery system design for large rockets

1. shouldn t have been allowed to wander underneath the rocket s trajectory in the first place Author Rick Newlands 1 updated 09 12 14 Technical papers Part 1 recovery system design The traditional HPR design The problem is that model rocketeers are very conservative when it comes to recovery system design They want to be sure the recovery system works to save their expensive flight computers so they almost never experiment with new designs they re still using designs only suitable for small Estes powered model rocket vehicles and simply beef them up to withstand the horribly large loads that then ensue Considering the amount of innovation spent on the vehicle as a whole remarkably little progress has been achieved in recovery system design When I was tasked in the early days of Aspirespace to devise recovery systems the world of HPR rocketry was still very new there was nothing really to draw upon so researched how the big boys did it and my research is detailed herein Fourteen years and l m appalled to find still that almost no progress has been made in HPR recovery The issue of scale Perhaps I m being a tad unfair the model rocket and HPR world have evolved recovery systems particularly suited to that scale of vehicle which does not mean that it s suitable for larger scales The physical thickness of ripstop chute canopy material nomex heatshield material and bridal lines means that sma
2. where subscripts p parachute and s store p 50 5 Cd Vierminal Standard sea level atmospheric density o is 1 225 kg m and gravity g is 9 81 True airspeed The actual speed through the air See Equivalent airspeed Vent hole A small hole at the apex that s designed to allow some of the air trapped in the inflated canopy to leak out This tends to prevent air spilling over the edges of the canopy which would otherwise cause the canopy to oscillate sideways quite dramatically Author Rick Newlands 43 updated 09 12 14 Technical papers References Ref 1 has a different title for each re issue e U S A F Parachute handbook March 1951 e Performance and Design Criteria for Deployable Aerodynamic Decelerators Jun 1963 e Recovery Systems Design Guide Irvin Industries Inc AFFDL TR 78 151 Dec 1978 downloadable on the internet Ref 2 Notes on the design of spacecraft deployment and separation systems Guy Gratton Vladimir M Shakhmistov with Marina A Kulinik Guy Gratton UKSEDS Ref 3 A semi empirical theory to predict the load time history of an inflating parachute RAE TR 79141 by J S Lingard Ref 4 Sport Parachuting Technology Applied to Rocketry Apogee newsletter 279 www apogeerockets com Ref 5 Safety design for space systems Musgrave Larsen Sgobba International Association for the advancement of space safety ISBN 978 0 7506 8580 1 E
3. Technical papers Parachute recovery system design for large rockets Introduction Any engineer who s seen a video taken from a camera onboard an HPR rocket vehicle tends to wince when the parachute recovery system fires Bits of airframe are tossed all over the Sky and there s usually the ominous clonk of one piece of airframe bouncing off another HPR rocketeers have followed an evolutionary approach to recovery systems if it works who cares how messy it is and there s the old engineer s adage if it ain t broke don t fix it But by doggedly applying small model rocketry parachute recovery system design to ever larger vehicles the loads occurring when the recovery system deploys are often enormous by far the largest the vehicle has to deal with A properly designed recovery system reduces these loads considerably This guide describes the design of recovery systems applicable to HPR class rocket vehicles and larger As most HPR vehicles use a two stage recovery system drogue and main chute l II concentrate on this Examples of the more common methods and devices used in the parachute industry are given and parachute industry nomenclature is used covered in the glossary at the end of the paper words in bold are listed in the glossary Aspirespace can t be held responsible for the information contained herein if your recovery system fails and somebody is hurt by falling vehicles it ain t our fault and the somebody
4. 0 02 04 06 08 1 0 1 2 1 4 1 6 18 20 MACH NO Drag Coefficient of Several Ribbon Parachutes as Function of Mach Number 1962 Data Supersonic chutes suitable for airspeeds up to about Mach 1 5 can be bought from Ky Michalson s website www the rocketman com chutes html Author Rick Newlands 31 updated 09 12 14 Ballutes At even higher Mach numbers a more radical design is required The ballute or attached inflatable decelerator AID is an inflatable device similar in shape to the space hopper children s toy The balloon shaped rear and centre part is a tension shell the conical forward part carries the loads to a junction point for connection with the body A burble fence around the equator of the ballute creates a uniform flow separation thereby eliminating destabilizing side forces Technical papers ORAG COEFFICIENT Cp BURBLE FENCE RISER AAR SCOOPS 4 TOTAL o DRAG COEFFICIENT Co 0 8 RELATES TO INFLATED BALLUTE DIAMETER 0 4 Dp 0 2 AN eS BUNTEN 0 1 0 2 03 04 08 10 2 0 30 40 80 MACH NO Drag Coefficient Versus Mach Number for Goodyear Ballute The burble fence and the inverted conical front part together provide good stability Air scoops in front of the burble fence ram air inflate the ballute Inflation with stored gas or gas generators has been investigated but was replaced with the simpler ram air inflation method The figure above sho
5. I initiated at l REEFING CUTTERS REEFING LINE CUT REEFING canopy stretch bon y ape REEFING RINGS pull cords ON EACH SUSPENSION REEFING CUTTERS t 1 LINE SKIRT ATTACHMENT TWO MINIMUM o attached to the i l om F SUSPENSION LINES y ines or to the 1 j canopy After a iT preselected t gj Erg time the cutter TE amp 1 fires and the Aa Du 1i knife severs 2E i the reefing line allowing the LEGENO ARACHUTE REEFED PARACHUTE FULLY OPEN DIAMETER OF REEFING parachute LINE CIRCLE REEFED OF REEFING LINE CIRCLE FULLY OPEN fully Oy VENT DIAMETER Parachute Skirt Reefing Reefing line cutters can be bought expensive or home made Ref 4 describes a small reefing cutter made from two concentric metal tubes the inner tube is fired along the inside of the outer tube and its sharp edge cuts the reefing line BLACK Pow DEZ __2 YT Fe TYPE Pive Inter A IN THIS E TH ts eb rue Ene gt THIS EN offen A j OPEN o i a ot Toe COR sand aai N S DOS Link goes THACUsH HOLE IN OVIZLA reuse The inner tube must be a snug fit within the outer tube Author Rick Newlands 23 updated 09 12 14 ED Technical papers Control line reefing A reefing method that is simpler for HPR rocketeers to implement is skirt reefing with a cont
6. say 3mm long by 3mm diameter which can be filled with expulsive powder then blocked at each end These can be activated by a hot nichrome wire or an igniter The benefit of using a nylon bolt instead of a metal one is 1 it s lighter and 2 the shattered fragments are not razor sharp Plastic is viscoelastic a shock load will shatter it like glass whereas a constant or slowly applied load will be restrained Here s one made earlier the components are restrained by a plug of araldite epoxy secured by a wooden dowel nail made dozens of these even double ended ones twin igniters for redundancy and every one performed flawlessly Be warned the fragments of plastic fly far and fast upon x ignition quite a loud bang too so i du shield your eyes FRACTURE ZONE E P T y i ai 3 Author Rick Newlands 15 updated 09 12 14 Technical papers The power source The stored energy source used to provide the motive power to deploy the chute can be of almost any type even the simple big spring In a multiple stage system the chute from the previous recovery stage is often used to pull the next chute out for simplicity In rocketry pyrotechnic power sources such as hot gas expulsion tubes or rocket motors are used to launch chutes or their container if used because pyrotechnics have very low mass for their power and are simple and compact They
7. within the hole to provide a cutting edge ouitable sizes of plastic rod shear pins per diameter of airframe are courtesy of UKRA 38mm 1 5 diameter 2 x 1 6mm shear pins 54mm 2 diameter 2 x 1 6mm shear pins 68mm 2 6 diameter x 1 6mm shear pins 75mm 3 diameter 3 x 2 5mm shear pins 137mm 6 7 diamete 167mm 77 diameter 290mm 11 4 diameter Author Rick Newlands 22 updated 09 12 14 a i J fom F i E ae Sam Technical papers The most common plastic used seems to be the Evergreen styrene rod Packs are about 2 ish for four or five 12 inch rods from any model shop When using shear pins more expulsion charge is needed Reefing A reefed chute is one who s canopy mouth has been restricted so that the canopy cannot open fully This reduces the chute drag so in effect gives a staged recovery at some set time reefing line cutters or other dis reefing devices releases to allow the chute to fully open Skirt reefing is the most common reefing method Reefing rings are attached to the canopy skirt on the inside of the canopy at the connection point of each suspension line The reefing line a continuous line that restricts the opening of the canopy is guided through the reefing rings and several reefing line cutters Each cutter contains a Oy pyro time train 17 PARACHUTE canopy I 1 Lp N
8. 12 14 Technical papers The drag figures quoted for large chutes are often higher than they actually are due mainly to a poor measuring method and also expect small replicas of big chutes to have lower drag coefficients Cd s say lower by about 20 percent Materials and construction Parachutes are best made from Apex ripstop nylon as sold in kite shops Crown For extra strength for drogues use a Uf double thickness of material or use Vent t hot air balloon gr ri which i F NS ia ie oon grade ripstop which is Skin 4 N Remember to make a vent hole at the apex of circular canopies of roughly 1 percent of the canopy area This will need reinforcing Good practice is to continue the Suspension Lines bridal lines right across the canopy across the vent hole and down the other side as this adds strength Confluence Point Riser fret Chutes can be bought off the shelf consult rocketry suppliers magazines and websites Construction plans for several common types are given below though as yet we don t have any data for streamers strips of material about 10 times as long as they are wide that are popular in the HPR world as subsonic drogues though they appear to be size limited they don t work at larger sizes Deployment bags should not be made of synthetic fabrics such as nylon as frictional heating between the parachute bay walls and the bag during a vicious extrac
9. P 3607 hs can 0887 N e 2h tan 180 N Generally Sy 01 S Verena Tane Mom O85 to 02 w 067 D GORE LAYOUT Sue hoof Line Str Line Parachute f Goes Length Weight D ft N Ibe faeit Ike 32 30 36 2250 10 60 44 300 4p 4000 10 108 Author Rick Newlands 36 los 10 000 Velocity 150 Bai Brake 180 kts 8 52 Broke duped updated 09 12 14 Technical papers Conical Ribbon The constructed shape of this Canopy is cdtaimed in the same manner s that described tor solid cloth conical parachures Gores like the flat circular ribbon design are composed af a gid of horizonte ribbons spaced ang retained l a close intervals by marrow vertical tapes Radial tapes which extend from the vant ro the skirt gre sewn to gether in the joning of adjacent gores The conical ribbon parachute shows higner drag than the fist circular ribbon just as tne solid cloth conical parachute does over the solid Hat parachute CONSTRUCTION SCHEMATIC of equal area Data tor several specific conical ribbon parachute and load configurations are listed below 02 1 Vaned Porosity other parachutes of the i conical ribbon classification the gore of the 14 2 ft i diameter drogue parachute in the table below is cor structed with geometric porosity varied three levels incre
