Em drives

[tomthenose]

anyone know anything about these propellant free Em drives? They're pretty weak just now (mN's) but apparently with super conductor technology could be made a lot more powerful: a 'static specific thrust of 3.15 x 104 N/kW (3.2 tonnes / kW).'

Alot of the the science is a little over my head, could this be a game changer for space travel? looks pretty interesting

http://emdrive.com/

http://www.wired.co.uk/news/archive/2013-02/06/emdrive-and-cold-fusion

 

[Hielor]

Boeing's Phantom Works, which works on various classified projects and has been involved in space research, went as far as acquiring and testing the EmDrive, but say they are no longer working with Shawyer.

If this was going to be a game-changer, Boeing would've been all over it, I suspect.

 

[T.Neo]

Reactionless drive, it's like a perpetual motion machine. It won't work, because it happens to be physically impossible.

 

[tomthenose]

its a propellant free drive which means it doesn't work on the newtonian principle of firing stuff out the back makes you go forward, its still based on scientific principles. read the links! apparently it does work because they've built several.
'The latest paper describes their latest thruster and gives the test results in details, showing that with a couple of kilowatts of power they can produce 720 mN (about 72 grams) of thrust.' Of course i haven't seen this evidence

i'm no expert i only found out about it because i liked the picture...

from emdrive.com

'Principle of Operation

At first sight the idea of propulsion without propellant seems impossible. However the technology is firmly anchored in the basic laws of physics and following an extensive review process, no transgressions of these laws have been identified.

The principle of operation is based on the well-known phenomenon of radiation pressure. This relies on Newton’s Second Law where force is defined as the rate of change of momentum. Thus an electromagnetic (EM) wave, travelling at the speed of light has a certain momentum which it will transfer to a reflector, resulting in a tiny force.

If the same EM wave is travelling at a fraction of the speed of light, the rate of change of momentum, and hence force, is reduced by that fraction. The propagation velocity of an EM wave, and the resulting force it exerts, can be varied depending on the geometry of a waveguide within which it travels. This was demonstrated by work carried out in the 1950’s. (CULLEN, A.L. ‘Absolute Power Measurements at Microwave Frequencies’ IEE Proceedings Vol 99 Part 1V 1952 P.100)

Thus if the EM wave travelling in a tapered waveguide is bounced between two reflectors, with a large velocity difference at the reflector surfaces, the force difference will give a resultant thrust to the waveguide linking the two reflectors. If the reflectors are separated by a multiple of half the effective wavelength of the EM wave, this thrust will be multiplied by the Q of the resulting resonant cavity, as illustrated in fig 1.'

 

[Hielor]

its a propellant free drive which means it doesn't work...

You had a bunch of extra stuff in your post. I cut out the rest of it for you, and left only the part you need :lol:

 

[Artlav]

Sounds too good to be true.
It's too energy efficient to be a photon drive, so there is something else.

Basically, a true reactionless drive is ruled out on quite a fundamental level - if you look at it from different frames of reference, there will be an inconsistency in total energy.

Is there something coming out of it?
I can't quite find it in the description - they just use "because of special relativity!" phrase as "stop asking questions!", and make a dubious comparison with a laser gyroscope.
If it's an open system, then there should be an effect on something outside of the engine - where is that effect?

Another curious thing about it is that the thrust is supposed to decrease as the thing is approaching a given fraction of speed of light, which looks like a direct violation of special relativity - assuming a privileged frame of reference for that velocity.

Also, if it's working, why aren't NASA, RSA, the army, and so on on it already?

However, all of the above might be made moot if they would show a working engine, especially their claimed 3 tonnes per KW one. Science always bends it's head when hit on it by an anvil of evidence.

 

[Linguofreak]

If the same EM wave is travelling at a fraction of the speed of light,

This is a big red flag: EM waves *always* travel at the speed of light in *all* reference frames.

 

[jedidia]

Any drive that uses em for propulsion is basically a photon drive... which means, yes, it works without propellant, but not at the magnitudes described here. bouncing an EM-wave between mirrors sounds a lot like pulling myself up by my hair, at least as long as the mirrors are in the same reference frame.

But there is of course this:

The propagation velocity of an EM wave, and the resulting force it exerts, can be varied depending on the geometry of a waveguide within which it travels.

which I am completely unqualified to make any statements on. As linguofreak, I am of the impression that photons always travel at the speed of light. However, that isn't constant either... it varies by medium. What would happen if you let the light cross through vacuum to the rear mirror, and lead it back through water, in which it travels slower (if such a thing could be engineered)?

Beyond this, because photons travel at the speed of light does not necessarily mean that the wave propagates at that speed, I suppose, and if it is in the end the wave that imparts the momentum, and not the individual photons, this might work on the currently known principles... Which would probably mean that something in our principles somewhere is wrong and in the end it wouldn't work after all, because it's a violation of energy conservation.

So unless they can explain where exactly in the equation the surplus energy of the vessel equals out, I can't see this work despite not really understanding the exact nature of em waves and their properties...

 

[Urwumpe]

which I am completely unqualified to make any statements on. As linguofreak, I am of the impression that photons always travel at the speed of light. However, that isn't constant either... it varies by medium. What would happen if you let the light cross through vacuum to the rear mirror, and lead it back through water, in which it travels slower (if such a thing could be engineered)?

