Combustion

Part 10 - Ram jets

Pulsejet engines are simply tubes that produces thrust intermittently from a resonant combustion chamber. They typically have low compression ratios but they are lightweight, have few or no moving parts and do not need a compressor or forward motion of the engine to force air into the combustion chamber. 

A valveless pulsejet engine has no moving parts.   Gases move through the engine because of its shape.  Start-up requires compressed air mixed with fuel which is ignited by a spark plug. Most of the burning gases leave through the wider cross section of the exhaust and, having more inertia, this produces a partial vacuum for a fraction of a second after each detonation. This sucks in a fresh charge of an air/fuel mixture which is ignited by the tail end of the previous fire ball (the spark plug is disconnected after start up). The incoming air is slightly compressed at the resonant frequency of the tube, typically many times per second and this determines the pulse rate of the engine.

Valved pulsejet engines use simple flap valves to stop the hot gases escaping back through the air intake which allows more fresh air and fuel to enter the combustion chamber as the hot exhaust gases leave.

A retired Russian artillery officer Nikolaj Afanasievich Teleshov patented a pulsejet engine in 1867.  Between 1906 and 1932, Russian engineer, V.V. Karavodin, built a working model pulse-jet, while French inventor, Georges Marconnet and Ramon Casanova both patented pulsejet engines and, in America Robert Goddard, demonstrated a bicycle powered by a pulsejet engine. 

In 1934, Georg Madelung and Paul Schmidt proposed building a "flying bomb" to the German Air Ministry and the world's first cruise missile, the Argus As 109-014  (later known as the V-1) flew on December 10, 1942.

Their engine design was an improved version of Georges Marconnet pulsejet, and it was a simple, low cost design that ran on gasoline. The ratio of exhaust pipe length to diameter was 8.7 to 1, which caused it to resonate at a frequency of 43 cycles per second, the frequency of the combustion cycle. 

The engine was started with acetylene gas and high pressure air ignited by a single automobile spark plug and, as soon as the engine was running, the fuel was switched to gasoline (petrol) and that was ignited by the tail of the previous fireball. The engine produced a static thrust of 2,200 N (Newtons) (490 lbf (pound force)) and an in-flight thrust of about 3,300 N (740 lbf). The thrust was not sufficient for take-off so the V-1 was launched from a ramp by a steam catapult powered by hydrogen peroxide and potassium permanganate. 

In 1945 U.S. engineers reverse-engineered a V-1 that crashed without exploding and created the JB-2 Loon cruise missile, forerunner of later missiles.

Unlike a pulsejet, a ramjet uses a continuous combustion process but it cannot produce thrust at zero airspeed and must be accelerated to a speed where it begins to produce thrust, typically by a catapult, rocket, gas turbine jet or artillery shell.  Ramjets generally outperform gas turbine-based jet engines and work best at supersonic speeds (Mach 2–4).  Although inefficient at slower speeds, they are more fuel-efficient than rockets over their entire useful working range up to at least Mach 6 (2,041.7 m/s; 7,350 km/h).  Ramjets performance above Mach 6 falls off because of pressure loss caused by shock as the incoming air is slowed to subsonic velocities for combustion.

Ramjets can operate up to speeds of Mach 6 (4,600 mph; 7,400 km/h) but are most efficient at supersonic speeds around Mach 3 (2,300 mph; 3,700 km/h).  As the speed increases and efficiency declines, the air intake geometry must be changed so that the incoming air is not compressed (and therefore heated) as much. At this point, the air flow through the combustion chamber is supersonic and the ramjet becomes a supersonic combustion jet or scramjet.