10. aerodynamic cars affect the airflow around them to quite a distance away from the vehicle so the airflow around the chute may well be travelling at an airspeed Temp Comp and direction quite different to what is expected especially in the vehicle s wake A pyramidal framework of poles bolted to a roof rack or suchlike should raise the test parachute at least two metres above the roof Tabs Wrapped 2 of the car and Glued P Fabric Tabs e Definitely finite mass deployment unless a representative store mass is released with the chute Strain Gauges 1 4 In OD Stainless Steel Tubing Slits for Tab Measuring the load versus time curve during the Attachment deployment of a recovery system requires fixing strain gauges to a recording device and letting the whole system fall free The recording device has to have a fast scan rate 1 L i 1 I i Bottom View of Omega Sensor Without Tabs Author Rick Newlands 21 updated 09 12 14 Technical papers A force transducer known as an Omega sensor is shown above These can be made extremely small The variation of tube strain with force is a simple engineering formula and is easily calibrated Testing of expulsion tubes or drogue shells or whatever is required too much powder and the snatch load will be excessive A videocamera web camera and a freeze frame player can be used to discern the exit velocity provided you know the frame rate number
11. cause dangerous build up of internal pressure that could rupture the expulsion tube or send the expulsion powder past its detonation pressure Only ever use thin balsawood sheet for burst diaphragms Author Rick Newlands 17 updated 09 12 14 Technical papers In order to get a dangerous pressure buildup between a chute or drogue shell and an expulsion tube the seal would have to be ludicrously tight Just tight enough so that it won t slide out when the expulsion tube is held upside down will be sufficient Adjust the fit of a drogue shell in its launch tube by wrapping adhesive tape around its perimeter Glue a strip of paper or thread across the mouth of the expulsion tube as an added restraint if required or use a shear pin see later High altitude problems Its been reported that several rocket vehicles have suffered ejection charge failures at very high altitude It s not clear why but it s thought that the near vacuum of very high altitudes is preventing the propagation of heat flame across the loose pile of expulsion powder the bulk of the powder doesn t burn Black powder like other propellants has what is called a deflagration limit which is a minimum pressure at which combustion is barely self sustaining If the pressure drops too low combustion will cease or be erratic at best Also at sea level to 20 000 feet more or less air contributes significantly to the heat transfer from the igniter to the pow
12. multiple stage recovery system a smaller chute or drag device is opened first to slow the rocket vehicle down to a lower airspeed that the main chute can then be safely opened at e When done correctly the maximum loads generated by any stage s chute in a multiple stage recovery system is considerably less than for a single stage chute alone e Due to the higher dynamic pressure at opening the initial drag devices known as drogues or first stage chutes can have high canopy loadings small surface areas and yet still create a reasonable drag e Typically the drogue is opened at apogee The system then reaches terminal velocity and descends fairly rapidly reaching low altitude in too short a time for wind drift to be significant The main chute is then opened at this low altitude This is referred to in HPR rocketry as Close Proximity Recovery CPR as the rocket vehicle hopefully lands not far from where it was launched The Snatch Load Whether forcibly expelled pyrotechnically or not by the time a chute has travelled to the full extension of the riser the chute has built up a sizable difference in velocity relative to the rocket vehicle it deployed from Author Rick Newlands 4 updated 09 12 14 Technical papers This velocity difference has been increased by the deceleration of the chute due to its drag which will be much higher if the chute is allowed to partially open before lines taut as in a tradit
13. relative to the rocket vehicle at point 1 and is equal to Vp V6 This can be computer simulated based on the on the expulsion tube exit speed of the chute and the subsequent deceleration of the deployment bag or drogue shell due to its drag This equation reduces to AV km if the mass of the chute canopy is much less than the mass of the rocket vehicle mc m These equations assume that the masses of the riser and bridal lines are negligible compared to the mass of the canopy which may not be correct As a rough approximation one can assume that the riser mass and mass of the bridal lines are roughly equal so that the centre of mass of the combined riser and lines can be taken to be halfway between vehicle and chute From geometry this centre of mass is therefore travelling at Vp Vc 15A Vinax when the lines go taut and so has a momentum of VY2A Vmax Mriser m A i M iser Mines so in the above equation substitute m for Me Author Rick Newlands 10 updated 09 12 14 Technical papers Hopes analyses To obtain a value for k in the Average New i 60 Webhina Londi equations above the following force ebbing Loading versus strain e graphs are broadly 60 50 50 a l T aol Average New representative of nylon chords and Ew webbing x 30 30 20 20 k dF dF oL 10 10 Average Used 1 de m Webbing L
14. the chute which itself is based on the nominal diameter Do see the glossary for details Constructed Shape Inflated Drag Opening Average Shape Coef Load Angie of General C5 Factor Oscillation Application Plan Profile o C X Range Inf Mass Z8 LN to 1 8 Descent Circular 80 75 Conical to Descent 90 Descent 78 Deceleration 45 Drogue to Descent Flat Ribbon Deceleration T 50 to Descent Q 55 Deceleration The above table is for chute types that have constructional details given in part 3 of this paper Parachute Types and Characteristics Parachute System rue configuration and least severe for the modified ringsail system E P Ribless Characteristics ign Cross Guide Surface Drag coefficient Q 5 to 0 55 0 52 to 0 58 0 52 to 0 8 0 6 to 0 78 0 3 to 0 34 Deening load 7 1 05 to 1 3 e 1 3 0 1 1 2 1 4 actor Syerage angle to 3 3 to 6 w O to 3 O to 3 gf oscillation WT tests canopy stability Beginning of These parachutes were characterized by partial No data severe collapse and fluctuations of the canopy available pulsation and immediately after the first inflation peak at ribbon flutter Mach numbers M gt 1 4 The partial collapse at M 1 5 was most severe for the disk gap band Mach range 01 lt M lt 20 eke xe ee 1 LM 1 Author Rick Newlands 29 updated 09
15. the distance d to the drop in Kinetic energy K E of the system between points 1 and 2 Canmapy Miri Lines qo taut m A work AK E Recall that work is the integral of force F with distance x thus 2 E dx AK E From the law of conservation of momentum between 1 and 2 when the rocket vehicle and chute have reached a common velocity at point 2 in the diagram this velocity is V m V m V ee m t V where m mass V velocity subscript r rocket vehicle and subscript chute canopy Author Rick Newlands 9 updated 09 12 14 Technical papers Approximating the actual stretch force versus distance graph of the combination of lines and riser as a linear function gives a good enough result in practice 2 dx 4F d AK d where k Fj d xo is the effective spring constant and p is the gradient of the force versus distance graph shown here 0 Note the offset this is used to obtain a better line fit to data from many synthetic textiles such as the used webbing strain graph b below In the ideal case or for steel cable xo would be equal to zero The energy equation is then left hand side point 1 right hand side point 2 mV mV m mJV k d x Y TUR quje m uo m W3 Ts Kd Xo 2 2 2 2 Rearranging and substituting for F this gives the snatch load F m where AV max is the maximum velocity reached by the chute canopy
16. the same reason to ensure that the vehicle continues pointing nose first and isn t subjected to large angles of attack and therefore large airframe loads by going sideways Once the vehicle s airspeed has been markedly reduced by one or more drogues then the main chute can be fired out the nose in the traditional way although more often it s deployed sideways out of a bay on the side of the vehicle by opening a door larger vehicles can provide proportionally much larger internal volume for their size therefore folded main chutes don t require nearly so high a fraction of fuselage tube length as minimum diameter HPR fuselage main chute bays the folded main chute is much squatter not a sausage Design philosophies Having listed the issues with traditional HPR designs it s time for me to suggest improvements for larger vehicles but first a brief reminder of the recovery system ethos Recovery system design is very much an exercise in assuming that anything that can go wrong will go wrong and then designing all the flaws out of the system Aerospace design practices should be used especially in light of the large loads occurring and the high reliability required of the system e Redundancy Try to duplicate vital systems especially timers and igniters in case of component failure If the primary system fails is there an independent backup system Obviously too many backups will lower the overall reliability by adding more c
17. to achieve the same Lift force at altitude as opposed to if it were flying at Sea Level The aerodynamics of the aircraft will dictate several key airspeeds such as best glide airspeed best climb airspeed and above all maximum safe airspeed that the structure can withstand and the pilot will want to know how these airspeeds increase with increasing altitude Altitude Equivalent Airspeed performs the conversion for him if he flies at 100 Knots Equivalent airspeed then the aircraft will perform and feel the same as if it were flying at a True actual airspeed TAS of 100 Knots at Sea level the aerodynamic loads on the vehicle lift drag hull pressure will be the same The conversion factor from True airspeed TAS to Equivalent airspeed EAS comes directly from the aerodynamic force equation T VS Cf LASS Cf p atmospheric density Rearranging and canceling Pa altitude i Vas Vas where sea level atmospheric density p is 1 225 kg m Psea_level It would be convenient for the pilot if his Airspeed Indicator showed Equivalent airspeed rather than True airspeed and happily it so happens that the mechanics of a traditional Airspeed Indicator do exactly that The displayed airspeed is then called Indicated airspeed IAS Expulsion charge An amount of pyrotechnic material designed to generate hot expanding gas in order to expel a parachute Filling time The time taken for the canopy to fully i
18. 1 2 Impact loads The above graphs were plotted by gently hanging successively heavy weights off of a rope however materials behave differently under sudden impact loadings such as will occur with snatch loadings and also opening shock loads see below The following graphs were obtained by dropping a 60 First bounce heavy mass on the end of a new riser varies with vertical distance D 140 First bounce The k values due to impact loads in these graphs are actually higher than the static load graphs shown above WEBBING LOAD RATED LOA 0 0 05 0 010 0 015 400 Lb nylon chord STRAIN 1500 Lb 5 8 inch nylon webbing This is due to the visco elastic properties of polymers wherein the e g nylon fibres actually get stiffer as the loading rate increases Author Rick Newlands 11 updated 09 12 14 Technical papers If dynamic load strain data isn t available for your particular riser or bridal line but static data is the following empirical curve of peak dynamic load F to static load Fe versus the inverse of strain can be used to correct your LEAST data note the large corrections This curve was averaged for a wide range of impact speeds on a nylon bridal line but should be okay for most polymer ropes You can then re plot the force strain graph to estimate K as before by re scaling the y axis by the factor at the 00 ye 2 30 inverse of the strain that occurred at
