MIT researchers study electro-hydrodynamic thrust
Imagine an aircraft that is silent, invisible to infrared detectors, has zero emissions and can hover in an eerie manner that helicopters can’t. Now imagine it coming from technology currently used to suck dust out of living room air. That’s what a team of researchers at MIT is doing. They've conducted a study that indicates that ionic thrusters, currently a science fair curiosity, might one day take to the skies.
Ionic thrusters sound like something you’d find on a spacecraft, and the principle is similar to that of the ion drives being developed by NASA and other space agencies. However, where an ion drive works like a rocket in the vacuum of space, an ionic thruster is more like a jet engine.
If you want to see an ionic thruster in action, just have a look at one of those electrostatic dust collectors found in many homes. These work on the very simple idea of using an electrostatic charge to pull dust motes out of the air and collect them on metal panels. What does this have to do with flying? Put your hand against the grille of the dust collector and you’ll feel a very slight breeze – despite the fact that the collector has no moving parts. What’s moving it? Ionic wind.
The proper name for “ionic wind” is ElectroHydroDynamic (EHD) thrust. It’s been known since the 18th century that electricity can kick up a tiny air movement, but it wasn't until the 1960s that EHD was identified and developed by scientists and engineers such as air pioneer Major Alexander Prokofieff de Seversky, who developed much of the physics and patented the basic technology.
Severskey used EHD to propel what he called an “ionocraft,” which are still built by students and hobbyists to this day. It works by using an negative anode to charge air particles. These charged particles or ions are drawn down to a positively charged cathode. As the ions move toward the cathode, they bump into other air molecules and push them down, creating the ionic wind.
In a working model, such as the one used by the MIT team, the anode is called the “emitter” and is made from a thin copper electrode. The cathode is of a thicker aluminum tube called a “collector.” These are mounted with a gap between them using a very light framework and powered by means of a wire connected to an outside electricity source.
Seeing an ionocraft in flight is slightly unnerving. Ionocraft aren't very large, being little more than bench top models, but when they take off, they don’t make a sound. Instead, they float up and hover on the wispy breeze forced down by the ion stream. The ionocraft can even be steered by varying the voltage, to turn and tip it like a helicopter.
In the ‘60s, the ionocraft seemed like a revolution in aviation. There was talk about them being used in all sorts of small aircraft, and the military were interested because ionocraft give off no heat, so there’s no infrared signature. Ionocraft were seen as replacing helicopters, as silent commuter ferries, as craft capable of operating at the edge of space, as traffic monitors or anti-missile platforms.
The problem was, the technology didn't scale very well. What worked for a small model that was built like a kite didn't do at all well as the ionocraft got bigger. It couldn't even carry its own power supply, so it wasn't long before ionic thrusters became the denizens of science fairs and the obsession of anti-gravity cultists.
Where MIT came in was at the point that the researchers realized that very few rigorous studies of ionic wind as a viable propulsion system had ever been carried out, and exactly what the ionic thruster is capable of hadn't been measured. So, they devised a test where an ionocraft was hung under a digital scale and tens of thousands of volts with enough amperage to run a light bulb were run through the craft.
The results were surprising. The team discovered that the ionic thruster turned out to be remarkably efficient compared to, for example, jet engines. Where a jet produces two newtons of thrust per kilowatt, the ionic thruster punched out 110 newtons per kilowatt. Furthermore, the thruster was most efficient at low thrust, which meant that power wasn't being wasted.
“It’s kind of surprising, but if you have a high-velocity jet, you leave in your wake a load of wasted kinetic energy,” said Steven Barrett, an assistant professor of aeronautics and astronautics at MIT. “So you want as low-velocity a jet as you can, while still producing enough thrust.”
Despite these promising findings, don’t expect to see any ionocraft in the skies soon. One problem with ionic propulsion is that even with its remarkable efficiency, it requires incredible amounts of voltage. Even a small craft would need megavolts to lift it, so a lot of work needs to be done to build up thrust while bringing down powerplant weight.
However, the characteristics of the ionic thruster means that increasing its thrust means increasing the gap between the anode and cathode. For an ionocraft to get off the ground with its own power supply and payload, the engine would need to be so large that the craft would be inside the engine. What that means is that an ionocraft would probably be large, round, carry its workings and payload in a bulgy middle section, and take off in vertical silence.
In other words, we might one day see flying saucers.
The team’s results were published in the Proceedings of the Royal Society.