5th Iteratioal Coferece o Laucher Techology 23, S15.2 Progresses i Simulatig the Advaced I-Air-Capturig Method Marti Sippel, Josef Klevaski Space Laucher Systems Aalysis (SART), DLR, 5117 Cologe, Germay Marti.Sippel@dlr.de, Josef.Klevaski@dlr.de Recetly a ew, iovative approach for the retur of o-ssto reusable space trasportatio vehicles has bee proposed by DLR: The wiged stages are to be caught i the air ad towed back to their lauch site without ay ecessity of a ow propulsio system. This pateted procedure is called i-air-capturig. The performace gai by this advaced method shows a possible icrease i delivered payload betwee 15 % ad 25%, assumig the same structural techology level of the stages. Alteratively, the sie of a reusable system ca be sigificatly reduced compared to the stadard approach, without ay loss i payload mass. The paper presets a detailed descriptio of the proposed method, givig data of umerical simulatios regardig the omial approach maeuvers. The most promisig choices for the capturig procedure with respect to flight time, approachig velocity, ad loads are studied. Trajectory simulatios with a represetative cotrol loop are performed, takig also ito accout perturbatios by atmospheric gusts. The secod part of the paper compares i-air-captured lauchers with covetioal systems ad quatifies the advatages. Nomeclature D Drag N H altitude m M Mach-umber - S distace m T Thrust N V velocity m/s W weight N k cotrol system coefficiets s -1 load factor - q dyamic pressure Pa α agle of attack - flight path agle - part sigal of cotrol s -1 η geometrical agle - σ bak agle - ψ aimuth - Subscripts, Abbreviatios 3 DOF three degrees of freedom ACCD aerodyamically cotrolled capturig device CAD computer aided desig GLOW Gross Lift-Off Weight GTO Geostatioary Trasfer Orbit IR ifrared LEO Low Earth Orbit LFBB Liquid Fly-Back Booster LH2 Liquid hydroge LOX Liquid oxyge MECO Mai Egie Cut Off RLV Reusable Lauch Vehicle RP-1 Rocket Propellat (kerosee) SSTO Sigle Stage to Orbit TSTO Two Stage to Orbit AC (capturig) aircraft R reserve WS as sep wiged stage assiged separatio 1 INTRODUCTION Oe major problem i the itroductio of multiple stage reusable space trasportatio systems, is to fid a adequate method for the stages' retur to the lauch site. A simple glide-back is oly achievable with either ocearoud-earth vehicles (very high -V requiremet close to SSTO) or small booster stages (oly small icremet to laucher's total -V). I ay other case secodary ladig sites have to be selected or precautioary measures for a powered retur flight are to be icluded i the reusable stage. Obviously, both approaches are closely boded to serious drawbacks. Ufortuately for future reusable stages, today s etire lauch sites are located such that oly scarcely populated areas (e.g. oceas) are foud dowrage. This is obviously due to the fact, that ay cosiderable damage o earth by the fall-back of expeded stages or destroyed lauchers has to be strictly avoided. Therefore, it is highly difficult to fid existig reachable ladig fields, or i case of a ocea it is eve quite impossible to costruct them at all. Cosequetly the requiremet to reach a alterative ladig site has a strog impact o the laucher's trajectory ad hece performace. I ay case, this method requires a cosiderable amout of additioal ifrastructure to ship the reusable stage back to its origial lauch site. Techiques of powered retur flight obligate a propulsio system ad its fuel, which raises the stage's iert mass. If the rocket mai egies should be re-igited, oe faces the poor specific impulse of such systems resultig i a (i terms of mass) prohibitively expesive amout of propellat just for deceleratig. Usually a more attractive solutio to the problem seems to be, to itegrate a secod type of air-breathig propulsio, reducig the propellat mass by a order of magitude but addig the weight of the additioal egies. 1
This paper proposes a differet approach: The wiged reusable stages are to be caught i the air, ad towed back to their lauch site without ay ecessity of a ow propulsio system. This so called i-air-capturig method is supported by large cargo trasports, offerig sufficiet thrust capability to tow a wiged laucher stage with restraied lift to drag ratio. I-air-capturig has bee pateted by DLR [1] ad is uder theoretical ivestigatios for almost three years [2]. lauch site. Close to the airfield, the stage is released, ad autoomously glides like a sailplae to earth. 