EmDrive

Rocket engines operate by expelling propellant, which acts as a reaction mass and which produces thrust per Newton's third law of motion. All designs for electromagnetic propulsion operate on the principle of reaction mass. A hypothetical drive which did not expel propellant in order to produce a reaction force, providing thrust while being a closed system with no external interaction, would be a reactionless drive, violating the conservation of momentum and Newton's third law. Reactionless drives, like other forms of perpetual motion, do not exist in nature, and physicists regard claims that a drive is reactionless as pseudoscience. In 2016, Harold White's group at NASA's Advanced Propulsion Physics Laboratory reported in the Journal of Propulsion and Power that a test of their own model had observed a small thrust. In late 2016, Yue Chen of the communication satellite division of the China Academy of Space Technology (CAST), said his team had tested prototypes, and would conduct in-orbit tests to determine if they could observe thrust. Martin Tajmar's group at the Dresden University of Technology started testing prototypes in 2015, and by 2021 concluded that observations of thrust were false positives, reporting in the CEAS Space Journal they had refuted all EmDrive claims by "at least 3 orders of magnitude". Media coverage and responses Media coverage of experiments using these designs has been polarized. The EmDrive first drew attention, both credulous and dismissive, when New Scientist wrote about it as an "impossible" drive in 2006. following White's first tentative test reports in 2014. Scientists have continued to note the lack of unbiased coverage. In 2006, responding to the New Scientist piece, mathematical physicist John C. Baez at the University of California, Riverside, and Australian science-fiction writer Greg Egan, said the positive results reported by Shawyer were likely misinterpretations of experimental errors. Baez and Carroll criticized this explanation, because in the standard description of vacuum fluctuations, virtual particles do not behave as a plasma; Carroll also noted that the quantum vacuum has no "rest frame", providing nothing to push against, so it cannot be used for propulsion. In the same way, physicists James F. Woodward and Heidi Fearn published two papers showing that electron−positron virtual pairs of the quantum vacuum, discussed by White as a potential virtual plasma propellant, could not account for thrust in any isolated, closed electromagnetic system such as a quantum vacuum thruster. In 2015, physicists Eric W. Davis at the Institute for Advanced Studies in Austin and Sean M. Carroll at the California Institute of Technology concluded that the thrust measurements reported in papers by both Tajmar and White were indicative of thermal effect errors. In May 2018, researchers from the Institute of Aerospace Engineering at Technische Universität Dresden, Germany, concluded that the dominant effect underlying the apparent thrust could be clearly identified as an artifact caused by Earth's magnetic field interacting with power cables in the chamber, a result that other experts agree with. In March 2021, Tajmar's group published a definitive analysis of their own past experiments and those of others, showing that all could be explained by and reproduced via outside forces, refuting all EmDrive claims.When power flows into the EmDrive, the engine warms up. This also causes the fastening elements on the scale to warp, causing the scale to move to a new zero point. We were able to prevent that in an improved structure. Our measurements refute all EmDrive claims by at least 3 orders of magnitude. == Designs and prototypes ==

