acatalepsy
Junior Member
Not Currently In Space
Posts: 97
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Post by acatalepsy on Nov 21, 2016 16:02:25 GMT
This is exactly what I thought after reading about the EM Drive, ECat. The thing that everyone else also said was impossbile. Maybe it really works too. I would advise caution rather than optimism. But it isn't helpful to simply dismiss the EM drive as impossible because it doesn't conform to your understanding of physics or assumptions about the drive. It's possible that something genuinely interesting is going on, but the theories proposed by the EM Drive's inventors are somewhere between crank and con-man, and even assuming there's some genuinely physics-interesting stuff going on (rather than an experimental setup that's highly prone to different types of instrument error), that doesn't mean the EM Drive itself will actually be useful. In fact, given the implications of the claims, some extremely strong evidence needs to be presented to raise it above "this almost certainly doesn't work", and the evidence has been very weak at best. It's perfectly helpful to dismiss the core EM Drive claims because they don't conform to physics as we understand them (and we understand physics pretty good), for the exact same reason we (correctly) dismiss perpetual motion machines.
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Post by cuddlefish on Nov 21, 2016 16:09:50 GMT
This is exactly what I thought after reading about the EM Drive, ECat. The thing that everyone else also said was impossbile. Maybe it really works too. I would advise caution rather than optimism. But it isn't helpful to simply dismiss the EM drive as impossible because it doesn't conform to your understanding of physics or assumptions about the drive. It's possible that something genuinely interesting is going on, but the theories proposed by the EM Drive's inventors are somewhere between crank and con-man, and even assuming there's some genuinely physics-interesting stuff going on (rather than an experimental setup that's highly prone to different types of instrument error), that doesn't mean the EM Drive itself will actually be useful. In fact, given the implications of the claims, some extremely strong evidence needs to be presented to raise it above "this almost certainly doesn't work", and the evidence has been very weak at best. It's perfectly helpful to dismiss the core EM Drive claims because they don't conform to physics as we understand them (and we understand physics pretty good), for the exact same reason we (correctly) dismiss perpetual motion machines. The fact that if it works as described it's just a sufficiently large scale kinetic energy capture setup away from a PMM certainly doesn't hurt.
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Post by concretedonkey on Nov 21, 2016 19:04:17 GMT
That doesn't really help with the physics problem, though. If P-In produces T-Out without needing to haul propellant, conservation of energy is dead, because of that damned squared term in kinetic energy. The faster relative to a thing the object is going, the more energy it 'gains' from every additional m/s, so if power in produces m/s, sooner or later you'll get a net positive. My running theory on how the EM Drive works is that it smacks the aft end of the resonant chamber with so many microwave frequency photons that it starts losing electrons, which are launched away from the engine at high speed, with the truncated cone shape causing this to preferentially happen towards the wider end of the cone. This means that while it does produce thrust, it isn't technically reaction-less, as it uses the copper end plate of the cone as reaction mass. Granted, my understanding of physics comes from browsing through this forum, Spacebattles, Sufficient Velocity, along with watching Scott Manley videos and reading Atomic Rockets, so if some smart person (paging illectro , paging illectro) could tell me if I'm being an idiot, that would be much appreciated. I think you got your wish www.youtube.com/watch?v=JGcvxg7jJTs
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Post by ash19256 on Nov 21, 2016 20:39:54 GMT
My running theory on how the EM Drive works is that it smacks the aft end of the resonant chamber with so many microwave frequency photons that it starts losing electrons, which are launched away from the engine at high speed, with the truncated cone shape causing this to preferentially happen towards the wider end of the cone. This means that while it does produce thrust, it isn't technically reaction-less, as it uses the copper end plate of the cone as reaction mass. Granted, my understanding of physics comes from browsing through this forum, Spacebattles, Sufficient Velocity, along with watching Scott Manley videos and reading Atomic Rockets, so if some smart person (paging illectro , paging illectro) could tell me if I'm being an idiot, that would be much appreciated. I think you got your wish www.youtube.com/watch?v=JGcvxg7jJTsActually, that was the video I created my theory after watching, mostly doing my best to take into accounts complaints leveled in the video against other theories as to how the EM Drive works.
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Post by n2maniac on Dec 17, 2016 7:17:08 GMT
Time to be a naysayer. They got 1.2mN/kW out of it. That is 0.83kW/mN, or effectively 0.83 Mm/s exhaust velocity. Light would be 300 Mm/s, so it is producing 360x more thrust than a photon thruster. Well, this sounds kinda nice on the surface (other than the immense amounts of power needed to get anywhere anytime soon as many have pointed out). Now, the experimental setup itself is detailed in the paper. Sources of error are gone through but omit one detail: they put an RF emitter inside a metal chamber without any RF absorber, noted that it was difficult to get low reflectance back into the emitting antennae, and solve it with an auto-tuner. They have a Q-factor over 1000 on what should be an open thruster to open space (which would "absorb every photon moving in that direction)! If the driving antenna was effective at driving the cone, this would imply they had 1000x more photons bouncing back and forth than the cone was spitting out. Yea, they might be measuring photon pressure exerted against the cone. With ~99.3% of it sourced from just bouncing around the chamber. I'll believe it when they have RF absorbers lining the chamber (or, heck, just a few scattered throughout the chamber would be an indicator. If the thrust persists, there may be something. If it then becomes impossible to reproduce the same 360x photon thruster levels, they are fooling themselves with a resonant cavity that is partly the vacuum chamber). /rant Edit: I would like to emphasize to anyone interested in this and/or remotely familiar with RF to read the paper and decide for themselves. Don't take my word for it, and if you see a problem with my argument please point it out. Thanks
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Post by Easy on Dec 19, 2016 16:31:52 GMT
Chinese have stated they will try it in LEO. Don't have a link or info, sorry.
Hopefully they will publish their results.
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Post by n2maniac on Dec 20, 2016 6:54:12 GMT
I remember hearing someone was going to try it in orbit. At least that removes the vacuum chamber = RF reverberation chamber error source.
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Post by Easy on Dec 21, 2016 0:56:59 GMT
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