A pulse detonation engine is a propulsion device that uses controlled explosions to create thrust. These types of engines have been researched for use in supersonic aircraft. Theoretically, pulse detonation engines are more efficient than traditional jet engines at high velocities, and can drive an aircraft to four or five times the speed of sound.
Normal jet engines ignite fuel in a process known as “deflagration.” This means that as long as air is present, the fuel burns at a fairly constant rate. The expanding gases produced during deflagration move slower than the speed of sound. This is an efficient use of fuel at low speeds, but does not provide sufficient energy for sustained supersonic propulsion.
Pulse detonation engines rely on a much more violent and vigorous expansion of gases. Fuel in this type of propulsion system literally explodes and creates a shockwave that moves faster than sound. When these detonations are cycled or pulsed, thrust is created. A special valve is required to produce a recurring pulse of controlled explosions. The most common design for this valve is a rotating disk that allows air to enter a combustion chamber at a set interval.
It is important for the pulse timing to be consistent. Too long an interval between detonations will prevent usable thrust from building up, while too short a time will result in one large unsustainable explosion. Due to the high pressure shockwave created by a pulse detonation engine, the materials must be much stronger and heavier than a traditional engine housing. High levels of vibration and sound are also factors that must be minimized using dampeners and insulation.
Several U.S. research groups, including the National Aeronautics and Space Administration (NASA) and Defense Advanced Research Projects Agency (DARPA), have experimented with different variations of pulse detonation engine technology. The first flight of an aircraft powered by this type of propulsion occurred in 2008. This test aircraft did not reach supersonic speeds, however, and engine research continued.
Some researchers believe that pulse detonation engines can be combined with other types of engines. This type of hybrid system might allow an aircraft to use a fuel-efficient jet engine during takeoff, and then switch to the more powerful detonation engine for high-speed cruising. Many designers speculate that pulse detonation propulsion will allow the military to build advanced reconnaissance aircraft. At very high supersonic speeds, such an aircraft may be able to evade and outrun enemy defenses. Efficient supersonic engines also have civilian applications, and could lead to extremely fast cross-country or inter-continental flights.