register | NASA has tested an "impossible" electric space drive that uses no
propellant, and found it works even when it is designed not to –
sparking immediate skepticism of the technology.
The system is designed to use microwave energy reflected along a
specially designed chamber to produce thrust. The idea first appeared as
the Emdrive by
British inventor Roger Shawyer in 2001, who designed a motor that he
showed could produce power in this way. But critics scoffed, saying it
would violate the laws of momentum.
The EmDrive, we're told,
generates thrust by using the properties of radiation pressure. An
electromagnetic wave has a small amount of momentum which, when it hits a
reflector, can translate that into thrust, Shawyer found, and this
apparently can be used to power flight in the near-frictionless
environment of space.
The idea languished, but a decade later the
Chinese Academy of Sciences published a paper saying that it too had
built an EmDrive-like which, when fed 2.5kW, generated 720mN of thrust –
a tiny amount, admittedly.
But this got the attention of NASA
boffins, who in 2013 commissioned a series of tests on the drive and got
some surprising results.
In an eight-day trial held by US engineering firm Cannae, researchers
found that by using a reflective chamber similar to that proposed by
Shawyer, the team was able to use solely electrical input to generate 30
to 50 micro-Newtons of thrust. Again, incredibly tiny, enough to move a
grain of sand, but apparently significant.
"Test results indicate
that the RF resonant cavity thruster design, which is unique as an
electric propulsion device, is producing a force that is not
attributable to any classical electromagnetic phenomenon and therefore
is potentially demonstrating an interaction with the quantum vacuum
virtual plasma," the team reported in a paper to the 50th Joint Propulsion Conference in Cleveland, Ohio, at the end of July.
11 comments:
There is "virtual vacuum quanta blabla momentum" (last paragraph above), two mirrors repel (attract?) each other in the dark, but that's always been a very low background-noise effect, and it's far from the only possible explanation for a surprising result in the lab. Or they say "plasma" so maybe they're fantasizing about virtual electron-positron pairs. Another effect that has been studied to death and conserves momentum like it's going out of style.
Shades of Fleischmann and Pons: "we've ruled out everything else, so it must be fusion."
Yang Juan say different http://www.emdrive.com/yang-juan-paper-2012.pdf
Yeah, and they really do say milli Newtons not micro Newtons. Seven hundred mN you should be able to feel with your hand.
You get some thrust for sure. The question is how much. They get 100,000 times more than my decaffeinated estimate. They also say their thrust is consistent with calculations, so maybe it's my caffeine deficiency talking.
To be sure I understand your point, the working EMDrive models are producing measurable thrust for which there is no existing and generally agreed upon explanation, correct?
No, the PRC group say their measurements are "consistent with calculations." I just don't understand what their calculations are.
Yang et al. only very vaguely sketch a calculation. They don't actually calculate a number.
I could say more if anybody really cares, but there's nothing new in the principle they say they're relying on, and it should simply give
Force = Power / c
where c is the speed of light. http://en.wikipedia.org/wiki/Radiation_pressure#Radiation_pressure_by_emission
The tapering of the chamber does increase the radiation pressure at the narrow end, but it decreases the area that pressure acts over. There's no net effect.
In my opinion it's a fan-powered sailboat. You don't have to analyze the shape of the sails to figure out how that works.
Care to make it interesting? $100?
The point isn't that you can't power a boat with a fan. Those Everglades boats use fans. You can deflect with the sail and reverse the thrust, sure. That should be uncontroversial. I question what "physics books" said that wouldn't work, lol.
The point is you can't multiply the thrust using sails. And you can't multiply momentum by bouncing a ball between two of those pitch-back things. Not unless you add energy some other way -- move them inward or something.
It's Yahoo. It is not true that they had jets that deflected exhaust long before the physics community figured out conservation of momentum. Newton's law comes from sometime in the 1600s. It explains things like the Harrier just fine.
The momentum disappearing from the cavity mode is the momentum carried by the radiation losses in the metal reflectors.
In steady state the radiation losses equal the power supplied by the microwave source.
It's going to be at best the same magnitude of momentum that you get by just pointing the microwave source backwards. Same exact deal as the fan-sailboat -- the sail can't be used to produce power, only to change the direction of the thrust.
The measured thrust reported by Yang & them is too high for me to explain by radiation pressure.
Now if they're actually evaporating something off of a hot surface, that could be another story.
http://en.wikipedia.org/wiki/Ion_thruster
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