Space missions rely on cryogenic fluids — extremely cold liquids like liquid hydrogen and oxygen — for both propulsion and life support systems. These fuels must be kept at ultra-low cryogenic temperatures to remain in liquid form; however, solar heating and other sources of heat increase the rate of evaporation of the liquid and cause the pressure in the storage tank to increase. Current storage methods require venting the cryogenic propellant to space to control the pressure in fuel tanks.
NASA’s Zero Boil-Off Tank Noncondensables (ZBOT-NC) experiment is the continuation of Zero Boil-Off studies gathering crucial data to optimize fuel storage systems for space missions. The experiment will launch aboard Northrop Grumman’s 23rd resupply mission to the International Space Station.
Even with multilayer insulation, heat unavoidably seeps into cryogenic fuel tanks from surrounding structures and the space environment, causing an increase in the liquid temperature and an associated increase in the evaporation rate. In turn, the pressure inside the tank increases. This process is called “boil-off” and the increase in tank pressure is referred to as “self-pressurization.”
Venting excess gas to the environment or space when this process occurs is highly undesirable and becomes mission-critical on extended journeys. If crew members used current fuel storage methods for a years-long Mars expedition, all propellant might be lost to boil-off before the trip ends.
NASA’s ZBOT experiments are investigating active pressure control methods to eliminate wasteful fuel venting. Specifically, active control through the use of jet mixing and other techniques are being evaluated and tested in the ZBOT series of experiments.
ZBOT-NC further studies how noncondensable gases (NCGs) affect fuel tank behavior when present in spacecraft systems. NCGs don’t turn into liquid under the tank’s operating conditions and can affect tank pressure.
The investigation, which is led out of Glenn Research Center, will operate inside the Microgravity Science Glovebox aboard the space station to gather data on how NCGs affect volatile liquid behavior in microgravity. It’s part of an effort to advance cryogenic fluid management technologies and help NASA better understand low-gravity fluid behavior.
Researchers will measure pressure and temperature as they study how these gases change evaporation and condensation rates. Previous studies indicate the gases create barriers that could reduce a tank’s ability to maintain proper pressure control — a potentially serious issue for extended space missions.
The research directly supports Mars missions and other long-duration space travel by helping engineers design more efficient fuel storage systems and future space depots. The findings may also benefit scientific instruments on space telescopes and probes that rely on cryogenic fluids to maintain the extremely low temperatures needed for operation.
The investigation could improve tank design models for medical, industrial, and energy production applications that depend on long-term cryogenic storage on Earth.
