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.
Shawyer's calculations have not convinced everyone. Depending on who you talk to Shawyer is either a genius or a purveyor of snake oil. David Jefferies, a microwave engineer at the University of Surrey in the UK, is adamant that there is an error in Shawyer's thinking. "It's a load of bloody rubbish," he says. At the other end of the scale is Stepan Lucyszyn, a microwave engineer at Imperial College London. "I think it's outstanding science," he says. Marc Millis, the engineer behind NASA's programme to assess revolutionary propulsion technology accepts that the net forces inside the cavity will be unequal, but as for the thrust it generates, he wants to see the hard evidence before making a judgement.
To review the project, the UK government hired John Spiller, an independent space engineer. He was impressed. He says the thruster's design is practical and could be adapted fairly easily to operate in space. He points out, though, that the drive needs to be developed further and tested by an independent group with its own equipment. "It certainly needs to be flown experimentally," he says.
Armed with his prototypes, the test measurements and Spiller's review, Shawyer is now presenting his design to the space industry. The reaction in China and the US has been markedly more enthusiastic than that in Europe. "The European Space Agency knows about it but has not shown any interest," he says. The US air force has already paid him a visit, and a Chinese company has attempted to buy the intellectual property associated with the thruster. This month, he will be travelling to both countries to visit interested parties, including NASA.
What's crucial 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.
He has shown that it generates about 16 millinewtons of thrust, using 1 kilowatt of electrical power. Shawyer calculated that his first prototype had a Q of 5900. With his second thruster, he managed to raise the Q to 50,000 allowing it to generate a force of about 300 millinewtons 100 times what Cosmos 1 could achieve.
Engineers in Germany have already developed superconducting cavities as part of next-generation particle accelerators, and Shawyer hopes to have his own superconducting thruster ready within two years. Without electrical resistance, currents in the cavity walls will not generate heat. Engineers in Germany working on the next generation of particle accelerators have achieved a Q of several billion using superconducting cavities. If Shawyer can match that performance, he calculates that the thrust from a microwave engine could be as high as 30,000 newtons per kilowatt enough to lift a large car.
Why haven't physicists stumbled across the effect before? They have, says Shawyer, and they design their cavities to counter it. The forces inside the latest accelerator cavities are so large that they stretch the chambers like plasticine. To counteract this, engineers use piezoelectric actuators to squeeze the cavities back into shape. "I doubt they've ever thought of turning the force to other uses," he says.
No doubt his superconducting cavities will be hard to build, and Shawyer is realistic about the problems he is likely to meet. Particle accelerators made out of niobium become superconducting at the temperature of liquid helium only a few degrees above absolute zero. That would be impractical for a motor, Shawyer believes, so he wants to find a material that superconducts at a slightly higher temperature, and use liquid hydrogen, which boils at 20 kelvin, as the coolant. Hydrogen could also power a fuel cell or turbine to generate electricity for the emdrive.
In the meantime, he wants to test the device with liquid nitrogen, which is easier to handle. It boils at 77 kelvin, a temperature that will require the latest generation of high-temperature ceramic superconductors. Shawyer hasn't yet settled on the exact material, but he admits that any ceramic will be tricky to incorporate into the design because of its fragility. It will have to be reliably bonded to the inside of a cavity and mustn't crack or flake when cooled. There are other problems too. The inside of the cavity will still be heated by the microwaves, and this will possibly quench the superconducting effect.
Then there is the issue of acceleration. Shawyer has calculated that as soon as the thruster starts to move, it will use up energy stored in the cavity, draining energy faster than it can be replaced. So while the thrust of a motionless emdrive is high, the faster the engine moves, the more the thrust falls. Shawyer now reckons the emdrive will be better suited to powering vehicles that hover rather than accelerate rapidly. A fan or turbine attached to the back of the vehicle could then be used to move it forward without friction. He hopes to demonstrate his first superconducting thruster within two years. [The moving and losing thrust issue is why it could be good for countering friction and gravity for getting to orbit and make less of a difference in space]
Prior article on propellant free microwave space propulsion