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Is EmDrive actually just gibberish?

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nzoomed

Sun Nov 27 2016, 12:19AM

Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288

I think the media are reporting it wrong though.

In our news the reporter claimed its an amazing device that gets its energy from "nothing" that can only be understood as over-unity.

Dr. Slack

Thu Dec 08 2016, 12:25PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

with the straplines ...

The 'impossible' EM Drive is about to be tested in space

Time to get to the bottom of this crazy thing

Interesting. Looks like there are not one, but two, efforts to put this damned thing into orbit.

Let the reckoning begin!

nzoomed

Thu Dec 08 2016, 11:17PM

Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288

My question is where does the electricity come from?
Solar panels?

You would need quite a large solar array to power it wouldnt you?

BigBad

Fri Dec 09 2016, 01:55AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

I still don't believe a word of this. All the previous experiments by others have now been retracted. The NASA team have almost certainly just fucked something up.

Their experimental time/thrust curve is about 50% thermal expansion; even the NASA team admit that. That there's even any real signal beneath that is IMO super unlikely. The thrust curve they obtained takes *seconds* to decay; but given the Q-factor the microwaves are gone from the frustrum in *microseconds*.

I just don't buy it. Extraordinary claims require extraordinary evidence, and this is really, really ordinary.

Uspring

Fri Dec 09 2016, 11:29AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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I once puzzled about the energetics of a rocket engine. The power supplied to the rocket is F*v, where F is the thrust and v the velocity of the rocket. Thus for high v this can be larger than the power of the combustion. How can that be?

The solution is, that the kinetic energy of the exhaust fumes has to be taken into account. When the rocket is moving at high speed, the exhaust fumes will be trailing it a lower speed. Thus the propellant as exhaust fume looses speed and kinetic energy compared to the time when it is still in the rocket. So energy conservation is restored. A quick calculation will confirm this.

Now if you have a rocket drive without a propellant the above argument can't be appled anymore. Energy won't be conserved, i.e. an EM drive is a perpetual motion machine.

hen918

Fri Dec 09 2016, 01:48PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Uspring wrote ...

...
Now if you have a rocket drive without a propellant the above argument can't be appled anymore. Energy won't be conserved, i.e. an EM drive is a perpetual motion machine.

Now I'm an EM drive sceptic, but it isn't a rocket engine. So E=F*D like normal.

Uspring

Fri Dec 09 2016, 03:46PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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I'm sorry, if I haven't been clear enough. With power I meant the rate of change of energy, not energy itself. E=F*D is equivalent ot P=F*v.

If the EM drive is not similar to a rocket engine in the way, that it propels something out of the back, then this makes it a perpetual motion device.

hen918

Fri Dec 09 2016, 04:19PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

@Uspring

Ahh, I see, that's just me, misreading.
Yes, that's the problem I noticed with the EM drive: to obey the laws of motion *something* (quite heavy) will have to be propelled out of the cavity.

BigBad

Sun Dec 11 2016, 01:37AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

There is also some wiggle room perhaps if it works like a linear momentum wheel; you fill the cavity with microwaves, and it starts to move, and you switch it off, and it stops again. That might not even destroy the whole of physics. But it would also be a lot less useful.

But I'm still firmly in the 'it doesn't work at all camp'.

Karmaslap

Sun Dec 11 2016, 10:51AM

Registered Member #58215 Joined: Wed Dec 30 2015, 11:27AM
Location: Boise, Idaho
Posts: 65

BigBad, that would heavily violate conservation of energy and would be highly impossible with what we know of physics, with no loopholes that as current explanations afford.

If it works, conservation of momentum is sort-of broken with the radiation pressure theory which also has problems.

I'm glad they are testing the device in space, but think it will prove to not actually operate.

I'm much more excited for the Japanese satellite that will extend a cable much further into orbit in an attempt to create a satellite which will not expend fuel to keep its orbit.

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