RF resonant cavity thruster

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EmDrive/Cannae drive
Controversial invention
Inventor Roger Shawyer, Guido Fetta
Theory violation Conservation of momentum, Newton's Third Law

RF resonant cavity thrusters are a proposed type of electromagnetic thruster, such as the EmDrive and Cannae drive, which requires an electrical power source but unlike traditional electromagnetic thrusters, does not require reaction mass or propellant. While designed initially with space travel in mind, the technology could potentially be used for travel in many environments.[1][2][3][4][5][6]

As of 2015, a few prototypes have been built. These include the EmDrive, invented by aerospace engineer Roger Shawyer,[7][8] and the Cannae drive, invented by chemical engineer Guido Fetta.

The design of such thrusters, and theories attempting to explain how they work, are all controversial. As of 2015, there are still arguments about whether the experimental results seen so far are simply misinterpretations of spurious effects mixed with experimental errors. Many theories of operation have been proposed, and all are criticized on the grounds they violate the conservation of momentum, a fundamental law of physics, though its proponents assert that it does not.[9]

Chinese researchers from the Northwestern Polytechnical University (NWPU) in Xi'an in 2008, published a report in their university's journal on the theory behind such devices, and in 2012-2014 they reported building a prototype and measured net thrust in a series of tests.[4][10][11][12][13] They made it clear that their work was still preliminary, however this work was done at higher power levels than anyone had previously attempted.

In 2014 and 2015, a NASA research group at Johnson Space Center tested models of both the EmDrive and Cannae drive. They reported observing net thrust from both, at low power levels.[6][14] A research group at the Dresden University of Technology also tested a small EmDrive in a hard vacuum and reported predicted as well as unexpected thrusts.[15][16] However, as of 2015 none of these experiments and results have been peer reviewed.

History[edit]

Electromagnetic propulsion designs have been around since the start of the 20th century. In the 1960s, extensive work was done on a variety of such drives: from ion thrusters that strip ions from propellant, accelerate them, and eject them; to plasma thrusters that work in a similar way with plasma currents, but do not require electrodes. The plasma in a plasma thruster can be generated from an intense source of microwave or other radio-frequency (RF) energy, in combination with a resonant cavity tuned to resonate at such a frequency.[17]

EmDrive[edit]

In 2001, Roger Shawyer founded Satellite Propulsion Research Ltd, in order to work on the EmDrive. He thought he could produce a drive that used a resonant cavity to produce thrust without propellant. The company was backed by a "Smart Award" grant from the UK Department of Trade and Industry.[5] The DTI grant totalled £250,000, spread out over three years.[18] By December 2002, he was demonstrating a working prototype, reporting a total thrust of about 0.02 newtons powered by an 850 W magnetron.[19] It was later reported that the device could only operate for a few tens of seconds before the magnetron failed, due to overheating.[20] In 2003, Shawyer reported a move toward higher thrust, stating that "An extrapolation of the [EM Drive] theory has been carried out for high thrust engines and the concept appears feasible. Initial design work on a lift engine has been undertaken."[21]

Second device and New Scientist article[edit]

In October 2006, Shawyer conducted tests on a new water-cooled prototype, which increased thrust to 0.1 newtons and ran on 300 W of microwave power.[20] He planned to have the device ready to use in space by May 2009, and was considering making the resonant cavity a superconductor.[20]

Shawyer submitted a theory paper to New Scientist, a weekly popular science consumer magazine,[22] and the EmDrive was featured on the cover of the 8 September 2006 issue of the magazine. The article portrayed the device as plausible, and emphasized the arguments of those who held that point of view.

