EHD Thrusters

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A simple ionocraft, generating 15 gram-force (0.147 Newtons) of thrust at 54kV
A simple ionocraft, generating 15 gram-force (0.147 Newtons) of thrust at 54kV

EHD thruster stands for electrohydrodynamic thruster. This is the general and most appropriate term used for high voltage devices that propel air or other fluids, to achieve relative motion between the propulsion device and the propelled fluid. EHD thrusters, unlike the related ion thruster family, do not need to carry their own supply of propellant gas, although they still need to carry their own electrical power source or generator. Also, unlike related propulsion devices, they need a fluid for their operation and cannot operate in space or vacuum.

Contents

What is an EHD thruster

An EHD thruster is a propulsion device based on ionic fluid propulsion. The principle of ionic (air) propulsion with corona-generated charged particles has been known since the earliest days of the discovery of electricity, with references dating back to year 1709 in a book titled Physico-Mechanical Experiments on Various Subjects by Francis Hauksbee. The first publicly demonstrated tethered model was developed by Major De Seversky in the form of an Ionocraft, a single stage EHD thruster, in which the thruster lifts itself by propelling air downwards in accordance with Newton's laws of motion. De Seversky contributed much to its basic physics and its construction variations during the year 1960 and has in fact patented his device U.S. Patent 3,130,945 , April 28 1964). Only electric fields are used in this propulsion method. The basic components of an EHD thruster are two: an ioniser and an ion accelerator. Ionocrafts form part of this category, in which the number of stages is one, and the ionised fluid is air, but their energy conversion efficiency is severely limited to less than 1% by the fact that the ioniser and accelerating mechanisms are not independent. Unlike the ionocraft, within an EHD thruster, the air gap in its second stage is not restricted or related to the corona discharge voltage of its ionising stage. Also, EHD thrusters are not restricted to air as their main propulsion fluid, and work perfectly in other fluids, such as oil.

EHD thruster operation

The first stage consists of a powerful ioniser which, when supplied by high voltage in the kilovolt to megavolt range, ionises the intake fluid into ion clouds which flow into the second stage of the device. The second stage consists of one or multiple stages of ion accelerators, powered by voltages in the kilovolt or megavolt range, in which the ionised fluid is moved on a straight path along the length of the accelerating unit. Movement of the ion clouds can be electronically controlled to increase the effective efficiency. Within this path, the ions travel at a constant drift velocity and multiple impacts occur with the neutral fluid molecules present in the accelerating unit, which is open to the surrounding fluid. In accordance with Newton's Third Law of motion, the thruster will be acted upon by an equal and opposite force to the total force exerted by the ions over the neutral fluid within the second stage.

Optionally, the temperature, pressure and fluid constituents may be synthesised within the accelerating stage to increase the efficiency of momentum transfer between the charged ions and the neutral fluid molecules. The charged ions are then neutralised on their exit from the second stage. The electrical to mechanical conversion efficiency is equal to the ratio of the velocity of the neutral fluid to that of the moving ions. In a single stage EHD thruster, this ratio is typically equal to 1 m/s:100 m/s or 1%. A well engineered EHD thruster can achieve a much higher degree of electrical to mechanical conversion efficiency with the correct design parameters, indeed very close to 100%. The remaining losses would be mainly due to the mechanical drag of the thruster physical structure.

An EHD thruster, or the single stage ionocraft, does not require any combustion or moving parts. The term "Ionocraft" dates back to the 1960s, an era in which EHD experiments were at their peak. In its basic form, it simply consists of two parallel electrodes, one in the form of a fine wire and another which may be formed of either a wire grid, tubes or foil skirts with a smooth round surface. When such an arrangement is powered up by high voltage in the range of a few kilovolts, it produces thrust. The ionocraft forms part of the EHD thruster family, but is a special case in which the ionisation and accelerating stages are combined into a single process, and the fluid is exclusively air.

EHD thrusters require many safety precautions due to the high voltage required for their operation, and also the risk of premature death from heart or lung disease due to the inhalation of their ionised air product, ozone. A large subculture has grown up around this simple EHD thrusting device and its physics are now known to a much better extent.


Simple Ionocraft construction

Typical ionocraft construction
Typical ionocraft construction

A simple triangular shaped ionocraft derivative, also known as a lifter, can be easily constructed by anyone with a minimal amount of technical knowledge. The model in its simplest form has the shape of an equilateral triangle with sides generally between 10 and 30 cm. They basically consist of three parts, the corona wire (or emitting wire), the air gap (or dielectric fluid), and the foil skirt (collector). The electrical polarities of the emitting and collecting electrodes can be reversed. All of this is usually supported by a lightweight balsa wood, plastic drinking straws or other light electrically isolating frame so that the corona wire is supported at a fixed distance above the foil skirt, generally at an air gap of more than 1 mm per kilovolt. The corona wire and foil should be as close as possible to achieve a saturated corona current condition which results in the highest production of thrust. Failure to observe the minimum air gap distance will result in air dielectric breakdown and cause arcing, which can destroy both thruster and power supply.

Ionocraft components

The corona wire

The corona wire is usually, but not necessarily, connected to the positive terminal of the high voltage power supply. In general, it is made from a small gauge bare conductive wire. The thinner the wire (the higher the wire gauge number), the lower is the corona inception voltage and the larger the corona current. The corona wire is so called because of its tendency to emit a purple corona-like glow while in use. This is simply a side effect of ionization. Excessive corona is to be avoided due to the associated health hazards for excess of inhalation of ozone produced by ionisation.

The air gap

The air gap is simply that, a gap of free flowing air between the two electrodes that make up the structure of an ionocraft.

