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Given 10-1000 Watts of electrical power (and no other consumables), what is the current best way to turn it into thrust? Just running it through a heater on an insulating pad would result in an IR thruster, but has bad focus. A laser has good focus but only for a small percentage of the energy.

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Is it supposed to happen in the atmosphere or in vacuum? – Deer Hunter Jun 14 '13 at 21:16
I'm thinking space (so a box fan is out). – BCS Jun 15 '13 at 2:24
Note that the Pioneer spacecraft unintentionally became a photon rocket, causing the Pioneer anomaly:… – Ben Crowell Aug 25 '13 at 22:12
@BenCrowell countering that sort of things (or more accurately torques caused by them) is the application that got me thinking about this. – BCS Aug 29 '13 at 1:09

As you suggested yourself, in a vacuum environment, turning electrical energy into photons can be done at near 100% efficiency using a heating coil. A lot of the energy will be in the infrared band. All photons, regardless their wavelength can be focused by placing the heating coil at the focus of an aluminum parabolic mirror. A total $P = 300 \ W$ of electrical power would create close to $\frac{P}{c}=\frac{300 \ kg \ m^2 \ s^{-3}}{3\ 10^8 \ m/s} = 1 \mu N$ of thrust.

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I guess the question devolves to a balance between mass of the device and how well it collimates the radiation. I suspect that a dish would have to be VERY deep to do a lot better than a hot plate and will be a lot more complex. – BCS Aug 29 '13 at 1:15

In my opinion the most efficient way to turn electricity into thrust is by using lasers. From what I know laser beams are the most collimated forms of light.

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What about accelerating ions / electrons? – Brandon Enright Jan 9 '14 at 22:55
the laser beam might be highly collimated but the waste heat (IR) won't be. Besides, it adds a relatively large mass that it has to accelerate (which might be an issue is your power supply has a high enough specific power). – BCS Jan 10 '14 at 21:57

The keyword here is Ampere-Maxwell.

Like this: enter image description here

The first thing you see is a circuit, with wires coiled around a nail, then it the circuit it is puut inside a box (as seen in the picture below it) with the nail protruding, and its magnetic field gives thrust to the spaceship., A little, at least.

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I could see how that would work is it was an AC power source, but it raises the question of how efficient it is. I'd expect this becomes a "directional antenna" problem: i.e. the more directional an antenna you build the more efficient a thruster it becomes. The question then becomes is it easier to make an X Watt EM radiator highly directional in the RF band or the IR band? – BCS Jul 15 '13 at 18:14
You are assuming the energy source isn't carried with the ship, the OP assumed the ship would carry the source. You can definitely push a ship from outside using a mounted energy source, that's not the issue, the issue is to do it when you don't have a momentum-sink, you need to conserve momentum, so you need to eject stuff, and since OP doesn't want to eject stuff, he need to eject photons. – Ron Maimon Aug 22 '13 at 22:29

One possible solution is a heater pad with a honeycomb (made out of a materiel that is a good blackbody and a poor conductor) stacked on it. I don't know the math needed to show that it will work but I suspect that this will result in an "effective surface temperature" that is strong a function of the angle from the axis of the honeycomb.

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