The effect of sloshing on the controllability of a conventional aeroelastic launch vehicle

Abstract

Examples of control problems occurring during flight tests of fighter aircraft are well documented. In many cases, the cause of the problem could be characterised as inadequate modelling or other inappropriate treatment of the aeroelastic effects on the vehicle dynamics and/or the flight-control design. However, such problems are not restricted to just aircraft. Especially long and slender bodies, such as (small) conventional launch systems, may suffer from an unwanted coupling between the rigid body and its flexible modes. In addition, due to fuel consumption during the flight, the change in atmospheric environ-ment, and aerodynamic effects, the entire flight profile should be examined, rather than a single worst-case point, to identify the stability and controllability performance of the launch vehicle. The current research treats the launch vehicle as a flexible beam with lumped masses to account for the subsystems and fuel, using a three-dimensional assumed-modes method with longitudinal and lateral effects. Sloshing is added to the system as a spring-mass-damper, where the liquid oxidiser and fuel are modelled separately. Multiple discrete flight points are considered for aeroelastic analysis at different stages of the ascent profile. These individual points are analysed as quasi-static and quasi-steady inputs for a continuous flight trajectory simulation. The stability and controllability for the entire flight under the influence of wind gust and turbulence is estimated based on this simulation. While simulating the manoeuvres with the sloshing model included it became apparent that the (very) simple PD controller was not able to stabilise the vehicle. Results concerning the influence of sloshing can therefore not be shown. The development of a more robust adaptive controller is currently on its way, and it is expected that this non-linear controller will be better suited to handle the strong non-linearities of the combination of flexibility, sloshing and a demanding wind environment.