Zero Boil-Off Tank (ZBOT, ZBOT-NC)
Rocket fuel, spacecraft heating and cooling systems, and sensitive scientific instruments rely on very cold cryogenic fluids. Heat from the environment around cryogenic tanks can cause their pressures to rise, which requires dumping or “boiling off” fluid to release the excess pressure, or actively cooling the tanks in some way. Zero Boil-Off Tank (ZBOT) uses an experimental fluid to test active heat removal and forced jet mixing as alternative means for controlling tank pressure for volatile fluids.
Investigation on Space Station to Test Minimizing Pressure of Space Travel
Spacecraft rely on liquids for everything from fuel to life support systems for astronauts. Storing these liquids at the correct temperature and pressure is essential to prevent loss of fluids or failure of a storage tank. Human life in space is a balancing act of reliable systems and meticulous planning.
Rocket fuel and other liquids used in space are stored at cryogenic temperatures of –423 to -243 degrees F. As these liquid cryogens are warmed by the environment, they evaporate, which increases pressures inside storage tanks.
NASA Glenn Research Center’s Zero Boil-Off Tank (ZBOT) experiment on the International Space Station will study ways to relieve that pressure without the loss of fluid. ZBOT launched aboard Orbital ATK’s Cygnus spacecraft on its seventh contracted resupply mission for NASA from… Continued
- Cryogenic propellants are rocket fuels that are stored as liquids at very cold temperatures. However, because they are kept at such cold temperatures, heat from the surrounding environment evaporates the liquid in the tank into vapor causing the pressure in the tank to rise. In order to limit the pressure from increasing to a catastrophic level that can rupture the tank, several techniques may be used to keep the pressure manageable.
- One technique is venting or dumping fluid overboard to relieve the excess pressure resulting in the loss of the volatile fluid. Actively cooling the tank contents by circulating a portion of the tank contents through a system that absorbs the heat from the liquid. This is a vent-less technique that does not lose cryogenic fluid by using active cooling.
- Another technique is dynamic pressure control, another vent-less technology that mixes the bulk liquid with or without active cooling. Both tank pressurization and pressure control are governed by interactions among the forced mixing, and the condensation and evaporation process at the vapor-liquid interface.
- Because gravity plays a significant role in the motion and position of the vapor and liquid phases, a dynamic pressure control system for space applications cannot be accomplished solely based on analysis and computational models, especially since there is a lack of relevant microgravity data.
- Since a large scale test in space is costly and the type of instrumentation and sensors that are suitable for use with liquid cryogens are limited, small scale ground based and microgravity experiments using simulant fluids are needed to understand the underlying physical phenomena influencing tank pressurization in space and then to optimize and scale up the pressure control method for microgravity storage.
- This investigation performs both normal gravity and microgravity small-scale experiments on earth and aboard the ISS, respectively, to gather relevant data, and uses the 1g and microgravity experimental data to validate numerical models to optimize design of scaled-up pressure controlled storage systems.
- Relevant microgravity data are benchmarked to verify and validate computational fluid dynamic (CFD) models for fluid tank pressurization. These models can be used to design future, larger storage tanks of highly volatile liquids, such as cryogenic propellants. This research ultimately reduces the risk and costs of future space expeditions.
- ZBOT addresses some of the limitations of previous experimental efforts including definition of initial and boundary conditions, long-term experiments that permit comparisons with thermodynamic models, and point-wise and field measurements of temperatures and fluid velocities for numerical validation.
- ZBOT investigates the role of transport and phase change phenomena on stratification, pressurization, and pressure control of a volatile fluid, Perfluoro-normal-Pentane (P-n-P).
- Comprehensive ground based and microgravity tests are conducted to study the effects of heat flux, fill level, and mixing on thermal stratification, pressurization and pressure control. • Development of a state-of-the-art two phase CFD model for the storage tank to aid future scale-up tank pressure control designs.
- Development of a multi-zonal thermodynamic model for the storage tank for quick and efficient engineering analysis.
- Validation and Verification of the models by the acquired ground based and microgravity data.
- Both the experimental data and modeling efforts are used as benchmarks for designing tanks for the long-term storage of cryogenic liquids
Long-term storage of cryogenic fluids is necessary for spacecraft propulsion and life support. Scientific sensors aboard space telescopes and other space probes also require operation at cryogenic temperatures, but can only work as long as the cryogenic fluids last. ZBOT carries out small-scale microgravity tests to enable further research for lightweight, efficient and long-duration cryogenic storage in space.
Cryogenic tanks require complicated storage and flow solutions for fluids that act as both liquid and gas, depending on their temperatures. ZBOT investigates the role of phase change physics and heat transport on the pressure control of these volatile fluids. Results from the investigation improve models used to design tanks for long-term cryogenic liquid storage, which are essential in biotechnology, medicine, industrial, and many other applications on Earth.
Operational Requirements and Protocols
The ZBOT test points are performed at three different fill levels: 70%, 80%, and 90%. The test tank is launched at a fill level of 70%. Particles are then injected into the fluid. The test fluid, P-n-P, is pre-conditioned to a nominal starting point temperature prior to each test. Tests fall into three categories: jet-mixing, subcooled jet mixing, and self-pressurization tests. Once tests are completed, the Fluid Reservoir are used to increase the fluid level in the test to the 80% and 90%. Dissolved gas are removed from the test fluid after each fill adjustment. Also, additional particles are injected to the fluid adjustment. There are a total of 52 test points, 23 each at the 70% and 90% fill levels, and 6 test points at the 80% fill level. Data are downlinked periodically. Return of the hardware is not required for data retrieval.
On-orbit procedures cover the installation of the hardware into the Microgravity Science Glovebox (MSG). During installation the crew must evacuate air from hoses and fill the water loop. The hoses are evacuated using the Microgravity Science Glovebox vacuum exhaust system (MSG VES). The water loop is filled from the ZBOT Water Reservoir. After the hardware is installed, the crew inject particles into the Test Section between 3-5 times. Experimental runs are controlled from the ground.
Project Manager: William Sheredy, NASA Glenn Research Center
Project Scientist: John McQuillen, NASA Glenn Research Center
Principal Investigator: Dr. Mohammad Kassemi, NCSER/GRC
C0-Investigator: Dr. David Chato, NASA GRC
Engineering Team: ZIN Technologies, Inc.