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Sorption Compressor Assembly Design

The compressor assembly is comprised of the six compressor elements, high-pressure stabilization tanks, the low-pressure stabilization bed, check valves, and refrigerant flow manifolding. A nominal switch time is 667 s, leading to an overall cycle time of 4000 s. This switch time can be varied substantially, depending on the voltage of the electrical power available from the spacecraft, and to compensate for changes in the instrument cooling requirements and sorption cooler performance over time. The compressor assembly mounts directly onto the heat rejection radiator. This radiator is sized to reject the cooler input power at 270 K +10 K/-20 K. The low heat rejection temperature was selected to ensure precooling of the 4.5 K RAL cooler at less than 19 K. The compressor assembly mass is 40 kg. The volume of a sorption compressor assembly is 0.25 m x 0.8 m x 0.8 m. The engineering breadboard compressor is shown below, during assembly.

Engineering Breadboard Compressor
The engineering breadboard compressor pictured here was used to identify design issues and to verify the performance of the cooler and it's components. Mike Schmelzel is shown assembling this compressor. (Image credit: Fig. 2 from Pearson et al., 2007)

A single compressor element is comprised of two concentric cylinders closed with end caps. The inner of these tubes contains the La1.0Ni4.78Sn0.22 hydride material and the outer forms a vacuum jacket around the inner cylinder. This vacuum jacket is used as a gas-gap heat switch. The hydrogen gas for the gas-gap heat switch is supplied through a tube penetrating the side of the outer cylinder. The heater passes through the hydride material and is designed to uniformly distribute heat to ensure a high degree of temperature uniformity when at the maximum temperature of 465 K. Heat transfer to the hydride is provided by aluminum foam that fills the inner cylinder and makes tight contact to the heater. The foam is 89% empty, and is cut to allow penetration by the various other components, which are located in the inner cylinder. A vent tube passes through the center of the hydride material. This vent tube is made of sintered 316 stainless steel, and has a sub-micron porosity, which excludes the powdered hydride from the hydrogen gas flow.

A heater, thermocouple, and vent-tube lead run from the end cap of the inner cylinder to that of the outer cylinder. Each has a small bend for stress reduction, which defeats the potential for any of these elements to carry load. Therefore the only elements taking significant load are those designed to be structural, load-carrying elements. There are two of these structural end supports: one at either end of the tube assembly. At one end provision is made for thermal compliance, without strain, between the inner and outer tubes. The outer tube assembly is primarily fabricated of 6061-T6 aluminum. This outer tube also provides the primary structural attachment point for the single compressor bed. Nearly all parts of the compressors and the gas-handling system which come in contact with hydrogen are made of 316L vacuum arc remelt (VAR) stainless steel which has been electropolished on the surfaces exposed to the hydrogen. This choice of material also serves to prevent the degradation of the compressors structural parts by reaction with the hydrogen. There are two components which are made of other materials. The aluminum foam, which provides heat conduction to the hydride in the compressor, and the seals of the check valves, which are made of Viton. Stringent cleaning and assembly methods are used during construction.

The space between the inner compressor vessel and the outer tube assembly is used as a gas-gap thermal switch. It is preferred that this switch be operated in a closed cycle using a hydride to pump the gap to approximately 0.01 Torr when "off" and to approximately 10 Torr when "on".[7]
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