This program will result in an advanced engineering model of a two-stage linear Stirling cycle cooler for use by instruments on the Earth Observing System (EOS). However, the cooler will be of use to many other NASA programs in earth science, astronomy, microgravity sciences, interplanetary sciences and the Human Exploration Initiative.
The cooler must have long life, high reliability and low vibration, as well as being small, light weight, and efficient. The key cooler specifications are as follows:
The two-stage Stirling coolers from Ball and Creare have been functionally tested at Goddard. Both appear to perform adequately thermodynamically and have characteristics that should ensure long life in a flight model. The Creare diaphragm cooler provides just under 0.3 W of cooling at 30 Kelvin, which was the specification, while the Ball flexure-bearing cooler exceeds the specification be more than 50%. This Ball cryocooler produced 0.45W of cooling at 30 Kelvin at a total input power of 95 W. This 95 W input power includes 20 W of losses associated with breadboard electronics that should be eliminated in the engineering model electronics. (The design of the engineering model electronics have been completed.) The overall efficiency of the Ball cooler at its design point is 50% more efficient than any other cooler to date.
The Ball cooler weighs less than 15 kg while the Creare diaphragm cooler weighs over 25 kg. The Ball compressor incorporated radial position sensors which have provided proof that the compressor has non-contacting bearings. This feature is unique to the Ball cooler. It essentially guarantees that the cooler will not " wear out" . The Creare diaphragm cooler also should have long life. However, because of limited funds and the superior weight and power performance of the Ball cooler, the Creare cooler development has been halted. The Ball cooler development is proceeding and completion of the engineering model is expected during the summer of 1995.
The Ball two-stage Stirling cooler, which has historically been called the " 30K cooler" , has turned out to be much more than was expected. Everyone, including Goddard personnel, believed that a single-stage Stirling cooler would be ready for flight before a two-stage machine. It now appears that the Ball two-stage cooler will be the first cooler ready for flight with true clearance seals (and therefore with high probability of long life).
Goddard has recently completed a set of performance tests with the technology demonstration model cooler. These tests indicate that the Ball cooler has excellent thermodynamic performance even in the 55 - 80 Kelvin temperature range. Thus, the Ball cooler could be useful to a wide variety of programs.
The goal of this program is to develop long life miniature coolers. Users include the instruments flying on small spacecraft such as the Small Explorer series, the Earth Probe series and Small Spacecraft Technology Initiative (SSTI) satellites. Additional small spacecraft series are expected in the future.
A second goal of the program is to produce miniature coolers at low enough cost that they could be used commercially. Commercial coolers require long life times similar to those of the aerospace coolers. They must be miniature and have low vibration as well. Miniature coolers are an obvious example of a " dual use" technology. The first use of these coolers might be on small commercial satellites for earth sensing, weather forecasting and wireless communications. However, if the cost of mass produced coolers could be reduced sufficiently, ground based, mass markets include low vibration coolers for cryopumps on silicon chip production lines, microwave communications applications and cooling of digital electronics.
The objective is a light weight, low cost cooler which will meet the needs of both the small satellite and ground-based commercial cooler markets. The existence of such a cooler would allow major advances in several aerospace areas, including advanced weather satellite instruments, earth resource remote sensing, military applications and possibly space-based microwave communication applications. Goddard is also consulting with a variety of companies and other government laboratories on potential ground-based applications. These include low vibration coolers for cryopumps on silicon chip production lines, microwave communications applications and cooling of digital electronics.