The air gap is a vital necessity to the functioning of this device as it is the medium through which momentum is being transferred during the operation. Saturated current conditions occur at an air gap of about 1 mm to every kV.

The collector

The collector may take various shapes, as long as it results in a smooth equipotential surface underneath the corona wire. Variations of this include a wire mesh, parallel conductive tubes, or a foil skirt with a smooth round edge. The foil skirt collector is the most popular for small models, and is usually, but not necessarily, connected to the negative side of the power supply. It is usually conveniently made from cheap, lightweight kitchen aluminium foil.

The foil skirt is named simply because it is shaped much like a skirt, and is made from aluminium foil. It is by far the most fragile part, and must not be crumpled to work properly. Any sharp edges on the skirt will degrade the performance of the thruster, as this will generate ions of opposite polarity to those within the thrust mechanism.

Reversing the polarities of the corona wire with that of the foil does not alter the direction of motion. Thrust will be produced regardless of whether the ions are positive or negative. For positive corona polarity, Nitrogen ions are the main charge carriers, whilst for negative polarity, Oxygen ions will be the main carriers and ozone production will be higher. The slight difference in their ion mobility, results in slightly higher thrust for the positive corona polarity case.

How they works

The generated thrust can be explained in terms of electrokinetics or, in modern terms, electrohydrodynamics propulsion and is given by the equation:

F=Id/k
where
F  is the resulting force, measured in newtons,
I  is the current flow, measured in amperes,
d  is the air gap distance, measured in metres, and
k  is the ion mobility coefficient of air, measured in m2/(V s). (Nominal value 2×10−4 m2/(V s))


In its basic form, the ionocraft is able to produce forces great enough to lift about a gramme of payload per watt, so its use as a vertical thruster is restricted to a tethered model. Ionocrafts capable of payloads in the order of a few grams usually need to be powered by power sources and high voltage converters weighing a few kilograms, so although its simplistic design makes it an excellent way to experiment with this technology, it is unlikely that a fully autonomous ionocraft for vertical take off will be made with the present battery technology. Further study in electrohydrodynamics, however, show that different classes and construction methods of EHD thrusters and hybrid technology (mixture with lighter than air techniques as those shown on Blaze Labs Research website, can achieve much higher payloads or thrust-to-power ratios than those achieved with the simple lifter design. Practical limits can be worked out using well defined theory and calculations such as those given on the 'Ionocraft mathematical analysis and design solutions' paper (see external links). Thus, a fully autonomous EHD thruster is theoretically possible.

When the ionocraft is powered up, the corona wire generates a very high electric field gradient. The user must be extremely careful not to touch the device, as it can give a nasty shock. At extremely high current, well over the amount usually used for a small model, contact could be fatal. When the corona wire is at approximately 30 kV, it causes the air molecules nearby to become ionised by stripping the electrons away from them. As this happens, the ions are strongly repelled away from the anode but are also strongly attracted towards the collector, causing the majority of the ions to begin accelerating in the direction of the collector. These ions travel at a constant average velocity termed the drift velocity. Such velocity depends on the mean free path between collisions, the external electric field, and on the mass of ions and neutral air molecules.

The fact that the current is carried by a corona discharge (and not a tightly-confined arc) means that the moving particles are diffusely spread out into an expanding ion cloud, and collide frequently with neutral air molecules. It is these collisions that create a net movement. The momentum of the ion cloud is partially imparted onto the neutral air molecules that it collides with, which, being neutral, do not eventually migrate back to the second electrode. Instead they continue to travel in the same direction, creating a neutral wind. As these neutral molecules are ejected from the ionocraft, there are, in agreement with Newton's Third Law of Motion, equal and opposite forces, so the ionocraft moves in the opposite direction with an equal force. There are hundreds of thousands of molecules per second ejected from the device, so the force exerted is comparable to a gentle breeze. Still, this is enough to make a light balsa model lift its own weight. The resulting thrust also depends on other external factors including air pressure and temperature, gas composition, voltage, humidity, and air gap distance. The heavier and denser the gas, the higher the resulting thrust.

Patents and publications

U.S. Patent 3,120,363  — Flying apparatus — G.E. Hagen
U.S. Patent 3,120,363  — Flying apparatus — G.E. Hagen

Below are patents and publications related to ionocrafts and electrohydrodynamic apparatus intended for flight operation.

American patents

Issued

Applications

Non-American patents
  • GB300311 — A method of and an apparatus or machine for producing force or motion — T. T. Brown
Journals and articles
  • Talley, R .L., "Twenty First Century Propulsion Concept". PLTR-91-3009, Final Report for the period Feb 89 to July 90, on Contract FO4611-89-C-0023, Phillips Laboratory, Air Force Systems Command, Edwards AFB, CA 93523-5000, 1991.
  • Tajmar, M., "Experimental Investigation of 5-D Divergent Currents as a Gravity-Electromagnetism Coupling Concept". Proceedings of the Space Technology and Applications International Forum (STAIF-2000), El-Genk editor, AIP Conference Proceedings 504, American Institute of Physics, New York, pp. 998-1003, 2000.
  • Tajmar, M., "The Biefeld-Brown Effect: Misinterpretation of Corona Wind Phenomena". AIAA Journal, Vol 42, pp 315-318 2004.
  • DR Buehler, Exploratory Research on the Phenomenon of the Movement of High Voltage Capacitors. Journal of Space Mixing, 2004
  • FX Canning, C Melcher, E Winet, Asymmetrical Capacitors for Propulsion. 2004.
  • GV Stephenson The Biefeld Brown Effect and the Global Electric Circuit. AIP Conference Proceedings, 2005.

References


External links

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