You might have seen in recent months, the MIT students and professor are testing “ion propelled plane”. A plane capable of sustained powered flight with no moving parts in its propulsion system. Ion-propelled aircraft, Aircraft with no moving parts in propulsion system
They have been working on it from 7 years. Just as the Wright Brothers announced to the world that powered flight was possible, this flight lays down a milestone for ion drive technology that could pave the way for future investment and development.
It has the potential to drastically improve propulsion technology. Having no moving parts is a benefit that cannot be understated. Parts can be made lighter as they no longer need to survive the stress of movement. Reduced stress means reduced maintenance and costs, but perhaps the most immediate benefit we can reduce noise from this technology.
With no noisy combustion or rotating aerodynamic surfaces stirring up the air, these planes are like gliding owls. Characteristic military contractors will be eager to take its advantage.
But with current limitations, this may take some time to come to the market. Let’s investigate just how this new technology works, and where it needs to improve in order to compete with current technology.
This technology has been in development for decades now with many spacecraft already using variations on the idea to achieve highly efficient thrust systems. These engines work on a similar principle to the ion propulsion of the MIT plane, albeit in a very different environment that lends itself to the technology.
Take the NSTAR (NASA Solar Technology Application Readiness) ion drive aboard the now-retired Dawn spacecraft. This spacecraft used xenon as a propellant because it has a high atomic mass allowing it to provide more kick per atom, while being inert and having a high storage density lending itself to long term storage on a spacecraft.
The engine releases both xenon atoms and high energy electrons into the ionization chamber, where they collide to produce a positive xenon atom and more electrons. These electrons are then collected by the positively charged chamber walls, while the positive xenon atoms migrate towards the chamber exit which contains two grids.
A positive grid called the screen grid, and a negative grid called the accelerator grid. The high electrical potential between these grids causes positive ions to accelerate and shoot out of the engine at speeds up to 145, 000 kilometers per hour.
At that speed, even the tiny xenon atoms can provide a decent bit of thrust, but even still this engine provides a maximum of 92 milli Newtons of force. About the same force a piece of paper will exert while resting on your hand. But in the vacuum of space, there is no air to sap away the precious energy we provide. With no drag or friction to remove energy we gradually build up our kinetic energy and gain speed.
The dawn spacecraft weighed about 1220 kilograms at launch with a dry mass of 750 kilograms after the propellant had been expended, so let’s say it has an average weight between the two of 1000 kilograms. Rearranging the force equals mass by acceleration equation, we can calculate the acceleration this engine could provide at 0.000092 m/s2
A tiny acceleration, but multiple by a week (604800 seconds) and our spacecraft is flying at 55.6 m/s. Multiple it by a year and it’s flying at 2898 metres per second, that’s 8.5 times. The latest generation ion drives, dubbed the NEXT engine, can produce three times the force and has been tested continuously without stopping for 6 years straight here on earth.
That’s enough force to accelerate that 1000 kilograms to 44651 m/s, 130 times the speed of sound. This is an incredible technology, that will revolutionize how we explore space in the near future, but here on earth, it has a completely different set of challenges. Like-
Air will continuously sap away any energy we input into our vehicle through drag, and so we need to create an ion drive that can provide more energy than air can remove while traveling fast enough to achieve flight. Not an easy task and the fact MIT has managed it is mind-blowing.
How MITans did it?
They first needed to optimize their plane design for the application. Reducing weight to minimize the energy required to maintain height, and minimizing drag to reduce any energy losses to the air.
They did this using something called geometric programming optimization, which allows designers to specify constraints and design criteria to a program that will then find the optimal design.
After running multiple computer simulations they settled on a plane with a 5-meter wingspan and a weight of 2.56 kilograms. It would require a flight speed of 4.8 meters per second with a thrust of 3.2 Newtons. 3.2 Newtons is vastly more than anything achieved by NSTAR or NEXT engines, but they do not work in entirely the same way.
Ion drives for space need to carry atoms to be bombarded, within earth’s atmosphere there is no shortage of atoms to ionize and accelerate and this helps counteract some of the negatives of the drag they also induce.
The planes propulsion comes from an array of ion drives carried below the wing. The positive anode was a thin steel wire, which helped minimize the drag it induced. While the cathodes were foam aerofoils covered in thin aluminum, these being light and capable of producing lift to offset their weight. In this case, nitrogen is ionized and attracted across the electric field induced by the 20,000 volts of electric potential between them.
The nitrogen ions collide with neutral air molecules along the way to provide additional thrust. Creating something called ionic wind. Getting that 20 thousand volts of alternating current is really the most difficult part and the team had to design their own lightweight high-power voltage converter to step-up the 200 volts of direct current drawn from their lithium polymer batteries.
So how does this differ from a conventional one?
Jet Engines have a very high thrust density at over 10,000 Newtons per meters squared, so we can produce a great about of force with relatively low area. This plane achieved a thrust density of 3 Newtons per metre squared, so it is generating very little force over a very large area.
The issue here is the same issue that prevents batteries from being a viable solution for planes, power requirements do not scale linearly with the mass of the plane, they increase with the square of the mass. You can find about this in detail here: While the power requirements to overcome drag increases with the cube of the velocity. So our ion propulsion power will need to scale with it, but we cannot simply hang racks and racks of these electrodes beneath our plane.
They, along with the structures required to support them would cause far too much drag, and in turn, flight surfaces would need to scale to counteract the pitching moment this would cause, causing even more drag. This technology is still in its infancy and there are engineers are working day and night over it hope you could hear some great news soon.