Prepared by Steve Castles, NASA Goddard Space Flight Center, June 7, 1993.
The Ball two-stage Stirling cooler has turned out to be much more than expected. The entire cooler community, including the Goddard cooler group, assumed that a single-stage Stirling cooler would be available for flight before a two-stage machine. It now appears that the Ball cooler will become the first cooler ready for flight with clearance seals and therefore with a high probability of achieving long life. This cooler has been develop under contract to GSFC under an OACT program called the 30K cooler program. It is anticipated that OACT will fund the remaining effort on the Ball cooler in FY95.
The Air Force is sufficiently impressed with the Ball technology demonstration model cooler that last winter they started planning a follow-on to the Ball two-stage Stirling cooler. This program would develop a single-stage version of the cooler using the same compressor being developed for the two-stage cooler. As an alternative, the Air Force is recently giving consideration to funding Ball for a follow-on two-stage Stirling cooler. The follow-on two-stage cooler would have a new regenerator optimized for higher temperatures than the 30K machine. This follow-on effort is predicated on the continuing success of the 30K cooler program.
Goddard, Ball and other cooler experts have recently become convinced that two-stage coolers have several inherent advantages over single-stage machines. First, the two-stage machine is more efficient at all temperatures, even for higher temperatures that are within the normal temperature range of a single-stage machine. Second, many payloads have designed for a thermal shield around the cold stage to reduce the thermal radiation on the detectors and to cool low noise pre-amplifiers. A two-stage machine can easily and efficiently provide cooling at the warm-stage (first-stage) for such a shield while still providing adequate cooling for the detectors with the cold-stage (second-stage). As a result of recent analyses and tests performed by Goddard and Ball, these possibilities are starting to be taken seriously.
The Ball two-stage cooler may be the only cooler with demonstrated clearance seals available for flight for a number of years. Therefore it may turn out that the Ball two-stage Stirling cooler will become the " workhorse" aerospace cooler.
The most difficult technical problems with long life mechanical coolers are in the compressor. Therefore, while concentrating on the development of the Oxford cooler technology, the Goddard cooler group has also been working with companies to develop improved compressor technologies. This effort was carried out via SBIRs and OACT " Technology Infusion" money. We believe that one company is making good progress on new, low cost compressor technology.
The advantage of the new compressor technology is that it greatly reduces the alignment tolerances and most piece part tolerances. A low vibration version of the new compressor technology has not yet been fabricated. In fact, low vibration tactical/commercial coolers are not available from any company, regardless of compressor technology. We are presently working with two companies to produce the first low-vibration version of their respective coolers. The company with the most promising compressor technology does not have adequate expertise to develop a low vibration cooler by themselves so Goddard is producing the vibration control electronics and has specified the required alterations that the company must make to their commercial cooler concept to allow us to adapt our vibration compensation system.
With this new compressor technology, it may be possible to decrease the cost of a space-based cooler by an order of magnitude from its present cost of nominally $1,000K per cooler. It should also be possible to produce long life ground-based coolers at much lower cost. To accomplish the goal of a low cost, long life cooler, both the cooler technology must be demonstrated to be viable and the cost of the electronics to control the cooler must be made much more simple. The low cost cooler development program that Goddard and Ames are proposing could accomplish these goals by 1999.
A second advantage of the new compressor technology is that it should be possible to reduce the mass from about 14 kg to about 5 kg. The reduction in volume is even more dramatic in that the cooler itself is the size of a 16 oz can. Obviously, the combination of a miniature cooler, coupled with low cost, makes this cooler important to the small spacecraft missions such as Earth Probes and Small Explorers. These attributes will also be important to the commercial space market, both the communications satellite market and the earth sensing market.
The new compressor technology is the only viable technology that the Goddard cryocooler group has been able to identify that has a good chance of resulting in a low cost, long life cooler. Both low cost and long life are required to meet the needs of the commercial ground-based and space-based cooler market. The commercial market for mechanical coolers has recently received great attention. The push is coming from both the communications industry and the digital electronics industry. Two ARPA TRPs were released last year for the development of high temperature superconducting microwave communication products. I am working with Kul Bhasin at LeRC to stay abreast of his TRP; I am working with the people at Philips Labs to stay abreast of the other TRP; and I have been in contact with numerous companies involved in microwave communications. Relative to cryogenic electronics, a new Cryoelectronics subdivision of the Emerging Technology Division of the Electronic Industries Association (EIA) has just been chartered. Its members include semiconductor chip, computer, instrumentation, medical instrumentation and cryocooler manufacturers. A number of companies are already pursuing high speed computing applications.
I am working with ARPA, NRL and NIST to have a new topic added to the next TRP cycle. The new topic will assist commercialization of cooler applications and will include any system requiring cryogenic temperatures. However, it will probably be dominated by electronics and communications applications. As mentioned above, there are space-based commercial applications as well as ground-based applications, namely microwave communications and remote sensing.
It is beyond the scope of this note to provide extensive technical details about the potential applications for commercial coolers. Let me just say that I believe that enormous commercial markets will begin using commercial coolers as soon as coolers can be produced with long life, low vibration and at low cost.
Goddard and Ames Research Center are proposing a joint program for the development of Low Cost, Dual Use Coolers. Goddard would work with industry to develop a low cost, long life compressor and vibration control electronics. I believe that Goddard is presently the world leader in low cost vibration control systems. We are confident that a vibration control system adequate for most commercial and scientific applications can be produced at low cost. The long life, low cost compressor is still to be demonstrated but the new compressor technology appears to be very promising.
The low cost and high reliability of the pulse tube make it a good candidate to mate with a low cost, long live compressor. ARC would develop a pulse tube commensurate with the compressor developed by Goddard. The commercial cooler industry is presently pursuing passive (un-driven) displacer Stirling cooler configurations so the efficiency of the pulse tube configuration could be directly compared to the efficiency of the commercial, passive displacer Stirling configuration. The cost and reliability benefits of the pulse tube can then be weighed against potential lower efficiency.
Goddard has been receiving funds from SDIO/BMDO (through the Air Force) to fund Creare, Inc. to develop the reverse Brayton cycle turbo-cooler. This cooler uses small high speed turbines with self adjusting gas bearings.
Creare has just finished the first technology demonstration model cooler. It produced 5W of cooling at 65K with 215W input power. This cooler is still too large for space-based applications. Goddard has an on-going Phase 2 SBIR with Creare to develop a miniature turbo-alternator which is progressing very well. With this turbo-alternator, a small single-stage cooler with 100W input power should be possible. This cooler should be capable of reaching 40 K. A two-stage version of this cooler can reach 10 K and can serve as the upper-stage for a magnetic cooler presently under development at Ames. The Ames magnetic cooler is an extremely efficient cooler operating between 2K and 10K. This vibration free 2K cooler would meet the needs of missions such as SIRTF and SMIM that require liquid helium temperatures. Also, the upper-stage could be used to provide cooling at intermediate temperatures for missions that require higher temperature vibration free coolers. Such missions include next generation HST instruments and space-based interferometers to search for planets around nearby stars.
Working with industry, Goddard will develop a vibration free cooler that will be capable of operating at any temperature from 100K down to 10K. This cooler will serve as the upper-stage to a magnetic cooling stage to be developed by Ames. The magnetic cooler would provide at least 25 mW of cooling at 2K.
Steve Castles Head, Code 713