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ToSeek
2008-Sep-24, 05:02 PM
Chinese Say They're Building 'Impossible' Space Drive (http://blog.wired.com/defense/2008/09/chinese-buildin.html)


Chinese researchers claim they've confirmed the theory behind an "impossible" space drive, and are proceeding to build a demonstration version. If they're right, this might transform the economics of satellites, open up new possibilities for space exploration - and give the Chinese a decisive military advantage in space.

To say that the "Emdrive (http://www.emdrive.com/)" (short for "electromagnetic drive") concept is controversial would be an understatement. According to Roger Shawyer, the British scientist who developed the concept, the drive converts electrical energy into thrust via microwaves, without violating any laws of physics (http://emdrive.com/faq.html). Many researchers believe otherwise. An article about the Emdrive in New Scientist (http://technology.newscientist.com/article/mg19125681.400) magazine drew a massive volley of criticism. Scientists not only argued that Shawyer's work was blatantly impossible, and hat his reasoning was flawed. They also said the article should never have been published (http://www.newscientist.com/blog/fromthepublisher/2006/10/emdrive-on-trial.html).
"It is well known that Roger Shawyer's 'electromagnetic relativity drive' violates the law of conservation of momentum, making it simply the latest in a long line of 'perpetuum mobiles' that have been proposed and disproved for centuries," wrote John Costella, an Australian physicist. "His analysis is rubbish and his 'drive' impossible (http://www.assassinationscience.com/johncostella/shawyerfraud.pdf)."

cjameshuff
2008-Sep-24, 05:28 PM
Chinese Say They're Building 'Impossible' Space Drive (http://blog.wired.com/defense/2008/09/chinese-buildin.html)

Yeesh. It doesn't take any great effort to see how it's flawed. If it worked, any asymmetrical container would experience constant thrust. Take an empty water bottle...internal pressure is equal on all surfaces, and the bottom has more area than the cap...by his analysis, if tossed into vacuum with some air inside, it would accelerate bottom-ward. It doesn't, of course, because the bottle also has sides which can not be entirely vertical...

So...what could the Chinese possibly be thinking?

tdvance
2008-Sep-24, 05:34 PM
The last link in ToSeek's post is a good explanation of what's wrong.

I suspect this is a case of someone with more authority than scientific knowledge declaring this a funded project, while ignoring the closed-minded scientists who tell him it won't work. Either that, or "we must do this in case it works, before the evil Americans do it".

JustAFriend
2008-Sep-24, 11:57 PM
Nothing at all wrong with letting them build it.

Let 'em dump a whole bunch of money in it and PLEASE let them publish the results if it doesn't work (the Chinese are notorious for not admitting to failures).

Then we can put the whole thing to bed as a blind alley....

Superluminal
2008-Sep-25, 01:44 AM
Typical of communist countries. The Soviets dabbled in all sorts of woo woo stuff.

Doodler
2008-Sep-25, 01:56 AM
Does it run on melamine?

Drunk Vegan
2008-Sep-25, 04:30 AM
Typical of communist countries. The Soviets dabbled in all sorts of woo woo stuff.

This statement strikes me as unnecessarily political and in fact downright wrong.

There are plenty of foolhardy perpetual motion machine and cold fusion inventors right here in the U.S. merrily wasting their lives in pursuit of the impossible.

nauthiz
2008-Sep-25, 04:53 AM
Yeah, but the US only started trying to train telepathic spies when when the powers that be became worried about an ESP gap. ;)

stutefish
2008-Sep-25, 05:17 PM
Yeesh. It doesn't take any great effort to see how it's flawed. If it worked, any asymmetrical container would experience constant thrust. Take an empty water bottle...internal pressure is equal on all surfaces, and the bottom has more area than the cap...by his analysis, if tossed into vacuum with some air inside, it would accelerate bottom-ward. It doesn't, of course, because the bottle also has sides which can not be entirely vertical...

So...what could the Chinese possibly be thinking?
I confess to believing that a water bottle, tossed into a vacuum with some air inside, would in fact accelerate bottom-ward, as the air escaped out the back.


