Pulse Detonation Engine Technology

In January of this year, PopSci published an article, Scram!, about the hottest new design for hypersonic aviation engines. They describe the conditions in which the aircraft would fly in;

You don’t run this wind tunnel. You fire it. With the help of air pumped to 160 times atmospheric pressure and a highly explosive combination of hydrogen, oxygen, and a megawatt generator, the tunnel, set in a suburban Long Island business park, can reproduce the hellacious conditions an aircraft would encounter while traveling 20 miles above Earth at 5,300 mph–Mach 8–a speed at which the violent airstream packs enough energy to soften and melt solid nickel alloys.

Great obstacles must be met in order to get a plane to survive the trip. The idea of producing enough power to move a plane at Mach 8 is theoretically easy but very tricky to build. One boost to the program is a new type of jet fuel that will let the engine fire at the temperatures needed to produce the hypersonic airflow. Today’s metallic materials cannot withstand the 4000 degrees of heat that the engine produces, only a few exotic ceramics can.

The hypersonic (at least five times the speed of sound) velocities that the engine is striving for have until now only been feasible with single-use hydrogen- or hydrocarbon-fueled ramjet engines that in most cases would be destroyed by the end of the flight, making them suitable for missiles but not much else. Scramjet test engines –which improve upon ramjet performance and reliability but are much harder to engineer–have been around for decades, but their success rate has been dismal, and they have usually burned dangerous and unwieldy hydrogen for fuel. (Ramjets slow air traveling through them to subsonic speeds, while scramjets keep airflow above supersonic speeds–hence, the difficulty keeping them lit.) But this new engine, being tested at the cutting-edge facilities of aeronautical engineering firm GASL, runs on JP-7, a significantly more manageable kerosene-like jet fuel, and it has a unique cooling system that is key to its performance under hypersonic conditions. HyTech is slowly edging toward success, with several tests over the summer that have come close to a previously unattainable milestone: actually starting and sustaining combustion amid supersonic airflow.

Simply testing for that achievement is almost as hard as accomplishing it. Producing Mach 8 airflow through a 6-foot-diameter test chamber like the one at GASL takes a lot of energy–its facility is a labyrinth of tanks, ducts, and valves all designed to spit out a blast of air at incomprehensible speeds for up to a minute at a time.

With the listed difficulties, advances would have to be made in several aspects of plane building. Also, the benefits of such a engine would be enjoyed by the military and the government. There has to be a better solution.

That solution may be the Pulse Detonation Engine. The idea has been around for many years, since 1930s. The Germans were playing around with the idea in the 30s but technology was not advanced enough for them to have a successful experiment. In the 90s the idea was resurrected. Pulse Detonation Engine (PDE) does not promise the speed that a Scramjet would but it is a hypersonic engine.(Scramjet, up to MACH 15 or 10,000 mph, PDE, around MACH 5) The beauty of the PDE is that it is ultra efficient, fuel-wise. PDE also suffers a few of the problems that Scramjet does namely thermal fatigue. Our metals need to be able to handle the heat. With both technologies, noise is a very big problem. When operating in the supersonic and above range, we can’t seem to avoid the sonic boom.

The concept behind the PDE is deceptively simple. In short, there are two kinds of combustion: the old, familiar, slow kind of burning, called deflagration, and another, much more energetic process called detonation, which is a different animal entirely. Imagine a tube, closed at one end and filled with a mixture of fuel and air. A spark ignites the fuel at the closed end, and a combustion reaction propagates down the tube. In deflagration–even in “fast flame” situations ordinarily called explosions–that reaction moves at tens of meters per second at most. But in detonation, a supersonic shock wave slams down the tube at thousands of meters per second, close to Mach 5, compressing and igniting fuel and air almost instantaneously in a narrow, high-pressure, heat-release zone.

That zone is where the highly efficient combustion that the Pratt & Whitney and General Electric engineers hope to harness takes place. To bring it into existence, one must precisely coordinate fuel input, airflow and the ignition spark to create a “deflagration-to-detonation transition,” or DDT, the process by which an ordinary flame suddenly accelerates into an immensely more powerful detonation. And one detonation is only the beginning, because while it generates more thrust for the amount of fuel combusted than a deflagration, it also combusts only a tiny amount of fuel. To make a PDE work–to get any practical thrust out of it–one needs dozens of detonations every second, a detonation wave.

PDE technology is planned to be applied in the military, government and commercial airline industries. If the engine design progresses and matures, flying will never be the same. Commercial airlines will use less fuel and travel to the farthest places in under 4 hours. As with the Concorde, noise will probably be the biggest deterrent for the commercial airliners, but if we learn to suppress the sonic boom airplanes as we know them will become dinosaurs. Commercial space passage will also be in the mix thus cutting our travel times by even more. Imagine the day in which a normal flight from New York to Tokyo would take 2 hours. The aircraft would take of and head straight for space, breach the atmosphere and make a mad dash through frictionless space. Reenter and land. The day will come, unfortunately, it won’t be for nearly 20 years.

Sources: PopSci articles Scram! and Pulse Detonation Engine