One truly fascinating aspect of rocket engines I read about is the
acoustic engineering of gaseous thermodynamics.
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.
Wow, that is crazy. You are the first one I see mention this. Surprised that it is not something mentioned more often. It is such an interesting factoid. So you are saying this applies to ALL rocket engines or just a particular subset of them?
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.
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.