Propulsion of an interstellar probe by a laser-pushed lightsail has been proposed by Forward and others. A difficulty of this approach is that, while the specific impulse is effectively infinite, the energy efficiency is very low, and hence a lightsail requires extremely high power. Since the limiting factor in the construction of a laser-propelled interstellar probe is the amount of laser power required, it is useful to consider alternatives that increase the energy efficiency. An alternative is the laser-powered rocket, where the laser is converted into electrical energy, which is used to power an electric propulsion system.
The trade-off between a laser lightsail and a laser-electric propulsion depends on the mission velocity. The optimum energy efficiency occurs when the exhaust velocity is equal to 0.63 times the mission velocity, at which point the mass ratio of the vehicle is 4.92, independent of the mission velocity. For higher exhaust velocity, energy is wasted in the exhaust energy; for lower exhaust velocity, energy is wasted in accelerating propellant. For example, for a flyby probe with a velocity of 0.2c, the optimum exhaust velocity is .126c, corresponding to a specific impulse of about 3.8 million seconds. While this is out of the range of possibility of even nuclear rockets, it is possible to achieve such high exhaust velocities with ion engines.
As an example mission, a probe at 0.2c to Alpha Centauri using a 15 GW laser is analyzed. Such a probe would return information well within the original experimenters' professional lifetimes.
As the mission velocity increases, specific impulse becomes more important, and the lightsail becomes more attractive. A lightsail increases in energy efficiency as the velocity increases, approaching an efficiency of 100% as the speed of light is approached. On the other hand, if it is desired that the probe stop at the target system, the efficiency of a lightsail in the braking mode is very low. Thus, the laser-electric system is favored for lower velocity missions (under roughly 0.5c), and for missions which require braking at the target. For higher velocity systems, staged propulsion, using a laser-electric engine for the initial acceleration and switching to laser-lightsail propulsion at high velocities, is optimum.
In the limit of extremely high specific impulse of the electric engine, the photon momentum can be comparable to rocket thrust. In this case, extremely high specific impulse can be achieved with energy efficiencies considerably better than that of a pure lightsail.
Laser-propelled Interstellar Probe (schematic)
Fixed laser, at left, illuminates a light-weight solar array, shown here as a centrifugally-tensioned thin-film membrane supported by tension wires. Power from the array is fed to an ion engine.
The text portion of the presentation made at the conference can be found here.