Is it able to convert electricity into propulsion force?


by NGC 6751
Tags: electricity, energy, force, propulsion, space travel
NGC 6751
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#1
Jul4-10, 11:51 AM
P: 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

thx

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my_wan
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#2
Jul4-10, 11:59 AM
P: 863
Ion drives 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.
Bob S
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#3
Jul4-10, 04:37 PM
P: 4,664
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

http://en.wikipedia.org/wiki/Magnetohydrodynamic_drive

Bob S

Dickfore
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#4
Jul4-10, 04:38 PM
P: 3,015

Is it able to convert electricity into propulsion force?


So, what does an electric motor do?
pallidin
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#5
Jul4-10, 06:06 PM
P: 2,292
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.
Naty1
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#6
Jul4-10, 07:05 PM
P: 5,634
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.

http://en.wikipedia.org/wiki/Conduction_electron


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"....
K^2
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#7
Jul5-10, 03:42 AM
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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.
Lsos
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#8
Jul5-10, 11:27 AM
P: 768
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.
Dickfore
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#9
Jul5-10, 11:35 AM
P: 3,015
How about something like this device



Attached Thumbnails
ionmotor.png  
Bob S
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#10
Jul5-10, 11:59 AM
P: 4,664
Quote Quote by Naty1 View Post
Sounds like you mean using electrons directly for propulsion?? good idea. as far as it goes...
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

http://en.wikipedia.org/wiki/File:Ion_engine.svg

Bob S
pallidin
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#11
Jul5-10, 09:34 PM
P: 2,292
Quote Quote by Bob S View Post
...propulsion will cease.
Exactly. That's the problem.
Regardless of method, mass must be expelled.
Additional propulsive mass must be made available.
Bob S
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#12
Jul6-10, 10:57 AM
P: 4,664
Quote Quote by pallidin View Post
Exactly. That's the problem.
Regardless of method, mass must be expelled.
Additional propulsive mass must be made available.
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
AJ Bentley
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#13
Jul6-10, 11:36 AM
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Quote Quote by Bob S View Post
inefficient form of propulsion.Bob S
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)
Bob S
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#14
Jul6-10, 08:39 PM
P: 4,664
From Bob S

"Massless photons are..... an inefficient form of propulsion."
Quote Quote by AJ Bentley View Post
Lol, Not so!

It is highly efficient. Just terribly, ternribly puny.

Like the Stirling engine (very efficient but simply can't burn enough fuel to provide power for most purposes)
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
BrianConlee
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#15
Jul6-10, 09:23 PM
P: 65
I'm working on it ;)
AJ Bentley
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#16
Jul7-10, 02:38 AM
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Quote Quote by Bob S View Post
converting energy into momentum
Excuse me?!
georgir
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#17
Jul7-10, 05:55 AM
P: 128
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.

Quote Quote by Bob S View Post
So a low energy ion beam is more efficient by a factor of 2/β in converting energy into momentum (thrust).
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.
cjameshuff
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#18
Jul7-10, 07:53 AM
P: 210
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.


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