19. CHARGE DUPLICATE IGNITEHS Commercial pyrotechnic devices for HPR rocketry can now be purchased Pyrodex expulsion charge For example the Pyrotechnic HiRMI Electric Match igniter Release mechanism from fo Toggle Black Sky Research consists of J yf Fiston an expulsion powder powered S p Spring 6 32 x 1 PHP Screw piston that releases a metal A fp toggle from a slot in the piston barrel upon actuation Although highly reliable problems have been encountered if too much powder is used as this can Piston _ give the piston enough momentum to rebound off of its IL end stop back down the barrel before the toggle has moved clear A new more expensive PRM S PHP 2 has been released but all MERI that is required to prevent this IRAN RUN RR problem is to absorb the piston s momentum with blue Spring tack or wet tissue paper placed just ahead of the end stop The tether release system from Defy gravity is similar but more versatile and can restrain much larger loads until separation is required www defyg com tether html Author Rick Newlands 14 updated 09 12 14 fet Technical papers A typical HPR installation is shown here a conventional Drogue parachute deployment aharge is used to deploy a Drogue parachute The Drogues parachute is attached to tha top of tha Main parachute which is also attached to the Toggle of the PAM In operation the syste
20. D j 067 D i hz 1441 2 p i i E i fi to E ro GORE LAYOUT Des gnation Canopy Woof Line St Parachute Max Rateot andiz miom Goms nylon Weignt Vega Deploy Geert Condit DAR N lbs Du ad 05 bs Vico fps n Ca 2B 1 28 550 B2 i3 Pesone 00 Z ks 202 Gi 64 225 100 80 1907 2200 280 204 100 16 120 d A 215 Cano 3500 150 250 204 15 16 160 550 1 28 AX Cao 50000 take 220 Clusero tes Author Rick Newlands 33 updated 09 12 14 Technical papers Comical The canopy is constructed as ie surface of reguiar pyramid of M sides anc pase angie p Dy ONG gares having 3 Vertex i JOQ JM Ita design is 3 minor variation of ine fi dicular The conici s af simple ana economical to construct handle and a the ilat circuigr and serves similar applications Asa Mult of drop tesis with models conducteg in 1849 conical parachutes with up to 30 cone angies snowed approximately ten percent nioner drag than cold flat sarachures af Wie same surface area Sub segeent fuli scale tests using 28 ft ana 32 E diameter parachutes confirmed these results for zeeciliz conical parachute ang losa centiauratiang gre sted below uu 2 an isin TP J eos a S s N tan BvD B 2 tan Bis Generally S 001 S Je 180 ro LF Bs GORE LAY
21. Fe Peak impact load to Static load Testing It needs to be said that a lot of the above mathematical and graphical analyses can be replaced by recording the force versus time results of suitably inventive testing methods For example one can use an RDAS as an acceleration recording device The opening shock load Some milliseconds after the snatch load peak is past the canopy opens If a drogue shell was used the drogue has just been pulled free of the shell as the shell s inertia kept it going The chute rapidly fills and inflates creating a momentary peak drag load known as the opening shock load this peak can be 2 or 3 times the steady drag of the chute and is caused by the mouth of the canopy swallowing a mass of air which it then decelerates Infinite mass assumption The unsteady aero fabric mass dynamics of this opening process confounds researchers even today as the mathematical modelling requirements are excessive What is known is that if the chute s canopy loading is higher than about 1436 N m 30 lb ft then the parachute rocket vehicle system won t decelerate noticeably during the period of canopy inflation because the rocket vehicle s mass is huge assumed infinite in comparison to the available drag and therefore the velocity of the system can be assumed to be constant during the opening period Opening If this high canopy loading condition known as hs sex infinite mass occurs then calculati
22. OUT CONSTRUCTION SCHEMATIC h Sue Gone Canopy Lines Line Parachute Payload Max Ange mion Gons inven Length Oof N le Do lig 25 11 24 55 4 8 1 7 S ilL 8 TD 12 B6 a gt 186 we su 10 163 7 25 Author Rick Newlands BM Peso 200 Jri Mele 1E Bobi Mise 500 Z5 34 DE 1 i Fd tp oF P gg INFLATED PROFIL EE updated 09 12 14 Technical papers Cross The cross parachute a French development is finding increased use for deceleration in applications that nequi Good stability and icw cost Th design i simple The canopy consti of two identical clon frecqyangles Crossed end joined t3 other ai the Square intersection to fonm a flat surface four equal arms Suspension lines ane attached to the nuter edges of four arms Some versions employ tie cords between corners of adjacent arms The Cross para chute is similar in stability performance and drag efficiency to the ringsiot paracnute Gut nas a wndency to rotate is popular as a deceleration parachu for ground vemcles dragsters Recent agalicati ns include stabilization and deceleration of air dropeed naval weapons 22 11 ang iow rats of descent high altitude probe experiments 5 2D a e CONSTRUCTION SCHEMATIC Generally i T z 1102 EF Li n 0 263 to 0 333 INFLATED
23. PROFILE Parachute Payload Vehicle Max Rate of Application Fef Size Canopy Dimension Line Str No of Line D s Lins Length Weight Denloy D ft R LU ibs Fi 1 0 ls Velocity Did 03k 63 1 100 2B 1156 175 Pacicage 120 56485 60 Hi AtProbe 210 6 38 Minyon 14 3 4X0 16 1 70 00 Author Rick Newlands 35 updated 09 12 14 Technical papers Slotted Canopy Parachutes Fiat Circular Ribbon Tne canopy is a flat circular amd consists of ccnoiniric nibbons usually two ches sm With supported by smaller For zontally spaced tapes ang radial ribbons gore edges Fib Dons and are accurately spaced t or vide the desired of space to solid fabric over the entre canopy Gores are triangular and dimensions derarmineg cn ihe img manner a amp for the solid Cloth flat circular parachute The flat circular ribbon parachute has lower drag efficiency ign ihe solid cloth O rachutes However its stability is excellent and the maximum opening force is low in compari son The canopy i relatively slow in openang and its performance reliability depends on specific design Parameters Compared to solid cloth parachute canopies the flat circular ribbon can gy 15 more difti cult ta manufacyure Data for specific flar circular parachute and load conthmurations given regular polygon of AY sides J 3
24. accelerate Unless it s launched axially out of the nose the effect of its hitting the airflow side on while still travelling slowly could without an extendable launch rod deflect it onto an unexpected trajectory Choose a rocket motor with a high boost thrust short duration burn for the tractor don t know whether a fin stabilised tractor rocket deployed rearwards would fly straight at all It is possible to obtain spin stabilised rocket motors for distress flares that don t need fins so would be better in this application Expulsion powder Commercial rocketry expulsion powder aka ejection charge is stable shock insensitive and fairly static insensitive It burns rapidly but at a fairly moderate temperature The main types available to the rocketeer are black powder gunpowder and pyrodex One cubic centimeter of powder is about five times as much as you ll need to expel a drogue on an HPR sized rocket a good rule of thumb is that you require 1 gram of powder per 200 cubic inches 3277 cubic cm of expulsion tube to be pressurised Expulsion powder is only effective if a reasonably gas tight seal exists between for example the expulsion tube and the chute and or wadding to allow a sufficient build up of pressure Estes commercial rocketmotors use powder sealed in by a cap of plaster of paris which only finally fractures at high pressure N B If you overdo the plaster of paris in homemade burst diaphragms you ll
25. ading edge of the parachute is equal to six to nine times the maximum body diameter to get the chute well behind the body wake and the suspension line length is equal to two times the nominal parachute diameter Do BOW TYPE SHOCK WAVE TRAILING SHOCK mnt ts EXPANSION WAVE VISCOUS WAKE TRAILING SHOCK Wave HL 3 3 A amp T i j hs EDGE YNE 23 7 RECIRCULATION TS SUSPENSION LINE Supersonic Flow Around a Vehicle Parachute System Conical ribbon parachutes are suitable up to the Mach 2 to 2 5 range Several new canopy designs have been developed including hemisflo ribbon equiflo ribbon and hyperflo parachutes The hemisflo ribbon parachute proved to be the most practical design for velocities up to Mach 3 Low altitude high dynamic pressure application of nylon parachutes is limited to about Mach 2 2 because at higher speeds aerodynamic heating starts to melt the leading edge of the canopy and lightweight canopy parts such as ribbons and tapes Whatever the chute type its drag coefficient reduces with Mach number eae F BEGINNING OF SEVERE CANOPY 05 PULSATION AND RIBBON FLUTTER ON FLAT CIRCULAR RIBBON TYPE c CANOPIES Y 04 SEVERE PULSATION ON 2 CONICAL RIBBON U 03 e FLAT CIRCULAR i 25 4 02 em CONICAL 30 kg 26 01 HEMISFLO the 26
26. afety factor of about 2 This may over engineer the main chute system but without the comfort of extensive testing this may be no bad thing In Ref 3 Lingard relates how all parachutes have a unique opening signature a characteristic load versus time graph Peak opening force scales directly with a dimensionless parameter known as Froude number see glossary So if you can measure the peak opening force for one size of parachute perhaps using an onboard accelerometer then you can calculate the peak force for other sizes and or other opening airspeeds provided it s the same design of chute Deployment from a vertical trajectory increases the peak opening load quite significantly at low Froude numbers because gravity is trying to re accelerate the system In fact for a Froude number of 10 and above a mass ratio of about 3 gravity dominates and the system actually accelerates during the opening process which increases the opening shock load to higher than the infinite mass case Another effect that significantly increases the peak opening load of main chutes is to open them at high altitude Generally aircraft flying at the same Indicated airspeeds or Equivalent airspeeds will experience the same aerodynamic forces whatever altitude they re flying at But canopy inflation forces depend upon True airspeed at very high altitudes a moderate Equivalent airspeed can have a very large True airspeed so the opening shock forces get lar
27. ag force available from the drogue to pull the release pin increases greatly But tried this system on a minimum diameter 54 mm diameter fuselage vehicle and it didn t work l ve also tried it on a much larger main chute where it did work size dictates the success of this system The drogue shell The drogue shell system tends to be RECESSED CHANNEL a more reliable system for drogue deployment from a vehicle travelling FOR DRDGUE LINE at high subsonic or supersonic airspeeds The mass of the shell can get excessive for main chute applications hence the name EXPULSION TUBE tube wherein the chute is e compressed into a hollow shell like BLAST PLATE container sealed by a blast plate This system is basically an expulsion The shell sides can be hinged at the nose to eventually split apart as shown but are closed and locked by the blastplate during expulsion The shell has noseweight to give it enough momentum and aerodynamic stability to clear the fins if fired sideways out of the fuselage or to clear the vehicle s base wake region of dead air if fired rearwards Author Rick Newlands 7 updated 09 12 14 Technical papers Pros e he drogue can be tightly packed inside the shell keeping its cross sectional area to a minimum which is important for reducing the snatch load The shell encloses the drogue and can be made of insulated material to shield the drogue from the heat of expuls