The last link in ToSeek's post is a good explanation of what's wrong.
Also, I'm not sure how good this explanation is. It's certainly an angry explanation, and it certainly insists that advanced theoretical understanding isn't necessary to debunk the original claims, but I'm having trouble figuring out exactly where the problem is.

You have a microwave generator, it pumps microwaves into a cone-shaped vessel, and then the waves vent out the back. How is this any different, in principle, from any other rocket, other than in the nature of the reaction "mass" being used?

And where's the violation of conservation laws? It's not like the SSME violates conservation laws by venting exhaust product out the mouth of its nozzle, right?

cjameshuff
2008-Sep-25, 05:41 PM
I confess to believing that a water bottle, tossed into a vacuum with some air inside, would in fact accelerate bottom-ward, as the air escaped out the back.

Put the cap on first. Yes, many real water bottles would have issues with containing an atmosphere of pressure, but that's entirely aside from the issue.



You have a microwave generator, it pumps microwaves into a cone-shaped vessel, and then the waves vent out the back. How is this any different, in principle, from any other rocket, other than in the nature of the reaction "mass" being used?

His "space drive" doesn't vent the microwaves anywhere. Doing so would make it a photon drive, which is entirely possible, if very low thrust. The cavity is closed...microwaves go in, but don't exit. His claim is that photon pressure in a closed cavity in the form of a truncated cone results in a net force toward the larger end.

Larry Jacks
2008-Sep-25, 05:51 PM
Originally Posted by Superluminal
Typical of communist countries. The Soviets dabbled in all sorts of woo woo stuff.

This statement strikes me as unnecessarily political and in fact downright wrong.

There are plenty of foolhardy perpetual motion machine and cold fusion inventors right here in the U.S. merrily wasting their lives in pursuit of the impossible.

Inventors working on their own is one thing. All they do is delude themselves and whomever is foolish enough to invest in them.

When a government adopts something radical in the name of ideology, a lot of people can suffer. China under Mao did things (e.g. Great Leap Forward (http://en.wikipedia.org/wiki/Great_leap_forward) and Cultural Revolution (http://en.wikipedia.org/wiki/Cultural_revolution)) in the name of ideology that resulted in the deaths of millions of people (I've heard figures of over 40 million deaths). Likewise, the Soviet Union adapted a formal theory of agriculture and genetics based on communist ideology (such as the work of Trofim Lysenko (http://en.wikipedia.org/wiki/Trofim_Lysenko) set back their biological and agricultural efforts for decades.

stutefish
2008-Sep-25, 06:51 PM
Put the cap on first. Yes, many real water bottles would have issues with containing an atmosphere of pressure, but that's entirely aside from the issue.




His "space drive" doesn't vent the microwaves anywhere. Doing so would make it a photon drive, which is entirely possible, if very low thrust. The cavity is closed...microwaves go in, but don't exit. His claim is that photon pressure in a closed cavity in the form of a truncated cone results in a net force toward the larger end.
Oh!

Yes, I see the problem quite clearly now.

How did the Chinese government ever fall for this? Or, how did they think they would gain face by publically endorsing it?

cjameshuff
2008-Sep-25, 07:33 PM
Oh!

Yes, I see the problem quite clearly now.

Given how utterly wrong it is, your confusion about what they meant is quite understandable...

Any asymmetrical stray emissions will produce an uneven force, though. It would be far less than predicted, but perhaps enough to convince an official eager for some great accomplishment he can take credit for, or allow them to claim "promising" results and get further funding. The un-leaked RF energy will be lost as heat, which they could blame on losses in the resonant cavity due to shape and materials, or the method of injection...something that could surely be corrected with further development. Also, if the microwaves are produced in stationary equipment, there will be net photon pressure on the drive, allowing it to accelerate away from the microwave source...exactly countered by the recoil on the microwave source, of course.

JustAFriend
2008-Sep-25, 09:40 PM
Hey, just a thought:

They could team up with Mythbusters and make it a planetary pay-per-view.

....they might even make a load of bucks off the experiment and this could be a way to help finance real missions.....

