How do you know that OST can be cooled to 4 K?
Mike DiPirro/OST Technical Lead
OST makes extensive use of the lessons learned in earlier programs. The Concept 2 architecture is similar to that of Spitzer, which had a 5 K telescope and 1.3 K instruments and had 5.5 year life because it limited the heat load to the cold package to less than 6 mW. Today's high TRL cryocoolers are able to cool ~50 mW at 4 K, which enables a much larger telescope/instrument to be flown. OST will have minimal deployments, simplifying testing, and increasing reliability and cost.
Design Principle 1: Use Staged Cooling. Thermodynamics tells us that it is more efficient to intercept the parasitic radiated and conducted heat from warmer sources at as high a temperature as possible. This means that instead of trying to remove heat directly from 4 K with a room temperature refrigerator, we take the heat out in stages: Using reflectors at >100K, passive blackbody radiators to deep space at >30 K, and mechanical cryocoolers to reach 4 K.
Design Principle 2: Keep it Simple. Avoid putting warm objects in cold regions. Design for analysis and test. Design a deterministic system that has large margins wherever possible, rather than requiring detailed analysis. A 1x105 node model is not a flexible tool for analyzing changes to the design. Make sure that the design can be testable before flight.
Design Principle 3: Maintain a Large Thermal Margin. OST is being designed for heat loads that are 100% higher than the Current Best Estimate (CBE). Adopting this conservative approach at the beginning of the design allows thermal trades for better performance in other areas as the design matures.
The cryocooler development undertaken by NASA at the turn of the century has resulted in 3 to 4 high TRL designs in the US (TRL 4-5) of flight-worthy mechanical cryocoolers. One of these has been taken to TRL 7 for cooling MIRI to 6K on JWST. Sumitomo Industries has already flown two versions of a 4 K cooler. These technologies are very robust and all US coolers use versions of compressors (sometimes the only moving part) that have many years of flight heritage on previous NASA and DoD missions for higher temperature cryocoolers (30 K - 70 K). A survey of all of these flight coolers showed that the vast majority outlive the missions that they are on. At this stage of cryocooler development high reliability with long life is expected.
The OST test plan utilizes lessons learned and facilities used by JWST for cold testing. The XRCF at Marshall Space Flight Center and Chamber A at Johnson Space Center are existing facilities that were used for JWST testing and will meet the requirements for OST. Lessons learned give OST an advantage in the duration of testing: the infrastructure has already been developed and qualified, so the test duration will be significantly shortened from the 5 years of JWST to 1-2 years for OST.
Finally, a disclaimer: Cryogenics is not easy in the sense that anyone can do it. However, by employing general cryogenic principles, using developed and developing technology, and paying careful attention to cryogenic design, implementation, and test, this is very achievable at a reasonable cost and in a reasonable time frame.