A Carbon Dioxide Purge and Thermal Protection System for Liquid Hydrogen Tanks of Hypersonic Airplanes
With supersonic flight already achieved, studies of hypersonic flight in the atmosphere are underway [1–3]. A study of a hypersonic transport  indicates that the use of liquid hydrogen fuel with its high heat of combustion may result in twice the cruise efficiency attainable with hydrocarbon fuel; in fact, this and other advantages  suggest that liquid hydrogen fuel is mandatory for efficient hypersonic cruise airplanes. Hypersonic flight then not only results in high external surface temperature from aerodynamic heating (about 1500°F compared with about 500°F for supersonic flight), but requires tank temperatures as low as — 423°F. This simultaneous existence of high and low temperatures for long flight times requires an efficient thermal protection system to prevent excessive fuel boil-off; moreover, since liquid hydrogen has a very low density, large surface areas must be protected. Insulation alone is not adequate since the cold tank can cause air to liquefy continuously . Experiments  have shown that the resulting cryopumping action can release heat at a rate as high as 3600 Btu/hr-ft2, which represents a fuel boil-off rate of 18 lb/hr-ft2—a totally unacceptable rate for a flieht vehicle.
KeywordsThermal Protection Liquid Hydrogen Hypersonic Vehicle Tank Wall Thermal Protection System
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