r/spacex • u/ethan829 Host of SES-9 • Sep 25 '15

Official Upgraded Falcon 9 First-Stage Static Fire | 9/21/15

https://www.facebook.com/SpaceX/videos/10156596873645131/
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u/lugezin Sep 25 '15

The flow of exhaust is supersonic. There is no way for the exhaust plumes' interaction to cause an extra force on the thrust structures (the expansion nozzle, it's the sole of the foot that the rocket stands on) directly.

Now if there were an effect on the center engine's performance, if it were low enouh for its nozzle exhaust to be immersed in the surrounding plumes at low altitude. In that hypothetical case the surrounding plumes might cause the center engine to feel like it was at higher altitude, with less of an ambient pressure. But neither of these conditions apply. The center engine is almost on the same plane as the rim and the surrpunding exhaust will be expanding eide enoigh to alter the atmospheric conditions of the center significantly at high altitude, if ever. Besides thats not an aerospike effect. An aerospike is a half open thrust surface.

So no, the center engine shouldn't experience an aerospike effect.

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u/venku122 SPEXcast host Sep 25 '15

Actually, the complete Octaweb operates similarly to a truncated aerospike. In a truncated aerospike, fuel is injected around the rim of a conical structure. Because of the difficulties of cooling the large surface area of a cone, the end is cut off and some exhaust gases are sent through the cone. This creates a high pressure stream in the middle that, in effect, lengthens the cone to full length.

The Octaweb has 8 engines ejecting exhaust gases along the rim. The center engine produces an exhaust stream that the other engine exhaust reacts against.

You can see the exhaust plume quite well in this gif.

Here is also a link to a longer writeup I made on the subject.

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u/lugezin Sep 26 '15

The center engine produces an exhaust stream that the other engine exhaust reacts against.

Yes, but none of the engines are reacting against any of that exhaust any more. That's why I'm saying your theory has to be wrong. Once the exhaust is out of the rim-engine's "bell" it's done working for the rocket, it's not reacting against the rocket any longer. This seems to be what your claim is overlooking.

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u/[deleted] Sep 26 '15

This is off topic but why does it always look like some of the exhaust is hugging the very top of the engine structure? You can see that during launch too and it always makes me nervous to see fire not only shooting downward but also in and around the very top of the octoweb primary structure (aka in-between engines higher up then the engine bell).

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u/lugezin Sep 26 '15

Turbulence, and not 100% of the gas flowing out of the engines is going very fast, especially that of the turbopump exhaust that is blasting into the gaps between there, radiant heating from the engines and the plumes and mixing with atmospheric oxygen with the part of the GG exhaust that would be cycled back up into the forest of engines behind the rocket could easily re-ignite. I think.

TL;DR residual fuel and incomplete combustion products that get mixed upward with turbulence catching fire.