developed as a part of the “SPS Demonstration Program" utilizing the funds presently in the NASA demonstration phase cost estimates. At this point in the session, a vigorous discussion ensued as to the apparent contradictory results arrived at by ESA (very large HHLV) and Boeing (much smaller HHLV) as to who was correct and who was wrong. The realization finally came that both answers were probably correct, in that a 30-year SPS operational program justifies and demands more than one generation of space transportation systems. The smaller vehicle is appropriate to hold down investment costs and conduct the early part of the SPS placement and a larger vehicle, developed from revenue generated by early SPSs, may employ more advanced technology and dramatically reduce the recurring transportation costs of the later SPS transportation operations. As to the issue of the recovery mode for this vehicle, the presence of wings and the high cost landing facilities in the present “Reference System” is largely due to uncertainties as to the costs of refurbishing a vehicle landed in the sea. ESA currently has an approved program to recovery the first stage of the Ariane launch vehicle beginning in the summer of 1983. The Ariane program manager indicated at the 1979 IAF Congress in Munich last fall that this first stage recovery was expected to reduce Ariane launch costs by at least 15%, even though the booster refurbishment operations will be conducted in Paris. My conclusion from this discussion is that the ESA choice of recovery modes may be correct, but that there will not be sufficient confidence in ballistic recovery to commit to this potentially lower cost approach until the mid-1980s. This date is obviously well in advance of the decision date for the investment phase of the SPS program and, therefore, should remain in consideration until more knowledge is gained. The next paper was presented for NASA by Rockwell International (RI). The topic was “The Orbit-To-Orbit Transfer of SPS Space Transportation.” The RI studies resulted in the selection of electric propulsion for transfer of the massive elements of the power satellite from the low Earth orbit delivery point of the HLLV to the operational geostationary orbit. Their studies have indicated that the selection of low orbit construction with “self-powered” transfer as opposed to a dedicated electric propulsion transfer vehicle is not a strong driver on transportation costs. The choice of construction methods and type of photovoltaic cells will probably determine the selection of orbit transfer mode. Since the RI “Reference System” SPS uses gallium arsenide solar cells which operate at 125°C and are continuously selfannealing, they prefer a dedicated cargo orbit transfer vehicle (COTV) for their SPS. If, however, silicon photovoltaic cells are the choice for the SPS, use of those cells in an OTV will either require accepting the severe power loss consequent to radiation damage in passage through the trapped radiation belts or annealing the array at 500°C to restore output. Choice of silicon cells for the SPS may drive the system to perform construction in low Earth orbit and employ the self-powered mode of orbit transfer. The Rockwell studies indicated that electrical propulsion as a class is preferable to chemical orbit transfer for the reason that the large mass of propellant required for chemical orbit transfer more than doubles the mass which must be placed in low orbit to achieve the program. The Rockwell studies have considered both ion and magneto-plasma-dynamic (MPD) arcjet engines and have considered argon and other gases as the working fluid. Their selection is an ion engine utilizing argon supplied with beam voltage at 2,000 volts. The engine beam density was determined by RI to be 1,000 amp/m2 in consultation with the NASA-Lewis Research Center. The result-
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