28. are surprisingly reliable when properly ignited Expulsion tubes These have the initiator and energy source combined The humble party popper is such a device DROGUE CHUTE When the small amount of internal expulsion powder is ignited it burns ICNIITED dl rapidly filling the expulsion tube with ees expanding gas which launches the j Y chute A HEATPROOF EXPULSION TUBE In rocketry the fuselage body tube is WADDING EXPULSION POWDER traditionally used as a large expulsion tube N B The use of metal expulsion tubes is illegal in the U K because of the shrapnel that can occur if the expulsion tube overpressures and fractures Composite tubes are less of a hazard and weigh less in traditional HPR recovery systems the composite fuselage tube is used as the expulsion tube A 3mm wall thickness cardboard tube will withstand typical expulsion pressures and being insulative rarely chars as the hot gasses aren t resident within the tube for long enough for sufficient heat to build up Restrict the use of adhesives to the outside of the tube as many adhesives are flammable Pros e Simplicity Cons e The chute must obviously be heat protected Sheets of Estes wadding can be bought from model shops to roll into a ball and place between the expulsion powder and the chute but this is merely tissue paper soaked in a solution of water and powdered alum or aluminium sulphide the
29. as it helps build the pressure up before popping off the nose V T Obviously the mass of the CO2 cartridge s and actuator adds to the Break 1 TIE BAG Some ejection options MAIN CHUTE Forward ejection The traditional method used in model rocket recovery systems the chute deploys in the direction of travel Pros e Simplicity of design thrust forces keep the chute within its compartment during ascent therefore little restraint is required Author Rick Newlands 18 updated 09 12 14 Technical papers e he same forces keep the expulsion charge at the bottom of the compartment as required Cons e When the chute opens it naturally decelerates much more than the rocket vehicle and ends up behind the vehicle The riser if anchored to a hardpoint within the fuselage therefore gets bent back nearly 180 degrees round the edge of the deployment bay and can zipper e Even if these don t occur severe compressive loads are imposed down the fuselage which being slender is much weaker in compression than in tension The chutes deploy side on to the Sideways ejection N direction of travel d ROCKET ATN Pros OS APOGEE e Ensures that the chute cannot impact with the fins e Ensures that the riser has a EXPULSION TUBE FIRES sufficient moment arm to combat DEPLOYING DROGUE the rocket vehicle s aerodynamic uum stability to flip the vehicle aro
30. asing from vant to skirt e g the upper one third o the gore uses closer spacing and the lower one LM third wider ribbon spacing than the center section With this parachute a drag coefficient CD 64 was Obtoined in wind tunnel test witnour loss ef MA d di stability However the opening load tector increased 1 fr of iH isee Table 2 2 INFLATED PROFILE wid Ig Tw i M j Es d i A d i i MET F ran ge e 2 tan Bie Vertical Tape B Pain po coe Rodio Tape ii Generally 3 lt 001 So Horizonte Ribbon NE 100 to 2 0 d D GORE LAYOUT Sue Coe fbior Nool Geom Line St Line Parachute Payload Payload Max Fate of Soacial Ref Ange nylon Gores Weng Weight Deoloy Descent Cond PU n ii N X ix gm i fgs 65 29 xO xm 265 2x0 20 252 Anais 3000 204p ib Grose 5 a ED 16 17 140 Bome 164000 200 s Cue 170 SE 35 Inno 10 75 Orden 715 amp Ook 71 42 27 180 20 75 Bipi go 15 Orga 213 1000 1600151 ain Author Rick Newlands 37 updated 09 12 14 Technical papers Hemisflo Ribbon Parachute Hemisflo ribbon parachutes have been used at velocities up to Mach 3 primarily as drogue and stabilization devices and for applications where the parachute must operate for longer periods of time in the supersonic region and often in
31. cess of the uncoiling of the bridal lines from the unfolding of the chute itself This prevents tangling The chute and lines are then deployed neatly into the air in an orderly sequence Cenopy Conmpasrimenmt m zi a xe Line Compartiment This figure shows line lengths inside such a deployment bag held tidily in place by loops of elastic until pulled out Sometimes the individual lines are stored separate from one another in individual sleeves again to prevent tangling d od aispeniios ge Bungie 7 If the lines are extracted first this is referred to as lines first deployment The opposite is canopy first deployment but this is best avoided as the deployment is messy and the loads are large Author Rick Newlands 5 updated 09 12 14 Technical papers This figure shows a drogue deployed quarter bag i e only the lines are enclosed in a bag while the canopy is inside a sleeve which gets concertina d then stored under a protector flap as shown Sleeve Mouth Locking Loops Suspension Line Bights Drag Assist Pocke Protector Flap Shown Open You can buy rather simple deployment bags from rocketry vendors though the smaller they are the less flexible they are so don t work effectively These bags are often nomex to shield the chute from the heat of the expulsion charge Compression Compression packing by whatever method is often used for large aerospace parachutes The Ap
32. common C lanyard Alternatively secure the joint with shear pins PTF Ee GROMMET gt TO RISER Pros SECT END VIEW e Simple OF NOSECONE BASE e Reliable e Well tested Cons e Fuselage tube often needs local reinforcement to withstand high bending moments From Ref 2 here are two popular joint designs that are used on large commercial sounding rockets and spacecraft The Separation band and the Bearing lock Separation band This consists of a tight strap holding the two halves of BAND the fuselage together The band has C shaped 8 A channel section to grip protrusions from the lip of both A EXPLOSIVE halves ff BOLTS The band is made from equal segments which are usually joined by several explosive bolts for redundancy If only one explosive bolt is used with a hinge diametrically opposite it this is known as a Manacle clamp and resembles a handcuff The successful Skylark sounding rocket used wire tensioned separation bands whereas the Black Arrow satellite launcher used a manacle clamp to hold the 3 stage on Pros e Moderately simple construction e Very reliable if multiple explosive bolts are used as the firing of any one bolt will free the band with the help of separation springs e withstand very high loads and bending moments Cons e The original design needs explosive bolts e Careful design is needed to allow access to the bolts for assembly e Needs an aerody
33. d have a very dangerous effect on the trajectory Using an extremely tight fitting chute or shell could cause an overpressure which fractures the tube and so is ill advised A special latch or suchlike must secure the chute in place during motor firing and furthermore the system must be designed fail safe so that any pyrotechnics cannot be armed let alone fired until this latch is released after motor burnout Shear pins see later will do the job of restraint Author Rick Newlands 19 updated 09 12 14 Technical papers Pros e Vehicle is not swung off of its nose first trajectory at deployment A large mass optimised fuselage suddenly flying sideways at high airspeeds may fail due to excessive drag deceleration and rotational accelerations Cons Limited space available for installation around the motor oafety latch or shear pins required A good expulsion speed is required to avoid the chute getting caught in the recirculating region of dead air that occurs behind the blunt base of the vehicle e expulsion tube installed in the fuselage near a rocket motor for rearwards expulsion will need heatproofing of itself and the chute from the heat radiated from the rocket and its exhaust or it might go off prematurely Fuselage separation prior to ejection In this traditional HPR method whole sections of the fuselage are separated at a designated point Known as a separation plane using some kind of join
34. der Above that there is significantly lower assisted convective and conductive heat transfer so a much more energetic igniter is required to set off the powder than at sea level The first way to correct this problem is used on military and civilian high altitude rockets they use sealed canisters to contain the powder containing ground level pressure air with burst diaphragms for motor igniters and deployment devices The container is designed to burst at a set pressure when the powder burns and expands Whatever material is chosen for the burst diaphragm should be tested to make sure it will break at a 20 psi overpressure to prevent fragment damage to the rocket since confined black powder can generate 25 000 psi pressure or higher A second though heavier option is to use pressurised carbon dioxide CO2 to power the recovery system The Rouse Tech gas ejection system is such a system that is available commercially Read the user s manual at http www rouse tech com pdfs CD3 Manual06 pdf and curse the Americans for using imperial measurements The Rouse Tech calculation method says that for every gram of Black Powder you would use in a rocket you should use 5 grams of CO2 obviously then you should round up to the nearest cartridge 12 gram 16 gram 25 gram or 38 gram Richard Brown reports that in his experiments the figure is nearer 10 V gram of CO2 and he also says that shear pins see later are a must for CO2
35. e or sausage like bag used to contain the packed chute during deployment to reduce snatch loads by reducing cross sectional area The bag is usually pulled out by the previous stage s riser When the current stage s riser goes taut a release pin is pulled out or laces are cut opening the bag and allowing the chute to deploy Drag equation Aerodynamic drag is equal to the drag coefficient Cd times dynamic pressure times some reference area For the rocket vehicle this reference area is the maximum cross sectional area of the fuselage ignoring the fins or small local structures whereas for aircraft it s the total wing area For chutes the reference area is the nominal area 50 this is why the drag coefficients of chutes are quoted as Cdo Author Rick Newlands 30 updated 09 12 14 Technical papers Drogue A small area chute used as the first stage of a recovery system sometimes called a pilot chute Dynamic pressure q All aerodynamic forces scale directly with the kinetic energy term v p y p being volume specific mass i e the air density at the current rocket vehicle altitude and V is the vehicle s airspeed This kinetic energy term is known as Dynamic Pressure q to distinguish it from its Potential energy counterpart of Static pressure P Equivalent Airspeed The density of the atmosphere decreases with altitude which means that an aircraft must fly faster at the same angle of attack