EmDrive In 2001, Shawyer founded Satellite Propulsion Research Ltd, to work on the EmDrive, which he said 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. In December 2002, he loosely described a prototype which he alleged had produced a thrust of powered by an 850 W cavity magnetron. He reported that the device could operate for only a few dozen seconds before the magnetron failed from overheating, however details were never published or replicated. Second device and New Scientist article In October 2006, Shawyer claimed to have conducted tests on a new water-cooled prototype with increased thrust. He reported plans to have the device ready to use in space by May 2009 and to make the resonant cavity a superconductor, featured the EmDrive on the cover of 8 September 2006 issue. The article portrayed the device as plausible and emphasized the arguments of those who held that point of view. Egan, a popular science fiction author, 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". Especially, Egan said he was "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 Baez and posted on his blog. New Scientist editor Jeremy Webb responded to critics: New Scientist also published a letter from the former technical director of EADS Astrium: A letter from physicist Paul Friedlander: Later work In 2007, the UK Department of Trade and Industry granted SPR an export license to Boeing in the US. According to Shawyer, in December 2008 he was invited to present on the EmDrive, and in 2009 Boeing expressed interest in it, at which point he stated that SPR built a thruster which produced 18 grams of thrust, and sent it to Boeing. Boeing did not license the technology and communication stopped. In 2012, a Boeing representative confirmed that Boeing Phantom Works used to explore exotic forms of space propulsion, including Shawyer's drive, but such work later ceased. They confirmed that "Phantom Works is not working with Mr. Shawyer", nor pursuing those explorations. While no functional prototype of the first-generation drive had yet been produced, and no detailed schematic of a new device was provided, it loosely described models for a superconducting resonant cavity and for thrusters with multiple cavities. In 2016, Shawyer filed further patents and launched a new company, Universal Propulsion Ltd., as a joint venture with Gilo Industries Group, a small UK aerospace company. In 2016, Fetta announced plans to eventually launch a CubeSat satellite containing a version of the Cannae Drive, which would run for 6 months to observe how it functions in space. No followup was published. In China, researchers working under Yang at NWPU built a resonant cavity thruster in 2008, and tested it for a number of years. A 2012 report claimed they had observed thrust, but in 2014 they found it to have been an experimental error. A second, improved prototype did not produce any measured thrust. and a design with a cavity that was a semicylinder instead of a frustum. That December, Chen announced that CAST would conduct tests on a resonant cavity thruster in orbit, == Theoretical inconsistencies ==

All proposed theories for how the EmDrive works violate the conservation of momentum, which states any interaction cannot have a net force (i.e., the net sum of all forces is zero); a consequence of the conservation of momentum is Newton's third law, where for every action there is an equal and opposite reaction. An often-cited example of apparent nonconservation of momentum is the Casimir effect, in the standard case where two parallel plates are attracted to each other. However the plates move in opposite directions, so no net momentum is extracted from the vacuum and, moreover, energy must be put into the system to take the plates apart again. has been postulated in an inhomogeneous vacuum, but this remains highly controversial as it will violate Lorentz invariance. Both Harold White's theories of how the EmDrive could work rely on these asymmetric or dynamical Casimir effects. However, if these vacuum forces are present, they are expected to be exceptionally tiny based on our current understanding, too small to explain the level of observed thrust. In the event that observed thrust is not due to experimental error, a positive result could indicate new physics. == Tests and experiments ==

Tests by inventors In 2004, Shawyer claimed to have received seven independent positive reviews from experts at BAE Systems, EADS Astrium, Siemens and the IEE. The technical director of EADS Astrium (Shawyer's former employer) denied this in the strongest terms, stating: In 2011, Fetta claimed to have tested a superconducting version of the Cannae drive, suspended inside a liquid-helium-filled dewar, with inconclusive results. None of these results were published in the scientific literature, replicated by independent researchers, or replicated consistently by the inventors. In a few cases details were posted for a time on the inventors' websites, but no such documents remained online as of 2019. In 2015, Shawyer published an article in Acta Astronautica, summarising seven existing tests on the EmDrive. Of these, four produced a measured force in the intended direction, three produced thrust in the opposite direction, and in one test thrust could be produced in either direction by varying the spring constants in the measuring apparatus. Northwestern Polytechnical University 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, China, said that they had developed a valid electro-magnetic theory behind a microwave resonant cavity thruster. 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, they reported a maximum thrust of 720 mN at 2,500 W of input power. The group investigated ideas for a wide range of untested and fringe proposals, including Alcubierre drives, drives that interact with the quantum vacuum, and RF resonant cavity thrusters. In 2014, the group began testing resonant cavity thrusters and in July, White reported tentative positive results for evaluating a tapered RF resonant cavity. This did not happen. They later conducted experiments in vacuum at 40-80W of input power, publishing the results in November 2016 in the Journal of Propulsion and Power, under the title "Measurement of Impulsive Thrust from a Closed Radio-Frequency Cavity in Vacuum". The study said their system was "consistently performing with a thrust-to-power ratio of 1.2±0.1mN/kW", but also enumerated many potential sources of error. Dresden University of Technology 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. Testing was performed first on a knife-edge beam balance able to detect force at the micronewton level, atop an antivibration 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 . They used a conventional ISM band 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 micronewtons, much larger than the expected result of zero thrust. 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 the device. In 2018, they published results from an improved test rig, which showed that their measured thrust had been a result of experimental error from insufficiently shielded components interacting with the Earth's magnetic field. In new experiments, they measured thrust values consistent with previous experiments and again measured thrust perpendicular to the expected direction when the thruster was rotated by 90°. Moreover, they did not measure a reduction in thrust when an attenuator was used to reduce the power that actually entered the resonant cavity by a factor of 10,000, which they said "clearly indicates that the "thrust" is not coming from the EMDrive but from some electromagnetic interaction." They concluded that "magnetic interaction from not sufficiently shielded cables or thrusters are a major factor that needs to be taken into account for proper μN thrust measurements for these type of devices," and they planned on conducting future tests at higher power and at different frequencies, and with improved shielding and cavity geometry. After 2017, no further updates were announced. In 2023, a new company, IVO Limited, claimed to be developing a similar drive, which they would test in space later that year on a cubesat, but in the end did not do so. == Experimental errors ==

