Frames of reference for launch-vehicle trajectory simulation

Abstract

The selection of efficient coordinate systems is an important task in the simulation of launch-vehicle trajectories. The primary aim is to decouple the translational and rotational motions of the vehicle. Decoupling improves the accuracy of the computation in real time or fast hybrid computer simula tions. Our study led to the choice of an inertial frame for describing translational dynamics and the body frame for rotational dynamics.

We derived our proposed scheme from Rubin's work on air craft and short-range missiles. ^17,18 A set of additional reference frames essential for launch-vehicle trajectory simulation is also presented. We compared three methods for computing the trajectory of a launch-vehicle using the same input data and the same numerical techniques. Results in dicate that the proposed scheme yields higher computing speed than approaches employing flight-path axes or body axes for integrating the translational dynamics.

These simulations show that, as the calculations progress, tra jectory variables obtained using either flight-path axes or body axes differ increasingly from those obtained using iner tial axes. Comparing the post-flight trajectory simulation results (using the initial conditions of the flight test) with flight test data for the launch vehicle indicates that the iner tial axes yield the most accurate results. The appendices sum marize the dynamic equations for the reference frames.