Liquid Hydrogen as a Supersonic Transport Fuel

The properties that make hydrogen an ideal fuel for long-range supersonic transport aircraft are very high combustion energy per pound relative to kerosene, or any other fuel, and a very high cooling capacity per pound. When hydrogen is burned in air, a pound of hydrogen will release 51,000 Btu/lb of hydrogen burned compared to 18,500 Btu/lb—a typical number for hydrocarbon fuels such as kerosene (Fig. 1). The ratio of these two numbers is 2.75:1. A feeling for the impact of this ratio on aircraft design can be obtained by considering that if everything else in the airplane stayed the same, an airplane would go more than 2.75 times as far with hydrogen as it would with kerosene. Generally, things do not stay the same, however, because of volume penalties and because it is economically more advantageous to take out this increased performance by putting more payload in the airplane than it is to increase the range of the airplane to this extent. Some compromise must be reached between extending the range of the airplane and increasing its payload in order to obtain the lowest possible operating cost.