2 MATHEMATICAL MODEL OF THE SIMULATION The lauch vehicle's ascet trajectory ad ballistic flight phase is calculated by a stadard 3 DOF optimiatio ad simulatio program. The flight path regards the usual costraits of safety, dyamic pressure, ad heat flux. Sice this approach is fully cosistet with all other laucher aalyses, it is ot further described here. The mathematical model used for the simulatio of the capturig procedure icludes a complete set of oliear dyamic equatios of motio i three-dimesioal space for both vehicles (the wiged reusable stage ad the capturig aircraft) with atmospheric simulatio ad a mathematical model of the wiged stage's cotrol system i modular structure. 3 DESCRIPTION OF THE PROPOSED IN-AIR- CAPTURING METHOD This chapter presets a detailed descriptio of the proposed method, givig data of umerical simulatios regardig the omial case. 3.1 Pricipal fuctioality A schematic of the reusable stage's full operatioal circle is show i Figure 1. At the laucher's lift-off the capturig aircraft is waitig at a dowrage redevous area. After its MECO the reusable stage is separated from the lauch vehicle, ad afterwards performs a ballistic trajectory, soo reachig deser atmospheric layers. At aroud 2 km altitude it decelerates to subsoic velocity ad rapidly looses altitude i a glidig flight path. At this poit a reusable returig stage usually has to iitiate the fial ladig approach or has to igite its secodary propulsio system. Withi the i-air-capturig method, the reusable stage is awaited by a adequately equipped large capturig aircraft. Both vehicles have the same headig still o differet flight levels. The reusable upowered stage is approachig the airlier from above with a higher iitial velocity ad a steeper flight path, actively cotrolled by aerodyamic brakig. The time widow to successfully perform the capturig process is depedet o the performed flight strategy of both vehicles, but ca be exteded up to about two miutes. The etire maeuver is fully subsoic i a altitude rage from aroud 8 m to 2 m. The upper costrait is set by the requiremet to reach full aerodyamic cotrollability of the wiged stage, which is foud to cause some time delay. After successfully coectig both vehicles the wiged reusable stage is towed by the large carrier aircraft back to the Figure 1: Schematic of the proposed i-air-capturig A basic requiremet is that trackig of the returig lauch vehicle is always possible by radar, or satellite, ad is commuicated via direct data lik. Therefore, a realtime optimiatio of the aircraft's geographical positio is maageable. Sice the ballistic phase of the stage exteds to several hudreds of secods, a correctio of up to 1 km is achievable, if separatio-coditios uexpectedly differ otably form the omial case. 3.2 Approach of the RLV to the capturig aircraft After deceleratio to subsoic speed at a altitude aroud 2 km, the wiged stage is actively headig towards the capturig aircraft. Uder omial circumstaces the latter is assumed to be i a 'passive' mode, just cruisig at costat altitude (e.g. 8 m) ad relatively low flight Mach-umber of about.55 which correspods to the equivalet earth speed 4 km/h. It has to be assumed that both vehicles are ow permaetly i commuicatio with each other. Durig descet the reusable stage is able to perform some positio-correctio maeuvers, ad to dissipate kietic eergy, if required. It plays the 'active' part i the approachig maeuver. After peetratig the deser atmospheric layers, the wiged stage is aerodyamically cotrolled by the agle of attack α, the trajectory bakagle σ, ad the air-brake deflectio agle δ FB. I this simulatio the speed brake is assumed to be of Space Shuttle rudder type although alterative lay-outs exist. Cotrol should be achieved as far as possible without major iterferece betwee α, σ, ad δ, acquirig 'quasiidepedet' parameters. The stage's avigatio system computes the assiged flight aimuth ψ as, which should always poit at the capturig aircraft, the curret distace, ad the altitude differece betwee both vehicles. The cotrol system of the wiged stage implemets the followig cotrol law regardig the lateral movemet: σ ψ ( as ) + k σ si σ (equ. 1.) σ 1 / s 2
where k are suitable cotrol system coefficiets, ad ad ψ are the curret flight parameters. The realiatio of this cotrol law results i a precise headig of the reusable stage i the directio of the capturig aircraft, ad is maitaied util capturig. The wiged stage firstly glides with a very steep agle gl (e.g. aroud -18 ) ad reduces gradually its velocity, while the capturig aircraft flies i the flight level H 8 km with the costat equivalet velocity VEAS 4 km/h (Mach- Number M.55). The cotrol system stabilies the flight path agle by adaptatio of α: α = + ; with (equ. 2.) ( gl ); (cos );.15 / s ;.1 / s ; with curret flight data ad, ad the cotrol system coefficiets k as well as partial cotrol fuctios. Further the cotrol system of the wiged stage permaetly computes the vertical geometrical agle η, which represets the agle betwee the lie