building an actual launcher. We have, for example, calculated the control loop response involved in guiding a laser-driven vehicle from the ground, and demonstrated that such ground-based guidance is stable over a wide range of conditions. If the planned test with single pulse pairs at 2 kJ are successful, the Laser Propulsion Program will be ready to proceed to tests with a repetitively pulsed laser of significant average power. Unfortunately, few such lasers are available, and none provide our desired pulse format. The Program currently plans to modify the Humdinger CO2 laser at Avco Research Laboratory, but we are still seeking other options. The Program will also begin work on tests using Nd:glass lasers at 1.06 /zm, both to determine the wavelength scaling properties of the double-pulse thruster and specifically to see how laser propulsion could be adapted to use the large 1.06 /zm FELs now under development by the SDIO. Program for Laser Propulsion There are several possible routes to a working laser launch system. Assuming continued development of large lasers by the SDIO, it is likely that lasers (and optics) sufficient for launch-to-orbit will be built in the next decade. If these can be adapted (primarily through extended run times and improved durability) to routine use, laser launching may be a major peacetime application of strategic defense technology. Alternatively, a dedicated laser launcher using CO2 technology could be built. This would require a modest expansion of the current research efforts to demonstrate higher efficiencies, select and optimize a propellant, and demonstrate sustained performance with repetitive pulses. This would be followed by the design and construction of a subscale launch facility with 1-2 MW average power and a 4 m class telescope; the laser in particular could serve as a prototype module for a larger modular laser. As noted above, this sub-scale system could find immediate practical applications in satellite maneuvering. It would also answer essentially all questions about the viability of a larger system, particularly with respect to transmitting a beam through the atmosphere. Finally, using the engineering experience and proof-of-principle results from the sub-scale launcher, a full 20 MW launcher could be designed and built. The time required to do this depends on the priority given to the project, but an overall time scale of five years appears feasible. Conclusions A working ground-to-orbit laser launch system could be built by the middle of the coming decade. Such a launcher would be capable of launching tens of thousands of small (20 kg) pay loads into low Earth orbit every year, at an incremental cost approaching US$100/lb. The capital cost of the system, including development costs, would be approximately US$0.5 billion—comparable to the cost of a handful of shuttle or expendable rocket launches, whose total payload the laser could launch in a few months. Such a laser system could significantly lower the cost of many space operations, from space station resupply to the launching of small communications satellites. It would also provide unique capabilities for prompt launch of, for example, emergency spares or small sensor satellites. Even a sub-scale laser system, costing roughly onetenth of the price, could provide new capabilities, notably for maneuvering satellites using thrusters with two to three times the specific impulse of chemical rockets. The basic operation of a laser propulsion thruster has been demonstrated in the
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