ing specific impulse (Isp) is 7,963 seconds and thrust of 56 Newtons. One hundred twenty such engines are utilized for the RI OTV, which has a dry mass of 759 tons, a propellant weight of 849 tons and transfers a payload of 6,860 tons from the low Earth orbit (487 km, 31.6 degrees inclination) to geostationary orbit with an outbound trip time of 120 days. Current RI estimates are that a fleet of 21 such vehicles will be required to deliver SPS elements to geostationary orbit. The recurring cost of OTV is largely due to the high unit cost of the vehicle with its large solar array and multitudinous electric propulsion rocket engines. The next presentation was by BARDI, Inc. (a subsidiary of Brown and Root, Inc.) reporting upon the design and cost analysis performed for NASA of a sea platform HLLV launch and recovery facility. This facility, termed the “offshore space complex" (OSC) provided a 300 ft wide by 15,000 ft long runway, platforms and industrial facilities, living quarters and three remote launch facilities with the necessary propellant storage and servicing provisions. Since hurricanes do not occur within 7 degrees of the equator, the design wave for this facility need only be in the vicinity of 10 to 12 ft. Design waves for offshore platforms for North Sea is 80 to 90 ft. The sea mount selected for the BARDI analysis is located approximately 90 degrees west longitude and 3 degrees north latitude and is suitable for pile mounting of these structures in 600 ft deep water. The costs for providing the runway and “real estate" necessary to support SPS flight operations was estimated to range from 3.3 to 3.9 billion dollars not including buildings, technical facilities and equipment. BARDI stated that this facility is technically feasible and can be delivered within six years from go-ahead. The long runway is the most significant determinant for OSC cost and can be reduced by improved definition of the aerodynamics and arresting systems of the launch vehicle recovery. Current design practice for the life of such structures is 30 to 40 years without continuous maintenance. BARDI indicated that the life of this facility could be extended to well beyond 40 years by the application of improved corrosion control practices. In answer to a question, BARDI indicated they do not have an authoritative cost estimate for the preparation for a land site to an equivalent state of readiness for construction as is provided on the sea mount by the OSC. It is clear, however, that the marginal costs of operating the HLLV from a sea platform built near the equator is significantly less than 4 billion dollars more than from a remote land site. Boeing expressed the opinion that the offshore facility costs may actually be no more than preparing a remote equatorial land site. This comment resulted in a strong response by ESA. It is the ESA opinion that the present Kourou facilities should be the launch site of choice for an SPS program. The OSC concept offers an interesting alternative to the use of Kourou but, in my opinion, it is premature to select between these options. The next presentation was given by the NASA Jet Propulsion Laboratory (JPL). The subject of the presentation was the magneto-plasma-dynamic arcjet thruster. The MPD engine, the application studies of this engine and the work in progress on this system at JPL and Princeton University was described. The MPD engine is inherently a much higher thrust per unit device than the more highly developed ion bombardment engine. Its primary virtue to SPS is, therefore, the potential to significantly reduce the investment in each of the electric orbit transfer vehicles required in the fleet. This vehicle investment cost is the largest contributor to the orbit transfer costs of the “Reference System" SPS. The MPD engine, therefore, constitutes a significant opportunity for reducing this portion of the SPS transportation costs. In the JPL paper, a second concept for applying electric propulsion to the orbit transfer problem. JPL proposed using microwave beamed power from three power
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