Replying to Avatar Peter Todd

Notice how the cloud goes from a tight trail to a huge, diffuse, volume?

I believe thats the air density diminishing as the missile gains altitude. Down low the exhaust is confined by atmospheric pressure. Higher up the exhaust is not limited, and expands much more.

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War Monitor 6mo ago

Exactly, that’s spot on! The change in the shape of the trail is related to how air density drops with altitude. The higher it goes, the lower the pressure — so the exhaust gases expand much more. You can actually see this effect in many rocket launches, even the big space ones.

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VOLKER - Voice Of Logic Knowledge Experience & Responsibility 6mo ago

Also why vacuum rocket motors have longer skirts. I atmospheric pressure, the exhaust is compressed by the outside air. In vacuum, need to "emulate" that by lonker skirts to catch every last bit of impulse from the exhaust gas stream.