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Acta Astronautica 65 (2009) 1723–1737 www.elsevier.com/locate/actaastro

A panorama Philippe Caissoa,∗, Alain Souchiera, Christophe Rothmunda, Patrick Alliota, Christophe Bonhommeb, Walter Zinnerc, Randy Parsleyd, Todd Neille, Scott Fordee, Robert Starkee, William Wangf , Mamoru Takahashig, Masahiro Atsumih, Dominique Valentiani,1

aSnecma Vernon, bCNES, France cEADS ST Ottobrunn, dPratt and Whitney , USA eAerojet, USA f Corporation, USA gIHI Aerospace, Japan hMHI, Japan iFrance

Received 13 January 2009; received in revised form 20 March 2009; accepted 26 April 2009 Available online 26 May 2009

Abstract Liquid- are widely used all over the world, thanks to their high performances, in particular high -to- ratio. The present paper presents a general panorama of liquid propulsion as a contribution of the IAF Advanced Propulsion Prospective Group. After a brief history of its past development in the different parts of the world, the current status of liquid propulsion, the currently observed trends, the possible areas of future improvement and a summarized road map of future developments are presented. The road map includes a summary of the liquid propulsion status presented in the “Year in review 2007” of Aerospace America. Although liquid propulsion is often seen as a mature technology with few areas of potential improvement, the requirements of an active commercial market and a renewed interest for has led to the development of a family of new engines, with more design margins, simpler to use and to produce associated with a wide variety of thrust and life requirements. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction in 1903, Tsiolkovsky suggested the use of liquid pro- pellants for in order to achieve greater range. In 1898, a Russian schoolteacher, Konstantin Tsi- Early in the 20th century, an American, Robert H. olkovsky [Fig. 1]—(1857–1935), proposed the idea of Goddard (1882–1945), conducted practical experiments space exploration by rocket. In a report he published in rocketry. While working on -propellant rock- ets, Goddard became convinced that a rocket could be ∗ Corresponding author. propelled better by liquid . No one had ever built E-mail address: [email protected] (P. Caisso). a successful liquid-propellant rocket before. It was a 1 Consultant. much more difficult task than building solid-propellant

0094-5765/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2009.04.020 1724 P. Caisso et al. / Acta Astronautica 65 (2009) 1723–1737

Fig. 1. Konstantin Tsolkiovski. Fig. 2. HM4 (source: C. Rothmund, Snecma). rockets. Fuel and tanks, turbines, and combus- tion chambers would be needed. In spite of the dif- DYNE, CALTECH) and Europe (ONERA and SEP). ficulties, Goddard achieved the first successful flight This contributed to the successful development of the with a liquid-propellant rocket on March 16, 1926. Fu- propulsion of well-known launchers, as SOYOUZ (RD elled by and gasoline, the rocket flew for 107 and RD 108), (LR 87), (RS 27), only 2.5s. , and 1 (). These engines Goddard’s gasoline rocket was the forerunner of a used the generator cycle. whole new era in rocket flight. Nowadays, liquid propulsion relying on storable, LOX based or cryogenic is a mature 2.2. LOX/liquid field and a core technology for most launchers in service. Tsiolkovski identified one century ago liquid hydro- gen and liquid oxygen as the most promising propellant 2. History combination for rocket engines. The first drops of liq- uid hydrogen were obtained just a few years before in 2.1. LOX/hydrocarbons 1905. It took more than 50 years to see the first prac- tical application of this combination to an upper stage LOX fed and fed engines were already (). In the mean time, large thrust engines, tested before WW II. The first large thrust LOX/alcohol operating with storable propellants or with LOX hy- engine was flight tested on the A4 in 1942. The cham- drocarbons combinations, were already in use since the ber pressure was still moderate and the development of 1950s. large thrust engines was hampered by low frequency In the 1950s, Nuclear Thermal Propulsion was hydraulic instabilities or high frequency in- extensively studied in the USA, consequently a bet- stabilities in the case of storable and LOX– ter knowledge of hydrogen application for propulsion engines. and of hydrogen based thermodynamic cycles became The of a liquid propellant rocket available. Combined with the development of liquid engine was designed by the German scientist Eugen hydrogen fuelled , this favoured the start of in the early 1930s. This discovery proved to cryogenic engines, first of all the RL10. be a vital asset for the development of all subsequent The early sixties showed the development of cryo- high-thrust rocket engines, beginning with that of Von genic engines in France (HM 4 [Fig. 2])andinRussia Braun’s A-4. (RD 56 and RD 57). A great deal of work on combustion instabilities In Japan, the first cryogenic engine (LE-5) was de- was performed in Russia, USA (, ROCKET- veloped in the 1970s for upper stage application. P. Caisso et al. / Acta Astronautica 65 (2009) 1723–1737 1725

