As I've said on the AttO intro. page, it is important that there be investigations into speculative science, both in terms of performing it and in terms of investigating the performance of those experiments. It is very important that the results of speculative science be properly looked into for flaws so that quality control standards are enforced on its investigators. Otherwise valuable lines of speculation may well descend to the level of junk science through lack of proper "adult supervision". Most ventures into the speculative should rightfully be expected to lead to dead ends or other failures -- and I think the whole category of "reactionless drives" fits this description.







So, getting right to it. Starting a few years ago there have been an ongoing series of stories about a so-called reactionless thruster for space travel circulating on the internet. In fact, there were several stories, about several variations of a reactionless thruster. Some of them were called Q-Drive or Cannae Drive or EmDrive, with the most recent set of stories being about the one at the NASA Eagleworks Lab called the EmDrive. The EmDrive is the one I'll be critiquing here. However, I expect that the critiques apply to all the other variations of the reactionless drives because, as far as I can tell, they are all being tested in the same flawed way.







In August of 2014 there was a set of news stories about the so-called emDrive being tested at a NASA facility called the Eagleworks. The Eagleworks tests were in response to research performed at the Northwestern Polytechnical University (NPU) in Xi’an, Shaanxi, China, on microwave-driven resonant cavity thrusters from 2004 through 2013. The Eagleworks tests studied variations on what they propose use the (supposed) quantum vacuum plasma thrusters (QVPT). In that same month of August 2014 I posted a brief critique of the tests to the Hackers Conference's mailing list after the news items surfaced there. Thus, this weblog article is largely derived from my Hackers list posting and its followups because the issues I cited then are still relevant now.







The news item then was a 2014 Wired story titled 10 questions about Nasa's 'impossible' space drive answered (http://www.wired.co.uk/news/archive/2014-08/07/10-qs-about-nasa-impossible-drive) and it referenced a paper titled Anomalous Thrust Production from an RF Test Device Measured on a Low-Thrust Torsion Pendulum (https://ntrs.nasa.gov/search.jsp?R=20140006052) by D. Brady, H. White, P. March, J. Lawrence, and F. Davies. The paper provided details about the test rig and about the objects being tested. The test procedures seemed, at first reading, to be careful and controlled. The more recent paper about the EmDrive that is inspiring me to post is titled Measurement of Impulsive Thrust from a Closed Radio-Frequency Cavity in Vacuum (http://arc.aiaa.org/doi/10.2514/1.B36120) by Harold White, Paul March, James Lawrence, Jerry Vera, Andre Sylvester, David Brady, Paul Bailey again seems to be a very careful study.







However, there is a significant physical effect that the vacuum isolation chamber itself is likely causing and that the experimenters are misunderstanding as a "reactionless" thrust result that is present in all of the test setups.







The WikiPedia article on RF resonant cavity thruster (https://en.wikipedia.org/wiki/RF_resonant_cavity_thruster) does a good and reasonable job of presenting the history of testing on this class of so-called "reactionless" drive devices. The WikiPedia article even has a subsection titled "Radiation Pressure" that almost details the source of the errors being made in all the tests. Almost, but not quite.







Here are my observations about the Eagleworks test apparatus and procedures together with my suggestion for how to prove me wrong.







Firstly, the Eagleworks have done some COMSOL modeling of the electromagnetic response of the test objects. However, the modeling seems to assume free-space conditions for the test object and I am unconvinced that they are modeling the resonant response of the test objects together with the electromagnetic environment in which they are being tested. Which leads to the next observation.







Secondly, the tests were carried out within an enclosed, conductive chamber. I know from my own (1D) experience with laser tubes and optical resonators that electromagnetic emissions within an enclosed high Q-factor cavity can set up some counterintuitive behaviors. For example, a HeNe laser cavity that is not energized can serve as a very highly reflecting "filter" to an externally applied pulse that has its center frequency tuned to the resonance on the tube -- if the pulse is short enough in duration. The reason for the transient reflective properties of such a laser tube is that the light entering the cavity within the tube takes a while to "ring up" to a strong enough resonance that the quantity of light exiting the far end of the tube becomes significant. This is sort of the inverse example of what I'm going to describe, but it is one that I have direct experience with, so I'm including it. The transmissive properties of the laser's cavity depend on its resonant Q being able to overcome the reflectance of its mirrors. If the cavity is a poor resonator, then the mirrors bounding it are just that -- mirrors. In which case, incoming light is reflected from the laser without passing through it. Perhaps this is a poor example which does not serve to illustrate well. A better example is as follows.







