Not that we don’t love Star Trek, but the writers could never decide if ion propulsion was super high tech (Spock’s Brain) or something they used every day (The Menagerie). Regardless, ion propulsion is real and we have it today on more than one spacecraft. However, MIT recently demonstrated an ion-powered airplane. How exciting! An airplane with no moving parts that runs on electricity. Air travel will change forever, right? According to [Real Engineering], ion-propelled (full-sized) aircraft run into problems with the laws of physics. You can see the video explaining that, below.
To understand why, you need to know two things: how ion drive works and how the engines differ when using them in an atmosphere. Let’s start with a space-based ion engine, a topic we’ve covered before. Atoms are turned into ions which are accelerated electrically. So the ion engine is just using electricity to create thrust exhaust instead of burning rocket fuel.
The downside is that the thrust is very tiny. The upside is that, in space, that tiny thrust adds up so that in a few days or weeks you can be moving very fast. With no moving parts, keeping an ion engine running constantly is no real problem. Keeping a massive rocket burning fuel for months is problematic.
So the MIT flyer uses the same technology, right? Sort of. Spacecraft carry around their own ion fuel in the form of xenon (although some older engines used mercury). Xenon is good because it is relatively heavy which provides more thrust and is easy to store.
So what if you clamped an ion engine to an airplane? Well, you’ll need to put the xenon fuel tanks on it, too, which is going to make the plane heavier. You also have two problems. You need a certain minimum amount of speed to get your wings to create lift. In addition, your tiny thrust won’t add up like it does in space because of things like wind resistance. If a spacecraft’s engine stops it just stops accelerating but keeps going at its current speed and heading. If an aircraft loses power, that’s not the case.
Obviously, the MIT engineers had to create a very light airframe that could generate a lot of lift at low speeds. The resulting plane had a 5 meter wingspan and weighed less than 3 kilos. But what about fuel? A spacecraft carries their own, but an aircraft can consume nitrogen which is everywhere in the atmosphere. Sure, it doesn’t have the mass of xenon, but not having to carry your fuel is a big plus.
So why won’t you be boarding that midnight ion plane for Georgia anytime soon? Scale. The video does a good job of explaining the trades, but in the simple view a heavy plane is going to take more power and you get in a vicious spiral where more weight needs more power, but more power adds more weight.
You can see MIT’s video about their solid-state airplane (we like that name) after the first video, below. Sure, it is possible future advancements will make ion-powered aircraft more practical. But it probably won’t be in the next year or three. However, there are other ways to run an aircraft off of air, and you never know when some breakthrough will make something practical. After all, in 1950 who could imagine computers that cost a few hundred dollars and fit in your pocket?