2.3. Storable propellants

Storable propellants were initially used during World War 2 on German rocket-plane and early air-launched propulsion systems that relied mostly on nitric acid/furalin. This technology was later taken over by SEPR (France) and applied on their successful SEPR-844 en- gine that helped power the Mirage 3 interceptors and became the world’s only reusable used on operational fighters. In the United States, more powerful storable pro- pellants were developed and used: tetroxide, , UDMH or MMH. They led to the development of the Titan family of missile propulsion and many spaceflight applica- tions (, space probes and manned vehicles). The Titan I–IV was the workhorse for the Air Force for over 50 years, with LR87 first stage engine and LR91 second stage engine developed by Aerojet. Storable propellant upper stage and en- gines proved to be highly reliable and are still flown today. The Aerojet OMS engine has suc- cessfully flown over 120 missions and the Delta II up- per stage (AJ10-118) engine has successfully flown over 200 flights. In Europe, storable propellants were used on sound- ing rockets (VERONIQUE and VESTA), on the DIA- MANT launch-vehicle first stage engines VEXIN (nitric acid/turpentine) and VALOIS (N2O4/UDMH). They were also implemented on the French-built second stage (“Coralie”) of the launch vehicle. In addition, SEPR initiated the development of a storable propulsion engine (NTO with a fuel consisting of 50% and 50% UDMH) for the German- designed third stage (“”) of the European launch system “EUROPA”. The storable propellant technology has later on also been applied by EADS Astrium ST to the current Ari- ane 5 upper stage engine with NTO/MMH as Fig. 3. Viking engine (source: Snecma). propellant combination. Another well-known representative of the storable propellant family is the VIKING engine [Fig. 3], de- the influence of pumps efficiency or in-flight variations veloped by SEP, which remains one of the most suc- of pump inlet pressure on the mixture-ratio. Built under cessfully produced rocket engines, with more than 1100 licence as the , it is still in use in India (PSLV built (–4). Its uniqueness resides in its reg- and GSLV launchers). ulation system that relied on two regulators. First the Storable combinations are still widely used. main regulation which equalizes the chamber pressure Cleaner propellant combinations (e. g. nitrous oxide– to a reference value by “throttling” the flow to the gas hydrocarbons) are tested at small scale to identify their generator, then a “balance regulation” that eliminates potential. 1726 P. Caisso et al. / Acta Astronautica 65 (2009) 1723–1737

Fig. 5. RD-180 (source:PWR).

innovative technology which allowed the design of higher pressure combustion chambers. A copper inner wall with milled cooling channels was associated to Fig. 4. P111 engine (source: MBB). a structural nickel jacket, the nickel shell being ob- tained by electro-deposition over the copper core. This 2.4. Engine cycles break-through technology for high pressure combustion chambers was applied in the joint MBB/Rocketdyne Subsequently to the early era of en- LOX/LH2 project BORD (1966–1968). Test results gines, the development of very high pressure engines showed that adequate LH2 cooling could success- was felt necessary for ascent stages and boosters: high fully be obtained at a nominal pressure of 210bars pressure meant more compact engines with better sea (3045psia) and even at pressures as high as 286bars level specific . (4150psia), the highest known pressure ever achieved Practically, this was only possible with a staged com- for a LOX/LH2 rocket engine. bustion engine. Incidentally, the staged combustion pro- In Europe, this regenerative cooling technology was vides another advantage: as the turbine exhaust (either used for the HM7 [Fig. 6] on the third stage of the Ar- oxidiser rich or fuel rich) is gaseous, the main cham- iane 1–4 launchers, and later for , AESTUS ber combustion is consequently very stable (gas/liquid and . The Space Shuttle Main Engine (SSME combustion). [Fig. 7]), developed by Rocketdyne, was the first staged Early staged combustion work was performed in Ger- combustion cryogenic engine used operationally that many (MBB P 111 [Fig. 4]) and in Russia. Most Rus- relied on this technology. The RD 0120 (CADB) and sian engines in use today are relying on this technol- LE 7 (MHI) are the other representatives of this family ogy: RD 253 and RD 275 for UDMH/NTO, RD 171, of engines. In Europe, MBB and SEP studied a staged RD 180 [Fig. 5], RD 190 for LOX/Kerosene (ZENITH, combustion cryogenic engine of 20ton thrust for EU- ATLAS 5, ). ROPA 3 upper stage, using the same thrust chamber In the late 1960s the Messerschmitt–Boelkow–Blohm technology based on nickel electro-deposition, but this GmbH (MBB) in Ottobrunn (Germany) developed an development was stopped. This project was also highly P. Caisso et al. / Acta Astronautica 65 (2009) 1723–1737 1727

