MIT’s AeroAstro labs is working to develop a drone that can fly at speeds up to Mach 0.8, or roughly 988 km/h. Dubbed the Firefly, the drone is essentially a mini-rocket and has a similar shape to a zeppelin. It is designed to launch from a fighter jet and collect data or distract enemy weapons systems.
The Firefly is designed to meet a challenge presented by the U.S. Air Force: develop a UAV no more than 63,5 millimetres wide and 431,8 millimetres long that could be air-launched from a fighter jet and fly at Mach 0.8 for 2 to 5 minutes. After MIT’s recent success building the Peredix swarming drones that also launch from a fighter, the team feels confident they are up to the new challenge.
“There was no vehicle with this speed, at this size, that could deploy off an aircraft,” John Hansman, MIT professor of aeronautics and astronautics, said to Aviation Week. “It is too small for a turbine and too fast for electric, while a pulse-jet presents thermal problems.”
A solid rocket presents some unique challenges, and the team had to slow the fuel burn rate to meet the requirement that the rocket fly at Mach 0.8 for 2 to 5 seconds. A typical rocket this size burns its fuel in just a few seconds, and slowing the burn rate can reduce the pressure to the point that combustion is not possible at all.
The solution was found by mixing ammonium perchlorate propellant with an oxamide inhibitor. “We use this burn-rate suppressant, which—via chemical decomposition—cools the flame and changes the flame structure so that it actually burns slower,” says doctoral student Tony Tao to MIT Technology Review. The slow-burn propellant will power the 900 to 1 300 gram Firefly for up to 3 minutes, well within the Air Force’s requirements.
The Firefly’s motor burns its fuel from the aft end forward, rather than the inside out, with exhaust travelling through a graphite/ceramic nozzle. The top half of the drone contains its payload, avionics, and a flight control system, while the bottom is outfitted with a pop-out wing and a deployable tail for guidance.
Built out of titanium through additive manufacturing, the Firefly is one of the first 3D-printed rockets. Difficulties emerged due to the combustions close proximity to the electronics, as the temperatures could overwhelm the drone’s controls. After considering design modifications, the MIT team decided to put an insulating layer between the engine and electronics instead.
The Firefly is still a year or so away from being ready for a flight test. Work on the engine is proceeding in partial burn tests, and an igniter charge is used to increase chamber temperatures and pressures to start the motor.
Half drone and half missile, the Firefly could be the start of a whole new kind of fast-flying military UAV.