Review On Laser Lightcraft Research At DLR Stuttgart

Abstract

A review on 15 years research on remote laser propulsion with a parabolic thruster at DLR is presented. Mission scenarios were analyzed for nanosatellite launch to Low Earth Orbit (LEO) using a ground-based high energy laser as energy supply significantly optimizing the mass-to-payload-ratio. Experimental work was carried out using a home-made, electron-beam sustained CO2 high energy laser in the 10 kW class with around 10 μs pulse length. The parabolic thruster was compared with the Lightcraft Technology Demonstrator in air-breathing mode as well as with Polyoxymethylene (POM) as an ablative propellant with respect to laser pulse energy and beam profile taking into account for standardization issues of ballistic pendula. Experiments showed good performance of pure air-breathing mode without propellant down to 200 mbar ambient pressure allowing for a drastic propellant reduction for the initial flight phase during dense atmosphere. The commonly used hydrodynamic point explosion model with a strong shock wave was analyzed with respect to the optimization of the impulse coupling coefficient in geometric scaling by the adaptation of nozzle diameter and length to the range of the applied laser pulse energy. The usage of ablative propellants like POM, inevitable in the vacuum of space, yields enhancement of impulse coupling under atmospheric conditions which can partly be attributed to combustion. Various polymer-metal composites were developed and analyzed in order to achieve a higher specific impulse, but failed due to material inhomogeneity. Started up with wire-guided flights, using only air as propellant in a laser-induced breakdown, the detonation reproducibility by means of an ignition pin on the axis of symmetry of the thruster was proven in the free flight experiments yielding an altitude up to 8 m, limited by the laboratory ceiling. Nevertheless, flight dynamic analysis of a tilted pin as steering gear and hovering experiments near ground level revealed crucial coupling of lateral and angular motion and the demand of spin-stabilization for a beam-riding flight. A review of related publications, in cooperation with US AFRL, University of Stuttgart and Nagoya University, is given as a compendium.