INSPACE PROPULSION--WHERE WE STAND AND WHAT'S NEXT

Robert L. Sackheim Assistant Director and Chief Engineer for Propulsion NASA Marshall Space Flight Center, Marshall Space Flight Center, Alabama USA

GBSTRGCT launch vehicles, many spacecraft suppliers, such as Baeing (formerly Hughes), Loral, and Northrop- The focus of this paper will be on the three stages Grumman (formerly TRW), have developed integral of in-space transportation propulsion systems, now propulsion systems (IPSs) on board their spacecraft commonly referred to as in-space propulsion (ISP); to perform many of the more conventional orbit i.e., the transfer of payloads from low-Earth orbits transfer functions. Some examples include the into higher orbits or into trajectories for planetary Hughes 601 and 702 commercial series of satellite encounters, including planetary landers and sample buses and the Northrop-Grumman IPS for the return launchers, if required. Functions required recent Chandra final orbit insertion functions. There at the operational location where ISP must provide have only been three complete upper stages devel- thrust for orbit include maintenance, position oped over the last 30 years: (1) The basic Centaur control, stationkeeping, and spacecraft altitude con- and all the associated upgrades, (2) the advanced trol; i.e., proper pointing and dynamic stability in Centaurs for the Delta 3 and 4 and the Atlas 3 and 5, inertial space; and the third function set to enable and (3) the inertial upper stage which is the only operations at various planetary locations, such upper stage that is currently certified for launch as atmospheric entry and capture, descent to the in a shuttle. This has forced spacecraWpayload sup- surface and ascent, back to rendezvous orbit. The pliers to develop advanced onboard Ipssto economi- discussion will concentrate on where ISP stands cally meet their needs, primarily with their own today and some observations of what might be next financial investments. Unfortunately, major depen- in line for new ISP technologies and systems for near- dence on in-house development funding has mini- term and future flight applications. The architectural mized the capability of IPSs as well as significantly choices that are applicable for ISP will also be increased their operational risk in many cases. described and discussed in detail.

OF CURRENT STATUS

After the payload is placed in a low-Earth park- ing orbit by the launcher, an upper stage is frequently Table 1lists the primary upper stage and space- used to transfer the payload to its operational orbit. craft (in-space) propulsion options. Cold gas Upper stage designs, especially their weight and size, propulsion systems are inexpensive, low- are strongly driven by the performance and weight performance systems that are rarely used unless the= efficiency of theii primary propulsion system. The is an overriding requirement to avoid the hot gases specific impulse (I ), propellant density, and over- and/or perceived safety concerns for liquid and solid sp. all stage mass fiactlon of the primary propulsion systems. Solid propellant-based systems have been system are key parameters. However, because of the used extensively for orbital insertion, but the lack of development of higher performance upper spacecraft's propulsion subsystem must be stages, especially those that would be compatible augmented with another technology to provide with reusable launch vehicles as well as expendable orbital maintenance maneuvering, velocity control,

1 ,', 1 .> , ..

Table 1. Principal options for spacecraft propulsion subsystems.