36. ectory Narrow envelopes are very sensitive to opening airspeed V prediction and because of the V dependence of drag a reasonable estimate of recovery system loads requires accurate prediction of the vehicle airspeed at chute deployment which can only be gained from a trajectory simulation that doesn t just model purely vertical ascents see our paper a dynamic rocket simulator for a suitable sim but estimates the horizontal speed component too It s wise to include the effects of wind and wind gusts on the rocket s trajectory see our papers Dynamic stability analysis rocket simulator and Rocket vehicle loads and airframe design on the Aspirespace website To minimise airspeed the drogue chute must be opened at apogee Modern commercial rocketry flight computers such as the RDAS can sense apogee to allow this Wide envelopes by definition aren t so critical of opening airspeeds and hence trajectory prediction can be cruder Wider envelopes usually require more rugged or complex recovery systems Multiple stage recovery systems For a soft landing a main chute of large canopy area is required When opened even if at apogee such a large chute will generate enormous opening loads Often these loads are just too high the structural reinforcing of the rocket vehicle fuselage required to survive these loads adds excessive extra mass This is to be avoided as opening loads increase strongly with store mass In a
37. eefed below about 1096 reefed canopy mouth area compared to unreefed mouth area otherwise they won t open properly Heefing ratio To calculate the effect of skirt reefing first calculate the drag reduction you require reefing ratio The required reefing line ratio a can then be calculated from the graph here where the reefing line ratio is the diameter actually circumference of the reefed chute mouth compared to SOLID CIRCULAR the unreefed x RD diameter and TH describes how much reefing line needs to be pulled in to reef the chute The relationship isn t quite linear REEFING RATIO 0 2 0 3 Dp REEFING LINE RATIO r gt Reefing line forces Recfing Ratio Versus Reefing Line Ratio for Various Parachutes The shock load on the reefing line during reefed canopy inflation is surprisingly low around 596 of the opening shock load of the reefed chute Landing speeds The landing speed of a store suspended under a parachute can simply be calculated by assuming that the system has reached terminal velocity This landing vertical velocity should be about 5 metres per second A velocity much higher than this could be dangerous to persons underneath the store and might break the fuselage Author Rick Newlands 25 updated 09 12 14 Technical papers The drag of the store is negligible in comparison to the drag of a chute large enough to attain this terminal
38. elding must be continuous and terminated to the shell of connectors and components The shield must be joined electrically to the shell of the connector or component around the full 360 degrees of the shield The shell of connectors or components must provide attenuation at least equal to that of the shield e The electrical circuit to which the system electrically actuated pyrotechnic device is connected must be isolated from vehicle ground by no less than 10K ohm e All circuits must be designed with a minimum of two independent safety devices Any time personnel are exposed to a hazardous system a minimum of two independent safety devices are required to be in place e he system electrically actuated pyrotechnic device must be protected by an electrical short until its programmed actuation This requirement does not negate the use of solid state switches e Any electrical relay or switch electrically adjacent to the system initiator either in the power or return leg of the electrical circuit must not have voltage applied to the switching coil or the enable or disable circuit for solid state relays and switches until the programmed initiation event Author Rick Newlands 28 updated 09 12 14 a i J fom F i E ae Sam Technical papers Part 3 parachute design Parachute types In the following tables note that chute drag coefficients are based on the nominal area So the actual fabric area of
39. en shears the riser in half or if the riser remains intact it cuts a slot down the fuselage like a cheese wire which is known in rocketry circles as zippering Then the two sections of fuselage are often joined by a length of shock chord This chord is often very long as this makes it more elastic which is reckoned to lower the recovery loads Actually the opposite is true the long length gives much more time for the separate bits of fuselage with their own individual drags to attain markealy different airspeeds relative to each other which causes a large shock load when the line finally goes taut A short length of elastic bungee would be much better Alternative design for larger vehicles How are large commercial sounding rocket vehicles recovered Certainly not like the above For start off hurling large bits of airframe around is clearly not acceptable large rocket fuselages can be made much more delicate to reduce their mass but the downside is that they d simply fold up if flying sideways Also larger fuselage tubes have proportionally much larger inertia for their size therefore colliding tubes would break up on impact with each other For these reasons the drogue chute that comprises the first stage of the recovery system is usually fired sideways or rearwards out of the bottom of the fuselage its lines are connected to the base of the vehicle Rear eject is the system used by aircraft and dragsters and for
40. ge Note that this altitude effect depends upon canopy loading only low canopy loadings such as main chutes suffer from this altitude effect whereas drogues are insensitive to altitude Author Rick Newlands 13 updated 09 12 14 Technical papers Part 2 recovery system notes and components The following are some notes and information that discovered in my parachute design research Positive deployment For reliable parachute opening it is essential to physically pull or throw the chute away from the rocket vehicle at a reasonable initial relative speed otherwise the chute may flop against the fuselage or snag or rip on the fins before it has a chance to open and may subsequently not open fully or even open at all The initiator SEPARATION MN P KEVLAR BAND The device that initiates the deployment can be of many forms electromechanical solenoid or geared electric motor servo thermal bi metallic strip or melt through plastic restraint sewn with nichrome hot Wire as shown here SEWN TOGETHER WITH KAPLAN NICHROME WIRE THREAD TO POWER SUPPLY or whatever The pyrotechnic variety such as explosive EXPLOSIVE BOLT EXPLOSIVE bolts or hot gas expansion powered T 3 devices piston driven line cutters and RELIEF CHANNEL latches burst diaphragms have very low mass for their power so are extensively used EXPULSIVE CUTTER SLED ALP au SLA ALLA AE AF AE AA AM AEA a DUPLICATED D i EXPULSION
41. ion e This system has been tried successfully on all sizes of vehicle model rocket small HPR and much larger Cons e Slightly higher complexity e The shell is jettisoned completely so it must be designed to have a low terminal velocity for the safety of people on the ground below fit it with its own streamer if necessary While it would initially appear that a weighty drogue shell would generate a higher snatch load the shell parts company with the drogue before the maximum snatch load can build up and of course the shell keeps the drag area low Here s the drogue shell expulsion tube and lines tube for our ADV1 rocket Few properly designed recovery systems fail because of the snatch load Force With the use of a drogue shell or 1000 F Snatch Force deployment bag the snatch load m A will be equal to or more likely Fors less than the opening shock load when the chute opens as 0 1 2 3 4 shown in these comparative TIME SECONDS TIME SECONDS graphs a Without Deployment Bag b With Deployment Bag Canopy First Deployment Lines First Deployment In a poorly designed traditional system however the snatch load might be several times higher than the opening load The snatch load will cause inertial loads on any devices attached directly to the chute canopy these must be secure or they ll tear off If an auxiliary or previous stage s chute is used to haul out another chute
42. ional system There will be a large momentum built up relative to the rocket vehicle due to the drogue s admittedly small mass multiplied by the difference in velocity The less mass the drogue has the higher its velocity difference tends to be In consequence when the riser connecting the chute to the rocket vehicle finally goes taut there will be a sudden whip load down the riser caused by the deceleration of this momentum This dynamic twang is known as the snatch load and if no attempt has been made to restrain the canopy from partially inflating before this snatch load has concluded this can be the highest load the recovery system has to suffer You might think that the small mass of the tiny drogues used in HPR rocketry couldn t produce a significant snatch load but you d be surprised Reducing the snatch load The way to reduce the snatch load is to reduce the difference in airsoeed between the chute and the vehicle at lines taut Apart from going easy on the amount of expulsion charge used and keeping the riser short the other way of doing this is to reduce the drag of the chute keep it tightly compacted until after lines taut to reduce its drag area by containing it in a bag or shell The deployment bag A rucksack like or sausage like bag used to contain the packed chute prior to and during the initial stages of deployment Often the bag has two compartments that are opened in sequence to separate the pro
43. lands 38 updated 09 12 14 Technical papers Glossary Items in bold are cross referenced to other glossary entries to save repetition Apex The geometric centre of a canopy where the bridal lines converge which requires reinforcing around the vent hole Apogee The highest altitude reached by a body on a trajectory launched from or passing close to or orbiting the Earth Latin apo geos The corresponding lowest point is the perigee but this term isn t usually used if the perigee would be within the thicker lower atmosphere or worse underground Bridal lines or Suspension lines The many individual lines running from the canopy to the confluence point Canopy The fabric drag producing area of the chute Canopy loading Cd SO The ratio where Cd is the drag coefficient of the canopy and 50 its Nominal Area is the total system mass parachute plus store g is gravity Confluence point Where the bridal lines converge at the riser sometimes at a large knot or keeper Constructed area Sc The cross sectional area of the mouth of the canopy when constructed based on the Constructed diameter Dc Constructed diameter Dc The diameter of the mouth of the canopy when constructed For a flat circular canopy this is the same as Nominal diameter DO whereas for a conical canopy Dc will be less than DO depending on how steep the cone is Deployment bag see main section A rucksack lik
44. ll HPR chutes don t package well they require a proportionately larger internal volume to accommodate the folded chute in its bag For minimum diameter HPR vehicles e g 54 mm diameter fuselage tubes the folded main chute becomes a very long sausage that requires a very large fraction of the fuselage length to accommodate it the only way to get such a long thin chute out is to split the fuselage across a diameter and draw it lengthways out of the tube Any bag that has to contain such a chute is also long and very narrow and due the aforementioned minimum material thicknesses the system of chute within bag simply isn t nearly flexible enough to function properly the chute won t come out of the mouth of the bag Therefore the entire fuselage diameter is used as the chute container instead of a bag More on bags later Then there s the drag area scaling effect the fact that small rockets suffer a disproportionately much larger drag effect than large rockets and furthermore the fact that they often go transonic at very low altitudes which causes a huge drag spike This then requires seriously robust fuselage tubes and nosecones for small rockets to withstand the drag of the nose compressing the tube These small rigid fuselages and nosecones can then withstand much higher accelerations it s acceptable to fling these fuselage sections all over the sky Until that is a thin fin meets an upper section of fuselage and punctu