of sight of the two vehicles ad the local horio H (equ. 3.) η = arcsi S where H is the curret altitude differece ad S is the curret total distace. Whe the returig laucher's positio relative to the aircraft comes to a certai vector poit, the ed phase of the approachig maeuver is iitiated. The the aircraft itself starts a descedig glide path, still i frot of the stage. I the simulatio described hereafter a descet glidig of both vehicles is chose, with a flight path agle capt slightly below the maximum L/D ratio of the wiged stage. The more or less parallel descedig of both vehicles eables a smoother approach maeuver, ad a extesio of the duratio available for the capturig. It further makes it possible to correct the distace betwee both vehicles ad to adapt the flight velocity of the wiged stage by air-brakig very precisely. But the almost colliear flight requires that the ormally higher lift/drag ratio of the capturig aircraft (about 15...16) has to be adapted to the L/D of the reusable wiged stage (aroud 4...6). This ca be achieved by usig air-stream spoilers, a air brake, ad /or by lowerig the ladig gear to icrease drag. Similar maeuvers are carried out by Gulfstream II jets operated as Space Shuttle traiig aircraft [6]. The absolute value of the agle betwee both vehicles η decreases gradually durig descet. At the istat whe this agle becomes smaller tha the wiged stage's glide path ( η < gl ), the capturig aircraft receives a sigal from the reusable stage to also start its descet flight at a pre-chose capturig agle as = capt. The laucher stage itself adapts the path agle such, to follow the capturig aircraft. As a result, after a short time both vehicles fly i lie with a icliatio agle correspodig to the chose capturig glide path agle. The wiged stage follows the capturig aircraft reducig its distace S ad its velocity V. The cotrol law is the followig: as H = arcsi ; S ( ); (cos );.15 / s ;.1 / s ; (equ. 4.) α = + ; with (equ. 5.) as δ FB = v + ; with (equ. 6.) δ v FB x 3 / s; V (V x ws x x ; δ V FB AC 9 V V S; (equ. 7.) R R where δ FB is the deflectio agle of the air-brake, ad V R is the assiged excess or reserve velocity of the stage. Note, that equatio 5 is differet from equatio 2 because the flight path agle as is ulike the iitial glide agle gl. The approach maeuver has bee simulated for differet reusable stages (see chapter 4 for type descriptio) accordig to the above explaied method, ad the cotrol costraits are applied. A example of a reusable stage with separatio velocity aroud 2 km/s is described here. Figure 2 depicts the trajectory data altitude vs. time, ad geographical positio of both flight vehicles from laucher separatio to the accomplished capturig. At first the wiged stage performs its ballistic flight, followed by a commo glide slope with the aircraft right after 6 s. At this time the geographical distace betwee them is less tha 3 km. Altitude [km] Latitude [ ] 1 9 8 7 6 5 4 3 2 1 5.3 5.2 5.1 5 4.9 4.8 4.7 R 2 4 6 8 ); Capturig Aircraft Wiged Stage 4.6-53 -52-51 -5-49 -48-47 -46-45 Logitude [ ] Capturig Aircraft Wiged Stage Figure 2: Simulatio of the reusable stage's approach procedure to the capturig aircraft startig with laucher separatio 3
Altitude [km] Velocity [km/s] Flight Path Agle [ ] 16 14 12 1 8 6 4 2 5 55 6 65 7 75 8.35.3.25.2.15.1.5 Capturig Aircraft Wiged Stage 5 55 6 65 7 75 8 5-5 -1-15 Capturig Aircraft Wiged Stage -2 5 55 6 65 7 75 8 Capturig Aircraft Wiged Stage Figure 3: Simulatio of the reusable stage's fial approach procedure to the capturig aircraft startig 5s after separatio from laucher The aerodyamically cotrolled approach as show i Figure 3 is iitiated, whe the reusable stage reaches the deser atmospheric layers ad decelerates to the subsoic regime. The steep glide agle of aroud 18 degrees is performed with a slowly decreasig air speed of aroud 265 m/s. After the capturig aircraft has received the appropriate sigal, both vehicles are descedig o early the same glide slope (6s). As ca be clearly see, the returig stage is still the active vehicle, sice it is subject to some cotrol deviatios i flight path agle capt. The wiged stage actively reduces velocity up to the poit where its miimum safety distace is achieved (675s). As ca be see from Figure 4, the total distace betwee the two flyig craft falls short.5 km aroud 655 s after separatio. Subsequetly the distace could be cotrolled i this simulatio at a miimum rage betwee 155 ad 2 m for duratio of 13 s. The upper boudary is ot set by vehicle cotrol, but by a miimum acceptable level above groud. The fial altitude i this simulatio is as low as 1.2 km. A time for capturig up to at least oe miute is evertheless well withi reach, sice the altitude after this period