;-)

tdvance
2008-Sep-25, 10:33 PM
Also, I'm not sure how good this explanation is. It's certainly an angry explanation, and it certainly insists that advanced theoretical understanding isn't necessary to debunk the original claims, but I'm having trouble figuring out exactly where the problem is.

You have a microwave generator, it pumps microwaves into a cone-shaped vessel, and then the waves vent out the back. How is this any different, in principle, from any other rocket, other than in the nature of the reaction "mass" being used?

And where's the violation of conservation laws? It's not like the SSME violates conservation laws by venting exhaust product out the mouth of its nozzle, right?

I found it a humorous, not angry, explanation. Nothing more than high-school physics plus a tiny amount of vector arithmetic is needed to understand it. The paper being debunked doesn't argue that the thrust is from escaping microwaves (and you'd need an awful lot!!!). The paper argues that an enclosed container with microwaves bounding around inside will somehow put more pressure one one side than the others, all because of the conical shape of the container. The link I referenced explains why that is not the case.

nauthiz
2008-Sep-25, 10:40 PM
It took me a moment to visualize what the paper was trying to explain, too. I think it would have been easier to understand if he had included some visual aids of his own. That or maybe I should have read the paper it was debunking first.

ETA: Now the only nagging question left in my head is, why is the chamber flat at both ends? According to the principle that's supposed to make it work, it seems you'd get a much more efficient design by having it taper to a point. That way you don't have microwaves bouncing off the small end and creating a drag force (Fg2 in the figure on page 2 of Shawyer's paper (http://emdrive.com/theorypaper9-4.pdf).

cjameshuff
2008-Sep-26, 12:09 AM
The paper being debunked doesn't argue that the thrust is from escaping microwaves (and you'd need an awful lot!!!).

Off on a tangent...one interesting thing about photon drives is that because the momentum of a photon is directly proportional to its energy, all that matters is the power output...a watt of UV, a watt of IR, and a watt of microwaves give the same thrust. A laser or maser and reflectors to redirect its waste heat won't do any better than a big tungsten filament (or nuclear lightbulb) and parabolic reflector.

(Technically, there'll be a small difference due to differences in collimation, but considering the inefficiencies in highly collimated photon sources and resulting difficult-to-collimate waste heat radiation, I don't think it makes a practical real-world difference.)

Solfe
2008-Sep-27, 05:23 AM
Might I ask a silly question about this trick? I am trying to figure out how you could get enough of a result to make someone want to pay for it.

Could a system like this move the bottle, make it rattle around but not really go any where? Sort of like a rocking chair?

Take a balance and press down on one side. It moves. Now press the other side. It moves again. It seems to me this device would do the same if you pulsed the energy; the bottle should move with the firing and then in the other direction when the wall is struck just like the balance.

With the balance you are not fooling anyone; it moves when you touch it. This device seems like it would move, but obviously won't go anywhere. You could claim that it moves and it is just a matter of getting it to go in the one direction all of the time.

Is that correct or really bad logic?

Solfe

nauthiz
2008-Sep-27, 05:57 AM
It'd take one heckuva pulse of photons to get the device to move perceptibly. More likely he's just duped people into investing without ever having seen it.

Given that it's been two years since the emdrive made the cover story of New Scientist there still hasn't been anything along the lines of independent confirmation of what Shawyer has claimed (namely, .1N of thrust), it's probably safe to assume that nobody on the planet has ever seen it do anything interesting. I might be willing to briefly entertain the idea that he has been keeping such demonstrations behind closed doors if not for the fact that he put it on the cover of New Scientist and has a paper on the Web describing exactly how the thing is supposed to work even though the device is covered by absolutely no patent protection. It's hard to imagine that anybody would be so open and so secretive about something like this (unless they're just playing games).

cjameshuff
2008-Sep-27, 07:24 AM
Could a system like this move the bottle, make it rattle around but not really go any where? Sort of like a rocking chair?

