The EmDrive: Theory, Engineering & Applications

September 27, 2026
|In Field Effect
|By Tim Ventura

Roger Shawyer’s EmDrive poses a question with enormous consequences for spaceflight: could a spacecraft accelerate using electricity without ejecting propellant? In his presentation to the Alternative Propulsion Engineering Conference (APEC), the British engineer connected that proposition to a sweeping vision of superconducting thrusters, personal air vehicles, lunar tourism, and reusable spaceplanes. Along the way, he laid out the proposed physics, demanding engineering requirements, and reported experiments behind his electromagnetic propulsion system. The resulting story reaches well beyond an unusual microwave cavity—it’s about the distance between a laboratory measurement and a technology that could change how people travel through space.

How Roger Shawyer’s EmDrive Is Supposed to Work

Shawyer began with a distinction he considers fundamental: the EmDrive is “not reactionless but propellantless.” Introduced as a chartered electrical engineer with 48 years of experience in the space and defense industries, he presented it as an electrical machine, not an exemption from the laws of physics. Its central component is a tapered, resonant microwave cavity. Electrical power supplies electromagnetic waves that travel back and forth inside the enclosure, where he proposes that their interaction with its surfaces produces directional thrust without a conventional exhaust stream.

The taper is essential to his explanation. Near the cavity’s larger end, Shawyer described the wave’s group velocity as approaching the speed of light; near its smaller end, he placed it below one-tenth of that speed. During the Q&A, he clarified that he wasn’t claiming to alter the speed of light itself. His argument concerns the guide or group velocity associated with the waveguide geometry—the propagation of energy and momentum within the cavity—and how that changes as the smaller end approaches cutoff conditions.

His starting point was the radiation-force expression F = 2P/c, where P represents electromagnetic power and c is the speed of light. Drawing on research by Professor Alex Cullen, Shawyer described a modification involving group velocity that changes the calculated force on an end plate inside a waveguide. He then introduced resonance and the cavity’s quality factor, Q, which characterizes its ability to retain electromagnetic energy. In his model, the combination of resonance and unequal end-plate forces produces the proposed thrust.

Shawyer rejected the familiar analogy of a passenger trying to move a car by pushing against its windshield. Instead, he described moving magnetic fields, induced currents, and an electrical-machine interaction. His explanation also invokes momentum accounting and relativistic velocity relationships to argue that the wave and cavity should be treated as an open system despite the physical enclosure. That’s the central theoretical claim: useful propulsion without violating conservation of momentum or energy. Establishing that connection experimentally, rather than simply asserting it, remains the crucial distinction between the proposed mechanism and a demonstrated propulsion system.

From Cold War Research to EmDrive Hardware

The idea’s origins reach back decades before the EmDrive became a subject of public controversy. Shawyer traced his interest to Professor Eric Laithwaite’s 1974 Royal Institution lecture on gyroscopes and unconventional propulsion. At the time, he was working for Sperry Gyroscope, which was involved in Britain’s Chevaline defense program. In January 1975, his team was asked to investigate unconventional solutions to a propulsion problem. While others pursued mechanical approaches, he returned to Cullen’s 1951 electromagnetic research and proposed a microwave-based alternative.

Another influence was Professor Roger Jennison, whose papers on electromagnetic momentum began appearing in 1976 and continued through 1989. Shawyer’s propulsion concept subsequently took a back seat to his regular defense and space-industry work before resurfacing in the 1990s alongside Project Greenglow. The main British experimental program followed between 2001 and 2010, supported by government funding and private investment. Shawyer credited Defence Science and Technology Laboratory monitor David Fernando with insisting that the team identify and calibrate out spurious forces that could affect its measurements.

The reported performance increased substantially during that period. A 2003 experimental thruster had a Q of approximately 6,000 and produced about 19 millinewtons per kilowatt, according to Shawyer. By 2006, a demonstrator engine had increased Q by roughly an order of magnitude and reached an average of 243 millinewtons per kilowatt across numerous experimental runs. Both systems used magnetron microwave sources. Later designs moved toward narrower-band signal generators and amplifiers, bringing more demanding requirements for manufacturing accuracy and alignment.

Development then divided between a flight-oriented thruster and an experimental superconducting device. Shawyer said the flight-thruster work was completed in 2010, with a Boeing contract concluding that July. He also described a technology transfer conducted under a Technical Assistance Agreement involving the U.S. State Department and British Ministry of Defence. These milestones form part of the development history he presented to APEC; a flight-oriented design and a completed contract aren’t, by themselves, evidence of a successful propulsion demonstration in orbit.

