orbit an object as large as a space shuttle external tank. It is also sufficient to push tonsized satellites from low orbit to geosynchronous transfer orbit on time scales of weeks, while saving half to two-thirds of the mass of a standard liquid- or solid-fueled upper stage. If a high enough laser flux can be achieved in orbit, the laser could also clear away space junk. Small bits of debris would be evaporated. The surface of larger pieces would ablate, producing enough thrust (at low specific impulse) to deflect the junk into orbits that re-enter the atmosphere. A megawatt-scale laser facility is also a necessary step in developing a laser launcher. While not capable of putting anything in orbit, it could launch small ‘sounding rockets’ to several hundred kilometers in altitudes, and provide detailed information on atmospheric absorption, turbulence and blooming. It could also aid other space experiments by providing very high levels of burst power to satellites passing overhead (although this function might be better served by a short wavelength laser, whose light could be efficiently converted to electricity by ordinary solar cells). Status of Laser Propulsion Research The SDIO Laser Propulsion Program has conducted experiments at several industry and federal laboratories, and both industry and university groups have carried out theoretical analysis and computer modelling of the double-pulse planar thruster and related schemes. We have demonstrated experimentally that the double pulse thruster concept works, producing higher thrust efficiency (exhaust kinetic energy/laser pulse energy) and higher specific impulse than can be achieved with single laser pulses under similar conditions. This was done with single pairs of CO2 laser pulses, with pulse energies of a few joules and pulse widths of 50-100 ns. Specific impulses of 700-800 s have been demonstrated using both single and double pulses. The actual thrust efficiencies achieved with double pulses are only about 10%, while the launch system specifications cited above assume an efficiency of 40%. However, theory and computer modeling suggest that substantially higher efficiencies will be obtainable with longer pulses. Several energy loss mechanisms involve characteristic time or distance scales comparable to the scale of the current experiments, and will be much reduced at larger scales. We are currently preparing for experiments using a 2 kJ, 1 /zs laser at Avco Research Laboratory, in which we hope to demonstrate efficiencies of 20% or more. Note that varying the efficiency changes only the size of the laser needed to lift a given payload. Even at 20% efficiency all of the applications described above are practical, although the launch system cost would be somewhat higher. We have identified several promising propellant candidates, including lithium hydride and other light hydrides, water ice, and certain C-H-0 plastics, notably polyacetals (trade names Delrin and Celcon). More important, we now understand many of the properties required of a good propellant, such as short optical absorption depth in the solid (for efficient evaporation during the first laser pulse) and at least one component with a low ionization potential (for efficient absorption of the second pulse, which is absorbed by electron-ion and electron-neutral interactions). We have demonstrated our ability to modify propellants to achieve desired properties, for example by mixing wavelength-sized metal flakes into a plastic propellant to serve as plasma ignition sites; these lower the flux needed to achieve efficient heating during the second laser pulse. Finally, we have analyzed many of the critical systems-level problems involved in
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