EmDrive: China's radical new space drive

Scientists in China have built and tested a radical new space drive. Although the thrust it produces may not be enough to lift your mobile phone, it looks like it could radically change the satellite industry. Satellites are just the start: with superconducting components, this technology could generate the thrust to drive everything from deep space probes to flying cars. And it all started with a British engineer whose invention was ignored and ridiculed in his home country.

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Shawyer aimed to develop an EmDrive: a closed, conical container which, when filled with resonating microwaves, experiences a net thrust towards the wide end. It seems to violate of the law of conservation of momentum, implied by Newton, which says that no closed system can have a net thrust. However, Shawyer says net thrust occurs because the microwaves have a group velocity which is greater in one direction than the other and Einstein's relativity comes into play. Group velocity, the speed of a collection of electromagnetic waves, is a tricky business -- a pulse of light can even have a group velocity which is greater than the speed of light -- but can it really cause net thrust?

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Meanwhile, the EmDrive was picked up by Yang at Xi'an, who has a background in space propulsion systems.

The Chinese team took a cautious approach. They started with a new analysis in terms of quantum theory in 2008 which indicated that the theoretical basis was sound and net thrust is possible. The next paper in 2010 quantified the amount of thrust that could be produced, and stated that the team was getting positive experimental results. The latest paper describes their latest thruster and gives the test results in details, showing that with a couple of kilowatts of power they can produce 720 mN (about 72 grams) of thrust.

It may not sound very much, less than three ounces, but in space a little thrust goes a long way. Boeing's advanced XIPS thruster, which fires out Xenon ions at high speed, generates less than a quarter as much thrust from twice as much power. It's used to maintain satellites in position, or move them to a slightly different orbit. Crucially, Xips weights about twenty kilos, more than an equivalent EmDrive, and the propellant for prolonged operation can weigh much more.

http://www.wired.co.uk/news/archive/2013-02/06/emdrive-and-cold-fusion