Fig. 6. HM7B engine (source: Snecma). innovative with a single-shaft axially mounted turbop- ump. Fig. 7. SSME engine (source:PWR). In Japan, this cooling technology was also applied to the open expander engine (LE-5B) in order to enhance the structural and operational margins, thus increasing Following a focus on performance and the devel- engine reliability. LE-5B proved its robustness and re- opment of technologies which brought engine per- liability in actual flights of H-II and H-IIA. formances very close to their theoretical specific im- The most recent commercial developments in large pulse limit, current developments have placed more cryogenic engines were aimed at providing a high per- emphasis on: formance level at a reduced cost with an emphasis on • robustness: RS 68 in USA, and VULCAIN 2 [Fig. 8]in reliability (reduced failure occurrence); • Europe as well as LE7A in Japan. Based on these cri- cost reduction (both —simpler teria, the gas generator cycle was sometimes preferred processes and reduced parts number—as well as in- to the closed cycle (RS68). direct: simplified operation and reduced system com- The liquid propulsion milestones are summarized in plexity) [11,44];and • Fig. 9. improved endurance and life increase (with indirect benefit on reliability). 3. Current status The requirement for reliability is becoming increasingly The design of a propulsive system involves a com- the prime requirement due to: promise between potentially conflicting objectives: reliability, performance, low recurring cost and low • the high cost of (especially the scientific and development cost. institutional ones) and 1728 P. Caisso et al. / Acta Astronautica 65 (2009) 1723–1737

• the visibility of a launch failure and the resulting de- The relative weight of performance and cost param- terioration of the climate of confidence among the eters may vary from a project to another: actors of the space industry: customers, in- • surers, public authorities. the performance requirement may be essential for up- per stage: every additional second of specific impulse results in a significant payload increase; • the recurring cost parameter is dominant for (expend- ) booster engines; and • the development cost constraint can be very important for a dedicated scientific mission.

Lessons learnt from recent developments have shown that the hierarchy of these design requirements should be clearly expressed at the beginning of a project and maintained along the duration of the project, avoiding shifts from a priority to another. An increasingly visible trend is the importance of international cooperation [24,32,39]. Numerous projects involve cooperation between companies belonging to multiple countries. The development of ARIANE is one of the earli- est examples of international cooperation, mostly in the frame of Europe. RD 180 [Fig. 5] engine implementation on ATLAS 5 is another well-known example of this trend. International cooperation requires solving the follow- ing difficulties:

• exchange of large amount of data in compatible for- mats, especially for CAD models; • compatibility between different technical standards or Fig. 8. Vulcain 2 engine (source: Snecma). norms; and

ConceptConcept EarlyEarly tests FlightFlight CommercialCommercial operationoperation

VULCAIN HM7VULCAINHM7 (AR 5) (ARIANE 1) TsiolskovskiRLTsiolskovski 10, HM4 RL 10, HM4 Centaur(ARIANECentaur 1) (AR 5) LOX –LH2

LE-5 SOYOUZ, ATLAS Goddard A4SOYOUZ,A4 ATLAS (H-I) LOX / HC

PROTON Storable GIRDWaltherPROTONGIRD WaltherGoddard VIKING (AR1) TITANTITAN AESTUS (AR5)

CZ2CZ2 X1 Pumps fed Goddard

P111 RD 170 RD 0120(0120() ENERGIA)

Staged LE-7 RD180 RD 253RD 253 () (PROTON) SSMESSME LE-7 RD180 combustion (ATLAS 5) (H-II)(ATLAS(H-II) 5) 1900 1921 1935 1942 1957 1969 1979 1995 2008

Fig. 9. Liquid propulsion milestones. P. Caisso et al. / Acta Astronautica 65 (2009) 1723–1737 1729

new instrumentation technologies have transformed the field of component and engine testing. The last 10 years have seen the emergence of new type of instrumentation, which helps test engineers in extracting as much information as possible from the tests:

• flush pressure gauge with high bandwidth have helped characterizing fluid excitations of structures which are the source of numerous high cycle fatigue failures in rocket engines; • laser, optical and magnetic instrumentation have con- tributed to the knowledge of behaviour; • thermal instrumentation and infra-red camera have led to a more accurate evaluation of heat fluxes; • etc...

