Via advanced nanotechnology, the news that the Solid State Heat Capacity Laser (SSHCL) has achieved 67 kilowatts of average output power in the laboratory. Six to eight months of additional work are needed, it is believed, to reach the 100 kW mark. Which sets Brian Wang to pondering a “…proof of concept photonic laser propulsion system using mirrors to bounce laser light and multiply the effectiveness of lasers generate 35 micronewtons of thrust using low wattage lasers and 3000 bounces.”
Wang then quotes from a paper on multi-bounce methods by Geoffrey Landis and Robert A. Metzger. A major problem in laser lightsail techniques is reducing the power requirement, which can be onerous:
It has been proposed that extremely small payloads (10 kg) could be delivered to Mars in only 10 days of travel time using laser-based lightsail craft (Meyer, 1984), but in order to do so, would require a 47 GW laser system.
And if we start thinking interstellar, the laser numbers go sharply up. We’re talking about lasers in the range of 65 GW to 7.2 TW, power levels approaching the total power generating capabilities of Earth. A limiting factor is the sheer inefficiency of momentum transfer from a photon to a lightsail. But bounce methods can dramatically alter power requirements.
The bounce method Landis and Metzger discuss seems straightforward. A laser beam sent to a lightsail is directed back to its source, from which it is reflected again to the lightsail. Landis and Metzger see the limitations on the number of bounces feasible being dictated by our ability to re-aim the reflected beam, the efficiency of the spacecraft (and the reflector at the laser site) in reflecting the beam, and the heating of the lightsail.
Can multi-bounce methods make the difference? Landis and Metzger’s conclusion: “The use of a multi-bounce approach radically reduces the power requirement of the laser system (by a factor of 1000) as compared to conventional single bounce laser sail schemes, making the possibilities of such multi-bounce lightsail craft feasible within the coming decades.”
So 67 kW for a laser in a laboratory setting is getting us into interesting territory. Wang goes on:
One thousand 100 kilowatt laser modules and 2000 bounces would be equal to a 200 Gigawatt laser. This would be 4 times the 10 kg system and could deliver 40kg payloads to Mars in ten days. Ten thousand modules would allow for 400 kg payloads to Mars in ten days.
All of which makes for fascinating reading. The Landis and Metzger paper is “Multi-bounce laser-based sails,” Space Technology and Applications International Forum 2001, in AIP Conference Proceedings, Vol. 552, with abstract here. Fortunately, the full text is also online. Interesting to study this one in connection with Young Bae’s work on laser bounce techniques as recently discussed in these pages.