Cryocoolers to Monitor Global Surface Temperatures from Space

by Stuart Watson, Airbus Defence and Space, stuart.watson@astrium.eads.net and Dr. Adam L. Camilletti, Airbus Defence and Space, adam.camilletti@astrium.eads.net

The Cryocooler Development and Engineering Group at Airbus Defence and Space, formerly known as Astrium, has delivered the first of two space cryocooler systems, scheduled to launch in mid-2015 on the European Space Agency’s twin Sentinel 3 satellites. Post-delivery acceptance testing and integration is now underway. The UK-based team have been working on the project for more than five years. These are the first new European systems of this nature for over a decade.

The Sentinel 3 satellites are part of the European Space Agency’s (ESA) Copernicus program for global monitoring for environmental and security purposes. The spacecraft, manufactured by Thales-Alenia Space France (TAS-F) are tasked with measuring sea surface topography, sea and land surface temperature and color with high accuracy and reliability. To do this, it is equipped with two instruments. To perform the temperature measurements, the Sea Land Surface Temperature Radiometer (SLSTR) is fitted.

The SLSTR, being produced by Selex ES, Italy, is capable of measuring surface temperatures to an accuracy of less than 0.3K at a resolution of down to 500 m. To achieve this, the instrument’s optical assembly and sensors must be cooled to below 85K. This, combined with a mission life of more than 7.5 years, is where the need for a mechanical cryocooler system (CCS) comes from.

The Airbus DS CCS can be broken down into three distinct sections:

Mechanical Cryocoolers: Each CCS features two 50-80K Stirling coolers. This workhorse of space mechanical cooling is Airbus DS’s core recurrent cryocooler product and it has a strong heritage. The unit can trace its development back to the original concept design at Oxford University in the 1980s, where the flexure bearings it employs were invented. In total, over sixty 50-80K Stirling coolers and its derivatives have been produced to space flight quality standards by Airbus DS. These units have accumulated over 145 years of in-orbit service, with several individual units having passed the 12-year mark.

The 50-80K Stirling cooler is of split-cycle configuration, with a 300 mm transfer line linking the compressor and displacer assemblies. The linear compressor is of typical Oxford-type design, with a moving coil supported between two sets of flexure bearings, or diaphragm springs as they are also known. These bearings see more than 10 billion deflections during life and therefore special precautions must be taken to prevent the possibility of fatigue failures. These include photochemical etching rather than conventional machining and individual part inspection.

Mechanically, the displacer is of the same design as the compressor but on a smaller scale, reflecting its reduced power consumption. Dynamic gas seals are of the clearance type, with clearances on the order of five microns made possible by the flexure bearings’ high radial stiffness. Position monitoring is provided by linear variable differential transformers and the working fluid is helium-4.

Cryocooler Assembly: If the mechanical cryocoolers are the system’s engine, then the cryocooler assembly (CCA) can be viewed as the chassis and all ancillary equipment. The structure comprises a series of match machined brackets. These have to balance several sometimes-conflicting requirements: accurately locating the cold fingers within the instrument half a meter away from the CCS’s baseplate mounting; providing an adequate thermal path between the compressors and heat pipe interfaces; mechanically supporting the coolers during vibration and coming within the mass budget. Consequently, a range of aluminum alloys and titanium have been used, with each bracket individually assessed based upon its specific thermal and structural needs.

Linking the 50-80K Stirling coolers and the instrument are two thermal link assemblies, supplied by Space Dynamics Lab, Utah. These were specially developed for the project from Space Dynamics’ existing range, and are of the aluminium foil type. Cold tip monitoring is achieved through use of Cernox sensors.

Due to its close proximity to the instrument, a tight exported vibration limit was placed on the CCS. This has been achieved through pairing the mechanical coolers’ mechanisms and placing them in opposed orientations. Active control is provided through the use of Kistler 8203A50 piezoelectric accelerometers, which provide feedback to the drive electronics. These were specially space-qualified for this system.

Cooler Drive Electronics (CDE): The CCS is controlled by a custom CDE, built by Airbus DS Spain. This fully redundant unit is the electrical interface which links the cryocooler system to the instrument and spacecraft.

The CDE incorporates a number of closed-loop control algorithms. As well as controlling mechanism motion, the CDE can be ordered remotely to control cold tip temperature and mechanism stroke and to actively minimize self-induced vibration. Other CDE functions are conditioning high accuracy thermistors and accelerometers; built-in protections to prevent over-stroke or over-current events within the 50-80K Stirling coolers, and a diagnostic capability that allows in-flight health checking of the cryocooler mechanisms.

The overall system is designed to run in a hot redundant mode: Both coolers are nominally run at approximately half power to achieve the required lift. Should there be a failure in one side of the cooling chain, the other side is then ramped up to full power to partially compensate. In addition to featuring two separate cryocoolers, all sensors on the CCS are installed in primary and redundant pairings.

In order to meet ESA standards and qualify the CCS for space flight, a full environmental test program has been run. The system has been vibrated to the levels generated by the launch vehicle and thermally cycled between survival temperatures of -40˚ and 60˚ C. At the completion of this test program, the system was confirmed capable of providing a 1.5W heat lift at 78K with a rejection temperature of 20˚ C and an electrical power supply of 80 W.

The small but capable team at Airbus DS are very proud of what they have built and their focus has now shifted to completing assembly and test of the second flight unit, scheduled for delivery later this year.

To contact the Cryocooler Engineering and Development Group at Airbus Defence and Space in the UK, please contact stuart.watson@astrium.eads.net or adam.camilletti@­astrium.eads.net.

This work was produced with the financial assistance of the European Union. The views expressed herein can in no way be taken to reflect the official opinion of the European Union.