beam, but that hardware can reside on the ground, indefinitely reusable and straightforward to build, test and maintain. Laser propulsion, as originally conceived by Kantrowitz [1], uses a large ground- based laser to supply energy to a small rocket vehicle. The laser beam heats an inert propellant, which is exhausted to provide thrust. Because the propellant exhaust velocity is not limited by its chemical energy content, specific impulses in excess of 1000 s can be achieved. Ground- or space-based CW lasers and space relay mirrors have been suggested as a way to power orbital maneuvering thrusters [2, 3], but proposed CW laser thruster designs have been relatively complex, using regeneratively cooled nozzles and liquid hydrogen propellant. Such systems are competitive with other advanced orbital-maneuvering concepts such as solar-electric or solar-thermal, but are not suitable for a small-scale Earth-to-orbit launcher. If a pulsed laser is used, the engineering temperature limits of conventional thrusters do not apply. High Isp is thus available from propellants much heavier (on an atomic scale) than hydrogen. Also with appropriate design no nozzle is needed to produce efficient thrust, and, ideally, a thruster can consist of only a block of suitably formulated solid propellant. Since the spring of 1987, the US Strategic Defense Initiative Organization (SDIO) has sponsored a research program on laser propulsion, managed through the Lawrence Livermore National Laboratory. The program has focused on a particular type of laser propulsion thruster, the double-pulse planar thruster [4], This thruster uses a solid propellant block composed of one of several inert materials, such as plastic or water ice, seeded with additives to control its optical and chemical properties. An ‘evaporation’ laser pulse ablates a layer of propellant of a few microns in thickness forming a thin layer of gas which is allowed to expand to roughly atmospheric density. A second laser pulse then heats this gas layer to approximately 10000 K. The hot gas layer expands rapidly, producing thrust. The entire process takes a few microseconds, and is repeated at 102-103 Hz rates. This process is illustrated in Fig. 1.
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