A force for space with no reaction

Tom Shelley reports on an extraordinary concept, which if it turns out to be as good as it seems, could change a large part of engineering

A reactionless force motor, designed for space use, has the potential to drive objects on until they reach speeds close to that of light.

Looking at first sight as if it cannot possibly work, a prototype has been developed by one of our leading engineers, and is endorsed by academics and government.

Defying intuitive understanding, it is low cost to make, is said to obey the laws of physics as they are currently understood, and in the longer term, could revolutionise transport and actuation.

The Emdrive is the brainchild of Roger Shawyer, who, in the past, has had charge of some of Britains most advanced aerospace projects. He says the germ of the present idea started when he was at Sperry Gyroscope, and was asked to look for a reactionless system for missile guidance. Conventional engineers, may at this point, throw up their hands and say that a reactionless system is not possible, citing Newton's Third Law. However while light photons obey Newton's Laws in some respects, the idea of the solar sail being a typical example, in other respects, light and objects travelling at or near light speed do not obey them.

In essence, the Emdrive is a resonating bottle full of microwaves. Because microwaves are a low frequency form of light, their behaviour is governed by Einstein's Theory of Special Relativity. And while microwaves and other forms of electromagnetic radiation may be thought of as very fast moving particles, they also have to be thought of as waves. At the same time that the constituent particles are moving at light speed, or their phase velocity, energy is transferred by the wave aspect travelling at group velocity. Group velocity is the result of waves of different wavelengths interacting with each other. While, according to Einstein, the phase velocity of electromagnetic waves is the speed of light in the appropriate medium whatever happens, and in whatever moving frame of reference the observer happens to be, group velocity varies. Group velocity can be any speed from stationary to light speed (a few physicists suggest the additional possibility of faster than light), and this varies the amount of momentum striking an impenetrable barrier, and thus the force exerted on it. Hence, it is possible to have a bottle full of electromagnetic waves exerting more force on one end than the other, whereas this is not possible for anything else that an engineer would normally be expected to encounter.

In the case of the prototype Emdrive, the closed resonating cavity is wider at one end than the other. Mathematical analysis shows that group velocity is higher at the wide end than the narrow end and that consequently, there is a net force exerted on the wide end. Furthermore, the net force exerted is proportional to the Q, or the effectiveness that the cavity shows as a resonator.

Most academics have run away frightened at the very idea of getting involved in an idea which is so controversial. One, however, Dr Richard Paris, a reader in mathematics at the University of Abertay in Dundee has endorsed the calculations. While the theoretical analyses, may of course, still be wrong, there is no denying that the prototype device appears to behave as predicted.

A curiosity of the constructed prototype is that when switched on, it takes some seconds to apparently build up to full thrust. Shawyer at first suspected that the apparent thrust might be due to some buoyancy effect arising from heat generated within the EMC enclosure. Careful modelling and analysis, however, shows that the effect arises purely from the time constants of the pulsed output of the microwave source, and the way these interact with the time constant of the balance system used to measure the forces developed.

The device uses a resonator made of copper, filled with microwaves from a commercial magnetron running at 2.5GHz, delivering 850W at an efficiency of around 70%. Enclosed in an EMC enclosure for safety reasons, the total weight of the box of apparatus is 15kg. When the box is placed upon a balance one way up and is switched on, it exerts a downward force of 15kgf + 2gf, and when it is placed the other way up, it exerts a force of 15kgf - 2gf. (The force motor and microwave generator weigh only 9.4kg, the remaining weight is that of the EMC enclosure).

A force of 2gf, or about 0.02N, may not sound much, but on a spacecraft, it is dramatic, because it can be constantly applied for hours, days, weeks, months or years. A 3 tonne satellite typically carries 1.7 tonnes of propellant. If it did not need to do this, its weight would be halved, and so would the launching cost of each satellite, currently a minimum of 80 million (Russian launcher). It would also greatly increase the working life of satellites, since this is presently ended when they run short of propellant, and are no longer able to keep themselves in their correct orbit. The end result is the increased economic viability of satellite communication and navigation systems, especially those that presently have marginal economics, tipping the balance between fibre optic ground based and space based systems. Wrist mounted communication systems and PDAs which would never lose signal, unless underground, would be the most immediate result noted by the 'man in the street'. Even on the basis of present satellite launch programmes, projected cost savings of 15.5 billion over the next 10 years earned the idea a DTI SMART Award in August 2001, and are encouraging the raising of serious money for the next stage in development.

Assuming that the measured effects are as real as they appear to be, and that the theoretical analyses are correct, it is possible to speculate where the technology might go after this. In space, it would reduce the journey time to Mars from nine months to three, rendering feasible the proposed NASA/ESA manned mission to Mars program, presently a pipe dream because of cost.

On the ground, it may be possible to make the engine much more powerful, even powerful enough to lift a flying machine against the force of gravity. We have been asked not to say how this might be done, but we can reveal that it involves a drastic improvement in the 'Q' factor, possible by a means which is available using present day technology, but one which would require a fair amount of expenditure to develop. Shawyer insists that such an engine would not be an anti gravity machine, which it may or may not be possible to construct, but would certainly behave like one.

One of the curiosities of the idea is that as the size goes down, the working frequency goes up. Hence, it may one day be possible to make very small force motors working on the same principle, but powered by light. These would be more compatible with very small scale robotics than trying to build very small mechanical actuators.


A means seems to have been found to generate force and thrust without a reaction and without a propellant

The technology has been developed for use as a satellite thruster, potentially halving the launch weight of satellites, but could well have many other uses in the longer term

The idea exists as a demonstrable working prototype.

UPDATE: MAY 2004 (News item in Eureka

Emdrive powers towards space

The Emdrive, which can drive spacecraft without the need for rocket propellant, originally revealed in the December 2002 edition of Eureka, has received further support.

Roger Shawyer, the Emdrive's inventor has written to us to tell us that: "The study results were independently reviewed and further support was then given by the DTI in the form of a Research and Development grant. This has enabled work to be started on the design, manufacture and test of a complete Demonstration Engine. The 250,000 programme is presently on target to provide a convincing demonstration of the technology. A fully representative spacecraft engine is being built and will be 'flown' on an air bearing to demonstrate the performance. A paper giving the theory and a summary of the experimental work is available free by e mail application to , giving name and affiliation. More information may be found at . TS

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