Alternatives of the scramjet

PDE (Pulse Detonation Engine)

A pulse detonation engine, or PDE, is a type of aircraft engine An aircraft engine, as the name implies, is an engine developed to be used in an aircraft. More specifically it almost always refers to internal combustion engines as opposed to jet engines.

Unlike an automobile engine, aircraft run at high power settings for very long times. In general the engine is run at maximum power for a few minutes while taking off, then at a slightly reduced power for climb, and then spends the vast majority of its time at a cruise setting, typically 65% to 75% of full power. In contrast a car engine might spend 20% of its time at 65% power while accelerating away from a red light, followed by 80% of its time at 20% power while cruising. that is designed primarily to be used in high-speed, high-altitude regimes. To date no practical PDE engine has been put into production, but several testbed engines have been built that have proven the basic concept. In theory the design can produce an engine with the efficiency far surpassing gas turbine

All regular jet engines and most rocket engines operate on the deflagration of fuel – that is, the rapid but subsonic combustion of fuel. The pulse detonation engine is a concept currently in active development to create a jet engine that operates on the supersonic detonation of fuel.

The basic operation of the PDE is similar to that of the pulse jet engine A pulse jet engine is a very simple form of aircraft engine wherein the combustion occurs in pulses. It falls somewhere in between true jets on the one hand and rockets on the other.

A pulse jet is comprised of a one-way air inlet valve, a combustion chamber, and an (acoustically) resonant exhaust tube (tailpipe). It also has a means of admitting and mixing fuel with the intake air (or injecting fuel into the combustion chamber), and a means of ignition when the engine is started. Once the engine is running there is no need to provide further ignition. There is also some means of providing combustion air to start the engine from a compressed air supply air is mixed with fuel to create a flamable mixture that is then ignited. The resulting combustion greatly increases the pressure of the mixture, which then expands through a nozzle for thrust. To ensure that the mixture exits to the rear, thereby pushing the aircraft forward, the pulsejet uses a series of shutters or careful tuning of the inlet to force the air to travel only in one direction through the engine.

The main difference between a PDE and traditional pulsejet is the way in which the airflow and combustion in the engine is controlled. In the PDE the combustion process is supersonic, effectively an explosion instead of burning, and the shock wave .

In compressible fluids such as air, disturbances such as pressure changes caused by a solid object moving through the medium propagate through the fluid as pressure waves traveling at the speed of sound. When the cause of the disturbance is moving slowly relative to the speed of sound, the pressure wave enables the fluid to redistribute itself to accommodate the disturbance, and the fluid behaves similarly to an incompressible fluid.
of the combustion front inside the fuel serves the purpose of the shutters of a pulsejet. When the shock wave reaches the rear of the engine and exits the combustion products are ejected in “one go”, the pressure inside the engine suddenly drops, and air is pulled in the front of the engine to start the next cycle. Some designs require valves to make this process work properly.

The main side effect of the change in cycle is that the PDE is considerably more efficient. In the pulsejet the combustion pushes a considerable amount of the fuel/air mix (the charge) out the rear of the engine before it has had a chance to burn (thus the trail of flame seen on the V-1 flying bomb and even while inside the engine the mixture’s volume is continually changing, an inefficient way to burn fuel. In contrast the PDE deliberately uses a high-speed combustion process that burns all of the charge while it is still inside the engine at a constant volume, a much more efficient process. Detonation is inherently more efficient than deflagration, thus while the maximum energy efficiency of most types of jet engines is around 30%, a PDE can attain an efficiency theoretically near 50%.

There are aircraft-drones prototypes testing this tech??????