A resistojet thruster schematic is shown in figure VI-D-1-7. The resistojet is a simple, relatively low performance electrical thruster that has had much development work and has some flight experience. The factors limiting the performance of a resistojet are the propellant molecular weight and the propellant temperature as indicated from the relation Since the maximum temperature is restricted by material limits of the heating element, low molecular weight is important for high Isp. This leads to the use of hydrogen for a resistojet, in spite of its disadvantages of low liquid density and low boiling point. Predicted values for delivered Isp as a function of gas temperature for a hydrogen resistojet is shown in figure VI-D-1-8. For the thruster characteristics shown in table VI-D-1-4, an Isp of 1000 seconds is thought to be attainable within the life constraints of a two-month trip time. Although the performance of an arcjet thruster is governed by the same relation as given above for the resistojet, the heat source is an electrical arc rather than a metallic heating element (see schematic, figure VI-D-1-9). This allows much higher temperature operation and thus higher performance. A considerable amount of experience exists with arcjets as plasma sources for materials testing, and a good deal of effort has been put into arcjet thrusters for space applications. Arcjet thruster characteristics are shown in table VI-D-1-5 which result in predicted performance values of 1500 seconds for NH^ and 3000 seconds for h^. 1.3.2 PARAMETRIC DATA Figure VI-D-1-10 and VI-D-1-11 illustrate the trends of WP/WPL for the electric COTV options considered for KSC and equatorial launches. The effect of high Isp systems on reducing both the effect of stage mass fraction and the plane change penalty for a KSC launch is notable. At a aV of 19,000 ft/sec, the electrochemical staqe has a ratio of WP/WPL in the range of 3 to 4, approximately the same as a conventional chemical stage and considerably higher than the other electric options. For the resistojet, WP/WPL ranges from 1 to 1.8, for the NH^ arcjet, .6 to 1.0, and for the arcjet .23 to .4 (all values based on 19,000 ft/sec aV). At the performance levels of these stages (except for the electrochemical system) staging would probably not be an attractive option. For example, if a two-stage resistojet COTV could attain the same mass fraction as a single-stage system, the reduction in propellant required would be approximately 10 percent. 1.3.3 OPERATIONAL CONSIDERATIONS All of the COTV's considered here have a propellant requirement considerably in excess of the assumed capability of the HLLV; multiple HLLV flights will be required to emplace the COTV propellant in LEO.
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