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 in which electromagnetic radiation is confined to a microwave cavity, and provides thrust to the cavity in a particular direction as the radiation reflects within the cavity.

Such a thruster would be a type of reactionless drive, providing thrust from electricity without consuming a propellant. This would apparently violate Newton's Third Law and the conservation of momentum,[1] leading most physicists and propulsion experts who look at such devices to dismiss them as impossible.[2][3] Despite this, inventors try to discover such drives, because if they existed they could support long voyages in space, where propellant is a primary limiting factor.[4]

RF resonant cavity thrusters in particular have been promoted by two inventors who say they have designed such devices: Roger Shawyer designed a thruster he called the EM Drive, and Guido Fetta designed a thruster he called the Cannae Drive.[5][6] Neither has demonstrated thrust from their device in a public test.[citation needed]

A few groups of physicists have tried to build and test their own resonant cavity thrusters, based on the designs published by Shawyer and Fetta. Juan Yang at Xi'an's Northwestern Polytechnical University (NWPU) built and tested three thrusters from 2008 to 2014. Initial observations of thrust[7] were later retracted after identifying a measurement error[8] with an improved setup they did not measure any significant thrust.[8][9] Sonny White's group at NASA's Eagleworks laboratories, which tests unusual rocket designs, has built and tested versions of both the EM drive and the Cannae drive. In 2014 they reported observing a small thrust,[10] close to their margin of error, but at the same time observed thrust from a control device.

This concept for a thruster drew attention in the 2000s when a few popular science magazines wrote articles about it as an "impossible" drive.[11][12] There has been particular criticism of misleading claims in the US media[13] that the drive had been "validated" by NASA.[12]

History and context[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.[14]

A low-propellant space drive has long been a goal of space exploration, since the propellant is dead weight that must be lifted and accelerated with the ship all the way from launch until the moment it is used (see Tsiolkovsky rocket equation). Solar sails, gravity assists, and propulsion by beams from the ground are useful specifically because they allow a ship to gain speed without propellant. However, these techniques do not work in deep space. Shining a light out of the ship provides a small force from radiation pressure, i.e. using photons as a kind of propellant, but the force is far too weak (for a given amount of electrical power) to be useful in practice.

A true zero-propellant drive is widely believed to be impossible, but if it existed, it could potentially be used for travel in many environments including deep space. As a result such drives are a popular concept in science fiction, and their very improbability contributes to enthusiasm for exploring such designs.[4][5][6]


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 it is an example of a reactionless drive, in violation of conservation of momentum.[6][15]

The concept has been criticised by physicists including Eric W. Davis, at the Institute for Advanced Studies in Austin and Sean Carroll at the California Institute of Technology,[16] who have said that the thrust measured in the original Eagleworks publication is indicative of thermal effects. John C. Baez, a mathematical physicist at the University of California and Australian science fiction writer Greg Egan have also said that positive results are misinterpretations of experimental errors.[17] Since those criticisms were written, the significant positive results, primarily the work at Northwestern Polytechnic University, have indeed been identified as experimental errors.

The initial results claimed by Shawyer in selling the idea to potential investors, have never been replicated. Even the most optimistic replication yielded far less thrust than he initially claimed. Moreover, Shawyer repeatedly claimed that his successive models were getting better, and would soon be ready to use in space, none of which turned out to be the case. After a brief burst of publicity for these claims, the hyperbole contributed to general controversy around the possibility that such thrusters could work at all.

To the extent that there are theories that could explain thrust from such devices, the predicted thrusts are tiny for significant input energy, and the experiments designed so far have had many sources of noise. So even when respected labs have tried to experiment with such a thruster, the results have been difficult to evaluate or replicate.

