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Is it able to convert electricity into propulsion force?

  1. Jul 4, 2010 #1
    Is it possible to convert electricity into propulsion force?

    My friends and I are discussing about this subject. Is it possible to build a device which can convert electricity into propulsion force?

    just so you know, we have incessant electricity produced by solar cells and if that electricity could be stored and somehow converted into propulsion force:I think we(perhaps) might be able to create a new renewable driving 'fuel' for our spacecrafts...... Therefore(if possible) the hardship of carrying enormous amount of chemical fuels for our spacecrafts might be avoided and space travel might come in handy.

    Is it possible to do so? sorry if I misunderstand something.

    Looking forward for your responds


  2. jcsd
  3. Jul 4, 2010 #2
    http://en.wikipedia.org/wiki/Ion_thruster" [Broken] work this way. It still requires ejecting some kind of mass like any rocket, but the ejection impulse is supplied by electricity in this case. Basically a magnetic rail gun ejecting ions instead bulk metals.
    Last edited by a moderator: May 4, 2017
  4. Jul 4, 2010 #3
    Maglev (magnetic levitation) trains use an electromagnetic propulsion system to propel the trains along the rails. Another electromagnetic (magnetohydrodynamic or MHD) propulsion system is one proposed for submarines, wherein an ion current is created in seawater in a channel that has a very strong orthogonal magnetic field through which the seawater flows. The Lorentz I x B force on the ions creates a thrust. See


    Bob S
  5. Jul 4, 2010 #4
    So, what does an electric motor do?
  6. Jul 4, 2010 #5
    Well, first of all, electrons in this scenario are not "produced". What IS produced is the potential for movement.
    So, where do you get more electrons to thrust out after the initial thrust? Solar cells won't work; they DON'T MAKE ELECTRONS. They only move the available electrons.
  7. Jul 4, 2010 #6
    sounds like you mean using electrons directly for propulsion?? good idea. as far as it goes...

    Sure, but is it economical and practical????

    A vacuum tube DOES accelerate electrons directly from cathode thru grid to anode, producing a "force" against the emitting cathode but that process requires a complete circuit.....perhaps limiting what you have in mind.

    Post #5 IS a constraint, not necessarily an obstacle.

    Solar cells do move loosely bound (available) conduction electrons, but no simple mechanism produces them in abundance.


    So I don't know of a practical way to produce electrons in abundance and accelerate them out the back of a rocket ship without some sort of intermediate fuel storage. (but then again I don't know all that much!!)

    One way might be to create a horizon ( black hole, cosmic, accleration) and use that to separate virtual electrons from positrons and utilize the real electrons. Another theoretical way might be to collect electricity from from black hole emissions, then eject them as a power/propulsion force....one theory posits a society/outpost with a huge hollow space station (ring) surrounding a black hole and extracting "free' power from it....

    The solar wind may be a more direct source of propulsion.

    But as you can infer, there are a vast array of Nobel prizes available for such stuff. go get one!!

    PS: Have you thought about utilizing photons (light) directly...why bother with low efficiency solar cells when the sun sends us lots of free photons 24 x 7??
    I suspect their momentum is small per photon, but you can't beat "free"....
    Last edited: Jul 5, 2010
  8. Jul 5, 2010 #7


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    You can do anything you want, so long as momentum is conserved. If you can come up with something that will get the impulse exactly opposite to the impulse attained by your craft, you have a plausible drive. Rest is engineering. If you can't, then you are in violation of Momentum Conservation Law, and authorities know where you live.
  9. Jul 5, 2010 #8
    Currently Ion drives do a pretty good job at converting electricity into propulsive force with least amount of propellant. But, you still need some propellant.

    If you're talking about turning electricity into force without using reaction mass, the only way I can think of is to turn the electricity into light, which would give you a net force.

    The only thing is that in order to get a useful amount of net force, you would need an absolutely preposterous amount of electricity, and a stupidly powerful flashlight. You're not going to get anywhere within a lifetime using solar panels. You're going to need exotic energy sources, at least utilizing fission, but you'd probably be better off waiting for fusion or annihilation to be available.
  10. Jul 5, 2010 #9
    How about something like this device


    Attached Files:

  11. Jul 5, 2010 #10
    You can use electrons, or positively charged ions, for propulsion of a spacecraft, but as soon as the spacecraft builds up a residual charge of a few microCoulombs, the electrons or ions will be attracted back to the spacecraft and the propulsion will cease.