Science fiction author Greg Egan, who holds a Bachelor of Science degree in Mathematics from the University of Western Australia, distributed a public letter stating that "a sensationalist bent and a lack of basic knowledge by its writers" made the magazine's coverage unreliable, sufficient "to constitute a real threat to the public understanding of science". In particular, Egan found himself "gobsmacked by the level of scientific illiteracy" in the magazine's coverage of the EmDrive, stating that New Scientist employed "meaningless double-talk" to obfuscate the relation of Shawyer's proposed space drive to the principle of conservation of momentum. Egan urged those reading his letter to write to New Scientist and pressure the magazine to raise its standards, instead of "squandering the opportunity that the magazine's circulation and prestige provides" for genuine science education. The letter was endorsed by mathematical physicist John C. Baez and posted on his blog.[23][24] Egan also recommended[23] that New Scientist publish a refutation penned by John P. Costella (a data scientist with a PhD in theoretical physics)[25] of Shawyer's paper.[22]

The following month, the New Scientist editor addressed the ensuing controversy over the article, stating, "[w]e should have made more explicit where it apparently contravenes the laws of nature and reported that several physicists declined to comment on the device because they thought it too contentious."[26]

Cannae and other drives[edit]

The Cannae Drive (formerly Q-drive),[27] another engine designed to generate propulsion from a resonant cavity without propellant, is another widely known implementation of this idea. Its cavity is also asymmetric, but is flatter than that of the EmDrive. It was designed by Guido Fetta in 2006 and has been promoted within the US through his company, Cannae LLC, since 2011.[27][28][29][30] Shawyer has said the Cannae drive "operates along similar lines to EmDrive, except that its thrust is derived from a reduced reflection coefficient at one end plate," which he says would reduce its thrust.[6]

A team at Chinese Northwestern Polytechnical University (NWPU) has developed and tested their own prototype, starting in 2008. They built on these ideas but did not explicitly replicate either EmDrive or Cannae designs. As of 2014, they had published results from tests running at 100x the power of other prototypes.

Theory[edit]

Like other resonant cavity designs, these drives use a magnetron to produce microwaves which are directed into a metallic, fully enclosed conically tapered high Q resonant cavity.

These drives have a greater area at one end of the device, and a dielectric resonator in front of the narrower end. They are designed to generate a directional thrust toward the narrow end of the cavity. They require an electrical power source to run the magnetron, but no other propellant.

Any apparently reactionless drive is treated with skepticism by the physics community, since a truly reactionless drive would violate the law of conservation of momentum. Proponents claim these drives do not violate conservation of momentum and are not reactionless.[9] Shawyer has self-published theory papers about the EmDrive.[31] These include the fundamental assertion underlying the theory: "[t]his force difference is supported by inspection of the classical Lorentz force equation F = q(E + νB). (1) If ν is replaced with the group velocity νg of the electromagnetic wave, then equation 1 illustrates that if vg1 is greater than vg2, then Fg1 should be expected to be greater than Fg2." This statement makes two assumptions which Shawyer does not substantiate and which may explain the discrepancy between Shawyer's predictions and those of conventional physics. For example he assumes that radiation pressure is the result of the Lorentz force acting on charged particles in the reflecting material. This is analyzed by Rothman and Boughn[32] who point out that the standard theory of radiation pressure is more complicated than the simplified analysis suggests.

Various hypotheses and theories have been proposed explaining the underlying physics for how these drives might be producing thrust. Shawyer claims that thrust is caused by a radiation pressure imbalance between the two faces of the cavity caused by the action of group velocity in different frames of reference within the framework of special relativity.[33] Yang from NWPU calculated the net force/thrust using classical electromagnetism.[12] Harold G. "Sonny" White, who investigates field propulsion at Eagleworks, NASA's Advanced Propulsion Physics Laboratory, speculated that such resonant cavities may operate by creating a virtual plasma toroid that could realize net thrust using magnetohydrodynamic forces acting upon quantum vacuum fluctuations.[34] Likewise, the paper describing the Eagleworks tests refer to a possible interaction with a so-called "quantum vacuum virtual plasma".[14] This reference has been criticized by mathematical physicists John Baez and Sean M. Carroll because in the standard description of vacuum fluctuations, virtual particles do not behave as a plasma.[24][35][36]

Testing and replication claims[edit]

Static thrust tests[edit]

Shawyer has reported seven independent positive reviews from experts at BAE Systems, EADS Astrium, Siemens and the IEE.[18] In 2006 he speculated that, with adequate funding, commercial terrestrial aircraft incorporating EmDrives as lift engines could be ready by 2020.[37][38] He proposed that very high Q superconducting resonant cavities could produce static specific thrusts of about 30 N/W, which is 3 tonnes-force of thrust per kilowatt of input power − "enough to lift a large car".[39] As of 2015, no EmDrive has been tested in microgravity.

