The use of slush hydrogen (SLH) at the triple-point pressure (7.042 kPa or 1.02 psia) provides several advantages and several disadvantages compared with liquid hydro-gen.129 On the positive side, a 50% solid mass fraction in slush has 15% greater density and 18% greater heat capacity than normal boiling point liquid hydrogen. The increase in density allows a more compact tank, and the increased heat capacity reduces the amount of venting required for a given heat leak. Slush also has the advantage that the solid mass fraction can be allowed to vary to accommodate heat leaks without venting. During storage, heat leaks cause some of the solid in the slush to melt, reducing the solid mass fraction (some of the vapor condenses as well to maintain the tank at the triple-point pressure). However, it is not clear how to perform vapor extraction with vapor at 1 psi. A thermodynamic vent system, with its Joule-Thomson expansion device, might become clogged by the solid-hydrogen particles in the slush.
While the vapor extraction problem may possibly be surmountable, the low vapor pressure of slush hydrogen (the triple-point pressure) presents the same problems in compressor design as for the solid storage concept. In addition, the ground handling of slush is complicated by the fact that the volume of the slush increases as heat enters and the solid melts. As heat leaks into the slush, some of the solid will melt and the overall volume of the slush will increase. If left unchecked the volume of the
129 "Long-term hydrogen storage and delivery for low-thrust space propulsion systems,'' Paul J. Mueller, Brian G. Williams, and J. C. Batty (all of Utah State University, Logan), 30th ASME, SAE, and ASEE Joint Propulsion Conference and Exhibition, Indianapolis, IN, June 2729, 1994, AIAA 1994-3025; Benefits of Slush Hydrogen for Space Missions, Alan Friedlander, Robert Zubrin, and Terry L. Hardy, NASA TM 104503, October 1991; "Technology Issues Associated with Using Densified Hydrogen for Space Vehicles,'' Terry L. Hardy and Margaret V. Whalen, AIAA/SAE/ASME/ASEE 28th Joint Propulsion Conference. Nashville, TN, July 1992, NASA TM 105642, AIAA 92-3079.
slush will exceed the volume of the tank and it will come out the vent. If the slush melted completely it would occupy the volume of the same mass of liquid. Making the tank large enough to accommodate this (or only filling it partially) negates the density/volume savings of using slush in the first place. Thus, slush conditioning will be required on the pad to maintain adequate solid fractions as heat leaks into the tank. Many of the ground-handling concerns were addressed in the National Aerospace Place (NASP) design effort, but developing the infrastructure for slush handling at launch facilities will be very expensive.
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