Part II : 1950-1957
Air-Breathing Engines for High-Speed Flight
 After panel discussions of inlets, exits, and cooling, an engine panel headed by H. M. Henneberry analyzed four types of engines for the Mach 4 manned mission and two types of ramjet for the Mach 7 unmanned missile.* The Mach 4 engines were: turbojet, fuel-rich turbofan, hydrogen expansion, and air-turborocket. All four had common elements of air inlet, fan or compressor for increasing the pressure of the incoming air, afterburner where additional fuel was burned, and nozzle. The fan or compressor was driven by a gas turbine, but the turbine and its driving gas differed among the four engines. In the turbojet the driving gas was primarily air, in the fuel-rich turbofan and air turborocket the gas was hydrogen rich, and for the hydrogen-expansion engine the gas was all hydrogen.
Of the four engines, the hydrogen-expansion type is of particular interest because it was under development in a super-secret Air Force project to be discussed later. The hydrogen-expansion engine described by Henneberry had a complex flow system which will be described with the aid of figure 23. Air entering the engine was compressed by a two-stage fan driven, through suitable gearing, by a high-speed turbine. A small amount of air was directed to the primary combustor; the main stream flowed directly to the afterburner. The liquid hydrogen was raised to a high pressure by a pump and served as a coolant for various purposes (such as cooling hot vehicle surfaces) prior to entering the engine. The heat absorbed during these cooling functions converted the hydrogen to a gas. In the engine, the hydrogen flow was split, with one part flowing directly to the afterburner. The other part flowed through a heat exchanger where its temperature was increased substantially. The hot hydrogen was used to drive a 3-stage turbine which, in turn, powered the fan for compressing the air. After leaving the turbine the hydrogen entered the primary combustor where it burned hydrogen-rich with air. The hot, hydrogen-rich combustion gas entered the other side of the heat exchanger where it provided the heat for the separate flow of hydrogen gas....
....for the turbine previously mentioned. After leaving the heat exchanger, the hydrogenrich combustion gas flowed to the afterburner where it and the other part of the hydrogen flow burned to completion, after which the hot gases expanded through the nozzle to provide thrust. Henneberry and his panelists estimated the weight of the hydrogen-expansion engine to be 10 percent heavier than a turbojet, and to have many difficult development problems.**
The turborocket, being pushed by W. C. House of Aerojet-General Corporation, used a small rocket to provide the hot gases for driving a turbine, with the turbine driving the air compressor or fan. The rocket used either a monopropellant or bipropellants-the latter being fuel-rich with additional burning in the afterburner, as in the other engines. The panel described a turborocket using liquid hydrogen as the fuel. After being compressed, part of the incoming air was diverted through a heat exchanger, chilled on its other side by liquid hydrogen. The air was liquefied and pumped at a high pressure to the rocket chamber. The main air-stream flowed directly to the afterburner. Liquid hydrogen, after serving to liquefy the air in the heat exchanger, went directly to the rocket chamber where it mixed and burned fuel-rich with the air. The combustion products drove a turbine (which drove the air fan or compressor). After leaving the turbine, the hydrogen-rich gas flowed to the afterburner where it mixed and burned in the main air-stream. A potential problem in this type of engine was icing from moisture in the incoming air.
 After comparing the four types of engines, the Henneberry panel concluded that the turbojet was the simplest, would require the least development, and was adaptable to a wide variety of fuels.
Of the two ramjets considered by the Henneberry panel, one was described as conventional, the other fuel-rich. In the former, enough fuel was provided for complete combustion with the oxygen available, while the latter used an excess of fuel. The excess fuel helped to offset decrease in thrust at speeds above Mach 10. The panelists found that hydrogen was superior to diborane and methane for cooling and for performance at high supersonic speeds. They concluded that a fuel-rich ramjet using liquid hydrogen could extend the usefulness of air-breathing engines to speeds up to Mach 18.
** As will be
discussed later, Pratt & Whitney built a hydrogen-expander
engine and tested it for the first time two months before the NACA
conference. The engine development was part of a highly secret Air
Force project, and it is very doubtful that the Henneberry panel
was aware of it. Another, somewhat similar, type of cycle-the Rex
engine-was known to the Lewis laboratory and the Henneberry panel
may have drawn on this knowledge. Interview with A.V. Zimmerman,
Roger Luidens, and Richard Weber, NASA Lewis Research Center, 30