Philip W. Garrison
Electric propulsion (EP) is an attractive option for unmanned orbital transfer vehicles (OTVs).Vehicles with solar electric propulsion (SEP) and nuclear electric propulsion (NEP) could be used routinely to transport cargo between nodes in Earth, lunar and Mars orbit. See figure 28 [Earth-to-Moon Trajectory for a spacecraft Using Electric Propulsion]. Electric propulsion systems are low-thrust, high-specific-impulse systems with fuel efficiencies 2 to 10 times the efficiencies of systems using chemical propellants. the payoff for this performance can be high, since a principal cost for a space transportation system is that of launching to low Earth orbit (LEO) the propellant required for operations between LEO and other nodes. See figure 29 [A Lunar Ferry Using Solar Electric Propulsion] and 30 [An Advanced Nuclear Electric Propulsion System] .
The performance of the EP orbital transfer vehicle is strongly influenced by the power-to-mass ratio of the nuclear or solar electric power system that supplies electricity to the propulsion system because the power plant must be carried along with the payload. The power requirement for cargo OTVs will be high 0-5 MWe ) for useful payloads and trip times. Advances in space power technology will reduce mass and make possible systems producing higher power. These systems, coupled with electric propulsion, will provide faster trips and permit the use of this technology for manned as well as unmanned transportation.
Candidate Systems
Electric propulsion systems of various types have been proposed for space missions. Such systems can produce much higher exhaust velocities than can conventional rockets and thus are more efficient. In a conventional rocket system, a fuel is oxidized in an exothermic reaction; the exhaust velocity is limited by the temperature of the reaction and the molecular weights of the exhaust gases. In an electric propulsion system, an electrical current is used to ionize the propellant and to accelerate the ions to a much higher velocity. In the simple case of an ion thruster, ions are generated, accelerated across a voltage potential, and emitted through a nozzle. Because of the high velocity of the ions, such a device has a very high specific impulse (a measure of engine performance or efficiency; see p. 90).
With existing power systems, electric propulsion devices can produce only low thrust. However, emerging high-power r systems will enable both ion engines that can produce higher thrust and other types of electric engines. Magnetoplasmadynamic (MPD) thrusters use power systems operating at 10-20 kV and at 12000 amperes. The large current creates a magnetic field that can accelerate ions to 15-80 km/sec. An alternative system, called an arc jet, uses a high voltage arc, drawn between electrodes, to heat the propellant (hydrogen) to a high temperature.
The principal focus of the U.S. electric propulsion technology program ha been the J-series 30-cm mercury ion thruster. This technology is reasonably mature but not yet flight qualified. Mercury may not be an acceptable propellant for heavy OTV traffic operating from Earth orbit. Ion thrusters are currently being developed for argon and xenon (see fig. 31). specific impulses between 2000 and 10,000 seconds are possible, but a value less than 3000 seconds is typically optimum for these missions.
Magnetoplasmadynamic thruster technology is also being developed in the United States and elsewhere, but it is significantly less mature that mercury ion or arc jet technology. MPD thruster (see fig.32 and fig.32 concluded [Magnetoplasmadynamic Thruster]) can operate with a wide range of propellants providing specific impulses of approximately 2000 sec. using argon and up to 10,000 sec using hydrogen. MPD thrusters operate in both pulsed and steady-state models. A steady-state MPD thruster is a high-power device (approximately 1 MWe ) and is an attractive option for EP OTV applications.
Extensive work was done on arc jet and resistojet technology in the 1960s, but this technology has received little attention in recent years. The arc jet (see fig. 33 [Arc Jet]) is also a high-power device and provides a specific impulse between 900 and 2000 sec. The arc jet, like the MPD thruster, can operate with a wide variety of propellants.
Research conducted at the Jet Propulsion Laboratory since 1984 (see Aston 1986, and Garrison 1986) has demonstrated the successful operation of (1) a 30-cm ion thruster at 5 kW and 3600 seconds with xenon propellant, (2) a steadystate MPD thruster at 60 kW with argon propellant, and (3) an arc jet for 573 hours at 30 kW with ammonia propellant. NASA's Lewis Research Center has recently initiated programs to develop the technology for 50-cm, 30-kW xenon ion thrusters and low-power arc jets. The Air Force is funding research in MPD thrusters at Princeton University and MIT and in high-power arc jets at Rocket Research Corporation.
Technology Needs
Because of the difficulty of developing larger ion thrusters, large numbers of ion thrusters are required for a multimegawatt OTV. Steady-state MPD thrusters and arc jets are likely to be better suited to the cargo OTV application. Of the two, the arc jet is the more mature technology.
The funding for each of the above EP technologies is nearly subcritical because there is no established mission requirement for the technology. Increased funding will be necessary to make this technology available for the scenarios under consideration.
Impact of Scenarios Utilizing Nonterrestrial Materials
Nonterrestrial material utilization has two potential impacts on EP technology needs. If a demand for large quantities of lunar materials is established, electric propulsion is a highly competitive option for transporting both the bulk materials needed to construct the bases and factories for such an operation and the raw materials and products output by it. Electrically propelled OTVs, such as the lunar ferry described in figure 29, can beneficially supplant chemically propelled vehicles when cargo traffic to and from the Moon reaches some level, perhaps 100 metric tons (100,000 kg) per year. The second impact concerns the ability of the transportation system to rely on nonterrestrial resources for resupply of consumables. All other aspects being equal, a system that can be resupplied from local resources is clearly preferred.
However, the most readily available lunar propellant, oxygen, is not well suited to EP operations. Significant technology advances are required to operate any of the EP devices with oxygen, the principal technology barriers being the development of techniques to prevent the rapid oxidation of high-temperature thruster components. On the other hand, if hydrogen could be obtained from lunar (or asteroidal) sources, it would significantly enhance the performance of the EP OTV as well as benefit the oxygen-hydrogen chemical propulsion vehicles needed for high-thrust surface-to-orbit operations.
Aston, Graeme. 1986. Advanced Electric Propulsion for Interplanetary Missions. Paper AAS-86-259, 33rd Annual
Meeting of American Astronautical Society, Boulder, Oct. 26-29.
Garrison, Philip W. 1986. Advanced Propulsion Activities in the USA. Paper IAF86-170, 37th Congress Int.
Astronaut. Fed., Innsbruck, Austria, Oct. 4-11.