45. lsevier publishing Other References e Help and advice from Irvin Parachutes U K Rocket Services Dorset the United Kingdom Rocketry Association and friends from the Black Arrow launch vehicle programme and the High Power Rocketry community Author Rick Newlands 44 updated 09 12 14
46. m alactronies Aras tha Drogue charge and deploys tha Drogue at peak altitude and now pulls on the top of the Man Tha Main is retained in the Ling fom the to the crown of the Main airframe by the line going from the PRM Toggle to the top of the Main At low E M altitude the PAM fires releasing the Toggle and allowing the Drogue to pull the this ineisiietion saser Main from the airframe and daploying it to allow cafe recovery of your rocket MAIN PARACHJTE rmi 8 m GS 2 RU 5 UR c m Um mom omo a J oe P I te Drague deployment charge A few pyrotechnic initiators of the hot gas type can allowably be homemade but their reliability is only as good as the testing and quality control applied Installing identical backup devices redundancy in such a way that the failed device will not hinder operation of the backup is advised Backup igniters are advised too wired in parallel or better yet fired from a completely separate circuit Frangible bolts prefer the name frangible bolt rather than the name explosive bolt because the latter term is too emotive N B Homemade explosive bolts are illegal in the U K if manufactured from metal because of the shrapnel hazard Instead nylon bolts can be purchased from hardware or electronics stores then drilled to form a small cavity
47. n e Each completely separated part will need its own recovery system and the required reliability of each part s recovery system will increase if any expensive payloads rocket motors or equipment are housed within it e The chances of the ground crew successfully recovering all completely separated parts increase if they all land fairly close together which depends upon how late in the recovery sequence they separate Separation joint design A separation joint needs forethought in its design Creating a separation joint that is lightweight but can withstand the forces acting on it is tricky in itself but the biggest technical challenge is to design a mechanism that won t jam if actuated while the joint is suffering any sideways or bending forces and in fact will work every time Author Rick Newlands 20 updated 09 12 14 Technical papers The locating ring or socket As used in model and HPR rocketry this simply consists of a FUSELAGE EE HS coupler tube or ring that acts as an internal collar linking the two THEE ARR fuselage sections together f INN Fixed rigidly to one section it is a simple slide fit into the other H _ Moa N A particularly heavy forward section might decelerate less than the Qs 4D rearward section after burnout causing premature separation If this N 7 8 jj could be a problem the sliding parts can be secured prior to N 7 deployment via release pins which are pulled out by a
48. n allowed to dry Or use nomex shields e The chute riser and or bridal lines should be protected too Author Rick Newlands 16 updated 09 12 14 Technical papers The tractor rocket This is a small auxiliary rocket tied to a FUSELAGE heatproof lanyard so that the rocket exerts a pull One design is shown here PAPER COVERS AT BOTH ENDS OF LALINOCH TUBE There is an obvious fire risk if launched from TRACTOR ROCKET inside the fuselage so a tractor rocket would usually be fired from inside an insulated tube The exhaust from the rocket being contained by the rear of the tube exerts a back pressure on the rocket s nozzle that can reduce thrust so although there is an expulsion tube effect also the net thrust is lower ANNULAR FIN N B There is the temptation to try putting a little expulsion powder inside the tube but don t as this could crack the rocket s nozzle or worse its brittle block of solid propellant which would cause the motor to explode Use a tube open at both ends as shown above to reduce the back pressure on the nozzle Any paper end covers should be glued onto the tube otherwise they will blow out during ascent of the main rocket vehicle Pros e It has an extremely high power to weight ratio e There is no recoil on the vehicle as the system deploys Cons e will exit the vehicle initially at a much lower speed than a drogue shell for example although it will then continue to
49. namic shroud or causes high drag Author Rick Newlands 2 updated 09 12 14 Technical papers Bearing Lock When the inner piston is fired upwards pyrotechnically the ball bearings can roll inwards into the now exposed recessed channel in the piston freeing the outer tube Hydraulic quick release couplings incorporate a bearing lock so can be RM TWO PARTS TO BE SEPAHA TED used SHEAR Pros BEARINGS e Reliable especially if multiple RECESSED expulsion charges are used CHANNEL e Very little force is required to move the piston whereas the lock can successfully restrain very heavy loadings prior to separation e The ball bearings can be used to give a low friction release if more than one set is used C vons e Has to be manufactured to a reasonable tolerance to work e Unacceptable debris hazard from flying ball bearings unless they re captured after leaving the recessed channel Shear pins It has become standard practice in the UK HPR world to use shear pins to hold sections of fuselage together These are small pieces of plastic that shear when the expulsion charge Brass insert fires due to the visco elastic properties of plastics which means that they can absorb heavy loads but only small Shear pin shock loads The pins are inserted snugly into holes drilled through the coupler joint to be restrained and glued in place Often a small insert of very thin brass plate or tube is mounted
50. nflate Froude number An aerodynamic scaling factor equal to 2 n where V is the snatch velocity g is gravity and DO is the Nominal diameter 8 of the canopy The higher the snatch velocity the higher the Froude number Author Rick Newlands 40 updated 09 12 14 fee Technical papers Gore One of the fabric panels sewn together to make the canopy The number of gores used to make a chute tends to vary linearly with increasing canopy diameter and is equal to the number of bridal lines minus one Hardpoint The strengthened attachment point on the store that the riser is attached to High powered Rockets Rocketry HPR Non commercial hobbyist rocket vehicles powered by motors of H class or above would suggest that large vehicles above N class require non HPR recovery system designs The governing body for HPR in the UK is the United Kingdom Rocketry Association www ukra org uk believe that the techniques and devices described in this document comply with UKRA rules and legislation though ask their Safety and Technical Committee for advice Indicated airspeed See Equivalent airspeed Lanyard An auxiliary pull line used to haul or actuate something Line Cutters or knife cutters Used for general line cutting such as cutting through the laces of a lace packed deployment bag as an alternative to the release wire They re in essence small washers who s inner edge has been sharpened to a k
51. ng the c opening shock of the chute is trivial as empirical int Mass values relating the opening shock force ae I a 0 coefficient to the eventual steady state drag Flat SS 1 8 Circular coefficient are known for most chute types Several values are given below note that the simplest canopy designs give the highest shock Conical load oimply multiply the steady state drag coefficient Cdo based on the nominal area see glossary by the peak opening load factor Cx given in the diagram here and then plug the resultant coefficient into the drag equation as usual The opening shock is therefore Cx times the drag force at the opening airspeed The table of Cx values is given here is for the types that have constructional details given in part 3 of this paper Author Rick Newlands 12 updated 09 12 14 Technical papers Main chute opening shock The canopy loading of drogues are almost always Infinite mass but main chutes must have much lower canopy loadings than drogues to keep their vertical descent speed low and strictly require a Finite mass analysis The store will decelerate during the canopy inflation process which lowers the dynamic pressure progressively during the filling This lowers the peak opening shock force considerably compared to the Infinite mass case perhaps by more than 50 therefore a conservative design philosophy is to calculate the infinite mass value as before which therefore gives a s
52. nife edge perhaps by careful countersinking Two small holes drilled in the rim of the cutter allow you to weakly sew the cutter as if it were a button to some part of the recovery system to temporarily restrain it The diameter of the cutter s inner hole should be slightly bigger than the line that s to be threaded through it for clearance and for safety if the cutter vibrates or moves a little during flight before deployment The cutters have a slot to allow connection to a lanyard so that a strong pull on this lanyard breaks the cutters free and further pulls the cutters through the line cutting it BLADE Another shape of cutter which is easier to manufacture out of disposable razorblades is LINE TO BE CUT Mass ratio M The ratio of the store mass to the mass of air trapped within the chute which varies with the m cube of the Nominal diameter 00 M S where p is atmospheric density p Main chute The large final stage chute also called the landing chute Model rocketry Rocket vehicles powered by motors of G class or less Nominal area SO The actual area of fabric of the chute Author Rick Newlands 4 updated 09 12 14 Technical papers Nominal diameter DO D0 T This is defined from the Nominal area SO as 50 for all canopy types though this is only actually the case in the real world for flat circular canopies Opening shock load Some milliseconds after the
53. oading 0 05 0 010 0 015 0 05 0 010 0 015 so multiply the gradient of these STRAIN STRAIN graphs by li the unstretched line a New Webbing Loading b Used Webbing Loading length Note that the area under an average used rope curve is much less than the area under the new rope i e the energy absorbing properties of a rope are less after the first stretch so a used dissipates a snatch or opening shock load less therefore those loads will affect the store more To select k for an old rope use the gradient of a tangent to the curve for the range of working loads designed for i e it would be wise to construct the riser from rope that is twice as strong as will be required so one would use the value of k derived from the tangent to the 50 rated load point in the above graph b whereas if one wanted to work out the failure load of the system i e the load that would just snap the riser use the higher value of k at the 10096 rated load point In the above graphs the 5096 and 10096 gradients are probably similar but if you were using a safety factor of 5 the gradient at the 2096 load is lower Note that you ll get higher k values for an old rope which in the equations for Fs above gives higher snatch loads as expected For ropes bundled in parallel bridal lines simply add the k s of each rope together whereas for ropes in series e g riser connected to bridal line s add the K s as 1 Ktota 1 k1
