and soon it'll be clusters of boosters, there's space to fit another 4 in a hexagonal pattern. you could lift a mini hexagonal mars base fully assembled that way, land it and land the next one quite close.
Musk explains why SpaceX prefers clusters of small engines
21–30 of 104 posts
Re: Musk explains why SpaceX prefers clusters of small engines
#22There 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.
Yes. One of the biggest predictors of reliability in aerospace systems in general is time in operation---the longer you've actually run something, the closer it approaches the upper limit of reliability for that component. Running 27 small copies gives you 27x the time in operation vs a single big-ass engine.
The downside is complexity, and in particular unknown failure interactions which could bring down the entire system in a cascade. As long as many of the components are identical, though, an improvement in reliability of that component should translate directly into lower probability of failure interactions since there are fewer failures, period.
Re: Musk explains why SpaceX prefers clusters of small engines
#23> The Russian Soyuz rocket has five engines, each of which has six thrust chambers.
RD-107 on the four side blocks has 6 chambers, RD-108 on the central block has 8 chambers.
Re: Musk explains why SpaceX prefers clusters of small engines
#24There 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.…
> When you're landing a rocket, you need to be able to throttle down quite low.
When R-7 was developed in 1950-s, as a weapon, one of requirements was the precision of attained speed at the moment of engine cut-off. It was solved by shutting down main chambers on the upper (second) stage, powered by RD-108 - 4 chambers about 20 tons of thrust each - and maintaining thrust from steering chamber, 4 of them, 3 tons of thrust each.
This is another example of deep throttling.
Re: Musk explains why SpaceX prefers clusters of small engines
#25>For computers, Musk said, using large numbers of small computers ends up being a more efficient, smarter, and faster approach than using a few larger, more powerful computers. I always figured, in theory, a super-super-powerful single threaded single processor would out compete the multi-threaded multi processor design on efficiency, because of the hardware and software inefficiencies in inter processor hardware con…
Depends on the task - it turns out rocket thrust is quite parallelizeable
Re: Musk explains why SpaceX prefers clusters of small engines
#26and soon it'll be clusters of boosters, there's space to fit another 4 in a hexagonal pattern. you could lift a mini hexagonal mars base fully assembled that way, land it and land the next one quite close.
Imagine seeing 6 boosters land simultaneously. Just nuts.
Re: Musk explains why SpaceX prefers clusters of small engines
#27It all depends on how an engine fails. A failure to provide thrust is survivable with more engines. But a vulnerability that causes a fuel line to go boom, taking out the whole rocket, is made more likely by having more engines.
IIRC, the engines in the Falcon 9 are pretty well separated from each other, such that even a catastrophic failure of one engine is survivable by the rest. Indeed, that seems to be what happened the one time an engine did fail (albeit on a very early version of the Falcon 9 with the "square" engine layout): https://www.youtube.com/watch?v=dvTIh96otDw
I believe that was actually the reason why they moved to a "round" configuration.
They realized that the "corner" engines were doing more work, and if one of them failed it caused larger issues than they were expecting.
However in the "round" layout, all of the engines are doing the same amount of work and have a similar amount of control over the rocket, so one failing can be more easily compensated for by the surrounding engines.
Re: Musk explains why SpaceX prefers clusters of small engines
#28and soon it'll be clusters of boosters, there's space to fit another 4 in a hexagonal pattern. you could lift a mini hexagonal mars base fully assembled that way, land it and land the next one quite close.
[deleted]
Re: Musk explains why SpaceX prefers clusters of small engines
#29It 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 abou…
Just an addendum, while Musk in fact compares the rockets to computers, I have found that what the management takes away from conversations with engineers is often not the most important part.
But typically everyone on the team has opinions about success or failure of the project and we all discount something important. Managers doubly so. They rarely credit the spotters who keep them from falling when their process has huge holes in it.
Re: Musk explains why SpaceX prefers clusters of small engines
#30Would it then make sense to plan for even more smaller engines? Like while BFR is meant to have 31 Raptor engines, could it have 60 Merlin engines instead or something?
They certainly could use more smaller engines for the BFR, but just because one dimension of the design benefits from smaller engines doesn't mean the optimal design uses a nano-scale rocket array.