Designs and prototypes[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.[6] 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 operate for only a few dozen seconds before the magnetron failed, due to overheating.[18]

Second device and New Scientist article[edit]

In October 2006, Shawyer conducted tests on a new water-cooled prototype and claimed increased thrust.[1] He planned to have the device ready to use in space by May 2009 and was considering making the resonant cavity a superconductor.[1]

New Scientist magazine[19] 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.[17][20]

Egan also recommended[17] that New Scientist publish a refutation penned by John P. Costella (a data scientist with a PhD in theoretical physics).[19][21] 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."[11]

New Scientist 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",[22] and a letter from physicist 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."[23]

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.[24] 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),[25] 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.[25][26][27][28][29] 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.[30] In 2016, Fetta announced plans to eventually launch a cubesat satellite containing a version of the Cannae Drive, which they would run for 6 months to observe how it functions in space.[31]

Researchers working under Juan Yang (杨涓) at the Northwestern Polytechnical University (NWPU) in Xi'an developed their own prototype resonant cavity thruster 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, however in 2014 they found that this had been an experimental error introduced by a power cable. In a revised study with an improved model, they found that any actual thrust was too small for their setup to measure (less than 1mN for a 230W power source).[5][32][33]

Replication efforts[edit]

Three science labs have tried to build and test at least one such thruster: at NWPU, at the Dresden University of Technology, and at NASA's Eagleworks research group. None has been able to repeatedly measure thrust from a thruster, and all observations were dominated by noise and experimental design problems.

Mechanism and hypotheses[edit]

There are no known physical mechanisms for such thrusters to work. They have been studied because two inventors claimed to have working models, however those models have not worked in public and those inventors have also no proposed physical mechanisms for their function. Various attempts have been made to explain theoretically why some experiments might conceivably detect thrust.

Device structure[edit]

All these devices 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 require an electrical power source to run the magnetron, but no other propellant.


Noise and experimental error[edit]

The simplest explanation is that any thrust detected is due to experimental error or noise. In all of the experiments set up, a very large amount of energy is going into generating a tiny amount of thrust. Even the smallest stray signal – for instance from thermal or magnetic effects – could produce what looks like a thrust of that size. These experiments require more shielding from their environment than any of the experiments as of 2015 were able to provide, as each of those experimenters has noted. The strongest early result, from Yang's group in Xian, was later reported to be caused by a large experimental error.[8]

Measurement error[edit]

A similar explanation is that imprecisions in measurement, variation in measurement, or publication bias, have led to occasional positive observations with no statistical significance.[16] For instance, those who have tried running such experiments have only written up in detail the experimental runs where they believe to have at least some positive result. This is often a factor with experiments that have large variation, small studies and sample sizes, and few replications.[34]

Radiation pressure[edit]

Shawyer claims that the drive is not reactionless, and instead 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.[32] He has self-published non-peer-reviewed papers describing how he believes the drive works: "[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[35] who point out that the standard theory of radiation pressure is more complicated than this simplified analysis suggests.

Virtual plasma[edit]

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.[36] Likewise, the paper describing the Eagleworks tests refer to a possible interaction with a so-called "quantum vacuum virtual plasma".[37]

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.[20][38][39]

Quantized inertia[edit]

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).[40] 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.[41][42][43][44][45] While this model allows the device to create thrust without breaking Newton's third law, it assumes that Unruh radiation is real, and requires the 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.[46] Unlike other hypotheses used to explain the device, McCulloch's hypothesis is testable and McCulloch has suggested building a cavity where the length of the cavity is the same as the diameter of the small end, to cause the Unruh radiation to fit better in the small end, resulting in a reversal of thrust.[44]

True reactionless drive[edit]

This would be the most unlikely result of all. These are considered impossible for many reasons; most reactionless drives are a form of perpetual motion machine. If thrust were produced without expelling momentum from the system in the opposite direction, the lack of momentum expulsion would make the device not only propellant-free, but a "reactionless" drive in the sense that it violates Newton's Third Law and the conservation of momentum. Any such drive would require a new undiscovered physical law, which somehow had not been observed under any other conditions. The experimenters listed above do not believe the drives are reactionless, and are trying to test one of the previous hypotheses.

