Flying Qualities and Controllability of Hypersonic Spaceplanes

Abstract

Spaceplanes represent a new promising concept for space flight. A spaceplane is a reusable, safe, efficient and economical space transportation system that can generate lift during its atmospheric flight, analogously to an aircraft, and it is able to travel in space as a spacecraft. What makes spaceplanes so attractive is the possibility of reusing the system for more than one mission, and the flexibility that they allow in mission. The growing interest in hypersonic spaceplanes requires that these vehicles have adequate properties of safety of flight and ease of controllability in nominal and off-nominal conditions. From this it follows the need to study their flying qualities and controllability characteristics. The current thesis addresses the flying-quality and controllability analyses, together with the development of a control system capable of improving these properties. The analyses are conducted along the ascent and re-entry trajectory of a representative single-stage-to-orbit spaceplane, the Festip System Study concept 1. The stability, trim capabilities and flying qualities of the open-loop system are analysed by studying the eigenvalues and eigenvectors of the state matrix of the state-space model. Comparing the obtained results with the requirements specified in the military documents MIL-F-8785C and MIL-HDBK-1797 for subsonic vehicles, it is possible to conclude that the reference vehicle is dynamically unstable. Thus, the need for an advanced control system arises. One concept seems particularly interesting for this application: the adaptive control system, which is characterised by a low sensitivity to disturbances thanks to its adaptive gains. Not only the control system is design to be optimal in terms of integrated control error and effort, but also a robust design methodology is applied to identify a control design that is as insensitive as possible to uncertainties of the input and design parameters. The responses for both longitudinal and lateral control in nominal and off-nominal conditions are simulated and evaluated. It results that the system behaviour is strongly related to the control system performance. The robust and advanced control system is able to stabilise the vehicle with relatively low control effort and minimise the effect of disturbances, guaranteeing safety of flight and mission success.