still accouts for more tha 2.8 km. Cotrol deflectio of the brakig flap is show i Figure 5. The assumed brake effectiveess icreases vehicle drag up to 25 % at full deflectio. The 9 upper limit is ever reached i the performed simulatio. Distace [km] 5 4.5 4 3.5 3 2.5 2 1.5 1.5 5 55 6 65 7 75 8 Figure 4: Total distace betwee the two stages i fial approach procedure startig 5s after separatio from laucher Air Brake Deflectio [ ] 9 8 7 6 5 4 3 2 1 5 55 6 65 7 75 8 Figure 5: Air-brakig deflectio agle δ FB of reusable stage's fial approach procedure 3.3 Compariso of Suitable Capturig Methods The capturig techique itself has bee systematically ivestigated i 3 DOF simulatios. The process ad the ecessary mechaics are by far ot optimied yet, but prelimiary aalyses give a idicatio of the most promisig techique. The homig head might be ehaced to fid its target with multiple redudacies. Radar o the capturig device or a active traspoder at the coectio poit of the returig stage are such improvemets to help safely coect both flight vehicles. Aerodyamics of the capturig devices is assessed by the DLR code CAC [4] ad the mass, iertia, ad the frictioal force of the rope are cosidered i the simulatios of the procedure. Cotrol of the missile is performed by the self-homig priciple, calculatig the required thrust vector agle of attack ad yaw always headig the device versa the movig target. Four differet types of capturig methods have bee studied: The first procedure proposed i [2] is the harpoo priciple with a missile lauched from the capturig aircraft ad directly shot versa the returig stage, a variat requires for the missile to perform a loop maeuver, ad approach the RLV from behid to cosiderably reduce the impact loads. The third optio fires the missile from the reusable stage versa the capturig aircraft, also decreasig relative velocity ad hece loads. The last alterative employs a aerodyamically cotrolled capturig device (ACCD), which is to be released by the airplae ad the towed, cautiously approachig the laucher. At aroud 12 m total distace S, a small pilot-rope is shot from the trasport aircraft to the returig stage. The 4
harpoo cosists of a ati-armor IR-guided missile with attached capturig device ad mechaics to rail the pilot rope. The relatively slow MILAN is chose to limit relative velocity at 3 m/s. The rope is stored o a sychroied powered drum system, located i the aft compartmet of the cargo trasport. The missile mass, cosiderably icreased by its payload, will reach a fial velocity i the high subsoic regime. The delivery duratio of the harpoo exteds to 6 s. A possible desig for the coectig poit is show i Figure 6. The aular structure is lowered from the reusable stage's ose sectio. Such a desig allows the powered missile to fly through the hole after detachig the capturig device. The missile s approach parallel to the stage s fuselage should be achievable avoidig collisio with the RLV. A shock absorbig sprig costructio o the stage ad the capturig device is to be desiged to withstad relative velocities up to 3 m/s. After achievig the primary coectio, a strog towig rope is draw out, while the wiged stage is still approachig the carrier aircraft. At a miimum total distace of approximately 2 m the cotrol system should prevet further approachig of the stage for safety reasos. The full deploymet ad coectio of the towig rope (v rope = 1 m/s) is assumed to take aother 2 s. Figure 6: Pricipal sketch of a coceivable coectig procedure with approachig missile ad coectig poit of the reusable stage i deployed positio The alterative approach by flyig a loop maeuver has to be carried out by much faster air-to-air missiles like AIM 9, Magic 2, R-73, or ASRAAM. Although highly agile, the assumed costrait of 1 g icreases the miimum radius to a few thousad meters ad hece the rope legth beyod 1 km. Obviously, such a extet is completely impractical. Therefore, the secod capturig optio has to be dropped. The third procedure fires the missile i the opposite directio, from the reusable stage versa the towig aircraft. The coectig device has to be deployed from the airlier. While the pilot rope ad missile are to be carried by the stage, the towig rope evertheless should be carried iside the aircraft. The latter requiremet is beeficial for the stage's iert mass, but demads a more complex rope mechaism. Aother ati-armor rocket with icreased rage, the TRIGAT is selected. Iitiatig the process at a distace of 23 m, the target is reached i less tha 4s. The loads o the missile are beig ad do ot exceed 6 g i ay directio. Relative velocity at target impact is sigificatly reduced (see Table 1). Figure 7: Rederig of the ACCD ad the returig stage cautiously approachig each other time for capturig [s] time for tow coectio [s] Aircraft to RLV (MILAN) RLV to Aircraft (TRIGAT) ACCD 5.7 3.73 14.3 2 2 check time [s] 5 5 5 total i-aircapturig time [s] 3.7 28.73 19.3 margi 29.88% 317.68% 521.76% relative impact velocity [m/s] 291.4 116.6 4.9 kietic eergy [kj] 242.43 131.4.96 Table 1: Characteristic data of differet capturig methods The last alterative employs a aerodyamically cotrolled capturig device (ACCD), which is to be released ad the towed by the airplae. This device (Figure 7) cotais the coectig mechaism ad simply advaces the stage by its ow drag ad lift, provided by small wigs (spa 1.6m). Actuators cotrol the ACCD s orietatio ad the approachig velocity is further cotrolled by brakig of the towig rope from iside the aircraft. Also iitiated at 23 m distace betwee the two crafts, the whole maeuver takes about 14 s i the omial case. All loads remai below 3 g ad the fial relative velocity is at 5 m/s. It has to be oted that although the time to achieve capturig is loger tha those with missile fired harpoos, full coectio by a towig rope ca already be achieved at the very momet of capturig. Therefore, the total time for the i-air-capturig is the shortest with less tha 2 s (see Table 1). Table 1compares the mai data of the three feasible capturig optios. The performed comparative aalyses of the capturig procedures clearly idicate that the aerodyamically cotrolled capturig device (ACCD) offers the largest time margi (wrt. the parallel flight duratio of 12 s) as well as the lowest loads. It further 5
offers a cost advatage compared to the expedable missiles sice this device ca easily be reused for each capturig. 3.4 Requiremets for the Towig Aircraft Techical requiremets of the tow-aircraft maily ivolve istallatio of the rope mechaism, availability of sufficiet thrust, ad i some special cases probably additioal spoilers to act as aerodyamic speed brakes. The rope ad its mechaism have to be desiged to withstad the pullig stress with regard to dyamic loads. The maximum values are most likely beig reached durig pull-up of the assembly after capturig. A towig rope diameter of 1.6 cm is estimated to be sufficiet for up to 2 kn load. The thrust requiremets of the capturig aircraft are depedet o the reusable stage's mass ad its L/D-ratio. The reetry mass of a wiged booster stage icludes the dry mass, ad if ot draied - propellat residuals ad reserves. This may accout for a mass betwee 25 ad 8 Mg. A trimmed subsoic L/D of blut stages with low aspect ratio of about 4 to 5 is achievable [5]. The thrust reserve of the capturig aircraft therefore has to exceed 5 to 2 kn i a adequate flight altitude. Before performig a detailed aalysis, it ca be stated, that a four egie jetlier without ormal cargo loadig offers sufficiet thrust margis. This is correspodig to a Airbus A-34 or Boeig-747-class jet, which have bee produced i large umbers. Moreover, a cosiderable quatity of these airplaes is already available at a affordable price, sice some of them have bee retired from commercial airlie service. There should be o disadvatage i operatig used airliers for the i-aircapturig role, because the daily flight hour demad is modest. Oe questio of cocer might be that of crew safety i a aircraft performig a ear midair collisio. This catastrophic crash has to be avoided by fully automatic ad redudat cotrol avioics of both vehicles operatig i a sychroied mode. Ay pilot iterferece i this maeuver from the capturig aircraft is by far too slow, to have a positive impact. Sice o real demadig pilot work is foreseeable, oe should seriously cosider redesigig the capturig ad towig aircraft as a umaed aerial vehicle. This is ot such a exotic idea, if oe realies that eve combat traiig missios are flow umaed by coverted F-4 fighters [7]. By givig up pilot cotrol for all capturig missios, it might be also possible to broade the flight evelope, which will ot be acceptable with me o board. This further eables high risk maeuvers if ever required - which are otherwise excluded ad would result i the loss of the returig stage. Hece a umaed towig aircraft will augmet overall reliability ad safety of the i-air-capturig method. 4 PERFORMANCE GAIN BY THE IN-AIR- CAPTURING METHOD The iterest i the advaced capturig method ca best be demostrated by its possible performace gai. The calculatios are based o detailed simulatios, assumig the same structural techology level of all regarded stages, to be compared. Detailed mass models ad ascet optimiatio are icluded i the ivestigatio. Nevertheless, it was ot possible withi this study to fully iterate the desig of the advaced reusable stages. It is quite likely, that further optimiatio potetial exists for the captured lauch vehicles. 