There might be an impressive hum from transformers, anything else will be special effects added by them. The device won't produce any thrust, but if the microwave source is stationary (by which I mean not rigidly attached to the device), there will be a detectable force on the device. A very tiny one, due to simple photon pressure...3.33 micronewtons per kilowatt, assuming the microwaves are absorbed completely rather than reflected back out...but measurable, and with sufficient ingenuity they could spin that into "positive results". Like I said...they could claim the Q of the resonator isn't good enough, and that the reason the thrust is so low is that it's absorbing the microwaves, and that all they need is a little more time and money to make better resonators...or maybe something's wrong with the microwave source, etc...

Jerry
2008-Sep-27, 07:24 PM
It isn't impossible that this conic design is coupling with air molecules outside the chamber, and creating a measurable thrust when the system is not in a vacuum. (I don't see why it would, but I can see that a demonstration could fool an observer who is not critical enough to assess this possibility.) Is the 'nozzle' conductive or not?

novaderrik
2008-Sep-27, 09:43 PM
seems to me that this is a bit like mounting a fan at the back of a sailboat pointed up at the sail to provide thrust.

cjameshuff
2008-Sep-27, 10:35 PM
It isn't impossible that this conic design is coupling with air molecules outside the chamber, and creating a measurable thrust when the system is not in a vacuum. (I don't see why it would, but I can see that a demonstration could fool an observer who is not critical enough to assess this possibility.) Is the 'nozzle' conductive or not?

Also, the interior of the resonator may not be airtight. If they can only run it for brief pulses (due to overheating caused by the "poor geometry of the resonator" or something similar that could surely be solved with a bit more time and money...certainly nothing to do with the fact that there's just nowhere else for the microwaves to go), it might simply eject heated air and achieve a measurable thrust.

mugaliens
2008-Sep-28, 03:31 AM
"All ahead, full impulse, Mr. Sulu."

Professor Tanhauser
2008-Sep-29, 04:52 AM
Personally I hope this thing does work, just to knock some of the smug off some people.

ryanmercer
2008-Sep-29, 11:00 AM
All I will say is idiots... and in the event it works, I can delete this reply, and thus write history :)

Grey
2008-Sep-29, 03:18 PM
Personally I hope this thing does work, just to knock some of the smug off some people.I wish that we could develop some kind of reactionless drive, too. Unfortunately, the results of physics experiments that I've seen (including many that I've done myself) from numerous subfields leave me no doubt that conservation of momentum is one of those rules that we're not going to get around.

nauthiz
2008-Sep-29, 03:30 PM
Even if we do assume that this thing works, it's even harder to see why it should be any more efficient than a straight photon drive.

cjameshuff
2008-Sep-29, 03:49 PM
I wish that we could develop some kind of reactionless drive, too. Unfortunately, the results of physics experiments that I've seen (including many that I've done myself) from numerous subfields leave me no doubt that conservation of momentum is one of those rules that we're not going to get around.

Laser light sail propulsion is almost as good, perhaps even better since you no longer have to make compromises to reduce the mass of the power plant, and can reuse it for other launches. Be nice if you could avoid the complication of deploying huge reflective film sails, though. And while the drive lasers would be reusable, they would not be cheap.

One concept I've seen that could work at short ranges is a variation on light sails in which a reflector on the craft an another on a base station form a cavity in which light bounces multiple times before escaping, applying more momentum to the craft (and the base station, of course, which presumably has other ways to handle it, or is just massive enough for it not to matter much) from the same energy input. If you can make big and lightweight mirrors with the required precision, this would at least give an initial boost out of orbit. If only the diffraction limit didn't exist...

Grey
2008-Sep-29, 03:57 PM
Even if we do assume that this thing works, it's even harder to see why it should be any more efficient than a straight photon drive.A photon drive is hideously inefficient. If the numbers from Shawyer's site were correct, this would be more than 5,000 times more efficient than a photon drive. Which is a good reason to suspect that he's made just a teensy mistake somewhere along the way. :)

Grey
2008-Sep-29, 04:09 PM
Laser light sail propulsion is almost as good, perhaps even better since you no longer have to make compromises to reduce the mass of the power plant, and can reuse it for other launches. Be nice if you could avoid the complication of deploying huge reflective film sails, though. And while the drive lasers would be reusable, they would not be cheap.And this is the right way to handle the situation. Stop trying to get around the conservation of momentum, since we know that isn't going to work. Instead, take a look at what that tells about the problem (largely, that we should stop looking at closed systems), and come up with a better solution. For the laser light sail, you essentially leave the heaviest parts of your vehicle (the power plant and the fuel) on the ground, so that you can make them as big and as powerful as you want without having to worry about lifting them into space. It's a very nice design.


