液氢
Liquid hydrogen was theoretically the best possible propellant, but in the early 1950s it was expensive, and available only in small quantities. In 1952, the AEC and the National Bureau of Standards had opened a plant near Boulder, Colorado, to produce liquid hydrogen for the thermonuclear weapons program. Before settling on liquid hydrogen, LASL considered other propellants such as methane () and ammonia (). Ammonia, used in the tests conducted from 1955 to 1957, was inexpensive, easy to obtain, liquid at , and easy to pump and handle. It was, however, much heavier than liquid hydrogen, reducing the engine's impulse; it was also found to be even more corrosive, and had undesirable neutronic properties.