An improved system for densifying (by cooling) the liquid hydrogen and liquid oxygen used as propellants for the space shuttle has been proposed. These propellant liquids are cooled to minimize the sizes of tanks needed to store them and to reduce maximum operating pressures. The proposed system would densify these liquids 7 to 8 percent more efficiently than does the present propellant-densification system, and would reduce the absolute vapor pressure from about 15 psi (≈0.1 MPa) to about 1.5 psi (≈10 kPa). Sizing analysis indicates that the combination of the increase in density and the decrease in vapor pressure would reduce the weight of the space shuttle by 10 to 20 percent. Similar reductions in the required volumes and weights of tanks for storing cryogenic liquids could be beneficial in any industry in which the sizes of such tanks are cost factors.

Figure 1. The Tank Recirculation Subsystem would continuously circulate the warmer cryogenic propellant liquid from the tank along with a supercooled stream of the same liquid.

Densification of the space-shuttle propellant liquids is done during stable replenishing operations. Heretofore, these liquids have been cooled through surface-evaporation heat transfer and convective mixing. This method of cooling, while simple, is time-consuming and constrains the tank pressure to one atmosphere (≈0.1 MPa). The proposed system - a product of research at Rockwell International - would supercool the liquids to lower temperatures and vapor pressures than does the present system, and in a fraction of the time. An added advantage of the proposed system is that a vent valve would be relocated from the flight vehicle to the ground, with consequent further reduction in vehicle weight and simplification of design.

Figure 2. The GSE Cooling Unit would vent heat and keep the propellant liquid subcooled. The GSE unit could be designed according to a heat-exchanger-bath concept or a thermodynamic-vent principle.

The proposed system (more precisely, a pair of systems - one for each liquid) would comprise two subsystems: (1) a tank recirculation subsystem (see Figure 1), which would continuously recirculate the initially warm liquid in the affected propellant tanks with subcooled liquid; and (2) a ground support equipment (GSE) cooling unit (see Figure 2), which would vent heat and keep the liquid subcooled. The GSE unit could be designed in one of two ways: as a simple heat exchanger or based on the thermodynamic-vent principle. The heat-exchanger design would involve use of a liquid bath as a boiling liquid medium. In the thermodynamic-vent version, a fraction of the recirculation fluid would be expanded to a lower pressure without changing its internal heat content. This throttling of the liquid, or moving from high pressure to low pressure, would cause the liquid to flash to a low temperature. The flashing, in turn, would cool the recirculating fluid. The low pressure on the colder side of the heat exchanger would be maintained by a compressor/blower unit, which would reject the vented gas from the low pressure to ambient pressure.

The GSE design would be determined by the thermodynamic properties of the liquid being recirculated and by cost constraints. The heat-exchanger GSE design, while less effective, would be less costly to build, especially for a fixed-structure system like the space shuttle. The thermodynamic-vent GSE design would be most beneficial in a new system because the vehicle could be designed according to the reduced volume and weight requirements associated with the improved cooling system.

This work was done by Tibor I. Lak, Steve P. Petrilla, and Martin E. Lozano of Rockwell International for Johnson Space Center.

Title to this invention has been waived under the provisions of the National Aeronautics and Space Act {42 USC 2457 (f)}, to The Boeing Co. Inquiries concerning licenses for its commercial development should be addressed to

Danielle Bartoli
The Boeing Co.
Canoga Park, CA 91309-7922
Tel. No. (818) 586-1367

Refer to MSC-22723

NASA Tech Briefs Magazine

This article first appeared in the November, 1999 issue of NASA Tech Briefs Magazine.

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