Also called ionic wind, electrohydrodynamic thrust was first recognized in the 1960s. Ionic thrusters are simple in design: They feature one thin copper electrode, known as an emitter, and one thicker tube of a metal-like aluminum called a collector. A lightweight frame supports the wires, which connect to an electrical power source, and keeps them apartthe gap between them is vital to creating ionic wind.
When voltage is applied to the wires, the resulting field gradient pulls electrons away from surrounding air molecules, ionizing them. The ionized air molecules are strongly repelled by the emitter and strongly attracted to the collector. As they move toward the collector, they push the other air molecules around them, creating thrust.
Lightweight lifters made of balsa, aluminum foil, and wire can be powered by ionic wind, called ionocrafts, and they are easy to make and cool to watch. But most people have assumed this kind of thrust is too inefficient for large aircraft. Steven Barrett, assistant professor of aeronautics and astronautics at MIT, has now shown that ionic thrusters may, in fact, be perfect for aerospace applicationsespecially, he says, for surveillance vehicles.
"I first had the idea as an undergrad," Barrett says, "because it was interesting to me that hobbyists were making small lifters, which showed this worked on some level. And I found out that these hobbyists were all wondering if it could be efficient enough to power a larger craft." He picked up the project again when he became a faculty member and had more creative freedom.
Why pursue ionic thrusters? For one thing, Barrett says, they have the potential to outperform current jet engines. In a series of experiments in which Barrett fed electricity to a simple ionocraft attached to a digital scale, which allowed him to measure the exact thrust produced each time the craft left the ground, the model produced 110 newtons of thrust per kilowatt, versus a jet engine's 2 newtons. Ionic thrusters are silent and, because they give off no heat, completely invisible to infrared sensors.
The system was most efficient at a low velocity, but Barrett explains that this is actually a positive. "You want to produce the most thrust you can at the lowest velocity," he says. The amount of kinetic energy producedenergy that's totally wastedis half the amount of thrust times the jet velocity. So to waste less energy, you want to make a larger thruster with a lower velocity. "That's why jet engines have gotten bigger over time," Barrett says. "The larger, lower-velocity jet is a more efficient way to produce thrust."
However, thrust densitythe amount of thrust produced per given areapresents a major obstacle for ionic windpowered jets. The power of an ionic thruster increases with the space between the electrodes, because this allows a buildup of more air molecules. To lift a plane, you'd need a thruster that took up the entire planenot exactly an ideal design solution for, say, an airliner.
"The most fundamental question was whether this could be efficient," Barrett says, "and the answer is yes, in theory. So the next most fundamental question is whether you can produce enough thrust per volume for the technology to be useful, and we don't know that yet."
Producing the voltage needed to get such a plane airborne would also be a concern. While tiny balsa models need only a few kilovolts to produce enough ionic wind to take over, Barrett estimates that a small jet could need thousands. And some kind of battery would be needed on board to supply continuous power, adding to the weight of the craft.
David McGrath, director of systems engineering for aerospace contractor ATK, told PM that he finds the idea of using ionic thrusters to be an interesting one. "It's nice to see some detailed testing being done to characterize the effect," he says. While he thinks that applications are "iffy" given the limited research, he says "for small unmanned aerial vehicles, it might work."
Barrett says the same. And while he doesn't expect to figure out how the technology could be commercially viable anytime soon, he's not ready to give up on wider applications. "In the near term, the best application would be for UAVs," he says, "as they have a large surface area on which to fit the propulsion device. Longer term, it's harder to say. We don't yet know how well it scales upso it might scale up to much, much larger applications."