Figure 1. Secondary combustion augmented thnrster-bimodal spacecraft attitude and velocity control RCS thruster. and attitude control. Liquid systems are divided into to those that are readily available, storable, and easy monopropellant and bipropellant systemswith a third to handle. Also, we must weigh the lead time needed alternative, dual mode; i.e., a bipropellant derivative. to develop new hardware against any limitations hm Monopropellantsystems have successfully provided using a combination of existing components or orbital maintenance contml and attitude control func- stages. Finally, limits on payload acceleration may tions for hundreds of spacecraft but lack the perfor- dictate the maximum permissible thrust levels. mance to provide high-efficiency, large AV maneuvers required for orbital insertion. Bipmpel- Liauid Rockets for UDper Stape - and Smcecraft lant systems are attractive because they can provide Propulsion Svstems all three functions with one higher performance sys- tem, but they are more complex than the historic solid In a liquid rocket system, propellants are stored rocket and monopropellant combined systems. as liquids in tanks and fed on demand into the com- Dual-mode systems are integrated monopropel- bustion chamber by gas pressurization or a pump. lant and bipropellant systems fed by common fuel Bipropellant engines chemically react a fuel and an tanks. These systems are actually hybrid designs that oxidizer, and monopropellant engines catalytically use hydrazine ("4)as a fuel for high-performance decompose a single propellant. Bipropellant engines bipropellant engines; i.e., nitrogen tetroxidehydra- deliver a .higher Zsp but involve additional system zine (N204/N2H4), and as a monopropellant with complexity and cost. Table 2 shows liquid rocket conventional low-thrust catalytic tbrusters. The N204 engines currently in use for upper stages or PSs on feeds into the bipropellant engines and the mono- spacecraft. For up-to-date and more detailed propellant thrusters from a common fuel tank. In this information, contact the respective developers. manner, they can provide high Zsp for long AVburns at high thrust €?om a single propulsion system; e.g., Electric Propulsion apogee circularization, and reliable, precise, mini- mum-impulse bums by the monopropellant thrust- Electric propulsion (FP) uses externally provided ers for attitude control and tight pointing. An electrical power, either from the Sun (converted additional capability to enhance dual-mode propul- through photovoltaic solar arrays-100% to date) or sion is the development of a bimodal-thrust device, from nuclear and thermodynamic conversion ther- which can operate in a bimodal manner, either as a mal engines (to accelerate the working fluid to pro- simple catalytic-monoprop&llant thruster or as a high- duce useful thrust). For example, in an ion engine, performance, bipropellant thruster, known as the an electric field accelerates charged particles that exit secondary combustion augmented thruster (SCAT) at high velocity. Alternatively, in a magnetoplasma- shown in Figure 1. dynamic thruster, a current-carrying plasma In the propulsion system selectionprocess, prac- interacts with a magnetic field, resulting in abrentz tical consideration may restrict the propellant choices acceleration to expel the plasma.

2

thruster and power processor. (The masses of the into the future for a variety of medium to high propellant subsystem, gimbals, and other mission energy propulsion functions. specifics are not included.) Pulsed plasma thrusters (PPTs) are inherently Resistojets have been used for North-South pulsed devices that operate at -847-s ZSp. They were stationkeeping and orbital insertion of communica- built in the Applied Physics Laboratory at Johns tion satellites in the United States (U.S.) and for or- Hopkins University and have successfully main- bital control and attitude control system (ACS) tained precision control of three NOVA spacecraft functions on Russian spacecraft, respectively. Pro- for many years. PPTs feature very small (14x10"5Ns) pellant temperatures are fundamentally determined impulse bit capability, use of a solid propellant by material limits in resistojets, which implies mod- (Teflon@by DuF'ont@), and the ability to operate at est (propellant-specific) maxima for Zsp of -300 s near constant performance over large power ranges. for the 0.5- to 1-kW class resistojets. An improved version PPT that operates at =1,200-s Resistojets have several desirable features, Zse (Table 3) was developed under NASA leader- including (1) values of thrust and power far higher ship, and a flight test was recently c!onducted on the than other EP options, due to their high efficiencies Earth Observer 1 spacecraft that very successfully and modest Zsp; (2) the lowest EP system dry masses, demonstrated propulsive ACS. The characteristics primarily due to the lack of a requirement for a power of PPTs will likely limit their power operating range processor; and (3) uncharged (benign)plumes. These to under a few hundred watts and, as suggested by features will continue to make resistojets attractive Table 5, they have large specific masses. Within their for low to modest energy applications, especially operating capability, however, PITSpromise a com- where power limits, thrusting times, and plume im- bination of low power, high ZSp, and a small impulse pacts are mission drivers. In addition, resistojets can bit that is unique. Based on recent successes on flight operate on a wide variety of propellants, which leads vehicles, it is expected that PPTs' use for ACSs and to their proposed use as a propulsion and waste gas for modest energy AV applications in small space- management concept on the International Space craft where the power and thrust limitations of PPTs Station (ZSS) and as an Earth-orbit insertion system are acceptable and,desirable. (operated on hydrogen). Hall effect thrusters (HETs) and ion thrusters Arc jets provide about twice the Z, of resistojets (ITS)represent the highest performance El? options; while still maintaining some desirabfe electrother- characteristics of mature versions of both concepts mal features, such as use of standard propellants and are shown in Table 5. HETs were developed and relatively low dry masses. The increased Z, coupled flown on dozens of Russian space missions for.vari- with relatively low efficiencies of about 6.3 to 0.4 ous functions and are under intense development for leads to significant decreases (% times) of thrust use on other nations' spacecraft. Flight-type HETs and power relative to resistojets. In addition, because have been produced by Fakel Enterprise (Fakel), we must control the complex plasma and arc phe- Keldysh Research Center (KeRC), and TsNIIMASH, nomena, arc jets require relatively complex power all of Russia, and quite aggressive HET research and conditioning, resulting in dry masses about twice development programs are in place in Europe, those of resistojet systems. Japan, and the United States. Table 5 lists three €ET Significant efforts, including development concepts to illustrate the state of the art. The of novel materials, were necessary to define and vali- 1,600-s Zsp concept was developed to flight-ready date the 600-s N2H4arc jet. It is expected, therefore, status by a team, including International Space Tech- that 600-650 s represents the upper range of ZSp that nology, Inc., Loral, and Fakel. The 1,638-s Zsp can be expected from low-power arc jets using stor- device was built, qualified, and delivered for flight able propellants. Arc jets do provide major mass test (at reduced levels of power and Zsp) by a NASA/ benefits for many spacecraft, are relatively simple industry team. The high-power HET was built to integrate, and are the least complex and costly by Space Power Inc. and KeRC for a 1999 flight test of any plasma propulsion device. For these reasons, on a Russian GEOSAT. In addition, two versions it is also expected that low-power arc jets will un- of 1.5-kW class HETs traceable to the Fakel con- dergo evolutionary improvements and be used well cept are planned to provide stationkeeping on

