orbit. Estimates of weights for the solar-photovoltaic power station g range from 23 million pounds (10.5 x 10 kg) (Ref. A4) , to 40 million g pounds (18 x 10 kg) (Ref. All). For solar-thermal approaches, the esti- 6 mates range from 285 million pounds (13.0 x 10 kg) to over 400 million 6 6 pounds (180 x 10 kg), and to over 600 million pounds (270 x 10 kg) for the nuclear-Brayton approach. Weight estimates for the Power Relay Satellite (PRS), while less, are still large. They range from 500,000 pounds (0.23 x 10 kg) (Ref. A4) to 925,000 pounds (0.42 x 10 kg) (Ref. All). In addition, maintaining the propellant supply for attitude control and stationkeeping for all approaches imposes transportation requirements. One estimate, for instance, of propellant needs for a 5 GWe photovoltaic SPS is approximately 3 30,000 pounds (14 x 10 kg) per year (Ref. A2), other estimates range up 3 to 100,000 pounds (45 x 10 kg). Besides the payload weight capacity needed to transport such massive amounts of material, the payload volume capacity is expected to be a critical parameter. Since much of the material is expected to be lightweight but bulky, this parameter can strongly influence the number of flights required. At this point, quantitative values for the volume parameters do not seem to have been generally derived, although Ref. E6 does estimate a payload density for SPS elements of 1.2 to 2.0 lbs per cubic 3 foot (20-30 kg/m ). Other HLLV requirements that will influence vehicle parameters and design constraints can be summarized as follows: • Launch loads and accelerations must not cause cargo damage. • The launch vehicle must be reusable to the extent required for cost-effectiveness. • There is no requirement for an appreciable return capacity. • The HLLV may be required for transport to either LEO or GEO, depending on the selected assembly approach.
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