By M. Petach, M. Michaelian T. Nguyen, R. Colbert, and J. Mullin, all from Northrop Grumman Aerospace Systems.

The James Webb Space Telescope’s (JWST) Mid InfraRed Instrument (MIRI) cooler subsystem features aclosed cycle helium Joule-Thomson (JT) cryocooler pre-cooled by a three-stage pulse tube cryocooler. The cooler subsystem itself consists of four major subassemblies (Figure 1), each integrated into the spacecraft separately and then interconnected: the Cryocooler Electronics Assembly (CCEA), Cryocooler Compressor Assembly (CCA), Cryocooler Tower Assembly (CTA) and Cold Head Assembly (CHA).

The CCA, or flight model sub-assembly, compresses and precools the system’s helium working fluid. It resides within the JWST observatory’s spacecraft bus. The major assemblies of the CCA are a three-stage pulse tube precooler, a JT precooler helium compressor and a series of recuperators that provide thermal isolation between the room temperature helium from the JT compressor and the precooler’s three stages.

The CCA’s JT compressor and PT precooler were tested independently before being integrated on the CCA’s thermal interface plate. The JT compressor contains a single stage that provides a nominal helium gas flow of 25mg/s at approximately 12 bar on the high side and 4 bar on the low side. The PT precooler combines the PT compressor and three-stage pulse tube cold heads that provide cooling at the nominally 150 K, 50 K and 18 K helium precooling stages. The three recuperators associated with the PT precooler provide thermal isolation between the aforementioned precooling stages.

The Flight Model and Flight Spare JT compressors were tested as standalone components for performance and exported forces. The performance of each compressor was tested by pumping helium through a load valve with a pneumatic impedance that was varied to mimic the range of flow impedances experienced with the flight system. The performance of each compressor is shown in Figure 2. The summary includes key pertinent performance characteristics, including reject temperature, frequency, pressures, mass flows and compressor powers. The two flight compressors met subsystem requirements.

The JT compressor exported force was tested on a six-axis force dynamometer. The JT compressor was operated at acceptance level power with a representative pneumatic restriction used in place of the nominal system JT restriction. The x-axis of Figure 3 represents the fundamental drive frequency and the 2nd Harmonic of the drive frequency. The amplitudes of the various exported force axes are plotted for each compressor in units of mN peak, with Flight Model and Flight Spare results adjacent to one another for each axis.The key result from this testing was that the exported forces due to the JT compressor drive axis were on the order of 50 mN and less for the drive frequency and 1st harmonic, whereas the off-axis exported forces were all less than 200 mN.

The JT compressor launch vibration testing was not done at the component level and was instead deferred to CCA level vibe tests. The basis for this was that the JT compressor is the same as TRL9 NGAS High Efficiency Cryocooler (HEC) compressor, with over a dozen units in flight operation, and the reed valve design was validated with valve module level vibration tests.

The PT precooler design was derived from the NGAS 10 K pulse tube cooler. The precooler contains a three-stage pulse tube with stages operating at 150 K, 50 K and 18 K. The PT cooler was first tested at the cooler-only level, which contains only the compressor and the pulse tube cold heads. It was tested again at a higher level of assembly, containing the cooler along with flight recuperators and the two flight thermal shields that its upper stages cool (Figure 4).

The second test was completed in order to verify minimal impact to the system performance as a result of the added flight hardware. In the case of the cooler plus flight recuperators/thermal shields, the refrigeration performance was initially measured without helium gas flow to verify minimal impact of the recuperative and flight shield hardware.

Later, using the same cooler configuration, the precooler performance was measured with helium gas flow through the JT circuit using a representative pneumatic restriction at approximate flight mass flow rates and pressures, with electrical heat added to measure lift. Figure 5 shows Pinch point condition load lines for the Flight Model and Flight Spare PT precoolers for the third stage at the cooler-only level.

The environmental condition for all data collected had a constant second stage load of 275 mW applied at approximately 50 K to simulate the expected flight loads on the second stage shield.

The power applied to the PT compressor in all cases was an equivalent compressor power at Flight CCEA AC terminals of 300 W.

Figure 6 shows the pinch point condition load line performance at the next higher level of assembly with cooler plus flight recuperators/thermal shields and Figure 7 shows the combined plot of Pinch Point load line data from the two assembly levels. The precooler refrigeration performance data at both assembly levels was better than the requirement and was consistent with previous end-to-end cooler tests using the DM precooler and FM cold head assembly.

The three-stage PT precooler was also tested to quantify its exported forces. The three-stage PT precoolers were tested on the same six-axis force dynamometer as the JT compressors and were tested at the cooler plus flight recuperator/thermal shields level with the recuperative heat exchangers and flight thermal shields attached.

As in the refrigeration performance testing, the external loads on the precooler were kept constant at 275mW at 50 K and greater than 500 mW at 18 K. All six axes of the Flight Model and Flight Spare assemblies were measured, with X, Y and Z forces reported.

In Figure 8 the X axis shows a nominal drive frequency of 30.5, with the 1st through 5th harmonics shown with increasing frequency. The 1st and 2nd bars in each frequency correspond to the drive, or X-axis exported forces, the 3rd and 4th bars delineating the Y-axis exported forces and the 5th and 6th bars corresponding to the Z-axis exported forces, as measured on the 6-axis dynamometer.

The key result from this figure is that the exported forces due to the PT compressor drive axis were less than 100 mN throughout the drive frequency through the 1st five harmonics, whereas the off-axis exported forces were all less than 600 mN with many around 100 mN or less.

The three-stage pulse-tube precooler was additionally exposed to a random vibration environment. The Flight Model and Flight Spare were tested at acceptance levels of approximately 0.1g2/Hz. All post-vibe checkouts indicated nominal results.

The PT and JT compressors are the only shock sensitive components in the CCA, and thus needed to be qualified for flight. As previously mentioned, the HEC compressor design used in the JT compressor for the JT loop had been qualified for this shock environment on several previous flight programs, so additional flight qualification for that compressor was not needed. The HCC compressor used for the three-stage PT precooler was qualified for the MIRI program via a ringing plate test of an NGAS-owned compressor. Figure 9 shows the intended shock profile with the results of the actual profile applied to the NGAS-owned compressor.

At this time, the MIRI CCA components have all been characterized by in-process tests before integration onto the CCA. And the Flight Model and Flight Spare CCA sub-assemblies have been successfully delivered to acceptance testing at the CCA level. The component level characterization data acquired as a result of the MIRI cooler CCA sub-assembly test campaign are applicable to future space missions that might use a different configuration of these components.

ACKNOWLEDGMENT
This work was performed for the Jet Propulsion Laboratory, California Institute of Technology, sponsored by the United States Government under a Prime Contract between the California Institute of Technology and NASA.