Comparison of oxygen liquefaction methods for use on the surface of Mars
One of the specificities of exploring Mars is the tremendous mass of propellant necessary for the ascent from the planet's surface. Due to the large mass of the Mars Ascent Vehicle – the space shuttle to be used by astronauts to evacuate the surface of Mars in a future crewed Mars mission – and the resulting large quantities of propellant (generally oxygen and methane) required, it is more efficient to produce propellants in situ from the atmosphere of the planet.
Since the nominal rates of propellant production are much lower than the required rates to feed the engine in real time, a methodology must be developed to contain and store the propellants while they await use by the engine. Typical chemical propulsion oxygen and methane-based systems store the propellants as cryogenic liquids due to the increase in density by several hundred times, which significantly decreases the volume (and thus the mass) of the tanks required for the spacecraft.
For a liquid oxygen and liquid methane-based propulsion system, over 75% of that propellant mass is liquid oxygen. Thus, by producing oxygen on the surface, the mass of the return vehicle required to be delivered to the surface is reduced by more than 50%.
An open-access article1 from NASA researchers published in Cryogenics reviews the various methods by which the oxygen (and methane) could be liquefied and stored on the surface of Mars. Five different architectures or cycles were considered: Tube-on-Tank (also known as Broad Area Cooling or Distributed Refrigeration), Tube-in-Tank (also known as Integrated Refrigeration and Storage), a modified Linde open liquefaction/refrigeration cycle, the direct mounting of a pulse tube cryocooler onto the tank, and an in-line liquefier at ambient pressure. Models of each architecture were developed to give insight into the performance and losses of each of the options.
The result was that Tube-on-Tank and Tube-in-Tank architectures were the most attractive solutions, with NASA’s engineering management choosing to pursue tube on tank development rather than further differentiate the two.
In the tube on tank cycle, a reverse turbo-Brayton cryocooler is connected to a tubing network that is anchored to the tank wall. The working fluid from the cryocooler then circulates through the cooling network, which is the thermal load of the cryocooler. This allows for direct integration of the cryocooler to the tank and more efficient cooling. An added benefit is that the tube-on-tank methodology creates a nearly isothermal tank wall.
As a result, NASA is focusing its Martian surface liquefaction activities and technology development on Tube-on-Tank liquefaction cycles.
1 JOHNSON W. L. HAUSER D. M. PLACHTA D. Comparison of oxygen liquefaction methods for use on the Martian surface. Cryogenics [online]. 2018, vol.90, p.60-69. Available following this link.