Why Do Rocket Engines Look So Complex?
erikexplores.substack.com
erikexplores.substack.com
The combustion chamber is a resonant cavity, a Helmholtz oscillator, and burning fuel in it sets up a standing wave that would shake the engine to pieces within seconds.
To counter this the fuel is modulated in anti-phase to the chamber pressure. Basically rocket engines are giant noise-cancelling headphones.
Light a candle about 3 inches tall. Put it inside a cardboard toilet-roll tube leaving about 2 inches empty at the top. The flame will start to go into periodic oscillation.... pop, pop, pop.... On each cycle it over-consumes available oxygen, almost goes out, air rushes in to replace the hot, low density, depleted air and the flame comes alive again.
Can't remember the name now but there was an awesome documentary doing the rounds on YouTube about Russian and US space race and how they adapted German V2 technology to get into space and to the moon.
The early "pulsejet" is actually designed to oscillate the same way, flooding the chamber with fuel and air in cycles. Forcing air increases the frequency and as the frequency goes to the limit it becomes a "ramjet" (a continuous wave supersonic weapon) but you need to get it to about Mach 3 first. Arthur C Clarke though they would be the first things capable of getting escape velocity.
This information I read was about designs in the 1950s. Specifically early high power liquid fuel types that had an oxygen, hydrazine mix in the chamber. Very precarious. That happened because the pulse/ramjet wouldn't scale up. Apparently you can't build an Apollo sized version that won't just explode. Designs have vastly improved since.