4 I.

Table 5. Hall effect thruster status summary. or greater thrust and power levels than those of ITS and are considerably lighter, but €€ET operations above ~2,500-spose major lifetime or redesign chal- lenges. Both concepts eject high-velocity, charged plumes and present approximately the same issues regarding spacecraft integration. HETs and ITSpro- vide extreme benefits for emerging space missions and the choice between them will be very mission specific. In general, however, lTs become increas- ingly beneficial as mission energies increase, and HETs appear optimum for many time- constrained situations typical of Earth-orbiting space missions. Electric propulsion has come a long way in the last decade. For example, as mentioned above, the development highly successful flight dem- the French Stentor spacecraft for 9 years. Table 5 and onstration of a long-life, high-perfomce ion EP summarizes the status of HETs. system, NASA Solar Electric Propulsion Five mature ion thrusters are also shown in known as Technology Application Readiness (NSTAR) (part Table 3. The 2,585-s Zsp system was built by the of the flight demonstration project as Deep Boeing Space Systems and is operational on a com- known Space l), was part of the discovery series of mercial communication satellite (COMSAT) that was missions. effort was mainly by NASA, launched in 1997. 2,906-s Zsp concept was built This funded The although private investment was also provided by by a team from Mitsubishi Electric Corporation and the contractor who now uses a derivative xenon ion Toshiba Corporation, Japan, and was flown on the propulsion system (XIPS) for their family of com- ETS-VI spacecraft in 1994. An orbital insertion mercial COMSATs. issue prevented the system from performing its planned stationkeeping function, but in-space This ion propulsion system demonstration for deep space exploration missions NSTAR pro- characterizations were performed in 1995 and an (the gram) was conducted jointly by Glenn Research identical system was flown on the Japanese COM- Center and the Jet Propulsion Laboratory with in- ETS spacecraft. The 3,250-s and 3,400-s Z, systems dustry participation.2 Flown on the Deep Space 1 ' were built in Europe by teams headed 6y Matra Marconi Space and DASA, respectively. These spacecraft under the new millennium program, devices were baselined for stationkeeping on the launch in 1998, NSTAR was the first interplanetary spacecraft to use ion thnisters propulsion. European Space Agency's Arternis spacecraft that as main The NSTAR engine uses xenon propellant, has a was launched in July 2001. The 3,280-s, 2.5-kW 30-cm optics diameter, and was operated at device is the highest power mahue IT for which data 0.5-2 kW in flight. engine was designed for are available and was very successfully demonstrated The 8,000 horn life and has perfonned well in flight, on NASA's New Millennium DS-1 mission. already exceeding 25,000 hours of reliable opera- HETs and are the highest Z options avail- ITS tion in space. Also noteworthy is that commercial able for mission planners; many Ayseshave been COMSATs now routinely utilize ion propulsion conducted to evaluate their use for high-energy for stationkeeping; first of these was launched missions. Comparison of the two devices is difficult the in 1997. because of the relative lack of maturity of devices In chemical propulsion for upper stages and built to comparable power levels and standards. ITS onboard IPSs, only minimal investment has been operate more reliably at higher Zsp than HETs, and their performance and specific mass are deeply provided by the U.S. Government. The paucity of new technology developments for the high lever- penalized by operation at Zsp less than ~2,500s be- age of increased performance ISP systems greatly cause of the constraints imposed by the ion optics has ' hampered the growth of Earth orbital and deep space systems. On the other hand, HET systems have 30%

