Air/LH2

Air/LH2 propellant. The propellants tested and proposed for use in environmentally-friendly or high-speed jet engines. Ambient air (78 % nitrogen, 21% oxygen, etc.) is scooped up by air intakes and used in turbojet, turbofan, ramjet, scramjet, or other airbreathing engines as an oxidizer. Liquid hydrogen has not been used as a fuel for aircraft to date due to its big drawbacks - it was highly cryogenic, and it had a very low density, making for large tanks. The United States mastered hydrogen technology for the highly classified Lockheed CL-400 Suntan reconnaissance aircraft in the mid-1950's. It is mainly proposed in air-breathing engines for high-speed scramjet aircraft, and mixed propulsion reusable single-stage-to-orbit designs, where use of hydrocarbon propellants creates coking and other issues. It is also proposed for, and has been tested as, the fuel for all commerical jet aircraft, as part of a post-petroleum 'hydrogen economy'.

The delivered cost of liquid hydrogen in 1960 was approximately $ 2.60 per kg. Large-scale production was expected to reduce the cost to $ 1.00 per kg. In the 1980's NASA was actually paying $ 3.60 per kg.

Fuel: LH2. Fuel Density: 0.071 g/cc. Fuel Freezing Point: -259 deg C. Fuel Boiling Point: -253 deg C.