F-1 Exhaust on Saturn V

"John Pelchat" wrote in message
om...
|
| What is the explanation for the dark band in the exhaust
| of the five F-I engines of the Saturn-V?

That's the preburner (or gas generator, if you prefer) exhaust. It's being
used for film-cooling the nozzle.

The gas generator is what powers the pumps and other mechanical systems of
the engine. A portion of the fuel and oxidizer is diverted to a small
burner where it is ignited and used to drive a turbine. This turbine is
geared to the fuel pumps, etc. This gas generator is run significantly
fuel-rich in order to keep the combustion temperature down (at a cost, of
course, of suboptimal combustion). As you can guess, if you burn kerosene
without enough oxygen, you get a very sooty combustion product. And, of
course, a cooler gas. If memory serves, it's around 700 F.

So after it has driven the turbine, what to do with it? In other engines
it's simply dumped overboard from an auxiliary exhaust port. It's
relatively non-propulsive.

In the F-1 engine, however, it was injected into the nozzle as a form of
film cooling. Film cooling is where you arrange for a layer of cooler gas
to form between the hot principal combustion products and the inner surface
of the nozzle. This reduces the heat loading on the nozzle by making a sort
of boundary layer. The preburner exhaust doesn't seem cool at 700 F, but
compared to the primary gas, it's quite cool enough.

The flow of this cooler (and less incandescent) boundary layer remains
annular for several feet after it leaves the nozzle until turbulence causes
it to mix with the hot inner core of the plume.

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The universe is not required to conform | Jay Windley
to the expectations of the ignorant. | webmaster @ clavius.org