One concept I've seen that could work at short ranges is a variation on light sails in which a reflector on the craft an another on a base station form a cavity in which light bounces multiple times before escaping, applying more momentum to the craft (and the base station, of course, which presumably has other ways to handle it, or is just massive enough for it not to matter much) from the same energy input. If you can make big and lightweight mirrors with the required precision, this would at least give an initial boost out of orbit. If only the diffraction limit didn't exist...Will this trick work? For any sail, you start out with photons going in the same direction as your vehicle, you end up with photons going in the opposite direction. That should be the maximum momentum boost you can get out of those photons, and it shouldn't much matter how many times they reflect to achieve that.

cjameshuff
2008-Sep-29, 04:47 PM
Will this trick work? For any sail, you start out with photons going in the same direction as your vehicle, you end up with photons going in the opposite direction. That should be the maximum momentum boost you can get out of those photons, and it shouldn't much matter how many times they reflect to achieve that.

Those reflected photons are no different from the original ones, other than a bit of redshift and a momentum vector of slightly lower magnitude pointed in the opposite direction. Reflect them back at the craft, and you can use them to transfer a bit more momentum to it. The light bounces back and forth between the two reflectors, exerting photon pressure on both. Energy balances out as red shift on the photons, and the momentum imparted to the base reflector, the vehicle and what is carried off by photons that miss the reflectors all balances out.

So yeah, it'll work...as long as you can get a decent proportion of the light to bounce multiple times. When you get a few thousand km apart, this will become *really* hard. The diffraction limit demands big mirrors, the geometry demands extremely precise mirrors, and the tiny amount of thrust per incident watt demands very lightweight mirrors, at least on the vehicle side. Perhaps a smaller high-precision launch mirror that gets jettisoned when distance makes it just a heavy one-bounce mirror.

Grey
2008-Sep-29, 06:01 PM
Those reflected photons are no different from the original ones, other than a bit of redshift and a momentum vector of slightly lower magnitude pointed in the opposite direction. Reflect them back at the craft, and you can use them to transfer a bit more momentum to it. The light bounces back and forth between the two reflectors, exerting photon pressure on both. Energy balances out as red shift on the photons, and the momentum imparted to the base reflector, the vehicle and what is carried off by photons that miss the reflectors all balances out.

So yeah, it'll work...as long as you can get a decent proportion of the light to bounce multiple times. When you get a few thousand km apart, this will become *really* hard. The diffraction limit demands big mirrors, the geometry demands extremely precise mirrors, and the tiny amount of thrust per incident watt demands very lightweight mirrors, at least on the vehicle side. Perhaps a smaller high-precision launch mirror that gets jettisoned when distance makes it just a heavy one-bounce mirror.Ah, I understand what you mean. You're reflecting them back to the craft from your base station on Earth, or from some other intermediate mirror. I didn't read your original description closely enough, and thought it was referring to multiple bounces within a cavity contained within the vehicle itself. What you're actually describing makes perfect sense.

cjameshuff
2008-Sep-29, 06:54 PM
Ah, I understand what you mean. You're reflecting them back to the craft from your base station on Earth, or from some other intermediate mirror. I didn't read your original description closely enough, and thought it was referring to multiple bounces within a cavity contained within the vehicle itself. What you're actually describing makes perfect sense.

I was thinking more along the lines of a large orbital structure as the base, maybe using electrodynamic tethers or something to counter the recoil of launching craft, or just launching them in different directions so it averages out over time (if you have to deal with the atmosphere as well, I don't think you'd get enough of the light to bounce multiple times to be worth the trouble)....but yeah, that's the general idea.

