BPCU installed in vibration isolation stand.
Vibration testing was conducted on an operating closed-cycle Brayton power-conversion unit (BPCU) located at the NASA Glenn Research Center. Results from this test were compared with those from a mechanical dynamics computer model of the same unit. This work showed that dynamic forces developed by the BPCU were small and that the model could accurately predict torque and vibration levels. Accurate predictions of BPCU characteristics are essential for spacecraft control/structure interaction analyses and for understanding the operational capabilities of scientific instrumentation onboard.
The BPCU is a fully integrated power-conversion system including a turbo-alternator, recuperator, and gas cooler with helium-xenon working fluid designed for operation up to 2 kWe. The heat source used in the test was a series of silicon-carbide electrical resistance heaters contained in a shell and tube heat exchanger that heated the helium-xenon gas to over 1000 K, simulating a fission reactor source. A commercial chiller with a pumped ethylene glycol cooling loop provided waste-heat rejection. The BPCU utilizes multifoil insulation, which requires a vacuum environment.
The test approach for the 2-kWe Brayton engine required careful planning of measurement sensor types and locations. Test-stand modifications necessary to minimize environmental noise and to ensure accurate dynamic measurements were identified and implemented. Because the analytical model predicted that the 2-kWe BPCU would be a very quiet machine,the Microgravity Emissions Laboratory (MEL) at Glenn was given the test-instrumentation and data-acquisition task. The MEL team utilized a highly sensitive acceleration measurement system at Glennís Vacuum Facility 6 to record the mechanical dynamics signature. Facility background vibration (due mostly to vacuum pump systems) was measured and found to be too high to discern the very low-level vibration of the BPCU. Consequently, an isolation test stand was designed and built to utilize existing mounting pads within the vacuum tank, and a two-cable suspension system was used to isolate the BPCU from facility vibration. Pretest background noise checks confirmed the effective-ness of the isolation standís ability to attenuate environmental vibration.
Testing showed minimal excitation (10-15 to 10-10 g2/Hz) in the frequency band of interest (0 to 600 Hz). This is important input to the understanding of on-orbit spacecraft control/structure interactions for a large, flexible spacecraft. With such low disturbances coming from the operating 2-kWe BPCU, issues with spacecraft control/structure interactions are not expected. In addition, on the basis of the experimental data, an operating BPCU would not impact scientific measurement onboard the notional spacecraft application.
Find out more about the research of Glennís Thermo-Mechanical Systems Branch: http://www.grc.nasa.gov/WWW/TECB/
Dr. Michael J. Barrett, 216-433-5424, Michael.J.Barrett@nasa.gov
Analex Corporation contact: David S. Hervol, 216-433-9624, David.S.Hervol@nasa.gov
Authors: David S. Hervol, Damian R. Ludwiczak, Dr. Dzu K. Le, Anne M. McNelis, Albert Y. Yu, and Sergey Samorezov
Headquarters program office: Exploration Systems
Programs/Projects: Project Prometheus
Last updated: October 16, 2006
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