"shpuker" (shpuker) 11/23/2016 at 12:20 • Filed to: EM Drive, Ion, Thruster, NASA, Vaporware

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So I originally typed this up as a response to the post on the main page but it got a bit lengthy so I’m gonna give it it’s own space. The “EM Drive” does a good job of raising questions about our current understanding of physics though so in order to get an idea on what it might be (whether that just a fancy bit of vaporware or something significant) is to address the questions it raises and look at a variety of potential explanations.

What this drive actually does is question the conventional understanding of light emission, electromagnetism and how they relate to momentum. Example, if I asked you what the net force on a piece of wire that had 1A flowing through it from left to right what would your answer be? Zero? Physics would tend to agree, but is it actually right? I’ll put it this way, our basic understanding of current is that it is the flow of electrons through a conductive material. Makes sense.

Well now think of inducing a current in an object. since you’re inducing the movement of electrons (via inducing a current in the object, shouldn’t there be a resulting force? Well yea but we don’t really think of it that way, we think of it more as a “resistance” to a changing current.

Example: Say you’re dropping a magnet through an induction coil. It’s generally understood that this causes a measurable back EMF through the wire. Ok how about dropping a metal bar through an induction coil that’s charged? If setup properly it will actually stop its fall, levitate it, and begin heating it. Well why is that? Essentially there is a current being induced in either the coil or the metal (depending on which you choose to pick as your reference) and a force upon the metal. This same principle provides the basis for a rail gun. Well if you think about it as electron emission resulting in an upward force on the metal you have the basic principle of an ion thruster as well (more specifically an electron bombardment thruster, albeit by combining the electron with a gas, typically xenon) in order to create a significant amount of measurable thrust. Now we’re getting somewhere.

So back to this EM Drive concept. So how do they say it functions? Well let’s look at the description from !!!error: Indecipherable SUB-paragraph formatting!!! .

“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.“

So is this what’s actually happening? Well as we understand yes, however this shouldn’t cause any sort of positive thrust, hence this isn’t what anyones really trying to prove or disprove at this point. Rather the issue is figuring out where the fuck the counter force is, and where any sort of net thrust could possibly come from.

There are a number of theories as to what this is, most of the published ones are pretty much non-conclusive though. So why the hell is NASA researching it, and do they actually believe the thing is creating thrust? Honestly I think they do, and I think they’re trying to figure out what the environmental interaction is that causes it. Time to fire up up my theory (Don’t worry it ties back to the background earlier in the post I swear).

So what they’re saying is that essentially you have electromagnetic waves coming through this thing and bouncing off each end of it, and it uses this principle of radiation pressure to create a velocity difference and thus causes a greater momentum in the waves striking the wide end of the device. Yea I’m calling bullshit on that actually creating any sort of net thrust. Rather what I believe (and what I think NASA believes as well, though obviously I’m just guessing there) is that it doesn’t create thrust from this at all, but rather through a bit of an obscure method of electron bombardment, just with no real target.

Here I have the black arrows indicating the waves, and the red indicating a hypothetical electron emission. Now lets pull in a bit of the photo-electric effect. So you have these electro-magnetic waves bombarding each end of the shroud, supposedly at two different energy levels. Now if the material you’re using has a work function that happens to allow electron emission at the high energy end but not the low energy end, now you have something that might be usable. Whether or not this will actually result in any practical amount of thrust is a whole different topic, but essentially what you’ve done is create an incredibly inefficient method of electron emission, however you no longer violate the conservation of momentum nor energy, as you now have a mass emission to explain your thrust.

TL;DR: No it doesn’t actually violate the conservation of momentum, and technically it doesn’t do anything that hasn’t been done already by a different type of thruster. It simply does it in a way that we haven’t really thought of yet.

Disclaimer:I’m an engineer and not a physicist so some of my explanations could be off, as well as some of my reasoning. Feel free to let me know if you disagree, I’d love to discuss it. Also if you made it this far and actually read the whole thing, you should 1) probably get back to work as I’m sure your boss noticed by now, and 2) go have a cookie for lunch because you earned it.

Batman the Horse > shpuker 11/23/2016 at 12:30

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Spooky action up close.

MrDakka > shpuker 11/23/2016 at 13:01

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I am hoping this is the real deal as we can start to move away from hohmann transfers and go to brachistochrone trajectories under full burn. Of course it still won’t solve the other 50% of space travel, but propellantless interplanetary travel is very appealing to us rocket folks

BvdV - The Dutch Engineer > shpuker 11/23/2016 at 13:03

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Great write-up, also bonus internet points for going through the pain of using Visio to make a drawing.

