Because the hot gas layer is only a few millimeters thick while a typical vehicle is two meters across, no nozzle is needed to confine the expanding gas. The expansion generates thrust uniformly across the flat base of the vehicle (hence the ‘planar’ thruster). In addition to making the vehicle design extremely simple, this scheme has two other advantages. First, the thrust direction is independent of the laser beam direction; the vehicle can fly at an angle to the laser beam. Second, the thrust can be varied across the base of the vehicle by controlling the beam profile. The vehicle can therefore be steered from the ground, and does not need its own guidance system. Properties of a Ground-to-orbit Launcher Figure 2 illustrates the components of a minimum size ground-to-orbit launch system which could be constructed in the next four to five years. The laser is a 20 MW average power electric discharge CO2 laser, producing 500 kJ, 2 /zs pulses at 40 Hz. This would be a very large laser, but the technology for such large CO2 lasers was well developed in the 1970s. Because of the physics of the double-pulse thruster itself, the 10 /zm wavelength is preferred over short wavelengths, although a laser propulsion system could operate at wavelengths as short as 1 /zm. A high power free electron laser (FEL) would be even better, offering higher electrical efficiency (20-25% vs 15%) and possibly greater reliability. FEL technology is still new, however, and may not be available at competitive prices for several years. The laser requires roughly 150 MW of electricity, which can be obtained from the national power grid or produced locally, e.g., by diesel generators. The laser beam is focused by a 10 m diameter beam projector telescope onto a 2 m diameter vehicle. This combination gives a useful range of approximately 1000 km. The maximum payload mass is proportional to the system range (other factors being equal), but 1000 km approaches the maximum practical range, both because of limits on telescope and vehicle size, and because the vehicle must stay well above the laser’s horizon during the launch. The telescope could be a variant of a conventional astronomical telescope, similar to the 10 m Keck astronomical telescope now being built by Cal Tech and the University of California [5], or it could be a more specialized design, for example a phased array of smaller mirrors. An adaptive optics system is needed to correct for atmospheric turbulence and thermal blooming, but the combination of long wavelength and a cooperative vehicle (which can even telemeter back information about the beam profile) keeps the complexity of this system well within the state-of-the-art. However, a mountaintop (3 km altitude) launch site is needed to reduce absorption of the laser beam by atmospheric water vapor and CO2 The vehicle consists of 120-150 kg of propellant and 20 kg of payload, with a few kilograms of structural support, primarily a stiff baseplate to support the thin propellant block. A throwaway air-breathing stage improves performance by lifting the vehicle to 20 km or higher with a ‘laser pulse-jet’. The vehicle then drops the air- breathing hardware and accelerates vertically to about 100 km, where it ‘turns over’ and accelerates downrange to 400-500 km altitude and 1000 km range. At that point it runs out of propellant and enters a circular or elliptical orbit. The maximum acceleration is about 6 g. The time from launch to entering orbit is 15 min or less. Launcher Cost and Scaling The cost of the 20 MW-20 kg system described here is estimated at US$450m; this is roughly broken down in Table I. The incremental cost of launching a single vehicle is
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