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Looking to explore the inverse point of this question:

What challenges are there for more, smaller liquid fuel engines instead of one large?

As stated in the answer to the above question, fewer, bigger engines reduces parts count and increases reliability at the expense of thrust range (relevant for reusable boosters such as Falcon 9), but what happens when you want to go really big?

The largest single combustion chamber liquid-fueled engine ever successfully flown was Saturn V's F1, last flown 45 years ago. It has no equal today (not considering solid-fuel boosters such as the Shuttle SRBs); even the Space Shuttle main engines develop only about half as much thrust.

The Falcon 9 demonstrates that nine engines are enough to provide the variable range of thrust necessary to land an expended booster. So what trade-offs lead SpaceX to employ large numbers of engines for their proposed big heavy boosters (one proposal called for 42 engines, another calls for 31) rather than develop bigger engines to use in smaller numbers?

I know that one problem faced during development of the F1 was combustion instability. Is this the dominant factor inhibiting the development of bigger engines, or are there others, and if so, what? The modern "benchmark" seems to be the SSME; what is impeding development of anything substantially larger, like, for example, that of the Sea Dragon (again, limiting the question to liquid-fueled engines)?

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  • $\begingroup$ For SpaceX, it seems likely that they will use Raptors in Falcon-sized rockets at some point -- possibly starting with a methane upper stage for Falcon 9 -- so that commonality may be driving their engine sizing. $\endgroup$ – Russell Borogove Mar 18 '18 at 19:49
  • $\begingroup$ The F-1 took a lot of time for development. Development was started in 1955 and tests continued at least to 1965. Combustion instabilities were addressed from 1959 through 1961. See Wikipedia F-1. The first tests of the combustion chamber were done using pressure feed before the propellant pumps were ready. $\endgroup$ – Uwe Mar 18 '18 at 19:53
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    $\begingroup$ In the specific application to the BFR/BFS this is a duplicate of this question: space.stackexchange.com/questions/25158/…. Short answer they need at least 6 engines on the spaceship for redundancy (plus ground and space nozzles). That determined the engine thrust and it turnd out they need 31 of them for the booster. $\endgroup$ – Steve Linton Mar 18 '18 at 20:00
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    $\begingroup$ I think common use of the euphemistic phrase "combustion instabilities" really makes it harder for the broad public to understand what sort of problem it actually is, at these scales. It evokes the idea of combustion instability in a car engine, like the engine coughing a bit, exhaust smoking a little, a small shake etc. The photos at the beginning of "Ignition" depict the effects in rocketry much more evocatively. $\endgroup$ – SF. Mar 19 '18 at 9:00
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    $\begingroup$ @AnthonyX Meh! Good weasel with single combustion chamber. :) But it sort of makes the point that it was easier, cheaper, and more sustainable (F-1 flew only on S-V, RD-170 is still in series production, sort of, after 20+ years of flight. Or however long Zenit has been in service, since at least the mid-90's). $\endgroup$ – geoffc Mar 19 '18 at 13:12

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