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Leroy Sparr, Michelle Sartor & Robert Boyle Cryogenics, Propulsion and Fluid Systems Branch NASA/Goddard Space Flight Center Greenbelt, MD, USA 20771
Stuart Banks & Edward James McDonnell Douglas Corp. Seabrook, MD, USA 20706
NASA, the U.S. Air Force and industry have invested considerable money during the last decade to develop low vibration mechanical cryocoolers for space applications with typical mean time between failures (MTBF) of five years or more at a cost of about $1.0 million per cooler.
Militarized, unbalanced tactical cryocoolers are readily available at comparatively low cost (i.e. $20,000.00) with typical MTBF's of about 5000 hours. NASA/GSFC personnel have tested tactical cryocoolers from Magnavox, Texas Instruments & Hughes Aircraft. All of these cryocoolers were of the linear motor type with opposed compressors and a free piston, unbalanced expander. Typical vibration levels are 6 newtons of force at the expander and 2 newtons of force at the compressor at the fundamental and harmonic frequencies. NASA/GSFC personnel designed an active counterbalance to mount on each expander to null vibrations. NASA/GSFC's vibration control algorithm was then used to null vibrations below 0.1 newtons.
Characterization tests for each of these cryocoolers include load curves, power measurements, thermal vacuum testing and axial and lateral vibrations measurements with and without an active vibration control system. Baseline characterization testing has been completed on all three cryocoolers. Life testing is currently underway. Characterization testing will be repeated every 1500 hours until cooler failure to monitor degradation processes. Conclusions and lessons learned are presented.