Hypersonic Research Engine

With little doubt, the most ambitious47 of the

X-15 experiments was the Hypersonic Research Engine (HRE) from the Langley Research Center. At the time that researchers began to consider supersonic-combustion ramjet engines during 1954, the X-15 was not an approved program and played no major role in the engine’s conceptual development. However, events soon transpired that made flight testing of a supersonic ramjet engine desirable, and the Flight Research Center and Langley proposed a joint project to accom­plish just that. The 1962 crash of the X-15-2 opened the door for extensive modification aimed primarily at providing a platform for development of the Mach 8 air-breathing HRE. Then, as now, no tunnel facility existed wherein such an engine could be realistically tested, and rocket boosters could not give steady-state tests or return the equipment.48

The actual prototype engine was to be carried attached to the lower ventral of the X-15A-2. Twenty-nine inches were added to the fuse­lage between the existing tanks for the liquid hydrogen to power the HRE, two external fuel tanks were added, and the entire aircraft was coated with an ablative-type insulator.

During 1965, Garrett-AirResearch was put under contract to provide six prototype engines by mid-1969. As would happen, the development effort necessary to produce a workable engine had been severely underesti­mated, and Garrett quickly ran into problems that caused serious delays in the project.

In the meantime flight-test evaluations were made of the modified aircraft itself and of a dummy HRE attached to the X-15A-2. On the first and only maximum-speed test of the X-15 A-2 in 1967, shock impingement off the dummy HRE caused severe heating damage to the lower empennage, and very nearly resulted in loss of the aircraft. Though quick­ly repaired, the X-15A-2 never flew again. Hindsight would place the blame for this design oversight on haste and insufficient flow interaction studies. A key lesson learned from this episode was not to hang external stores or pylons on hypersonic air-

craft, at least not without far more extensive study of underside flow patterns. As John Becker later observed, “Flight testing on the X-15A-2 would have been long-delayed, hazardous, very costly, and fortunately never came about.”4V

When the X-15 flight program was terminat­ed, the HRE degenerated into a costly wind tunnel program using partial-simulation test models. The HRE was eventually tunnel test­ed in 1969, and the primary objective of achieving supersonic combustion was met, although the thrust produced was less than the drag created. HRE engineers nonetheless claim a success in that the objective was supersonic combustion, not a workable engine. The program continued until 1975 and never achieved a positive net thrust, although it still contributed to the technology base, albeit at a very high cost. A hindsight study conducted in 1976 concluded that the HRE’s fuel-cooled structure was its main contribution to future scramjets.50