Study

Joint Study Group:

ADVANCED

INTERPLANETARY MISSIONS

USING

NUCLEAR–ELECTRIC

PROPULSION

Reference Missions:

Asteroid Sample Return
Mercury Lander
Pluto/Charon Rendezvous

Contents

Summary (in English, Russian, and German)

1. Introduction 1

1.1. The Third Major Step in Rocket Propulsion (E. Stuhlinger) 1

1.1.1. Initiative for electric propulsion space missions 1
1.1.2. Nuclear–electric propulsion development 2
1.1.3. Nuclear–electric propulsion missions 3

1.2. Prospectives for the New Stage of the Solar System Planetary Study (T. M. Eneev) 4

1.2.1. Major objectives of solar system flights 4
1.2.2. Missions to the minor bodies 5
1.2.3. New spacecraft 7
1.2.4. Asteroid missions and other projects 7

2. Basic Ideas (H. W. Loeb) 9

2.1. Background of the Study 9

2.1.1. First epochs of solar system exploration 9
2.1.2. Future solar system exploration and NEP 9
2.1.3. Former objections against NEP 11
2.1.4. New situation of NEP 12

2.2. Joint Study Group Policy 13

2.2.1. History and activities of the JSG 13
2.2.2. Goals of the JSG 14

2.3. NEP–Principles 15

2.3.1. General rocket propulsion principles 15
2.3.2. Chemical propulsion: a high–thrust, low–energy drive 17
2.3.3. Electric Propulsion: a low–thrust, high–energy drive 18
2.3.4. EP–thrusters 19
2.3.5. EP–power sources 24
2.3.6. Apprehensions and reservations against nuclear energy 26

2.4. EP–Spacecraft and Missions 28

2.4.1. NEP–system components and concepts 28
2.4.2. NEP–system adaptation and characteristics 30
2.4.3. NEP–, SEP–, and SNEP–missions 35

3. Scientific Objectives and Model Payloads 41

3.1. Introduction and Survey (H. W. Loeb) 41

3.1.1. Objectives of the primary reference mission 41
3.1.2. Objectives of secondary reference missions 42
3.1.3. Survey of model payloads 44

3.2. Scientific Objectives and the Model Payload of Asteroid Sample Return Missions
(I. A. Gerasimov, G. Schwehm, and I. T. Zotkin) 44

3.2.1. Survey of asteroids 44
3.2.2. Scientific objectives 48
3.2.3. Model payload 49
3.2.4. Asteroid sample return 50
3.2.5. Meteorite parent bodies 50
3.2.6. Mixing in the solar system 50
3.2.7. Planetary processing 51
3.2.8. Sampling strategy and flight concepts 51

3.3. Scientific Objectives and Model Payloads of Planetary Missions (G. Schwehm) 52
3.3.1. Survey of planetary missions 52
3.3.2. Mercury´s scientific objectives and model payload 52
3.3.3. Pluto´s scientific objectives and model payload 55

4. Mission Scenarios 57

4.1. Separation of Orbit Optimisation Problem (T.M. Eneev) 57

4.2. Missions to Primitive Bodies
(G. B. Efimov, T. M. Eneev, M. S. Konstantinov, M. Hechler, and V. G. Petukhov) 58

4.2.1. Asteroid rendezvous missions 58
4.2.2. Multiple Main Belt asteroid rendezvous and flyby 61
4.2.3. All–NEP asteroid sample return missions 63
4.2.4. Hybrid propulsion asteroid sample return missions with NPP 65
4.2.5. Hybrid propulsion asteroid sample return missions with SPP 67
4.2.6. Comet rendezvous missions 67

4.3. Asteroid Reference Mission Scenario (K. V. Evdokimov and L. A. Latyshev) 70

4.3.1. Choice and description of the reference mission 70
4.3.2. Comparison of NEP and SEP missions 71
4.3.3. "Fortuna" rendezvous with high–thrust rockets 71

4.4. Planetary Mission
(G. B. Efimov, T. M. Eneev, M. Hechler, M. S. Konstantinov, A. R. Martin, and V. G. Petukhov) 72
4.4.1. Missions to Mercury and Giant Planets 72
4.4.2. Pluto/Charon rendezvous 75