I think you’re onto something here. Also being an engineer(of the electical/automotive kind), I strongly doubt the whole functioning as it is being explained everywhere, though that could be stubbornness. Though I’m sure NASA will find mass emission causing the trust in the near future, as from what I’ve seen the current paper only deals with the fact that it can indeed generate a thrust.

shpuker > MrDakka 11/23/2016 at 13:10

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When it comes to “propellantless” thrusteres you still have the issue of comparable magnitude, or in other words the fact that the static-mass of an electron to that of a thruster is in the neighborhood of 1e32:1 while something like a xenon gas is more around 1e23:1 (obviously still tiny but that’s still an improvement by a factor of 1 billion)

gogmorgo - rowing gears in a Grand Cherokee > shpuker 11/23/2016 at 13:13

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I still remember the story from a few years ago where NASA studied it and found no measurable thrust, even with equipment sensitive enough to detect waves hitting the ocean shore some 30 miles away.

shpuker > BvdV - The Dutch Engineer 11/23/2016 at 13:19

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Yea i’m pretty new to using Visio but it’s probably the best drawing software I’ve used so far. I’d like to get my hands on some actual graphics design software though

I’ve been a bit irritated by the articles put out about it as well. Pretty much all of them just talk about it being impossible and creating thrust from nothing. That’s not what anyone researching the damn thing is even looking into though, everyones actually looking at it and just trying to figure out where the actual (potential) thrust is coming from. There’s obviously a mass transfer, we just haven’t found it yet.

One experiment I’d like to see though is someone trying to confirm that you can create reliable electron emission from electro-magnetic bombardment. or EMBe - E to give it an abbreviation.

MrDakka > shpuker 11/23/2016 at 13:21

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Ah but that still requires xenon as propellant, so you’re cheating XD

Compared to stuff like solar/laser sails and photon rockets, the EM thruster is much better for the thrust to power ratio whose ratios are around the micronewton per kilowatt range

shpuker > MrDakka 11/23/2016 at 13:26

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That it does. Personally I don’t think we’ll ever achieve any version of propellant-less travel that is significant enough to achieve proper, guided, deep space exploration. Things like solar sails and prop-less thrusters work well as supplementary thrusters and as orbital correction but I don’t really see the potential for the amount of thrust needed (but I would love to be proven wrong).

What I would like to see is someone combine the concept of electro-magnetic waves with plasma thrusters. That could be fun.

BvdV - The Dutch Engineer > shpuker 11/23/2016 at 13:46

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I tried my hand at using Visio a couple of times, but if I try to make somewhat complicated drawings, it always messes up. Which is why I prefer to use Adobe Illustrator for such things nowadays, it lacks the pre-defined blocks(but it is very easy to re-use parts of a drawing), and it needs some getting used too, but in the end it is able to reach much higher levels.

I guess that’s just publications trying attract to the layperson to visit their pages, since a layperson(or semi-layperson for that matter) will likely be stunned by the suggestion of such a ‘fact’ as a breach of one of Newton’s laws. But I agree, it’s very irritating.

That would be interesting to see indeed, I think(I could be wrong though) that a reliable EMBe-E could have quite a big influence on the scientific/technological state of the art.

MrDakka > shpuker 11/23/2016 at 15:44

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Its not just thrust that’s relevant for space travel (it is extremely important for getting into orbit and in system delta-v manuevers). Specific impulse of an engine (commonly in units of seconds) is a measure for deep space exploration; it is a measure of a drive’s efficiency, sort of like mpg. Specifically it is the impulse produced per unit of propellant. With any sort of rocket, you’re ultimately limited by how much propellant you can carry. Even the most advanced theoretical antimatter or fusion rockets are subject to this.

However, with zero propellant drives like solar/laser sales or potentially EM drives, you can continue to accelerate as long as you have power. The specific impulse is theoretically infinite since you’re no longer expending mass to accelerate. Practically you’re bound by other things but you’re no longer bound by how much propellant you can carry, freeing you to dedicate the mass you would used for propellant for something else, like fuel or payload capacity. It bests any ion/plasma thruster because of this. This is what makes zero propellant drives so attractive for long range relativistic journeys. Granted, it’s not very useful where thrust is needed very quickly, such as getting into orbit, but it’s more or less unbeatable for long distance journeys.

All this is moot if the EM drive turns out to be the newest in a long line of failed reactionless drives. Personally I want to believe, but I won’t be surprised if it’s all for naught.

shpuker > MrDakka 11/23/2016 at 16:06

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This is true. My main concern with the EM drive really comes down to the fact that I don’t think its actually a zero propellant drive. Yes it technically doesn’t consume an intentionally supplied fuel, but rather I think it’s emitting electrons from the surface of the device.

MrDakka > shpuker 11/23/2016 at 16:48

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Then technically photon rockets and solar sails don’t count either. Damn you wave particle duality!