Capitalizing on these new instrumentation technologies, design and test engineers have access to an immediate understanding of the behaviour, reduce the number of tests, and avoid unnecessary design loops induced by wrong corrective actions due to incomplete or faulty interpretation of the physics. Furthermore, miniaturization, new instrumentation techniques and availability of computing power will boost the use of regulation and health monitoring tech- nologies, thus significantly contributing to performance Fig. 10. Vinci engine (source: Snecma). and reliability of propulsive systems. An extensive use of “smart” system and health mon- • barriers arising from international trade regulations itoring technique, which will be able to provide the sta- when dealing with sensitive technologies. tus of propulsive system with increased accuracy and correct its deviations, is likely to be seen in the coming Improvements in design (especially with simulation years. tools) and manufacturing methods, which have trans- formed the automotive, and aircraft industries in the 4. Trend for and future launchers late 1990s and early 2000s have equally transformed the field of space propulsion. The requirement of the commercial market, Numerical simulations and computer-aided design which dictates a regular increase in satellite mass and a have contributed to significantly shorten development stronger than ever demand for reliability, will probably duration by eliminating the trial and error process, lead to a family of new or improved engines with more which led to long developments in the past. design margins, simpler to use and to produce. Recent demonstration engines such as the Vinci The regular rate of mass increase for telecommu- [Fig. 10], upper stage engine, have reached nication satellites over the past few years can be reliable steady state operation in a much smaller num- explained by the restriction of use of geo-stationary ber of tests than would have been required in the 1970s positions which induce telecommunication operators to or 1980s. concentrate as much transmitting capability as possible Additionally, increased focus on productivity, repro- in one single . ducibility and environmental concerns has also modi- Additionally, a common goal of all existing commer- fied working methods in the space industry as much as cial launch suppliers is to possess an array of launch in any other industrial sector. vehicles, which enable them to provide a wide variety Just as numerical simulation has transformed the de- of launch services. This is obtained by simultaneously velopment of propulsive systems, miniaturization and operating a fleet of heavy, medium and small launch 1730 P. Caisso et al. / Acta Astronautica 65 (2009) 1723–1737 vehicles as much as by increasing the flexibility of ex- isting launchers, very often through the development of new upper stages offering new features or additional performance such as a restart capability or an increased propellant load. In order to maintain the quality of their launch service, launch suppliers need to rely on a dedicated and skilled work force which should be ready to deal with any anomaly or solve any production mishap that may endanger their reliability record. Maintain- ing a research and development effort, starting new developments may help in keeping motivated teams of engineers and technicians which are as much es- sential in serving today’s need as in preparing the future. At least in Europe, next launchers are foreseen to be improved versions of existing and expendable ones. Improving the upper stage capability is a very efficient way to increase the overall launcher performance (pay- load), hence the interest of new cryogenic upper stage engines like VINCI [1,23], in Europe [Fig. 10], but also Fig. 11. J2X engine (source:PWR). MBXX [32] in the US and Japan and RD0146 [30] in Russia. The next launcher generation (2020–2025) is less The role of private entrepreneurship in promoting in- clearly identified. A programme dedicated to the prepa- novative designs combining performance and low costs ration of future launcher, the Future Launchers Prepara- is still to be demonstrated. tory Programme (FLPP), began in Europe in February In the US, manned space transportation came back to 2004 and aims at having a Next Generation Launcher the forefront with the development of I and V. (NGL) operational around 2020. The FLPP is focused In 2006 NASA made significant progress on on developing concepts for various launch vehicle sys- and V system development, selecting to build tems together with the technology needed to realize the upper stage of Ares I and Pratt & Whitney Rock- them. etdyne for design, development and testing of the J-2X The debate is still open concerning the choice of engine [Fig. 11] that will power the upper stages of Ares upper stage propellants: cryogenic, LOX– or IandV[46]. For the booster of it is planned to storable. use a version of the RS-68 cryogenic engine currently In Europe, this trade-off is focused on small and used on the Delta IV launch vehicle. medium launchers, while in Japan all types of launchers Propulsion is also being developed to support the in- are considered. space portion of the exploration architecture. Aerojet The interest in using hydrocarbons, especially is developing a pressure-fed engine for the new human methane, for rocket propulsion, is mainly driven by the transport service module. This engine is based high fuel , high boiling point, reduced handling upon the Space Shuttle Orbital Manoeuvring Engine, constraints, and reduced need for safety precautions which uses storable propellants. The plan is to also use relative to hydrogen. this Orion Service Module engine to perform the as- The emergence of very cheap launchers can be no- cent function on the lunar module using storable pro- ticed, especially for small payloads, but the recent pellants, although methane/LOX options are also being experiences (e.g. ) show that it does not considered. seem so easy to simplify too much the engine de- For the descent stage of the lunar exploration archi- sign (e.g. the go back from radiative to regenerative tecture NASA has identified pump-fed hydrogen per- on Merlin engine) and to reduce formance levels as being needed. Readiness for this ap- very significantly the development process cost, with- plication is being pursued along two fronts: first Pratt out discovering unforeseen phenomena during the first & Whitney Rocketdyne is supporting deep throttling flights. demonstrations in the CECE programme [15]; Northrop P. Caisso et al. / Acta Astronautica 65 (2009) 1723–1737 1731

