Why does the IMAGE satellite have to be shaken before launch?

Every mechanical structure has a set of frequencies that it has a natural resonance with, like someone swinging on a playground swing. At these frequencies, a slight push delivered with the same frequency can escalate into a major vibration of the structure. When a rocket launches a payload, the fuel leaving the engines causes vibrations at certain frequencies, and if a satellite or its components also have 'resonances' at these frequencies, energy can be transferred from the rocket's vibrations into the spacecraft and damage it. There are specific guidelines for each type of rocket, that certain resonant frequencies are to be avoided to prevent damaging the payload. Usually, sophisticated models of the spacecraft structure and contents can identify all of the natural frequencies, but the final physical test confirms just how well the spacecraft was actually designed.

According to the current 'mass model' for the IMAGE satellite, it has resonances at 37 cycles per second, however a recalculation of the mass model by Lockheed, who will be doing the final spacecraft vibration and thermal tests, says that there is a 27 cycle per second axial resonance in the current IMAGE spacecraft. For the Delta launch vehicle we will be using, the requirements state that there may be no resonances below 35 cycles per second. Some last-minute redesign work will need to be done to bring the IMAGE satellite into compliance with this launch requirement, otherwise we will not be able to fly the satellite. It will literally be shaken apart during launch!


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