At hypersonic speeds, the turbojet engine gives way to a much simpler air-breathing engine: the ramjet. The high-speed compressors and turbines of the turbojet are forgotten in the ramjet because the highvelocity air scooped up by the engine intake reaches
 pressures sufficient for engine operation as it "rams'' into the combustion chamber. Langley Research Center, in association with the Garrett Corporation, designed a hypersonic ramjet research engine for inflight testing on the X-15 rocket plane at Mach 6. This engine, however, was strictly experimental and incorporated several untested features, such as the use of hydrogen fuel in a combustion chamber operating at about 5000° F. Rather than risk a pilot by mounting this veritable bomb on the X-15, NASA asked Lewis Research Center to test the engine on the ground first. In 1970 NASA-Lewis already had the key elements for a large blowdown hypersonic tunnel in place at its Plum Brook station on Lake Erie, 50 miles west of Cleveland. The relative remoteness of the Plum Brook facility made it an ideal place to test what was bound to be a very noisy engine. For heat transfer research, a large 5000-psi tank farm had been installed there plus an induction-heated, graphite pebble-bed heater capable of raising a test gas to 3 500 ° F. To solve the chronic problem of oxidation of the graphite heater at extreme operating temperatures, inert gaseous nitrogen was initially passed through the heater. The controlled addition of....
 oxygen to the nitrogen stream downstream of the heater but ahead of the test section provided a test medium that matched the constituents of atmospheric air in supporting combustion.
To convert the heat-transfer equipment into a hypersonic tunnel, Lewis personnel added three 42- inch water-cooled nozzles sized for Mach 5, 6, and 7 operation. A large steam ejector served to reduce tunnel pressures to those typical of high altitudes.
A full-scale "boilerplate" hypersonic engine was installed in the Plum Brook tunnel. Aerodynamically it conformed to the basic design, but there were no restrictions on structure weight for the ground tests. The engine did operate properly, burning hydrogen fuel successfully, but the thrust levels were lower than anticipated. Nevertheless, the tests were considered successful, auguring well for the eventual construction of a flight-model hypersonic ramjet and Mach 6 operation with air-breathing rather than rocket engines.