Laser Propulsion - an Innovative Launcher Technology

Abstract

Laser power can be transformed into propulsive power to set into motion various kinds of vehicles and other objects on the ground, in the air and in space. The transformation process can occur indirectly or directly. For instance by producing electricity via photovoltaic cells to power an electric motor, by producing thermal power in the medium of a thermodynamic cycle or via a heat exchanger. A promising technology is the generation of impulses by the ablation of matter from a solid body, or by the initiation of a high-pressure plasma breakdown wave in a fluid medium, called pulsed thermal laser propulsion. This technique can become a very efficient, relatively near term scheme for launch-ing small satellites at low costs into LEO with payload masses on the order of 5 to 10 kg. Using a pulsed, electron beam-sustained multi-wavelength laser with pulse energies as high as 410 J in CO₂ laser gas and possible repetition rates up to 100 Hz the launch of a parabolic thruster, called “Lightcraft” has been demonstrated in the laboratory. In addition measurements of fundamental pro-pulsion parameters in the atmospheric environment as well as at reduced pressures down to 1 mbar have been performed for a Lightcraft. The propulsion mode was either air breathing or ablative, using a plastic material as solid propellant. A requirement for efficient pulsed laser propulsion from ground to LEO is the achievement of a spe-cific impulse of up to 800 s at a jet efficiency of at least 50 %. These numbers cannot be attained by classical laser ablation using CO₂ laser radiation at pulse lengths in the range of 10 microseconds and polymers as propellant because the impulse formation by laser ablation is limited by the premature absorption of the incident laser radiation in the initially produced cloud of ablation products. In order to investigate this process Schlieren photography as well as absorption experiments with a CO₂ probe laser beam have been applied. Based on recent laser propulsion experiments in our laboratory at DLR we studied the mission charac-teristics for a 50 kg satellite launch. This includes considerations of the available or required laser power, the transition of the Lightcraft through the atmosphere, the laser range, the transmissivity of the atmosphere for the laser radiation, the turning of the vehicle for final orbit insertion and some general restrictions to the launch site.