ENGINE PROPOSALS

Three companies-Aerojet, Bell, and Reaction Motors-submitted proposals for the X-15 engine on 9 May 1955, the same day as the airframe competitors. North American had already asked the Air Force and NACA to dismiss the NA-5400 as an alternative. A copy of the Aerojet XLR73 proposal could not be located.

Bell was conservative in its engine proposal and stated that "modifications have been limited to those necessary to permit the engine to be used in a piloted aircraft." The changes to the XLR81 were made primarily in the starting and control systems, mostly to provide additional safety margins. The modified engine would be capable of multiple starts with a safety system based on a similar device provided for use during ground testing. The modifications provided an engine that could operate at an 8,000-lbf thrust level in addition to the normal 14,500-lbf full thrust. The modifications included the addition of a propellant bypass valve just in front of the injector so that, at the reduced thrust level, approximately one-half of the propellants would return to the tanks instead of being injected into the thrust chamber. This eliminated the need to change the pump discharge pressures, and allowed the same amount of propellants to flow through the cooling system. Only one engine in each airplane would have the capability to provide the 8,000- lbf level, although this reflected the removal and capping of the bypass valve and not any major change in engine configuration. Bell also proposed changing the fuel as a safety measure. In an attempt to minimize the risk of mixed propellants accumulating and exploding, Bell wanted to exchange the jet fuel normally used in the XLR81 with a mixture of 40% unsymmetrical dimethylhydrazine (UDMH) and 60% jet fuel (Bell called this combination "JP-X"). This would make the two propellants hypergolic, eliminating the hazard. Bell also pointed out that these propellants would not need to be topped off from the carrier aircraft, since neither had an appreciable vaporization rate. Bell noted that "since tests of the major components of the XLR81-BA-1 engine have been successful, extensive development tests of these components will not be required for the X-15 engine program."10

Like the Bell proposal, the proposal from Reaction Motors was brief (Bell used 15 pages, and Reaction Motors used just 14). The XLR30 would be modified to "1) emphasize safety and minimum development time, 2) start, operate and shutdown at all altitudes and attitudes, and 3) be capable of at least five successive starts without servicing or manual attention other than cockpit controls." Instead of the thrust-stepping proposed by Bell, Reaction Motors offered an infinitely variable thrust ranging from 13,500 to 50,000 lbf at sea level. Reaction Motors believed that "the highly developed state of the major engine components, i. e., turbopump, thrust chamber and control valves allows RMI to meet the schedule…." Unlike Bell, which extensively discussed the modifications required to make its engine meet the X-15 requirements, Reaction Motors instead gave a technical overview of the XLR30, and it was not possible to determine what the modifications were. Nevertheless, the overall impression was that the state of XLR30 development was far along.-1111