I know of persons who have experimented with cell phones enclosed within metal pipes -- both ends sealed by screw on caps -- who have found that the cell phones can still communicate with the outside world. This happens because the EM radiation trapped within the pipe's enclosure "rings up" to a very high intensity such that the rate of loss from the trapped fields balances with the rate of generation of the radiation within the enclosure. A significant portion of the losses are in fact radiation escaping from the non-perfect seals of the enclosure, while the rest are resistive losses of the walls of the pipe. The solution to the high intensity radiation field that causes the EM leakage is the inclusion of an absorber within the enclosure to reduce its Q factor. By reducing the cavity's Q-factor the internal resonances are prevented from building up to the point where a significant RF signal can get out of the shielded chamber (pipe). Conversely, the inclusion of damping material will prevent incoming RF from leaking in in small quantities and building up to a large enough resonant level that the cell phone inside the pipe can "take the call".







Finally, in the case of the EmDrive (and similar), if the RF fields inside the vacuum test enclosure itself are allowed to build up -- because the enclosure acts like an electromagnetic "hall of mirrors" within which the test object is placed -- by a factor of the enclosure's Q, which is probably on the order of 10^4, then we can account for the observed forces produced by the test objects. And here is where the irony lies.







In the WikiPedia article on RF resonant cavity thruster (https://en.wikipedia.org/wiki/RF_resonant_cavity_thruster), in the subsection on Radiation Pressure, we have the following material. It is a rationale for the apparently observed thrust by one of the inventors, Roger Shawyer:







Shawyer has suggested thrust is caused by a radiation pressure imbalance between the two faces of the cavity. He gave a presentation on this at the International Astronautical Congress 2014, later publishing it in the peer-reviewed Acta Astronautica. In it he wrote, In an EmDrive engine, microwave energy is converted to mechanical force according to the thrust equation, derived from the basic radiation pressure equation: F = 2 P0 / c. Shawyer's thrust equation, derived from Allen Cullen's equations, is given by:







where F is the force, P0 is the incident power, c is the speed of light, Qu is the unloaded Q factor of the cavity, {lambda}0 is the wavelength of the microwaves in free space, and {lambda}g1 and {lambda}g2 are the wavelengths at the end of the largest and smallest cross-section, respectively.



Shawyer insists the EmDrive is an open system. However, physicists point out that relying only on special relativity, without emitting anything and with no interaction with an outside field or matter, makes his drive a closed system. Since the two end plates are part of the thruster and the microwaves are trapped inside the cavity, standard Einstein–Maxwell equations and the conservation of momentum show no effective thrust can occur due to any force on the cavity caused by internal electromagnetic energy.







Unfortunately, in his analysis here, Shawyer was intending to discuss the interactions of the electromagnetic radiation on the inside of the test object, not that portion of it escaping out of the test object and into the interior of the test chamber itself! He was on the right track, he just did not apply his reasoning to the external cavity that the test objects were encased within. Let us take a whack at that now.







By the equation given above, F = 2 P0 / c, electromagnetic radiation gives a force of 6.7*10^-9 N/W.







The test objects were experiencing a force of about 10^-6 N per Watt of input power, which is about 300 times higher than the expected "raw" thrust one would expect from just the recoil from the EM radiation emitted from the test objects (assuming it all went in one direction).







However, the environment that the test objects are in is an enclosed conductive cavity that will have a complicated structure of high intensity standing waves built up in it by any radiating source placed within. Worse yet, the newest incarnation of the test setup has an automatic tuning circuit that is used to find the frequency of highest power resonant behavior of the test ensemble. Which is to say, if there is any leakage power between the interior of the test object and the interior of the test chamber then the automatic tuner will tune the system "correctly" to match that coupled resonance. None of this cavity coupling is being accounted for because if it were we'd see some form of effective absorber incorporated into the test chamber.







I expect that the "thrust" effects that they are seeing are the result of the intensified microwave radiation field within the test chamber itself. If the test chamber's cavity Q is on the order of several thousand (not unreasonable, given that microwave ovens have a Q something like 50,000) then the resulting electromagnetic field intensity will be correspondingly higher than the few Watts they are inputting into the device. The intense field is acting very much like the fields in an optical tweezer -- pushing the test object around in the gradients of the (complicated) standing wave pattern. Therefore, the resulting "thrust" is simply an electromagnetic force acting between the object under test and the walls of the enclosure.







This assessment of the observed force's origin is buttressed by the reports in the 2014 work that the only time there was no force was in the case where a 50 Ohm resistor was used in place of the test object. That resistor would supply "proper termination" to the RF feed and kill most of the leakage from the test object into the test chamber.







If they were to include a few square feet of absorbing material on the interior walls of the test enclosure I'd bet that the "thrust" effects they are reporting would diminish by about 99%.







Back in 2014 I tried to contact Harold White to make my suggestions for damping the test chamber. I telephoned and left voice mail, I sent email (titled Of Space Drives and Stabilized Lasers), I even asked friends at NASA to "hunt him down" and relay what I've presented here. No effect. I've waited for over two years for someone, anyone, to do the test of reducing the cavity Q of the test chamber, or even just suggest doing it. No effect.







Eagleworks: I want to be proven wrong. Do a test run with a proper RF absorber in the test chamber environment and see what happens.

