For example, the picture below illustrates an "air breathing" vehicle being powered by a space-based laser system; optical windows on the top of the vehicle admit the beam to internal mirrors which direct the beam to various thrust chambers for vertical or horizontal flight. (In this example, laser supported "combustion" is used to heat the propellant [air].) A small amount of on-board propellant is used for final orbit insertion upon exit from the atmosphere.

Beamed Energy Launch Concept

Beamed Energy Lauch Diagram

Finally, microwave powered vehicles can also make use of "indirect" thruster modes (in addition to the microwave analog of laser supported "combustion" modes) by using lightweight rectennas for beam-to-electricity power conversion.

BEAMED-ENERGY SOURCE REQUIREMENTS

Relatively near-term beam power levels (ca. 1 MW) can be used with orbit transfer vehicles because they can operate at a low thrust-to-weight (T/W) ratio. However, very high power levels are required for launch vehicles. For comparison, a typical Earth-launch vehicle (of any kind) requires "jet" powers on the order of 0.1 MW per kilogram of vehicle; thus, beam powers on the order of GWs will be required for even small vehicles (even assuming ideal conversion of beam power into jet power and zero-mass air-breathing propulsion systems).

Various VIS/IR laser systems (e.g., chemical, solar-pumped, free electron laser [FEL]) have been demonstrated. Interestingly, microwave power transmission over modest Earth-to-LEO distances is somewhat more technologically mature; for example, transmission of 100-kW microwave beams over distances of many kilometers and subsequent conversion of the microwave power into electricity was demonstrated in the 1970s. GW-class pulsed microwave sources have been demonstrated; GW-class steady-state power beaming systems were studied extensively as part of the NASA Solar Power Satellite (SPS) study.

SYSTEMS / INFRASTRUCTURE ISSUES

As with the Mass Driver Catapult launch concepts, beamed energy launch systems attempt to lower launch costs by placing the complex and massive parts of the propulsion system on the ground (or in orbit) for easy construction, supply, repair, etc. Although there are no intrinsic technological "show stoppers" to beamed energy Earth-to-orbit propulsion, there are serious issues associated with development and infrastructure costs. This is due to the high beam power levels (many GW) required for launching a vehicle from the surface of the Earth. Thus a similar situation is found to that of the Mass Driver Catapult launch concepts where a potentially very expensive infrastructure must be amortized over many launches to be attractive.

One way to amortize this infrastructure that is unique to beamed-energy systems is that they can supply many users. For example, as shown in the fourth image, a beamed-energy system could be envisioned filling a capacity like that of a terrestrial power grid. Power could be supplied to high-T/W Earth or Moon launch vehicles, orbit-to-orbit or Earth-orbit escape low- or high-T/W vehicles, and lunar base power needs, thus broadening the scope of the user base over which the infrastructure is amortized. Finally, VIS/IR beamed-energy orbit transfer vehicles share many technologies with their solar-thermal propulsion counterparts (e.g., inflatable optics, thrusters, cryogenic H₂ storage and feed systems, etc.). This suggests a potential technology investment strategy starting first with demonstration of solar-thermal propulsion orbit transfer vehicles, followed next with development of MW-class lasers for laser-thermal orbit transfer vehicles, and concluding with development of GW-class laser or microwave systems for Earth-to-orbit launch vehicles.

BEAMED ENERGY AS A SPACE POWER GRID

CURRENT RESEARCH

The Air Force Phillips Laboratory (AFPL) is currently conducting a series of proof-of-concept experiments to demonstrate the feasibility of air-breathing Earth-to-orbit laser propulsion. Because of the availability of only modest laser power levels (currently 30 kW with a planned 1 MW demonstration in 1998), only small, simple vehicle designs can be tested. For example, as shown in the images, the current program uses a lightweight 8 in. diameter vehicle. The bottom of the vehicle is shaped to reflect the incoming pulsed laser beam into a ring-shaped focus to initiate laser "combustion" (heating) of the air. (A simple guide wire is used to control the vehicle's orientation in the laser beam.)