by guest columnists Wesley Johnson, NASA Glenn Research Center, firstname.lastname@example.org, and Tara Polsgrove, NASA Marshall Space Flight Center, email@example.com
Cryogenic fluids are the basis for many different in-space systems, and several recent publications from NASA examine potential roles for cryogenic fluids in human missions to the red planet [2-5]. Propulsion, in-situ resource utilization and power storage and generation represent the main systems using cryogenics.
Perhaps the most fundamental enabling technology for Mars exploration is In-Situ Resource Utilization (ISRU). In layman’s terms, this means “use as much stuff as you can find on Mars so you don’t have to take as much with you from the start.”
A simple evaluation of ascent vehicle designs [2,4] shows that propellant accounts for between 75 and 80 percent of ascent vehicle mass and, depending on the fuel that is used, the mass of liquid oxygen can be anywhere from 75 to 85 percent of that figure. By using oxygen already present on Mars, NASA can cut the mass of its landers in half, as the crew ascent vehicle is the largest payload that has to be landed . To accomplish this, oxygen from the Martian atmosphere could be extracted, liquefied and then stored in the ascent vehicle tanks . Carbon dioxide is 95 percent of the atmosphere on Mars and simple chemical reactions of carbon dioxide can free one of the oxygen atoms. If water can be found—NASA has measured it in small quantities in the regolith and some experts think there may be large underground glaciers in addition to polar ice caps—it can be combined with carbon dioxide through the Sabatier process to produce both oxygen and methane; or direct electrolysis can produce oxygen and hydrogen. These gases could then be liquefied and stored for later use.
The process seems straightforward but there are challenges beyond just obtaining resources. ISRU production and liquefaction systems would have to be pre-positioned with enough time to generate the propellant necessary for ascent before the crew arrives, for example, as some abort scenarios may require that the crew leave Mars almost immediately after they land.
Launches from Earth to Mars can only happen every 26 months due to the alignment of the planets, so the ISRU system would use that time to produce oxygen at approximately 90 K. Both the ISRU production and liquefaction systems would have to operate autonomously at an average production rate of 2.2 kg/hr to produce the over 23 tons of oxygen required to lift an ascent stage off the surface of Mars .
NASA is considering several different transportation stage propulsion options  to get to Mars. One option has different propulsion for cargo and crew, where solar electric propulsion stages transport the cargo and a separate liquid oxygen/liquid methane chemical rocket transports the crew.A second option has a system that is part chemical (either oxygen and methane or “hypergolic” propellant—hydrazine and nitrogen tetroxide) and part solar electric propulsion. The third option uses liquid hydrogen in a nuclear thermal propulsion stage, while a fourth includes liquid oxygen and liquid methane for both crew and cargo.
Each option has different operational parameters. Some are refueled to support multiple flights while others are assembled in lunar orbit. A single mission may have a duration of up to five years and the hardware may need to last for up to three missions—depending on whether the stage is designed for reusability—so no losses due to boiloff during quiescent operations can be allowed. Even losses of 5 to 10 percent per year add up unless the in-space stages are refueled in Martian orbit.
Heat load will also be an issue. Previous Mars mission concepts proposed the use of up to 200 layers of MLI in an attempt to minimize boiloff losses , but current plans require cryocoolers to eliminate heat load into the tank and control the pressure in the tanks as well.
These stages will additionally operate for long periods of time in microgravity, so methods are needed to keep track of propellant quantity and to enable extraction of the propellant to supply to the engine as a sub-cooled liquid, preventing cavitation in the turbopumps that feed the main engines.
The hydrogen-based nuclear stages would be unique in their requirements for 20 K refrigeration, with perhaps a 90 K stage included to reduce heat loads at 20 K as well as to increase system efficiency. However, many of the requirements and even conceptual operations of the nuclear stages have yet to be defined.
In all the transport scenarios, a separate stage for landing on Mars—currently assumed to be powered by liquid oxygen and methane—will require many of the same technologies as the transportation stage.
Concepts for application of cryogenic fluids for energy storage and generation are still in development. A Martian outpost would most likely be powered by a nuclear or solar power station with long cables for power distribution. It will take some time, however, to get each element of the outpost connected to the surface power supply.
It has been assumed by NASA researchers that some of the leftover propellant from the landing stage will be used to power fuel cells while astronauts acclimate to gravity and connect the power system to all of the different elements that were previously landed. This project could take some time, however, as astronauts who spend six months on the International Space Station can need up to several weeks for acclimation on Earth.
There have also been discussions about the ISRU plant continuing to make oxygen for the crew to breathe—either in living quarters or during Mars excursions away from the habitat—since the facility will have finished all of its tasks for the propulsion elements once the crew lands. However, depending on the amount required, the benefits provided by the higher density of the liquid storage may not outweigh the extra processes required to store and use the liquid.
