JPL Experiments with Hydrogen-Oxygen, 1948-1949
 It is ironical that Young's experimental team at Aerojet, early in getting started with hydrogen-oxygen in 1945-even building a liquefier to get a supply of liquid hydrogen-was not the first to experiment with liquid hydrogen in a rocket on the West Coast. Baker, using Aerojet-furnished liquid hydrogen, beat them by four months. J PL had been interested in hydrogen-oxygen as a high-energy propellant combination since starting a study for the Bureau of Aeronautics in 1945.*
 When Aerojet queried JPL in 1947 for interest in using liquid hydrogen, JPL responded with an estimated need for 600 to 900 kilograms for a year of experimentation. While Aerojet's liquefier was under construction, a 100-liter dewar was built for use in transporting liquid hydrogen from the Aerojet plant to the JPL test cell. When Aerojet produced liquid hydrogen on 21 September 1948, Baker was ready and waiting. Aerojet provided 75 liters of liquid hydrogen to J PL and Baker used it in a rocket run the same day. The results were first reported in the JPL Combined Monthly Summary No. 8 for the period 20 August-20 October 1948:
With these words, JPL became the second U.S. laboratory to report rocket experiments using liquid hydrogen, a little over a year after Ohio State University's first test.
The performance obtained in the first JPL test with liquid hydrogen-oxygen was 2717 meters per second, within 15 percent of theoretical-not bad for the first attempt. The average heat transfer rate was 3.6 joules per second per square meter, much lower than measured by Aerojet but in agreement with the data from Ohio State University.
Baker was appalled at how little liquid hydrogen he was able to use in the rocket firing. Only 37 percent was burned in the rocket chamber. An estimated 21 percent was lost in cooling the transport dewar, 16 percent evaporated during transit from Azusa to Pasadena, and 26 percent was lost in cooling the propellant tank of the test rocket. If Baker had not already precooled the hydrogen containers and system with liquid nitrogen, the liquid hydrogen loss would have been much greater. This experience led JPL to use gaseous hydrogen for injector testing while reserving liquid hydrogen for heat transfer and cooling tests. They were already conducting some experiments with gaseous hydrogen which also were reported in Monthly Summary No. 8.
The gaseous hydrogen-liquid oxygen rocket experiments were conducted with a 445-newton (100-lb-thrust) chamber and the results indicated that liquid oxygen above its critical pressure cooled two-thirds of the combustion chamber, with water cooling the rest. At that time, cooling with liquid hydrogen was a big unknown, for fundamental heat transfer data on hydrogen above its critical pressure were missing. Walter B. Powell, who had built an electrically heated tube for heat transfer research, agreed to obtain the missing data. This was given first call on the next available supply of liquid hydrogen while injector testing continued with gaseous hydrogen-liquid oxygen at a higher thrust (2.2 kN or 500 lb). Baker was to use the data Powell obtained to design a regeneratively cooled thrust chamber, possibly using both liquid hydrogen and liquid oxygen as coolants.
Early in 1949, Baker succumbed to enthusiasm, confidence, or impatience and decided to go ahead with designing and testing a hydrogen-cooled thrust chamber without waiting for Powell's results. He had already calculated that liquid hydrogen had twice the heat absorbing capacity of liquid oxygen at their relative flow rates and  therefore would be a better coolant. He designed a rocket engine of 445 newtons (100-lb thrust) for operation at 20 atmospheres chamber pressure. On 15 April 1949, Baker became the first person in the United States, if not the world, to operate a liquid hydrogen-liquid oxygen rocket thrust chamber that was cooled with liquid hydrogen. The test ran for 77 seconds and performance was relatively low (2630 meters per second), succeeding runs, however, established beyond any doubt that high performance and regenerative cooling with liquid hydrogen were realizable . Sixteen runs were made through 10 June 1949 over a range of hydrogen-oxygen mixture ratios, with an average running time of 69 seconds for the series. Three runs were made at a combustion pressure of 33 atmospheres and three sizes of combustion chambers were used during the series. Maximum performance was an exhaust velocity of 3420 meters per second at 33 atmospheres combustion pressure and an oxygen-to-hydrogen mass ratio of 4. Baker encountered no serious difficulties and concluded that large size, regeneratively-cooled rocket thrust chambers using liquid hydrogen-liquid oxygen were practical.34
Although Baker had no serious problems with burning hydrogen or cooling with it, he was still concerned over the supply of liquid hydrogen. The cost was about $45 per kilogram and he was able to burn half or less of the amount purchased, with the rest lost in transit and cooling. The hydrogen delivered was about half orthohydrogen and half parahydrogen Baker was aware that the spontaneous conversion of orthohydrogen into parahydrogen released heat, and suggested that savings could be made if all the hydrogen were converted to parahydrogen by means of a catalyst at the liquefier. With this sensible suggestion, he anticipated developments during the 1950s.
* JPL, was also interested in the possible use of nuclear energy to heat hydrogen. In 1947, Walter B. Powell of, JPL attempted to measure the performance of gaseous hydrogen heated electrically in a tube, but found that the thrust and flow rates were so low that accurate measurement was impractical.