The optimum small satellite (SmallSat) cryocooler system must be extremely compact and lightweight, achieved in this paper by operating a linear cryocooler at a frequency of approximately 300 Hz. Operation at this frequency, which is well in excess of the 100-150 Hz reported in recent papers on related efforts, requires an evolution beyond the traditional Oxford-class, flexure-based methods of setting the mechanical resonance. A novel approach that optimizes the electromagnetic design and the mechanical design together to simultaneously achieve the required dynamic and thermodynamic performances is described. Since highly miniaturized pulse tube coolers are fundamentally ill-suited for the sub-80K temperature range of interest because the boundary layer losses inside the pulse tube become dominant at the associated very small pulse tube size, a moving displacer Stirling cryocooler architecture is used. Compact compressor mechanisms developed on a previous program are reused for this design, and they have been adapted to yield an extremely compact Stirling warm end motor mechanism. Supporting thermodynamic and electromagnetic analysis results are reported.
Development of the Dual-Use Cryocooler (DUC) system has progressed substantially over the past two years, including
the design, build and testing of a brassboard thermo-mechanical unit (TMU). Early design efforts were undertaken with
simplicity as a goal, and as a result the brassboard TMU contained significantly less parts than typical space-level
cryocoolers. Build time for the brassboard unit was extremely short, with the compressor being built in a matter of days
as opposed to the more traditional timescale of weeks. The brassboard TMU was subjected to characterization testing in
both horizontal and vertical orientations (to address sensitivity of the pulse-tube cold head to gravitational effects), and
results from that set of tests have been correlated to the thermodynamic model. Several lessons were learned as the
testing and correlation activities progressed, and improvements necessary to meet the intended performance objectives
were identified for implementation in the deliverable system.
Significant progress was made in terms of electronics development as well. Existing tactical assets were heavily
modified for use with the DUC, including the addition of separate drive circuits for each compressor motor. The
operating software was modified to enable features not found in typical tactical systems such as first-order active
vibration cancellation. Ultimately, the brassboard electronics were used to drive passive loads as well as an actual
(representative) tactical Stirling cryocooler.
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