    A typical spacecraft will have a self-capacitance of ~100 to 1000 picoFarads, so a charge buildup of a few microCoulombs will be significant. Dickfore's solution will work, because it accelerates an equal # of electrons and ions.

    [added] See electron injection ion neutralizer in


    Bob S
    Last edited: Jul 5, 2010
  12. Jul 5, 2010 #11
    Exactly. That's the problem.
    Regardless of method, mass must be expelled.
    Additional propulsive mass must be made available.
  13. Jul 6, 2010 #12
    Nearly correct.
    Momentum must be "expelled", but not necessarily mass. Massless photons carry momentum, and can be used as an inefficient form of propulsion.

    Bob S
  14. Jul 6, 2010 #13
    Lol, Not so!

    It is highly efficient. Just terribly, terribly puny.

    Like the Stirling engine (very efficient but simply can't burn enough fuel to provide power for most purposes)
  15. Jul 6, 2010 #14
    From Bob S

    "Massless photons are..... an inefficient form of propulsion."
    The photon momentum, expressed in terms of the photon energy E is

    p = E/c

    The ion momentum for a low energy ion beam with ion energy E = ½mv2 is

    p = 2E/v = (2c/v) E/c = (2/β) E/c

    So a low energy ion beam is more efficient by a factor of 2/β in converting energy into momentum (thrust).

    Bob S
  16. Jul 6, 2010 #15
    I'm working on it ;)
  17. Jul 7, 2010 #16
    Excuse me?!
  18. Jul 7, 2010 #17
    There are only two options. One, as mentioned, is to use electricity to accelerate some ejected reactive mass. This obviously can not continue indefinitely, unless in some far-future sci-fi scenario you generate matter/antimatter from energy, but that would be quite pointless compared to emitting that energy as light directly.

    Emitting light is the other option, yes. It has impulse much like matter and can function equivalently to reactive mass. The case where you convert light to electricity with solar cells and then back to light for propulsion however is obviously not optimal for efficiency. You'd be better off just directing/reflecting the incoming light in the first place, i.e. use solar sails instead of solar cells.

    Bob, you are only taking into account the kinetic energy of the ions, not the full energy that you are losing. That is why it seems to be more efficient.
    You are losing the mass of those ions, which by Einstein's famous formula is also energy. If you had for example the corresponding antimatter ions and could annihilate them, and release the full energy as light in a single direction, then that would be the most efficient propulsion.
    Last edited: Jul 7, 2010
  19. Jul 7, 2010 #18
    Using Newtonian mechanics (good enough for current rockets and ion engines)...
    Momentum: P = m*v
    Kinetic energy: E = 0.5*m*v^2

    The important parts are the scaling with mass and velocity. You can achieve the same momentum change by ejecting half the mass at double the velocity, but it takes four times the energy (assuming equal efficiency in converting energy from the power source into kinetic energy of the exhaust).

    High exhaust velocities are more mass efficient and less energy efficient. It's a tradeoff. You can get away with smaller propellant tanks with ion engines, but you need a dense, high power source of electricity. You can get a lot more thrust for a given energy input with electrothermal or chemical engines, but you need much bigger propellant tanks. A photon rocket would be the most mass efficient, least energy efficient option...no propellant needed, but enormous amounts of power required to produce any useful thrust.

    Photon sails use an external source of light so they don't have to carry around a massive power source and insanely bright light source, instead carrying a lightweight reflective sail, and are rather more practical than photon rockets. They may use sunlight, but will also work with laser beams that could be produced electrically at a fixed installation.
  20. Jul 7, 2010 #19
    I was wondering about the whole photon rocket concept...if matter/antimatter annihilation produces pure energy, wouldn't it be more practical to simply annihilate matter/antimatter (or even detonate a nuclear bomb) inside some sort of mirror-lined combustion chamber, instead of going through the whole nuclear reaction-electricity-preposterously bright flashlight process?
  21. Jul 7, 2010 #20
    Antimatter annihilation actually releases about half the energy in the form of neutrinos. These are essentially impossible to stop, and so about half the total energy is lost.

    Despite this, given highly reflective gamma ray mirrors, an antimatter photon rocket would be the most mass efficient rocket possible. The power output required to achieve any notable acceleration would be enormous, though, and the antimatter extremely energy intensive to produce. With antimatter, you could of course achieve high power outputs rather simply, but you want to stay intact while doing so...and you also need those magic gamma ray mirrors.

