I know this is a very old thread, but just in case someone is still watching it, this seems to be the correct explanation. I had also incorrectly thought the F-1 engine itself was run rich. I got the right answer from one of the ALSJ contributors, a very knowledgeable bunch. The hot gas that drives the turbine is produced in a "gas generator" -- a RP-1/LOX burner -- that deliberately burns a very rich mixture. As it passes through the turbine, it expands and cools (as in any heat engine). This relatively cool, oxygen poor exhaust then flows through large ducts visible on the outside of the nozzle into the nozzle, cooling it and (I think) forming a boundary layer that actually insulates the nozzle from the main plume. After being ejected from the nozzle, the excess fuel eventually burns on contact with atmospheric O2.

If you ever see film of a F-1 static firing, watch as the engines are shut down. The orange/brown section of the plume shrinks until the incandescent part nearly touches the end of the nozzle. I'd say this is due to the turbopump slowing down and sending less exhaust gas to the nozzle.

I think this makes it very clear that Bill Wood's claim of "injecting kerosene" is a classic case of a little knowledge (combined with paranoia) being dangerous. Yes, the engine did in a sense inject extra kerosene into the plume, but for a perfectly good reason that evidently he did not understand or want to understand.

Regarding ignition, I think the gas generator had its own ignition system so as to first get the fuel and LOX flowing into the injectors. Then the hypergolic "cartridge" ruptured to ignite the mixture in the combustion chamber. I love that ignition sequence in the film - the streams of LOX falling through the engine followed by the ignition fireballs. "Apollo 13" really botched this one. The F-1s looked like a bunch of CO2 fire extinguishers because that's exactly what they used to create the effect. At least they got the Venturi effect that sucks the fireballs back past the engines. And they got the dark plume segment just fine.

Re pogo and combustion instabilities, they're two different things. Combustion instabilities occur inside an engine either in flight or on the ground. The pressure fluctuations can tear it apart. Waiting for them to occur spontaneously during test firings could take a long time, so they provoked them by firing small explosives inside the engine. If it was prone to an instability, the explosion would often trigger it.

Pogo is an entirely different effect caused by pressure fluctuations in the propellant lines due to changes in vehicle acceleration. The vehicle accelerates, increasing propellant flow, increasing thrust, etc until the process reaches its limit and abruptly reverses: decreasing thrust reduces acceleration reducing propellant pressure and flow, reducing thrust, etc. This makes it hard to duplicate on the ground so it often has to be solved by analysis and simulation.

The solution to pogo in most cases was to provide accumulators -- "buffers" -- in the propellant lines to damp out the pressure fluctuations. It was enough to do this on the LOX lines, LH2 just wasn't heavy enough to cause problems.

Old it may be- the threads exist for discussion
And quite a post too- very informative.
Welcome to BAUT

The hot gas that drives the turbine is produced in a "gas generator" -- a RP-1/LOX burner -- that deliberately burns a very rich mixture. As it passes through the turbine, it expands and cools (as in any heat engine).

Yes, the open-cycle gas generator concept is fairly basic. It's just fairly unknown to conspiracists. The relative coolness of the exhaust derives less from its expansion in the engine and more from the deliberately rich formulation of the gas generator fuel mix that specifically keeps it from burning too hot. That would require a cooling solution for the turbopump motor itself. That is, the designers specifically ran the gas generator fuel-rich in order to hold down the temperature. The downside of this is that the turbine tends to soot up fairly quickly, limiting the entire engine's run time. This is why you don't see any reusable RP-1 engines.

I contribute to ALSJ too.

...cooling it and (I think) forming a boundary layer that actually insulates the nozzle from the main plume.

Yes, the industry term is "film cooling." The optimum F-1 nozzle was too big to build entirely with Rocketdyne's oven brazing process. So the nozzle skirt was bolted on, precluding its participation in the regenerative cooling scheme that Rocketdyne accomplishes by brazing its nozzles out of small tubes. Von Braun used film cooling in his original V-2 rocket engine. Every engine will use film cooling techniques to some extent by considering the perimeter of the injector plate a special case. This causes either faster annular flow or cooler annular combustion.

Regarding ignition, I think the gas generator had its own ignition system so as to first get the fuel and LOX flowing into the injectors.

Yes, the easiest way to start up a turbopump-fed engine is simply to drive the turbine with a primer charge of gas, usually from some auxiliary combustion. You spin up the pumps with a small hypergolic or solid-fueled rocket, then when it reaches critical flow you can light off the main chamber.

