Given fuel usually makes up the majority of the mass of any spacecraft trying to break the bonds of gravity, developing propellantless propulsion systems might be the key we need to really open up space travel. The EMDrive appeared one of the most promising, after several experiments showed it can generate thrust from basically nothing – in apparent violation of Newton's Laws of Motion. Unfortunately, those laws look set to remain intact after a German team built and tested their own EMDrive, and found that environmental factors may have been responsible for false positives.

British engineer Roger Shawyer first proposed the concept of the EMDrive in 2001. More technically known as a radio frequency resonant cavity thruster, the idea goes that by beaming microwaves into a cone-shaped cavity, the radiation pressure at the small end should be different to that at the larger end, creating a net thrust force. In theory, that should allow it to propel a craft without ever needing any fuel whatsoever.

If that sounds too good to be true, that's because it probably is. Critics pointed out that the thruster would violate Newton's Laws of Motion, specifically the conservation of momentum. Every action has an equal and opposite reaction, so to push something forward, mass would need to be ejected out the back. This "Impossible" EMDrive just wouldn't work.

And yet, it apparently did. Somehow, thrust was detected in past experiments on prototypes of these designs, including the similar Cannae Drive. Granted, not all attempts have worked, and those that do generate very small amounts of thrust, but that's still impressive when it shouldn't work at all.

To check it out for themselves, researchers on the SpaceDrive Project have built and tested the EMDrive, as well as another propellantless design known as the Mach-Effect thruster. The EMDrive was set up in a vacuum chamber, and firing it up the team was able to register about 4 microNewtons of thrust.

But it was their next results that dealt a blow to the idea. The team used a stepper motor to rotate the thruster 90 degrees and 180 degrees from its original position. As expected, turning it 180 degrees yielded about the same level of thrust in the opposite direction. At 90 degrees, the EMDrive shouldn't have produced any thrust – but it still did.

That led the team to suspect that the thrust signals were false positives, caused by something else. To make doubly sure, they dampened the amount of microwave power going into the cavity by five orders of magnitude – and the thrust signal remained almost exactly the same.

"This clearly indicates that the 'thrust' is not coming from the EMDrive but from some electromagnetic interaction," the team concluded.

This interference, the researchers suggest, was probably the result of the prototype's power cables interacting with the Earth's magnetic field.

The team doesn't outright say that the experiment proves that the EMDrive doesn't work. Any actual thrust generated could be masked by this electromagnetic interaction, so in future tests the team plans to properly shield the setup to reduce that interference. It may cast doubt on the results of previous positives, though.

"Such shielding was not present in any of the previous tests, which should be carefully re-analyzed," the team says.

Tests with the Mach-Effect thruster followed a similar pattern. Thrust signals were detected, but later ruled to be the result of electromagnetic interactions. The researchers plan to continue their experiments.