NASA/Goddard Space Flight Center is pursuing the development of 3 classes of cryocoolers to support NASA scientific missions. These classes of coolers are called "large coolers", "miniature coolers" and "vibration free coolers".
Large coolers supply a relatively large quantity of cooling power in the 20 to 150 Kelvin temperature range. This class of cooler includes 3 versions of the Ball Aerospace multi-stage Stirling cycle cooler and the low cost Sunpower M77 cooler. The miniature cooler is designed to support the operation of a low power focal plane assembly in the 60 to 150 Kelvin temperature range. The miniature cooler will have a mass of nominally 1 kg and an input power of 10 to 20 watts, depending on the cooling power requirement. The vibration free cooler is designed to support instruments whose performance would be degraded by the residual vibration from linear coolers. Because of the extremely high efficiency of the turbo-Brayton cooler in the 4 to 10 Kelvin temperature range, payloads requiring cryogenic cooling at 10 Kelvin or below may choose to use this vibration free cooler technology. There are presently three versions of the miniature turbo-Brayton cooler under development by Creare, Inc., covering the temperature range from 5 Kelvin to 65 Kelvin or above.
There are 3 versions of the Ball Aerospace multi-stage Stirling cycle cooler. The two-stage version, capable of producing 0.45 watts at 30 Kelvin for 70 watts of input power, has been qualified for flight. A protoflight model two-stage cooler has completed 9 months of an on-going life test at Goddard (as of 3/98). A three-stage version of the Ball cooler is being developed by the Air Force Research Laboratory. The three-stage version is designed to cool two separate focal plane assemblies at two separate temperatures. Specifically, it provides 0.5 watts at 35 Kelvin plus 0.5 watts at 65 Kelvin for 75 watts of input power. It is scheduled to be qualified for flight by June, 1998. Ball has also developed a single-stage version of this cooler that will be capable of up to 2 watts of cooling power at 65 Kelvin. This version of the cooler is being fabricated for the EOS-PM HIRDLS instrument.
The Sunpower M77 cooler is a low cost, commercial Stirling cycle cooler. Goddard has procured 10 of these coolers that were designed to allow active vibration cancellation and to survive launch conditions. While Sunpower is still selling the M77 cooler as field test units, one of the coolers delivered to Goddard has been flight qualified. It has survived a 15 grms random vibration test; it has been thermally cycled; it has operated over the temperature range from less than -30°C to +50°C; and it has operated 1.5 years since the last servicing of the cooler, with no degradation in performance.
The Sunpower M77 cooler uses gas bearings to support the piston and displacer. Goddard has used very sensitive instrumentation in an attempt to detect rubbing contact within the cooler and has not detected any signs of touch contact under proper operating conditions. See attached sheet for further information.
The miniature long life cooler is intended to cool a low power detector assembly to temperatures as low as 60 Kelvin. It is being developed by lockheed-Martin. It is designed to produce minimum impact on the mission by having low mass and low input power, as well as being relatively inexpensive. Specifically, it will have a net mass of approximately 1 kg and an input power of 10 to 20 watts or less. It is designed to produce 0.3 watts of cooling at 65 Kelvin for approximately 12 watts of input power. It will produce up to 1 watt of cooling power at 80 K for approximately 20 watts of input power. It will also have low residual vibration. The cooler is being designed from the beginning as a low cost cooler for both the commercial and space market. This cooler development is at an early stage, with a flight cooler expected to be available in 2000 or 2001.
Goddard Space Flight Center is contracting with Lockheed-Martin for the development of the cooler but also has a parallel "in-house" effort to develop a new set of low cost, low mass electronics to operate the cooler. The present generation of coolers have flight electronics that would dwarf the mass and volume of the miniature cooler. Goddard has recently demonstrated a concept for electronics to both power a miniature cooler and to provide vibration compensation. This new cooler electronics configuration is well matched to the miniature cooler in that the electronics provide an order of magnitude reduction in the mass and volume of the electronics required to drive a low vibration cooler. Furthermore, it should be possible to produce the new cooler electronics at low cost. Therefore, the electronics are being developed for both the commercial and the space market.
Goddard has been pursuing the development of the miniature turbo-Brayton technology for over a decade. TurboBrayton cooler technology has many excellent features, including essentially vibration free operation, large cooling power per unit mass and volume, high thermodynamic efficiency at low temperatures, and ease of integration. Unfortunately, it has historically not been possible to adequately miniaturize the technology for use in space. That is, only very large cooling capacity coolers could be produced.
Recent technical breakthroughs by Creare, Inc. now enable this technology to be miniaturized while maintaining high thermodynamic efficiency. The cooler consists of three major components, a compressor, a counterflow heat exchanger and a turboalternator (expander). The compressor and the turboalternator use identical technology, namely high speed miniature turbines supported by gas bearings. The compressor turbine typically rotates at up to 1,000,000 RPM. Creare uses specialized robotic electron discharge milling machines to micro-machine the tiny turbines with high efficiency aerodynamic surfaces.
The use of an efficient counterflow heat exchanger, instead of a regenerative heat exchanger as required by Stirling cycle coolers and pulse tube coolers, enables reverse Brayton cycle coolers to achieve extraordinary thermodynamic efficiency in the 4 to 10 Kelvin temperature range. It is the high thermodynamic efficiency of the turboBrayton cooler, along with its very small mass and size, that make this cooler technology so appealing for space missions with detectors and other hardware operating at or below 10 Kelvin.