An application of robust H₂/H[infinity] control synthesis to launch vehicle ascent by Tyler Nicklaus Hague.

Abstract

This thesis explores the application of H2/H [infinity] control synthesis methods to launch vehicle ascent, specifically the pitch-plane control of the Kistler Aerospace launch vehicle, K1. A classical single-input, single-output design is also presented in order to assess the true applicability of a modern control synthesis approach to launch vehicle ascent. In addition, the K1 dynamics are developed to include aerodynamic, fuel-sloshing, tail-wags-dog, and body-bending effects. The objective of the modern control synthesis approach presented here is to design compensation for pitch tracking and disturbance rejection. It combines techniques in optimal H2, optimal H [infinity], and sub-optimal control synthesis to create pitch control laws that provide 6 dB of gain margin and 302 of phase margin. The sensitivity and high-order of traditional H2/H [infinity] synthesis are addressed through the implementation of uncertainty in the design model and the application of balanced, model order reduction on the resulting controllers. To reduce the complexity in applying these methods, a hierarchical approach and design strategy is employed. Additionally, a graphical user interface is presented that exploits the capabilities of commercial software during the design. A comparison of the two design methods, classical and modern, reveals that each design architecture is capable of creating controllers of equivalent order with both nominal and robust performance. The advantages of the modern approach are realized in the design process itself. The hierarchical methodology and intuitive nature of the modern approach helps to manage the selection of design parameters. Whereas, classical methods provide less insight into strategies for parameter selection and control design. Additionally, the ability to address disturbances and uncertainty in the modern approach offers a more direct alternative to the ad hoc and iterative nature of classical methods, and although not fully exploited here, the modern approach does allow coupling between channels to be accommodated. These conclusions confirm the viability of a modern control synthesis approach and establish the foundation for future development of modern-based, ascent control laws.