And actually, you just gave me an idea...the vehicle could carry multiple mirrors, and jettison one and use it to form a "base" reflector at much closer range, greatly reducing the precision requirements and allowing lightweight sail mirrors to be used. The "base" mirrors would be decelerated in the process, basically acting as reaction mass and possibly remaining in orbit to be recovered. The geometry would be tricky to work out, but I think it'd work...

Jerry
2008-Sep-30, 12:01 AM
I wish that we could develop some kind of reactionless drive, too. Unfortunately, the results of physics experiments that I've seen (including many that I've done myself) from numerous subfields leave me no doubt that conservation of momentum is one of those rules that we're not going to get around.
We might be able to exploit the existing electromagnetic partitions (plasmas, if you will) and develop weak momentum in a chosen direction without ejecting particles. (In a way, we already do, Messenger is using the solar wind to trim her approach to Mercury. Could we do something similar in deeper space? Conceptually, this would require an ability to identify existing potentials in otherwise empty space and exploit them...I see two or more main modules interconnect by very long composite cables...

cjameshuff
2008-Sep-30, 01:01 AM
We might be able to exploit the existing electromagnetic partitions (plasmas, if you will) and develop weak momentum in a chosen direction without ejecting particles. (In a way, we already do, Messenger is using the solar wind to trim her approach to Mercury. Could we do something similar in deeper space? Conceptually, this would require an ability to identify existing potentials in otherwise empty space and exploit them...I see two or more main modules interconnect by very long composite cables...

Nitpick: the solar wind has nothing to do with Messenger's maneuvers, the probe is using radiation pressure on its solar panels...using them as very stubby solar sails. I don't know how useful something like electrodynamic tether propulsion would be in the weak magnetic field between planets, let alone between stars, but it's an interesting idea.

cjameshuff
2013-Feb-09, 01:43 AM
David Hambling (the author or the article mentioned in the first post) is back at it: http://www.wired.co.uk/news/archive/2013-02/06/emdrive-and-cold-fusion

Just as blindly accepting of the claims and dismissive of critics as he was before.

For those wondering about the Chinese tests, the translated paper gives me little confidence in their measurement procedures. The apparatus design, for example...a movable resonant cavity is linked to a stationary magnetron via a flexible waveguide. There will obviously be radiation pressure affecting the waveguide, and could be any other number of interfering factors...thermal expansion, air currents, etc. From the description, they only tested one configuration...there is no mention of them testing it with a symmetrical resonant cavity of similar Q factor, or trying different orientations.

Shawyer also released an updated paper on his site, in which he claims the EmDrive is equivalent to an electric motor...including acting as a generator if accelerated in the opposite direction. Consider the outcome of putting one on end in a gravity field, and it becomes clear how badly this thing violates conservation of energy as well as conservation of momentum.

Van Rijn
2013-Feb-09, 01:59 AM
Yes, I saw a thing on the EmDrive on Arstechnica and was wondering why they were bringing this up again. The article was fairly skeptical, but not skeptical enough: It should have started with the requirement that this would break conservation of momentum, and would require very good evidence before anyone should even get interested in the subject. At least there were some good comments (and good responses to the bad comments).

As for David Hambling, this is the guy that wrote articles with titles like "What to make of Andrea Rossi's apparent cold fusion success" and "Success for Andrea Rossi's E-Cat cold fusion system, but mysteries remain" . . . like the mystery of why anyone would take Rossi's obvious hoax seriously.

JustAFriend
2013-Feb-09, 08:07 PM
The biggest problem with the Chinese and Russian governments is that if the drives DONT work,
they will be too embarrassed to admit it and the results will never be published. (way too many
stories of their scientists 'disappearing' after things like that...)

neilzero
2013-Feb-10, 03:06 AM
If we have lots of space craft traveling about the inner solar system, each laser beam can accelerate several space craft, before there is no craft that can benefit form acceration in the last direction of the beam. Does any one know the minimum spot size possible at a range of one AU from a B ultraviolet laser, or another wave length? Possibly solar orbit colonies don't care (within reason) their next path, but do like to change the seanery? Neil