The enigmatic EmDrive (or Cannae Drive) – an impossible propulsion engine that scientists still cannot explain

// April 26th, 2016 // News


imageThe enigmatic propulsion system known as EmDrive was designed in 2001 by Roger Shawyer and promoted through his company, Satellite Propulsion Research. A short time after its development, chemical engineer Guido Fetta designed a similar mechanism he called the Cannae Drive (aka Q-Drive, Q-Thruster). Both systems were considered scientifically implausible and disregarded by scientists who noted that the propulsion system violated one of physics most powerful laws – the law of conservation of momentum, which states the total momentum of two objects before and after collision must remain constant. In other words, it’s impossible to produce thrust without consuming any sort of propellant. The problem is, the EmDrive really works – and in 2015, to their surprise, NASA was inexplicably able to duplicate Shawyer’s findings leaving scientists scratching their heads and unable to explain the impossible “engine” Shawyer has discovered.

Shawyer develops the EmDrive

A low-propellant space drive has long been the goal of space explorers. If a zero-propellant drive such as EmDrive could exist (thus allowing exceptional travel distances), space travel would be revolutionized. And that’s what Shawyer set out to create – a drive that could produce thrust using only electrical current and no propellant. His EmDrive design uses a resonant cavity and a magnetron, a high-powered vacuum tube that generates microwaves using electrons and a magnetic field. The microwaves produced by the magnetron are directed into a metallic, fully enclosed conically tapered resonant cavity which is wider on one end and contains a dielectric resonator on the narrower end.  Other factors potentially impacting the EmDrive’s impossible design include a mysterious interference pattern created by the shape of the object and the microwaves bouncing around inside and the operation frequency of the device which produces a strange resonance within the object’s cavity. Both are unusual interactions and not fully understood.

Shawyer’s first EmDrive design consistently burnt out after about 20 seconds when the magnetron overheated and melted. As a result, in 2006 he created a water-cooled prototype which not only kept the engine from burning out, but to his surprise, also increased the thrust it produced.

On September 8, 2006, New Scientists magazine ran a cover article touting the device which generated a feverish frenzy of critique within the scientific community. Notable physicists lambasted the article explaining that it violated the most basic law of physics and thus, was impossible.  They countered that something must be inherently wrong with the Shawyer’s measurements.

Scientists duplicate Shawyer’s impossible findings

Two years later, researchers at Northwestern Polytechnical University in Xian set out to create a prototype of the EmDrive and disprove Shawyer’s findings. In 2012, they reported that the device worked – a series of tests did indeed produce a measurable thrust but they could offer no explanation as to how it did so.

Taking note of the NWPU’s findings, NASA set out to build their own EmDrive. By 2015, the NASA Eagleworks research group at Johnson Space Center tested models of both the EmDrive and the Cannae drive and reported observing a net thrust from both devices. Skeptics quickly denounced NASA’s findings claiming the experiment should have been conducted in a vacuum. NASA redid their experiment six months later, this time in a vacuum, and produced the same results. For reasons unknown to modern-day scientists, the EmDrive does the impossible.

The EmDrive today

To date the scientific community is still working to explain how the EmDrive works. Once they do, it is expected that larger scale devices will be constructed and space travel as we know it, will be changed forever.


Additional information

Principle of Operation

At first sight the idea of propulsion without propellant seems impossible. However the technology is firmly anchored in the basic laws of physics and following an extensive review process, no transgressions of these laws have been identified.

The principle of operation is based on the well-known phenomenon of radiation pressure. This relies on Newton’s Second Law where force is defined as the rate of change of momentum. Thus an electromagnetic (EM) wave, travelling at the speed of light has a certain momentum which it will transfer to a reflector, resulting in a tiny force.

If the same EM wave is travelling at a fraction of the speed of light, the rate of change of momentum, and hence force, is reduced by that fraction. The propagation velocity of an EM wave, and the resulting force it exerts, can be varied depending on the geometry of a waveguide within which it travels. This was demonstrated by work carried out in the 1950’s. (CULLEN, A.L. ‘Absolute Power Measurements at Microwave Frequencies’ IEE Proceedings Vol 99 Part 1V 1952 P.100)

Thus if the EM wave travelling in a tapered waveguide is bounced between two reflectors, with a large velocity difference at the reflector surfaces, the force difference will give a resultant thrust to the waveguide linking the two reflectors. If the reflectors are separated by a multiple of half the effective wavelength of the EM wave, this thrust will be multiplied by the Q of the resulting resonant cavity, as illustrated in fig 1.

Fig 1. Diagram of an engine concept.

The inevitable objection raised, is that the apparently closed system produced by this arrangement cannot result in an output force, but will merely produce strain within the waveguide walls. However, this ignores Einstein’s Special Law of Relativity in which separate frames of reference have to be applied at velocities approaching the speed of light. Thus the system of EM wave and waveguide can be regarded as an open system, with the EM wave and the waveguide having separate frames of reference.

A similar approach is necessary to explain the principle of the laser gyroscope, where open system attitude information is obtained from an apparently closed system device.

Leaked NASA document

The following NASA document leaked in November 2016 and confirms what everyone already knew – NASA proved the EmDrive works.  The document contains a technical description of the engine’s operation.

