September 26, 2009

Are EMdrives Really Asymmetric Capacitor Thrusters ?

This site has covered the EMdrive before.

The EmDrive is highly controversial research in propulsion which would be reactionless and uses superconducting cavities.

Rocketeer comments: Here's my take on it. It "works" after a fashion (actually generates thrust), but not in the way that Shawyer thinks it does. It's actually a form of Asymmetric Capacitor Thruster (ACT), which generates thrust by the Biefeld-Brown effect -- charged metal surfaces ionise the surrounding air by corona discharge, and create an '"ion wind" which pushes the apparatus along. Good for the continued integrity of the laws of physics, but bad for space applications of the EmDrive, because in a vacuum it would do precisely nothing.

Shawyer claims that this and other effects have been taken into account.

"Stray electromagnetic effects were eliminated by using different test rigs, by testing two thrusters with very different mounting structures, and by changing the orientation by 90 degrees to eliminate the Earth’s magnetic field," he writes. "Electrostatic charges were eliminated by the comprehensive earthing required for safety reasons, and to provide the return path for the magnetron anode current."

When I asked him about the possibility of "ion wind" being the real cause of thrust, Dr Shawyer patiently explained that it had been addressed at a very early stage:

"Air currents from whatever source were eliminated in the first Proof of Concept project by testing the experimental thruster mounted in a hermetically sealed box. The experiment was reviewed and accepted by professional government scientists." [The research was being supported by the British government at the time.]

He also points out that real ion drives need much higher voltage and that "Anyone who thinks they can create grammes of thrust from ion wind at the voltages we work at clearly doesn’t understand physics." He does not believe a vacuum chamber test would show anything, as ion drives function in a vaccum and there would still be the question of wehther some ionised material was somehow being ejected. However, the hermetically sealed box test should have negated that possibility.

Understanding Asymmetric Capacitor Thrusters
Biefeld-Brown effect at wikipedia

The Biefeld–Brown effect is an effect that was discovered by Paul Alfred Biefeld (CH) and Thomas Townsend Brown (USA). The effect is more widely referred to as electrohydrodynamics (EHD) or sometimes electro-fluid-dynamics, a counterpart to the well-known magneto-hydrodynamics. Extensive research was performed during the 1950s and 1960's on the use of this electric propulsion effect during the publicized era of the United States gravity control propulsion research (1955 - 1974). During 1964, Major Alexander Procofieff de Seversky had in fact published much of his related work in U.S. Patent 3,130,945, and with the aim to forestall any possible misunderstanding about these devices, had termed these flying machines as ionocrafts. In the following years, many promising concepts had to be abandoned due to technological limitations and were forgotten. The effect has only recently become of interest again and such flying devices are now known as EHD thrusters. Simple single-stage versions lifted by this effect are sometimes also called lifters.

23 page pdf NASA study of Asymmetrical Capacitors for Propulsion

Asymmetrical Capacitor Thrusters have been proposed as a source of propulsion. For over eighty years it has been known that a thrust results when a high voltage is placed across an asymmetrical capacitor, when that voltage causes a leakage current to flow. However, there is surprisingly little experimental or theoretical data explaining this effect. This paper reports on the results of tests of several Asymmetrical Capacitor Thrusters (ACTs). The thrust they produce has been measured for various voltages, polarities, and ground configurations and their radiation in the VHF range has been recorded. These tests were performed at atmospheric pressure and at various reduced pressures. A simple model for the thrust was developed. The model assumed the thrust was due to electrostatic forces on the leakage current flowing across the capacitor. It was further assumed that this current involves charged ions which undergo multiple collisions with air. These collisions transfer momentum. All of the measured data was consistent with this model. Many configurations were tested, and the results suggest general design principles for ACTs to be used for a variety of purposes.

A series of careful tests have been performed on Asymmetrical Capacitor Thrusters
(ACTs). In the past, several mechanisms have been proposed for the thrust that they produce. These mechanisms were considered, both on theoretical grounds and by comparison with test results. All of the mechanisms considered were eliminated except one. A simple model was developed of ions drifting from one electrode to the other under electrostatic forces, and imparting momentum to air as they underwent multiple collisions. This model was found to be consistent with all of our observations. It predicted the magnitude of the force (thrust) that was measured. It also predicted how the direction of the thrust changed when the location of the ground wire changed. Furthermore, it also predicted that the direction of the thrust was independent of the polarity of the applied voltage. Finally, it qualitatively predicted how the magnitude of the thrust varied as the design of the ACT (its shape, etc.) varied, over many such design changes. It may be concluded that that the ion drift model explains how a thrust is developed by ions pushing on air. Tests were also performed in nitrogen and argon, and were performed at reduced pressures. A thrust was also produced at moderately reduced pressures, when the ACT produced a current flow without causing a breakdown of the air or other gas. In spite of decades of speculation about possible new physical principles being responsible for the thrust produced by ACTs and lifters, we find no evidence to support such a conclusion. On the contrary, we find that their operation is fully explained by a very simple theory that uses only electrostatic forces and the transfer of momentum by multiple collisions.

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