The Problem With Current Space Propulsion

Space propulsion is currently caught between a rock and a hard place for two reasons:

High exhaust velocities dissipate more energy per unit thrust. Momentum increases with the square root of energy. Exhausted atoms with 3 times the impulse have 9 times the kinetic energy. This can overheat the spacecraft – and the crew. Increased exhaust velocity also reduces the thrust chamber’s ability to elastically reflect high energy atoms, as more high energy atoms/ions embed themselves in the lattice structure of the thrust chamber wall.

At low exhaust velocities, the spacecraft must accelerate to multiple exhaust velocities. Thus, at lift-off, most initial work accelerates the fuel, and not the payload. As exhaust velocity multiples increase, the ratio of fuel to payload grows exponentially. To reach twice the exhaust velocity, the fuel must be 8 times the rocket mass; to reach 5 times the exhaust velocity, the fuel must be 150 times the rocket mass.

The Solution to High Thrust, High Specific Impulse Spaceflight

A laser powered plasma rocket is a straightforward solution. A ground based propulsion laser on Earth beams light onto a focusing mirror attached to the spacecraft. This mirror focuses light to an intense hot spot at the target. The target would be a small piece of matter at the center of a magnetic nozzle. The intense, highly concentrated laser light would turn this target into a plasma – an electrically conducting gas. At one end of the magnetic nozzle, the magnetic pressure exceeds the plasma pressure. At the other end of the nozzle, the plasma pressure would exceed the magnetic pressure. The plasma would then force open the nozzle end facing out into space and plasma exhaust would be thrust out into space.

If the time the plasma spends inside the nozzle is short compared to the skin time of the plasma, at those temperatures, then the plasma will act as a super conducting balloon with ions bouncing against the magnetic field and then out into space. The magnetic field would insulate the spacecraft from most of the plasma exhaust heat from the laser powered rocket. Furthermore, if the focusing mirror is highly reflective, its rate of heating compared to the heating of the target will be low.

Research To Date

Researchers have investigated energy remotely beamed by microwaves and lasers with some success. Beam-powered propulsion has also been experimentally investigated and is one of the few ways to power vehicles, unconnected to the electricity grid, that need higher energy densities than batteries can supply. Beamed energy could enable planes or container ships, to operate without burning fossil fuels. Leik Myrabo’s Lightcraft is a small prototype for a laser driven plasma propulsion system. I am not aware of any existing laser driven plasma propulsion experiments that insulate the spacecraft from the heat of the thrust chamber with a magnetic field to achieve a high specific impulse.

Is A Laser Powered Rocket Better Than A Solar Sail?

Yes.

The problem with light is that the amount of momentum a given amount of light energy contains is miniscule. Reduced momentum per unit energy is an unavoidable feature of a higher exhaust velocity, yet a laser powered plasma thruster lies in the sweet spot where the exhaust velocity is very high compared to standard chemical rockets yet still low enough to provide a much larger “kick” per unit energy than a solar sail.

The other way to get more momentum out of light would be to reflect light multiple times between a ground-based mirror and a space-based mirror. The required alignment precision, however, is insanely high. A laser powered plasma thruster needs far less precise alignment compared to a system that required multiple bounces between a mirror in space and a mirror on the ground.

Is A Laser Powered Rocket Better Than An Ion Thruster?

Yes.

There is a limit to the density of plasma which ion thrusters can emit as, beyond a certain density, the plasma will screen the acceleration grid. The plasma thruster system proposed in this article is charge neutral and so can be ejected at much higher densities. Furthermore, ion thrusters require an onboard electricity source which creates waste heat. Furthermore, the acceleration grid itself is liable to be hit by the exhaust atoms.

Conclusion

A laser powered plasma thruster is a straightforward design that could simultaneously achieve both high specific impulse and high thrust. Delivering a combined performance that far exceeds any other thruster design currently in use. It’s also safer than existing chemical rockets as there is no store of explosive material aboard the spacecraft. Laser powered plasma thrusters could open the solar system up to manned exploration. First with an Earth-based laser powering trips to the moon and then with a moon-based laser powering trips to the rest of the solar system. The moon is more tectonically stable and so should make the demanding alignment required over multiple astronomical units more feasible.

John

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