    Another, more practical approach would be to perform antimatter annihilation inside a gamma ray absorber, which would then heat to incandescence (or build a nuclear reactor that directly radiates its output rather than first converting it to electrical power)...there's nothing to gain by making a gamma ray rocket over a visible-UV one, the thrust is proportional to the total power output, not wavelength. The absorber/radiator has a more limited power output (there's a maximum temperature it can operate at before vaporizing), but it's far easier to reflect the re-radiated light (or convert a fraction of it to electricity to power the craft).

    Neither option is really very practical though. Photon rockets maximize mass efficiency, but require enormous amounts of power to produce as much thrust as even an ion drive. Rather than building enormous facilities for turning vast amounts of energy into waste heat to produce the needed quantities of antimatter to fuel these craft, you'd be better off building laser stations for sailcraft. Lasers are very inefficient at turning energy into thrust, but the lasers and their power sources are fixed installations, they don't have to move themselves with the thrust they produce...this approach frees the sailcraft from having to carry propellant or large power sources.
  22. Jul 8, 2010 #21
    How far can you shoot a laser though before it spreads out? And then of course if you have a destination, you need to slow down...

    I started getting interested in the whole photon rocket thing because it seems like the only option for getting anywhere outside of the solar system. Sure it takes a lot of power, but I'm sure that problem will be resolved in the future. Whether in 30 years or 300 years...it's good to know that eventually, it most likely WILL happen. Just a few months ago I was questioning whether we'll be stuck here forever.
  23. Jul 8, 2010 #22
    There's no hard limit on distance, it depends on how big you make the output optics of the laser station. (An aside: arrays of smaller lasers spread out over a wide area are not a useful approach for this...you can reduce the spot size by doing this, but not increase its intensity. Read up on the "thinned array curse" for details. Fortunately, the reflectors don't have to withstand a strong gravitational field.)

    As for stopping at the destination...for many purposes you won't need to, a flyby sufficing. And for many destinations it would be practical to exploit aerobraking, or to set up a beam propulsion station using slower missions able to stop themselves. Or if control of the sails is good enough and the laser's reach long enough, you could jettison the larger part of the sail and use it to reflect light back onto the smaller portion remaining on the craft, decelerating it while the bulk of the sail continues to accelerate.

    In fact, it would probably be more effective to make a one-shot beam power station that is accelerated with the craft and then released ahead to decelerate the main craft, than it would be to make a photon rocket that could brake itself with the main craft as payload. The sailcraft would effectively get double the thrust by reflecting light back toward its origin instead of just emitting it, and during the deceleration phase, it wouldn't have to carry the power source or light source, greatly reducing its mass. (even with antimatter as a power source, the extremely high power equipment required to make a photon drive/laser station will be heavy)

    A solution to the power requirement issue seems rather physically implausible, no matter what the timeframe. Even if you use antimatter to produce the power...there's the simple issue of needing to emit and direct light at enormously high power without the craft vaporizing or proving hopelessly overweight. Low thrust photon drives can certainly be built (they're really just scaled up flashlights), but even if their total delta-v is enormous, missions using other drives will be able to get to the destination and back before even an advanced photon drive craft will have accomplished any significant part of its acceleration.

    There are several alternatives for interstellar probes that seem far more plausible than photon rockets, beam propulsion (laser and particle beam) among them. Photon drives just don't look competitive, unless you're talking million-year missions.
  24. Jul 8, 2010 #23
    About 30 years ago NASA did an experiment where they unfurled a long cable in an attempt to generate electricity from the orbital velocity of the spacecraft in the magnetic field of the earth. I think they tried it twice and it failed both times. At the time I wondered what possible use that could be because they already obtained electricity from solar cells and any energy it produced would come at the expense of orbital altitude.

    Then it struck me that perhaps they were trying to determine if it was feasible to do the reverse - use the electricity from the solar cells to force the spacecraft into a higher orbit or at least keep the orbit from decaying. Some years later Scientific American suggested the same idea and some ways that it might be accomplished.
  25. Jul 8, 2010 #24
    Sometimes, a decrease in orbital altitude is what you want. An electrodynamic tether could be a cheap way to deorbit decommissioned satellites or used upper rocket stages.

    BTW: generation of power succeeded rather well. They had deployment reel problems and failures that were likely due to problems with the insulation (pinhole failures causing arcing that cut the cable). On the TSS-1R mission, they deployed 19 km of tether before it broke, and the tether remained in orbit for a few weeks.
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