The current state-of-the-art in liquid cryogen storage technology is the vacuum jacketed dewar with multi-layer insulation (MLI) in between the inner and outer shells to slow the rate of radiation heat transfer. While shell materials typically consist of thin gauge stainless steel and aluminuam, the mass fraction of cryogen to tank ranges from 6% for 1 kg tanks to 15% for 8 kg tanks . Other storage technologies such as high pressure room temperature gas and metal hydrides provide gravimetric storage of up to 6% and 4%, respectively. The performance of current cryogen storage technologies is insufficient for small scale (<100 kg) flight applications, and thus requires a new evaluation of recent advances in materials and manufacturing technology that can be adopted into novel cryogen storage solutions.
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Hydrogen has long been viewed as a competitive energy carrier in aerospace applications due to its high specific energy that is greater than any other fuel currently in use (e.g. 2.8x kerosene). Because mass is a primary design constraint for airborne vehicles, liquid hydrogen was the fuel of choice for early aircraft such as the Lockheed CL-400 Suntan and modern space plane concepts such as NASA’s National Aerospace Plane and the X-33 sub-orbital space plane. Unmanned aerial vehicles (UAV) are sophisticated platforms for conducting remote observation of weather phenomena, agricultural and forestry health monitoring, and fire tracking and observation. The UAV sector of NASA’s Aeronautics Research Mission Directorate has identified the need for electric power and propulsion development along with development of lightweight, long-life cryogenic propellant research . Hurricane monitoring is of great importance to NASA which concluded that the multi-day flight duration afforded by PEM fuel cell and liquid hydrogen propulsion systems were compelling reasons to pursue development of these respective technologies for both heavier-than-air (HTA) and lighter-than-air (LTA) high altitude long endurance (HALE) vehicles .
While hydrogen is directly oxidized by combustion in most aerospace propulsion applications, its utility is expanded to other power generation sources such as proton exchange membrane (PEM) fuel cells. Additional benefits of hydrogen include non-toxicity, a fast dispersion rate that mitigates risk posed due to leaks, and multiple generation paths. These paths include both renewable and non-renewable sources including steam methane reformation, electrolysis, and biomass through gasification. However, engineering complications related to liquid hydrogen storage must be solved to achieve higher tank system gravimetric capacity, specific energy, and energy density than current storage technologies provide.
Due to the continuous demand of hydrogen gas for a fuel cell powered UAV, a higher boil-off rate compared to storage dewars is acceptable, permitting an increase in tank heat flux and a commensurate reduction in insulation. Advanced cooling systems for space satellites use the sensible heat from boiled cryogen vapor to cool radiation shields and decrease boil-off from the liquid hydrogen tank by a factor of 6.5 . Therefore, a further reduction in insulation mass can be achieved by integrating vapor cooling into the sidewalls of the UAV fuel tank. By using low density polymers in lieu of metal, additional mass reduction is possible due to the low pressure of liquid hydrogen storage tanks (~30 psig) that do not require the higher tensile strength of metals. The low thermal conductivity of polymers, 20x less than stainless steel and 200x less than aluminum at 20K, provides an added benefit in terms of increased thermal performance.
The objective of this innovative research is to demonstrate the cyrogenic compatibility of 3D printed polymers that will not only benefit liquid hydrogen fueling solutions for UAVs, but also the manufacture of complex parts for medical devices and spacecraft components that require operation and strength in cryogenic environments. We will demonstrate this potential by building a small (5 L) liquid hydrogen tank for a UAV that incorporates vapor cooled channels into the tank walls to reduce heat load into the cryogen. The cold hydrogen vapor is then warmed to near ambient temperature as it moves along the outer tank walls. Integrating the heat exchanger eliminates the need for a discrete unit, which would otherwise reduce key performance metrics such as gravimetric capacity and specific energy of the storage system. We use selective laser sintering (SLS) of nylon blends to create the complex geometry necessary, along with aerogel based insulation materials, to achieve the required heat flux to maintain a boil-off rate sufficient for nominal cruise velocity. Power requirements above that necessary for nominal cruise engage a 10W heater in the tank to increase the boil-off rate.
 Daugherty, M. A. et al., 1996, A Comparison of Hydrogen Vehicle Storage Options Using the EPA Urban Driving Schedule, Advances in Cryogenic Engineering, Vol. 41
 NASA Civil UAV Assessment Team, 2006, Earth Observations and the Role of UAVs: A Capabilities Assessment, NASA version 1.1, www.nasa.gov/centers/dryden/pdf/175939main_Earth_Obs_UAV_Vol_1_v1.1_Final.pdf
 Nickol, C. L. et al., 2007, High Altitude Long Endurance UAV Analysis of Alternatives and Technology Requirements Development, NASA/TP-2007-214861
 Muratov, C. B., Osipov, V. V., and Smelyanskiy, V. N., 2011, Issues of Long-Term Cryogenic Propellant Storage in Microgravity, NASA/TM-2011-215988