Produces Desired Surface Topology and Finish of Cryogenic propellant storage tank Interior Surfaces to Enable Spacecrafts to Conserve Cryogenic Fuel During Long-Duration Missions and Reduced Gravity Environments
This parylene C coating reduces the cryogenic propellant loss by coating the interior walls of cryogenic storage tanks to lower the boil-off rates. NASA’s biggest challenge for the next millennium is the extension of human space exploration from a low earth orbit to a high earth orbit, then to the moon, Mars, and possibly asteroids. The missions will use the high thrust and high efficiency of cryogenic chemical propulsion. So, minimizing cryogenic propellant loss is critical to the success of these missions requiring long-term storage of cryogens. However, maintaining super cold fuels is a major challenge as even small amounts of heat entering a tank cause the liquid fuel to slowly turn into vapor. This fuel loss, known as boil-off, reduces mission capability, increases cost, and forces spacecraft to carry extra propellant waste that needs to be vented overboard. Additionally, vital equipment, such as the space tug and tankers, have long periods of loitering where the boil-off of cryogenic propellant could lead to significant losses in fuel reserves. This is a more significant issue for longer duration mission, such as a trip to Mars. There is a need for technologies that help reduce and minimize propellant losses due to boil-off.
Researchers at the University of Florida have developed a storage tank interior coating for making the inner tank surfaces of a spacecraft less likely to initiate boiling evaporation. By creating an ultra-smooth, low-thermal-conductivity surface, the coating forces the liquid to require a much larger degree of superheat before boiling can initiate. This results in substantial boil-off reduction during ground tests and in reduced-gravity parabolic flight environments, contributing directly to better mission efficiency and long-term cost reduction.
Application
A thin-film Parylene-C coating used to minimize or eliminate boil-off in cryogenic propellant tanks for spacecraft, orbital depots, and deep-space missions
Advantages
- Reduces fuel loss during long-term cryogenic storage
- Performs effectively in Earth and reduced-gravity environments, supporting mission needs beyond low Earth orbit
- Low-thermal-conductivity coatings, reducing heat transfer to cryogenic fluids
- Compatible with commonly used tank metals, allowing easy integration with standard aerospace systems
- Improves mission endurance, helping spacecraft retain more usable fuel
- Enables significant mission mass savings and near zero-boil-off capability
Technology
The Parylene-C coating improves the interior surface of a cryogenic tank to makes fuel evaporation less likely to begin. It lowers the boil-off rate by 70% and 65% against the bare surface aluminum 6061 and stainless 304Ltank surface materials in terrestrial conditions, respectively. In metal tanks, tiny natural imperfections on the surface can encourage the formation of vapor due to boil-off, especially when the tank is exposed to heat from the surrounding environment. When these small sites trigger fuel to turn into gas, boil-off increases and usable fuel is lost.
With this new surface treatment, the inside wall surface of the tank becomes more uniform and stable, reducing the likelihood that these early vapor pockets will form. This helps the fuel remain in liquid form longer, even when exposed to the same external heating conditions. The effect is especially valuable in situations where gravity is low, because the movement and behavior of boiling fuel become harder to predict under those conditions.
Testing with very cold liquids showed a consistent pattern: tanks with this improved interior coated surface lost noticeably less fuel than untreated tanks. This trend held true in both: Terrestrial experiments using coated heater elements submerged in liquid nitrogen and Parabolic flight tests at 1–5% Earth gravity performed in reduced-gravity conditions, demonstrating that the coating maintains its performance in realistic mission environments. Results demonstrated dramatically lower heat-flux-driven boil-off, confirming that this coating is a promising enabling technology for zero-boil-off (ZBO) cryogenic storage systems in space, contributing to longer mission endurance.
Brochure
