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Musk explains why SpaceX prefers clusters of small engines

arstechnica.com

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Re: Musk explains why SpaceX prefers clusters of small engines

#11

Post by simonh »

One thing not mentioned is throttle range. Larger engines can’t throttle down as low as smaller ones. That’s irrelevant for launches, but matters crucially for landing.

I know the F9 engines can’t throttle down to a hover, even on a single engine with the fuel tanks mostly empty, but I’m sure they did all their landings up until the last few on a single engine that throttled down to its lowest setting for a reason. Controllability matters and while max power suicide burns are theoretically ideal in practice landing at full thrust on all engines would be highly unlikely to ever be workable. If F9 had say 3 larger engines, I doubt landing would be possible at all. Also you need to be able to build a configuration with an engine in the middle.

Re: Musk explains why SpaceX prefers clusters of small engines

#12

Post by mhandley »

There are two other reasons which SpaceX has mentioned before:

- When you're landing a rocket, you need to be able to throttle down quite low. Even a single Merlin 1-D engine, throttled down, is too much thrust to be able to hover with a nearly empty booster. It's really hard to get stable combustion at very low throttle settings. Having only one engine out of nine running for landing makes this much more manageable.

- There are economies of scale and reliability when you're building large numbers of something. Ariane 5 only launched 6 times in 2017. It uses one first stage engine, so they're only building one engine every 2 months. Falcon 9 launched 18 times in 2017, with 9 first stage engines per launch, that's roughly an engine every 2 days. More continuous construction, better economies of scale, more repeatable.

Re: Musk explains why SpaceX prefers clusters of small engines

#13

Post by russdill »

nine_k wrote: post #5
craig1f wrote: post #2

TL;DR; - You get more fault-tolerance with a large number of smaller engines

Interestingly, the Soviet Н1 Moon-bound rocket [1] used a similar setup, and it was plagued be reliability problems: making many smaller parts work reliably at the same time is harder for obvious reasons. Either reliability of engines went seriously up, or software control (impossible in early 1960s) made it possible to operate a bunch of less-than-ideal engines successfully. [1]: https://en.wikipedia.org/wiki/N1_(ro…

A more accurate description would be that it's really hard if you never test things and rush them into a launch because of political pressure

Re: Musk explains why SpaceX prefers clusters of small engines

#15

Post by orbital-decay »

nine_k wrote: post #5
craig1f wrote: post #2

TL;DR; - You get more fault-tolerance with a large number of smaller engines

Interestingly, the Soviet Н1 Moon-bound rocket [1] used a similar setup, and it was plagued be reliability problems: making many smaller parts work reliably at the same time is harder for obvious reasons. Either reliability of engines went seriously up, or software control (impossible in early 1960s) made it possible to operate a bunch of less-than-ideal engines successfully. [1]: https://en.wikipedia.org/wiki/N1_(ro…

Actually, N1 failures had little to do with the complexity itself, and engines were reliable enough for the time.

Due to the lack of funding (the soviet lunar program was given the priority well into the moon "race") they were using the old methodology of testing it in the actual flight, not doing any static fires and only doing a bare minimum of ground testing. Saturn V, on the other hand, heavily relied on the ground testing before the launch. That's why Saturn V mostly worked and N1 failed. Energia worked perfectly much later, because it was developed with the proper amount of ground testing.

Re: Musk explains why SpaceX prefers clusters of small engines

#17

Post by IshKebab »

nine_k wrote: post #5
craig1f wrote: post #2

TL;DR; - You get more fault-tolerance with a large number of smaller engines

Interestingly, the Soviet Н1 Moon-bound rocket [1] used a similar setup, and it was plagued be reliability problems: making many smaller parts work reliably at the same time is harder for obvious reasons. Either reliability of engines went seriously up, or software control (impossible in early 1960s) made it possible to operate a bunch of less-than-ideal engines successfully. [1]: https://en.wikipedia.org/wiki/N1_(ro…

It's almost like you read the article.

Re: Musk explains why SpaceX prefers clusters of small engines

#18

Post by Maybestring »

nine_k wrote: post #5
craig1f wrote: post #2

TL;DR; - You get more fault-tolerance with a large number of smaller engines

Interestingly, the Soviet Н1 Moon-bound rocket [1] used a similar setup, and it was plagued be reliability problems: making many smaller parts work reliably at the same time is harder for obvious reasons. Either reliability of engines went seriously up, or software control (impossible in early 1960s) made it possible to operate a bunch of less-than-ideal engines successfully. [1]: https://en.wikipedia.org/wiki/N1_(ro…

The N1 had insufficient component level testing. They decided to go straight to full up tests due to the cost of the test stand needed.

The big complication with the N1 was that they tried to steer it with variable thrust, rather than the usual approach of thrust vectoring.

Re: Musk explains why SpaceX prefers clusters of small engines

#20

Post by hinkley »

Klathmon wrote: post #3

It makes sense from a reliability perspective to have more smaller semi-redundant parts, especially since it sounds like they have the whole control system down, "scaling up" from managing and controlling 3 engines probably isn't all that different from managing 9, and eventually 31. (obviously this is still rocket science, and nothing is "easy") I'm curious if there are other benefits. I'd imagine that manufacturing…

But there has been talk in the past, including from Musk if memory serves, about the limits of reliablility.

We aren’t talking about hard drives here. You throw a couple more in and if one fails you just turn it off. Hard drives don’t explode and destroy the hard drive next to them or cause the enclosure to fail.

More rockets is more things trying to explode in only the same direction.

The other responder talked about the operational excellence that can’t be achieved if the numbers get too small. That seems more likely.

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