The EM drive 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.
UPDATE: Mr Shawyer has already built a prototype emdrive capable of generating about 16 millinewtons of thrust using 1 kilowatt of electrical power. The next version could be several orders of magnitude more effective. (Supercooled and using Niobium) So several newtons per kilowatt? Current ion engines have 100 millinewtons for one kilowatt. Here is a pdf that describes improved ion drives and thrust to power for existing and proposed ion drives.
The second phase produced a demonstrator engine which has recently completed its own set of independently reviewed static thrust tests.
‘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.’
This would be great if it works. No moving parts. No propellant. Sounds like it would make it a lot cheaper to get to orbit. From orbit there are a lot of potential systems for moving quickly around the solar system. Plasma magnets, M2P2, ion drives etc...