5 missions. Many of these new high-performance ISP regulation devices, that will enable appropriate technologies are literally required to enable future systems with high performance that offer improved planned space exploration missions. This need was high reliability for future deep space missions is also recognized by the NASA office of space science, critical for enhancing future missions. who then initiated a comprehensive series of pro- Above all, very low-density, high-temperature curement programs to develop these badly needed, capable, high-strength actively and passively cooled high-leverage, propulsion development programs. materials with their associated structural architec- For typical classes of spacecraft using today's ture will be critical. It is projected that toward the conventional ISP, as shown in Figure 3, it can end of the next decade, ceramic-based systems will be seen that 30% to 60% of all the mass launched find their way into more critical components into low-Earth parking orbit is for injection and and applications and will likely be one of the break- maneuvering propellant. However, in the case throughs that can be touted in a similar paper of a large, deep space probe spacecraft, such as the written 10 years fiomnow. Advanced metallics with Cassini (soon to be placed into a Saturn orbit), properties substantially improved over those launched by a Titan IV Centaur, almost all of the of today's operational materials will be employed launched msis spacecraft and upper stage propel- extensively. Another possibility for certain types lant (=95%, which allows little capability for of structures will be higher temperature, polymer- spacecraft power, avionics, structure, and the scien- based materials. tific instrument payload), as shown in Figure 4. By the end of the next decade, another revolu- Propulsion components will also be critical tion in design tool and design capability will to the success of future in-space transportation probably have taken place. A broader and deeper systems. The development of advanced, robust, light- understanding of physics is gained through advanced weight components, such as valves, tanks, and sensing techniques, coupled with improved numeri- cal and simulation methods; and further, with health management gains through better experimentation and technology demonstration should greatly increase the quantity of design options that can be addressed, the number of iterations that can be rapidly and accurately accomplished, the level of detail and fidelity in the analysis, and reductions in uncertainty and product design and development cycle times. In summary, by the end of this decade, the capabilities and tools should be well in hand that Figure 3. Typical spacecraft mass fractions- will allow major advancements toward the first over half of everything launched is integrated systems that incorporate all the key tech- on-board propulsion. nologies and demonstrate end-to-end mission, operational, safety, and economic performance potential.

Recent Advancements

Technology advancements for in-space transpor- tation have occurred in three primary categories: advanced materials, engine designs and propellants for chemical propulsion, and development and demonstration of EP and soon-to-be-flown Figure 4. In -space propulsion requirements: propellantless systems. These advancements are deepspace robotic exampk+Cassm... summarized in Figure 5.

6 ..:,

Electric

Substantial gains occurred in EP during the 1990’s. Electric propulsion has very low thrust but extremely high Z .Appropriate use through mis- sion design and avadabilitySP of adequate spacecraft power, coupled with advancementsmade in thruster technology, has brought widespread application within reach due to the substantial competitive advantage, cost reduction, or mission-enabling Planetary Aeroassist Tethers advantages provided. Often, EP can mean the dif- ference in launch vehicle class, substantial addition F’igure 5. In-space propulsion systems. to payload mass, or feasibility of a deep space exploration mission. In chemical propulsion, development of iridium- Another significant advancement in EP was rhenium chambers pioneered by NASA and achieved through development of the plasma industry started in the 1980’s. During the 199o’s, contactor for the ZSS. This device is essentially the this technology was brought from a low technology same as a hollow cathode required for neutraliza- readiness level of maturity to full operational use. tion in an electrostatic propulsion system. The ZSS This provided substantial advancement in perfor- hollow cathode assembly-designed, developed, mance parameters for monopropellant and bipropel- built, and tested at Glenn Research Center-accu- lant systems, offering long life at increased Zsp mulated 27,800 hours of operation during hfe test, (323 s in qualification testing)? The first flight demonstrating very long life for an essential occurred in 1999. In addition, major advancements propulsion system component. in spacecraft dual-mode propulsion systems; i.e., using N2H4as both a bipropellant fuel and as a mono- propellant fuel in the same system, have enabled higher spacecraft performance and operational Advancements have also been made in the benefits, such as was the case for the Chandra x-ray so-called propellantlth categories that include telescope. solar sail and tethers, shown in Figure 6. A series Although not yet implemented in a flight of systems tests on propellant-free propulsion tech- program, substantial progress has been made in nology have recently been completed at the NASA hydroxyl azide nitride (“)-based, monopropel- Marshall Space Flight Center (MSFC) in Huntsville. lant spacecraft propulsion thrusters which have The Propulsive Small Expendable Deployer System acceptable Z and good density-impulse character- (PrdEDS) is a tether-based propulsion system that istics. This is.sp important because increasingly strict draws power from the space environment around safety and environmental regulations as well as Earth, allowing the transfer of energy from the Earth operations costs make it steadily more difficult to to the spacecraft. Electrodynamic tethers used for utilize traditional monopropellants. During the the propulsion in low-Earth orbit and beyond could 19903, stable HAN-based monopropellant mixes significantly reduce the weight of upper stage rock- with laboratory thrusters using catalytic ignition were ets used to boost spacecraft to higher orbit. And demonstrated with over 8,000 s of cumulative because they requk no propellant, electrodynamic operation on a single catalyst/thruster combination. tethers substantially reduce spacecraft weight, The result was accomplished by an industry/Glenn providing a cheap, efficient method of reboosting Research Center team. The results will likely open the orbits of spacecraft and potentially the ZSS. the door to further development and eventual * An electrodynamic tether, which consists of a adoption for spacecraft use. long, thin wire deployed from an orbiting satellite or vehicle, uses the same principles as electric mo- tors in many household appliances and automobile

