IT IS NOT unusual for technologies from science fiction to join the real world, with examples ranging from video phones, via artificial intelligence, to self-driving cars. The ion drive is another such. Ion drives purportedly propelled the evil Empire’s TIE (twin ion engines) fighters in the original “Star Wars” movie. Real ion drives now reposition satellites and send space probes on interplanetary journeys. Surprisingly, according to a paper published this week in Nature, they may have an earthly role, too. For the first time an aeroplane has flown powered by an ion drive.
The reason for surprise is that an ion drive delivers only a tiny amount of thrust. This does not matter in the vacuum of space, because without friction to hold a spacecraft back, speed builds up gradually to a decent rate. On Earth, however, it had long been assumed that the atmosphere is too dense for an aircraft using an ion drive to get off the ground.
To be fair, the five-metre-wingspan model aircraft that Steven Barrett and his colleagues at the Massachusetts Institute of Technology (MIT) flew with an ion drive was launched with the aid of a bungee. Nevertheless, in a series of ten test flights (a time-lapse picture of one of which is shown below) it successfully traversed a 60-metre indoor sports track. This distance is comparable to that of the first flight, in 1903, of the powered heavier-than-air plane built by the Wright brothers—although theirs had a 12.3 metre wingspan and had also to carry the weight of a pilot.
Unlike the Wright Flyer, and all powered aircraft since, the use of an ion drive means the MIT craft contains no moving propulsion parts in the form of propellers or jet engines. It can fly silently and without direct emissions from burning fossil fuels. It works by using electrodes to ionise a gas in such a way that the resulting ions (charged particles) create thrust. In space, the gas to be ionised, usually xenon, has to be carried by the craft. The MIT aircraft, however, ionises nitrogen from the air.
The ion drive itself consists of strips of parallel electrodes suspended in a rack below the craft’s wing. These electrodes are powered by a pack of lithium-ion batteries. The nitrogen ions generated interact with neutral molecules in the air to form what is known as an ionic wind. The direction of this wind depends on the arrangement of the electrodes. In Dr Barrett’s design, the wind is directed backward, meaning that Newton’s third law of motion (to every action there is an equal and opposite reaction) carries the craft forward—and, crucially, does so at a speed sufficient for its wings to provide lift.
Scaling the design up will, admittedly, be tricky. Though the level of thrust generated is sufficient to fly Dr Barrett’s 2.5kg prototype, it is less than 1% of what would be required to keep an airliner airborne. There are, though, other sorts of aircraft which, with further development, ion drives might power. The MIT team reckon these include unmanned drones and high-altitude research craft.
Such devices might be built in entirely new ways. Although the test model used a conventional tail plane to provide stability and steering, Dr Barrett thinks future designs could do without it. Instead, the electrodes would shape the ionising electric fields in different ways, to control the direction of flight. Moreover, he adds, it might be possible to incorporate the electrodes into the skin of the aircraft so that there appears to be no distinctive power system. One day, this could result in something that looks a bit like a saucer, which could hover silently or zip through the air. Then another piece of science fiction would become science fact.