Debate Introduction
Engineers prepare the MESSENGER spacecraft for a vibration test on a shaker table at The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland. The purpose of this test is to verify that the spacecraft can withstand the launch vehicle environment during liftoff and ascent.
Flight configured spacecraft are subjected to base input vibration tests for certain programs. The spacecraft are often one-of-a-kind, so the vibration test is effectively a proto-qualification test covering both design and workmanship verification. The tests may be sinusoidal or random. The sine vibration is typically at low frequencies, below 100 Hz. The author’s colleagues are divided on whether these spacecraft system-level tests are prudent and effective.
The following assertions are made by A.M. Kabe and E. Perl from The Aerospace Corporation in informal working groups meetings.
- Vibration tables cannot replicate the impedance of the launch vehicle interface, nor the interaction that occurs between the launch vehicle and spacecraft when they are a coupled system; hence, the modes of vibration will not be the same as in flight.
- Only translational motions are applied at the base, one axis at a time, whereas during flight, the launch vehicle/spacecraft system will vibrate simultaneously in all six degrees of freedom at each mass point and at each interface point between the launch vehicle and spacecraft.
- The total acceleration load during powered flight also depends on the spacecraft rigid-body acceleration which a shaker cannot replicate.
- Derivation of a “base input” environment from a few accelerometer locations at the launch vehicle/spacecraft interface will generally lead to an over prediction of the motions at the interface, since local deformations are mapped on the assumption that the interface acts as a rigid plane.
- The use of (response)/(base motion) ratios to extract damping, a common practice, is not a valid approach for multi-degree-of-freedom systems it fails to account for the mode participation factor.
- The test article may not include the actual spacecraft launch vehicle adapter or the propellant mass in the tanks because of safety and contamination concerns.
- The test requirement forces the spacecraft organization to design its system to not only survive the launch environment, but also to survive an artificial test that more often than not produces overly conservative loads in many parts of the structure while not adequately testing others.
- The test can pose unnecessary risk of damaging flight hardware late in the program.
Note that A.M. Kabe advocates acoustic reverberant chamber testing of spacecraft as a workmanship screen, as an alternative to base shake testing. He also favors shaker table vibration testing on a component or subsystem level.
The following justification points are made by NASA engineers Daniel Kaufman, Scott Gordon, Steve Hendricks and Dennis Kern.
- Essentially all current launch vehicle organizations (Delta, Atlas, Taurus, Pegasus, Ariane, HII, Proton, Long March, Falcon, etc.) specify and require or strongly recommend a spacecraft sine or random vibration test.
- Testing is also required by NASA documents, such as References [45] and [46].
- Insurance companies require vibration tests on all commercial communications satellites.
- Some test facilities have the capability to perform simultaneous multi-axis vibration testing, as needed.
- The vibration test provides qualification for tertiary/ancillary hardware that would not otherwise be tested. This includes: Cable harnesses, bellows, connectors, actuators, plumbing lines, wave guides, brackets, dampers, shades and shields, articulation & deployment mechanisms, shunt heaters, louvers, purge equipment, hinges and restraints, blankets & supports.
- The test provides an opportunity to determine the structural linearity in the operational vibration range of response. Note that linearity is typically assumed in coupled loads analysis (CLA).
- Force limiting reasonably accounts for the interaction with the base motion, and has been effectively employed in spacecraft vibration testing, thus reducing the potential for an over-test at the spacecraft’s natural frequencies in the test configuration. The force limiting takes into account the CLA response levels.
- Force gauges under the spacecraft provide a very accurate method of measuring and limiting to the CLA loads during the vibration test for mid to high apparent mass modes.
- Numerous case histories have shown that vibration testing is effective for uncovering design or workmanship flaws which would have otherwise caused mission degradation or failure.
Note that NASA/GSFC typically uses sine vibration testing, whereas JPL tends to use random.
A few examples from sine testing at GSFC are:
- TRMM: During Observatory sine testing, found that the NASDA supplied PAF clamp band had insufficient tension and gapped during the test. As a result, the clamp band tension was increased for flight.
- GOES had a workmanship problem involving a missing or loose bolt which caused structural failure of a mission-critical antenna. It was detected during the lateral sine test.
- NOAA-K experienced IMU saturation during sine sweep testing. Because the spacecraft IMU provides guidance information for the Titan II launch vehicle during ascent, IMU saturation during launch would have resulted in a mission failure. Changes were made and launch vehicle restraints were implemented to resolve the problem, including wind restrictions at launch and a commanded first stage shutdown vs. fuel depletion.
- TDRS-H: During the sine vibration test, the first two modes for the Space Ground Link antenna (SGL) were lower than predicted by the model. The first mode dropped from 15 Hz to 11 Hz and the second mode dropped from 33 Hz to 25 Hz. It turned out that the mathematical model of this “simple” antenna was wrong and therefore the Verification Loads Cycle had to be rerun.
A few examples from random vibration testing at JPL are:
- Cassini: Experienced an RTG electrical short to its spacecraft mount in system random vibration test. Significant degradation in spacecraft electrical power could have resulted. Spacecraft mount was redesigned.
- CloudSat: Cloud Profiling Radar waveguide failure in spacecraft random vibration test due to apparent poor workmanship of adhesive bonding. Possible loss of science data averted.
- MER 1: Fundamental modes of the Rover in spacecraft random vibration test were 20% greater than predicted in all three axes. (Fixed base modal test had been performed on Rover, Lander, and Cruise Stage separately; FE models were then combined. Estimated stiffness of Lander attachment to Rover was too low.) FE model was updated just in time for the verification CLA cycle. Vibration test also revealed improper torque of bolts on some tanks in low level runs. Bolts were properly torqued and test completed successfully.
- MSL Rover: experienced several motor encoder screws backed out of at least one of the Rover actuators during Rover random vibration test. The actuators are used throughout Rover and the issue was unlikely to have otherwise been found before launch, which could have been a serious threat to the mission.
JPL prefers random vibration because it easier to control, particularly with respect to force limiting.
Note that JPL tested the SMAP spacecraft to the following workmanship PSD:
20 Hz to 250 Hz, 0.01 G^2/Hz
The overall level was 1.5 GRMS. The duration was one minute. The vibration was applied in the vertical axis and in one “45 degree” lateral axis.
Sine sweep vibration is more difficult to control than random especially for the case of lightly-damped modes. The sweep rate, compression factor, and tracking filter must be selected with great care.
INVAP experienced control issues during sine vibration testing of the ARSAT-1 structural test model. But note that dynamic simulators were used for the test, which can have much less damping than the actual flight hardware.