5. Technology Status – Nuclear Space Reactors 81

5.1. Nuclear Space Reactor Power Plant "Topaz" (G. M. Gryaznov) 81

5.1.1. Introduction: the "Topaz" system 81
5.1.2. "Topaz"–system mode of working 82
5.1.3. Choice of the NEP–power plant type 85
5.1.4. Main NPP–subsystems: composition and description 86
5.1.5. Incore thermionic reactor design of "Topaz–25" 88
5.1.6. Main performance of the NPP "Topaz–25" 88
5.1.7. State–of–the–art and endurance proof of "Topaz–25" components 92
5.1.8. "Topaz–25" module design and structural arrangement 93
5.1.9. Nuclear and radiation safety control in NPP–service 94
5.1.10. Conclusion 96

5.2. Western Activities in Space Nuclear Power System Development (A. Martin) 97

5.2.1. US programs 97
5.2.2. International programs 99
5.2.3. European programs 99

6. Technology Status — Electric Propulsion Systems 106

6.1. Electric Propulsion Thruster "ESA–XX" (C. Bartoli, H. Bassner, K. Groh, and H. W. Loeb) 106

6.1.1. Introduction and background 106
6.1.2. "ESA–XX" configuration and functional principle 108
6.1.3. "ESA–XX" performance requirements 110
6.1.4. "ESA–XX" assembly specifications — thrust unit, neutralizer and Xe–supply system 111
6.1.5. "ESA–XX" electronics specification 115
6.1.6. "ESA–XX" performance mapping 121
6.1.7. Conclusions 131

6.2. Stationary Plasma Thrusters "SPT–140", "SPT–200" and "SPT–290"
(A. Bober, V. A. Maslenikov, B. A. Archipov, and G. A. Popov) 132

6.2.1. Introduction 132
6.2.2. SPT physical principles of operation 133
6.2.3. Description of the SPT–structural composition and functional principle 137
6.2.4. Technical data of the large SPT–engines 137
6.2.5. Design description of the engine unit, the cathode–compensator
and the Xe–storage and supply unit 140
6.2.6. Description of the conversion and control system 143
6.2.7. Conclusion 146

7. Spacecraft Design 147

7.1. Automatic Spacecraft for Solar System´s Small Bodies Research —
Preliminary Study and Design (R. S. Kremnev and V. A Kudryashov) 148

7.1.1. Introduction 148
7.1.2. Mission scenario 149
7.1.3. Initital data and weight model of an independent spacecraft ("ISC") 150
7.1.4. Multipurpose automatic interplanetary spacecraft ("AIS") 151
7.1.5. Transport and power module 154
7.1.6. Trajectory unit ("TU") 154
7.1.7. Landing probe design concepts 160
7.1.8. Soil sample return concepts 163

7.2. Interplanetary Spacecraft with Electric Propulsion (H. Bassner and H. W. Loeb) 164

7.2.1. Introduction 164
7.2.2. Launch rocket for "ISC–2000" 165
7.2.3. Power plant systems of "ISC–2000" 167
7.2.4. Electric propulsion module "EPM" 169
7.2.5. Radiation shielding by the propellant 174
7.2.6. Payload of the "Fortuna" reference mission 176
7.2.7. NEP–spacecraft "ISC–2000" for the "Fortuna"–mission 177
7.2.8. SNEP–spacecraft "ISC–2000" for planetary missions 178
7.2.9. Conclusions of the preliminary spacecraft analysis 181

8. Conclusions and Recommendations of the Joint Study Group 183

8.1. Conclusions 183

8.1.1. Advanced missions and scientific objectives of nuclear–electric propulsion 183
8.1.2. Interplanetary spacecraft with nuclear–electric propulsion 185

8.2. Recommendations 187

8.2.1. Proposed preliminary implementation plan 187
8.2.2. Recommendations of a Pre–Phase A–Study 188
8.2.3. Recommendation of hardware development 189
8.2.4. Recommendation of a joint NEP–flight 189

9. Appendix 190

9.1. Pluto Tracking: Rendezvous with Pluto Using Jupiter Swingby
(T.M. Eneev, K.V. Evdokimov, G.G. Fedotov, G.B. Efimov, M.S. Konstantinov,
V.G. Petukhov, and G. A. Popov) 190

9.2. Acknowledgement 194

9.3. References 195

9.4. Abbreviations 199

9.5. Symbols and Units 200

STUDY REPORT

ADVANCED INTERPLANETARY MISSIONS

USING NUCLEAR–ELECTRIC PROPULSION

edited by the Joint Study Group:

Loeb, H. W., Prof. Dr., University of Giessen, Germany
Popov, G. A., Prof. Dr., Corresp. Member Academy, Director RIAME of MAI, Moscow, Russia

Achtermann, E., Dr., Dornier/Dasa, Friedrichshafen, Germany
Archipov, B. A., Dr., Fakel–Enterprise, Kaliningrad, Russia
Avduevsky, V. S., Prof. Dr., Academician, RIAME of MAI, Moscow, Russia
Bartoli, C., Dr., ESTEC, Noordwijk, ESA
Bassner, H., Dipl.–Ing., Dasa Munich, Germany
Bober, A. S., Director Fakel–Enterprises, Kaliningrad, Russia
Bodin, B. V., Russian Space Agency, Moscow, Russia
Bogdanov, A. A., Dr., Russian Ministry of Science, Moscow, Russia
Eneev, T. M., Prof. Dr., Academician, IMP named after Keldysh, Moscow, Russia
Efimov, G. B., Dr., IPM named after Keldysh, Moscow, Russia
Evdokimov, K. V., Dr., MAI, Moscow, Russia
Gryaznov, G. M., Prof. Dr., Director Krasnaya Zvezda Enterprise, Moscow, Russia
Hechler, M., Dipl.–Math., ESOC, Darmstadt, ESA
van Holtz, L., Dr., ESTEC, Noordwijk, ESA
Hopmann, H., Dipl.–Ing., Dasa Munich, Germany
Koelle, D. E., Dr., TCS–TransCostSystems, Ottobrunn, Germany
Konstantinov, V. A., Prof. Dr., MAI, Moscow, Russia
Kramer, P., Prof. Dr.–Ing., Dasa Munich, Germany
Kremnev, R. S., Dr. SPE named after Lavochkin, Chimki, Russia
Kudryashov, V. A., SPE named after Lavochkin, Chimiki, Russia
Latyshev, L. A., Prof. Dr., MAI, Moscow, Russia
Martin, A., Dr., AEA Technology, Culham Laboratory, U. K.
Messerschmid, E., Prof. Dr., University of Stuttgart, Germany
Meusemann, H., Dipl.–Phys., DARA, Bonn, Germany
Noack, E., Dipl.–Ing., DARA, Bonn, Germany
Sapozhnikov, V. I., Arsenal Design Bureau, St. Petersburg, Russia
Schleinitz, J., Dr.–Ing., Dornier/Dasa, Friedrichshafen, Germany
Schwehm, G., Dr., ESTEC, Noordwijk, ESA
Schwille, H., Dr., Dornier/Dasa, Friedrichshafen, Germany
Sinelshikov, M. V., Dr., Russian Space Agency, Moscow, Russia
Stuhlinger, E., Prof. Dr., Huntsville, USA
Zaritsky, G. A., Krasnaya Zvezda, Moscow, Russia

June 1995
to be presented in Bonn, Moscow, and Paris

published by the
"Joint Study Group on Advanced Interplanetary Missions Using Nuclear–Electric Propulsion"

contact addresses:

Prof. Dr. H. W. Loeb
1st Institute of Physics, University of Giessen
Heinrich–Buff–Ring 16
D–35392 Giessen, Germany
phone: 0641–702–2730, fax: 0641-702–2709

and

Prof. Dr. G. A. Popov, Corresp. Member Academy, Director of RIAME
Moscow Aviation Institute MAI
Volokolamskoe Sh. 4
125871 Moscow, Russia
phone: 095–158–00–20, fax: 095–158–0367

lay out:
1st Institute of Physics, University of Giessen,
Director: Professor Dr. Dr. h. c. mult. A. Scharmann
Heinrich–Buff–Ring 16, D–35392 Giessen, Germany
phone: 0641–702–27 10, fax: 0641–702– 27 09

printed by:

