The performance of conventional rockets is limited by the amount of chemical energy in their fuel. One way to improve the performance of rocket engines is to separate the energy source from the rocket. This can be accomplished by using a laser beam to transfer energy from a stationary source to the rocket. In laser propulsion, the rocket carries a tank of reaction mass, but a stationary laser supplies the energy. The laser can either be located on the ground, and beamed upward at the rocket, or in orbit, and beamed downward.
There are two approaches to laser propulsion to launch from the surface of Earth into space. In laser-thermal propulsion, a laser beam is used to heat a gas, which expands through a rocket nozzle to provide a thrust system. The laser beam is focused on a thermal receiver, consisting of a chamber with pipes through which the reaction fluid can flow. This thermal receiver then heats a fluid to vaporize it into a gas, and the hot gas expands through a conventional rocket nozzle to produce thrust. The advantage of the laser thermal system is that the fluid used for reaction gas can be an extremely light fluid-weight, such as liquid hydrogen, to result in very high performance.
A second approach to laser propulsion for launch is laser-supported detonation. In laser-supported detonation, a repetitively pulsed laser is utilized. Either liquid or solid reaction mass can be used. The reaction mass is vaporized by a pulse of the laser, and then a second laser pulse causes the reaction mass to explode into a high-energy plasma, a gas heated to the point where the electrons are stripped from the gas molecules, behind the rocket. The explosion pushes the rocket forward. An advanced laser propulsion system might use air as the reaction mass for the initial portion of the flight, when the rocket is still in the atmosphere.*
Laser propulsion systems require a high-power laser, a tracking system to follow the motion of the rocket, a mirror (or "beam director") to aim the laser at the rocket, and a lens or focusing mirror to focus the laser light onto the receiver. The difficulty of laser propulsion is that the system requires a laser with higher power than is available in currently existing laser systems. Laser propulsion can also continue to be used once the rocket is in space, to raise the vehicle to a higher orbit, or to boost it to a transfer orbit.
Another propulsion system is laser-electric propulsion, the use of a laser to illuminate a solar array to power an electric thruster. In laser-electric propulsion, a stationary laser (either based on Earth or in orbit) sends a beam of light to a photovoltaic array, which converts the beam into electrical power. This electrical power is then used as the power source for an electric thruster, such as an ion engine.
Further in the future, a laser might also be used to push a lightsail. This propulsion concept could be used as the engine for an interstellar probe.
see also Accessing Space (volume 1); Ion Propulsion (volume 4); Lightsails (volume 4); Power, Methods of Generating (volume 4).
Geoffrey A. Landis
Myrabo, Leik, and Dean Ing. The Future of Flight. New York: Baen Books, 1985.