System Point Input Random Excitation Testing (SPIRET) was developed as an alternative to both system level acoustics and base-shake random vibration testing of large spacecraft. The goal of SPIRET was to obtain an enhanced workmanship test of the spacecraft, prior to launch, by inducing a higher, more uniform dynamic acceleration response throughout the integrated system. The underlying SPIRET concept is to utilize the spacecraft modal characteristics to amplify the shaker input force and produce a high acceleration response throughout the flexible structure. SPIRET utilized commercial off-the-shelf vibration equipment coupled with satellite specific Mechanical Aerospace Ground Equipment (MAGE). Random vibration control was achieved using inline load cells and a Force Spectral Density (FSD) input spectrum which effectively provided both feedback control of the shaker and force limiting during the vibration test.
SPIRET effectiveness was verified using a hi-fidelity spacecraft simulator which was previously used for structural and acoustic testing. This enabled direct comparison of SPIRET response accelerations to acoustic response accelerations at common spacecraft locations. In general, SPIRET produced higher dynamic loading throughout the spacecraft than did acoustic testing at Protoqual (PQ) levels over the tested frequency range of 5Hz to 500 Hz. This resulted in a more thorough workmanship screen of the spacecraft prior to launch.
The benefits of SPIRET extend beyond the technical goal of achieving an enhanced system workmanship test. Significant schedule and cost savings were realized since the test was conducted within the spacecraft final assembly facility which enabled parallel vehicle processing activities and shared personnel resources. SPIRET has been successfully used as the final integrated spacecraft level dynamic test at Lockheed Martin Denver since 2001.