Evolution of attitude control law of an Indian re-entry launch vehicle

Reusable launch vehicle (RLV) is a goal pursued by different aerospace agencies, world-wide. Indian space research organization is also one among them. This paper covers the challenges faced and the solutions developed for control law design of a typical re-entry vehicle. The first stage of the test vehicle, used for technology demonstration mission, is a conventional solid rocket motor. The second stage of the test vehicle is a winged body vehicle termed as technology demonstrator vehicle (TDV). This TDV forms the orbiter stage of a conceptual two-stage-to-orbit vehicle. While developing control law for TDV, the experience gained in developing control law for expendable launch vehicles is fully utilized. Due to the presence of wingedsecond-stage, the solution for attitude control problem naturally inherits the procedure and technology used for designing control law of an aircraft. There are significant differences a re-entry vehicle has in comparison to both aircraft and launch vehicles. Due to these differences, the final control law depends on both aircraft and launch vehicle control law design methodologies. In this paper the authors share the insights gained in control law design of a RLV. The paper gives the reasons for the selection of a combination of co-ordinate systems, for the development of linear plant model of RLV. It emphasizes the difference in the plant modelling with respect to aircraft dynamics in accounting gravitational force in the linear plant model. Requirement of pseudo trim condition for force balance is described in the paper. The paper highlights the commands to be tracked and variables to be fed back during different regimes of re-entry flight. Issues in control law gain design with respect to selected sensors and their placement are elaborated in the paper. Insights on the aspect of trim scheduling, gain scheduling, and finalization of control law structure are also given in the paper.