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 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.
What is an EHD thruster
An EHD thruster is a propulsion device based on ionic fluid 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, a single stage EHD thruster, in which the thruster lifts itself by propelling air downwards. 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, 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 air ioniser which, when supplied by high voltage in the kilovolt to megavolt 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 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 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 mechanical drag of the thruster physical structure.