Advanced Interplanetary Missions Using Nuclear–Electric Propulsion

Joint Study Group:
H. W. Loeb1 and G. A. Popov2 (Chairmen), E. Achtermann3, B. A. Archipov4, V.S. Avduevsky2, C. Bartoli5, H. Bassner6, A. S. Bober4, A. A. Bogdanov7, G. B. Efimov8, T. M. Eneev8, K V. Evdokimov2, G. M. Gryaznov9, M. Hechler10, L. van Holtz5, H. Hopmann6, D. E. Koelle11, V. A. Konstantinov2, P. Kramer6, R. S. Kremnev12, V. A. Kudryashov12, L. A. Latyshev2, A. Martin13, E. Messerschmid14, H. Meusemann15, E. Noack15, V. I. Sapozhnikov16, J. Schleinitz3, G. Schwehm5, H. Schwille3, M. V. Sinelshikov17, and E. Stuhlinger18, G. A. Zaritsky9

Summary

Electric propulsion enables high–energy solar system missions of post–2000:

The pioneering epoch of spaceflight was based on chemical propulsion and already achieved several exciting scientific successes. Then, gravity assistance technique has been developed to enable more ambitious interplanetary missions with enlarged velocity increments. However, multiple swingby roundabouts increase the mission time remarkably; the overall projects run the risk of becoming too complicated. Moreover, the future solar system exploration will confront the mission planners with increasingly difficult, advanced, high–energy missions.

Already the pioneers of rocketry recognized the way to overcome the energy barrier inherent in chemical combustion: electric propulsion (EP) engines are able to produce exhaust velocities which exceed those of all chemical rockets by one order of magnitude. Consequently, EP–rockets enable

— significantly higher velocity increments and/or much higher payload ratios (see Fig. I), following directly by Tsiolkovsky´s law; alternatively, smaller launch rockets can be used,

— "direct" trajectories with simple mission profiles, with greatly reduced flight times, higher mission flexibilities and broadened launch windows; as an additional benefit, the often fatal spacecraft charging is avoided by EP–operation.

Thus, EP can significantly enhance or even enable advanced scientific missions (see Table I) and it could be considered to be the 3rd major step in rocketry development, following the introduction of solid propellant chemical rockets at the beginning of the 13th century and of liquid propellant chemical rockets in 1926.

The Joint Study Group proposes an international NEP–program for solar system exploration:

The Joint Study Group (JSG), an international group of scientists, engineers and managers from academy institutes and universities, from industry and space agencies, worked out this Study Report in order to promote the new, promising propulsion technology for the post–2000 solar system exploration programs.

Going beyond numerous worldwide studies published and proposals made in the past decades, the JSG emphasizes that nuclear–electric propulsion (NEP) missions are not only advantageous and advisable but also realistic and feasible within one decade. The JSG emphasizes:

— The approaching third epoch of solar system exploration by logically more and more difficult "advanced, high–energy missions", with high payload requirements and not too long mission times needs a high–specific impulse propulsion technique, namely nuclear–electric propulsion (NEP).

FIGURE I:Assessment of spacecraft payload ratio vs. velocity increment *v (without gravity assist); the "chemical propulsion" borderlines refer to UDMH + N2O4 and LH2 + LO2, respectively; the calculation of "nuclear–electric propulsion (current techology)" is based on "ESA–XX" ion thrusters and "Topaz" nuclear reactor power plants (see below); the "NEP (advanced technology)" refers to a nuclear power plant performance of only 6.5 to 10 kg/kW.

TABLE I: Advanced high–energy interplanetary missions with electric propulsion.

celestial target EP–potentialities and benefits
Inner Heliosphere,Sun Corona more payload,enlarged scientific instrumentation mass
Mercury more payloadlanding of instrument package possible
Primitive Bodies(Asteroids and Comets) more payload,multi–rendezvous and sample return possible
Giant Planets andtheir Satellite Systems multi–target missions possible,shorter mission time
Pluto/Charon rendezvous possible, enlarged instrumentation,shorter mission time
Outer Heliosphere,Deep Space, Kuiper Belt shorter mission time, higher data transmission,enlarged instrumentation mass

— A standardized–modular, multi–purpose interplanetary spacecraft for post–2000 missions, called "ISC–2000", could be developed, built, qualified and flown; since all essential NEP–spacecraft components are based on current technologies or are already space proven (as "Proton" launchers, "ESA–XX" and "SPT" thrusters, "Topaz" nuclear reactor power plants, etc.), D & Q–times and cost could be kept within acceptable limits.

— Like manned spaceflight, ambitious unmanned solar system exploration would benefit from an international cooperation, because space experiences, hardware developments, and technical potentialities would be combined.

