Design of a High Performance Cryocooler for Propellant Liquefaction and Storage on Mars

Human exploration of space is extending beyond low earth orbit and Moon as NASA is planning a human mission to Mars in 2014. Past studies indicate that In Situ Propellant Production (ISPP) on Mars is a key enabling technology in Mission to Mars. In ISPP, oxygen and methane, which are to be used as cryogenic propellants for any return vehicle from Mars to Earth, are produced on the Martian surface. Once the propellants are produced in gaseous state, they must be liquefied for storage and use in the ascent vehicle. Cryogenic cooling is needed for both liquefaction and storage since the storage temperature, around 90K for oxygen and 112K for methane, are considerably lower than the average temperature on the Martian surface, which is around 220K. This paper presents the preliminary design of a single-stage, lightweight and compact, cryocooler based on reverse Brayton cycle with performance comparable to, but more reliable than, a corresponding Stirling cycle. With the application of micro-scale highly-effective recuperative heat exchanger, the estimated COP of the miniature cryocooler can reach 0.2. Thermal cycle parameters that can influence the cycle performance are studied. The two key enabling components, an integrated and micro-fabricated compressor and motor and a micro-fabricated recuperative heat exchanger, are discussed and shown to be possible with modern technology.