Microwave Beams: the Only Way to Fly
Jetsons imagery aside, proposed 21st-century transports could whisk you off to the moon in less than six hours
Leik Myrabo wants to take the smoke and thunder out of spaceflight.
He would replace it with the virtual silence of disk-like craft propelled by microwave beams from satellites orbiting Earth.
The idea may seem like something out of a Star Trek production. His approach, however, is one of several the National Aeronautics and Space Administration is looking at as possible follow-ons to its reusable launch-vehicle (RLV) program. The program is designed to help the aerospace industry develop technologies leading to a fleet of economical, reusable, single-stage launch vehicles. The shuttle puts payloads into orbit for between $5,000 and $8,000 a pound. NASA hopes to bring that cost down to between $300 and $500 a pound.
''We have a series of projects under way to identify future highly affordable, reusable launch vehicles,'' says John Mankins, NASA's manager of advanced concepts studies.
The focus ''is to design a transport system for the 21st century, when we have a mature space industry,'' says Dr. Myrabo, associate professor of engineering physics at Rennselaer Polytechnic Institute in Troy, N.Y.
Myrabo also envisions using such craft for intercontinental air transport. Up to 500 solar-powered microwave satellites could be deployed to power a system that would replace one-fourth of today's international air traffic, he says.
The idea for these satellites has been around for years as a means of generating electricity on Earth. Although technically feasible, such a constellation could generate political heat as people debate real or imagined environmental effects and economic trade-offs.
For his part, Myrabo takes the long view. In a lightcraft world, ''airports probably won't exist,'' he says. ''People will be riding energy-beam highways. Aircraft would pick you up at your house and take you anywhere on the planet in 45 minutes and to the moon in 5-1/2 hours.''
Much of a conventional rocket's size and mass is devoted to the onboard fuel and oxidizer. With Myrabo's craft the fuel - microwave power - is external. The motor would consist of electrodes, magnets, and special antennas, rather than multi-ton arrays of fuel tanks, high-pressure pumps, combustion chambers, and nozzles. He figures that the launch mass of a five-passenger, beam-riding ''lightcraft'' destined for the moon would amount to 1,400 kilograms (3,080 lbs.), about the weight of a mid-sized sedan. By contrast, the 1969 Apollo-Saturn 11 mission, which put three astronauts on the moon, had a launch mass of 2.85 million kilograms (3,135 tons).
The lightcraft's antennas, dubbed rectennas, would convert part of the 10-billion-watt microwave beam into electricity for onboard systems. Rectennas inside the ship's rim would focus microwave energy along a line just beyond the perimeter. The air along that line would turn into a plasma, a gas in which electrons have been stripped from atoms.
The ionized atoms can be manipulated by superconducting magnets along the rim. Like hot gases escaping from the nozzle of a rocket, the high-pressure ionized gas would escape through a ''nozzle'' of magnetic fields, generating thrust. High-speed computers could change the direction of the magnetic nozzle electronically to steer the craft.
Myrabo points out, however, that the saucer-like shape produces excessive drag when the craft reaches Mach 3, or three times the speed of sound. ''And this is supposed to go into orbit?'' he asks. The answer: Rectennas would focus microwaves ahead of the craft to set up a cone- or wedge-like arc of plasma-heated air. The hot air inside these ''spikes'' is less dense than the surrounding atmosphere and presents less friction and stress. A lightcraft clocked at Mach 25 would be subject only to Mach-3 stresses and frictional heat.
The air spike also sets up a shock wave around the bow that can be harnessed by a unit known as a magneto-hydrodynamic (MHD) fan-jet motor. As compressed air from the shock wave passes electrodes on the hull, focused microwave pulses turn it into a plasma. The ionized gas conducts a current between the electrodes, which interacts with magnetic fields from the rim magnets. The result: forces that accelerate the passing air, propelling the craft up or sideways to speeds in excess of Mach 25. In effect, the lightcraft and the air combine to form an unconventional linear electric motor.
The air spike itself also could have nearer-term applications, says NASA's Mr. Mankins. Designers might find ways to use waste heat from large combustion engines to generate electricity. Then they could use air spikes to generate a synthetic shock wave ahead of craft such as high-speed commercial transports, reducing the likelihood of sonic booms.
Myrabo collaborated with Russian plasma physicist Yuri Raizer of Moscow's Institute for Mechanical Problems to develop the theoretical basis for an air spike. In April they tested the notion in RPI's Mach 25 wind tunnel. Experimenters not only generated the anticipated spike, but also demonstrated the anticipated bow-shock-wave effects.
''This proves the air-spike concept,'' he says. Two other key pieces - the high-power rectenna and MHD fan-jet have yet to be demonstrated. In the 1970s, however researchers at Raytheon Company demonstrated a low-power rectenna, while Westinghouse scientists demonstrated an external MHD accelerator, Myrabo says.
The next steps include more experiments and theoretical work with the air spike.
* Peter Spotts can be reached via electronic mail at firstname.lastname@example.org.