In the Study Report, scientific objectives and mission scenarios as well as the NEP–components and "ISC–2000" spacecraft schedules are described.

The JSG chose a NEP–pathfinder mission with respect both to its great scientific interest and to a reasonably modest NEP–performance requirement; this would guarantee a rather high chance of mission success, but demonstrate, nevertheless, the NEP–advantages.

A hybrid–propulsion sample return from asteroid "Fortuna" should be the primary reference mission:

A major goal of space science is the understanding of how the Sun and planets were formed. Hereby, a special role is played by investigation of relic matter preserved since the time of solar system origin or of bodies which have been relatively unaltered since then, i. e. of asteroids and comets. Besides the importance of these "primitive bodies" for the understanding of conditions in the primordial solar nebula and of planetary formation, we can also learn the evolution of the asteroid belt and the delivery of small bodies into Earth–crossing orbits, i. e. of meteorites, from their parent bodies.

The exploration of the primitive bodies was started with flybys (of "Giotto" and "Vega" at Halley´s comet and of "Galileo" at the asteroids Gaspra and Ida); as a next advanced step a comet rendezvous and the deposition of two surface stations (by "Rosetta") are in preparation. But, without disregarding the scientific value of these first steps, the mineralogists´ first choice would be to have a sample of this pristine matter in their laboratories on Earth. In order to accomplish this, a Comet Nucleus Sample Return Mission (CNSR) was planned by ESA and NASA. However, it turned out that with the chemical propulsion technology a CNSR would exceed mission duration and cost limits. NEP could provide the tools to make a mission like this become true.

As the sample return target body, the 221–km diam C–type asteroid "19, Fortuna", has been chosen. This is in the Main Belt, at 2.4 AU distance from the Sun. Fig. II shows one possible trajectory of the NEP primary reference mission.

FIGURE II:Hybrid trajectory of a NEP–flight to "Fortuna" with a chemical propulsion sample return (as one of several options, depending also on launch date and EP–specific impulse).

The mission scenario considers the launch of the 5.8–tons NEP–spacecraft by a "Proton–TM". By nuclear–electric interplanetary cruise, a useful mass (final s/c–mass minus mass of power & propulsion units) of about 2 tons can be taken to "Fortuna" within about 2 yrs. For the rendezvous manoeuvre, precision braking, hovering, and drifting near the asteroid surface (with remote monitoring), for deposing the landing craft, soil sampling, etc., one to three months are scheduled. For the escape from "Fortuna" and the return to Earth, bipropellant engines (with 3.2 km/s of exhaust velocity) will be used; the return vehicle of up to 200 kg of dry mass will reach Earth atmosphere within 1.3 to 1.6 yrs. After 3.6 to 3.7 yrs of total mission time, the reentry vehicle with the soil samples will be recovered, after aerocaputre and parachute descent.

Besides this reference mission, other "primitive bodies" missions have been analysed by the JSG, namely

— single asteroid rendezvous flights to "Vesta", "Fortuna", and "Massalia" with nuclear–electric and solar–electric propulsion (NEP and SEP; flight times 1.9 to 4.7 yrs, useful masses 1.5 to 2.0 tons),

— multiple NEP–rendezvous and flyby missions, e. g. with visiting of up to 8 asteroids of different classes (flight times 2.0 to 5.2 yrs, payload masses 0.3 to 0.9 tons),

— all–NEP sample return missions to "Vesta", whereby the payload advantage (up to 1780 kg of useful mass at Earth return), compared with the hybrid reference mission, must be balanced with the longer mission time (up to 5.5 yrs),

— NEP and SEP rendezvous missions with several comets, whereby the mission advantages (transfer times 2.2 to 4.4 yrs, payload masses up to 3.1 tons) would be significant over all the conventional projects.

In the most investigated cases, solar electric propulsion (SEP) enables smaller initial s/c–masses or higher payload capacities than NEP, thanks to the lightweight solar arrays, but SEP needs somewhat longer transfer times and it becomes ineffective for too great distances from Sun.

For both NEP and SEP, mission time and payload mass depend not only on launch date and launch window, but also on the EP–specific impulse. With increasing specific impluse, both the useful payload masses and, unfortunately, the EP–transfer times increase.

