Electrohydrodynamic thruster
EHD thruster stands for
electrohydrodynamic thruster. This is the general and most appropriate term used for high voltage propulsion devices. EHD thrusters, unlike the related
ion thruster family, do not need to carry their own gas supply, although they still need to carry their own electrical power source or generator. Also, unlike related propulsion devices, they need the atmosphere for their operation and cannot operate in space or vacuum.
An EHD thruster is a propulsion device based on ionic air propulsion, that works without moving parts, using only electrical energy. 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. De Severski contributed much to its basic physics and its construction variations during the year 1960 and has in fact patented his device (patent no: US3130945,
April 28 1964). Only electric fields are used in this propulsion method. The basic components of an EHD thruster are two: an
air ioniser and an
ion accelerator. The ionocraft and the
Lifter form part of this category, 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 Lifter, 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.
The first stage consists of a powerful
air ioniser which, when supplied by high
voltage in the kilovolt range, ionises the intake air 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 air 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 gases present in the accelerating unit, which is open to the atmosphere. 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 air within the second stage.
Optionally, the temperature, pressure and gas constituents may be synthesised within the accelarating stage to increase the efficiency of
momentum transfer between the charged ions and the neutral air 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 gas to that of the moving ions. In a single stage
ionocraft type 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 air drag of the thruster physical structure.
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Spacecraft propulsion*
Blaze Labs Research - Introduction to EHD thrusters *
Make your Own Electrohydrodynamic Thruster*
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