ENGINE OPTIONS

The engine situation was somewhat more complicated. Given that everybody now agreed that the General Electric A1 (Hermes) engine was unacceptable, the Power Plant Laboratory listed the Aerojet XLR73, Bell XLR81, North American NA-5400, and Reaction Motors XLR10 as engines the airframe contractors could use. The four engines were a diverse collection.-1321

The Aerojet XLR73-AJ-1 had a single thrust chamber that used white fuming nitric acid and jet fuel as propellants. As it then existed, the engine developed 10,000 lbf at sea level, but a new nozzle was available that raised that to 11,750 lbf. The engine was restartable in flight by electric ignition and was infinitely variable between 50% and 100% thrust. A cluster of several engines was necessary to provide the thrust needed for the new research airplane. At the time the Power Plant Laboratory recommended the engine, it had passed its preliminary flight rating qualification, with a first flight scheduled for April 1956.[33]

The development of the Bell XLR81-BA-1, usually called the Hustler engine, was part of Project MX-1964—the Convair B-58 Hustler. The B-58 was a supersonic bomber that carried its nuclear weapon in a large external pod, and the XLR81 was supposed to provide the pod with extra range after it was released from the bomber. The engine was a new design based on the engine used in the GAM-63 RASCAL missile. A single thrust chamber used red, fuming nitric acid and jet fuel to produce 11,500 lbf at sea level and 15,000 lbf at 70,000 feet. Sufficient thrust for the hypersonic research airplane would come from a cluster of at least three engines. The existing XLR81 was not throttleable or restartable in flight. Since ignition occurred after the B-58 dropped the weapons pod, the engine included a minimum number of safety components to save weight. At the time the Power Plant Laboratory recommended the engine, it had passed its preliminary flight rating qualification, with a first flight scheduled for January 1957.[34]

Although the Power Plant Laboratory included the engine on its list of candidates, and history papers often mention it, the NA-5400 apparently had little to offer the program. North American was using the effort as the basis for component development, with no plans to assemble a complete engine. If they had, it would only have developed 5,400 lbf at sea level (hence its company designation). The turbopump assembly was theoretically capable of supporting engines up to 15,000 lbf, and the power plant proposed for the new research airplane consisted of three separate engines arranged as a unit. The engine was restartable in flight using a catalyst ignition system. The propellants were hydrogen peroxide and jet fuel, with the turbopump driven by decomposed hydrogen peroxide.-1351

The Reaction Motors XLR10 Viking engine presented some interesting options, although Reaction Motors had already abandoned further development in favor of the more powerful XLR30 "Super Viking" derivative. As it existed, the XLR10 produced 20,000 lbf at sea level using liquid oxygen and alcohol propellants. The XLR30 then under development produced 50,000 lbf using liquid oxygen and anhydrous ammonia. The Power Plant Laboratory preferred to connect two XLR10 thrust chambers to a single XLR30 turbopump, believing this arrangement took better advantage of well-developed components and lowered the risk. The fact that the XLR10/XLR30 discussion used over two pages of the four-and-a-half-page engine report showed the laboratory’s enthusiasm. Interestingly, as designed, the engine was not throttleable or restartable in flight, nor was it man-rated.1361

In response to one contractor’s comment that three of the four engines appeared unsuitable because they lacked a throttling capability, the government indicated it would undertake any necessary modifications to the engine selected by the winning airframe contractor.-1371

Between the time of the airframe bidders’ conference and the 9 May submission deadline, Boeing, Chance-Vought, Grumman, and McDonnell notified the Air Force that they did not intend to submit formal proposals. This left Bell, Convair, Douglas, North American, Northrop, and Republic. It would seem that Bell and Douglas would have the best chances, given their history of developing X-planes. The Navy D-558-3 study would also appear to provide a large advantage to Douglas. On the other hand, although Convair, North American, and Republic had no particular experience in developing X-planes, they were in the process of either studying or developing high-speed combat aircraft or missiles. Northrop had little applicable experience of any sort, but had a long history of producing innovative designs.

During this period, representatives from the airframe contractors met with NACA personnel on numerous occasions and reviewed technical information on various aspects of the forthcoming research airplane. The NACA also provided data from tests in the Ames 10-by-14-inch and Langley 11-inch tunnels. Coordination on the NACA side became easier when Arthur W. Vogeley, an aeronautical research scientist from the Flight Research Division at Langley, became the NACA project engineer on 10 January 1955. Vogeley would act as a single point of contact for the NACA, with offices at both Langley and Wright Field.1381

On 17 January 1955, NACA representatives met with Wright Field personnel and were informed that the research airplane was identified as Air Force Project 1226, System 447L, and would be officially designated the X-15.1391 The Fighter Aircraft Division of the WADC managed the project since the requirements for the aircraft most closely resembled those for a contemporary jet fighter. In reality, except for some procurement and oversight functions, the division would have little to do because the X-15 Project Office and the Research Airplane Committee actually controlled most aspects of the project. The X-15 enjoyed a national priority of 1-B, with a category of A-1. The Air Force also announced that the WADC project engineer would be First Lieutenant (soon to be Captain) Chester E. McCollough, Jr. BuAer subsequently selected George A. Spangenberg1401 as the Navy project engineer.1411

Early in March the NACA issued a research authorization (A73L179) that would cover the agency’s work on Project 1226 during the design competition and evaluation. The contractors concentrated on preparing their proposals and frequently consulted with both the NACA and WADC. For instance, on 15 April John I. Cangelosi from Republic called John Becker to obtain information on the average recovery factors used for swept-wing heat transfer. Later that day Becker transmitted the answer to NACA Headquarters, which then forwarded it to each of the competing contractors on 26 April.1421

The Air Force and the NACA also were working on the procedures to evaluate the proposals.

During March the NACA Evaluation Group was created with Hartley Soule (research airplane project leader), Arthur Vogeley (executive secretary), John Becker (Langley), Harry J. Goett (Ames), John L. Sloop (Lewis), and Walt Williams (HSFS) as members.

In early February, ARDC Headquarters sent a letter to all parties emphasizing that the evaluation was a joint undertaking, and the ultimate selection needed to satisfy both the military and the NACA. The evaluation involved the X-15 Project Office, the WADC laboratories, and the NACA, while the Air Materiel Command and Navy played subordinate roles. The four evaluation areas were the capability of the contractor, the technical design, the airplane performance, and the cost.*43*

The Research Airplane Committee would begin evaluating the proposals when it met on 17 May at Wright Field. Slightly complicating matters, the Air Force raised the security classification on most X-15-related activities from Confidential to Secret. This restricted access to the evaluation material by some engineers and researchers, but mostly placed additional controls on the physical storage locations for the material.*441