No propellant drive prepares for space and beyond

Tom Shelley reports on progress with the controversial Emdrive and its potential applications in space and on the ground

The Emdrive - originally revealed in Eureka's December 2002 edition as a way of driving satellites and spacecraft using microwaves - is now demonstrating its ability to produce thrust on a consistent basis and is scheduled to be ready for space use by May 2009.
Meanwhile studies are underway into the design of a superconducting version, with a possible thrust of more than 30kN/kW. While it couldn't be used to accelerate a rocket, it might well be able to provide enough static lift for a flying vehicle propelled forward by other means.
Despite massive controversy, the project continues to be backed by DTI and private investors, and has now been shown to work. Roger Shawyer, who spent 20 years at Marconi Space Systems (now EADS Astrium), revealed details of the prototype engine and its development plans to an IEE chapter meeting in Portsmouth, where the audience included many ex-colleagues from his former company.
Shawyer stated that many of the claims he is alleged to have made about the Emdrive are untrue, particularly suggestions that it defies the principles of conservation of momentum, or Newton's Laws of Motion.
The crucial part, as he explained, is that it is a relativistic effect that arises because the waves being reflected at the two ends of the conical cavity into which the microwaves are injected have different effective velocities, and thus different frames of reference, and that a closed microwave wave guide is an 'open system' in terms of relativity. According to Einstein, all moving frames of reference are equivalent. Why this should be so, whether one is standing still or going at half the speed of light, nobody knows, and in effect Shawyer's engine could be chucking Dark Energy out of the back of it and functioning as a conventional rocket. On the other hand, there may be no such thing as Dark Energy, and Shawyer may have stumbled on what is really driving the galaxies apart.
But as he pointed out: "I am just a microwave engineer and all that matters is that it works."
In the present experimental engine and its immediate predecessor, the cavity is made in the form of a copper cone closed off by flat plates at the wide and narrow ends. The net thrust is proportional to the Q value of the cavity, where Q is the ratio of the amount of stored energy to the amount of energy lost per cycle. Acceleration extracts energy from the system and Q decreases. The development engine has a Q value of 50,000 and produces a specific thrust of 0.315N/kW. The original engine produced a thrust of 1.6 grammes, but could only run for tens of seconds at a time before the magnetron overheated and burned out. The present engine produces 9 grammes of thrust from 300W of microwave power and is continually water-cooled. The internal power density is about 17MW.
A video of the demonstration of the engine in its test cell involved mounting the engine and its cooling system on a beam, and supporting it on an air bearing. The test was undertaken in October last year, producing a thrust of 9.8 grammes, maximum speed of 2 cm/s and a movement distance of 185cm. According to Shawyer, the tests had involved accelerating from rest, deceleration to rest, forward and reverse engine mounting, energising at different start angles and using different input powers.

While 9.8 grammes of thrust from a 100kg of machinery may not sound very much, it is a much better power-to-weight ratio than the best competitive satellite and spacecraft propulsion technology, which involves using an ion engine.
For a 1500W DC input power, an ion drive produces 92mN thrust, whereas an Emdrive, based on current technology, should produce 330mN thrust. Furthermore, an ion engine of this size would weigh 112.5kg plus propellant, whereas an Emdrive would weigh 9kg. And while life under power for an ion engine is about six months, an Emdrive should run for 15 years - or virtually forever, if the microwaves were generated by some solid state device.
The big application is for commercial communication satellites - where 'Hotbirds' have a take-off weight of 3 tonnes, of which 1.7 tonnes is propellant - both to get them from Low Earth Orbit to Geostationary and to keep them pointing the right way once they get there. Using Emdrives, says Shawyer, should save 15 billion in launch costs over 10 years. While the thrust from the Emdrive would be small, it should get the satellite from Low Earth to Geostationary orbit in 36 days. Power would be from 6kW of solar cells fed to Travelling Wave Tube Amplifiers.
But what really captivated the audience was Shawyer's proposal for the next stage, which would be to use a superconducting cavity with a Q value of 5 billion and a thrust of 3 tonnes/kW. Unfortunately, one could not use the device accelerate without "causing the Q value to collapse, losing thrust in that vector", he conceded.
One serious consideration is to develop the technology so that it could be used gently to divert a large asteroid in danger of colliding with the Earth. In fact, prior to Shawyer's address, David Hall from EADS Astrium had discussed the use of microwave technology to study the internals of Near Earth Object asteroids. He said blowing up such a threat, Hollywood-style, was not really practical - the parts would still be likely to hit the Earth. Current ideas were mostly about finding ways of nudging asteroids into a safe trajectory. As acceleration would be so low, a superconducting Emdrive would be a possible option. A 1kW engine would require 24kW to keep it cool and shifting the asteroid would take somewhere in the order of 10 years, depending on its size.
Shawyer said his team was thinking of using superconducting cavities of a type already being developed and manufactured for a major accelerator project, and cooling them with hydrogen. If applied to lifting a vehicle, the boiling off hydrogen could be used to provide horizontal motion by feeding it to conventional turbofan engines, if in the atmosphere, or to rocket engines for use in space. Whether the technology will ever be used to produce hydrogen-propelled air cars or other wonders, only time will tell.

A full theoretical paper on the technology is available on the Emdrive web site ( )


* Novel non-propellant microwave drive has reached the point where it can be run continuously

* Demonstration engine and cooling system weighs about 100kg and produces just under 10 grammes of thrust

* Next engine for satellite propulsion should weigh a little less than 10kg and produce 330mN thrust

* A superconducting design able to deliver tonnes of lift thrust (but no acceleration) is being studied

For more technical developments see

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