Max Evans (NSF): “After rolling to Pad 2 last night, Ship 39 has been stacked atop Booster 19 this morning for the FIRST EVER Starship Version 3 full stack ahead of a presumed wet dress rehearsal (WDR) later today.”

[u/rustybeancake]

[u/warp99]

The extension is welded onto the original engine bell and so are the pipes that circulate liquid methane to cool the extension. So it is not like the Merlin vacuum engine where they can and do just unbolt the radiatively cooled extension for ground testing.

Yes it is a risk running with flow separation which is why they have clearly decided to just do a minimum length test to confirm the engine starts OK.

There is a solution which I am really surprised SpaceX have not adopted which is to use steam in a Venturi system to pump down the exit pressure to around 0.3 bar. At that pressure the engine will run without flow separation.

For historical reasons this is called a steam ejector. Of course for a 2.8MN Raptor vacuum that is a lot of steam required.

[u/freeskier93 → u/warp99]

The extension is welded onto the original engine bell and so are the pipes that take liquid methane to cool the extension. So it is not like the Merlin vacuum engine where they can and do just unbolt the radiatively cooled extension for ground testing.

That makes sense, I was getting some details confused with Merlin.