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EmDrive  (Source: emdrive.com)
It's a a propellantless microwave thruster that defies Newton's laws of motion

Chinese scientists have taken on a heavily criticized space drive idea that could one day launch satellites, deep space probes and even flying cars.

The research team hails from Northwestern Polytechnic University in Xi'an, and was led by Yang Juan. What they developed was a propellantless microwave thruster called EmDrive, which is a controversial idea because it goes against Newton's laws of motion -- and many have claimed to create a "propellantless" thruster before and failed.

Newton's laws of motion are all based on the idea that firing propellant out of the back at a high speed will push a craft forward. While solar cells offer infinite power, thrust is limited by propellant. Many have tried to get around this, but several scams have made this particular field a joke in the scientific world.

While space drives tend to rely on Newton's laws of motion, the EmDrive is a closed, conical container that has a net thrust toward the wide end when filled with resonating microwaves. This goes against Newton, who said that no closed system could have a net thrust. However, EmDrive works because the microwaves have a group velocity (the speed of a collection of electromagnetic waves) that is greater in one direction than the other -- which is where Albert Einstein's theory of relativity comes in.

British engineer Robert Shaywer, who began looking at the concept of a propellantless thruster when he opened his own company called Satellite Propulsion Research in 2001, is the original creator of the EmDrive. He made demonstration thrusters to prove it could be done, and even made sure the test results were accurate (meaning, the results weren't affected by friction, ionization, air currents, electromagnetic effects or interference). The first, made in 2003, had a thrust of 16 mN. This was enough to show it could be done.

However, he received a ton of criticism for his idea and was ridiculed in his own country. But the Chinese team at Northwestern Polytechnic University believed in his research and took the project head on to author the latest study, "Net Thrust Measurement of Propellantless Microwave Thruster."

The Northwestern Polytechnic University team was able to create the EmDrive with 720 mN of thrust with a couple of kilowatts of power.

So what purpose does EmDrive have? It could halve launch costs of satellites because as much as half the launch weight of these objects are attributed to propellant.

Shawyer is even working on a superconducting thruster that could be ready as soon as 2016. It would boost the Q value of the cavity, which determines the amount of thrust produced. He said it could be boosted by a factor of several thousand, possibly equating to a tonne of thrust per kilowatt of power.

Source: Wired



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RE: Violating laws of physics?
By ShieTar on 2/11/2013 8:14:50 AM , Rating: 2
The violated part of physics is the realation between impulse and energy. For a photon, regardless of its wavelength, this ratio is defined by the speed of ligth, i.e. p = E/c. In order to generate a thrust of 720 mN for the drive, you need to emit every second a packet of energy carriers with an impulse of 720mN*1s. If this is done with photons only, those photons have a corresponding energy of 720mN*1s*300,000,000m/s = 216 MJ per second, i.e. 261MW. Not "a few kilowatt" as discussed in this article. And it makes no difference how often you bounce these photons around in a cavity, they will never gain any additional impulse.

A fuel-based propulsion system on the other hand follows the relationship p=2E/v, which gives you significantly better Energy to Thrust ratios, as long as the velocity of the fuel is much smaller than the speed of light. Sadly, energy efficiency is best for a huge mass that is exhausted slowly, but that means a horrible fuel efficiency. And usually in space, energy is cheap, but mass is not. Thats why deep space vehicles are now often equipped with ion thrusters, which need a lot of energy but only very little fuel.


RE: Violating laws of physics?
By Jaybus on 2/11/2013 3:26:39 PM , Rating: 2
The photons do not cease to exist. Only a tiny fraction of the photon's energy is converted to kinetic energy. The impulse is the integral of force with respect to time. A force of 720 mN applied for 1 second is then an impulse of 0.72 N-s (Newton-seconds), which is equivalent to the change in momentum. A 0.72 kg mass would experience a 1 m/s^2 acceleration. So applied for 1 second, the change in kinetic energy would be 0.5*(0.72 kg)*((1.0 m/s)^2 = 0.36 J = 360 mW/s.

This very low efficiency (0.018%), doesn't look good at first glance. But let's say we have a spacecraft with a mass of 720 kg, 320 kg of which is a TOPAZ-I nuclear reactor generating 5 kW sustained for 3 to 5 years. Dedicating 4 kW to propulsion allows an impulse of 1.44 N-s, and so an acceleration of a paltry 0.002 m/s^2. However, this small force can be applied for years. In one week the spacecraft will have accelerated to 1.2 km/s. In 6 months it will be traveling at 30 km/s. In 3 years it will be traveling at nearly one third light speed.


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