The strongest early result, from Yang's group in China, was later reported to be caused by an experimental error. • Measurement error and noise. Most theoretical scientists who have looked at the EmDrive believe this to be the likely case. • Thermal effects. • Electromagnetic effects, including interaction with ambient magnetic fields and Lorentz forces from power leads. Other potential sources of error include confirmation bias and publication bias (discarding negative results). Measurement errors The simplest and most likely 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 goes into generating a tiny amount of thrust. When attempting to measure a small signal superimposed on a large signal, the noise from the large signal can obscure the small signal and give incorrect results. Shift in center of gravity due to thermal effects The largest error source is believed to come from the thermal expansion of the thruster's heat sink; as it expands this would lead to a change in the centre of gravity causing the resonant cavity to move. White's team attempted to model the thermal effect on the overall displacement by using a superposition of the displacements caused by "thermal effects" and "impulsive thrust" with White saying "That was the thing we worked the hardest to understand and put in a box". Despite these efforts, White's team were unable to fully account for the thermal expansion. In an interview with Aerospace America, White comments that "although maybe we put a little bit of a pencil mark through [thermal errors] ... they are certainly not black-Sharpie-crossed-out." Their method of accounting for thermal effects has been criticized by Millis and Davies, who highlight that there is a lack of both mathematical and empirical detail to justify the assumptions made about those effects. For example, they do not provide data on temperature measurement over time compared to device displacement. The paper includes a graphical chart, but it is based on a priori assumptions about what the shapes of the "impulsive thrust" and "thermal effects" should be, and how those signals will superimpose. The model further assumes all noise to be thermal and does not include other effects such as interaction with the chamber wall, power lead forces, and tilting. Because the Eagleworks paper has no explicit model for thrust to compare with the observations, it is ultimately subjective, and its data can be interpreted in more than one way. The Eagleworks test, therefore, does not conclusively show a thrust effect, but cannot rule it out either. White suggested future experiments could run on a Cavendish balance. In such a setup, the thruster could rotate out to much larger angular displacements, letting a thrust (if present) dominate any possible thermal effects. Testing a device in space would also eliminate the center-of-gravity issue. Tajmar's team later used such a setup to show that past results had all been artefacts of thermal effects. Electromagnetic interactions These experiments used relatively large electromagnetic inputs to generate small amounts of thrust. As a result, electromagnetic interactions between power leads, between power lines and ambient magnetic fields, or between the apparatus and walls of a test chamber, could all have significant effects. Yang reported in 2016 that an interaction with the Earth's magnetic field had caused the fairly large apparent thrust in their 2012 paper. Tajmar looked for potential Lorentz force interactions between power leads in trying to replicate White's experimental setup. Another source of error could have arisen from electromagnetic interaction with the walls of the vacuum chamber. White argued that any wall interaction could only be the result of a well-formed resonance coupling between the device and wall and that the high frequency used imply the chances of this would be highly dependent on the device's geometry. As components get warmer due to thermal expansion, the device's geometry changes, shifting the resonance of the cavity. In order to counter this effect and keep the system in optimal resonance conditions, White used a phase-locked loop system (PLL). Their analysis assumed that using a PLL ruled out significant electromagnetic interaction with the wall. == See also ==