Phase II Final Report - NASA's Institute for Advanced Concepts

Chapter 3.0 Vehicle Design
3.5 Fuel Storage and Production
can use more than 80% of its envelope volume. This increase in usable storage compared to a
conventional cylindrical tank is shown in Figure 3-150.
0.85
0.8
0.75
0.7
0.65
0.6
0.55
Cylindricl Tank
Conformal Tank
0.5
0.45
0.4
1 1.5 2 2.5 3 3.5 4 4.5 5
Space Envelope Aspect Ratio
Figure 3-150: Effect of Conformal Tanks on Available Space Utilization
[91]
The ability to construct a lightweight, high-pressure conformal tank is presently being investigated
by Thiokol Propulsion under a Department of Energy contract. [91]
3.5.3.1.1.2 Metal Hydride
Metal hydrides are metallic alloys that absorb hydrogen. These alloys can be used as a storage
mechanism because of their ability to not only absorb hydrogen but also release it. The release
of hydrogen is directly related to the temperature of the hydride. Typically, metal hydrides can
hold hydrogen equal to approximately 1% to 2% of their weight. If active heating is supplied to
remove the hydrogen this can increase to 5% to 7% of the hydride weight. If the temperature is
held constant, the hydrogen is released at a constant pressure. The metal hydride tank can be
used repeatedly to store and release hydrogen. The limiting factor on its ability to store hydrogen
is the accumulation of impurities within the tank. These impurities fill the spaces that would normally
store hydrogen, thereby reducing tank capacity.
The key trade off to utilizing a metal hydride storage system is whether there is sufficient heatgeneration
capability to extract the hydrogen from the hydride. The heat available to the hydride
must also address the inefficiencies associated with the heat transfer device used to move the
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