54. ock shearing force Snatch load Whether forcibly expelled pyrotechnically or not by the time a chute has travelled to the full extension of the riser the chute has built up a sizable difference in velocity relative to the rocket vehicle it deployed from In consequence when the riser connecting the chute to the rocket vehicle finally goes taut there will be a sudden whip load down the riser caused by the deceleration of this momentum This dynamic twang is known as the snatch load Snatch time The time when the Snatch velocity occurred Author Rick Newlands 42 updated 09 12 14 Technical papers Snatch velocity vs actually a scalar quantity airspeed What point in the canopy opening process should be defined as the start of opening One could use the speed the rocket vehicle was doing when the recovery sequence was activated but if the physics of the actual canopy inflation process are to be investigated the effects of varying riser lengths or varying expulsion velocities would preferably be removed from the equation so a more useful reference point is the velocity of the system just prior to inflating during the snatch load see snatch load in main text The snatch velocity is defined as the airspeed the system was doing at the peak maximum of the snatch load at whatever time that peak occurred This peak is used because it s easily spotted in recorded load time data such as a graph but if this isn
55. of pictures taken per second of the camera Recovery pyrotechnics electronics safety Any recovery pyrotechnics used must only be armed at launch preferably during lift off for the safety of ground personnel A pull out metal pin tied to the pad or simple break wire will tell the onboard electronics when the rocket is leaving the launchpad and can be used to arm the system and or initiate timers The RDAS flight computer can be armed with such a breakwire With more and more electronics being fitted into HPR rockets the potential for inadvertent recovery device actuation by stray electrical currents from other systems becomes a concern Here are a list of recommendations from Ref 5 for pyrotechnic electronics for spacecraft e electrical wiring and power source must be completely independent and isolated from all other systems They must not share common cables terminals power sources tie points or connectors with any other system e The system initiator must be isolated electrically by switches in both the power and return legs e All electrical circuit wiring must be twisted shielded and independent of all other systems The use of single wire firing lines having their shield as the return is prohibited e Shielding must provide a minimum 20 decibel safety margin below the minimum rated function current of the system initiator that is the maximum no fire current for electrically actuated pyrotechnic devices e Shi
56. ollo re entry capsule main chutes were compressed to the density of oak in a hydraulic press for compact storage e A chute deployed from a compressed deployment bag of reduced cross sectional area and hence reduced drag suffers a greatly decreased snatch force A compressed chute takes up much less volume e f decelerated too violently a chute s inertia acting on itself can throw its neat folding vital for clean deployment into disarray If the chute is tightly restrained by a compressed deployment bag then much higher decelerations can be withstood Should you wish to make a compressible deployment bag the method of lace packing has been found to be the easiest to construct wherein just as on a shoe laces threaded through eyelets sewn onto the bag are tightly pulled The laces are then cut at deployment time by line cutters Or the lacing is laced around a release pin as shown here in such a way that pulling the wire pin out opens the bag Author Rick Newlands 6 updated 09 12 14 Technical papers As found out the hard way this system doesn t work for small main chutes within minimum diameter HPR airframes because the number of lace loops becomes excessive for such long thin chute bags resulting in a high release pin friction used metal hoops instead of lacing to reduce the friction With larger vehicles the length to diameter of the bag reduces reducing the release pin friction and also the dr
57. omponents that could go wrong e Engineering factors of safety Recovery systems can often be significantly over strengthened with little increase in mass so do so Testing To discover any hidden flaws in the design e Simplicity Simplest is always best in terms of reliability and tends to weigh less though don t go too far the traditional HPR design is too simple As there is only finite internal space and mass to allocate to the recovery system you have to hypothesize possible failure modes and then prioritize in terms of likelihood of occurrence Author Rick Newlands 3 updated 09 12 14 Technical papers Only testing will show whether you guessed right The recovery envelope When parachutes inflate they exert huge forces down the riser to the store These opening loads like all aerodynamic forces vary directly with dynamic pressure which will obviously be a minimum at apogee Even with the steep trajectories of rockets fired at near vertical launch angles the horizontal airspeed at apogee can be surprisingly large creating opening loads rising to several Kilonewtons The recovery envelope is the range of airspeeds that one designs the recovery system to be able to function over The higher the allowable opening speed the less critical is the need to open at apogee and the more flexible the recovery system can be to deal with malfunctions such as unusually high airspeeds caused by an unexpected flatter traj
58. res it Traditionally the HPR recovery system comprises splitting the fuselage at a couplered socket joint to let the drogue chute out Typically the split is far rearwards of the nose causing the fuselage sections above and below the joint to become aerodynamically unstable they end up flying sideways on to the airflow This is reckoned to aid the deceleration of the vehicle maybe it does but only in a very brutal way the airframe loads are enormous Then the parachutes come out spewed out like so much untidy washing out of a spin drier canopy and lines all come out together in a mess most of the time the chute opens but not always sometimes the canopy gets tangled in the lines which isn t good enough Then there s the snatch load in parachute design this is the name given to the shock load that occurs when the chute riser goes taut and the mass of the chute which hasn t yet opened decelerates rapidly In traditional recovery systems this snatch load is often equal to or larger than the subsequent opening load when the chute opens It shouldn t be Author Rick Newlands 2 updated 09 12 14 Technical papers More on the snatch load later bear in mind that scaling effects cause larger chutes to have proportionately larger inertia which causes proportionally much larger snatch loads With HPR and model rockets the snatch load can often go through a riser that s bent over the lip of the fuselage joint This oft
59. rol line A two section reefing line is attached to the canopy skirt at points A see below guided around one quarter of the skirt and out of the canopy at points B to a confluence point C returning the same way but around the adjacent quarter of the canopy A second reefing line is run similarly around the second half of the canopy and is connected with the first line at point C The reefing system must allow full opening of the canopy Pulling the control line toward the confluence point of the suspension lines reefs the canopy paying out the control line dis reefs it PARACHUTE CANOPY REEFCO PARACHUTE CANOPY FULLY INFLATEO REEFING RINGS CONTROL LINE LINES DISREEFING DEVICE F DISREEFING DEVICE s REEFEO DISREEFED NOTE A TWO SECTION REEFING LINE IS ATTACHED TO THE CANOPY SKIRT AT POINTS A GUIDED AROUND ONE QUARTER OF THE SKIRT AND OUT OF THE CANOPY AT POINTS 8 A CONFLUENCE POINT C RETURNING THE SAME WAY BUT AROUND THE ADJACENT QUARTER OF THE CANOPY ISEE 563 Parachute Skirt Reefing With Control Line Here s this skirt reefing system added to a commercial HPR parachute using metal D rings bought from a sewing website Author Rick Newlands 24 updated 09 12 14 Technical papers used a Defy gravity tether for the dis reefing device for this chute www defyg com tether html Note that small HPR sized chutes can t be r
60. snatch load peak is past the canopy opens If a drogue shell was used the drogue has just been pulled free of the shell as the shell s inertia kept it going The chute rapidly fills and inflates creating a momentary peak drag load known as the opening shock load this peak can be 2 or 3 times the steady drag of the chute and is caused by the mouth of the canopy swallowing a mass of air which it then decelerates Projected area Sp The actual cross sectional area of the mouth of the canopy when inflated For originally flat circular canopies this is considerably less than their Nominal area Recovery system All components of the system designed to allow safe recovery of some store The main line attaching the chute to the store N This runs from the confluence point to the store hardpoint and is sometimes formed from extended bridal lines which are sewn Branched Riser 17 together at the confluence point and li dini m Pr NT bound further by a hoop of tough fabric or e ser pause webbing Metal at that point known as a keeper Stitching Keeper Sleeve Extended bridal line risers are usually protected by a fabric sleeve Keeper Separation plane The sectional plane across a joint that separates during part of the recovery sequence to allow chute s to exit Shear pin A pin of metal or plastic that holds some recovery component in place until the pin is shattered by a sh
61. t in order to provide an open compartment to allow the subsequent release of the chute The method popular in both model and High power rocketry is to use expulsion charge to pressurise the inside of the fuselage which then pistons apart at a slide collar joint Traditionally this collar is at the base of the nose which is thrown off as the chute below it cannons into it as a form of forwards ejection In an HPR system known as anti zippering chutes are rearwards deployed from the upstream section The expulsion charge used for separation also blows the chute out Points to consider when choosing the location of a Separation plane are e Inamultistage recovery system at which recovery stage should separation occur e Will the difference in drag to mass ratios of the two separated parts of the structure cause them to drift apart under aerodynamic forces after separation or collide Recently two separating parts of a K powered rocket collided near apogee embedding the fins of one half through the composite fuselage of the other half e the separated parts aerodynamically stable or unstable As well as causing fouling problems a tumbling section has a much higher drag than if not tumbling e What Normal Axial and Bending forces will the separation joint have to withstand at your chosen separation plane location prior to separation e If both sections are joined by a long length of riser there is a risk of collisio
