During the 1950s, interest in high-energy propellants for upper stages of rocket vehicles had steadily mounted. Such propellants were initially seen as a means for increasing the range of ballistic missiles, but this shifted early towards increasing the capability of rockets to launch satellites and space probes. The principal candidates were hydrazine-fluorine, hydrogen-oxygen, and hydrogen-fluorine, but none had reached the development stage. The coming of Sputnik and U.S. plans for a strong space program quickly produced action, the first high-energy upper stage authorized for development used liquid hydrogen-oxygen. The decision, and subsequent development, owed much to an earlier program, which had faltered-Suntan, the program to develop a high-altitude reconnaissance airplane fueled with liquid hydrogen (chap. 8).