EMdrive Microwave Propulsion

The Emdrive is an electromagnetic drive that would generate thrust from a closed system — “impossible” say some experts.


Principle of Operation

At first sight the idea of propulsion without propellant seems impossible. However the technology is firmly anchored in the basic laws of physics and following an extensive review process, no transgressions of these laws have been identified.

The principle of operation is based on the well-known phenomenon of radiation pressure. This relies on Newton’s Second Law where force is defined as the rate of change of momentum. Thus an electromagnetic (EM) wave, travelling at the speed of light has a certain momentum which it will transfer to a reflector, resulting in a tiny force.

If the same EM wave is travelling at a fraction of the speed of light, the rate of change of momentum, and hence force, is reduced by that fraction. The propagation velocity of an EM wave, and the resulting force it exerts, can be varied depending on the geometry of a waveguide within which it travels. This was demonstrated by work carried out in the 1950’s. (CULLEN, A.L. ‘Absolute Power Measurements at Microwave Frequencies’ IEE Proceedings Vol 99 Part 1V 1952 P.100)

Thus if the EM wave travelling in a tapered waveguide is bounced between two reflectors, with a large velocity difference at the reflector surfaces, the force difference will give a resultant thrust to the waveguide linking the two reflectors. If the reflectors are separated by a multiple of half the effective wavelength of the EM wave, this thrust will be multiplied by the Q of the resulting resonant cavity, as illustrated in fig 1.

Fig 1. Diagram of an engine concept.


The inevitable objection raised, is that the apparently closed system produced by this arrangement cannot result in an output force, but will merely produce strain within the waveguide walls. However, this ignores Einstein’s Special Law of Relativity in which separate frames of reference have to be applied at velocities approaching the speed of light. Thus the system of EM wave and waveguide can be regarded as an open system, with the EM wave and the waveguide having separate frames of reference.

A similar approach is necessary to explain the principle of the laser gyroscope, where open system attitude information is obtained from an apparently closed system device.

To critics, it’s flat-out junk science, not even worth thinking about. But its inventor, Roger Shawyer, has doggedly continued his work. As Danger Room reported last year, Chinese scientists claimed to validate his math and were building their own version.

Shawyer gave a presentation earlier this week on the Emdrive’s progress at the CEAS 2009 European Air & Space Conference. It answered few questions, but hinted at how the Emdrive might transform spaceflight — and warfare. If the technology works, that is.

The heart of the Emdrive is a resonant, tapered cavity filled with microwaves. According to Shawyer, a relativistic effect generates a net thrust, an effect confirmed by various Emdrives he has built as demonstrations. Critics say that any thrust from the drive must come from another source. Shawyer is adamant that the measured thrust is not caused by other factors.

While the argument over the drive’s impossibility continues, so does the engineering work. The problem is that nobody wants to talk about it. Even Shawyer gives little away.

Last year, professor Yang Juan of the College of Astronautics at Northwestern Polytechnical University (NPU) in Xi’an was happy to confirm that they were building an Emdrive which would be tested by the end of the year. But following the publication of this news in Danger Room, the situation changed. I was informed that the publicity was very unwelcome, especially any suggestion that there might be a military application. (Yang had previous published a study on the use of plasma as a weapon against low-orbiting satellites. [.pdf]) No further information has been forthcoming, and no Chinese papers have been published on the Emdrive, though Yang has recently published work on (unrelated) microwave plasma thrusters (.pdf).

Shawyer asserts that work is also being carried out in France, Russia and in the United States by a major aerospace company. But he cannot provide details beyond vague promises of “significant progress [that] has been made in both theoretical and experimental work, within these groups.” He also asserts that the British National Space Centre is said to be reviewing the Emdrive. Again, no details.

The CEAS 2009 paper outlines recent progress and plans. Previous thrusters generated relatively modest forces; the latest version now being built is based on a cooled superconductor and should generate more than 300 pounds of thrust for a 6-kilowatt input, Shawyer promises. (But does not yet appear to have done so.) The plan is to mount four of these thrusters on an unmanned demonstration vehicle that will weigh about 1,000 pounds. The craft will have no wings: It will be supported by the Emdrives and propelled by jet engines to about 230 knots. It will be capable of vertical takeoff and hovering silently in place. If successful, it will be adapted as a personal transport -– your very own flying car.

In the longer run, perhaps 10 years, Shawyer envisages a hybrid spaceplane using Emdrive technology — see the photo above of a 2-meter scale model. The idea is a craft capable of making the 10,000-mile run from London to Sydney, Australia in under three hours … or taking a 40-ton payload on the moon in about four days.

Aeronautical engineers have been dreaming of such a craft for decades; none have ever panned out. The theoretical advantage of the Emdrive spaceplane compared to rockets is that it allows a slow ascent with low acceleration rate. There is also no telltale rocket exhaust plume, and this may be the source of some of the interest. At present, the launch of a ballistic missile anywhere on Earth can be immediately spotted from space. An Emdrive-based launch system would be undetectable and could arrive from any direction, leaving the target of an attack no way of knowing who to retaliate against.

This is the kind of factor that might drive governments to put money into Emdrive projects. An investment in contested science is not a  probable winner — but  the payoff could be a big one.

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