[57] From 1945 to 1950, liquid hydrogen received considerable attention in analytical and design studies and in experimentation. The Jet Propulsion Laboratory of the California Institute of Technology and Project RAND at Douglas Aircraft Company compared rocket vehicle performance using hydrogen with the performance from other fuels. The superiority of liquid hydrogen was clearly indicated, but the biggest uncertainty related to the mass of vehicles using liquid hydrogen. North American Aviation and the Glenn L. Martin Company both made detailed designs of rocket vehicles using liquid hydrogen to obtain better vehicle mass values. Both incorporated thin-wall, pressure-stabilized, lightweight tanks as Oberth had proposed in 1923. Although not yet proven, this later became a key concept in the successful use of liquid hydrogen. Both the North American and Martin designs indicated superior vehicle performance with liquid hydrogen.

Concurrent with analytical and design studies were experiments on using liquid hydrogen-liquid oxygen in rocket engines. The Air Force sponsored experiments at Ohio State University on rockets, as well as scientific investigations of hydrogen's properties. At the same time, the Navy sponsored work at the Aerojet Engineering Corporation on liquid hydrogen-liquid oxygen rockets to determine the feasibility of launching a satellite with a single-stage-to-orbit vehicle. JPL, supported by the Army, also investigated the experimental performance of liquid hydrogen-liquid oxygen rockets and regenerative cooling.

All three laboratories conducting experiments had little difficulty obtaining efficient combustion and high exhaust velocities. Aerojet concluded that efficient combustion could be obtained with as little as 1/10 the volume normally used for rocket combustion. This, plus measurements indicating very high heat transfer, led them to propose and investigate an unusual thrust chamber design featuring a very small combustion volume and porous walls for transpiration (sweat) cooling, but difficulties with materials and cooling led to abandonment of the concept in favor of a more conventional design. Ohio State and JPL both used the more conventional thrustchamber design and obtained much lower heat transfer values than Aerojet. This led to the successful use of liquid hydrogen as a regenerative coolant, a major contribution to liquid hydrogen technology.

In the investigation of injection techniques for efficient combustion, it was found that a concentric tube design, where an annulus of hydrogen surrounds an oxygen stream, was superior to the conventional impinging stream concepts, and an injector with many such concentric tube elements gave good performance. This concept, verified by Aerojet, was another major contribution to liquid hydrogen technology for rocket engines.

Both Ohio State and Aerojet investigated the pumping of liquid hydrogen and both found it feasible with a centrifugal pump. Ohio State also found that ball bearings for [58] the pump could be operated without lubrication when immersed in liquid hydrogen, a very important finding for simplifying hydrogen pump design. The two investigations indicated liquid hydrogen could be successfully pumped.

Aerojet, using Herrick L. Johnston's design, built a hydrogen liquefier of 80 liters per hour, over three times greater than previous liquefiers. This showed that greater hydrogen liquefaction capability could be achieved through relatively straightforward engineering design. Dwight I. Baker of JPL found, however, that losses of liquid hydrogen prior to experimentation were too high to be tolerated and suggested that orthohydrogen be converted to parahydrogen at the liquefier by means of a catalyst-a key concept for practical use of large quantities of liquid hydrogen.

All the foregoing technical developments indicated that the basic technology for successful development of a rocket vehicle using liquid hydrogen-liquid oxygen was at hand, yet interest in using liquid hydrogen waned near the end of the 1940s. There are several explanations for this lack of interest. One is technical, for in spite of their successes, the experimenters encountered more than the usual number of difficulties in using liquid hydrogen, largely because of its lack of availability, very low temperature, explosive hazard, losses from orthohydrogen to parahydrogen conversion, and above all, the very low density. These were formidable obstacles for designer and experimenter alike, indicating that development of a hydrogen-fueled vehicle would be a long and costly development.

A second reason for lack of interest in hydrogen was the absence of a clear-cut need for its high performance. There were many other candidate fuels to be investigated including the boron compounds, hydrazine, and ammonia; and none had as many handicaps as liquid hydrogen. Of these, hydrazine looked particularly attractive.

A third reason was political. High Navy officials did not strongly support satellites. The Air Force made a major policy decision near the end of the 1940s to emphasize airbreathing propulsion rather than rocket propulsion.

Taking these reasons together, it is not surprising that interest in liquid hydrogen as a propulsion fuel receded in all but a few places where research-minded people remained interested in all high-energy rocket propellants. One such place was the Lewis Flight Propulsion Laboratory of the National Advisory Committee for Aeronautics at Cleveland, Ohio. The Lewis group was planning to conduct research with liquid hydrogen in 1950, but faced the same problem as Aerojet-the lack of liquid hydrogen. As they struggled with this problem, another development involving liquid hydrogen was begun on a crash basis and greatly advanced liquid hydrogen technology-thermonuclear research leading to the hydrogen bomb. These two contrasting activities-propulsion and explosives research-would renew interest in liquid hydrogen during the early 1950s.