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http://www.dvice.com/sites/dvice/files/emdrive.jpgEmDrive (also Relativity Drive) is the name of a spacecraft propulsion system proposed by British aerospace engineer Roger J. Shawyer, who develops prototypes at Satellite Propulsion Research Ltd (SPR), the company he created for that purpose in 2000.

 New Scientist ran a cover story on EmDrive in its 8 September 2006 issue. The device uses a magnetron producing microwaves directed inside a specially shaped, fully enclosed tapering high Q resonant cavity whose area is greater at one end, upon which radiation pressure would act differently due to a relativistic effect caused by the action of group velocity in different frames of reference. The inventor claims that the device generates a thrust even though no detectable energy leaves the device. If proven to work as claimed, the EmDrive could allow the design of spacecraft engines that would be electrically powered and would require no reaction mass. Such an engine would be a breakthrough in airflight and spaceflight.

The device and its mode of operation are highly controversial. As of 2014, it is still not proven if the EmDrive is a genuinely new propulsion method; a misinterpretation of spurious effects mixed with mathematical errors; or a scam. The proposed theory immediately received virulent criticism because it seems to violate basic Newtonian laws of physics, notably conservation of momentum, though the inventor insists on the contrary. Whatever it be, peer reviewed independent replication has been provided by Chinese researchers from the Northwestern Polytechnic University on both mathematical and experimental grounds. in 2008,2010, 2012, and 2013. NASA has followed suit with these claims by a validation of the experiment, replicated in Eagleworks Laboratories at the NASA Johnson Space Center.

Should the EmDrive produce a real thrust, various conjectures have been made to explain the underlying physics. Shawyer claims the thrust would be caused by radiation pressure imbalance due to group velocities of electromagnetic waves within the framework of special relativity. Dr. Yang predicts a resulting net force using classical electromagnetism. A more complete theory has been proposed in 2013 by Argentine physicist Fernando Minotti from CONICET, who explains the alleged forces on asymmetric electromagnetic resonant cavities by a particular class of scalar-tensor theory of the Brans–Dicke type. Dr. Harold G. "Sonny" White, a NASA mechanical engineer and physicist investigating field propulsion at Johnson Space Center, notes that such resonant cavities may operate by creating a virtual plasma toroid that would realize net thrust using magnetohydrodynamics upon quantum vacuum fluctuations.
Theoretical predictions

In 2010, the Chinese group published a second paper in another scientific journal where they quantify the theoretical maximum specific thrust of the prototype they have built (456 mN/kW) and announce their first positive experimental results: 315 mN/kW for a 1000W power input. The practical results were consistent with their theoretical predictions, both in term of thrust magnitude than in respect to the various shapes of resonant cavities tested. It is interesting to note that Yang's group calculated the theoretical thrusts of their experimental device applying a method involving a set of steady-state single-fluid MHD equations that was reformed by other Chinese researchers to quantify forces in a plasma generated in front of a vehicle embedded in a hypersonic flow, whereas Shawyer calculated similar predictions from the derivation of Cullen's equations. The original paper (in Chinese) is available on the SPR Ltd web site, as well as a professional English translation.

Experimental results- In 2012, more detailed results are published from the 2010 device in Acta Physica Sinica, also available on the SPR Ltd web site. Yang's team achieved a maximum thrust of 720 mN for an input power of 2.5 kW (specific thrust of 288 mN/kW). The paper also specifies the net thrust measurements were conducted on a patented aerospace engine test stand called the Rocket Indifferent Equilibrium Thrust Measurement Device usually used to precisely test spacecraft engines like ion drives. A magnified version of the diagram showing the evolution of thrust generated as a function of input power is available on the SPR Ltd web site.

By comparison, the NASA HiPEP ion thruster, intended for use on the cancelled JIMO mission, produced up to 670 mN of thrust at a power level of 39.3 kW (specific thrust of 17 mn/kW) while using 7 mg/s of xenon gas as propellant. When the xenon gas is depleted, the thruster ceases to function. It is claimed that the EmDrive, because it does not rely on reaction mass, would work indefinitely without any fuel other than that which might be required to generate electricity. Because the reaction mass typically forms 90% or more of the total weight of a spacecraft, a thruster that did not require reaction mass would represent a fundamental breakthrough in spacecraft design and propulsion.

Following these results, Dr. Yang and her team published another paper in English in an international scientific journal in 2013.

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