Measurement of impulsive thrust from a closed radio frequency cavity in vacuum (pdf)

Selected EmDrive FAQ (courtesy SPR)



Q. Is the thrust produced by the EmDrive a reactionless force?
A. No, the thrust is the result of the reaction between the end plates of the waveguide and the Electromagnetic wave propagated within it.

Q. How can a net force be produced by a closed waveguide?
A. At the propagation velocities (greater than one tenth the speed of light) the effects of special relativity must be considered. Different reference planes have to be used for the EM wave and the waveguide itself. The thruster is therefore an open system and a net force can be produced.

Q. Why does the net force not get balanced out by the axial component of the sidewall force?
A. The net force is not balanced out by the axial component of the sidewall force because there is a highly non linear relationship between waveguide diameter and group velocity. (e.g. at cut off diameter, the group velocity is zero, the guide wavelength is infinity, but the diameter is clearly not zero.) The design of the cavity is such that the ratio of end wall forces is maximised, whilst the axial component of the sidewall force is reduced to a negligible value.

Q. Does the theory of the EmDrive contravene the accepted laws of physics or electromagnetic theory?
A. The EmDrive does not violate any known law of physics. The basic laws that are applied in the theory of the EmDrive operation are as follows:

Newton’s laws are applied in the derivation of the basic static thrust equation (Equation 11 in the theory paper) and have also been demonstrated to apply to the EmDrive experimentally.
The law of conservation of momentum is the basis of Newtons laws and therefore applies to the EmDrive. It is satisfied both theoretically and experimentally.
The law of conservation of energy is the basis of the dynamic thrust equation which applies to the EmDrive under acceleration,(see Equation 16 in the theory paper).
The principles of electromagnetic theory are used to derive the basic design equations.

Q. Why does the EmDrive not contravene the conservation of momentum when it operates in free space?
A. The EmDrive cannot violate the conservation of momentum. The electromagnetic wave momentum is built up in the resonating cavity, and is transferred to the end walls upon reflection. The momentum gained by the EmDrive plus the momentum lost by the electromagnetic wave equals zero. The direction and acceleration that is measured, when the EmDrive is tested on a dynamic test rig, comply with Newtons laws and confirm that the law of conservation of momentum is satisfied.

Q. Is the EmDrive a form of perpetual motion machine?
A. The EmDrive obeys the law of conservation of energy and is therefore not a perpetual motion machine. Energy must be expended to accelerate the EmDrive (see Equation 16 of the theory paper). Once the EmDrive is switched off, Newton’s laws ensure that motion is constant unless it is acted upon by another force.

Q. Why does the thrust decrease as the spacecraft velocity along the thrust vector increases?
A. As the spacecraft accelerates along the thrust vector, energy is lost by the engine and gained as additional kinetic energy by the spacecraft. This energy can be defined as the thrust multiplied by the distance through which the thrust acts. For a given acceleration period, the higher the mean velocity, the longer the distance travelled, hence the higher the energy lost by the engine.
This loss of stored energy from the resonant cavity leads to a reduction in Q and hence a reduction of thrust.

Q. Has buoyancy been allowed for?
A. Buoyancy has been allowed for in the initial experiments and then eliminated by hermetically sealing the thruster.

Q. Are there any convection currents which might affect the results?
A. Convection currents did not affect the results, as measurements were taken with the thrust vector up, down and horizontal. Test runs were also carried out using a thermal simulation heater to quantify the effects of change of coolant temperature.

Q. Has stiffness in cables and pipes been allowed for?
A. The only connections to the balance were high flex electrical links

Q. Has friction in any pivots been allowed for?
A. Static thrust measurements were carried out using 3 different techniques – a counterbalance rig with a knife edge pivot, a direct weighing method using a 16kg balance (0.1 gm resolution), and with the thruster suspended from a spring balance with the weight partly offloaded on to an electronic balance.

Q. Have electromagnetic effects been taken into account? These include interactions between current-carrying conductors and between such conductors carrying RF currents and nearby metallic structures in which currents might be induced.
A. Stray electromagnetic effects were eliminated by using different test rigs, by testing two thrusters with very different mounting structures, and by changing the orientation by 90 degrees to eliminate the Earth’s magnetic field.

Q. Is there any ionization within the air, which might cause electrostatic charging and resulting forces?
A. Electrostatic charges were eliminated by the comprehensive earthing required for safety reasons, and to provide the return path for the magnetron anode current.

Q. Could RF pick-up measurement circuits have produced erroneous results?
A. EMC tests were carried out on the instrumentation to eliminate the effects of RF pick up.

Q. Could acceleration be caused by spurious torques generated by the air bearing?
A. Dynamic tests are preceded by an acceleration calibration test, using standard weights to determine the air bearing friction.

Q. Could acceleration be caused by anomalous thermal or electromagnetic effects?
A. Acceleration and deceleration tests have been carried out in both clockwise and anti-clockwise directions Acceleration from rest only starts when the magnetron output frequency matches the resonant frequency of the engine, following an initial warm-up period.

Sources: EmDrive, Wikipedia, New Scientists, CNN, Satellite Propulsion Research (SPR)

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