Researchers hope the first flight of an aircraft powered by a pulse detonation engine (PDE) will accelerate development of a propulsion concept that promises greater simplicity, lighter weight and a wider operating range than turbomachinery.

In late January, an experimental engine developed at the US Air Force Research Laboratory (AFRL) powered a modified Scaled Composites Long-EZ on a brief flight along the runway at Mojave, California, marking the first flight of a PDE-powered aircraft.

AFRL principal investigator Fred Schauer says the single flight, lasting "tens of seconds", proved the aircraft and its pilot could survive the noise and vibration of the PDE - concerns raised by an engine that delivers its thrust in pulses produced by the supersonic detonation of fuel and air in a tube.

Scaled PDE taxi
Small JATO gas turbine helped Long-EZ get moving                      © Alan Radecki

The experimental PDE, built by AFRL contractor ISSI, had four tubes, each firing at 20Hz. Producing a peak thrust of about 200lb (0.9kN), the engine powered the Long-EZ to just over 100kt (190km/h) at 60-100ft (20-30m) above the Mojave runway.

Schauer says the flight proved the structure could survive the Mach 5 pulses and that noise was "reasonable" in the cockpit. With the PDE running at 80Hz, noise was low frequency, sounding like an "engine brake on steroids", he says.

Scaled test pilot Pete Siebold wore specially developed earplugs. Vibration was mitigated by tuning the engine mounts to the operating frequency, Schauer says.

Scaled PDE fly
PDE-powered cruise lasted about 10 seconds            © AFRL

January's flight followed aircraft and engine modifications after an unsuccessful attempt to fly in 2004. In particular, Scaled designed a fairing for the engine to reduce drag, while the fuel was switched from gasoline to propane to broaden the PDE's operating limits and increase its thrust.

Built using off-the-shelf parts, the experimental PDE "was just a proof of concept", says Schauer. The cylinder head from an automotive engine provided valves to admit first purge air then the fuel/air mixture into each tube, dual superchargers overcoming the pressure drop across the valves. A spark plug ignited subsonic combustion in the tube, which accelerated into the supersonic detonation wave.

PDE animated gif
                                                                                                         © AFRL

"The engine is old technology. Now we have better initiation techniques, better valving, better injection schemes," says Schauer. Research continues within AFRL, but he expects the engine manufacturers to be next to fly a PDE. "We have shown the technical hurdles can be overcome," he says.