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According to a recent GAO report (p. 116), ULA's Vulcan rocket's BE-4 engines are "experiencing technical challenges" and there is some concern whether they will be ready / certified in time for the planned first launch.

Hypothetically, if ULA were to seek another engine to replace Blue Origin's BE-4 with minimal alterations to its existing hardware, the only high-thrust methalox engine I'm aware of that's more developed than the BE-4 is SpaceX's Raptor engine. Now, even if it were technically easy to swap in Raptors for BE-4s, there's no way either SpaceX or ULA would agree to such a deal.

But I am curious: if they somehow did agree to this, how easy would the swap be from a technical perspective? Would it be feasible to adapt to a Raptor plumbing designed to supply a BE-4? Would an additional Raptor be needed (the sea-level thrust is 2.2 MN for a Raptor vs 2.4 MN for a BE-4), and if so would there be any feasible way to add it (as I understand a fifth F1 engine was added to the Saturn V)? What other issues might arise (e.g., fuel mixture, fuel temperature, ...)?

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    $\begingroup$ I think this is fairly hard to answer objectively, because how easy it would be depends on a whole lot of technical details of both rockets. Both are proprietary systems full of closely guarded trade secrets. $\endgroup$
    – TooTea
    Jun 22 at 21:26
  • $\begingroup$ @TooTea Especially the BE-4. While Elon and other SpaceX engineers have been quite chatty about some technical details of Raptor, Blue Origin hasn't even publicly mentioned the specific impulse of the BE-4. $\endgroup$ Jun 23 at 15:32
  • $\begingroup$ I have a hunch that one of the main problems is going to be the vastly differing engineering cycles. The Raptor plumbing has changed drastically thrice in two years. True, they are still in development, so rapid changes are expected. But we see with the Falcon 9 that SpaceX is not above making significant changes even in the operational phase. This will apply even more to Starship / Raptor because of the much higher cadence. SpaceX will not want to freeze the design just because of ULA, and they can't very well have a separate production line just for ULA. $\endgroup$ Jun 25 at 7:00
  • $\begingroup$ @JörgWMittag: That's a good point, but in my mind it's closer to a cultural problem than the sort of technical challenge I was hoping to get information on. $\endgroup$ Jun 25 at 14:53
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The BE4 and the Raptor use pretty much the same fuel. They deliver very similar thrust, although the BE4 has a significant margin more.

The problem is, the entire Vulcan rocket has been designed around an engine that matches the BE4.

With the thrust deficit of the Raptor, they would need to add in a third engine. Or resize the whole launcher, invalidating a lot of their development work so far.
But adding an extra engine means completely redesigning the thrust structure and plumbing. And at the very least a complete overhaul of the avionics and control systems, possibly a complete rewrite. And with altered mass and thrust on the core, would the current design for the strapon boosters still be valid?

I think the Vulcan rocket could have been designed around the Raptor with no problem, but it is way too late in its development cycle for a changeover to now be viable. It could be done, of course, but it would be more a case of designing a new rocket and less of adapting the current Vulcan.

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Need to review the number of engines with all of the thrust, throttling, engine runtime, staging timing, payload and structural stress possibilities and implications. Probably needs 3 engines rather than 2 and could at least consider 4. So a number of variants would need to be considered. Following that review it might make sense to increase or even decrease the length of the first stage to optimize the flight plan and payload capability.

Need to redesign the propellant intake and the thrust structure to accommodate a different number of a different type of engine with a different thrust and different pipe work alignment. Need to consider vibration issues and harmonics. Probably all quite doable technically, but not so doable from a company politics perspective.

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