4.1 Medium Separatio Velocity The first quatified assessmet of the advatage is performed for a reusable first or booster stage with a separatio velocity aroud 2 km/s. The aalyses are based o symmetrical liquid booster cofiguratios attached to a future upgraded Ariae 5 expedable laucher. Such vehicles had already bee itesively ivestigated by DLR-SART [3]. The referece booster is a LOX-RP1 powered wiged stage usig kerosee fuel for the fly-back missio. A heavy lift double lauch ito GTO is regarded. Three sub-variats of the i-air-captured stage are cosidered, all derived from the origial JP-powered flyback cofiguratio: A. Abolishmet of all turbojet egies, their thrust structure, propellat supply, JP-taks, ad fuel. Additio of a special coectig structure, ad special commuicatio equipmet with the capturig aircraft. Due to the cosiderably reduced separatio ad atmospheric reetry mass (-28%), the wig is liearly reduced i sie by a factor of.9, which is a coservative approach. B. Abolishmet of the turbojet equipmet ad fuel as i A, while keepig the laucher's origial thrust-toweight ratio at lift-off. This ca easily be achieved by addig propellat for the rocket egies, burt durig a exteded boost time. Tak mass slightly icreases due to their volume elargemet of about 5 %. Although the reetry mass is reduced, the wig sie is uchaged to the referece vehicle to take ito accout higher reetry loads due to icreased separatio coditios. C. The aim of this cofiguratio is to keep the payload mass of the referece cofiguratio uchaged, while sigificatly reducig the sie of the reusable booster stage. Beside the measures already implemeted i A ad B, the ascet propellat mass ca be reduced, eablig the cotractio i tak, body, wig, fi, ad cotrol flap sies. This might be of iterest to miimie developmet as well as operatioal cost of the reusable stage. The resultig payload performace is icreased by aroud 15 % for the variats A ad B as show i Figure 8. Note, that case B is oly about 1.5 poits above A, sice oly a slight icrease i RLV ascet propellat mass is achievable for the regarded cofiguratio. 6
12.% 115.% 11.% 15.% GTO Payload Referece Case A Case B Case C Figure 8: Compariso of GTO payload of differet to be captured stage cocepts (A through C) with covetioal jet powered fly-back stage (referece) The stages to be captured offer a dry mass reductio of more tha 1 % just by the abolishmet of air-breathig egie equipmet. If the sie of the RLV is cosiderably reduced, holdig the payload costat, at least 17 % dimiishig of mass is achievable (Case C) i a still coservative assumptio. (see Figure 9) 15.% 85.% 8.% 15.% 85.% 8.% 75.% 7.% 65.% 6.% 15.% 85.% 8.% 75.% 7.% Reusable Stage Dry Mass Referece Case A Case B Case C Reusable Stage Separatio Mass Referece Case A Case B Case C Reusable Stage Lift-Off Mass Referece Case A Case B Case C Figure 9: Compariso of various masses of differet to be captured stage cocepts (A through C) with covetioal jet powered fly-back stage (referece) due to the lower ascet propellat mass. I case B this mass is slightly below the referece value, sice the requiremet of same laucher lift-off thrust-to-weight ratio has to be regarded with respect to a icreased payload mass. 4.2 High Separatio Velocity It is evidet that i case of a fly-back stage with cosiderably higher separatio velocity, the payload-gai by itroductio of the i-air-capturig method is further augmeted. Ofte, such lauch vehicles are ot desiged to perform a direct fly-back to the lauch site, sice the required fuel is beyod ay reasoable amout. If possible, these stages try to avoid ay secodary propulsio system ad try to reach a dowrage ladig site. Although, o a first look it might seem that i-aircapturig is ot of primary iterest for these RLVs, it ca be demostrated that cosiderable advatages exist. As metioed i the itroductio, availability of such ladig fields is very rare, ad to be able to reach those uder adverse coditios will pose a otable challege. The egative impact of the requiremet for acceptable dowrage ladig sites o the trajectory ad payload performace is icotestable. Obviously, i ay case of a chage i destied payload icliatio, the performace will be iflueced by the crossrage ad glide rage requiremets of the reusable stage. The referece laucher of this sectio cosists of a wiged reusable LOX/LH2 stage ad a expedable cryogeic upper stage for GTO ijectio. The referece reusable vehicle's MECO coditios close to 5 km/s are dictated by the requiremet to reach a dow rage omial ladig site i a simple glide trajectory. The aim of this ivestigatio is to show the ifluece of upper stage sie ad hece stagig coditios o the payload performace. The covetioal referece cofiguratio is compared to a more flexible i-air-capturig laucher variat with