Inside the EmDrive: Geometry, Resonance, and Microwave Control

Much of Shawyer’s presentation concerned details that disappear in simplified illustrations of a metal cone connected to a microwave source. His design process begins by solving guide-wavelength and wave-impedance equations along the cavity’s axis, using spreadsheet calculations with increments as small as 0.1 millimeter. One example involved a CubeSat cavity operating in the TE113 microwave mode. Near its smaller end, at an axial position around 140 millimeters, the calculations showed wavelength increasing sharply while impedance approached zero.

Shape was just as important as size. Shawyer argued that spherical end surfaces maximize the desired end-plate forces while suppressing opposing sidewall effects. Flat end plates, he maintained, introduce differences between axial and slant-path lengths that accumulate during repeated wave transits. The resulting phase errors would broaden the resonance and reduce Q. For his flight-thruster example, he reported that achieving a Q around 55,000 required keeping the mean path-length error down to approximately 0.1 millimeter—a demanding tolerance for an apparently simple enclosure.

Getting power into that enclosure creates another set of challenges. Shawyer placed the input loop near a magnetic-field maximum and the detector probe near an electric-field maximum. The coupling arrangement must match the cavity’s impedance at the intended operating frequency, but that match changes as the hardware heats. Because the input assembly and cavity respond differently to temperature, he advocated digitally controlled tuning using detector feedback and reflected-power measurements. Adjusting the input frequency would help maintain the match as operating conditions changed.

One of his strongest warnings concerned single-port measurements. An apparently well-tuned input, he argued, can conceal poor coupling into the main cavity: the measurement may characterize the input loop rather than the resonator it’s supposed to energize. His preferred approach therefore includes a separate detector in a two-port measurement system. Temperature measurements provide an additional diagnostic. In his account, rapid heating around the input connector while the large end plate remains comparatively cool suggests that energy is being dissipated in the feed assembly rather than delivered where the design requires it.

EmDrive Thrust Measurements and the Replication Dispute

The demonstrator program included a ground-based dynamic test intended to complement static force measurements. Shawyer mounted the engine on an air-bearing rig with a total rotating mass of approximately 100 kilograms. He reported acceleration from the cavity’s large end toward its small end and said the measured motion corresponded to the thrust obtained during static testing. For him, the agreement was significant: two different measurement approaches appeared to connect a reported force with acceleration of a substantial mechanical assembly.

He also highlighted research at Northwestern Polytechnical University in Xi’an, China. According to his presentation, the group disclosed a measurement of 720 millinewtons with 2.5 kilowatts of input power in April 2010 and published a peer-reviewed paper in 2012. He identified it as the highest published thrust result available to him at the time. The Chinese cavity also illustrated an alternative engineering approach, using a waveguide-slot input rather than the loop arrangement employed in his demonstrator.

Mechanical loading was another important—and unconventional—part of his testing argument. Numerical examples involving preload masses from approximately 2 to 500 micrograms showed how, in his model, the measurement apparatus could affect the buildup of stored cavity energy. An extremely lightly loaded system might produce only a brief force pulse rather than a sustained reading. His conclusion was that greater balance sensitivity doesn’t automatically produce a better EmDrive test. The response of the entire apparatus, including its mechanical loading, has to be included in the analysis.

Those design requirements became central to his response to null results associated with Martin Tajmar’s testing of a NASA-style cavity. Shawyer criticized its flat end plates and single-port measurement approach, arguing that a reported Q around 23,000 characterized the input arrangement rather than the traveling-wave behavior his model requires. He also questioned a reported 45-decibel return loss. That was his explanation for why the apparatus wouldn’t demonstrate the proposed effect. The APEC exchange didn’t include a response from the researchers he criticized or an experimental resolution of the disagreement.

Superconducting EmDrive: Three Generations of Propulsion

Shawyer organized his application studies into three generations, each with different performance assumptions. First-generation devices use uncooled cavities, with representative Q values around 50,000 and thrust-to-power ratios near 0.3 newton per kilowatt. Second-generation concepts introduce superconducting surfaces and liquid-hydrogen cooling. For those studies, he quoted approximately 470 newtons per kilowatt at accelerations around 0.05 meter per second squared. These advanced figures belong to his superconducting application concepts, distinct from the earlier laboratory measurements of 19 and 243 millinewtons per kilowatt.

The difficulty, in his model, is that higher Q makes the system more sensitive to acceleration-related frequency changes. Shawyer described a Doppler shift in the circulating waves that reduces stored energy as the cavity accelerates, lowering the available thrust unless the effect is compensated. He compared an accelerating cavity to an electric motor converting electromagnetic energy into motion. A decelerating cavity, he proposed, could behave like a generator. This motor-generator interpretation formed part of his theoretical explanation rather than a separately documented energy-recovery demonstration during the session.

