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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program Appendix C Electric Propulsion Considerations Electric propulsion can benefit the deployment of large payloads for orbit transfer. The mass and volume saved by using an electric propulsion system allows for the use of smaller launch vehicles or allows more satellites to be placed on a larger launch vehicle. Alternatively, more station keeping fuel can be carried for a single spacecraft, which would extend the on-station lifetime of the spacecraft. Ultimately, the benefit of electric propulsion, or any propulsion system, relies on the impact of the propulsion system on total mission cost. For some space missions requiring high power, the power system cost and mass can be partially offset by using electrical propulsion for orbit transfer and station keeping. Electric propulsion typically uses its fuel 4 to 10 times more efficiently than chemical propulsion. This efficiency results in a significant reduction in the mass of fuel required to complete certain space maneuvers. However, using electric propulsion systems requires that a spacecraft take more time to be placed into a final orbit. The increased amount of time it takes to reach orbit introduces other issues such as increased exposure to radiation while the spacecraft is in the Van Allen belt. The wide variety of electrical propulsion applications complicates the generalization of the benefits. Thus, for the sake of discussion, this appendix uses an example of how coupling power and electric propulsion significantly reduces mass and cost. Combining mission power requirements with electric propulsion for orbit raising or station keeping maneuvers creates a dual mode system, that is, a system that can satisfy more than one mode of operation. Of the various power systems that can provide dual mode operation, thermionic electric propulsion systems are unusual in that they can be designed to operate in a surge mode where the emitter temperature is increased from 1800 K to 2100 K. This temperature increase doubles the power output. This surge mode would be used during the propulsion portion of the mission, which raises an orbit. The surge in propulsion could be active for a relatively short time, from 30 to 90 days. The surge mode operation would result in a minor decrease in total expected life of a mission base-lined to last 7 years. The main advantage of surge mode operation is that it can be used to decrease the time required for orbit positioning or orbit transfer. However, primary orbit transfer using electric propulsion is still in the planning stage. APPLICATIONS Defense satellites must often be able to deal with contingencies such as changing inclination to observe a particular region on a timely basis, moving to a lower orbit to gain a better view of an area, moving to a higher orbit to avoid offensive damage, or maneuvering evasively to frustrate offensive measures. Electric propulsion would be one way to accomplish tasks such as these. However, electric propulsion is not appropriate for all DoD space missions. In a launch on demand situation where there is an urgent need to replace or deploy space assets, chemical propulsion would be the likely candidate for orbit transfer. For standard launches where the time it takes for a spacecraft to arrive on
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program orbit is important or for on-station maneuvering, the cost per kilogram to place a spacecraft on orbit is likely to be a key parameter. In this case, the cost to place a spacecraft on orbit includes such items as total propulsion cost, booster system requirements, command and control costs during orbit raising, and contingency for spacecraft loss because of propulsion failures. The trend to high power for several classes of satellites is causing electric propulsion to be considered. Commercial satellites are typically designed and programmed to perform for very specific lifetimes. The principal electric propulsion application for commercial satellites is station keeping using only the available power used for the main mission power (Sackheim and Byers 1998). An Example: Cost Savings Achieved by Dual Mode Operation Of the several electric propulsion systems competing for high power and orbit transfer applications, two offer high efficiency and long life at attractive specific impulses: gridded ion engines and Hall effect thrusters.1 Both devices accelerate noble gases, such as xenon and argon, to velocities in the 10 to 40 kilometers per second range. Xenon is safe, dense, and easily stored at ambient conditions. The Hall thruster is used here to illustrate electric propulsion payoff. A 25 kilowatt Hall thruster can be expected to operate as follows: Isp=15,680 meters per second (1,600 seconds),2 Efficiency=62 percent (58 percent after lead losses and power processing), Thrust: F ~ 1.6 Newtons (0.36 pounds force), Xe flow=0.12 grams per second.3 The changes in velocity for station keeping are less demanding than changes in velocity encountered during orbit transfer. Station keeping changes in velocity can be accomplished by a variety of mature electric propulsion systems, including: Arcjets, Electrothermal monopropellant systems, and Pulsed plasma thrusters. Although arcjets are not as efficient as Hall thrusters, they have the cost advantage of using hydrazine fuel, which is already required to be onboard the spacecraft for other propulsion (Sackheim and Byers 1998). To achieve a mass and cost comparison, a 100 kilowatt electric propulsion system made up of four 25 kilowatt Hall thrusters is fueled to match the total impulse of the Thiokol Star 75 motor, a state-of-the-art solid propellant space motor.4 To a first order approximation, the propulsion mass saved by the electric propulsion system will be considered as revenue producing payload. The Star 75 is 1.9 meters in diameter and contains 7,518 kilograms of propellant. The rocket provides approximately 200 kilo Newtons (45,000 pounds force) of thrust over 105 seconds. The cost is approximately $3.5 million. The equivalent electric propulsion system using four 25 kilowatt Hall thrusters and powered by a 100 kilowatt electric system would thrust at a combined total of 6.4 Newtons for about 33 days. Economies of Scale To place a kilogram of payload into low Earth orbit (LEO) costs between $6,000 and $10,000. The cost to reach geosynchronous Earth orbit (GEO) is at least $20,000 per kilogram and may go as high as $40,000, depending on the mission. When a chemical propulsion system is used, 60 to 70 percent of the mass that reaches LEO is the propulsion system needed to get the payload to GEO. Most of the mass consists of the propulsion system propellant. Using electrical propulsion, the ratio of propulsion mass to payload mass can be reversed. There are additional benefits if the power used for electric propulsion during orbit raising is also available and required for the main mission, thus creating a dual mode system. However, the lower thrust of the electric propulsion systems increases the orbit transfer time from hours to weeks. A LEO to GEO (1,500 to 36,000 kilometer) transfer with a 29 degree plane requires a satellite velocity increase of 3,500 meters per second using chemical propulsion and 4,050 meters per second using electric propulsion. The greater change in velocity required for electric propul- 2 A lower Isp is selected to shorten trip time. 3 Xe is xenon, the propellant generally used for gridded ion engines and Hall effect electric propulsion. 4 Total impulse is the integral of thrust over the thrusting time. 1 The rocket engine figure of merit is specific impulse (Isp), which in SI units is the velocity of the propellant exiting the nozzle. Meters per second is equivalent to thrust per rate of mass discharge or newtons per kilogram-second.
