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When you look at film of Saturn V launches where you can see the nozzles up close, the exhaust forms a "dark" band immediately below the nozzle extensions.

I understand that the F1 engine used film cooling, distributing the turbopump exhaust in a ring to create a film of relatively cool gas between the main exhaust and the nozzle extension.

But why does it appear dark? I would have assumed the gasses were essentially transparent, so the bright main exhaust should simply shine right through. Was there a lot of soot or other particulate in the turbine exhaust to make it so opaque?

enter image description here (image source, annotated)

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    $\begingroup$ It's also soot and other particulates that makes hot rocket exhaust look bright. Without soot, sea level exhaust is usually fairly transparent. 1, 2, 3, 4 $\endgroup$
    – uhoh
    Aug 18, 2019 at 23:47

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But why does it appear dark?

That's cooler gas from the turbine exhaust used to insulate the engine. The turbine exhaust is running rich (more kerosene, less oxygen) so it will burn cooler as to not melt the turbine blades running at 1500 F.

The gas generator mixture ratio, relative to the engine mixture ratio, is fuel-rich. This provides a lower combustion tempertaure in the uncooled gas generator and in the turbine.

As a result of running rich and cooler it contains a lot of unburned fuel. Soot.

Before it's used to cool the engine, the turbine gases pass through a heat exchanger, shown below. This uses the relatively cool turbine exhaust to heat up oxygen and helium to pressurize the tanks. This further cools the turbine exhaust.

enter image description here

The interior of the nozzle extension is protected from the engine exhaust gas environment (5800 Fahrenheit) by film cooling, using the turbine exhaust gases (1200 Fahrenheit) as the coolant. The gases enter the extension between a continuous outer wall and a shingled inner wall, pass out through injection slots between the shingles, and flow over the surfaces of the shingles forming a boundary layer between the inner wall of the nozzle extension and the hotter exhaust gases existing from the main engine combustion chamber.

See also

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    $\begingroup$ Intellectually, I understand using the turbopump exhaust to cool the nozzle. But it still amazes me that 1500 F is "cool" for that purpose. The temperatures in a rocket engine are amazing because they are so far outside of normal experience/ $\endgroup$
    – Wayne Conrad
    Aug 19, 2019 at 14:58

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