Ionic Thrusters Offer Quiet Flight

Gizmag and Science Daily both covered a propulsion system that’s been with us for many decades, but which is just now seeing practical applications in space flight, and may be adapted to terrestrial winged vehicles.

Your editor might have passed it over as overhyped, but the research came from the Massachusetts Institute of Technology (MIT) and was published in The Proceeding of the Royal Academy – two good indicators of veracity.

Jennifer Chu of MIT’s News Office explains, “When a current passes between two electrodes — one thinner than the other — it creates a wind in the air between. If enough voltage is applied, the resulting wind can produce a thrust without the help of motors or fuel.”  That phrase, “If enough voltage is applied…” is a significant qualification.

“Electrohydrodynamic thrust,” or “ionic wind” has been known since the 1960s, but limited to hobbyists and science fair projects.  This video from China demonstrates the use of electric thrusters to guide an indoor airship.

For years, such “engines,” that use two or more electrodes to ionize the ambient fluid and create an electric field, were considered to produce only low thrust levels – useful for models, but not practical for larger craft.  Researchers did not seriously look at large scale applications until recently.

Kento Masuyama and Steve Barrett found that such thrusters may turn out to be more efficient power producers than even conventional jet engines – by a factor of 55.  Concluding that ionic wind produces 110 Newtons of thrust (24.73 pounds-force) per kilowatt, compared to a jet engine’s 2 Newtons per kilowatt, Barrett, an assistant professor of aeronautics and astronautics at MIT, thinks the thrusters might be able to power “small, lightweight aircraft.”

The thrusters have other advantages, including silent operation, no heat generation, and infrared invisibility – excellent characteristics for a surveillance vehicle.

Thrusters are simple devices, but the high voltage and current make them potentially hazardous for home experimenters.  Watch this how-to-do-it video before getting out the aluminum foil and 30-gauge wire.  It includes safety tips that may make you reconsider allowing your kids to do this for a science fair.

MIT’s press release explains the construction and operation of the device.  “A basic ionic thruster consists of three parts: a very thin copper electrode, called an emitter; a thicker tube of aluminum, known as a collector; and the air gap in between. A lightweight frame typically supports the wires, which connect to an electrical power source. As voltage is applied, the field gradient strips away electrons from nearby air molecules. These newly ionized molecules are strongly repelled by the corona wire, and strongly attracted to the collector. As this cloud of ions moves toward the collector, it collides with surrounding neutral air molecules, pushing them along and creating a wind, or thrust.”

Barrett and Masuyama “hung the contraption under a suspended digital scale. They applied tens of thousands of volts, creating enough current draw to power an incandescent light bulb. They altered the distance between the electrodes, and recorded the thrust as the device lifted off the ground. Barrett says that the device was most efficient at producing lower thrust — a desirable, albeit counterintuitive, result.”

“”It’s kind of surprising, but if you have a high-velocity jet, you leave in your wake a load of wasted kinetic energy,’ Barrett explains. ‘So you want as low-velocity a jet as you can, while still producing enough thrust.’ He adds that an ionic wind is a good way to produce a low-velocity jet over a large area.”

Even small balsa thrusters require large openings and high voltages to achieve liftoff.  The large air gap required might “encompass the entire aircraft.”  Barrett thinks the voltages required would represent another hurdle.

“The voltages could get enormous,” Barrett says. “But I think that’s a challenge that’s probably solvable.” For example, he says power might be supplied by lightweight solar panels or fuel cells. Barrett says ionic thrusters might also prove useful in quieter cooling systems for laptops.

Ned Allen, chief scientist and senior fellow at Lockheed Martin Corp., thinks that despite their shortcomings, the technology “offers nearly miraculous potential.”

Allen notes, “[Electrohydrodynamic thrust] is capable of a much higher efficiency than any combustion reaction device, such as a rocket or jet thrust-production device,” explaining Lockheed Martin’s review of the technology for future propulsive needs.

“Efficiency is probably the number one thing overall that drives aircraft design,” Barrett says. “[Ionic thrusters] are viable insofar as they are efficient. There are still unanswered questions, but because they seem so efficient, it’s definitely worth investigating further.”

Masuyama and Barrett’s paper has been published In the Proceedings of the Royal Society under the title, “On the Performance of Electrohydrodynamic Propulsion.”

The abstract synopsizes the distinctions between single-stage and dual-stage thrusters. “We characterize the performance of EHD thrusters of single- (SS) and dual-stage (DS) configurations. SS thrusters refer to a geometry using one emitter electrode, an air gap and a collector electrode with large radius of curvature relative to the emitter. DS thrusters add a collinear intermediate electrode. SS thruster performance was shown to be consistent with a one-dimensional theory. Increasing the gap length requires a higher voltage for thrust onset, generates less thrust per input voltage, generates more thrust per input current and most importantly generates more thrust per input power. A thrust-to-power ratio as high as approximately 100 N kW−1was obtained. DS thrusters were shown to be more effective than their SS counterparts at producing current, leading to a smaller total voltage necessary for producing equal thrust. However, losses involving ion collection at the intermediate electrode led to reduced thrust-per-power compared with the SS thruster of equal length.

The high voltages involved would require huge advances in battery, supercapacitor or fuel cell technology, and might even require some form of nuclear power.  Note the Chinese thrusters are merely nudging a light-weight “blimp” around, and consider that a good sailplane has a 50 to 1 lift-to-drag ratio.   Such a machine would require only 1/50th the thrust required for levitation to keep it in level flight, and whatever additional power for takeoff and climb – certainly lower than lifting its entire weight vertically.  Otherwise, as some have speculated, ionic thrust machines may end up looking very much like flying saucers.

One kilowatt, possible with a small battery pack, could theoretically help the extreme example sailplane maintain level flight.  The dream of truly silent flight, if a configuration for the thrusters could be realized that would not add drag or affect control, could be a plausible reality.

Since this may be wildly speculative, comments and critiques are welcome and encouraged.