Doesn't matter: If the "water" travels with the spacecraft, the photons will still leave the engine at c into the vacuum of space. The part with higher optical density is part of the engine and not producing any net thrust.

Important for any rocket engine is the velocity at which the exhaust leaves the spacecraft. It doesn't matter if you slow it down to almost zero inside the engine (as it happens for turning dynamic pressure into static pressure during pumping).

Also, his claim about using differences in group velocity for producing thrust is nonsense for photons. The group velocity is equal to the particle velocity, which is still the local speed of light. The only velocity that can vary is the phase velocity... which carries no information and no energy.

A parabolic dish antenna would produce a significantly higher thrust than his claimed design.

 

[jedidia]

Doesn't matter: If the "water" travels with the spacecraft, the photons will still leave the engine at c into the vacuum of space. The part with higher optical density is part of the engine and not producing any net thrust.

Ah yes, silly me... in that case it wouldn't actually leave the reference frame of the vessel :facepalm:

 

[tomthenose]

The unqualified 'because of special relativity!' does ring of pseudo-science, theres a more extended paper by Roger Shawyer available on emdrives.com thats goes through some of the maths but its mostly lost on a pleb like me.

Apart from some rather nifty looking pictures of something that looks like a space age bug zapper there's not much solid proof of them i can find. Did the Chinese team publish the results from their experiments? Or did they just say they where good?

shame its all starting to look a bit sketchy

btw i think this should have been posted in 'maths and physics' rather than 'spaceflight news'

 

[Urwumpe]

The unqualified 'because of special relativity!' does ring of pseudo-science, theres a more extended paper by Roger Shawyer available on emdrives.com thats goes through some of the maths but its mostly lost on a pleb like me.

Don't get lost on pseudo-math, it is just there for distracting you. Especially on the first page are many claims, that he later doesn't prove in the math.

His big big error appears very secretly then already on the next page (4). For the thrust equation, he uses radiation pressure by the particle impulse of a photon. But for the group velocity, he suddenly switches to waves and their properties, and happily declares that the speed of the photons (The propagation velocity of the EM waves) will equal the group velocity and thus there will be thrust - but there isn't. The propagation velocity is still c. What changed is really the group velocity of the waves, not of the particles that generate the radiation pressure.

What he really describes with his waveguide is geometric electromagnetic dispersion. Dispersion only matters for waves made of multiple different wavelengths. There will be no dispersion at all since he uses a single frequency/wavelength.

http://en.wikipedia.org/wiki/Dispersion_(optics)#Dispersion_in_waveguides

So, his waveguide does alter the group velocity, since we have no free-plane vacuum conditions (there group velocity = speed of light), but it does not alter the propagation velocity. It does also not alter radiation pressure.

 

[Face]

The question of how the theory might work is not so important as results IMHO. If the thruster is capable of producing thrust in space - measurable beyond every reasonable doubt - it just works.

So far, however, there is absolutely no evidence that it does. If there were, the results would be all over the internet already. But let's wait and see, as those theories tend to fade away real quick these days.

 

[Urwumpe]

Practice: It works, but you don't know why.
Theory: It doesn't work, but you know why.

 

[jedidia]

Practice: It works, but you don't know why.
Theory: It doesn't work, but you know why.

Coding: It doesn't work, and you don't know why

 

[Einion Yrth]

Coding: It doesn't work, and you don't know why

Or it works until you find the bug that should have prevented it ever doing so.

 

[Urwumpe]

Coding: It doesn't work, and you don't know why

Or it works until you find the bug that should have prevented it ever doing so.

I think Einion Yrth is closer to the reality of coding. :rofl:

 

[Face]

Back into the topic, I've read the linked article and searched for John Costella's "paper".

A nice read, and underlines one of the mistakes many reaction-less thruster designer make: misinterpreting temporal displacement for sustained movement. It is also often the reason for observed "thrust" from such asymmetrical construct when tested in water or air. If you wobble around an asymmetrical body in water or air fast enough, it will move due to simple medium replacement. That's why it is so important to test especially micro-thrust proposals at least in vacuum. IMHO even a pendulum test is not enough, if not done in vacuum.

Well, maybe they should just enclose their designs in perfect sphere shells :lol: .

 

[TR1978]

http://www.wired.co.uk/news/archive/2014-07/31/nasa-validates-impossible-space-drive

 

[jedidia]

We had a short excursion on that in the random comments thread.

Basically we're waiting for confirming and more rigurous experiments (they really should put the thing into a vaccum chamber and see what happens then... the more possible error sources can be excluded, the better).

If it turns out to work (something everyone is very sceptical of, but I do think it does deserve some real good looking into), it probably won't be for the reasons anyone assumed, even the guys that built the thing. All those are in pretty blatant violation of well-proven physics.
If it does work, it'll probably be for some shenanigans that we just didn't know about yet and that somehow makes sense in the larger context and the constructors of the thing stumbled over it by blind luck without actually understanding what's happening.

But, as several people said in the RTC, it's still very possible that the story will end the same way as the "neutrinos traveling FTL" some while back.