Ion engines are the best technology for sending spacecraft on long missions. They’re not suitable for launching spacecraft against powerful gravity, but they require minimal propellant compared to rockets, and they drive spacecraft to higher velocities over extended time periods. Ion thrusters are also quiet, and their silence has some scientists wondering if they could use them on Earth in applications where noise is undesirable.
Powered flight is noisy. Helicopters make a horrible racket, and screaming jet engines can make life near an airport almost unbearable. Even small propeller-driven aircraft are noisy. But what if ion engines could be used instead of these louder propulsion systems, at least in some applications where noise is an issue?
Steven Barrett from MIT thinks the idea has merit. Barrett is a Professor of Aeronautics and Astronautics at the Massachusetts Institute of Technology. He’s also the Director of the MIT Laboratory for Aviation and the Environment. “The aim of Steven’s research is to help aviation achieve zero environmental impacts,” the MIT website says. “This includes developing low emissions and noise propulsion technologies for aircraft…” This is where Barrett’s work on ion propulsion comes in.
Barrett’s been interested in an ion propulsion system for many years. In 2018 Barrett and colleagues published an article in the journal Nature titled “Flight of an aeroplane with solid-state propulsion.” Solid-state propulsion systems have no moving parts, so they’re very quiet. The power for flight comes from electroaerodynamics, where electricity moves ions and provides propulsion. Barrett and colleagues call the flow of ions the “ionic wind.” They’ve used it to propel a small test aircraft on steady, stable flights.
“This is the first-ever sustained flight of a plane with no moving parts in the propulsion system,” Barrett said in 2018. “This has potentially opened new and unexplored possibilities for aircraft which are quieter, mechanically simpler, and do not emit combustion emissions.”
This video from 2018 explains Barrett’s work up to that point.
So far, Barrett and his team have successfully demonstrated the concept with a 2.26 kg (5 lb) glider with a 5-meter (16.4 ft) wingspan. The wing is strung with wires like horizontal fencing. Lithium batteries in the fuselage supply current to the wires. The batteries supply a positive charge to the wires along the front and bottom of the wing, while wires along the trailing edge of the wing act as negative electrodes.
The unique battery system supplies 40,000 volts of electricity to the positive wires. The positive charges strip electrons away from air molecules, ionizing them. The newly ionized molecules are then attracted to the negative electrodes at the wing’s trailing edges. This polarity creates the ionic wind that forces air around the wings, creating lift and thrust. As the ionized molecules travel to the negative electrodes, they collide with million of other air molecules, propelling the aircraft forward.
Barrett’s been further developing the idea for a solid-state electroaerodynamic aircraft since publishing the paper in 2018. Now he’s working with the NASA Innovative Advanced Concepts (NIAC) program. In an article from Feb. 7, 2022, Barrett explained the current state of the idea.
“Advanced air mobility (AAM) is an aviation ecosystem that envisions small, electric, vertical takeoff and landing (VTOL) aircraft operations in urban areas,” he wrote. The problem with that scenario is noise: communities won’t welcome additional noise. Ion electroaerodynamics (EAD) could alleviate that problem.
EAD systems have no moving parts, so they’re nearly silent. The silence benefits several potential missions. “Example missions enabled by silent EAD propulsion include those near noise-sensitive urban communities, or time-critical delivery missions at night (e.g. for critical medical supplies) when community opposition to noise is most severe.”
Ion propulsion benefits from being silent, but it also has a drawback. It generates a low initial thrust. In space travel, this isn’t a problem. For example, NASA used a powerful conventional rocket to launch their DART mission from Earth because conventional rockets develop enough thrust to reach escape velocity. But once DART left Earth and its gravity behind, it used an ion drive for propulsion.
Barrett and his team demonstrated that an EAD aircraft could fly in sustained flight. But can one perform a VTOL flight?
Barrett thinks they can. “Novel multi-stage ducted (MSD) EAD thrusters, in which multiple EAD thruster stages are enclosed inside a duct, will be used to increase thrust enough to enable VTOL operations,” Barrett wrote in the February article. “Under this effort, we will design a VTOL-capable, near-silent aircraft powered by MSD thrusters.”
If they can bring their idea to fruition, it won’t be flying passengers any time soon. But VTOL EAD aircraft might fill other niches.
“The aircraft would enable package delivery missions in noise-sensitive areas or at night, where operations would otherwise not be allowed due to community opposition,” Barrett writes. So basically, they would be quieter drones.
Barrett’s thinking big, though. While EAD propulsion limits aircraft size at first, subsequent iterations could be larger, more powerful, and carrier heavier payloads, including passengers. “This effort would be in support of the long-term goals (1) to build and fly an MSD-powered aircraft, and (2) to assess the applicability of MSD thruster technology to other AAM use cases, including intra- and intercity passenger transport,” Barrett says.
Passenger transport is where the technology could pay off. There’s an enormous amount of air travel in the world. Collectively, air travel emitted 1.04 billion tons of CO2 in 2018, including passenger and freight. That accounted for about 2.5% of global CO2 emissions. New technologies like EAD propulsion could help with that.
With an idea like this, it’s not surprising that Barrett was a Star Trek fan. He was interested in the shuttles and how they travelled silently with no moving parts or exhaust. “This made me think, in the long-term future, planes shouldn’t have propellers and turbines,” Barrett said in 2018. “They should be more like the shuttles in ‘Star Trek,’ that have just a blue glow and silently glide.”
There’s a long way to go before EAD propulsion becomes a reality beyond quieter drones. But Barrett says just getting from concept to a limited flying prototype as proof of concept was a long road, too.
“It took a long time to get here,” Barrett says. “Going from the basic principle to something that actually flies was a long journey of characterizing the physics, then coming up with the design and making it work. Now the possibilities for this kind of propulsion system are viable.”