Optimisation of the EXPRESS Landing Dispersion

Abstract

Express was a multi-national capsule mission, which was put into orbit by a Japanese launcher. After a nominal orbital liftime of about 5.5 days the capsule was planned to be landed in South Australia. During this mission GSOC had the operational control and was also be responsible for the planning of the return manoeuvre never before exercised in Germany and Europe. In order to determine the re-entry manoeuvre a prototype software for re-entry calculations provided by ESA was used. This software provides a lot of optimisation options regarding de-boost manoeuvre size, acceleration peak, net heat rate peak, etc., whereas an optimisation (minimisation) of the landing error ellipse is not supported. As this minimisation is very important because of the time limitation for the capsule recovery,two approaches are possible:i) use the rigorous re-entry software and optimise the landing error ellipse by manually changing the input parameters; ii) use of semi- analytical algorithms. The second approach was selected using only a Keplerian orbit, a simple model for the atmosphere, a spherical shape of the Earth, and a mean ballistic coefficient for the capsule to determine the optimised thrust direction. The last orbit before the de-boost manoeuvre (given by a state vector within the last orbit) and the inertial radius vector to the landing point must be known as input. At each position within this orbit only one in-plane angle exists for a minimum landing dispersion. A comparsion of the results of both approaches shows that the difference in the semi-major Axes is about 200 m (length of the semi-major axis is in both cases about 80 km). Much more important is the speed improvement: about one hour would be necessary for the optimisation of the error ellipse using approach i), whereas only less than one second is needed by optimising it with the new algorithmÖ.