As can be seen from Exhibit 17, the HLLV envisioned by various contractors varies somewhat. Of the documents discussing it. Ref. E9 describes it as a fully reusable "space freighter" with a 500,000 pound (225,000 kg) payload capacity; Ref. A9 describes it as a fully reusable vehicle with the same capacity, but does not detail it further. Ref. A4 describes it as a "Reusable-Runway Recovered Space Transporter" with a payload capacity of 77,000 pounds (35,000 kg). Ref. E7, a recent (February 1976) document on the solar-thermal concept, briefly describes the HLLV as a fully reusable "LEO Freighter" with a 500,000 pound (225,000 kilogram) payload capacity. All ground handling would be by waterborne towing and a new lake at the Kennedy Launch Complex would serve as a landing basin for this vehicle. 0 Ref. E7 further states that the vehicle would weigh about 22 x 10 6 6 pounds (10 x 10 kg) at lift off, with an empty weight of about 2 x 10 6 pounds (1 x 10 kg). It would use LH /LO as the primary propellant and RP^/LO2 propellant during the first 110 seconds for boost assist and also for deceleration for a "soft landing." The technology and materials for this vehicle are said to be similar to those developed on the Saturn program, but with the added features of greater safety, a water-cooled reusable heat shield, and fully sealed engine compartments. The primary flight mode would be a "once-around" orbit with the payload being deployed on the backside of the orbit. In the area of the number of HLLV flights required, estimates are strongly a factor of the implementation approach and thus vary among the contractors. One estimate for constructing an operational solar-photovoltaic SPS is about 80-100 flights per year. Also, it is estimated (Ref. F2) that 50 transport vehicles, each flying once every day, would be required to build 10 space-based plants per year. With respect to the second vehicle category, i.e., an orbital transfer stage, a number of concepts have been advanced. An ion propulsion system is considered most attractive, especially since it could derive its power from the SPS. Such systems would have lengthy orbit transfer times, e.g.,
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