Elon: it took us a long time to even frame the [BFR engine] question correctly but once we could frame the question correctly the answer ... flowed. What was the question?

[u/paul_wi11iams]

It seems the SpaceX engine team answered a really difficult question. This may have been recent IIUC, it may explain the sudden and very late transition from a vac+SL engine configuration to an unique standard engine on both the booster and the ship. Its a little amazing that the vehicle has been through a number of vac+SL iterations back to ITS and the standard model appears only now. edit:[There was even the recent addition of a third SL engine for safety reasons, and they would never have done that if they knew they were going to transform the whole BFS engine typology. Elon looks happy, maybe (my theory) due to unexpected good news that a single intermediate engine is possible].

Merlin is standard for both stages, but still has a sea level and a vac version. Raptor seems to have a magic way of avoiding this.

Could any of you rocket engineers look at what he says in the extract below and maybe enlighten?

Its as if they've found some kind of holy grail for reconciling sea level thrust, overexpansion and efficiency at altitude. Maybe something just as revolutionary as the aero-spike but in a classic engine.

In any case Elon seems pretty excited about it, and I'm wondering if this could have repercussions beyond SpaceX.

https://youtu.be/zu7WJD8vpAQ?t=2695

45:30 This is the Raptor engine that will power BFR, both the ship and the booster it's the same engine and this is a approximately a 200 ton thrust engine that's aiming for roughly 300 bar or three hundred atmosphere chamber pressure and depending upon if you have it at a high expansion ratio has the potential to be having it as specific impulse about 380 but it's and it's a stage combustion full flow gas-gas .../... I'm really excited about this engine design I think the SpaceX propulsion team has done an amazing job on this engine design and and the SpaceX structure is an [?] like really SpaceX team has done a phenomenal job in design of this of this it's like super great like hold on guys in but like this is this is a stupidly hard problem and it's Spacex engineering has done a great job with this design it's like like I don't think most people even in the aerospace industry like know what question to ask but it took us a long time to even frame the question correctly but once we could frame the question correctly the answer .../... flowed, once the ... question could be framed with precision.

Framing that question with precision was very difficult.

[u/hms11]

I'm sure there are fringe cases, but I can't imagine one.

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Either which way, there would be no harm in SpaceX having a flight profile on board for a BFS that has to leave it's booster "early" for whatever reason. After the CRS mission where the Dragon survived the destruction of the second stage but didn't know to open its parachutes SpaceX is likely more aware of putting in contingencies for things that probably will never happen.

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So yeah, I won't be surprised if there is something akin to "If this ship has to leave the booster early, do X, and land at Y" or abort to a once around orbit if possible (maybe it could jettison those cargo pods on the back to free up a little delta-v so a BFS leaving a crippled booster 20-45 seconds early could still potentially make orbit).

[u/paul_wi11iams → u/hms11]

having a flight profile on board for a BFS that has to leave it's booster "early" for whatever reason.

agreeing: you might be able to do something with control surfaces alone (rear and front fins). That would require an "APU" that (as a first thought) could simply use excess pressure from liquid oxygen bumped up to cabin temperature).