Attitude Estimation for a Flexible Spacecraft in an Unstable Spin

An attitude reconstruction algorithm has been developed for a flexible sounding rocket whose spin vector nutates unstably about its minor inertia axis. This algorithm deduces attitude for use in post-flight analysis of the sounding rocket's science data. An additional motive for pursuing this work has been to show that a flexible-body dynamic model can be used in a Kalman filter/smoother-type attitude estimation system. The attitude reconstruction algorithm is a squareroot information filter/smoother. It uses a dynamic model of the rocket that includes a main rigid body and a flexure mode for a pair of elastic booms. Boom bending is the principal cause of the energy loss that leads to unstable growth of the nutations. The flexure of the booms is modeled by a Markov process, but the laws of mechanics are used to model the flexure's influence on the attitude dynamics of the main spacecraft body. The filter/smoother achieves an attitude accuracy that is significantly better than can be achieved using a rigid-body model. The peak attitude error is estimated to be 4 deg, and the majority of the error seems to be caused by the limited accuracy of the rocket's attitude sensors. ∗ Associate Professor, Sibley School of Mechanical and Aerospace Engineering. Associate Fellow, AIAA. # Graduate Student, School of Electrical Engineering. ** Professor, School of Electrical Engineering. ## Research Support Specialist, School of Electrical Engineering.