Part II : 1950-1957

Shift from Subsonic to Supersonic Aircraft

[133] When Rae received the final work statement of the engine contract, he was disappointed to find that it specified only a supersonic airplane and a shorter range than he had been urging.⁴⁷ Rae's interest was in very long-range, high-altitude aircraft, with speed a secondary consideration. His early proposals for essentially a lowpowered glider reflected this interest. His early engines used very small diameter turbines-on the order of 20 centimeters-and this was one reason he and Garrett had been attracted to each other, for small turbines were one of Garrett's specialties. During the long contract negotiations in the spring and summer of 1955, Rae still favored subsonic speeds, but the Air Force was more interested in a supersonic airplane at high altitude and would sacrifice range to get it. This was consistent with the Air Force desire for a superior airplane to supplant the subsonic U-2-superior both in altitude and in speed.

In the negotiations, Rae had gained funds for an airplane design study using the Rex engine. After receiving the work statement specifying a supersonic airplane, the Rex division prepared a "Problem Statement for Aircraft Studies" dated 7 November 1955 and negotiated a contract with Kelly Johnson at the Skunk Works. The problem statement specified Rex engines in pods for wing mounting. The size and weight of the pods with engines were given, as well as engine thrust and specific fuel consumption.⁴⁸ The pod diameter, essentially that of the engine inlet, was 122 centimeters, which meant an engine much larger than Rae's original concept; but the engineers of Garrett's Rex division did not feel that the larger sizes would be a major problem to develop.

During the course of the study, the engineers at the Skunk Works found that the thrust specified by Garrett was too low for their design needs. Agreements were reached with the Rex division on extrapolation of the engine data for engines of larger thrust and data giving specific fuel consumption as a function of Mach number. With these, the designers at the Skunk Works chose a cruise speed of Mach 2.25 and an engine with a thrust 50 percent higher than Garrett had originally specified. The larger thrust meant an engine with an inlet diameter of 150 centimeters. The engine now was considerably larger than machinery within Garrett's experience, but this did not deter the company. Some individuals within the Air Force, however, began to doubt whether Garrett was the best contractor for the engine.

The Lockheed study of a hydrogen-fueled airplane was completed and reported by the end of January 1956. Two configurations, both powered by Rex III engines of the larger thrust, were selected. The preferred design, designated CL-325-1, had a straight, thin wing and a slender fuselage. It was made of aluminum alloy with a single liquid hydrogen fuel tank in the fuselage. The second configuration, CL-325-2, used droppable auxiliary wing tanks, which reduced the airplane size and weight by about 15 percent. The CL-325 wing was lightly loaded, i.e., the wing had to support a [134] comparatively low weight per unit area. Its thrust per unit weight was also low, which required a long runway for takeoff. The characteristics of the CL-325, taken from the Lockheed report, are given by table 4. ⁴⁹

The Garrett engine contract, which began 15 October 1955, called for engine analysis and selection of the optimum one. The Rex division of Garrett, however, had been at work for some time before the contract and had already selected Rex III and had specified it for the Lockheed study beginning in November.⁵⁰

The first Garrett engine report, covering the first four months of the contract, was "Rex Engine Cycle and Selection," 15 February 1956, with the Lockheed report included as an appendix. The report concluded that the Rex III was the optimum engine and that Air Force mission requirements could be met. It also concluded that a Rex III with a thrust of 17 800 newtons at 30 500 meters altitude and Mach 2.25 was feasible; its specific fuel consumption would be less than 1360 kilograms per hour and dry mass less than 1995 kilograms. Other design data were given. The engine described had a thrust slightly greater than that selected by Johnson in the airplane design and reflected Garrett's confidence that they could develop the larger engine.

Garrett sent a team to Wright Field on 15 February to give an oral summary of the report. The members of the team were in high spirits, looking forward to a favorable reaction. They had been told that if the audience did not ask a lot of questions, it meant....

TABLE 4-Characteristics of CL-325-1 Hydrogen- Fueled Airplane

[135] ....trouble. Not many questions were asked; and that night, in the hotel room, the members of the team sat around trying to figure out what had gone wrong.⁵¹

What had gone wrong was that members of the audience, particularly those in the power plant laboratory, were beginning to realize that Garrett was talking about a whole new ball game. Gone was the simple, ingenious, new engine with its small turbines. In its place was a large, complex engine, with compressors and turbines about the size of those in contemporary turbojet engines. In addition, there were heat exchangers using hot combustion gases, something no one had yet attempted to develop. Rex III was considerably more complex than a turbojet engine and had other problems. Frank Patella, the laboratory's contract manager for the Rex engine study, noted in his log that Garrett apparently did not realize the problems involved in the proposed three-speed gearbox or the heat exchangers.⁵² The Wright Field experts may also have been troubled by Rae's assurance that liquid hydrogen production facilities would cover the development needs for the larger engine.