PORTLAND, Ore. Photonic thrusters have been imagined by engineers for about 35 years, but an experimental spacecraft propulsion system using a laser-based thruster will be described at the American Institute of Aeronautics and Astronautics's annual Space 2007 Conference (Sept. 18, Long Beach, Calif.). NASA funded the successful testing of the photonic laser propulsion (PLP) system earlier this year. This demonstration will be conducted by the Bae Institute (Tustin, Calif.) and its founder, Young Bae, a former SRI International, Brookhaven National Lab, and the Air Force Research Lab scientist.
Bae claims to have removed the last stumbling block to using photonic thrusters for spacecraft propulsion. The problem with other designs has been the minuscule thrust and difficulty in staying on target with the lasers. Bae claims to have solved these problems by integrating an optical cavity into a laser that traps the beamed photons, thereby amplifying their light pressure by 3000-times, while easing targeting with a centrally located laser in space.
|Laser light emerges from both top and bottom of the photonic thruster, keeping it stationary as it pushes outward against spacecraft.|
"Our approach to photonic laser propulsion is based on forming an active resonant optical cavity between two high-reflectance mirrors located separately in two space platforms," said Bae. "The breakthrough is in the fact that the laser gain medium in PLP is located within the optical cavity, in contrast to the previous failed attempts at passive resonant cavities, in which the laser gain medium was located outside the optical cavity."
Originally, PLP designers imagined using large ground-based lasers to generate enough intensity and to provide a backstop for pushing against, later moving to passive optical cavities located in space. Unfortunately, none of these designs was able to demonstrate enough thrust. In contrast, Bae's active optical cavity was demonstrated to supply milliNewtons (mN) of power using small, cheap lasers that can be operated from space so they don't have to suffer the distortion of cutting through the Earth's atmosphere. Rather than using the Earth as a backstop, the space-based laser platforms would send beams to push against spacecraft while keeping the central laser stationary with a balancing beam in the opposite direction, or with a conventional thruster attached to the laser platform.
For earth orbit use, Bae claims that satellites in space would no longer need a power supply for their own retro-rockets, but could use a centrally located laser platform to supply occational sychronized nudges from its laser beams to keep them in position. For deep-space missions, the feeble thrust from the pressure of light could build up over time. Since there is no fuel to run out when using photonic power--the spacecraft only requires mirrors to bounce the lasers off--once the laser is turned on, it can stay on for as long as necessary to continually accelerate spacecraft toward the theoretical limit of the speed of light.
Bae claims that the technique could be scaled up to accelerate a spacecraft to Mars in a week compared with the six months necessary using conventional rockets.
"For low thrust--on the order of mN [milli-Newtons]--applications such as precision spacecraft formations, the thrust that we obtained [from our demonstration prototype] is sufficient," said Bae. "For medium thrust--on the order of N--applications, such as spacecraft orbit changing and precision docking, we plan to scale-up [our current] PLP engines, which we believe can be accomplished in a few years. And for high thrust--greater than kN--applications, we plan to investigate new infrastructure technologies, such as in-space nuclear powered laser systems."