Grumman is also pursuing deep throttling technology Reliability will remain the number one design cri- for LOX/hydrogen propulsion based upon the Pintle in- terion in the future. But there will be an increased jector technology approach. awareness that achieving the best possible compromise Meanwhile Space exploration can also rely on robots between various objectives as early as possible is and automated vehicles. essential. Europe is focused on automatic planetary explo- New design tools may help in this task: parametric ration. Ascent and landing of heavy robotic payloads design methods, sensitivity analysis, probabilistic meth- will also require the development of new engines. ods (e.g. for structures). is an emerging field close to space In the field of car engines and turbojets, while the ba- propulsion for launcher in which most work is currently sic technologies have apparently remain the same over devoted to suborbital flights. SPACE- the last 50 years, the reliability and life duration made SHIP 2, the ROCKETPLANE XP and the ongoing tremendous progress thanks to use of new material, dig- EADS ASTRIUM project are the most well-known ital control and regulation. The same evolution could be illustrations of this activity. expected for rocket engines. Space tourism could also be seen as a way to mature New material processes such as metal deposition and very cheap and reliable propulsion techniques having new welding technologies will probably allow higher the potential to drastically reduce the launch cost. operating temperatures or facilitate the production of New concepts, like low thrust cryogenic propulsion, complex hydraulic shapes. may enable to extend the domain of cryogenic propul- The goal of increasing engine life duration-which will sion to lower propellant masses and smaller launchers be essential in the long term for reusable vehicles—also (orbital stage or upper stage of micro launchers with contributes to increase the reliability level when applied payload below 300kg) [10,14,17,43]. to expandable vehicles. Fully reusable launchers will probably not be de- For commercial launch vehicles, the reduction of veloped in a foreseeable future, but the introduction launch cost expressed in term of $ per kilogram or lb of reusable boosters (like LFBB: Liquid Fly Back of payload will remain essential. Boosters) could come earlier as a forerunner of full For large expandable launch system and infrastruc- reusability. tures, this goal can be obtained through a continuous in- Cleaner propellant combinations relative to usual crease in performance, for instance in order to keep the storable propellants (MMH, UDMH, NTO), com- dual launch capability for ARIANE, or by an increased monly designated as “green propellants” could come focus on design simplicity. to fruition, provided early demonstration at low thrust For instance, in the case of ARIANE or H2, every level are satisfactory [8,16,21,42]. second of main engine specific impulse brings around 100kg of additional payload. Green propellants could contribute to the cost reduc- 5. Areas of future improvements tion by lowering the direct cost of propellants and re- ducing the propellant handling cost. In the mid and long term, areas of future improve- In the long term, the launch cost reduction will be ments will probably represent a continuation of the ob- obtained by using partially reusable launchers (FLBB) jectives, which are already observed in the selection or totally reusable system. of propulsive solutions for ARES I and V in the US This future step will probably require a technological or in ESA Future Launcher Preparatory Programme in rupture in the field of aerospace material with respect Europe. to strength to density ratio as well as fatigue and creep These main areas of consolidation and improvement capability. are: It is difficult to predict when this transition could occur. • reliability; 6. Roadmap • cost reduction which should not be obtained at the expense of reliability; In this paragraph a distinction is made between a short • availability; and mid term future for which a predictable continuation • reduction of development duration; and of current programmes can be expected, and a long- • increase of performance considered in term of thrust term future full of unforeseeable technological ruptures and specific impulse, and also in term of life duration. and open to unbridle imagination. 1732 P. Caisso et al. / Acta Astronautica 65 (2009) 1723–1737

