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NASA Space

Instrument and Sensing Technology

GODDARD SPACE FLIGHT CENTER
CRYOCOOLERS
PROGRESS REPORT

July - September, 1995

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Two-Stage Stirling Cooler
Program Goals and Objectives

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:

• Long life: 5 years minimum; 10 year goal
• High reliability: 0.97 for 5 years
• 1000 start/stop cycles over mission life
• Low vibration: <0.05 lb force maximum; 0.02 lbf goal
• High efficiency: < 75 watts input power
• 0.3 watts of cooling power at 30 K
• Low weight: less than 15 kg
• Minimal size
• Simple interfaces
• Integration flexibility
• Designed to withstand Titan 4 or Shuttle loads
• Maximum cold finger displacement during operation of 0.001 inches
• 0.1 K temperature stability over a 24 hour period 28 volt dc input power
• Electromagnetic compatibility with EOS (GISS)
• Minimum magnetic signature
• Soft mounting cannot be used to meet vibration
• Cryocooler must operate in any orientation in a zero and 1-G environment.

Highlights of the Past Quarter:

The fabrication of the engineering model two-stage Stirling cooler continued at Ball Aerospace during the fourth quarter of FY95. The compressor has been assembled and operated. Parts for the expander are fabricated and final assembly is now in process and should be completed by the end of November. The boards for the engineering model electronics have been fabricated. The assembly of the electronics has recently commenced and should also be completed in November. The electronics are flight-like electronics except for the fact that they have commercial parts (each of which has a pin-equivalent flight part). The electronics box will be vibrated and will go through thermal/vacuum testing.

The concurrent build of the Air Force engineering model cooler is progressing well. The compressor has been assembled; the electronics boards have been fabricated; and the expander piece parts are being fabricated. A test set up for the new cold finger required for the Air Force cooler is being fabricated. Cold finger tests are expected to require at least 6 months and are the schedule critical item on the Air Force cooler.

Tests performed at Goddard on the Technology Demonstration model cooler and calculations performed by Ball both indicate that the Ball cooler can be used to cover a wide range of applications. The Technology Demonstration model was used in a test at Berkeley with the detector system proposed for HESSI. No degradation in performance of the HESSI detector was observed from either EMI or microphonics. Results were presented at the Cryogenic Engineering Conference in July. Based on the success to date with this cooler, several programs have expressed interest in using the Ball cooler.

Goddard continues to participate with industry in an attempt to produce a long life space and commercial (dual use) cryocooler. A consortium of Lockheed, Superconducting Technologies Inc. (STI), NIST, and NASA/Goddard is attempting to develop and test an inexpensive commercial cooler and mate it to a high temperature superconducting microwave filter for cellular telephone applications. This effort is funded through the Code X AITP program. A series of meetings have been held with the AITP members to determine the performance requirements for the cooler. Through these and other meetings, it has become obvious that to satisfy industry requirements, a commercial cooler must meet requirements that are equal in severity to the requirements of a space-based cooler. Goddard is actively working with industry to produce a cooler to meet these requirements.

Goal:

• Develop a long life, highly reliable two-stage Stirling cooler for use by Mission to Planet Earth and other space-based applications.

Approach:

• Contract with industry to develop the cooler, supported by Goddard- developed vibration control system and Goddard functional, environmental and life testing of the cooler.

Background:

• Two-stage Stirling coolers obtain lower temperatures than single-stage coolers and provide higher thermodynamic performance. They also offer the capability to cool multiple focal planes with a single cooler.

Status of Ball engineering model two-stage cooler:

• The assembly of the compressor is complete.

• The boards for the electronics have been fabricated. A flight box from another flight program will be used. The box provides a compact (3 inch x 8 inch x 8 inch) package.

• The assembly of the displacer is in process and will be completed next quarter. Approximately 3 months of functional and environmental tests are planned at Ball before the cooler is delivered to Goddard. Goddard will then perform tests that Ball can not economically perform. Specifically, Goddard will perform tests with the cooler heat sink temperature ranging from -30 degrees C to +30 degrees C. Load curves and vibration levels will be measured. Then a life test will be performed.

Goal:

• Develop an inexpensive, long life, highly reliable cryocooler for use in both space and commercial applications.

Approach:

• Work with industry and other government agencies to develop a viable long life commercial cooler. Goddard will provide expertise to industry on long life compressors and on the Goddard-developed vibration control system. Goddard will also provide functional, environmental and life testing of coolers in the existing Cryocooler Test Bed.

