ADVANCED PROPULSION CONCEPTS AT THE JET PROPULSION I~A~ORATORy

Current interest in advanced propulsion within NASA and research activities in advanced propulsion concepts at the Jet Propulsion Laboratory are reviewed. The concepts,which include high power plasma thrusters such as lithiun~-fueled Lorentz-Force-Accelerators,MEMS-scale propulsion systems, in-situ propellant utilization techniques, fusion propulsion systems and methods of using antimatter, offer the potential for either significantly enhancing space transportation capability as compared with that of traditional chemical propulsion, or enabling ambitious new missions. In addition to potentially addressing the propulsion needs of missions like those of the Human Exploration and Development of Space (HEDS) initiative, outer planet sample returns and orbiters, or an interstellar precursor mission, this research is aiding in fundamental scientific discoveries and developments in other technologies. I n t r o d u c t i o n ~ The Advanced Propulsion Technology (APT) group at the Jet Propulsion Laboratory was established in 1981 to identify and generate advanced propulsion concepts which offer theoretical performance significantly superior to that of state-of-the-art propulsion systems, and to evaluate the feasibility of these concepts through experiment and analysis. Support for this research has been provided by NASA independent of specific flight projects. In the last year, this activity has become part of the Propulsion Research program of the Advanced Space Transportation Program (ASTP) executed by the Marshall Space Flight Center. The Advanced Propulsion Concepts activity at JPL is sustained with a very modest investment by NASA of about one million dollars per year, yet can have an enormous impact on a variety of NASA programs, basic scientific research, and spin-off technologies. There are many examples of the impact advanced propulsion research has had on the aerospace community. Electric propulsion, considered an advanced technology when the Advanced Propulsion Concepts activity began, is now enabling missions of the New Millennium program. The first of these, Deep Space 1 (DS 1 ) [1], is scheduled for launch in July of 1998. The DS 1 spacecraft will flyby the near-Earth asteroid McAuliffe at just 5 to 10 kilometers above the surface, then pass by Mars for a gravity assist to enable an encounter with comet West-Kohoutek-Ikemura. Other examples of the impact advanced propulsion research has had include the fundamental understanding of plasma interactions with magnetic fields that has resulted from modeling work performed on fusion and electron-cyclotron-resonance thruster concepts. New insights into physical and chemical properties of fullerenes, which were considered as a possible ion engine propellant, have been gained. A previous research effort of the Advanced Propulsion Concepts activity at Brown University to design and build a magnetic levitation trap for liquid hydrogen was successfully completed. This work was a necessary precursor to the storage of antihydrogen, which may ultimately be achieved through present efforts in antimatter research. Research performed under this activity in high energy density materials (HEDM) for propulsion, specifically atomic hydrogen and rnetastable helium, led to the establishment of the Air Force Phillips Laboratory HEDM program. Research at the University of Arizona on the use of zirconia cells for C02 separation on Mars was precipitated by advanced propulsion research in in-situ propellant utilization. This technology is now one of the options available for both the 200 I and 2005 Mars flights. In what follows, present interest in and opportunities for advanced propulsion research within NASA are described. The research activities in advanced propulsion concepts presently underway at JPL are then presented. Advanced Propulsion Research Present Interest in Advanced Propulsion within NASA In July of 1997, NASA Administrator Daniel S. Goldin announced his interest in an interstellar precursor mission a mission to 10,000 AU in 50 years. Mr. Goldin has charged the Jet Propulsion Laboratory with investigating the possibilities for such a mission. Subsequent to his announcement, Mr. Goldin visited JPL on July 26, 1997 and was presented with some preliminary material outlining the technology needs and mission considerations for mounting the first interstellar precursor mission. Prominent among the identified needs was advanced propulsion. An interstellar precursor mission is the most recent and the most ambitious mission goal that could be enabled by the development of advanced propulsion systems. A high energy density concept, such as that which relies on antimatter initiated fusion reactions [2], would be required. Other mission goals, such as multi-body sample-~eturn missions, may be enabled by advanced solar sails with aerial densities between 1 and 5 g/m [3]. Such sails wou{d effectively be able to “turn off” solar gravity. Megawatt-level plasma thrusters could support piloted Mars missions as part of the NASA Human Exploration and Development of Space (HEDS) initiative. Possibilities for the use of electrodynamics tethers on the International Space Station for orbit raising maneuvers or even Earth return of small packages were recently presented at the Tether Technology Interchange Meeting in Huntsville, Alabama [4]. Pursuing the research that will result in the development of advanced propulsion systems is a great challenge at present. Regardless of stated objectives, NASA is not investing heavily in Advanced Propulsion Concepts. The entire Advanced Space Transportation Program with the exception of the Bantam Lifter activity, an effort to develop a small launch system capable of orbiting 100 kg payloads at a targeted cost of $1,000,000 per launch, remains unfunded in both 1998 and 1999. The Current Research Pro~ram The 1997 research activities at JPL are described below. Additional details can be found in Reference [5]. 1. In-Situ Propellant Utilization Reducing the initial mass in low-Earth-Orbit (IMLEO) of a spacecraft can enable a variety of robotic and piloted missions by dramatically reducing launch costs. One way to lower the IMLEO is to obtain some of the propellant required for the mission from extraterrestrial resources, many of which could provide oxygen. The principle barrier to the use of oxygen as propellant in plasma thrusters is the development of a cathode which can tolerate the oxygen environment. Field emitter cathodes are efficient, lowpower, and easily scaleable and have the potential to be functional in an oxygen environment. The successful demonstration of a cathode that operates on oxygen would enable In Situ Propellant Utilization (ISPU) for a variety of advanced propulsion concepts, including trans-lunar cargo propulsion systems.