Why does rocket exhaust appear to ‘pulse’ in little spheres?
In: Physics
It is called [shock_diamond or mach diamond](https://en.wikipedia.org/wiki/Shock_diamond) and happens in the jet engine too.
It happens when the pressure of the exhaust is lower than the pressure of the surrounding air The air will compress the exhaust and the result in a pattern of areas with compression and expansion of the exhaust gases.
Yes, the pressure of the exhaust of a rocket or jet engine can be lower than the atmosphere at sea level. You have a throat with a lower diameter after the combustion chamber that covert the high pressure to high speed in one direction. If all particles move in one direction the pressure drops. Hold a paper below your mouth and blow ower it and the paper rises as the air you blow have lower pressure than the air around it.
I have to say I have no idea of the exact mechanism but it is an interaction between where the exhaust gets compressed and where it expands. You can see that it happen quite cleary [in this image](https://en.wikipedia.org/wiki/Shock_diamond#/media/File:South_Carolina_F-16_taking_off_in_Afghanistan.jpg)
a jet or rocket engine has the highest efficiency if the pressure of the exhaust it the same as the surrounding air pressure. To high or to low pressure both decrease performance. Because pressure drop with altitude you have a nozzle sized to have the best performance at the altitude you require it most for aircraft. The nozzle can usually shrink and expand but not enough to get optimal pressure at ground level.
For a rocket, your size to get the best performance for the complete launch. You can have to low pressure as air from the atmosphere then leak in around the edge and it can cause vibration that destroys the nozzle. So engines for atmospheric use have smaller nozzles than the one for use in the vacuum of space where you like as wide nozzle as possible.
There is also size limitations because you often have multiple engines.
If you look at at Falon 9 you have 9 engines on the first stage and the engine can be gimbaled ( swiveled on two axes) so you need some space between them. This limits nozzle size.
The second stage that is used in a vacuum has a single engine with a nozzle that is almost as wide as the rocket. The engines are the same it is just the nozzle that is larger. In a vacuum the larger nozzle you have a higher efficiency but is need to fit in the rocket.
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