NewScientist has a better article, but I don't subscribe. Here are a few quotes though.The Engineer Online wrote:UK research into a propellant-free microwave engine designed for spacecraft propulsion is stirring interest from US and Chinese aerospace companies, its developer has claimed.
Despite sounding like the stuff of science-fiction, SPR (Satellite Propulsion Research), the company behind the EmDrive – which was first reported by The Engineer in November 2004 – insists its technology is gathering momentum in the international aerospace community.
According to the managing director of Roger Shawyer, the engine takes microwave radiation produced by a magnetron and feeds it into a specially shaped resonant cavity. The waves push against the end wall. Because of the difference in wave velocity, being higher at one end that the other, there is a momentum transfer. The result is a measurable net force from the cavity against its surroundings.
The EmDrive is just entering the third of three development phases. The first was an experimental thruster, which Shawyer claims provided a thrust of 2g in over 240 tests, shown as a drop in the engine mass.
The second phase produced a demonstrator engine which has recently completed its own set of independently reviewed static thrust tests.
‘These first two models were prototypes,’ said Shawyer. ‘As they operate at room temperature, the Q value, or the stored energy divided by losses, was restricted.
‘The next stage would ideally use technology employed by high-energy physics. If we approach a condition where we use the same shape cavity but at absolute zero and made from Niobium, the resistance drops to near zero and the Q value increases by several orders of magnitude.’
Shawyer cautions that the calculations only work for static thrusts. ‘You can’t beat the laws of physics. If it is used to accelerate, the Q value drops. It is best used to lift a body and oppose a force, for instance to counteract gravity. It cannot be used to accelerate further.’
Shawyer said the most practical application of the first generation is for thrusters used to lift a satellite into geostationary orbit form low-earth orbit. ‘The dramatic drop-off in Q doesn’t happen as quickly, so it makes little difference. It tends not to accelerate in the main velocity vector, only for thrusts up and down. It beats ion engines.’
The third stage of development will product a new generation of EmDrive, which Shawyer envisages coming into its own in 10 years or so and being used in a variety of terrestrial transport.
Shawyer is looking to US and Chinese aerospace industry to fund the next phase of development. The company hopes to license the first generation technology to one or more aerospace companies while they concentrate on the next phase, which will include experimental superconducting thrusters.
‘We are expanding the company, but we want to offload the marketing so we can concentrate on research,’ said Shawyer.
Thoughts?Take a standard copper waveguide and close off both ends. Now create microwaves using a magnetron, a device found in every microwave oven. If you inject these microwaves into the cavity, the microwaves will bounce from one end of the cavity to the other. According to the principles outlined by Maxwell, this will produce a tiny force on the end walls. Now carefully match the size of the cavity to the wavelength of the microwaves and you create a chamber in which the microwaves resonate, allowing it to store large amounts of energy.
What's crucial here is the Q-value of the cavity - a measure of how well a vibrating system prevents its energy dissipating into heat, or how slowly the oscillations are damped down. For example, a pendulum swinging in air would have a high Q, while a pendulum immersed in oil would have a low one. If microwaves leak out of the cavity, the Q will be low. A cavity with a high Q-value can store large amounts of microwave energy with few losses, and this means the radiation will exert relatively large forces on the ends of the cavity. You might think the forces on the end walls will cancel each other out, but Shawyer worked out that with a suitably shaped resonant cavity, wider at one end than the other, the radiation pressure exerted by the microwaves at the wide end would be higher than at the narrow one.
Key is the fact that the diameter of a tubular cavity alters the path - and hence the effective velocity - of the microwaves travelling through it. Microwaves moving along a relatively wide tube follow a more or less uninterrupted path from end to end, while microwaves in a narrow tube move along it by reflecting off the walls. The narrower the tube gets, the more the microwaves get reflected and the slower their effective velocity along the tube becomes. Shawyer calculates the microwaves striking the end wall at the narrow end of his cavity will transfer less momentum to the cavity than those striking the wider end (see Diagram). The result is a net force that pushes the cavity in one direction. And that's it, Shawyer says.
Hang on a minute, though. If the cavity is to move, it must be pushed by something. A rocket engine, for example, is propelled by hot exhaust gases pushing on the rear of the rocket. How can photons confined inside a cavity make the cavity move? This is where relativity and the strange nature of light come in. Since the microwave photons in the waveguide are travelling close to the speed of light, any attempt to resolve the forces they generate must take account of Einstein's special theory of relativity. This says that the microwaves move in their own frame of reference. In other words they move independently of the cavity - as if they are outside it. As a result, the microwaves themselves exert a push on the cavity.
Each photon that a magnetron fires into the cavity creates an equal and opposite reaction - like the recoil force on a gun as it fires a bullet. With Shawyer's design, however, this force is minuscule compared with the forces generated in the resonant cavity, because the photons reflect back and forth up to 50,000 times. With each reflection, a reaction occurs between the cavity and the photon, each operating in its own frame of reference. This generates a tiny force, which for a powerful microwave beam confined in the cavity adds up to produce a perceptible thrust on the cavity itself.
