Longitudinal Control Strategy for Hypersonic Accelerating Vehicles

Recent work on small scale access to space systems found that by incorporating a scramjet powered stage into the launch stack the percent payload into orbit can be improved significantly over conventional rocket only systems. However, in order to gain the most out of the airbreathing stage the flight dynamic pressure across the accelerating trajectory becomes an important design consideration. If it is too high then the structural loading and aerodynamic heating can be excessive, if it is too low scramjet performance will suffer and unreasonably large wings will be required to generate enough lift. A common compromise is to fly a constant dynamic pressure trajectory. This paper presents and compares three dynamic pressure controller methodologies that were developed for a scaled down version of the winged cone vehicle developed by NASA during the 90s. The controllers investigated are a constant gain PID, a gain scheduling PID and a pole placement PI. The performances of the controllers are demonstrated along a 50kPa dynamic pressure accelerating trajectory. It is shown that a unique criteria that must be satisfied is an initial flight path angle that satisfies a zero rate of change in dynamic pressure, and an underlying angle of attack for ‘lift equals weight’.