SPS Feasability Study SD76SA0239-2

4.2.1 Earth-LEO Logistics Scenario In the operational sequence described in Section 3, the construction and activation of four SPS's per year are contemplated. Thus, on the average, an SPS is activated about every 90 days. Because of contingencies, construction gaps, launch site weather constaints, etc., it was assumed that logistics flights would occur on only 75 of the 90 days. Based on the reference SPS weight of 27 metric tons (60 x 10$ lb), approximately 363,000 kg (800,000 lb) must be delivered to GEO each flight day. Previous studies of logistics that used a 28.5-degree inclined LEO orbit resulted in orbital burden factors of about 2.5 for recoverable common stage designs employing the sling-shot mode to GEO. Therefore, the total daily earth launch weight required to support the SPS construction/activation scenario would be 12.7 metric tons (2.8 x 10$ lb). This logistics requirement would require 14 flights daily of the ELV (91,000 kg/200,000 lb to LEO). Either ''salvo" launches or major on-orbit phasing maneuvers would be required to accomplish LEO rendezvous and docking of the OTV stages and the GEO payload. A similar phasing penalty would occur in the retrieval/return of the OTV stages. By ''flying" the ELV to an equatorial flight path, a compatible launch window without phasing would occur every 1.5 to 2 hours (depending on LEO altitude). A significant reduction in the phasing AV of the returning OTV stages also would result, and the opportunities for direct rendezvous for returning the OTV stage with an ELV becomes practical. Although optimization studies of earth-LEO logistics have not been conducted, it appears that the reference ELV can significantly reduce operations complexity. Also, on-orbit AV requirements can be significantly reduced by injecting into an equatorial LEO without severely penalizing the payload launch weight. Subsequent studies of the ELV concept should include analysis to determine the optimum LEO injection inclination as a function of launch payload capability, operations flexibility, and on-orbit AV requirements. 4.2.2 Orbital Transfer Vehicle OTV used as a reference in this study is a scaled version of the common stage LO2/LH2 OTV defined under Contract NAS9-14323 by BAC. The OTV is illustrated in Figure 4.2-1. A mass fraction of 0.906 and I$p of 470 was assumed for each stage. The gross mass of each stage is about 91,000 kg and equals the earth launch payload capability of the reference ELV. 4.2.3 LEO-GEO Logistics Scenario To minimize the total transportation costs associated with the construction and activation of an SPS, it is imperative that the full capability (payload weight) of the ELV be utilized each flight. One approach to maximum utilization of the ELV's capability would be to deliver 91,000 kg to LEO on each flight, assemble the OTV stages and whatever GEO payload the OTV could deliver from that altitude/inclination, and store the remainder of the launch vehicle GEO payload for delivery on a subsequent flight. This approach was deleted from further consideration because it (1) significantly complicated the GSO payload packaging, (2) introduced additional on-orbit operations, and (3) imposed a requirement for an additional program element—an on-orbit LEO staging base.