Planets and moons should be the targets of secondary reference missions:

The EP–potentiality exceeds by far the mission class of rendezvous or sample return from small, primitive celestial bodies. Table I indicates the broad EP–application range, covering all the space from close Sun approaches to flights towards the borders of our solar system. Several of the related missions are conventionally not only difficult to realize, but even beyond feasible and reasonable programs.

Especially the innermost and the outermost planets, Mercury and Pluto, are of great interest for planetary science; Mercury was only briefly encountered by a fast flyby mission; Pluto has never been visited.

It is true that there exist programs and proposals of a conventional Mercury–Orbiter mission (using 4 swingbys) and of a Pluto–flyby (with Jupiter´s gravity assist and 15 yrs of cruise time or directly in 8 yrs with a small Proton–launched spacecraft). However, electric propulsion would render possible the deposition of a lander on Mercury´s surface enabling in situ experiments, as well as a rendezvous with Pluto/Charon and a prolonged, detailed inspection of the Pluto system, including even a lander.

In both cases, the application of a hybrid solar/nuclear power source would be advisable, combining the two main advantages of both power systems, namely the high specific power of modern lightweight solar panels (between 0.5 and 2.5 AU of Sun distance) with the Sun–independent operation of nuclear reactors.

The solar wings would provide the EP–thrusters with energy during the first SNEP–thrusting phase and would be jettisoned when the reactor is started–up.

Detailed planetary EP–mission analyses may show whether a single gravity assist manoeuvre at Venus or Jupiter would improve the SNEP–performance for the two mentioned missions.

As the standardized NEP– or SNEP–modules as well as the "ISC–2000" vehicle are scheduled for multi–purpose applications, the JSG emphasizes that the "Fortuna" reference mission would be the opening act for a continuing chain of space achievements.

The NEP–components, i. e. thrusters and reactor power plant, are based on current technology:

For EP–power supply in deep space, there exists presently only one nuclear reactor power plant which already has been flown repeatedly and operated up to 1 yr in space, namely the 5–kWe system "Topaz" of Krasnaya Zvezda State Enterprise, Moscow.

Based on these experiments, a nuclear power plant "Topaz–25" can be manufactured with an electrical power increase no less than by a factor of 5 and with a minimum lifetime of 7 yrs. "Topaz–25" can provide a throttling ability down to 50 % as well as a power augmentation up to 200 % during 1 yr of operation.

Depending on the maximum power requirement, the "Topaz–25" mass will be between 2 and 3 tons.

The nuclear reactors of the "Topaz"–class are working with highly enriched U–235, ZrHx–moderator, in–core thermionic converters with Cs–feeding, NaK–coolant with an inductive electromagnetic pump, a shadow shield from U–238/LiH, and conical waste–heat radiators using heat pipes. For storage in launch position, one radiator part will be foldable.

Nuclear safety is provided by different measures (controllers, safety rods, locking mechanisms) in such a way that, in all cases, the reactor remains in a subcritical state until the spacecraft reaches escape velocity. Therefore, no radioactive hazard is to be feared for any NEP–spacecraft launch.

If additionally (or alternatively) solar arrays should be used for EP–supply, the standard and modular "SiBSFR"–panels of 10.6 kg/kW could be applied, representing already the state–of–the–art.

Within the countries from which most of the JSG–members come, two primary propulsion EP–thruster types are already under D & Q–programs:

— The 26–cm ionizer–diam Xe–ion thruster "ESA–XX" is working with an inductively fed rf–ionizer and a multi–hole three–grid accelerator system. This joint European thruster represents a combination of the "RIT–35" ionizer concept with the "UK–25" grid system; it is equipped with Italian neutralizers.
Smaller, secondary–propulsion European EP–engines were flown already ("RIT–10" on the "Eureca" satellite) or are to be applied for orbit control (two "RIT–10" and "UK–10" each, onboard the communication satellite "Artemis").

Recently, an "ESA–XX" prototype has been performance mapped by 4 test runs at Giessen University. The engine worked very stably and reliably. Thrust levels of 200 mN at nearly 50 km/s of exhaust velocity could be reached over extended periods at acceptable temperatures. The beam was well focused, and the measured total thruster efficiency reached 83 % exceeding even the scheduled value.
Programmatically, this testing prototype will soon be followed by an advanced and improved breadboard/engineering model; then, a qualification model will be built, thus leading to the final design and manufacturing of a flight model.
Based on its high efficiencies, its high specific impulse and its high lifetime expectation of 15,000 hrs (at 200 mN), the "ESA–XX" is very well suited to perform the long interplanetary propulsion phases with their high *v–requirements.

