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At about 04:58 in Scott Manley's new video NASA Might Be Giving Away A Saturn I Rocket - Here's Why I Love This Vintage Booster he says:

So the four engines in the middle of this thrust plate (plane?) on the bottom are fixed; they just hold position. The four on the outside they gimbal, and they provide all the roll, yaw, and the pitch control.

There’s another, there’s an interesting difference in these engines as well. The four engines in the middle, they exhaust their gas generator exhaust over the side; there’s these little flares coming out the side.

But the outside ones, they use a gas generator exhaust around the tip of the nozzle, so it looks like a film cooling… dark, and then you’ve got the light coming through here.

I think that’s great, and I never quite figured out why this decision was taken, and I suspect it’s got something to do with the engines gimbaling. They didn’t want the super-hot exhaust to impinge on anything.

Question: I don't understand what the challenge would be to point the gas generator exhaust from the four outer engines outward and away from the rocket so that even when they gimbaled it wouldn't hit something. Why would this have been so difficult? How could this be a proposed explanation for this 4 vs 4 configuration?

I've added some screen shots and a few arrow annotations of things referenced in the block quotes.

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You don't want an exhaust pointing sideways on a gimbaled engine, because that exhaust will exert considerable force on the gimbaling mechanism, making it much easier to move the engine in one direction than the other.

You also don't want the exhaust pointing sideways when you can point it downwards and take advantage of its thrust. So the more interesting question would be, what drove the decision to have the four inner turbopump exhausts pointed outwards?

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    $\begingroup$ Your point about torque is spot-on, but also note that in the videos the sideways exhaust is directed down by an elbow. The reason for piping it into the free air stream was so that it wouldn't collect in the lower pressure areas above the engines - but note that the S1-B inboards do not pipe it out the sides, but simply swirl it around the outside of the base of each H1-C engine bell. This must not be a low pressure area like the outside of the base of each H1-D. $\endgroup$ – amI Aug 17 at 11:00
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The design objective for the gas generator exhaust system on both types of H-1 engines was to preclude the fuel-rich gas generator exhaust from recirculating under the base of the booster in flight. The non-gimbaling engines had fixed plumbing to dump the exhaust out the side of the booster. The gimbaling engines couldn't have this fixed plumbing, instead they were fitted with an "aspirator" which directed the gas generator exhaust into the main exhaust plume where it was combusted.

The H-1C engine, the fixed inboard unit, had a curved exhaust duct to carry the turbine exhaust gases, and the H-1D engine, the gimbaled outboard unit, mounted a unit known as an aspirator. The inboard engines simply ducted the turbine exhaust overboard. The outboard engine exhaust was ducted into collectors, or aspirators, located at the exit plane of the nozzle. For the H-1D aspirator, designers chose a welded Hastelloy C shell assembly, mounted on the outside of the thrust chamber and extending beyond the thrust chamber exit plane. The aspirator prevented the fuel-rich exhaust gases of the gas generator from recirculating into the missile boat tail during flight. Instead, the gases merged into the engine exhaust plume.

Reference: Stages to Saturn Chapter 4 - The H-1 Engine General Description.

This image shows the fixed ducting used to exhaust the inner engine GG exhaust overboard from the engines not fitted with aspirators.

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Image source (emphasis mine)

This schematic shows the aspirator.

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Image source

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  • $\begingroup$ The H1-C (inboard) used on the S1-B (8 swept fins) dumps its turbine exhaust right next to the engine bell (look close at OP diagram, or Rocketdyne H-1 manual figs 1-5 and 1-20) and is braced by a small bipod. The 4 small plumes in the videos of finless and 4 fin versions must be doing something else. Anyway, piping the gimballed turbine exhaust sideways past the boat-tail would not allow a brace after the heat exchanger, which would be too flimsy. $\endgroup$ – amI Aug 17 at 9:57
  • $\begingroup$ @amI see diagram added to answer. $\endgroup$ – Organic Marble Aug 18 at 15:09

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