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program sion is a result of the persistent gravity from the longer time spent in LEO. For the purposes of this example, the costs of propulsion and power were estimated using assumptions for large constellations of communications satellites, for example Teledesic, where economies of scale come into play. Also, for missions in LEO, the losses in altitude due to gravity will reduce these benefits slightly. A satellite using an electric propulsion system takes longer to reach GEO so the effect of gravity acts on the spacecraft for a longer period of time. The costs per launch are as follows: Star 75 solid rocket motor: $3.5 million, Electric propulsion system+100 kilowatt space power system: approximately $9 million+$33 million =$42 million.5 Using the figure stated earlier, $20,000 per kilogram to reach a GEO orbit, the savings are as follows: Case 1. When the 100 kilowatt power source is used for electric propulsion only, the approximately 3,000 kilogram savings in mass yields approximately $60 million in additional payload for an approximate $21 million net savings. Case 2. When the 100 kilowatt system is used for both propulsion and primary mission power once on station, the approximately 6,000 kilogram savings in mass yields approximately $120 million worth of additional payload for an approximate $114 million net savings. The savings for the dual mode operation are substantial. Such savings are an integral part of the advocacy for more economical and higher space power. SPACE APPLICATIONS Even before the 1960s space race, the advantages of electric propulsion for deep space probes were recognized. However, neither power nor electric propulsion adequate for prescribed missions was available. Nuclear power, which is a candidate for certain missions that travel beyond Earth orbit, would enable elec TABLE C.1 Performance of Chemical and Electrical Propulsion Systems Typical Space Engine Specific Impulse (km/s) Chemical Solid propellant for spacecraft maneuvering 2.8 Storable liquid (N2O4 and MMHa) 3.3 Cryogenic oxygen and hydrogen 4.3 Electric Gridded ion engine 20 to 40 Hall thruster 10 to 25 aMonomethylhydrazine. tric propulsion systems to be used. During the 1970s and 1980s, NASA made considerable progress on several electric propulsion systems centered on the use of gridded ion engines and magnetoplasmadynamic (MPD) thrusters. In both the United States and Great Britain, research and development also concentrated on producing flight qualified ion engine systems in the thrust range of less than 5 kilowatts by the mid 1990s. The MPD engines would be suitable for large power systems producing more than half a megawatt. However, these systems are currently not developed to a point where they could be used. Gridded ion engines could potentially be useful for deep-space probes. The power requirements for these missions are generally low, in the hundreds of watts. If high power is not required for the mission, placing a high power system onboard the spacecraft for electric propulsion is usually not justified. The gridded ion engine offers potential advantages. These engines can operate at the lower levels of power that might be used on certain deep space missions. However, such missions would take several months to build the velocity needed to arrive at the far reaches of the solar system in a reasonable number of years. Gridded ion engines also provide higher efficiency by operating at a higher specific impulse, in the range of 40 to 60 kilometers per second.6 5 If the same 100 kilowatt system is onboard acting as a primary power system, some of these costs will be offset by the dual mode performance of such a system. 6 Thrust is fuel flow rate times exit velocity of the fuel. For constant power, reducing the flow rate permits the fuel to be accelerated to higher velocity, or higher specific impulse.
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Thermionics Quo Vadis?: An Assessment of the DTRA’s Advanced Thermionics Research and Development Program BIBLIOGRAPHY Caveny, L.H., ed. 1984. “Orbit-Raising and Maneuvering Propulsion: Research Status and Needs,” AIAA Progress Series in Astronautics and Aeronautics, Vol. 89, January. Sackheim, R.L., and D.C.Byers, TRW Space and Electronics Group. 1998. AIAA Journal of Propulsion and Power 14(5):669.
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