Testing and replication claims[edit]

Tests by the inventors[edit]

In 2004, Roger Shawyer reported seven independent positive reviews from experts at BAE Systems, EADS Astrium, Siemens and the IEE,[47] however these are disputed. 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."[22]

In 2011, Fetta tested a superconducting version of the Cannae drive. 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 these results have been published in the scientific literature, or replicated by independent researchers. They have been posted on their inventors' websites.[48]

In 2015, Shawyer published an article in Acta Astronautica, summarising 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 test, thrust could be produced in either direction by varying the spring constants in the measuring apparatus.[49]

Northwestern Polytechnical University[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, Shaanxi, China, claimed to have developed a valid electro-magnetic theory behind a microwave resonant cavity thruster.[7][50] 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,[6][32][33] they reported a maximum thrust of 720 mN at 2,500 W of input power.[33] Yang noted that her results were tentative, and said she "[was] not able to discuss her work until more results are published".[6] This positive result was over 100x more thrust per input power than any other experiment, and inspired other groups to try to replicate their work. However in a followup paper, Yang could not reproduce the 2010 observation and suggested it was due to experimental error.[8]

In 2016, Yang's team published a paper, in which they refined their experimental setup, using a three-wire torsion pendulum to measure thrust. They tested two different power setups. In one trial, the power system was outside the cavity: in this case, they observed a "thrust" of 8-10 mN. In a second trial, the power system was within the cavity, and they measured no such thrust. Instead they observed an insignificant thrust below their noise threshold of 3 mN, fluctuating between ±0.7mN with a measurement uncertainty of 80%. They concluded that they were unable to measure significant thrust; that "thrust" measured when using external power sources (such as in their 2010 experiment) could be noise; and that it was important to use self-contained power systems for these experiments.[8]

NASA/JSC Advanced Propulsion Physics Laboratory[edit]

White's team at the NASA/JSC Advanced Propulsion Physics Laboratory, also known as "Eagleworks", is devoted to studying advanced propulsion systems that they hope to develop using quantum vacuum and spacetime engineering.[51] The group has investigated a wide range of untested and fringe proposals, including RF resonant cavity thrusters and related concepts. In September 2016, White's team announced that a paper describing their work was reviewed and accepted for publication by the Journal of Propulsion and Power, for publication in December 2016.[52]


In 2011, the group built an RF resonant cavity thruster prototype for testing. In July 2014, the group reported tentative positive results for evaluating an RF resonant tapered cavity similar to the EmDrive.[37] 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).[37] 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), 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.[37] The experiment was criticized for among other things not having been conducted in a vacuum, to eliminate thermal air currents.

In 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.[53] 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, measurements are still scarce and need improvement before a new report can be published.[54]

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.[53]

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.[37]

Cannae drive[edit]

The same NASA test campaign evaluated a Cannae drive.[37] 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;[26] and a "null" test article lacking those radial slots. Both drives were equipped with an internal dielectric.[37] 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.[20][55][56] In the complete paper, however, Eagleworks concluded that the test results proved that "thrust production was not dependent upon slotting".[37]

Dresden University of Technology[edit]

In July 2015 an aerospace research group at the Dresden University of Technology (TUD) under Martin Tajmar reported results for an evaluation of an RF resonant tapered cavity similar to the EmDrive.[57] 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 0.1 mN, inside a vacuum chamber at ambient air pressure and in a hard vacuum at 4×10−6 mbar (3×10−6 torr).

They used a conventional 2.45 GHz 700 W oven magnetron, and a small cavity with a low Q factor (20 in vacuum tests). They observed small positive thrusts in the positive direction and negative thrusts in the negative direction, of about 20 µN in a hard vacuum. However when they rotated the cavity upwards as a "null" configuration, they observed an anomalous thrust of hundreds of µN, significantly larger than the observed results. This indicated a strong source of noise which they could not identify. This led them to conclude that they could not confirm or refute claims about such a thruster. 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."[16]

See also[edit]


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