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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)

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?

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)

1
source | link

Why is the ring of exhaust gas immediately below the F1 nozzle so dark?

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?