Third-generation designs would compensate by adjusting the cavity’s axial length with piezoelectric elements. Circular polarization would help distinguish incident and reflected waves, supporting control of frequency and geometry. Because the cavity can’t keep extending indefinitely, the arrangement alternates between two cavities: one supplies thrust while the other resets. For these compensated concepts, Shawyer quoted thrust-to-power ratios approaching 4 kilonewtons per kilowatt and accelerations up to approximately 1g. That projected combination of high thrust and greater acceleration underpins his most ambitious vehicles.

The materials discussion was more grounded in fabrication challenges. Shawyer addressed yttrium barium copper oxide superconducting films, sapphire substrates, and the difficulty of coating curved surfaces. He said the superconducting film must exceed the relevant electromagnetic penetration depth, while the substrate must withstand the mechanical forces involved. He also discussed partially superconducting layouts and an early assembly built from flat elements, although its full geometry wasn’t explained in the Q&A. The attraction is reduced surface resistance; the challenge is incorporating that advantage into a precisely shaped, mechanically robust cavity.

EmDrive Applications: CubeSats, Air Taxis, and Lunar Travel

The mission studies began with relatively modest spacecraft. Shawyer described a 12U CubeSat using approximately 50 millinewtons of thrust over an operating period around five years, illustrating possible exploration missions toward Mars or Pluto. For geostationary communications satellites, he proposed replacing primary and secondary propulsion with EmDrive systems. His attraction was eliminating the substantial mass allocated to propellant—approximately half the satellite’s mass in his example—and extending operational life beyond the limit imposed by that supply.

Second-generation applications ranged from a proposed 60-kilogram, battery-powered unmanned aircraft demonstrator to a 500,000-tonne supertanker. The smaller vehicle would provide a direct test of whether the superconducting concept could support flight. The tanker represented a very different problem, involving sustained operation and a closed-loop cooling system. Shawyer used the contrast to demonstrate the breadth of his proposed applications: the same underlying approach, scaled and configured differently, might serve both small experimental vehicles and large transportation systems.

His third-generation personal air vehicle used eight cavities arranged as four paired thrust units, giving it a control layout analogous to a four-rotor drone. The concept approached a one-tonne takeoff mass and carried two people. Unlike a rotorcraft, it wouldn’t depend on propellers or downward airflow for lift. In Shawyer’s vision, that distinction would allow an air taxi to operate close to buildings without the airflow restrictions he associated with conventional rotor-driven vehicles. The advantage was part of the proposed design, not a demonstrated urban-flight capability.

The personal space vehicle took the same vision beyond Earth. Shawyer outlined a concept intended to carry three people to the Moon and back in a single vehicle, with a launch mass slightly above 10 tonnes. He specified an acceleration around 0.01g and quoted approximately 72 hours of flight time for the study. The gentle acceleration was deliberate: the intended market was space tourism, and the design prioritized a mild flight profile rather than the intense launch experience associated with the conventional mission architecture he sought to replace.

A Reusable Spaceplane for Space-Based Solar Power

For heavy payloads, Shawyer outlined an unmanned spaceplane intended to deliver 50 tonnes to geostationary orbit. The concept had a launch mass of approximately 116 tonnes, including 15 tonnes of liquid hydrogen, and a wingspan approaching 20 meters. Its design target was 500 missions. Payloads would be carried beneath the wing, while the vehicle would combine vertical takeoff and landing with some gliding capability. Control in pitch, roll, yaw, and all three translational directions would support docking and payload deployment as well as transportation.

The proposed flight profile was just as unconventional. Shawyer quoted an acceleration around 0.014g, approximately six hours of continuous thrusting to reach geostationary orbit, and an atmospheric speed limit near 70 miles per hour. Those choices were intended to reduce mechanical, aerodynamic, and thermal loads on the airframe. Recalling an earlier description attributed to U.S. Air Force Colonel Coyote Smith, he characterized the ambition as a “space elevator without cables”—a reusable transportation system built around a slow, controlled ascent rather than a brief, high-intensity launch.

Space-based solar power supplied the economic motivation. One example required approximately 6,700 tonnes of hardware in geostationary orbit. Shawyer’s comparison used roughly 1,700 Atlas V launches at a combined transportation cost of $194 billion, or approximately $29,000 per kilogram. He cited a study requirement to reduce transportation spending to about $20 billion to make the project viable. These were the historical assumptions in his presentation, not a comparison with present-day launch-service pricing.