icreased payload performace: E: A reusable stage as for the referece, but icludig the ecessary capturig devices, coectig structure, ad commuicatio equipmet. The upper stage sie is o loger restricted by the dowrage ladig requiremet ad its propellat mass ca be icreased by 5%. (see [2] for the special case D!) Such a desig choice (or later growth versio) is most likely oly realiable with a i-air-captured stage, because the separatio coditios will be otably reduced due to the icreased upper stage's mass. As a cosequece the glide rage is degraded by 25 % or 6 km (Figure 1), posig isurmoutable problems for a reusable vehicle to safely reach its ladig field. It is judged quite ulikely to fid a alterate, closer ladig field with respect to today's lauch sites. The decrease i separatio mass is eve stroger (up to 3%), sice there is o eed for fly-back propellats. This will have a positive effect o the vehicle's reetry loads, strogly depedig o the hypersoic wig loadig [3]. The lift-off mass of case C is reduced by more tha 2 %, 7
15.% 85.% 8.% 75.% 7.% 65.% 6.% Reusable Stage Glide Rage Referece Case E Figure 1: Compariso of glide distace of to be captured stage cocept E with covetioal upowered glide stage (referece) Figure 11 shows the possibility of strogly boostig payload performace to GTO by 25 % usig the same iair-captured stage, ad a thrust ad propellat icreased upper stage (case E). It's uderstood that this example ca be alike trasferred to other laucher cofiguratios ad destiatio orbits varyig i altitude ad icliatio. The implemetatio of the i-air-capturig method will always cosiderably ease costraits o the RLV desig. 13.% 125.% 12.% 115.% 11.% 15.% Referece GTO Payload Case E Figure 11: Compariso of GTO payload of to be captured stage cocept E with covetioal upowered glide stage (referece) Moreover, i-air-capturig offers the opportuity to directly retur the reusable stage to its lauch site, reducig operatioal expeses ad tur-aroud time. More data o the differet stage masses of the high separatio velocity vehicles is provided i [2]. 5 ROBUSTNESS OF THE PROPOSED METHOD REGARDING ADVERSE CONDITIONS This part of the paper proofs the viability of the proposed i-air-capturig method by regardig its off-desig performace, assumig differet perturbatios of the ormal flight, icludig a chage i atmospheric wid or slightly altered stage-separatio coditios. Variatios of differet to be captured stages show that the RLV should have a horiotal distace to the capturig aircraft of about 2 km, whe startig aerodyamically cotrolled descet at 2 m altitude. A acceptable logitudial ad lateral deviatio of +/- 2 km ca be compesated oly by the returig stage, while the aircraft stays passive i omial mode. Without correctio, uexpected adverse coditios will produce larger deviatios, which ca ot be compesated durig the fial approach maeuver of the returig stage. These are otably a wid speed at sea level of 15 m/s (deviatio +/- 8 km) or a separatio coditio cosiderably below the omial case (deviatio aroud +/- 2 km). But suitable correctio maeuvers are well withi reach. The flight dyamic potetial of the descedig vehicle to dissipate eergy earlier i the trajectory is depedig o the specific desig - quite comfortable. Eve uder the most critical circumstaces, whe separatio time varies about 1.5 secods for the high velocity RLV (case E of chapter 4.2), the stage should be able to reach its regularly foresee redevous area, while stayig withi the loads evelope. This ca be achieved for example by limitig the maximum ormal acceleratio to 3.2 istead of 3.5 g, if maximum desig dyamic pressure is ot exceeded. Bakig maeuvers also offer some potetial to sufficietly correct the stage s place. Sice the capturig aircraft has a ow capability to improve its geographical positio, further margis exist. Trackig of the returig lauch vehicle is always possible by radar, or satellite, ad is commuicated via direct data lik. Wid coditios are meteorologically well kow ad ca permaetly be updated by the waitig airlier. Therefore, a real-time optimiatio of the aircraft's positio is maageable. Sice the ballistic phase of the stage exteds to several hudreds of secods, a adjustmet of up to 1 km is achievable. The ifluece of wid ad gust perturbatios o the missile or ACCD flight has bee examied. This effect is foud of mior importace uder the coservative assumptio that it oly impacts the missile. Usually the wid will similarly affect the airlier ad the returig stage, eve further reducig ay cocers. 