MIT researchers have flown the first airplane that has no moving parts. The aircraft, packed with lithium-ion batteries, used an ion thruster to fly the 60 meters that were available in the indoor flight area.
“This was the simplest possible plane we could design that could prove the concept that an ion plane could fly,” said Steven Barrett, associate professor of aeronautics and astronautics at MIT. “It's still some way away from an aircraft that could perform a useful mission. It needs to be more efficient, fly for longer, and fly outside.”
The plane weighs a little over 2 kilograms (5 pounds), and its engine has a thrust-to-weight ratio roughly comparable to that of a jet engine. Its lithium-ion batteries put out about 500 watts.
Ion drive was first demonstrated 101 years ago by famed rocketeer Robert Goddard, and it’s now routinely used in space, for instance to reposition satellites. Space applications are natural because of the great thrust that can be generated while using a very small amount of propellant, always in short supply up in orbit.
“For every kilogram [of propellant] you take up in space you have to have more propellant in the launch vehicle,” Barrett said, in a press conference run by the journal Nature, which today published the study. “So you use the propellant as efficiently as you can, throwing it behind you as fast as you can. In our application it’s sort of the opposite: You want a large volume of air, and the ion wind is a good way of achieving that objective.”
The researchers produce that wind by running 40,000 volts through a number of thin electrodes at the front of the plane’s 5-meter wingspan. That strips electrons off nitrogen molecules in the air, leaving behind positively charged ions. The ions then shoot toward a second electrode at the back; on the way, they collide with millions of air molecules, pushing them along as well—hence the large volume of air.
There’s a lot of electrodes sticking out, and one of the group’s next goals is to hide them away in the wing. While they’re at it, the researchers may want to lay on photovoltaic cells, which could recharge the batteries for use in high-flying applications where the plane functions like a relay station or a surveillance post. Such “pseudosatellites,” which would fly for extended periods on automatic pilot, could benefit from the great potential reliability of a system with no moving parts.
One more possibility cannot have escaped the notice of the military: Ion thrusters make no noise and are therefore stealthy, at least to the human ear.
Don’t look for them in people-moving aircraft anytime soon. Barrett speaks of “a low number of decades, not in five years or anything like that.”
People living near airports can hardly wait for the silence.
Philip E. Ross is a senior editor at IEEE Spectrum. His interests include transportation, energy storage, AI, and the economic aspects of technology. He has a master's degree in international affairs from Columbia University and another, in journalism, from the University of Michigan.