Background:

• No low cost cooler has demonstrated the necessary reliability and lifetime. The primary problem is the compressor. The only low cost, long life compressors are oil filled compressors similar to those used in refrigerators. After many years of effort, no organization has yet demonstrated a low cost method of using such a compressor without contaminating the cold portion of a cryogenic cooler. The alternative now being pursued is reciprocating compressors using flexure and gas bearings. Such compressors are being developed for the space program. Concepts to reduce cost while maintaining lifetime and reliability are being explored. These compressors can be used in either passive displacer Stirling coolers or pulse tube coolers.

• A few of the many commercial applications that will be enabled by a low cost, highly reliable cooler with a 5 to 10 year lifetime are listed below:
  • Miniature microwave filters for cellular telephone systems
  • Cooled CMOS to improve workstation and mid-sized computer performance
  • High Q resonators for radar systems. Applications include improved airport safety and military applications
  • High speed switches for digital communications
  • Low vibration cryopumps for fine line silicon wafer fabrication
  • Infrared detectors for law enforcement night vision systems
  • Dermatologist instrument for skin care.
  • Improved filters for communications satellites
  • Cryogenic detectors for improved earth resources satellites and weather satellites.

• An AITP contract has been issued to Lockheed which is working with Superconducting Technology Inc. (STI), NIST and Goddard to develop and demonstrate a long life commercial cooler and integrate it with a high temperature superconducting microwave filter. Lockheed will provide a flexure bearing-based compressor. STI will provide a gas bearing-based compressor (using technology licensed from Sunpower , Inc.) and the microwave filter (which is now an off-the-shelf item). NIST will provide the pulse tube and Goddard will provide testing services. For some time now, Goddard has been working with Sunpower to perfect the Sunpower gas bearing compressor technology for use both in space and for ground-based commercial applications.

Status of Goddard space/commercial cooler development program:

• Two meetings have been held to review the requirements that dictate the pulse tube design for the AITP cooler effort. STI stated that the heat sink for the cooler must be warmer than normal and the cooler must cool the superconducting filter to 70 Kelvin. The Sunpower Stirling cooler that is the predecessor for the STI cooler can provide the required cooling. However, both NIST and Goddard personnel are concerned that the inefficiency of the pulse tube for large temperature changes will not allow a single-stage pulse tube to obtain the necessary cooling. Therefore, both Goddard and NIST are advocating a two-stage pulse tube to increase the efficiency of the cooler at 70 Kelvin. This approach was overruled by STI because of concern about the cost and schedule of a two-stage cooler development. It was agreed to proceed with a single-stage design for the present. Detailed thermodynamic models are being used to guide the effort. Breadboard pulse tubes are being fabricated for preliminary testing.

• In 1994 Goddard procured 8 modified commercial coolers from Sunpower, Inc. The Sunpower coolers use gas bearing to maintain clearance seals. These coolers were modified to provide vibration control and to survive launch vibration. Four of these coolers have been under test for almost a year. Starting in Spring, 1995, these four coolers began to show signs of internal gaseous contamination. Goddard has used specialized gas analysis equipment to identify the contaminants within the cooler working fluid. The contamination is water. This is clearly from inadequate bake-out procedures. Sunpower has improved their bake-out procedures and is producing another batch of coolers. In addition, Sunpower is baking out the 8 Goddard coolers. Goddard will then put the coolers back on life test. The life test will resume next quarter.

  Contamination can also be created by the wearing of plastic bearings. Sunpower uses plastic bearings as back-up bearings to the gas bearings. Fortunately, the contaminants that typically are created by wear of the plastic bearings were not found during the gas analysis tests on the Sunpower cooler. Therefore the Goddard gas analysis results provide some evidence that the Sunpower gas bearing is working properly, at least under the conditions of the tests to date. As explained below, more severe testing is planned on some of the coolers.

  Functional tests of the Sunpower cooler indicate that it can operate at reduced heat sink temperatures. Reduced heat sink temperatures greatly improve the thermodynamic performance of cryogenic coolers. The reduced power requirement (by as much a factor of 4) could be a major benefit to space-based applications. Therefore, when the life tests on the Sunpower coolers are resumed, at least two coolers will be tested at reduced heat sink temperatures. This will be the first life test on any cooler with a reduced heat sink temperature.

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