2.2 CHEMICAL STAGE 2.2.1 PARAMETRIC DATA The high-thrust chemical COTV uses Og/H? propellants. Stage weight trends for these vehicles were based on the SWOP results referenced earlier (see figure VI-D-1-2) and on sizing data provided by Boeing for the FSTSA study. The engine size required is approximately that of the SSME; therefore, SSME derivative characteristics were used as shown in table VI-D-2-1. Staging options considered included a single-stage expendable vehicle, and one-and-two-stage reusable vehicles. Figure VI-D-2-1 illustrates the ratio of 'propellant mass to payload mass for these staging modes. The curves in figure VI-D-2-1 were generated for an orbit transfer aV consistent with launch of the HLLV from KSC. Figure VI-D-1-3 illustrates the effect of an equatorial launch versus the KSC launch for the single-stage expendable case. Figure VI-D-1-3 also illustrates the relatively dramatic reduction in aV for the high T/W orbit transfer. At a constant stage mass fraction of 0.93, the ratio of Wp/Wpl is 1.83 for the single-stage expendable, 2.16 for the two-stage reusable, and 2.86 for the single-stage reusable vehicle. Thus for a one million pound payload, the additional propellant required for return of the stage is 330,000 Ibm for the two-stage and 1.03 x 10° for the single-stage. The boost cost alone for this propellant is $6.6M for the two-stage or $20.6M for the single stage ($20/lb HLLV cost). To this must be added the propellant cost, the additional operations cost for stage retrieval and turnaround, and the additional costs for long-life reusable components. Other options are possible, including stage-and-a-half configuration in which most of the outbound propellant is carried in expendable drop tanks left at GSO; another option is to utilize ballistic entry and recovery for certain items such as the main engine and some avionics gear. These schemes for limited or partial reusability should be investigated to determine their economic viability. For the purpose of this study, an expendable single-stage vehicle will be used. 2.2.2 OPERATIONAL CONSIDERATIONS The basic consideration involved is whether the payload and its COTV are launched as a single HLLV payload or whether a LEO assembly is used to mate the payload and its propulsion unit following multiple HLLV launches. g For the former case, a total HLLV payload of 10° Ibm would result in a payload delivered to GSO of approximately 330,000 Ibm. Should it be required to transport an unitary payload exceeding that mass, multiple launches and LEO assembly would be required. The latter case, which admits to the requirement of LEO assembly of multiple HLLV payloads, seems to be the more conservative and flexible assumption.
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