‘7 I I generators. When a wire moves through a magnetic field, an electrical current results. As this current Figure 7. Solar sail and illustration of how flows through the wire, it experiences a push from thrust is derived from the reflection any external magnetic field. The force exerted on of the sunlight on the solar sail. the tether by the magnetic field can be used to raise or lower a satellite's orbit, depending on the at least some operational missions. Planetary explo- direction of the current's flow. ration will likely demand the extension of existing Researchers at MSFC also are investigating technologies to meet the unique needs of planetary the use of electrodynamic tethers to extend and en- transportation from surface to orbit. hance future scientific missions to Jupiter and its Power output for spacecraft will continue to moons. In theory, electrodynamic tether propulsion increase, affording the opportunity to substitute EP could be used near any planet with a significantmag- for chemical propulsion for main orbit transfer with netosphere, such as the enormous magnetosphere acceptable transit times. Taking advantage of the found arodJupiter. higher power capability in a most advantageous As mentioned earlier, another category fashion will demand increasing power capability of propellantless propulsion is the solar sail. Solar from the thrusters with high efficiency. In both HET sails use photon pressure of force on thin, lightweight and IT systems, 10-kW maximum power thrusters reflective sheets to produce thrust. The net force are under technology development. Planetary explo- or thrust on the solar sail is perpendicular to the ration as well as multipurpose Earth orbit thrusting surface, which is the result of a force tangential to requires a wide operating range at acceptable orbit and a force perpendicular to the orbit caused efficiency. In the case of IT systems utilizing solar by the reflective sunlight, as shown in Figure 7. Ideal power for planetary exploration, the power available reflection of sunlight from the solar sail surface is steadily reduced as the distance from the Sun produces 9 Nh2at 1AU. increases, placing important operational demands on the designs. The Next Decade Another emerging trend is the availability of advanced power sources that lead to the capabil- Chemical propulsion offers a unique combina- ity of performing ambitious missions that need tion of features that for some mission requirements to optimize at high power, very high I conditions. have no substitutes. It is likely that green (Le., Thrusters to meet such demand willSF likely be nontoxic) propellants will continue to be under tech- demonstrated with a substantial level of maturity nology development and begin to be introduced into that might provide a point of departure for flight

