Enhancing heavy lift booster performance by exploiting zeotropic condensation of air

A study of operating costs for aerospace transportation systems has resulted in the development of a cost model which suggests that the key to low-cost access to space is a low-power, low-thrust system. The best way to achieve heavy lift at low-thrust and, in turn, low-cost is with a horizontal take-off system. An analysis of liquid air cycle engines has shown them to be beneficial for horizontal flight systems, which cruise while onboard liquid hydrogen is used to liquefy and store liquid-air for use as an oxidizer during space flight. System specific impulse is always a very important performance measure; hence this paper examines a low-mass technique for extracting oxygen-enriched air during the liquefaction process. The condensation of air does not occur at a constant temperature, even at constant-pressure, because air is zeotropic. Furthermore, the condensation process occurs with an oxygen-rich mixture forming the liquid phase at the highest temperatures, and a nitrogen-rich mixture forming the liquid phase at lower temperatures. This behavior can be exploited by collecting liquid forming near the entrance of the liquefaction heat exchanger, where temperatures and oxygen concentrations are higher. A scheme for oxygen enrichment by zeotropic condensation in a liquefaction heat exchanger is described, and liquid air cycle performance for space launch applications is assessed.