NASA is not the only group interested in using cryogenic fluids for exploration. A consortium of industry, academia and government meets annually to discuss possible paths to Mars. Its fourth annual report details the technical difficulties or “long poles” for Martian exploration . Several of these companies are conducting independent analyses in addition to those proposed by NASA. SpaceX  also plans to use liquid methane or liquid natural gas and liquid oxygen based propulsion systems assuming production of both oxygen and methane on the surface of Mars, whereas Blue Origin  plans to use liquid hydrogen and liquid oxygen for its in-space stage and liquid natural gas as the fuel for its first stage landers. Lockheed Martin  favors an entirely liquid oxygen and liquid hydrogen propulsion system designed to generate both hydrogen and oxygen on the surface of Mars by electrolyzing water found on the surface on Mars and liquefying both. Meanwhile, NASA Jet Propulsion Laboratory  has been promoting a non-cryogenic option that delivers a smaller crew for a shorter duration, resulting in lower science returns in exchange for the lower propulsion efficiency of non-cryogenic propellants.
In preparation for the development of the various elements and pieces needed for these architectures, NASA and its partners have started planning the technology development required should the full approach be undertaken. The development of these technologies was discussed at the 27th Space Cryogenics Workshop, including recent two-phase forced convection heat transfer correlations by Dr. Jason Hartwig (NASA/GRC)* and Lingxue Jin (KAIST); passive thermodynamic venting to prevent the venting of liquid in microgravity by Junghyun Yoo (KAIST); multilayer insulation developments by Dr. David Chato (NASA/GRC ret.), James Fesmire (NASA/KSC), Christian Dubois (CNES) and Alan Kopelove (Quest Thermal Group); several developments in incorporating vapor cooling by Lauren Ameen (NASA/GRC); recent high power cryocooler develoments by David Plachta (NASA/GRC); liquefaction operations on the surface of Mars by Wesley Johnson; papers on computational modeling of large-scale cryogenic processes by Dr. Mo Kassemi (NASA/GRC/CWRU), Pooja Desai (NASA/JSC) and Mark Stewart (NASA/GRC); and NASA’s plans for developing a large 4 m diameter test bed by Wesley Johnson.
Additionally, Dr. William Notardonato (NASA/KSC) presented results of a large-scale demonstration of zero boiloff storage, densification and in-tank liquefaction of hydrogen. Recent presentations from NASA’s top administrators for exploration  show the progression of exploration using some of these technologies, first in lunar orbit (the use of the lunar surface is uncertain and not discussed), then towards Mars as early as the mid-2030s.
Notes and References
* All mentions: NASA Goddard Research Center (GRC), Korea Advanced Institute of Science and Technology (KAIST), NASA Kennedy Space Center (KSC), Cape Western Reserve University (CWRU), NASA Johnson Space Center (JSC), Centre National d’Etudes Spatiales (CNES).
 NASA Transition Authorization Act of 2017 http://www.congress.gov/115/plaws/publ10/PLAW-115publ10.pdf
 T. Polsgrove et al., “Mars Ascent Vehicle Design for Human Exploration,” presented at AIAA Space, 2015.
 T. Percy et al., “In-Space Transportation for NASA’s Evolved Mars Campaign,” presented at AIAA Space, 2015.
 T. Polsgrove et al., “Human Mars Ascent Vehicle Configurations and Performance Sensitivities,” presented at the 38th IEEE Aerospace Conference, 2017.
 T. Polsgrove et al., “Mission and Design Sensitivities for Human Mars Landers Using Hypersonic Inflatable Aerodynamic Decelerators,” presented at the 38th IEEE Aerospace Conference, 2017.
 R.J. Stochl, “Basic Performance of a Multilayer Insulation System Containing 20 to 160 Layers,” NASA TN D-7659, 1974.
 http://www.exploremars.org/wpcontent/uploads/2017/08/AMIV_2016_web.pdf, accessed Aug. 26, 2017.
 http://www.blueorigin.com/technology, accessed Aug. 26, 2017.
 http://www.spacex.com/mars, accessed August 26, 2017.
 H. Price, J. Baker and F. Naderi, “A Minimal Architecture for Human Missions to Mars,” New Space, Vol. 3, No. 2, 2015.
 T. Cichan, et al., “Mars Base Camp: An Architecture for Sending Humans to Mars by 2028,” presented at 2017 IEEE Aerospace Conference, 2017.
 http://www.nasa.gov/sites/default/files/atoms/files/nss_chart_v23.pdf, accessed Aug. 26, 2017. Note: the in-space propulsion stage is referred to as the “Deep Space Transport” or DST. ■