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

A radio frequency (RF) resonant cavity thruster is a proposed type of electromagnetic thruster. Unlike conventional electromagnetic thrusters, a resonant cavity thruster would use no reaction mass and would emit no directional radiation. Their design principles are not supported by prevailing scientific theories, apparently violating the law of conservation of momentum; as a result they are controversial.[1]

Aerospace engineer Roger Shawyer designed the EM Drive, or EmDrive, in 2001 and has promoted the idea through his company, Satellite Propulsion Research. Chemical engineer Guido Fetta designed the Cannae Drive, based on similar principles. Although generally considered implausible, if they are found to work as claimed, providing thrust without consuming a propellant would revolutionise many propulsion applications, particularly spaceflight.[2][3][4]

Independent teams of scientists, notably a team at Xi'an's Northwestern Polytechnical University (NWPU),[5] one at NASA's Eagleworks laboratories[6] and another at the Dresden University of Technology in Germany,[7] built prototypes of these designs. The NWPU team reported a small but significant thrust; NASA Eagleworks reported a much smaller thrust than the NWPU team; and the Dresden team reported a small thrust, but within experimental error.

Skeptics dispute claims that the drive has been independently validated.[8]

History[edit]

Electromagnetic propulsion designs which operate on the principle of reaction mass have been around since the start of the 20th century. In the 1960s, extensive research was conducted on designs from ion thrusters that convert propellant to ions and accelerate and eject them; to plasma thrusters that eject plasma ions in a similar way using plasma currents. The plasma in a plasma thruster can be generated from an intense source of microwave or other radio-frequency (RF) energy and in combination with a resonant cavity, can be tuned to resonate at a particular frequency.[9]

A low-propellant space drive has long been a goal of space exploration. If a zero-propellant drive existed, it could potentially be used for travel in many environments. This has contributed to the enthusiasm for exploring such designs, even if they seem impossible.[2][3][4]

Controversy[edit]

The design of such thrusters and the theories that attempt to explain how they might work are all matters of controversy, particularly the claims that this is an example of a reactionless drive, in violation of conservation of momentum.[4][10]

Inventors have not reliably demonstrated thrust from Emdrive designs and few scientists take the claims about these designs seriously. Critics such as John C. Baez and Australian science fiction writer Greg Egan assert that positive results are misinterpretations of spurious effects mixed with experimental errors.[11] Research teams continue to refine their designs and measurements and to explain their observations using traditional physics models.[12]

Designs and prototypes[edit]

EmDrive[edit]

In 2001, 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.[4] The DTI grant totalled £250,000, spread out over three years.[13][unreliable source?] In December 2002, he claimed a working prototype with a total thrust of about 0.02 newtons powered by an 850 W magnetron. The device could only operate for a few dozen seconds before the magnetron failed, due to overheating.[14]

Second device and New Scientist article[edit]

In October 2006, Shawyer conducted tests on a new water-cooled prototype and claimed increased thrust.[15] He planned to have the device ready to use in space by May 2009 and was considering making the resonant cavity a superconductor.[15]

New Scientist magazine[16] featured the EmDrive on the cover of the 8 September 2006 issue. The article portrayed the device as plausible and emphasized the arguments of those who held that point of view.

Science fiction author Greg Egan 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, alleging that it used "meaningless double-talk" to obfuscate the problem of conservation of momentum. The letter was endorsed by mathematical physicist John C. Baez and posted on his blog.[11][17]

Egan also recommended[11] that New Scientist publish a refutation penned by John P. Costella (a data scientist with a PhD in theoretical physics).[16][18]

New Scientist editor Jeremy Webb responded to critics, stating, "It is a fair criticism that New Scientist did not make clear enough how controversial Roger Shawyer’s engine is. We 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... The great thing is that Shawyer’s ideas are testable. If he succeeds in getting his machine flown in space, we will know soon enough if it is ground-breaking device or a mere flight of fancy."[19] It also published a letter from the former technical director of EADS Astrium, who stated: "I reviewed Roger’s work and concluded that both theory and experiment were fatally flawed. Roger was advised that the company had no interest in the device, did not wish to seek patent coverage and in fact did not wish to be associated with it in any way",[20] and a letter from Paul Friedlander, who stated "As I read it, I, like the thousands of other physicists who will have read it, immediately realised that this was impossible as described. Physicists are trained to use certain fundamental principles to analyse a problem and this claim clearly flouted one of them... The Shawyer drive is as impossible as perpetual motion. Relativistic conservation of momentum has been understood for a century and dictates that if nothing emerges from Shawyer’s device then its centre of mass will not accelerate. It is likely that Shawyer has used an approximation somewhere in his calculations that would have been reasonable if he hadn’t then multiplied the result by 50,000. The reason physicists value principles such as conservation of momentum is that they act as a reality check against errors of this kind."[21]

