Electron bombardment heating for electrothermal propulsion

Abstract

The technique of high-voltage, low-current electron bombardment heating for the heattransfer thrustor is investigated. The electron bombardment approach is shown to have inherent advantages for space application and to lack several restrictions found with resistance heating methods. A tantalum, electron bombardment- type gas heater was built to research thruster operation. Steady-state gas temperatures to 2700°K were produced with argon at a voltage-to-current ratio greater than 50. The device functioned successfully for a total operating time of approximately 50 hr. It was found that the electrical characteristics of the heater were adequately predicted by the Langmuir-Child law for space-charge limited emission. Possible causes of some electrical instabilities experienced during the tests are discussed. I. Introduction E LECTROTHERMAL propulsion engines produce thrust by the thermodynamic expansion of electrically heated gases through a supersonic nozzle. The heating may take place directly in the gas, as in an arc jet, or in an electrically heated material, over which the propellant is passed and heated by forced convection. These engines cover the lower specific impulse range of the electric propulsion family, with the arc jets having a possible useful range with hydrogen propellant to 2500 sec and the heat-transfer types up to 1000 sec with present materials technology. Figure 1 gives possible frozen flow specific impulse and efficiency values for hydrogen as a function of stagnation temperature and pressure.1.