8 demonstration in the second decade of the century. A major advancement will be enabled by the devel- opment and implementation of very high-power (hundreds of kW, to megawatts) nuclear electric propulsion (NEP) systems; Exploration of the outer planets in our solar system represents significant technological challenges in mission planning and vehicle design. Longduration missions currently utilize an opti- mized combination of chemical propellant, solar energy, and vehicle trajectories that require gravita- tional assistance. Using this conventional approach results in launch window time constraints, longer trip times, and limited power reserves for payload science. To mitigate these mission constraints, NASA's Figure 8. Conception of Bssion-powered nuclear power Prometheus program will focus on space craft. two types of nuclear power technology: (1) Nuclear fission electric propulsion and power systems The Jupiter moons are believed to contain water and (2) radioisotope power systems. NEP will beneath their icy crusts. The= is spectral evidence utilize a fission reactor, thermodynamic power that indicates salts and organic materials on their conversion system, and electric thrusters to provide surfaces, and geological evidence indicates that an propulsion for a new class of exploration spacecraft. ocean existed on Europa within the last 100 million The I" system will generate high Zsp (2,000- years. The kdalSurvey Report from the National 9,OOO s) that will reduce the amount of propellant Academy of Sciences identifiedthe Europa mission required for long-duration missions. as the top priority for solar system exploration. The development of the NEP spacecraft will Planetary surface exploration will be advanced eliminate the need to depend on complex trajecto- by the development of a new generation of RPSs. ries with the usual gravity-assist constraints. The RPS generates electricity from heat created The success of the Nuclear Propulsion and Power by the natural decay of plutonium. This advanced program will enable NASA to meet the higher power RF'S will provide reliable power for surface mobil- demand that will be required for exploration of the ity units, such as the Mars Smart Lander. Solar pan- outer solar system. Through the utilization of nuclear els nominally provide the lander with 180 days of technology, spacecraft will be able to operate over useful life; the advanced RPS would extend its life- a period of years and cover vast distances previously time to over 1,OOO days. The RPS would relieve the impossible to accomplish with traditional fuel tech- lander of its dependency on solar power during the nology. Without the double constraints of mass vehicle's greatly increased life on the planet's and power to encumber those vehicles, spacecraft surface, thus enabling a broader exploration of the utilizing nuclear propulsion will be able to maneu- planet's surface. ver robustly and with flexibility, once on-!3tation,and Multiple agencies will be involved in the NSP will also be able to provide vastly improved data program that will be funded through a combmation acquisition and transmission capability. of direct funding and competitive sourcing. These NASA's NEP protoflight spacecraft will be participants include NASA and the Department of developed as part of the Prometheus program. This Energy. NASA Field Centers involved include project will focus on the development of a fission- Marshall Space Flight Center, Glenn Research powered spacecraft, illustrated conceptually in Center, and the Jet Propulsion Laboratory. Figure 8, that will tour and orbit three of Jupiter's Near-term NSP goals will focus on the develop- moons-Europa, Ganymede, and Callisto. Science ment of advanced RPSs, specifically the data from this mission will provide full orbital multimission radioisotope thermoelectric generator characterization of all three icy moons.

9 t 3 .. ” .

and the Stirling radioisotope generator, and identifi- tant ISP developments. This paper did not attempt cation of candidate planetary science missions to rigorously sort through all major advancements enabled by NEP. and assign perceived impacts to select those to be In the past decade, achievements in ISP saw the highlighted. Instead, it highlighted those that were advent of Ep technologies that were steadily devel- noteworthy for the impacts they have on the direc- oped for 30 years. These provide revolutionary per- tion NASA has planned to take in the future of space formance improvements as well as necessitating exploration. changes in the way missions are performed to take advantage of the characteristics of the electric sys- tems. Chemical propulsion also was improved in important ways. Technology development efforts begun in the 1990’s became the basis for the propul- 1. Sackheim, R. L., and Cikanek, H. A. 111, “Evolu- sion advancements that are coming to fruition tion of Propulsion Requirements and Concepts in this decade. for Future Space Transportation Systems,” AAA/ NASA’s Integrated Space Transportation ESNCNES 6th International Symposium, Architecture Plan provides the basis for a very chal- Propulsion for Space Transportation of the xxlst lenging, creative, and productive next decade. Century, Versailles, France, May 1417,2002. The opportunity for a series of great strides in space 2. Cikanek, H. A. 111, and Sackheim, R. L., transportation is upon us. NASA now has an admin- “Space Transportation Technology Development istrator who strongly supports what we have identi- . at the U.S. National Aeronautics and Space fied and have planned to do. Although we have just Administration(NASA) Achievements of the Past come through some difficult, lean times and Decade, Vision for the Future,” AAA/EsA/CNES industry is still in a downturn, the future appears full 6th International Symposium, Propulsion of bright possibilities, which would not have been for Space Transportation of the XXIst Century, available without the many achievements Versailles, France, May 14-17,2002. of the 1990’s. 3. Wertz, J. R., and Larson, W. J., Space Mission NASA, along with its many worldwide partners, Analysis nnd Design, 3rd ed., Kluwer Academic has been at the forefiont of many of the most impor- Publishers, Dordrecht, The Netherlands, 1999.

10