6.1. Short and mid term the creation of the which com- bines the Delta launch system (Delta II and IV) and the 6.1.1. USA rival system. The space exploration programme objectives were Booster propulsion for these US Air Force systems clearly defined over the 2006–2007 period and the pro- include the RS-68 engine produced by PWR for the gramme is now well on track. Delta vehicle and the RD-180 kerosene booster engine Its main objectives are: produced by NPO-Energomash in Russia, but supplied for the Atlas vehicle through a Joint Venture with PWR. • to safely fly the Shuttle until 2010; For the upper stage, both launchers use models of the • to develop and fly the crew exploration vehicle before RL10 hydrogen/oxygen engine. Significant activities to 2014; and improve these propulsion systems are currently limited • to return to the Moon no latter than 2020. to a performance improvement for the RS-68 designated The propulsion options, which were retained to fulfil RS-68A. these objectives, are: Aerojet is providing the kerosene-fuelled AJ26, a highly modified version of the Russian NK-33 LOX- • the J2X, a 1300kN cryogenic engine [Fig. 11],based rich staged combustion engine, as main propulsion for on the era J2 engine and the powerpack of the the first stage of the Orbital Sciences Taurus II launch more recent X-33 aerospike demonstration [46]; vehicle, scheduled for first flight in 2010. • an upgraded version of the RS68; and Farther term technology readiness for the next gen- • development of the pressure-fed storable engine for eration of Air Force systems is being pursued under the the Orion crew vehicle service module, based on the IHPRPT (Integrated High Payoff Rocket Propulsion Shuttle orbital manoeuvre engine [45]. Technology) programme. During Phase I of IHPRPT, PWR and Aerojet successfully completed demonstra- In July 2007, NASA announced that the common ex- tion of new hydrogen/oxygen propulsion in a full-flow tensible cryogenic engine demonstrator (CECE) based . The recently initiated IHPRPT on Pratt & Whitney Rocketdyne RL10 engine was un- Phase II activity is focused on the kerosene/oxygen der development to support future space vehicles, with oxygen-rich staged combustion cycle and is being specific focus for the deep throttling lunar stage performed by Aerojet. [18]. Meanwhile developments of new engines for space In the lunar module, the lunar descent en- application and spacecraft control were on going. In gine from TRW was capable of throttling from 10,125lb 2007, a major accomplishment by Aerojet was the suc- down to 1250lb. It was a pressure fed storable system cessful hot-fire testing of an MR-80 series monopropel- that has limited performance for the new NASA lunar lant hydrazine engine. This is a Lander derived missions. engine tested as a proof of concept for an Ares roll con- The CECE will serve the same purpose. In its demon- trol engine. strator configuration, it is a 13,800lb engine fuelled Orbital Technologies (Orbitec) continues its devel- by higher performance liquid oxygen and hydrogen. Its opment of the Forward-1 reusable 7500lbf LOX–liquid main requirements include the capability to be throttled propane vortex engine. Forward-1 is a pump fed engine down to about 10% of maximum thrust. system that uses a regenerative-cooled and vor- NASA’s Propulsion and Cryogenic Advanced Devel- tex cooling in the chamber. opment (PCAD) programme is investigating the use of liquid oxygen and liquid methane technologies applica- 6.1.2. Europe ble for lunar ascent. Aerojet is currently developing a 6.1.2.1. ESA and the national space agencies (DLR, 5500lbf pressure-fed LOX/Liquid Methane high perfor- CNES, ASI, SNSB ... ). The short-term main goal of mance engine that will be tested in 2009. Aerojet also ESA and the national space agencies is the consolida- just completed the successful development and testing tion and improvement of ARIANE 5 and the comple- of a 100lbf LOX/Liquid Methane Reaction Control En- tion of the development of . gine for similar applications. When improvements of Ariane 5 deals mainly with LOX–methane RCS eliminates the handling of toxic a new cryogenic upper stage with the VINCI oxy- propellants. gen/hydrogen expander engine [Fig. 10], the com- At the same time, the consolidation of the existing pletion of the development of VEGA deals with the infrastructure was completed in 2006 with qualification of the AVUM storable 4th stage [7],and P. Caisso et al. / Acta Astronautica 65 (2009) 1723–1737 1733 the qualification the RACS, hydrazine The CNES launchers roadmap covers the whole range Roll and System [36]. of payloads from 30kg in LEO to more than 12ton in In parallel