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When dealing with launch vehicles (specifically heavy lifters), there have been a number of different mindsets in terms of engines. The Saturn V used some absolutely massive engines, which was great for reducing TWR, but the combustion stability was extremely problematic for alot of the F1's design and lead to a substantially lower ISP. Alternatively the engines like the RD170/RD180 were just as (if not more) powerful, had a higher ISP, and used a "shared" combustion chamber method to stabilize the combustion when producing this much force, with the tradeoff being a slightly lower TWR. Alternatively, rockets like the falcon and starship just use a ton of individual, completely separate engines that have very little effect on one another. While I know these engines are very different and thus hard to compare, after ~80 years of designing launch vehicles, are any of these potential configurations objectively "best", or are they all just situational? (again specifically for heavy launch vehicles where large thrust is required)

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    $\begingroup$ It's kind of naive to think there would be one best answer that covered every vehicle. Several of your assertions in the question are incorrect as well. $\endgroup$ May 3 at 0:06
  • $\begingroup$ The options for engines are quite sparse and shared by a series of launch vehicles. E.g. the new generation of Chinese rockets only have 2 engine options for all possible payload capacity. $\endgroup$ May 3 at 1:28
  • $\begingroup$ I know that everything would realistically be situational, (If there was one truly ideal option than everyone would probably already be doing it already), so I guess this is more based on historical data and your opinions than anything else. regardless, I am curious as to your insights about this. Also, @OrganicMarble what assertions are wrong? $\endgroup$
    – frank
    May 3 at 13:36
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    $\begingroup$ "...massive engines..." "...great for reducing TWR..."? "...combustion stability...problematic...lead to...lower ISP"? (It may be that this sentence is just poorly written and therefore confusing) "... engines that have very little effect on one another...."? $\endgroup$ May 3 at 13:49
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    $\begingroup$ The RD-170/RD-180 have multiple combustion chambers, not shared combustion chambers. And your equating of size with TWR is inaccurate: the Merlin 1D is smaller than either the F-1 or RD-180, and has about twice the TWR of the F-1. $\endgroup$ May 3 at 14:41

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I think the answer lies in analysis of failure, rather than optimum performance. Individual engine failures can produce a spectrum of knock-on events from simple single engine shut-down through thrust instabilities to fires and multi-engine catastrophes.

The single engine Ariane 5 typifies one extreme of the design spectrum. Engine failure means mission failure. Use a well designed and tested engine. Risk calculations are back-of-the-envelope.

The Soviet N1 rocket illustrates the extreme of the multi-engine design philosophy.

enter image description here

Failure of a single engine can potentially domino into mission failure as it did in 3 of the 4 N1 launches. The modes of failure were unforeseen for those launch failures. Unforeseen modes of failure means risk cannot be anticipated

This is a seldom mentioned issue for the crowded thrust dome of SpaceX’s booster. Look at all that plumbing. Elon hates flange joints for a reason. Every connection comes with a risk. The adjacent engine may be the casualty. Or, potentially, the entire mission.

enter image description here

The Saturn V design seemed to find the Goldilocks compromise. It could have safely completed its mission with one engine shut down, but this did not happen in any of the 13 Saturn V launches.

Development and testing of smaller engines is faster and cheaper, but locks into the risks of massively multiple engines.

Risk management depends critically on the number of engines and their potential modes of failure. Failure cannot be prevented unless the mode of failure can be predicted. Complexity breeds modes of failure.

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    $\begingroup$ On Saturn V middle engine was intentionally shut down early to limit g-loading, but your point remains valid (perhaps adjust the wording) $\endgroup$ May 3 at 11:06
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    $\begingroup$ There were Saturn V engine failures, but not on the first stage. $\endgroup$ May 3 at 12:35
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    $\begingroup$ Saturn V and the N1 flew in the 1960's and 70's. Things have moved on since then. Falcon heavy has already flown a number of times and has 27 engines. Computer simulation is now far better than it was in the 60's/70's by many orders of magnitude. $\endgroup$
    – Slarty
    May 3 at 22:26
  • $\begingroup$ Given Falcon Heavy has flown 3x as of May 2022 I wouldn't say it's been thoroughly tested. $\endgroup$ May 4 at 10:18
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    $\begingroup$ While it doesn't detract or invalidate your fundamental point in any way, I would like to point out that the Superheavy photo shows Raptor v1 engines, which are development engines. Raptor v2 is much cleaner or more compact, which means there will be less exposed plumbing and more space between the powerheads. (OTOH, SpaceX is using this compactness to squeeze even more engines in, the enter cluster will have 13 engines instead of the 9 in your photo, most like an inner circle of 3 surrounded by a ring of 10.) $\endgroup$ May 5 at 5:34

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