A new concept called "liquid shell insulation" has been proposed as a means of temporary thermal protection for scientific instrument probes that are required to operate for short times in hot, high-pressure environments. Liquid shell insulation was conceived to protect probes that would be dropped from spacecraft to great depths in the atmospheres of the outer planets. For example, at a depth of 1,000 km on Jupiter, a probe would have to withstand a pressure of about 4,000 Earth atmospheres (≈0.4 GPa) and a temperature of about 1,800 K. On Earth, liquid shell insulation might be useful for protecting probes that would be inserted in undersea volcanic vents or deep oil wells.
The liquid shell insulation would be placed outside the pressure-bearing outer wall of a instrument probe; this would be done so that, in addition to the delicate instrumentation within, the outer wall would also be thermally protected. Thermal protection of the outer wall would be necessary to prevent the crippling loss of mechanical strength that would occur if the wall were allowed to be heated to 1,000 K or more.
As its name suggests, liquid shell insulation would be a shell-like structure, with a liquid-filled porous insulation material surrounding the pressure-bearing outer wall (see figure). The basic function of the liquid shell insulation would be to provide a sufficiently long characteristic thermal time so that the protected instruments would not exceed the maximum allowable temperature after the required collection and transmission of data has occurred. In addition to the liquid shell, the probe would be equipped with a streamlined external shell for low drag, so that it could descend to the required depth in a minimum time. The inner surface of the pressure vessel would be lined with a secondary insulating layer consisting of fiberglass with xenon gas filling the interstices; this layer would keep the instruments much cooler than the pressure vessel, which, in turn, would be much cooler than the environment.
The design of the liquid shell must combine the following features in order to function properly in a high-pressure, high-temperature environment:
- Low thermal conductivity to limit heating of the pressure vessel,
- High heat capacity to absorb most of the heat conducted in from the environment,
- Equalization of the pressure in the liquid with the ambient pressure to prevent loading of the thin streamlined outer shell;
- Means to prevent hot ambient gases from entering into the shell and compromising its insulating ability.
According to the current design concept, the liquid shell would be made of a porous solid filled with a liquid. To minimize the transfer of heat through the shell, the design and construction of the porous solid component must minimize spurious convective motion of the fluid and attenuate the radiative transfer of heat energy, while allowing some convective motion because of expansion and venting of the liquid as described below.
Both ammonia and water have been considered for use as the liquid. Both of these fluids have very high specific heat capacities and moderate thermal conductivities. Equally important, in the original planetary-exploration scenario, either of these fluids would slowly expand during atmospheric descent because the expansion due to heating would more than offset the shrinkage due to pressurization. Therefore, the shell would have to be open to the atmosphere through a check valve in order to equalize the pressure across the thin streamlined outer shell. The check valve would not only prevent ambient gas from entering the shell, but would also allow the expanding heated liquid to leave the shell, carrying heat with it. Although the liquid ammonia or water would expand during the atmospheric descent, it would not undergo phase change because the ambient pressure would greatly exceed the critical pressure of either fluid [113 atm (11.4 MPa) for ammonia, 220 atm (22.3 MPa) for water].
If ammonia were used, some of it would dissociate into nitrogen and hydrogen at high temperature. The endothermic dissociation reaction would absorb heat, thereby improving the overall insulating performance of the liquid shell.
This work was done by Jeffery L. Hall of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.
This Brief includes a Technical Support Package (TSP).
Liquid Shell Insulation
(reference NPO-20648) is currently available for download from the TSP library.
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