# The “Em Drive” paper is out - need some help understanding it

Scott Manley tells us (see Engadget also) that the reaction-less drive peer reviewed paper is out, and nicely explains the basics. If it works, it would violate the long-held belief that linear momentum is conserved.

I looked at the paper and it didn't take long to find a strange word. (screen capture) The first sentence of the abstract reads:

A vacuum test campaign evaluating the impulsive thrust performance of a tapered radio-frequency test article excited in the transverse magnitude 212 mode at 1937 MHz has been completed.

What is a "transverse magnitude 212 mode"? Could it be a *transverse magnetic" mode actually? That's a real term, and would be written $TM212$, having three indices instead of two because it's a more complex shape than a cylinder. Or is it a new kind of resonant mode? Is this what publication in a peer-reviewed journal means? All reviewers, and the journal's editor approved this paper including the abstract?

Further, the RF amplifier is on the torsional balance, and DC power is supplied through sliding liquid metal contacts "to eliminate cable forces'. A current flowing through a closed circuit will experience a Lorenz force tending to push the loop to larger area. (See the "sliding bar problem, fig. 10.12.1) The magnitude of the force would scale linearly with current (and therefore with the RF power). I don't see any discussion of this force on the torsional balance, or estimation of it's magnitude, but it looks like when they tested reverse thrust, the sliding contacts were also moved to the other side as well. Was this force addressed in earlier publications, or did the reviewers miss this as well?

above: Example of a reactionless electromagnetic drive being tested inside a vacuum chamber, from National Geographic. Note, the drive is supposed to produce of the order of 30 to 100 micro-Newtons, which is about the weight of a few mosquito.

above: The "sliding bar" problem from here.

• Heh. From the second sentence in the actual article: "It is shown here that a dielectrically loaded, tapered RF test article excited in the transverse magnetic 212 (TM212) mode" [emphasis added]. – Chris Nov 22 '16 at 2:42
• Torsional balances make precise measurements of extremely small forces possible, but the elegance is defeated if you then make physical contact with it. The reviewers should have insisted on a description of where and how hard they were touching it. You go shopping, the grocer tells you a price that defies the laws of physics, then adds "by the way, I had my thumb on the scale"... wouldn't you at least want to ask "well how hard did you push?" – uhoh Nov 22 '16 at 5:29
• This is more suited for Physics than Space Exploration. – Hobbes Nov 22 '16 at 7:09
• @Hobbes no not really. We use high school physics in SX SE all the time! There is no question here about the physics at all. This is about the test of a thruster for space propulsion. Just because there is a diagram that has a magnetic field in it does not mean it is off-topic. While a discussion of the proposed physics behind the momentum non-conservation of the drive might go there, this is about correct thruster testing procedures. You wouldn't move all of the ion propulsion and photon propulsion questions to Physics SE too just because they have physics, would you? – uhoh Nov 22 '16 at 8:29
• I'm proposing to move it because your question is about the methodology of the experiment, which is more closely related to physics than to the application of this drive for space probes. Just trying to get this question in front of the audience most likely to be able to answer it. – Hobbes Nov 22 '16 at 9:38

Self-interacting Lorentz forces were addressed in section 8, point 3, along with other similar errors:

The third error is magnetic interaction, which has the potential for a false positive resulting from dc currents in power cables interacting during test article operation with ambient magnetic fields (e.g., local Earth field, magnetic damper) to generate a torque displacement on the pendulum. All dc power cables are a twisted pair or twisted shielded pair to minimize magnetic interaction. The test article is tested in forward, reverse, and null thrust orientations, but dc power cable routing and orientation is the same for all three configurations (power cables come in from the top of the test article), meaning any false positives will be the same magnitude and polarity for all three tests. This is not observed during the test campaign.

(Original paper)

Since the conductors were mounted in the same way for all tests, any Lorentz forces from the liquid metal would always appear in the same direction and with approximately the same amount of power as for any other test at the same DC current. But the test rig's measured thrust was highly sensitive to precise tuning of the RF PLL oscillator, and the thrust measured changed directions or disappeared appropriately with the different setups.

• It doesn't address self-interaction, it addresses interaction with "ambient fields", and it doesn't address changes of open loop area due to movement of the contacts in the liquid metal, but you've identified probably the closest thing to it. "...potential for a false positive resulting from dc currents in power cables interacting... with ambient magnetic fields (e.g., local Earth field, magnetic damper)... "but they do say that the configuration of the DC path does not change for the three tests. Your point on the large non-repeatability is well taken. Thanks for taking the time to dig in! – uhoh Nov 23 '16 at 3:10
• It should be tested not only with DC current power, it should also be tested using AC current. When AC current is used, thereshould be no Lorentz forces because positive and negative half-cycles will compensate. The earth magnetic field may be compensated using a Helmholtz coil. It is even possible to generate a local magnetic field in opposite direction of the earth magnetic field or to vary the field intensity. – Uwe Nov 24 '16 at 12:41
• @Uwe: I suspect AC-all-the-way would have been difficult to wire up, but in any case, can you identify a specific hypothetical error scenario that AC would avoid that the described setup would not? – Nathan Tuggy Nov 24 '16 at 20:28
• @Uwe et al. Unless we are dumping all of high school physics and not just conservation of momentum, all that's needed is a diagram or photo of the DC paths in order to estimate the resulting torque due to a variable loop area's self-interraction. If the two contacts are close and both near the rotation axis the will be smaller, if they are more widely spaced and located away from the axis it will be bigger. Either way it can be easily estimated assuming one still believe in science. Of course validation tests are still needed but one still provides straightforward estimates when possible. – uhoh Nov 25 '16 at 4:46

By parts,

What is a "transverse magnitude 212 mode"?

I didn't find anything in literature about "transverse magnitude". At a first glance you're right, probably they meant "transverse magnetic mode".

Is this what publication in a peer-reviewed journal means?

Peer reviewing is a long and painful process with numerous revision cycles that could last for years. It is understandable that they can overlook some typos and simple errors. Perhaps the authors can correct this in future.