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I would like to know, on a theoretical basis, if the electron flow entering into the anode of a carbon arc lamp would impart enough kinetic energy to the anode, and thus to the spacecraft, to help propel it through interstellar space.

The electron flow would be move in the same direction of the spacecraft as illustrated by the photo below:

enter image description here

I understand that a single carbon arc lamp, or even a large array of these lamps, would not produce enough force to accelerate a spacecraft up to the velocity needed for interstellar travel, yet I think that these lamps could provide a continuous force over a long period of time that will reduce the need for carrying a large supply of chemical propellant for the spacecraft's primary propulsion system.

As far as what would be the power source for these lamps, I think the most ideal power source would be an onboard nuclear reactor.

Could a carbon arc lamp help to propel a spacecraft through interstellar space?

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    $\begingroup$ This doesn't give any more thrust than pulling up on your bootstraps. $\endgroup$ – ikrase Apr 22 '20 at 1:01
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    $\begingroup$ Consider this: from where do the electrons receive the kinetic energy they impart on impact with the anode? $\endgroup$ – Christopher James Huff Apr 22 '20 at 1:47
  • $\begingroup$ @ChristopherJamesHuff, good point. I didn't consider that. $\endgroup$ – user35148 Apr 22 '20 at 1:50
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Yes! But perhaps not precisely in the manner you envisioned.

First, as has been discussed in the comments, the electrons emitted by the cathode can't be re-absorbed by the anode, or else no net momentum will be imparted to the craft.

If the electrons are instead emitted out the back of the craft, a 'la an electron gun, a small thrust will indeed be produced. Either because of the light weight of the electron (mv momentum vs mv^2 energy), losses due to synchrotron radiation, or some other physical effect, this force will almost certainly be negligible. A useful thruster will probably look much more like a particle accelerator.

If you were to measure this force for long enough, you may be surprised to find the needle drift from 'infinitesimal' all to way to 'decisively zero'. For, having gradually emitted negative charge, the craft will start to charge positively, and eventually will attract all the electrons back!

However, an arc lamp doesn't just involve electrons. Rather, the electrons slam into gas atoms or the carbon electrodes, producing a superheated soup of positively charged atoms. If this plasma is ejected appropriately, a significant thrust can be obtained without charging the craft.

Now we find that our arc lamp somewhat resembles a number of real-life propulsion systems: if using a gas source, it's probably most like an arcjet; if the carbon electrodes are used as the fuel, it could be a type of solid-fuel PPT.

Pure supposition on my part: the photon pressure produced by the mere light itself could also be harnessed - arclamps are very bright! A solar sail could perhaps be usefully driven by an inappropriately large lamp.

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  • $\begingroup$ Got any papers on the last bit about the solar sails + arclamps? $\endgroup$ – Magic Octopus Urn Apr 22 '20 at 16:21
  • $\begingroup$ @Magic Sorry, I should have been more clear in the answer - I have no idea if that's feasible; besides photon thrusters, it just seemed reminiscent of Breakthrough Starshot albeit with incoherent light rather than a laser. $\endgroup$ – 0xDBFB7 Apr 22 '20 at 18:24
  • $\begingroup$ Aww well it sounded interesting! $\endgroup$ – Magic Octopus Urn Apr 22 '20 at 20:08
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    $\begingroup$ I don't know if anyone's seriously suggested using incoherent light for beamed propulsion. For a photon drive, the inability to focus a tight beam means little...the lost thrust from a bit of divergence is outweighed by the improved efficiency over a laser. However, you still wouldn't use a nuclear reactor to generate electricity (losing most of the thermal power as waste heat) and then use the electricity to heat something up to produce light...you'd just use a glowing-hot reactor at the focus of the reflector. $\endgroup$ – Christopher James Huff Apr 23 '20 at 0:33
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    $\begingroup$ @ChristopherJamesHuff what an interesting idea! Here's some space to expand on that; Would an incandescent light bulb be more efficient than a laser for photon propulsion? $\endgroup$ – uhoh Apr 23 '20 at 3:11

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