Later work[edit]

In 2013 and 2014, Shawyer presented ideas for 'second-generation' EmDrive designs and applications, at the annual International Astronautical Congress. A paper based on his 2014 presentation was published in Acta Astronautica in 2015.[22] It describes a model for a superconducting resonant cavity and three models for thrusters with multiple cavities, with hypothetical applications for launching space probes.

Cannae and other drives[edit]

The Cannae Drive (formerly Q-drive),[23] another engine designed to generate propulsion from a resonant cavity without propellant, is another implementation of this idea. Its cavity is also asymmetric, but is flatter than that of the EmDrive. It was designed by Fetta in 2006 and has been promoted within the US through his company, Cannae LLC, since 2011.[23][24][25][26][27] 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]

Researchers working under Juan Yang (杨涓) at the Northwestern Polytechnical University (NWPU) in Xi'an developed their own prototype EmDrive in 2008, publishing a report in their university's journal on the theory behind such devices. In 2012–2014 they reported measuring net thrust in a series of preliminary tests.[3][28][29]

Replication efforts[edit]

In 2014 and 2015, the NASA Eagleworks research group at Johnson Space Center tested models of both the EmDrive and Cannae drive. They reported observing a small net thrust from both, at low power levels. There were two controls, the first, referred to as the 'null test article', was designed without the internal slotting that the Cannae Drive's creator theorised was necessary to produce thrust. This 'null test article' produced thrust, contrary to theoretical predictions, leading researchers to conclude that "thrust production was not dependent upon slotting". The second control device was built with the same RF load as all the previous devices but had no tapered cavity and did not produce any thrust, leading researchers to conclude that a tapered cavity is necessary for thrust production.[6][30] A research group at the Dresden University of Technology also tested a small EmDrive in a hard vacuum and reported that "we have performed a null measurement within our measurement resolution (which is on the order of prediction of the EMDrive thrust)."[7][31]

Hypothesis[edit]

These drives use a magnetron to produce microwaves that are directed into a metallic, fully enclosed conically tapered high Q resonant cavity. They 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.

If thrust is produced without expelling momentum from the system in the opposite direction, the lack of momentum expulsion would make the device a "reactionless" drive in the sense that it violates Newton's Third Law. 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. However, proponents claim these drives are not reactionless and do not violate conservation of momentum.

Shawyer has self-published theory papers about the EmDrive. 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 that Shawyer does not substantiate and that may explain the discrepancy between his 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 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. Yang from NWPU calculated the net force/thrust using classical electromagnetism.[28] 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.[33] Likewise, the paper describing the Eagleworks tests refer to a possible interaction with a so-called "quantum vacuum virtual plasma".[30] 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; furthermore, because the quantum vacuum has no "rest frame", there is nothing to push against, so it can't be used for propulsion.[17][34][35]

A paper in EPL by Mike McCulloch, a Lecturer in Geomatics at Plymouth University, claims that the EmDrive's thrust can be predicted using McCulloch's controversial theory of quantization of inertia (MiHsC).[36] McCulloch hypothesizes that inertia arises from an effect predicted by general relativity called Unruh radiation, that an accelerating object experiences black body radiation. Thus inertia is the pressure the Unruh radiation exerts on an accelerating body. At very small accelerations, Unruh wavelengths become so large they can no longer fit in the observable universe. When this happens, inertia is quantized. He claimed observational evidence for this in the form of the otherwise unexplained jumps in momentum observed in some spacecraft as they fly past Earth toward other planets.[37][38][39][40][41] While this model allows the device to create thrust without breaking Newton's third law, it does however assume that Unruh radiation is real, as well as require speed of light to change within the microwave cavity. This change in the speed of light is contrary to the central tenet of special relativity.[42] Unlike other hypotheses used to explain the device, the hypothesis put forth by McCulloch is testable and McCulloch has suggested that building a cavity where the length of the cavity is the same as the diameter of the small end should cause the Unruh radiation to fit better in the small end, resulting in a reversal of thrust.[40]

Testing and replication claims[edit]

Static thrust tests[edit]