Fetta tested a superconducting version of the Cannae drive on 13 January 2011. The RF resonant cavity was suspended inside a liquid helium-filled dewar. The weight of the cavity was monitored by load cells. Fetta theorized that when the device was activated and produced upward thrust, the load cells would detect the thrust as change in weight. When the drive was energized by sending 10.5 watt power pulses of RF power into the resonant cavity, there was a reduction in compressive force on the load cells consistent with thrust of 8-10 mN.

None of the above results have been published in the scientific literature, but have been posted on their inventors' websites.[40]

Chinese Northwestern Polytechnical University (NWPU)[edit]

In 2008 a team of Chinese researchers led by Juan Yang (杨涓), professor of propulsion theory and engineering of aeronautics and astronautics at Northwestern Polytechnical University (NWPU) in Xi'an, claimed to have developed a valid electro-magnetic theory behind a microwave resonant cavity thruster.[1][41] A demonstration version of the drive was built and tested under different cavity shapes and at higher power levels in 2010.[10] A maximum thrust of 720 mN was reported at 2,500 W of input power on an aerospace engine test stand usually used to precisely test spacecraft engines like ion drives.[5][11][12][13][42]

The editor of Wired magazine who covered the experimental results relating to reactionless drives reported that he received some comments from the Chinese researchers stating "the publicity was very unwelcome, especially any suggestion that there might be a military application"[3] and that Yang told him that "she is not able to discuss her work until more results are published".[5]

NASA/JSC Advanced Propulsion Physics Laboratory (Eagleworks)[edit]

White's team at Eagleworks is devoted to studying advanced propulsion systems that they hope to develop using quantum vacuum and spacetime engineering.[43] The group has investigated a wide range of untested and fringe proposals, including RF resonant cavity thrusters and related concepts.

EmDrive[edit]

In 2011, the group reported having an RF resonant cavity thruster prototype for testing.

In July 2014, the group reported positive results for an evaluation of a RF resonant tapered cavity similar to the EmDrive.[14] Testing was performed using a low-thrust torsion pendulum capable of detecting force at the micronewton level within a sealed but not evacuated vacuum chamber; the RF power amplifier used an electrolytic capacitor not capable of operating in a hard vacuum.[14] The experimenters recorded directional thrust immediately upon application of power.

NASA's first tests of this tapered RF resonant cavity were conducted at very low power (2% of Shawyer's 2002 experiment and 0.7% of the Chinese 2010 experiment), but a net mean thrust over five runs was measured at 91.2 µN at 17 W of input power. A net peak thrust was recorded at 116 µN at the same power level.[14] The experiment was criticized for not having been conducted under vacuum, which would have eliminated thermal air currents.[44]

Six months later, early 2015, Paul March from Eagleworks made new results public, claiming positive experimental force measurements with a torsional pendulum in a hard vacuum: about 50 µN with 50 W of input power at 5.0×10−6 torr, and new null-thrust tests.[45] The new RF power amplifiers were said to be made for hard vacuum, but still fail rapidly due to internal corona discharges, with not enough funding to replace or upgrade them, so measurements are still scarce and need improvement before a new report can be published.[46]

Glenn Research Center offered to replicate the experiment in a hard vacuum when Eagleworks manage to reach 100 µN of thrust, because the GRC thrust stand can only measure down to 50 µN.[45]

