How to annoy the neighbours!

[tnowak]

From Aviation Week and Space Technology:

Pulse detonation engine-powered flight demonstration marks milestone in Mojave

U.S. Air Force researchers will push for more development funding for pulse detonation engines (PDEs) following the successful first-ever manned demonstration flight of an aircraft powered by the exotic propulsion system.

The milestone flight was accomplished at Mojave, Calif., on Jan. 31 when Scaled Composites test pilot Pete Siebold flew a heavily modified Long-EZ above the main runway in straight-and-level flight at altitudes up to 100 ft. powered solely by the PDE. The engine was developed at the Air Force Research Laboratory (AFRL) by the on-site contractor, Innovative Scientific Solutions. At peak thrust at the start of the run, the PDE was producing more than 200 lb. thrust, says ISS research engineer John Hoke.

The prime objectives of the AFRL/ISS demonstration were to show that the PDE could be used to power an aircraft, the aircraft structure could survive the acoustic pressure of the Mach 5 shock waves exiting the detonation tubes, and that the noise level was not prohibitive for a manned flight. The test also sought to assess the durability of the detonation engine, which had already undergone more than 100 hr. of ground testing, plus a further 30 hr. on the integrated system.

The demonstration comes amid growing interest in the potential of PDEs for a range of simple, cost-effective missile propulsion systems, as well as in the possible development of combustion sources for commercial and military gas turbines. In its simplest form, the PDE is an air-breathing engine with few or no moving parts. Combustion takes place in an open-ended tube in which fuel is mixed with air and detonated. The detonation wave travels down the tube at supersonic speed, generating thrust and, at the same time, reducing inlet pressure to allow fresh air and fuel to be drawn in. Each pulse lasts milliseconds.

The AFRL’s prototype “off-the-shelf” PDE has been installed in a Scaled Composites-owned Long-EZ for the flight tests, and replaces a 100-hp. Textron Lycoming 0-235 piston engine driving a pusher propeller. The PDE system, integrated into the Long-EZ by Scaled Composites crew chief Rick Aldrich, includes a JATO rocket-assist pack that provides a few seconds of additional thrust to shorten the takeoff run.

The propulsion system consists of a Hirth-built two-stroke auxiliary power unit that controls the valves and compressors in the PDE, which is based on a heavily modified General Motors Pontiac Grand Am Quad 4, 16-valve dual overhead camshaft automotive engine. The detonation takes place in four tubes projecting from the back of the aircraft, and is designed to generate a mass flow of up to 1.6 lb./sec. to produce a combined force of roughly 150-200 lb. thrust for flight speeds of up to 130 kt. In the AFRL engine, the PDE fires at a frequency of around 80 Hz.

Assisted by the JATO, Siebold took off at around 75 kt. during his one-way run down Mojave’s 12,500-ft.-long Runway 12. Getting to grips with the unusual aircraft and its handling characteristics, Siebold encountered a “bit of Dutch roll, but corrected it by yawing,” says Hoke, who adds that “we were planning on flying at around 25 ft., but as a result he ended up between 60 and 100 ft. above the runway.” Siebold then “cruised for about 10 sec. using PDE power alone, and he thought he could have climbed using the engine.”

Engine noise levels were substantial but very directional and safe from the cockpit, says Hoke, who adds that at ground level close by the aircraft, “you could feel your chest pound a bit—but it wasn’t bad at all.” Despite the acoustic levels at the tube exit being in the 195-200-dB. range, “there were no issues at all” with structural response. Hoke says “people wouldn’t want to be directly behind it, but then what jet engine would anyone want to be standing behind?”

The flight also marked the success of several design changes and modifications made to overcome problems that prevented a flight test as far back as 2003 and early 2004. These included a change in fuel to improve the fuel-air mix detonation characteristics at the relatively lower atmospheric pressures found at Mojave’s 2,800-ft. altitude.

“We switched from avgas [aviation gasoline, a high-octane fuel] to propane, which is easier to detonate,” says Hoke. His recent work in hydrogen and hydrocarbon ignition at AFRL with PDE pioneer Fred Schauer compared the ignition and detonation performance of hydrogen, ethylene, propane, avgas, JP-8 and Fischer-Tropsch JP-8 (S-8). Hydrogen was found to have the best characteristics, followed by ethylene, propane, avgas, JP-8 and S-8.

Other changes included the addition of a fairing over the PDE to reduce vehicle drag, the fairing having originally been omitted owing to concerns over fuel leaks in earlier attempts, and changes in configuration to prevent exhaust from the Hirth motor being sucked into the PDE inlet.

Although there are no immediate plans for more flight tests, Hoke says fundamental research will continue. The demonstration will “hopefully be a springboard for more interest and funding,” he adds. “As little as 10-20 years ago no one could even detonate these hydrocarbon-air mixtures, and hopefully developments like this will lead to a revolution of technology where it will be used, perhaps as a combustor.”

[mikehallam]

1943 Buzz Bomb.

[Nick Allen]

A couple of years ago a Scrapheap Challenge involved a pulse-jet-driven vehicle. I remember at the time being highly impressed by both the simplicity and the noise -- but also the power! Look on YouTube under "pulse jet" for some further variations on the theme. How about a couple of units attached to the fuselage for take-off assistance? They could take their start-up airflow from the propwash. Might take while to get this past Engineering...