Beaming rockets into space
wait for a chance to share a ride with a large satellite,” Kare says.
Another cost advantage comes from larger payload space. While conventional propulsion systems are limited by the amount of chemical energy in the propellant which is released by combustion, in beamed systems you can add more energy externally. That means a spacecraft can gain a certain momentum using less than half the amount of propellant of a conventional system, allowing more room for the payload.
“Usually in a conventional rocket you have to have three stages with a payload fraction of three percent overall,” says Kevin Parkin, leader of the Microwave Thermal Rocket project at the NASA Ames Research Center. “This propulsion system will be single stage with a payload fraction of five to fifteen percent.”
Having a higher payload space along with a reusable rocket could make beamed thermal propulsion a low-cost way to get material into low Earth orbit, Parkin says.
Parkin developed the idea of microwave thermal propulsion in 2001 and described a laboratory prototype in his 2006 Ph.D. thesis. A practical real-world system should be possible to build now because microwave sources called gyrotrons have transformed in the last five decades, he says. One megawatt devices are now on the market for about a million US dollars.
“They’re going up in power and down in cost by orders of magnitude over the last few decades,” he says. “We’ve reached a point where you can combine about a hundred and make a launch system.”
Meanwhile, the biggest obstacle to using lasers to beam energy has been the misconception that it would require a very large, expensive laser, Kare says.
The smallest real laser launch system would have 25 to 100 megawatts of power while a microwave system would have 100 to 200 megawatts.
Building such an array would be expensive, says Kare, although similar to or even less expensive than developing and testing a chemical rocket. The system would make most economic sense if it was used for at least a few hundred launches a year.
In addition, says Parkin, “the main components of the beam facility should last for well over ten thousand hours of operation, typical of this class of hardware, so the savings can more than repay the initial cost.”
In the near term, beamed energy propulsion would be useful for putting microsatellites into low Earth orbit, for altitude changes or for slowing down spacecraft as they descend to Earth. The technology, though, could in the future be used to send missions to the Moon or to other planets and for space tourism.
Kare has looked into the possibility of using lasers to propel interstellar probes for NASA’s Institute of Advanced Concepts. A deep space launch would require higher power lasers with larger telescope systems as well as laser relay stations in space. Powering missions over interplanetary distance would require even bigger lasers and telescopes, as well as different propulsion techniques using propellants easier to store than liquid hydrogen.
Sending a spacecraft to a moon of Jupiter, for instance, would require a laser that gives billions of watts of power. “You’d have to have another couple generations of space-based telescopes to do something like that,” Kare says. “You can in fact launch an interstellar probe that way but now you’re talking about lasers that might be hundreds of billions of Watts of power.” Laser technology could reach those levels in another fifty years, he says.
