Effects of the initial conditions on the results of launch experiments with fibre armatures

Launch experiments with multi-fibre armatures conducted under apparently the same conditions result quite often in different values of the transition velocity. It is assumed that poor electrical armature-rail contact results in early arcing. A better quantitative control of the initial mechanical condition seems therefore to be necessary. The armature-rail contact resistance in the initial phase of a launch process is mainly determined by the preload force. To be able to perform a series of experiments under the same initial conditions, a method for the determination of the preload force is necessary.
Results of a 2D mechanical finite element model of the rail accelerator to simulate its static mechanical behaviour show the optimal location for measuring the stress condition in the armature region of the rail accelerator to control the armature preload force. A fibre-optic strain sensor has been developed to measure the mechanical stress condition. A priori knowledge of the stress-strain behaviour of the fibre armatures is required to determine the preload on the electrical contacts. A test set-up to determine the value of mechanical parameters of the fibre armatures was used. The 2D mechanical model of the rail accelerator is verified with experiments in the armature test bed (ATB). A simple optical method to determine the departure time of the armature during a launch experiment has been developed to establish the current at which the launch package starts moving.
In this paper a summary is given of the development of a method for controlling the initial conditions of a launch experiment. A measurement set-up used to determine the value of mechanical parameters of the multi-fibre armatures, and a fibre optic displacement sensor to measure the preload on the armature for controlling purposes is described too.