Direct Thrust Measurements of an EM Drive and Evaluation of Possible Side-Effects

 

Summary of Tests on Torsion Balance with Oil Fluid Damping (Time Axis shows only Part). Source: ARC

Abstract – The EMDrive has been proposed as a revolutionary propellantless thruster using a resonating microwave cavity. It is claimed to work on the difference in radiation pressure due to the geometry of its tapered resonance cavity. We [M. Tajmar and G. Fiedler] attempted to replicate an EM Drive and tested it on both a knife-edge balance as well as on a torsion balance inside a vacuum chamber. After developing a numerical model to properly design our cavity for high efficiencies in close cooperation with the EM Drive’s inventor, we built a breadboard out of copper with the possibility to tune the resonance frequency in order to match the resonance frequency of the magnetron which was attached on the side of the cavity. After measuring the Q-factor (amount of reflections inside the cavity before the radiation is absorbed) of our assembly, we connected the EMDrive to a commercial 700 W microwave magnetron. After a thermal mapping of the surfaces, we performed thrust measurements with a knife-edge balance as well as with a torsion balance in vacuum chamber. Our measurements reveal thrusts as expected from previous claims after carefully studying thermal and electromagnetic interferences. For the first time, measurements were also performed in high vacuum. Due to a low Q factor of <50, we observed thrusts of +/-20 μN. We identified the magnetic interaction of the power feeding lines going to and from the liquid metal contacts as the most important possible side-effect that is not fully characterized yet. Our test campaign can not confirm or refute the claims of the EMDrive but intends to independently assess possible side-effects in the measurements methods used so far. Nevertheless, we do observe thrusts close to the actual predictions after eliminating many possible error sources that should warrant further investigation into the phenomena.

EMDrive Concept

Conclusions – We have built and tested an EMDrive using a commercial standard magnetron with a resonance frequency of 2.44 GHz and 700 W of power in setups similar to the ones used in the past in order to assess possible side effects and their claimed thrust values. Our thruster had a considerably smaller Q factor (around 50 for the first tests and 20 at the end) compared to others (10,000 – 100,000), however our test facilities had a higher sensitivity as well.

Our first tests were done with a knife-edge balance configuration and we assessed different isolation scenarios in order to see any thermal or electromagnetic influence. As expected, we noticed a large thermal effect that could be significantly reduced by thermal isolation and by blocking any air circulation inside our measurement box. We indeed found thrusts that changed with the orientation of the thruster and magnitudes in line with the theoretical predictions for our low Q factor. After turning off the power, the thrust values in the order of several hundred μN remained and slowly degraded after power shut-off. Considering that the EMDrive and especially the magnetron mounted on it can get hot, such a setup does not seem to be able to adequately measure precise thrusts.

We continued with testing on a torsion balance inside a vacuum chamber. Here we also found thrusts but quickly realized that there was a strong interaction with our magnetic damping system. Still we used this setup to test an EMDrive for the first time in high vacuum down to 4×10^-6 mbar observing similar thrusts (although at somewhat lower power levels) ruling out any air influence in this configuration. After changing the position of the magnetron (outer position) and replacing the magnetic damping with oil fluid damping, surprisingly we could still observe thrusts that are indeed reversing with thruster orientation but with control runs in vertical direction producing similar thrusts compared to the positive direction. However, negative thrusts were only observed with firing the thruster indeed in a negative direction. Running the magnetron also in this direction at lower voltages produced similar positive values as the vertical control experiment. The thrusts observed with the oil-damped torsion balance were close to the original prediction taking our small Q factor into account (around +/- 20 μN for 700 W of microwave power – still an order of magnitude more effective than pure radiation thrust). We also observed that the thrust appeared not to go down to zero immediately after power is switched-off but rather noted a gradual decrease as if the EMDrive was charged up and slowly reduced its thrust effect.

The nature of the thrusts observed is still unclear. Additional tests need to be carried out to study the magnetic interaction of the power feeding lines used for the liquid metal contacts. (…) Next steps include better magnetic shielding, further vacuum tests and improved EMDrive models with higher Q factors and electronics that allow tuning for optimal operation. As a worst case we may find how to effectively shield thrust balances from magnetic fields.

Prototype with Waveguide, Magnetron

Torsion Balance in Vacuum Chamber