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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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  • $\begingroup$ Is it supposed to happen in the atmosphere or in vacuum? $\endgroup$ – Deer Hunter Jun 14 '13 at 21:16
  • $\begingroup$ I'm thinking space (so a box fan is out). $\endgroup$ – BCS Jun 15 '13 at 2:24
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    $\begingroup$ Note that the Pioneer spacecraft unintentionally became a photon rocket, causing the Pioneer anomaly: physics.stackexchange.com/questions/9727/… $\endgroup$ – Ben Crowell Aug 25 '13 at 22:12
  • $\begingroup$ @BenCrowell countering that sort of things (or more accurately torques caused by them) is the application that got me thinking about this. $\endgroup$ – BCS Aug 29 '13 at 1:09
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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 emitted, 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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  • $\begingroup$ 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. $\endgroup$ – BCS Aug 29 '13 at 1:15
  • $\begingroup$ It's a gross simplification, or even an exaggeration to say "All photons, regardless their wavelength can be focused by placing the heating coil at the focus of an aluminum parabolic mirror.". Gamma rays and hard X-rays will go right through aluminum. $\endgroup$ – Ross Presser May 30 '17 at 8:54
  • $\begingroup$ @RossPresser - hard X-rays from a heating coil? What temperatures do you have in mind? $\endgroup$ – Johannes Jul 7 '17 at 12:44
  • $\begingroup$ @Johannes - point taken -- but I think it makes sense to qualify your "all photons" right in the answer, rather than assume people understand what photons come from a heating coil. $\endgroup$ – Ross Presser Jul 7 '17 at 18:57
  • $\begingroup$ @RossPresser - ok thx. Have changed "all photons" into "all photons emitted". $\endgroup$ – Johannes Jul 14 '17 at 5:16
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The most efficient way to produce photon from electricity would be heating. Now the question is about collimating the emitted waves with the lowest mass possible.

How about a really narrow parabolic reflector made of flexible gold plated polymer. You may do away with supporting structures by using the light pressure to "inflate" the sail, supplemented with charging the whole spaceship, including the sheet, as to expend the sail by electrostatic pressure. This reflector could be used as a sail, putting any payload in a solid annulus around the boundary of the sheet. The second most important engineering consideration in this contraption, after material integrity, would be to keep the power density of the IR reflecting of the sheet below a certain maximum. This maximal flux would be given by the reflectivity of the gold layer and the maximum temperature of the polymer sheet, so that the excess heat can be radiated away (as loss). Maybe the reflectivity could be augmented by the use of specialized treatments of the sheet surface.

Another nice idea would involved not currently existing high-efficiency meta-materials arranged around the thermal source to collimate it. This may not be so farfetched as meta-materials have been constructed for microwave wavelengths. This contraption could be much more compact and robust than the first one, but a realistic implementation might end up heavier than the first proposition.

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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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  • $\begingroup$ What about accelerating ions / electrons? $\endgroup$ – Brandon Enright Jan 9 '14 at 22:55
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    $\begingroup$ 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). $\endgroup$ – BCS Jan 10 '14 at 21:57
  • $\begingroup$ @BrandonEnright you would loose mass... $\endgroup$ – G. Bergeron Dec 20 '16 at 11:24
  • $\begingroup$ @G.Bergeron You lose mass with lasers too. Except then the only mass you lose is in energy and you don't have the rest mass of the electron to contend with. $\endgroup$ – Brandon Enright Dec 20 '16 at 19:58
  • $\begingroup$ @BrandonEnright Agreed. But this is this saturates the absolute maximal ''mass efficiency'' of any propulsion system. $\endgroup$ – G. Bergeron Dec 21 '16 at 5:19
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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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