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I read a while ago about the famous Soviet N-1, called by some the most powerful rocket (in terms of thrust at liftoff) in history. It used an incredible 30 engines in its first stage. The American Saturn V, on the other hand, used only five F-1 engines in its first stage. The difference was that each F-1 was much more powerful than a single one of the (initially NK-15) engines used on the N-1. The advantage of the 30-engine system was that if one engine failed, there were backups, and more fuel could be shifted to or from others to compensate (although this led to some amusing mishaps with the KORD computer controlling the engines).

What are the other advantages/disadvantages for each of the systems (30-engines vs. 5-engines)? What made the Soviets choose a 30-engine system, while the Americans chose a 5-engine system?

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  • $\begingroup$ Wow! 11 million lbs of thrust on liftoff! Shuttle was only around 7 millions, Saturn V - 7.5 million, Energia 7.8 million. SLS will be around 7-8 million lbs. Wild. $\endgroup$ – geoffc Sep 22 '14 at 0:06
  • $\begingroup$ Related asparagus staging (yeah, I know - KSP). This is used in real life in the falcon heavy launch vehicle. $\endgroup$ – user5892 Sep 22 '14 at 6:32
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Combustion instability is a very hard problem to crack. It gets harder as the engine gets larger.

The Americans took on the challenge of a 1.5 million lb thrust engine and beat it (F-1, I suppose even the J-2 on the second (5) and third stages (1)). The Soviets tried and failed and went with a small (NK-15) engine. They needed the thrust so they clustered them.

The fuel lines at some point combine, and back pressure can cause vibrations in other feed pipes. Saturn V suffered from Pogo oscillations even with only 5 engines, you can imagine the N-1 would have had bigger issues with 30.

What would be interesting is to hear how this has affected Falcon 9 with 9 engines. I think it is the largest number of engines on a stage currently flying. (Or ever? Since the N-1? What had more engines?)

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    $\begingroup$ +1. I've talked with some of the people who were charged with attacking the pogo problem. They didn't really solve the problem, at least not in a theoretical sense. They just found ad hoc ways that appeared to make it go away. Good enough! $\endgroup$ – David Hammen Sep 21 '14 at 20:23
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    $\begingroup$ @DavidHammen My recollection of what killed the N-1 was basically Pogo writ large. 30 instances of pogo interacting with each other ripped the thing apart. $\endgroup$ – geoffc Sep 21 '14 at 23:18
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    $\begingroup$ Exactly. There's a huge difference between theory and practice. Theory hand-waves all those nasty problems away. Practice says you cannot do that, lest your vehicle act like a pogo stick. $\endgroup$ – David Hammen Sep 21 '14 at 23:48
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    $\begingroup$ @geoffc I've read all 4 volumes of Boris Chertok's Rockets and People. Chertok was responsible for designing the KORD. It was basically the electronic brain responsible for 30 engines. The major design point was if an engine was having trouble, KORD was supposed to shut it down and the opposite engine too to maintain symmetric thrust. N-1 was "overdesigned" with 30 engines such that it could suffer 4 shutdowns, or 2 pairs of shutdowns, and still make it to orbit. KORD never worked right. The analogue technology of the day apparently made it impossible. $\endgroup$ – DrZ214 Jan 3 '17 at 18:10
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    $\begingroup$ 30 engines with POGO sounds worse than 5 engines with POGO, but the rocket with 30 engines has smaller engines and the magnitude of their POGO effect is likely to be much smaller. Independently of that, there's a higher chance of their POGO canceling out or at least not reinforcing, just like 6 or 8 cylinder car engines are smoother than 4 cylinder engines. $\endgroup$ – Philip Ngai Apr 16 '18 at 17:54
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They might have been going for the advantage of you manufacturing a greater amount of individual engines, similar to what SpaceX is going for. SpaceX instead of manufacturing rockets with 1 or two engines goes for now with 9 per rocket and can improve on the manufacture using the methods that made so many industrialized things so much cheaper today. That is what I read about SpaceX take on it and it might have influenced the decision of this older rocket as well.

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  • $\begingroup$ The main factor was the Russians were unable to solve the problem of combustion instability in a large combustion chamber. Neither Russia or the US had good computer modeling capabilities but the US had more money to build prototypes and try different things until they stumbled across something that worked. Russia still uses the small combustion chamber concept as seen in engines like the RD-170 with 4 chambers driven by one turbopump. $\endgroup$ – Philip Ngai Apr 16 '18 at 17:57

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