Gauging Systems Monitor Cryogenic Liquids

Originating Technology/NASA Contribution

Rocket fuel needs to stay cool—super cool, in fact. The ability to store gas propellants like liquid hydrogen and oxygen at cryogenic temperatures (below -243 °F) is crucial for space missions in order to reduce their volumes and allow their storage in smaller (and therefore, less costly) tanks. The Agency has used these cryogenic fluids for vehicle propellants, reactants, and life support systems since 1962 with the Centaur upper stage rocket, which was powered with liquid oxygen and liquid hydrogen.

During proposed long-duration missions, super-cooled fluids will also be used in space power systems, spaceports, and lunar habitation systems. In the next generation of launch vehicles, gaseous propellants will be cooled to and stored for extended periods at even colder temperatures than currently employed via a process called densification. Densification sub-cools liquids to temperatures even closer to absolute zero (-459 °F), increasing the fluid’s density and shrinking its volume beyond common cryogenics. Sub-cooling cryogenic liquid hydrogen, for instance, from 20 K (-423 °F) to 15 K (-432.4 °F) reduces its mass by 10 percent. These densified liquid gasses can provide more cost savings from reduced payload volume.

In order to benefit from this cost savings, the Agency is working with private industry to prevent evaporation, leakage, and other inadvertent loss of liquids and gasses in payloads—requiring new cryogenic systems to prevent 98 percent (or more) of boil-off loss. Boil-off occurs when cryogenic or densified liquids evaporate, and is a concern during launch pad holds. Accurate sensing of propellants aboard space vehicles is also critical for proper engine shutdown and re-ignition after launch, and zero boil-off fuel systems are also in development for the Altair lunar lander.

Partnership

One company, in partnership with NASA, has developed a liquid-sensing system that monitors cryogens and densified propellants. Fremont, Ohio’s Sierra Lobo Inc. (SLI) specializes in cryogenics and propulsion, and, in particular, produces propellant storage systems. The Hispanic-American-owned company that began with only 9 employees in 1993 now has an ISO 9001:2008 registration and currently employs over 370 people in its Ohio, Florida, Texas, Alabama, Virginia, and California facilities.

In 2006, SLI developed the Cryo-Tracker Mass Gauging System (Cryo-Tracker MGS) with funding from a Phase III Small Business Innovation Research (SBIR) contract from Kennedy Space Center, after receiving Phase I and Phase II funding from the U.S. Department of Defense. The Cryo-Tracker (CT) probe—the key component of the Cryo-Tracker MGS—works in conjunction with the system’s other two components: electronics that provide power and signal management for each sensing element, and software that displays data received from the probe. Since winning the initial SBIR contract, SLI has successfully tested Cryo-Tracker MGS on parabolic flights, which simulate the reduced gravity of space flight without the high costs. On these flights, SLI researchers validated wicking technology used in the system’s sensors by capturing numerous images of the system’s CT probe operating in water during 90 reduced-gravity parabolas lasting 25 seconds each. Sierra Lobo also successfully tested a 33-foot CT probe and the Cryo-Tracker MGS in a simulation of typical pre-launch and flight operations in a large-scale expendable launch vehicle liquid oxygen tank.