Part II : 1950-1957
High-Level Air Force Interest in Rex
 The familiarity of top Air Force R&D officials with the Rex proposal and their desire for rapid contracting did not result solely from interest in a novel idea. The same month that Philip Richie learned of Air Force interest at ARDC Headquarters, the Fuels and Propulsion Panel of the USAF Scientific Advisory Board met at the RAND Corporation and considered superfuels.* The panel was impressed by the performance  potentials of two promising fuels-liquid hydrogen and boron hydrides. NACA experiments with hydrogen (pp. 97-98) were discussed -and the panel recommended active development of hydrogen fuel systems and engine combinations, as well as preliminary design studies of aircraft to use these fuels. The panel also met with the SAB panel on intelligence to consider vehicle requirements. The fuels and propulsion panel concluded that the Rex engine might contribute to this application and recommended further study.31
The Air Force motivation for rapid action on high altitude aircraft stemmed from the U-2. Many in that service were unhappy having the CIA manage that aircraft. Even before it flew, there were discussions within the Air Force about a follow-on airplane. The possibility that the U-2 might get shot down was recognized early, so attention was focused on airplanes capable of higher speeds and altitudes. One of the problems foreseen for the U-2 was its vulnerability from engine flameout at high altitude.32 If flameout occurred, the airplane had to descend to a much lower altitude-about 9000 meters-to restart the engine; at that time it was a sitting duck for antiaircraft fire.**
In addition to the flameout problem of the U-2, Kelly Johnson was faced with a problem of fuel loss from boil-off at very high attitudes. He had help on both problems from the Air Force and Pratt & Whitney, makers of the J-57-P37 engine. At the time, Col. Norman C. Appold, a combat pilot during World War II and holder of master's degrees in chemical and aeronautical engineering, was chief of the power plant laboratory at Wright Field. Earlier he had managed the Air Force contract with Pratt & Whitney for the J-57 and was very familiar with it. For this reason, and because he could draw on other propulsion and fuel experts in his laboratory, Appold was designated a "consultant" to Kelly Johnson. The father of the J-57 engine and chief engineer of Pratt & Whitney was Perry W. Pratt (no relation to the Pratt of P&W), and he too became closely involved with helping Johnson.
The J-57 turbojet engine normally operated on JP-4, a kerosene-like fuel. Johnson needed a fuel of lower volatility than JP-4 to minimize fuel loss during climb to the cruising altitude and during cruise. When the airplane took off, its fuel was at ground temperature. At high altitude, the combination of still-warm fuel and reduced pressure caused the more volatile portions of the fuel to boil away through the tank vents. Second, he needed a fuel with as high a combustion efficiency and flameout limit as he could get. Research showed that low volatility fuels had lower combustion efficiency than those of higher volatility, but this could be offset somewhat by improvements in the fuel injection system. Other research showed that fuels of low volatility had high flameout limits. In the end, Johnson, Appold, and Pratt selected a lower volatility fuel developed with the assistance of the Shell Oil Company research laboratories.33
During the course of studying the fuel-engine relationships for the U-2 and J-57, Appold and the fuel experts at the power plant laboratory considered a variety of fuels,  including some of high volatility such as methane and liquid hydrogen. Methane was available in quantity, but liquid hydrogen was quickly dismissed because it was not.34 This was the same period in which Rae was promoting the use of hydrogen in his Rex engine. Appold was well aware of Rae's Rex proposals and was involved in the actions regarding them. Sometime during the discussions between Appold, Johnson, and Pratt, the seed of the idea to use hydrogen was planted and grew.*** It matured into action in 1956 as we will see later.
** On 3 May 1960, two days after Francis Gray Powers was shot down over Russia. NASA put out a press release stating in part that "the pilot reported over the emergency frequency that he was experiencing oxygen difficulties." Propulsion engineers familiar with the altitude performance of jet engines assumed Powers had a flameout and descended to a lower altitude to relight. Powers, however, insists that he was shot down ata operational altitude. Gary Powers with Curt Gentry,Operation Over-Flight: The U-2 Spy Pilot Tells His Story for the First Time (New York: Holt, Rinehart and Winston), pp. 144, 201-202, 302, 323, 351-352.
Appold, Johnson, nor Pratt could recall definitely when or where
the idea originated (interviews with Appold 4 jan.,with Johnson 14
Feb. and 2 May, and with Pratt 14 May 1974). The origin of the
idea is less important than the interactions that occurred. Less
than four months after the first U-2 flight (Aug. 1954), the NACA
Lewis laboratory found that gaseous hydrogen in a turbojet
combustor did not flameout as easily as jet fuel and could burn at
pressures equivalent to 1600 m altitude (p. 98). No connection
between the U-2 problem and the Lewis experiments has been
establised, but the timing is interesting.