German Test Reveals That Magnetic Fields Are Pushing the EM Drive (arstechnica.com)

"Researchers in Germany have performed an independent, controlled test of the infamous EM Drive with an unprecedented level of precision," writes PvtVoid. "The result? The thrust is coming from interactions with the Earth's magnetic field." From the report: Instead of getting ahold of someone else's EM drive, or Mach-effect device, the researchers created their own, along with the driving electronics. The researchers used precision machining and polishing to obtain a microwave cavity that was much better than those previously published. If anything was going to work, this would be the one. The researchers built up a very nice driving circuit that was capable of supplying 50W of power to the cavity. However, the amplifier mountings still needed to be worked on. So, to keep thermal management problems under control, they limited themselves to a couple of Watts in the current tests. The researchers also inserted an enormous attenuator. This meant that they could, without physically changing the setup, switch on all the electronics and have the amplifiers working at full noise, and all the power would either go to the EM drive or be absorbed in the attenuator. That gives them much more freedom to determine if the thrust was coming from the drive or not.

Even with a power of just a couple of Watts, the EM-drive generates thrust in the expected direction (e.g., the torsion bar twists in the right direction). If you reverse the direction of the thruster, the balance swings back the other way: the thrust is reversed. Unfortunately, the EM drive also generates the thrust when the thruster is directed so that it cannot produce a torque on the balance (e.g., the null test also produces thrust). And likewise, that "thrust" reverses when you reverse the direction of the thruster. The best part is that the results are the same when the attenuator is put into the circuit. In this case, there is basically no radiation in the microwave cavity, yet the WTF-thruster thrusts on. So, where does the force come from? The Earth's magnetic field, most likely. The cables that carry the current to the microwave amplifier run along the arm of the torsion bar. Although the cable is shielded, it is not perfect (because the researchers did not have enough mu metal). The current in the cable experiences a force due to the Earth's magnetic field that is precisely perpendicular to the torsion bar. And, depending on the orientation of the thruster, the direction of the current will reverse and the force will reverse. The researchers' conclude by saying: "At least, SpaceDrive [the name of the test setup] is an excellent educational project by developing highly demanding test setups, evaluating theoretical models and possible experimental errors. It's a great learning experience with the possibility to find something that can drive space exploration into its next generation."

Comments

THIS is science

[Anonymous Coward]

This is what we should be teaching in schools and promoting in daily life/culture.

Re:THIS is science

[arglebargle_xiv]

As long as we teach them that science can be overruled by a voice vote in Congress, and the laws of mathematics by the Australian legislature.

Re:Thrust is coming from interactions with the Ear

[djinn6]

Almost all of the energy used in space travel is used near Earth. So if we have something that could boost a spaceship from low Earth orbit to escape velocity, and it doesn't use any fuel, that's still incredibly useful.

No surprise

[Michael Woodhams]

The EM drive, if it works, violates conservation of momentum, which can easily be used to also violate conservation of energy. (/. commenters on previous EM drive stories have gone into this at some length.)

The EM drive was originally designed using standard physics (I think electromagnetism and possibly special relativity) and the inventor's calculations showed it would produce thrust. They did not realize that as the input physics conserved momentum but their calculation result violated it, this guaranteed their calculation was in error.

The chances of this result being real were always really tiny. I'm happy there is now a good explanation for the anomalous experimental results.

Re:No surprise

[Michael Woodhams]

My understanding is that the 'thrust' comes from radiation pressure in a truncated cone cavity, and the false result comes about by accounting for radiation pressure on the ends of the cone but ignoring it on the conical sides. If you include the conical sides in your calculation, you find zero thrust.

However, it isn't actually necessary to point out the exact error. If you give me a list of numbers to add up, and all the numbers in the list is even, and I tell you I've calculated the total, and the total is an odd number, you know I've messed up. It isn't necessary to go over my calculations with a fine tooth comb to identify exactly where I went wrong. This case is the same - input physics conserves momentum, calculated result does not, calculations must be in error.

Re:Still useful for interplanetary flight?

[jythie]

Not really. One of the reasons this is such a nail in the coffin of the emdrive is it dashes their assumptions that ways will be found to scale up the effect. Proponents have been hoping that if they find the right material, right geometry, right frequency, they will hit on an effect that can produce lots of thrust for little power. If it is just the rig interacting with the earth's magnetic field, that puts very well understood limits on how much movement you will be able to get out of it.