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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Russ Ivanov says
Hi John,
Very long ago, back to soviet Union, after my graduation, I was involved into the development a powerful military laser system aimed at destroying airplanes . As I remember, one of the hardest challenges was the turbulence of the atmosphere, which prevented the laser beam from continuous focusing at the same spot on the target. The focal spot upon the target randomly moved around in the range of a couple of feet , when the distance to the target was as long as a few km . If the object in the outer space is supposed to be many thousands km away, how to solve the problem of focusing the beam?
Russ
admin says
Hi Russ,
That sounds bad. Very bad. It may be a show stopper for a Earth-based laser powering a spacecraft far away on the moon.
On the other hand a moon-based (or space based) laser might still be able to power airplanes and rockets on Earth. Because the dense part of Earth’s atmosphere is only a few kms, so you could aim a moon-based laser at any rocket or plane anywhere on Earth and it would only wiggle by a few feet, which sounds manageable. And once the airplane or spacecraft reaches high altitude, I imagine the refractive effect of turbulence might be less.
A compelling reason to colonize the moon?
John
Peter Denner says
To add to what Russ said, the closest precedent that I can think of is the Lunar Laser Ranging experiment. According to Wikipedia, “At the Moon’s surface, the beam is about 6.5 km wide, and scientists liken the task of aiming the beam to using a rifle to hit a moving dime 3 km away … Out of 10^17 photons aimed at the reflector, only one is received back on Earth, even under good conditions.”
Even assuming you could improve on the 1-in-10^17 figure by two or three orders of magnitude, this does indeed sound like a show-stopper for an Earth-based system, but as you say, maybe a Moon-based system would be feasible.
admin says
The key metric of importance is the airy disk which is inversely proportional to the beam diameter. I’ve calculated that in order to keep the laser beam to within a diameter of 20m a moon distance away, the diameter of the laser beam must itself be 20m.
So this is a massive engineering project!
There’s actually a clip from myth-busters on the lunar laser ranging project here:
https://www.youtube.com/watch?v=VmVxSFnjYCA&feature=youtu.be&t=140
Looks to me like the beam diameter of the lunar laser ranging experiment is about 10-20cm, 100 times less than the laser beam diameter of 20m that I propose. You would indeed expect that a laser diameter that was 100 times smaller, would give rise to a laser spot on the moon 100 times larger ( i.e. 20m X 100 = 2km )
Furthermore, from the video, the diameter of the retro-reflector looks even smaller. This would give rise to an even larger airy disk.
10cm out of 6 km= 1/(3.6*10^9)
Assume the same fraction getting back to Earth (1/3.6*10^9)^2 = 1/10*10^18 = 1/10^17
Which is the number you gave.
But if you increase the beam diameter 100 fold from 10cm, to 20m you get very different results and much less divergence.
But yes, we are talking about extreme engineering.