The vast majority of drones, regardless of how many rotors they have, steer and maintain stable flight by adjusting the relative speeds of each motor. If you want a drone to move left, you increase the speed of the motors on the right and decrease the speed of the motors on the left. That requires a lot of fine adjustments, which is why drone flight controllers constantly tweak the speeds through the motors’ ESC (Electronic Speed Control) modules to maintain a stable hover and only alter those to respond to your piloting commands. Tom Stanton wanted to see if it would be possible to use gas thrusters instead, and built a drone to try it out.
Gas thrusters are commonly used in space, where vehicles have to operate in a vacuum. That’s because there is no air for rotors to push against in order to provide thrust. Gas thrusters work because of Newton’s third law, which states that for every action, there is an equal and opposite reaction. So, as the gas is pushed out, the vehicle is also pushed away. That kind of propulsion system is rarely used in-atmosphere, however, because it isn’t efficient in overcoming air resistance. But drones need very little thrust to steer or correct for stable flight, and so Stanton thought that it might be possible to use gas thrusters to keep a drone hovering.
The drone design Stanton came up with has two counter-rotating rotors stacked on top of each other on the same vertical axis. Those provide upwards thrust, and the counter-rotation keeps the drone from simply spinning in place. But they don’t provide any pitch or roll control. For that, Stanton filled bottles with pressured air. Three nozzles were placed symmetrically around the drone and release the air when valves are opened. Those valves are opened by the flight controller through an Arduino. As Stanton demonstrates, the drone was able to hover somewhat well for several seconds. But the gas thruster design is hardly practical, as the pressure drops too quickly and the flight controller can’t predict the change in pressure — and therefore thrust. Still, it’s a very interesting experiment.