Against that baseline, he projected transportation spending of approximately $74 million, or about $11 per kilogram. His operating scenario called for 134 launches, a two-day turnaround, and construction of a solar-power satellite in less than a year. The same reusable vehicle would perform docking, maneuvering, and deployment work. It was an extraordinary cost projection, but one dependent on an entire proposed operating system: high-performance superconducting thrust, reliable cryogenic hardware, a reusable heavy vehicle, and scheduling closer to an airline than a conventional launch campaign.

Power, Cooling, and the Orbital Energy Question

“Propellantless” doesn’t mean independent of an energy supply. During the Q&A, Shawyer explained why liquid hydrogen featured so prominently in his advanced concepts. He proposed using it at approximately 20 kelvin to cool the superconducting material, then routing boiled-off hydrogen through a fuel-cell system to generate electrical power. Hydrogen therefore served two connected roles: cooling the cavity and supplying the energy system. The resulting vehicle would have to integrate refrigeration, power generation, and propulsion rather than treating the thruster as an isolated component.

Shawyer also emphasized the need to maintain stored electromagnetic energy while producing motion. He suggested an illustrative operating balance of approximately 50 percent conversion into kinetic energy, rather than claiming that all the cavity’s stored energy could become useful movement. Elsewhere in the presentation, he described pulsed operation when inertial loading dominates, with some first-generation examples using thrusting periods measured in seconds and interruptions measured in microseconds. Both ideas belonged to his proposed energy-conservation model, explaining how the device would replenish its field while accelerating a vehicle.

An audience member identified as Benedict pressed the question of whether the proposed spacecraft could carry enough energy to reach orbit. His objection concerned the vehicle’s energy requirement, not merely its propellant supply. He pointed to a velocity of roughly eight kilometers per second and noted that gravitational potential energy would add to the total. He also challenged the suggestion that a slower ascent settles the issue. Reducing airframe loads and demonstrating a sufficient orbital energy budget were, in his argument, separate questions.

Shawyer replied that the calculation should begin with Newtonian energy requirements rather than the conventional rocket equation, and he directed the audience to his published studies for the hydrogen-energy allocations. Benedict maintained that his objection already concerned kinetic energy rather than a rocket mass-ratio calculation. The exchange ended without a complete numerical reconciliation. For the spacecraft concepts, it identified a task beyond measuring thrust: demonstrating that the entire vehicle can supply the mission’s energy while accounting for conversion losses, cooling requirements, and the mass of all the equipment it must carry.

From EmDrive Mission Studies to a Demonstrated Vehicle

Shawyer’s vision extended beyond transportation to infrastructure between Earth and the Sun. He proposed moving large sunshades toward the Sun–Earth L1 region and using EmDrive attitude control to adjust their orientation and shading effect. Another concept combined shading with power generation, using laser transmission toward geostationary orbit and microwave transmission from there to Earth. Controllability was central to his argument. He acknowledged uncertainty about the consequences of shading Earth and said any deployment would need to proceed gradually, with the ability to adjust its effects.

He was also explicit about the limits of his knowledge when discussing reports of hardware already operating in space. The presentation referenced claims involving Russian satellites, the American X-37B, and China’s Tiangong-2 space station. But Shawyer stopped short of confirming them: “Whether these reports of in-orbit tests are completely accurate or not, I don’t know.” That qualification matters. Press coverage and suggestions of restricted information don’t establish that a particular spacecraft demonstrated the proposed propulsion effect.

Asked about practical next steps, Shawyer described a sequence beginning with first-generation thrusters on spacecraft, followed by small flying vehicles using second- and third-generation systems. Such demonstrations would bring together the issues explored throughout the presentation: microwave coupling, thermal management, electrical power, mechanical control, and sustained operation. He also emphasized isolation and field monitoring around high-power microwave equipment. Even in a propulsion concept as unconventional as the EmDrive, those practical engineering requirements don’t disappear.

The strength of Shawyer’s APEC presentation lies in the specificity of the case he put forward: a proposed mechanism, a development history, reported measurements, detailed design requirements, and a progression of mission studies. Its central tension lies in connecting them. A force measured in a laboratory, an explanation for that force, and a vehicle capable of reaching the Moon are different achievements. For the EmDrive, the decisive advance would be a clearly specified device producing repeatable propulsion, followed by a vehicle whose measured performance matches its engineering and energy budgets. That’s the bridge between an ambitious microwave experiment and a new way to travel through space.

References

APEC Presentation and Q&A

Related Technical Papers and Reports by Roger Shawyer

Independent Experimental Research