6 CONCLUSIONS A iovative method for the retur to the lauch site of reusable wiged stages by i-air-capturig has bee proposed ad aalyed. The major advatages demostrated i this paper, are icreased payload mass to orbit ad improved operatioal characteristics. The selected flight strategy ad the applied cotrol algorithms show a robust behavior of the reusable stage to reach the capturig aircraft. I the omial case the approach maeuver of both vehicles requires active cotrol oly by the glidig stage. The available time to achieve full coectio is strogly depedet o the aerodyamic ad flight mechaic characteristics of both vehicles, as well as the chose flight strategy. Simulatios (3 DOF) regardig reasoable assumptios i mass ad aerodyamic quality proof that a miimum distace below 2 m ca be maitaied for up to two miutes. Promisig capturig techiques have bee simulated ad techically assessed. Three feasible procedures are foud; two of them based o a guided missile acceleratig a harpoo, ad the third usig a aerodyamically cotrolled device towed by the capturig aircraft. Total time for i-air-capturig uder omial coditios is betwee 2 ad 3 s icludig the coectio of the towig rope with the returig stage. Although it has to be 8
ackowledged that these umbers are obtaied from theoretical aalyses ad ot from prototype demostratios, the huge margis foud (up to more tha 5%!) give a solid cofidece that implemetatio i a real system is achievable. The aerodyamically cotrolled capturig device (ACCD) shows the best performace ad lowest risk of all ivestigated procedures ad should therefore be used as the preferred cocept i future studies. The possible performace gai of the i-air-capturig method is calculated i detailed simulatios of selected example cases. Reusable stages with separatio velocity 2 km/s ad close to 5 km/s are regarded, represetig the rage of the i-air-capturig highest effectiveess. Itroducig the ew method istead of autoomous flyback with o-board propulsio ad propellat, offers a icrease of at least 15 % payload mass ito GTO. Alteratively the dry mass of the reusable stage ca be reduced by 17 % without loss i referece payload, hece cosiderably decreasig the sie ad cost of the vehicle. A reusable vehicle with omial dowrage ladig site usually is severely restricted by the availability of this field. Itroductio of a reusable i-air-captured first stage offers sigificatly better optios for desigig the upper stage tha seemigly possible with a dowrage ladig RLV. Uder these circumstaces a i-air-captured laucher ca achieve a calculated 25 % icrease i GTO payload. Aalyses of off-desig operatio show that the flight dyamic potetial of the descedig vehicle to dissipate eergy is quite comfortable. Eve if separatio time varies about more tha oe secod, the stage should be able to reach its regularly foresee redevous area, while stayig withi the loads evelope. Sice the capturig aircraft has a ow capability to improve its geographical positio, further margis exist. Keepig this i mid, a high degree of safety ad reliability of the i-air-capturig method is to be expected. I the last two years the uusual i-air-capturig procedure has bee uder thorough discussios ad icreasigly sophisticated simulatios at DLR. Cofidece i its feasibility is costatly risig ad it seems ulikely that severe problems might oe day be detected, which defiitely would exclude its operatio. Nevertheless, the techology readiess level (TRL) is still ot exceedig 'proof of cocept' (TRL=2). The ext steps should be compoet testig ad (most importat) subscale flight demostratio. Oce-agai it ca be stated, that i-aircapturig is able to strogly boost RLV performace, ad it offers a tremedous potetial of further improvemets. Ay commet ad suggestio will be highly appreciated by the authors. 8 REFERENCES 1. Patetschrift (patet specificatio) DE 11 47 144 C1, Verfahre um Berge eier Stufe eies mehrstufige Raumtrasportsystems, released 23 2. Sippel, M., Klevaski, J.; Kauffma, J.: Iovative Method for Retur to the Lauch Site of Reusable Wiged Stages, IAF-1-V.3.8, Toulouse 21 3. Sippel, M.; Ataassov, U.; Klevaski, J.; Schmid, V.: First Stage Desig Variatios of Partially Reusable Lauch Vehicles, J. Spacecraft, V.39, No.4, pp. 571-579, July-August 22 4. Klevaski, J.; Sippel, M.: Beschreibug des Programms ur aerodyamische Voraalyse CAC Versio 2, SART TN-4-23, DLR-IB 647-23/4, March 23 5. Lepsch, R.A.; Ware, G.M.; MacCoochie, I.O.: Subsoic Aerodyamic Characteristics of a Circular Body Earth-to-Orbit Vehicle, NASA TM 4726, 1996 6. Covault, C.: Astroaut Skills Hoed Flyig Steep STA Dives, i Aviatio Week & Space Techology, July 8, 22 7. Scott, W. B.: QF-4 Droes Emulate Eemy Fighters, i Aviatio Week & Space Techology, March 2, 2 Further updated iformatio cocerig the SART space trasportatio research is available at: http://www.dlr.de/sart 7 ACKNOWLEDGEMENTS The authors gratefully ackowledge the cotributios of Ms. Bärbel Schlögl who performed a large part of the capturig simulatios. 9