Computation of static disturbances experienced by a launch vehicle during the flight

Abstract

During its course from a launch site to an intended orbit, a launch vehicle faces an adverse environment and withstand a plethora of disturbances. The set of disturbance forces acting on each stage of the vehicle depends on the configuration of that stage. Different methodologies are used to predict the magnitude of static disturbances before the flight. To ensure the launch success, performance of a flight control system is validated using a high fidelity six degrees of freedom simulation testbed, in the presence of predicted magnitude of these disturbances. Magnitude experienced by the launch vehicle during its maiden flight, might vary from the predicted set. Actual magnitude of these disturbances is known only after the flight in post-flight exercise of matching launch vehicle dynamics using ground simulations. Correct information of the magnitude of these disturbances is of significant importance for subsequent developmental flights. Control power plant sizing, preflight actuator biasing to nullify lift-off disturbance and autopilot integrator limit setting are all dependent on the exact knowledge of these sources of disturbances. Flight data measurements of a launch vehicle offer a rich galore of information in this regard. This paper presents an analytical technique which uses flight measurements to compute static disturbances experienced during the flight by a launch vehicle. The main sources of steady disturbances considered in this paper are center of gravity, thrust-line offset and thrust misalignment.