these agencies are actively promoting GTO. All these developments are foreseen to be imple- and managing research and demonstration programmes mented in a European frame. aimed at initiating new technologies and upgrading CNES is promoting research on “green” and low cost the technology readiness level of emerging ones. The upper stage propulsive system for nano/micro launch previously mentioned Future Launcher Preparatory vehicles [9]. Programme (FLPP) led by the Germany is focusing on combustion with on-going (ESA) is one the most significant of these programmes. works on LOX/LH2 combustion, LOX/methane com- The FLPP is proceeding to mature technologies for bustion [29] and staged combustion. upper stages engines as well as high thrust engines The German Aerospace Agency is investigating [35,37]. various aspects of methane–oxygen combustion, such As part of ESA FLPP, an expander engine demon- as propellant injection, atomization, ignition, high- stration based on the VINCI engine is on going. pressure combustion, combustion instabilities, and In 2008 the cryogenic propellant VINCI engine tests performance of methane for regenerative cooling. continued with a goal of providing further information Italy is also promoting activities related to LOX– on the engine operation capability. The VINCI is an hydrocarbon engines [20]. upper stage engine with an increased performance and multiple firing specification, which is 6.1.3. Japan typical of what is currently required to enlarge the scope Production and management of the Japanese key of missions and increase the capability of heavy ex- rocket of H-IIA, the first flight of which successfully pandable satellite launchers. Its overall system design is occurred in 2001, was shifted from the Japanese Space under responsibility of Snecma (France) under ESA Agency (JAXA) to MHI on April 2007 when entering contract. It is a key element of European future launcher into the commercial launch market. JAXA along with evolution. MHI and other industrial partners has been continu- In parallel ESA and industry are preparing demon- ously improving the reliability of LE-5B and LE-7A strations activities for high thrust engines to be started engines to support H-IIA launch service. by the end of 2008 in order to meet the propulsion re- New design techniques such as Probabilistic Design quirements of post 2020 launchers. Approach (PDA) and sensitivity analysis were demon- The VULCAIN X is also one of the lead European strated with advanced Computer-Aided Engineering demonstration programmes in the field of liquid propul- (CAE) technology as a pilot programme for future sion [3]. It is using the VULCAIN 2, ARIANE 5 main engine development. stage engine, as a platform for implementing new tech- JAXA and MHI are still focusing on technology de- nologies in various field of liquid propulsion. velopment to improve engine reliability [26],whichis The VULCAIN X programme aims at demonstrat- the basis of the commercial launch service and will ing new technologies and sub-systems architecture for be also a major key driver for future manned vehicles, introduction in future developments: a fuel turbo pump which will be part of Japanese space activities in the with fluid bearings, a gas generator with tri-coaxial 21st century [22]. injection elements, a sandwich technology nozzle and The choice of the open expander cycle, which is more high band-width regulation valves. The VULCAIN tolerant to system failure, also contributes to improve X programme has been initiated by the French space reliability. For instance, the LE-5B engine can start up agency (CNES) and has been extended at a European to full power in spite of unexpected interface conditions level. such as low inlet LH2 pressure and poor chill conditions. As part of the VULCAIN X programme, VOLVO With a focus on system simplicity, robustness and tol- AERO of Sweden has developed a nozzle relying on the erance to failure, JAXA is applying this highly reliable, sandwich technology and advanced welding and metal flight-proven open expander cycle technology to next deposition processes. generation 100ton-class engine as designated “LE-X” In addition to European programmes involving in- for upgraded H-IIA family and next generation launcher dustrial partners of several countries, each national in 21st century [4,27]. agency is often pursuing specific goals, which are re- JAXA leads fundamental technology developments lated to historic field of expertise or specific national such as advanced inducer, combustion injector with the needs. latest high-speed visualization technology. 1734 P. Caisso et al. / Acta Astronautica 65 (2009) 1723–1737

