The mass fraction penalties for staging should be small for the propellant quantities involved; using a constant mass fraction, for the KSC launch going to a two-stage vehicle would reduce the propellant requirement from 3.25 to 2.96 pounds per pound of payload, an eleven percent reduction. If it were deemed desirable to return the first stage to low earth orbit for reuse, the aV for the first stage becomes approximately 6200 fps and the ratio of propellant to payload becomes 3.06 for the KSC launch case. For the equatorial launch case, return of the first stage degrades the two-stage propellant payload ratio from * 1.90 to 1.97 pounds per pound. The considerations discussed above show a strong incentive for the utilization of an equatorial launch site for a LO2/LH0 COTV; the 4000 fps reduction in AV resulting from the elimination or a 28.5 plane change gives a propellant savings of some 40 percent for a single stage vehicle; from that point, a two-stage system would result in an additional 6 percent reduction. 1.2.2 OPERATIONAL CONSIDERATIONS The various system weight characteristics, engine performance values, and staging options discussed above result in a range of the ratio of propellant/payload mass of from 1.9 to 3.3 pounds per pound. For the representative payload mass of 42 million pounds, this gives a LEO propellant requirement of from 80 to 140 million pounds for a LO2/LH2 COTV. Since this mass will obviously require multiple launches to emplace in LEO, the propellants will have to be stored in LEO for some time prior to their use. 6 Assuming an HLLV payload capacity of 10° Ibm, and making allowances for tankage, fairings, insulation, and other structure, to transfer the required propellant load to LEO will require some 90 to 160 HLLV launches, or 7 to 11 weeks at a two-a-day launch rate. Obviously propellant loading would only be initiated when everything was in readiness for the orbit transfer, but even so, a significant propellant storage problem will exist. Since the LH2 tank presents the most severe storage problem, the loading sequence would involve putting up the LO2 tanks first. Although this mode is definitely preferable to the inverse procedure of launching the LH2 first, it does impose a long duration LEO storage requirement on the LO2. Two basic approaches seem to be applicable to this problem; one would be to use a very low heat leak propellant tank to minimize the boiloff experienced during the LEO "hold" time while propellants are being accumulated; the other approach would launch a relatively high heat leak tank (optimized for the transit time from LEO to GSO) as well as a liquification plant to re-liquify the boiloff gases and return them to the propellant tank. It was found that a middle of the road approach was most favorable; a tank with sufficient thermal performance to limit the boiloff losses to a manageable level while not imposing an undue weight penalty on the stage appears feasible.
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