The line of reasoning followed for the chemical stage which led to the selection of a stage consisting of clustered propellant tanks, each of which comprises an HLLV payload, is also applicable to the electric stages. An additional consideration for the dependent electric COTV's is that the satellite must be configured and oriented such that sufficient power is available from the satellite to maintain the required thrust. This requirement could result in impacts to the SPS arrays, RCS, and structures that have not been assessed. These COTV options also have a problem with respect to occultation by the earth while in LEO. Preliminary estimates are that, until the orbit altitude reaches some 400 nmi, occultation will occur during each orbit. Although the AV budgets and trip time relations used were developed on the basis of no primary propulsion during occultation, some darkside thrust capability would seem desirable if not necessary. Without resorting to electrical energy storage, three options are available: (1) use of an independent power source up to the altitude where occultation is not a problem; (2) use of auxiliary chemical engines on the electric COTV for darkside thrusting; or (3) operation of the electric thrust systems in some type of tail off mode. As an example of the first option, figure VI-D-1-12 presents a plot of the total weight of a two-stage hybrid COTV (using a chemical first stage and an arcjet second stage) as a function of the first stage AV. At 7200 ft/sec, the total weight is found to be approximately 83 MLB, or some 27 MLB more than using a single-stage arcjet neglecting the occultation problem. Option (2) was favored by Boeing in their look at the SPS transportation problem in the FSTSA study. They estimated a required AV of 790 ft/sec, resulting in a propellant requirement of 3.24 MLB of 02/H2 propellants. The third option envisions using the inherent capability of the thruster itself in some type of tailoff mode. For the electrochemical system, a simple oversizing of the gaseous propellant accumulators would allow darkside thrusting with a moderate weight penalty and little or no performance degradation. For the arcjet thrusters, tailoff mode operation (with electrical power off) has been investigated. Figure VI-D-1-13 illustrates the type of performance that can be obtained; even after several thousand seconds, the thrust and Isp are considerably higher than cold-flow values. Assuming an effective Isp of 15 percent of nominal, the propellant penalty for operating in this mode would be on the order of 3.5 MLB. The resistojet probably does not have as good a tailoff performance characteristics as an arcjet; the performance would essentially be at cold-flow levels. 1.3.3 SYSTEM COMPARISONS The electric COTV concepts are compared in table VI-D-1-6. The thrust level was determined by arbitrarily setting the initial T/W to IO-4 g, except for the electrolysis stage, where the initial T/W was reduced to 5 x 10-5 g to avoid excessive weights of the
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