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 promises, could have a profound impact on 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. Upon first inspection it looks as if it cannot possibly work, however one of the UK's leading engineers has developed a prototype which has been endorsed by academics and government alike.
Defying conventional wisdom, it is low cost, 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. The germ of the present idea, he says, started when he worked at Sperry Gyroscope and was asked to look for a reactionless system for missile guidance. Many of you may, at this point, throw up your hands and say that a reactionless system is not possible while citing Newton's Third Law. However while 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. (Some people say that gyroscopes disobey the Third law - perhaps readers would like to comment).
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. 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, however, varies.
Group velocity can be any speed from stationary to light speed (with a few physicists suggesting the additional possibility of faster than light). This varies the level of momentum imparted when 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 unit, 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, as a consequence, there is a net force exerted on the wider end. Furthermore, the net force exerted is proportional to Q (Q being the effectiveness that the cavity shows as a resonator).
Most academics have blanched at the very idea of getting involved in such a controversial idea. 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 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 build up to full thrust. At first Shawyer 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 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 three 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 would be 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 systems 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 they 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 provide lift 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, which can be made possible 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.
Roger Shawyer at Satellite Propulsion Research
It seems to be possibe 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
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