Original Date: 04/26/1999
Revision Date: 01/18/2007

NASA requirements called for the liquid hydrogen (LH2) liquid level sensors for the space shuttle’s super lightweight external tank to undergo three-axis random vibration testing at -423° F. During testing, the vibration levels also need to be monitored to ensure that the power spectral density stays within specified values. Marshall Space Flight Center (MSFC), however, discovered that the accelerometers used to measure the vibrations were unreliable at this extremely low, cryogenic temperature. Specifically, the accelerometers exhibited random DC shifts and high frequency spikes below -200° F. The Center concluded that it was not possible to chill the accelerometers to the required temperature and still get an accurate output.

MSFC first approached this issue by performing random vibration testing at room temperature and using a power spectral density, six decibels down from the required vibration level to guard against hardware damage. One accelerometer was mounted on the shaker armature and another on the unit under test (UUT). The resulting power spectral density levels were then measured. Next, MSFC derived the transmissibility (Q factor) between the armature and the UUT (and its mounting configuration) by examining the difference between the responses of the two accelerometers. Using the Q factor to adjust the input vibration levels of the shaker table, MSFC predicted that the output vibration levels would stay within parameters. To test the concept, MSFC performed random vibration testing at -200° F, the lowest temperature at which the accelerometers had proved reliable. Results confirmed that measurements inside the UUT agreed with the test criteria. Based on these results, MSFC concluded that its approach was viable and would have no appreciable effects as the temperature was further lowered. The UUT was then successfully isolated and chilled to -400° F +30° F.

By developing a method to correlate the accelerometer response of the UUT with that of the shaker armature, MSFC successfully performed three-axis random vibration testing at extremely low, cryogenic temperatures. In addition, this method permits testing over a wider temperature range, allows the use of control points that may be inaccessible during testing, and avoids the need for specialized instrumentation. This technique was successfully used on the anti-vortex baffle sensors as well as an X-33 valve, and was extended to acoustic testing.

For more information see the Point of Contact for this survey.