Roger Shawyer has reported seven independent positive reviews from experts at BAE Systems, EADS Astrium, Siemens and the IEE.[13] However, in a letter to New Scientist, the then-technical director of EADS Astrium (Shawyer's former employer) denied this, stating "I reviewed Roger’s work and concluded that both theory and experiment were fatally flawed. Roger was advised that the company had no interest in the device, did not wish to seek patent coverage and in fact did not wish to be associated with it in any way."[20]

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, as predicted, 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. They have been posted on their inventors' websites.[43]

An article published by Shawyer in Acta Astronautica summarises the existing tests on the EmDrive. Of seven tests, four produced a measured force in the intended direction and three produced thrust in the opposite direction. Furthermore, in one of the tests, thrust could be produced in either direction by varying the spring constants in the measuring apparatus. Shawyer argues that the thrust measured in the opposite direction is the reaction force from the drive and therefore it is consistent with Newtonian mechanics.[1]

As of 2015, no EmDrive has been tested in microgravity.

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 NWPU, claimed to have developed a valid electro-magnetic theory behind a microwave resonant cavity thruster.[5][44] A demonstration version of the drive was built and tested with different cavity shapes and at higher power levels in 2010. Using an aerospace engine test stand usually used to precisely test spacecraft engines like ion drives,[4][28][29] They reported a maximum thrust of 720 mN (about 3 ounces of force) at 2,500 W of input power — roughly 0.3 mN/W, which they later pointed out to suffer from excessive force caused by heat related deformation of the flexible waveguide used to connect the microwave source and the cavity.[45]

The editor of Wired magazine who covered these experimental results reported that he received comments from the Chinese researchers stating "the publicity was very unwelcome, especially any suggestion that there might be a military application"[2] and that Yang told him that "she is not able to discuss her work until more results are published".[4]

In 2016, Yang et. al. published a paper, assessing an independent (self-contained) microwave thruster propulsion device with three-wire torsion pendulum thrust measurement system.[45] The measurement system was determined to be able to detect 3 mN force with uncertainty of 14%. For the independent microwave thruster propulsion device, they reported "Measurement results fluctuate within ± 0.7mN range under the conditions 230W microwave power output, and the relative uncertainty is greater than 80%." The same device was also tested with the DC power (about 30 amperes) supplied from outside. The excessive force in this case was found to be 8~10 mN, emphasizing the importance of using self-contained systems.

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.[46] The group has investigated a wide range of untested and fringe proposals, including RF resonant cavity thrusters and related concepts. As of March 2016, Popular Mechanics states that Eagleworks has a paper on the topic undergoing peer review which, they say, is unlikely to pass.[47]

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 an RF resonant tapered cavity similar to the EmDrive.[30] 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).[30] 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 (about 0.0004 ounces, or approximately the same weight as a grain of rice) at the same power level.[30] The experiment was criticized for not having been conducted under vacuum, which would have eliminated thermal air currents.

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.[48] 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.[49]

Glenn Research Center offered to replicate the experiment in a hard vacuum if Eagleworks manages to reach 100 µN of thrust, because the GRC thrust stand cannot measure forces lower than 50 µN.[48]

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 to design a new tapered cavity analytically determined to be in the 0.1 N/kW range. 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.[30]

Cannae drive[edit]

The same NASA test campaign evaluated a Cannae drive.[30] 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;[24] and a "null" test article lacking those radial slots. Both drives were equipped with an internal dielectric.[30] 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. 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 device with radial slots and the null test device without slots. The experimental control without a resonant cavity interior measured zero thrust, as expected. Some considered the positive result for the non-slotted device 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 how thrust was produced by the device.[17][50][51] In the complete paper, however, Eagleworks concluded that the test results proved that "thrust production was not dependent upon slotting".[30]

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 results for an evaluation of an RF resonant tapered cavity similar to the EmDrive.[31] Testing was performed first 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 lower. (Q < 50 in ambient air and later Q = 20 in vacuum tests after some oxidization of inner surfaces. The best resonance at that size would have been above 3 GHz, a frequency the magnetron used by Tajmar could not achieve.)

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.

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.

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.

This anomalous interaction was not fully understood. As a result, the authors conclude they can not confirm or refute claims about such a thruster and they recommend further investigation. They plan future experiments with better magnetic shielding, other vacuum tests and improved cavities with higher Q factors to increase thrust.

Eric W. Davis, a physicist at the Institute for Advanced Studies at Austin, noted "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."[52]

See also[edit]

References[edit]

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External links[edit]