Cryogenic Mass Gauging in a Free-Falling Storage Tank
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The most efficient shuttle would be an all-cryogenic propellant vehicle. One problem with on-board cryogenic storage is mass gauging. Even though, in principle, one can determine the amount of cryogen in a tank by its history of use, this is not practical. In free-fall, the exact configuration of the liquid and vapor in the tank is not known. Depending on the recent motion of the craft, the liquid could be in small, dispersed drops, a single glob, or some combination of these configurations. The configuration is known only when the vehicle is under significant acceleration, when a standard level meter may be used to detect the single liquid/vapor meniscus. Currently, the shuttle must be accelerated for 15 sec to make an accurate measurement of propellant volume remaining. We have considered various methods for determining the mass of liquid directly, and have pursued a technique that takes advantage of the large difference in specific heat capacity of liquid and gaseous hydrogen, both of which are nearly independent of the vehicle's motion and position with respect to any gravitational field. Small thermometers and heaters have been utilized that are rapidly responding and possess the necessary sensitivity to make a measurement in a 2,500 liter tank. System design criteria, with sensor construction, are discussed. Data on sensor sensitivity is presented.
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