Re pogo and combustion instabilities, they're two different things.

No, not really.

You're thinking only of high-frequency combustion instability ("screaming"), which has its causes primarily in the fluid dynamics of the combustion front inside the thrust chamber. The other forms, including but not limited to pogo, receive contribution from other portions of the vehicle.

The pressure fluctuations can tear it apart.

Not really. Low-frequency combustion instability ("chugging") breaks attachments like steering gimbals and fittings by pressure-induced vibration. Thrust chambers are ordinarily quite robust and don't just break apart because of overpressure or acoustical stress.

The principal concern of internal combustion instability at the higher frequencies is unplanned heat loading at the chamber walls. A "screaming" engine often creates standing or metastable waves (both transverse and longitudinal) that apply a disproportionate thermal load to certain parts of the chamber in excess of its heat rejection capacity. The wall softens, and then normal chamber pressure fails it mechanically.

If it was prone to an instability, the explosion would often trigger it.

You're speaking specifically of the F-1 tests, which were indeed so induced. I'm not aware of any engine in which a detonation at the throat would not induce an instability. So this technique is not limited to engines "prone to combustion instability."

Pogo is an entirely different effect caused by pressure fluctuations in the propellant lines due to changes in vehicle acceleration.

Quite true; however most of the medium- and low-frequency combustion instability, besides pogo, involves interaction with the propellant feed system. It is quite inaccurate to say that combustion instability as a whole occurs only within the thrust chamber.

(cf. Sutton and Biblarz, Rocket Propulsion Elements, pp. 342-353.)

The vehicle accelerates, increasing propellant flow, increasing thrust, etc until the process reaches its limit and abruptly reverses...

No. The propellant flow in a pump-fed is designed to be relatively constant regardless of nominal vehicle acceleration. Not all fuel flow in a rocket is longitudinal either. There is no automatic reversal (abrupt or otherwise) when the vehicle reaches some particular acceleration. There is an abrupt cessation of inlet pressure when the vehicle's engine stops, but as you've guess it doesn't matter then. But the operating inlet pressures are not the cause of pogo.

This makes it hard to duplicate on the ground so it often has to be solved by analysis and simulation.

And it is my profession to provide such analysis and simulation.

What makes it hard to reproduce on the ground is that pogo is almost always a combination of effects from the fluid dynamics of the fuel delivery system and the mechanical response of the vehicle itself as a whole in flight. It is not uncommon for longitudinal oscillations or fluctuations in the structure of the vehicle itself to induce relatively high frequency accelerations in the fuel flow or in the propellant lines themselves, resulting in pressure fluctuations that the natural characteristics of the propellant feed system cannot damp out.

Since it is obviously very difficult to study such effects properly in flight test, simulation is the better tool.

The solution to pogo in most cases was to provide accumulators -- "buffers" -- in the propellant lines to damp out the pressure fluctuations.

Yes, but be careful: a hydraulic accumulator will work very well as such a buffer, but those that are used strictly as fluid power accumulators have check valves that prevent them from operating also as shock dampers. Such use, where not intended, can damage pumps. The Saturn's pogo dampers were indeed hydraulic accumulators; but not all accumulators can be dampers.

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[AnonymousAstronautsVoice] "And you want me to sit on THAT while you light it? Well *I* have something you can sit on!" [EndAnonymousAstronautVoice]

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OK, in some long lens shots of the stage one S-V ascent, there is a notable "sootiness" to the exhaust. It's something I haven't see from other liquid boosters.
Might this be from the rich kerosene turbine exhause being injected into the exhaust stream?

I'd always seen that initial dark part to the exhaust, always figured it had something to to with the nozzle cooling scheme. Details are good!

All RP-1 plumes are a bit sooty at low altitude. In the F-1, the fuel-rich turbine exhaust sure doesn't help, but you'd get some soot even without it. These days you're also likely to have SRM strap-ons that mask any RP-1 plumes. The best way to see it in a modern rocket is via onboard cameras during the first stage boost, after the strap-ons jettison.

Somewhere on my drawing board is a half-finished cutaway illustration of the F-1 I'm working on, that shows the exhaust profile and the effect we're discussing. Worthy of National Geographic magazine. One of these months I'll finish it.

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How many of us have spent hours trawling through the ALSJ or the NASA photo archives to find a particular picture or a quote that answers a particular and obscure claim?
I have a love/hate relationship doing this. On one hand, I'm frustrated that I can't find what I'm looking for, and on the other hand I'm excited to find great pictures that I wasn't looking for.