The effects of preloads on the deployment separation performance of a spacecraft system

An important problem in spacecraft design is the method with which the spacecraft is released into orbit. The mechanism which releases the spacecraft must perform the following functions: the spacecraft must be released with very low angular velocity, the release mechanism must be 100% reliable even after years of storage, and a small forward axial velocity (tipoff error) must be imparted to the spacecraft to prevent bumping between spacecraft and the launch vehicle, called the bus. Two of the factors which influence spacecraft release behavior are 1) the effects of strain energy locked into the structure by deformations from assembly of the spacecraft and bus in a one-G field, and later separation of the spacecraft during deployment from the bus in a zero-G field, and 2) misalignments between the two structures which, when the structures are joined together, also cause strain energy to be stored. The release of this strain energy when the spacecraft and bus are separated can lead to tipoff errors. This analysis attempts to quantify these preloads and to assess their contribution to separation performance. Tipoff rates observed in flight tests have been on the order of 1.0 deg/sec. A comparison is made to see whether the two above factors can account for those rates. Strain energy storage has been quantified by modelling a bus and spacecraft using the finite element method, and assembling the two together in a deformed condition to solve for the static interface loads. These loads were then input to the model in a modal transient analysis to explore the effects of the stored strain energy on satellite tipoff rates. The analysis was performed using the MSC NASTRAN finite element program. The results give an upper bound to the tipoff contributions of preloads at the spacecraft-bus interface.