Encyclopedia Astronautica
Air/LH2


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 oxidiser. 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 single-stage-to-orbit designs, where use of hydrocarbon propellants creates coking and other issues.

The equilibrium composition of liquid hydrogen is 99.79 per cent parahydrogen and 0.21 per cent orthohydrogen. The boiling point of this composition is -253 deg C. Liquid hydrogen is transparent and without a characteristic odour. Gaseous hydrogen is colourless. Hydrogen is not toxic but is an extremely flammable material. The flammable limits of gaseous hydrogen in air are 4.0 to 75 volume percent.

Hydrogen is produced from by-product hydrogen from petroleum refining and the partial oxidation of fuel oil. The gaseous hydrogen is purified to 99.999+ per cent, and then liquefied in the presence of paramagnetic metallic oxides. The metallic oxides catalyse the ortho-para transformation of freshly liquefied hydrogen. Freshly liquefied hydrogen which has not been catalysed consists of a 3:1 ortho-para mixture and cannot be stored for any length of time because of the exothermic heat of conversion. 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.

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

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Associated Engines
Associated Stages
  • Sanger II-1 Mach 6.6 rocket launch aircraft. Winged scramjet using LH2 propellant. Loaded/empty mass 254,000/156,000 kg. Thrust 1,500.00 kN. Specific impulse 1200 seconds. Releases Horus @M 6.6 @ 37 km altitude @ 3100 km range More...

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