JAXA’s Engineering Digital Innovation (JEDI) Cen- in a more and more efficient way using the available tre is developing the advanced computer simulation technical Knowledge at the time when successive gen- technology to support these fundamental programmes. erations of engines were designed. As part of a “Propulsion for exploration” pro- These Concepts are the following: gramme, JAXA is also developing large range throttling LOX/LH2 engine for vertical assent/vertical landing • are one of the most efficient way to store reusable vehicle. chemical energy in a dense form; JAXA is also developing a LOX/LNG engine. Cur- • thrust and exhaust are generated by the ex- rently, the engine with gas generator and turbopump pansion of light molar mass ; the higher expan- will be applied for 2nd stage of GX rocket [47]. sion (high chamber pressure, high nozzle ), the higher the thrust; and 6.1.4. China and India • liquids should be stored at the lowest possible pres- China is expanding the family of Long March launch- sure and a system to increase their pressure may be ers adding increased capability and flexibility to this required. launch system. China is also actively engaged in a na- tional space exploration programme the recent high- This last point is linked to another question: the use lights of which were a manned flight around the Earth of densified cryogenic propellants. This technique has and the circumlunar flight of a scientific probe. been proposed since several years but not applied up to China develops new engine for the Long March now, except on Energia/Bouran. The application may be 5 (LM5) heavy lift launcher. Development has been interesting in the future, especially for in- propul- started for a 1200kN LOX/kerosene booster and 500kN sion (tank pressure below atmospheric pressure). LOX/LH2 upper stage engine. will be The available Knowledge obviously relies on state- in the class of Ariane 5 and Delta 4 and operation is of-the-art fluid mechanics, material and strength of ma- expected for 2014. terial science, electrical engineering for auxiliary power India is developing and upgrading the GSLV and and control. PSLV launchers. Using this Concept/Knowledge approach and trying As part of this effort, India is developing a new to project one self into a long term future, the questions 200kN thrust gas generator upper stage cryogenic to be asked are the following: engine [38]. • Will the basic concept change? • Will the available technical knowledge offer new so- 6.1.5. International landscape lutions to implement these concepts? One can see that each region/nation has its own propulsion system development for various political or A short sample of the questions arising from these gen- security reasons. eral considerations can be expressed as follows: In view of limited resources for each nation, one has to ask if the same kind of saving can be achieved • Could new energetic chemical combination further “commercially” in propulsion systems development improve the energy versus density ratio of today’s similar to commercial and gas turbines. known propellants? Energetic propellants are already However, there is a huge difference between aeronau- investigated relying on software that can help engi- tics and launchers: the series effect for commercial air- neering new and predict their properties. planes allows for a commercially funded development • Shall we see a wider application of the pulse detona- while the launch rate does not yet support this approach. tion engine? There is a general willingness among propulsion • Will smart material with a capability of providing companies to cooperate in order to spread the devel- their deterioration status and heal their damage be opment cost, but—except in the European case-this is used? somewhat hampered by export control rules. Besides technical aspects, as much as today, the evo- 6.2. Long term lution of liquid propulsion will remain driven by the “customers” needs (commercial, institutional or tourist). When looking at the evolution of liquid propulsion The planetary exploration (automatic or manned) over a century, one can observe that it relies on a few may be the main driver of liquid propulsion in the permanent simple Concepts that have been implemented future [5,13]. P. Caisso et al. / Acta Astronautica 65 (2009) 1723–1737 1735

The use of cryogenic propulsion for interplanetary This has led to a family of new engines with more mission will probably require active refrigeration, i.e. margins, simpler to use and to produce, which were Zero Boil Off (ZBO) [17]. initiated as demonstration programmes or are still on At least for LOX, In situ Propellant Production could the drawing board. drastically reduce the expenditure of recurring mission. Since these development activities are still heavily Another possible evolution—beyond the space relying on public funding, there is a need to engage, tourism-is the suborbital passengers transport—i.e. the inspire and educate the public concerning the benefit extension of liquid propulsion from launchers to hyper- that is coming from space exploration and from the use sonic, airline-like transportation. The vehicle may use a of space. There is also a need to strengthen existing and two-stage concept, with airbreathing propulsion on the create new international partnerships in order to share first stage and LOX–LH2 engines on second stage [34]. the cost of these developments. The considerable increase in the production rate and Almost a century ago, Tsiolkovsky and Goddard the requirements of reusability will have a deep impact came to the conclusion that liquid propulsion was the on the launcher business. most appropriate form of propulsion for long-range On a more modest scale, the increase of launch rate rockets. 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