62. t known the rocket vehicle s velocity at deployment often isn t greatly different Store Store is the preferred originally military recovery system term for the payload suspended under the chute i e everything else that isn t part of the recovery system itself In rocketry avoid using the term payload when referring to the store as payload also means the cargo carried by the rocket vehicle which causes confusion Strain Percentage stretch of a line per unit length StretchedLength UnstretchedLength d Or if dis the difference between stretched and unstretched length UnstretchedLength System The system is the chute and the store different from the Recovery system Terminal velocity As a falling object accelerates under gravity in an atmosphere its drag will increase until a point is reached where the drag force equals the object s weight and the net acceleration is zero resulting thereafter in a constant vertical velocity known as terminal velocity The drag reduces any initial horizontal velocity component of the trajectory to zero fairly quickly Depending upon the Nominal area 50 of the chute in relation to the total system mass m this terminal velocity could be higher or lower than the parachute deployment airspeed The terminal velocity is simply calculated by rearranging the drag equation as 2mg
63. the wake of a large forebody Typical applications are as stabilization and retardation parachutes for several types of ejection seats for the encapsulated seats of the B 58 and B 70 bombers and as first stage drogue chutes for the F 111 and the B 1 crew modules The canopy of the hemisflo parachute forms part of a perfect sphere with the suspension lines connected tangentially to the sphere see figure below where L D 2 0 h 0 916 D 1 20 Dd b My n he Bil iit N TT A 5 Typical Design of a Hemisflo Parachute The point where the lines contact the canopy becomes the canopy skirt resulting in a 210 degree canopy see above figure The hemispherical shape avoids the use of gores that can flutter in and out as on flat or conical canopies and eliminates the length difference in the leading and trailing edges of the horizontal ribbons This greatly reduces canopy breathing and high frequency ribbon flutter both sources of canopy damage and drag decrease on conical ribbon parachutes operating at supersonic velocities All detail design recommendations of conical ribbon parachutes also apply to hemisflo canopies The figure above shows horizontal ribbons on alternate gore sides As previously explained this arrangement may cause a venetian blind effect and can lead to canopy rotation Having both radials on the same side will decrease rotational tendencies Author Rick New
64. there must be a deliberately weak link connecting them that s designed to break when the latter stage s riser goes taut otherwise the drag of the actuating chute and its mass and the mass of air captured within it will seriously increase the snatch load if it stays attached permanently A better design is to have the previous stage s chute pull the shell or deployment bag off of the subsequent chute though the length of riser between previous stage chute and this bag mustn t be too long or high snatch loads will be generated due to excessive relative velocity Similarly the canopy must exit the bag or shell with little friction or the bag shell will pull on the canopy increasing the snatch load this is a problem for small bags and small chutes Author Rick Newlands 8 updated 09 12 14 Technical papers One important factor in determining the snatch load is the energy absorbing properties of the riser and bridal lines Note that a brand new rope will stretch and absorb a lot of the energy whereas a used rope is already partially permanently deformed and so is effectively more rigid and so will break more easily or will transmit more of the snatch load to the rest of the recovery system onatch load prediction as used by a parachute load prediction program This can be done on a spreadsheet but it s easier if programmed advise using a better method of integration than the Euler method to get velocities and displacemen
65. tion can melt synthetic fabric Heavy cotton sack cloth or linen is typically used instead such as heavy duty curtain lining Calico or nomex The bag may require axial strengthening with webbing or tapes Hopes and Lines Heavy duty webbing e g Dacron tape and strong lines can be bought either from kite shops or from shops supplying materials to make horse rugs and bridals Alternatively purchase from rocketry vendors Other lines and fastenings can be bought from yacht chandlers or mountaineering supply shops Hopes and lines are weakened considerably if forced to turn sharply through a large angle especially if they kink some sort of guide with a large radius such as a pulley wheel or grommet is advised Supersonic parachutes Currently our drogue chutes are deployed at low to moderate subsonic airspeeds But the time will come when our vehicles rise above then re enter the sensible atmosphere at supersonic airspeeds Subsonic chute designs forced to open at supersonic airspeeds will experience a shockwave across the canopy mouth which destabilises them they can flutter inside out and or tear apart Supersonic chutes therefore have to be designed differently Author Rick Newlands 30 updated 09 12 14 Technical papers This figure shows the supersonic flow field around a streamlined body with an attached aerodynamic decelerator at a velocity of approximately Mach 3 The distance between the body and the le
66. ts during a parachute deployment as simple integrations tend to numerically explode under large or sudden accelerations 2nd or higher order Runge Kutta integration methods are suitably stable oubject to the following restrictions a 1 dimensional analysis can be used e The deployment is reasonably parallel to the airflow not transverse so that the chute s deployment trajectory is pretty much a straight line i e deployed rearwards to the direction of flight e The chute is packed into a deployment bag or drogue shell to reduce its drag to a small value compared to its mass e canopy slides easily out of any drogue shell or deployment bag as soon as the lines go taut In the following diagram the remaining section of vehicle is travelling in a tail first attitude to the left After going taut at point 1 below the suspension lines and riser stretch as the chute decelerates relative to the vehicle and the chute canopy momentarily comes to rest at some maximum stretch point 2 before rebounding To analyse this situation a traditional loads analysis doesn t work because the loads are changing rapidly with time and the problem becomes intractable Instead you need to perform an energy analysis as this can capture the dynamics of the stretch and rebound This is the preferred method used by the parachute industry The Snatch load Fs can be calculated by comparing the work done in stretching the bridal lines riser bundle
67. und into a tail first attitude e Ifthe riser is short to keep the chute upstream of the fins to prevent snagging the vehicle must be rotated into a tail first trajectory to prevent the chute lying unopened against the fuselage as actually happened on some slender HPR vehicles Cons e AsSside hatch or door may be required which must withstand the pressure differential between the lower pressure of the air moving past the vehicle and the static pressure inside When we ran the NRC competition one team s side door had a weak catch and the door was sucked out e Limitation on hole size in the side of the fuselage as a large hole needs extensive edge reinforcement otherwise the fuselage is weakened e Holes for main chutes tend to be long and narrow which makes uniform deployment difficult We ve been experimenting with parabags These are calico see materials section or nomex airbags inflated by a small packet of expulsion charge and ensure an even deployment pressure along the length of the chute The bags need coated in liquid rubber or some other sealant to make them gas tight e Zippering is still an issue but avoidable with grommets radiusing etc Rearwards ejection Expulsion tubes or drogue shells require modification if they re to be used for ejection from the rear of the vehicle or they ll simply fall out due to the acceleration during motor thrust Having a chute open while the motor is thrusting coul
68. velocity and can be ignored in the terminal velocity equation see glossary so the vertical landing velocity is 2mg V see glossar V p C g y Squidding If opened at too high an airspeed simple main chute type canopies simply fail to open and streamer behind the rocket vehicle The canopy and lines then look remarkably like a squid Squidding seems only to affect very large chutes haven t heard of an HPR sized chute that went squid Load dissipation A device that can dissipate some of the high loads occurring within parachute risers during Opening are often incorporated into recovery systems Note that nylon rope stretches permanently above a certain load dissipating this load whereas Kevlar does not and simply snaps without prior stretch at too high a load Several long bungee chords or elastic straps connected in parallel with the riser are often used These are known in rocketeering circles as shockcord The frangible tie is shown opposite which is a strip of webbing folded lengthways and sewn together On opening both ends of the webbing get pulled apart and the graduated stitching tears smoothly and progressively dissipating shockloads The MARS rocket society used a variation on this idea they made loops in their risers and wound adhesive tape across the neck of the xar a loop The tape tore upon recovery deployment dissipating some of the load Testing This is
69. vital for ironing out the inevitable bugs in the recovery system For reasons that aren t terribly understood windtunnel testing never yields overly useful drag or opening shock results so other novel methods have to be used 1 Dropped from manned aircraft or hot air balloon Pros e Controlled experiment e High snatch velocity Cons e Expensive e Civil Aviation Authority waivers have to be acquired to allow dropping of anything e Possibly hazardous to pilots and ground personnel Author Rick Newlands 26 updated 09 12 14 Technical papers 2 Dropped from radio controlled aircraft rocket or large kite Pros e Cheaper No CAA waiver required e Horizontal deployments can be obtained by rocketry lowering the launch angle allows the same apogee velocities to be reached using lower power motors Cons e Complexity of remote release systems e Snatch velocity altitude attitude information etc must be obtained somehow e Possible construction and launch of another though simpler rocket vehicle 3 Dropped from tall building or cliff Cons e Vertical trajectory only gravitational effects have to be removed when extrapolating to deployments from horizontal trajectories Safety of those below Building or cliff might not be tall enough to obtain required velocity 4 Deployment from road vehicles 0 ros Cheap Controlled experiment in situ recording Cons e Even allegedly
70. ws the ballute drag coefficient as a function of Mach number The drag coefficient relates to the inflated area of the ballute Sp and not to the total surface area So as is customary on parachutes Another more modern ballute design is an inflatable ring this is easier to fabricate and avoids the wake of dead air behind the base of the body www gaerospace com projects Hypersonics aerodec elerators html Author Rick Newlands 32 updated 09 12 14 Technical papers Solid Cloth Parachutes Fiat Circular The canopy I5 8 regular polygon of i D Constructed as a flat surface wilh central veni design i the basis for mos circular Coulis OMEN types being variations in oore pattern and general geometry Flat circular parachutes are simple and economical to construct handle and CONSTRUCTION SCHEMATIC and often used in clusters They are in wide use for personnel and airdrop applications This parachute is very Data for several specific at circular parachute and load configurations are listed below for drag coefficient increase and impro vici infiation characteristics D i So 187 PL TEC 5 LW tan 780 AN NU CN i 4 A 2h tan 180 NEN i 29 tan 180 AN 1 1 Generally i L T Sy 007 5 o n INFLATED PROFILE Fi Ios O80 to 1 25 Da

Parachute recovery system design for large rockets

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