— A Russian series of 14–cm, 20–cm, and 29–cm ionizer–diam stationary plasma engines, namely "SPT–140", "SPT–200", and "SPT–290", are working with a magnetically effected closed electron drift inside the annular dc–ionizers and with a gridless Xe–ion acceleration by the discharge voltage itself.
Small, secondary–propulsion engines ("SPT–70" and "SPT–100") have been employed already more than 60 times onboard Russian satellites for orbit correction.

The "SPTs" are manufactured and tested at Fakel Enterprises, Kaliningrad. The larger engines reached thrust levels between 200 mN and 1 N at 15 to 30 km/s of exhaust velocity with up to 70 % thrusting efficiency. Lifetimes of 6000 hrs (for "SPT–140") to 12,000 hrs (for "SPT–290") are scheduled.
In comparison with the ion version, the stationary plasma–type consumes less power per unit thrust (16 to 22 kW/N vs. 31 kW/N) but more propellant (about 50 mg/Ns vs. 20 mg/Ns).

Therefore, large "SPTs" could be operated favourably on mission phases of augmented thrust requirements (at the limited power provision), i. e. for escape or approach manoeuvres, etc.

The modular NEP–spacecraft "ISC–2000" could be used for a variety of advanced missions:

The standardized NEP– or SNEP–module would be able to propel a multi–purpose interplanetary spacecraft "ISC–2000" for different solar system missions.

The spacecraft of 5 to 6 tons should be lifted–off to escape velocity by a "Proton–TM" rocket, which is based on the successful, more than 200 times flown "Proton" ground stages, equipped with a high–energy upper stage which will soon be in service. The maximum "Proton–TM" payload mass is about 6 tons; the maximum diameter available for the payload is 3.8 m, and the maximum payload length (in stowed position) 7.5 m. All these data fit to the JSG´s "ISC–2000" schedule (see Fig. III).

At one end of the oblong shaped, modular spacecraft, the nuclear reactor with its radiation shield, radiator and deployment system is arranged.

Subsequently, the 0.6 tons electric propulsion module is located, consisting of the thrusters, the propellant tank, the EP–power supply unit, the control unit, the box structure with thermal louvers, and 1 to 1.5 tons of Xe–propellant. A gross NEP–payload of 1 to 2 tons could be accelerated to *v–values up to 15 km/s.

If very high velocity increments should be needed, the nuclear power plant can be supplemented by two "SiBSFR" panels of e. g. 0.5 tons.

The gross payload ("trajectory unit") is located at the maximum distance from the reactor. It consists of the spacecraft–bus with telemetry system, attitude control, scientific instruments, etc. as well as of the asteroid lander/sample return subsystem. The latter includes the soil sampling equipment, surface scientific instruments, the return vehicle fuelled with bipropellant, and the Earth reentry vehicle.

Naturally, other mission classes (like the Mercury–lander or the Pluto–rendezvous, etc.) would require the modification of the sketched "Fortuna" payload.

To sum up:

* The Study Report deals with the future exploration of our solar system by advanced, high–energy missions using a new high–specific impulse propulsion method, namely nuclear–electric propulsion.

* The Joint Study Group members recommend an assessment study for this project to their governments and agencies.

* The Group emphasizes that NEP– and SNEP–missions are not only advantageous and advisable, but also feasible, because the new propulsion concept is based on current technologies.

* The project would benefit from an international cooperation, combining know–how, hardware developments, and activities.

* Following the proposed "Fortuna" pathfinder mission, a new era of space exploration with impressive future capabilities could begin.

FIGURE III: One of several possible "ISC–2000" configurations (in two views) as proposed by the JSG for the "Fortuna" reference mission; system analyses were done by Lavochkin State Enterprise, Chimki, other analyses by DASA–Munich/Giessen University (1 = nuclear reactor, 2 = radiation shield, 3 = partly deployable radiator, 4 = deployment system, 5 = Xe–propellant tank, 6 = gimballed EP–thrusters, 7 = spacecraft–bus, 8 = bipropellant sample return vehicle, 9 = instrumentation frame, 10 = narrow–beam aerial).