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The US Navy has a spacecraft program called GeoSat Follow-On (GFO). This satellite carries a radar altimeter that
is used to measure ocean wave heights and currents for Navy fleet forecasting. During launch on the Taurus launch vehicle,
dynamic loads imparted to the GFO spacecraft would have caused unacceptably low force/stress margins, if the spacecraft
were hard-mounted to the Taurus. A vibration isolation system was designed to reduce the dynamic loads on the GFO
spacecraft. The isolation system was designed, fabricated, tested, and delivered by CSA Engineering Inc., under an SBIR Phase II
contract with the Air Force Research Lab.
The main objectives for the isolation system were:
- Reduce dynamic loads imparted from the launch vehicle to the spacecraft due to the resonant burn load case. This was the most severe load case for the spacecraft.
- Reduce dynamic loads imparted from the launch vehicle to the spacecraft due to the other load cases, if possible. This is of secondary importance.
- Do not increase dynamic loads from any load cases.
- Do not introduce too much spacecraft to fairing relative displacement.
- Do not introduce modes which are too low in frequency or high in amplitude such that they interfere with the attitude control system.
The isolation system was designed, fabricated, tested and delivered in a four-month period. The GFO spacecraft
launch was very successful and the flight telemetry data confirmed that the isolation system performed as designed.
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The telemetry flight data from the launch of the GFO spacecraft on the Taurus rocket on February 10, 1998 is
shown in the figure. Specifically, two channels of the flight data measured acceleration in the longitudinal (thrust)
direction of the vehicle to assess the performance of the SoftRide vibration isolation system. One accelerometer was just aft
of the isolation system; this is referred to as "Below isolators" in the following plots and represents the "hard side" of the
isolation system. The other accelerometer was located just forward of the isolation system; this is referred to as "Above
isolators" in the following plots and represents the "soft side" of the isolation system. This transient data shows
significant attenuation across the SoftRide isolation system.
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PSD waterfall plots of the flight data show the frequency content as a function of time. From these plots and from the transient
data, it is clear that the SoftRide vibration isolation system performed very well to reduce structure-borne vibration levels
transmitted to the spacecraft. The isolation system was designed specifically to reduce the effects of solid motor resonant burn
in the 45 Hz to 60 Hz frequency range, which it did very well. It should also be noted that the SoftRide vibration isolation system
provided extreme reductions of structure-borne acoustics at higher-frequencies as shown in the plots up to 2000 Hz.
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