Eagleworks later announced a plan to upgrade their equipment to higher power levels, use vacuum-capable RF amplifiers with power ranges of up to 125 W, and design a new tapered cavity analytically determined to be in the 0.1 N/kW region. The test article will be subjected to independent verification and validation at Glenn Research Center, the Jet Propulsion Laboratory, and the Johns Hopkins University Applied Physics Laboratory.[14]

Cannae drive[edit]

The same NASA test campaign evaluated a Cannae drive.[14] They tested two versions: one device with radial slots engraved along the bottom rim of the resonant cavity interior, as required by Fetta's theory to produce thrust;[28] and a "null" test article lacking those radial slots. Both drives were equipped with an internal dielectric.[14] The null test device was not intended to be the experimental control. The control device was a third test article involving an RF load but without the resonant cavity interior.[44] Like the EmDrive tests, the Cannae drive tests took place at atmospheric pressure, not in a vacuum.

About the same net thrust was reported for both the standard and the null test devices. The experimental control without a resonant cavity interior measured zero thrust as expected.[44] Some considered the positive result for the non-slotted article as indicating a possible flaw in the experiment, as the null test device had been expected to produce less or no thrust based upon Fetta's theory of the designed mechanics.[44][47][48] In the complete paper, Eagleworks concluded, however, that the test results proved that "thrust production was not dependent upon slotting".[14] According to Baez, the fact that the results were not dependent of the slotting, which was claimed to be necessary for thrust according to the inventor, should be seen as an invalidation of the device.[24]

Dresden University of Technology[edit]

Martin Tajmar leads a research group in advanced space propulsion systems at the Institute for Aerospace Engineering, Dresden University of Technology (TUD).

In July 2015 he reported positive results for an evaluation of an RF resonant tapered cavity similar to the EmDrive.[16] Testing was performed preliminary on a knife-edge beam balance capable of detecting force at the micronewton level, on top of an anti-vibration granite table at ambient air pressure; then on a torsion pendulum with a force resolution of a tenth of a micronewton, inside a vacuum chamber at ambient air pressure and in a hard vacuum at 4×10−6 mbar (3×10−6 torr).

Tajmar used a conventional 2.45 GHz 700 W oven magnetron and attached it through a standard waveguide to a copper frustum cavity, which had distinctive features among other third-party replication experiments. The cavity was comparatively much smaller, with a height of only 68.6 mm; the entrance slit on the side for microwaves filled almost all that height; and the Q factor was considerably low (Q < 50 in ambient air and later Q = 20 in vacuum tests after some oxidization of inner surfaces) because the best resonance at that size would be above 3 GHz, a frequency the magnetron used by Tajmar could not achieve. The device produced positive thrusts in the positive direction and negative thrusts in the negative direction of about 20 micronewtons in a hard vacuum, consistent with the low Q factor.

Significant side-effects like air convection currents and buoyancy due to heat dissipated from the cavity and the magnetron were detected and taken into account by thermal insulation with glass wool for ambient-air tests. Electromagnetic interference was also shielded with high magnetic permeability iron sheets. Besides being tested horizontally in both directions on the torsion pendulum, the cavity was also set upwards as a "null" configuration. However, this vertical test intended to be the experimental control showed an anomalous thrust of hundreds of micronewtons that could be caused by a magnetic interaction with the power feeding lines going to and from liquid metal contacts in the setup.

Due to this anomalous interaction, not fully characterized yet, the authors conclude they can not confirm or refute the device claims and recommend further investigation. Future experiments are announced with better magnetic shielding, other vacuum tests and improved cavities with higher Q factors and electronics to increase thrust.

However, Eric W. Davis, a Senior Research Physicist at the Institute for Advanced Studies at Austin has stated "The experiment is quite detailed but no theoretical account for momentum violation is given by Tajmar, which will cause peer reviews and technical journal editors to reject his paper should it be submitted to any of the peer-review physics and aerospace journals."[49]

See also[edit]

References[edit]

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