Pulsejet

Propulsion, that relies on propellant (gas, plasma, etc...) for acceleration, divides into two types of categories - vacuum and atmospheric. Vacuum engines rely on fuel that is onboard the vehicle, while atmospheric engines rely on an atmosphere to generate thrust. We have covered some vacuum engines in the last editions so let us explore some of the atmospheric variety.

One of the simplest types of atmospheric engines is the Pulsejet. The Pulsejet, simply put, is a pipe with a fuel injector. Fuel combusts in a pulsed manner, with every pulse pushing hot gases outside while drawing additional air into the chamber for the next pulse. Every Pulsejet is tuned and works like a harmonic oscillator. The Pulsejet comes in two flavors, one that has a reed valve for the air inlet (as used on the V1 Flying Bomb during WW2 ) and a valveless version that relies on the acoustics of a "U-pipe" to draw in the air.

Simplicity and instant-start capabilities are perhaps the only advantages of the Pulsejet. In all other performance characteristics - these are inferior engines. A Pulsejet has very little compression ratio since the suction of air comes solely from the process of combustion. As a result, both thrust-to-weight ratio and power-density values are very low. Pulsejets are also not very fuel efficient and get extremely hot during operation.

Is there a future for the Pulsejet? The efficiency of the Pulsejet relies primarily on how much of the mixture can we actually get to ignite per every cycle. This is the very problem DARPA has been tackling for the past decade. Meet the 21'st century successor to the Pulsejet - the Rotating Detonation Engine (RDE) . In an RDE, a shockwave responsible for detonating the fuel moves around in a circular direction. Also, an RDE uses a better combustion process "detonation" (supersonic combustion) as opposed to "deflagration" (subsonic combustion). As a result, an RDE can provide high-efficiency continuous thrust while still relying on efficiency of many individual detonations. But don't expect these engines to make it on passenger aircraft anytime soon. Until technology vastly improves, applications will focus on short-range missiles and countless hobby projects.

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