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Size of engines
Jeff Findley wrote:
In article , says... SpaceX just designed brand spanking new Raptor engines. [...] They're gonna putbetwene 37 and 40 on the big Super Heavy stage 1. Big picture, what limits the size of engine you are building? One of the issues with very big engines is combustion instability. That was a huge issue during F-1 engine development. So, making many smaller engines is a way to avoid having to develop very large engines. There's also the Russian solution, RD-170 is an "F-1 class" engine with four combustion chambers because they deemed it too hard to solve the combustion stability. The Russians have a number of multi-chamber engines in that lineage including the dual-chamber RD-180 used in the Atlas V. It MAY... be that combustion instability may be less of an issue these days with all the advances in CFD but no one seems to want to go down that path. Either way I suspect that with modern computer control it's better to build and use multiple Raptor/BE4 sized engines unless the rocket is bigger than even Super Heavy! Or does growing beyond a certain size introduce fluid dynamic problems of how liquid methane and o2 behave in pipes, behave as they flow through large turbine with much mroe "space" to build, cause bubbles and cavitate etc) ? Would it be fair to state that the Raptors are sized to be easy to build? (so they need more). That was the philosophy with Merlin. Also, commonality between the second stage engines and the first stage engines naturally means you need far more engines for the first stage. Thanks rocket equation! Musk has said that SpaceX is "tracking to well under $1M for V1.0" and has a goal of $250k for V2.0 (250 ton thrust-optimized engine. It's always been the plan that Raptor should be producable in large numbers and that the per-unit cost should be lower than the smaller Merlin, AFAIK they've actually below the much smaller Merlin's cost? The benefit of starting with a clean sheet and thinking how to build it during the entire process I guess. They're still tweaking the design a lot but they're already producing a lot of engines (SN20 was on Starship SN4 when it was consumed by fire after ground equipment issues). Still, not sure that the Merlin size was choosen for ease of production reasons. Fundamentally only SpaceX knows ALL the reasons for their decisions, it's not impossible it could have been involved but there's so many other reasons why it makes sense for SpaceX to build Raptor around that size that I doubt it. I find it fascinating that there's so many rocket engines around the 1.6-2.4 MN (MegaNewton) range - Raptor, BE-4, RD-191, NK-33, RS-25 to name some well-known ones. And the per *chamber* thrust for RD-170/180 (Energia and Atlas V respectively) is also in that range. I can only think of one liquid fuelled rocket engine with more thrust per chamber, the F-1 with 6.7 MN in a single chamber (RD-170 is more powerful but needs 4 chambers). I discussed earlier why that might not be a size path others may want to go down. https://twitter.com/elonmusk/status/1179107539352313856 |
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Size of engines
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