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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

#91

Post by vermontdevil »

adventured wrote: post #74
allthenews wrote: post #69

There's something I don't understand about space travel. We made it to the moon in 1969, on the first computer to use ICs[1]. Since then we've seen monumental advances in computer science, material science, manufacturing, rocketry, and just about any other component of space flight. Why 50 years later is it still such a relatively difficult task to launch a rocket into space? Why is it still so expensive and failure…

Apollo cost ~$120-$150 billion in today's dollar. Falcon Heavy cost $500m-$1b to get right. Given some time, Musk could probably make it so a trip to the Moon costs $300 million, maybe less. We've ( they've ) accomplished an extraordinary cost improvement. It's still such a monumental task, because the laws of physics have not changed. Building a huge rocket, putting people in it, and firing it at the Moon, is not th…

Don’t forget Merlin design is based on an engine developed during the Apollo program. The lunar descent engine.

There has to be some savings in that when Musk and Mueller developed the current engine. But that’s fine. I’m good with our tax money from prior programs helping future endeavors.

https://en.wikipedia.org/wiki/Descent_Propulsion_System

Re: Musk explains why SpaceX prefers clusters of small engines

#92

Post by philipwhiuk »

mhandley wrote: post #12

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.…

It's slightly ironic that SpaceX has brought economies of scale to the space industry and Telsa is struggling with scale in the automotive industry.

Re: Musk explains why SpaceX prefers clusters of small engines

#93

Post by philipwhiuk »

ygra wrote: post #41
grondilu wrote: post #19

Would 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?

One goal of BFR is Mars, from which you can only get back if you can produce fuel on the ground. Which kinda rules out RP-1 as propellant. Hence the need for a methane engine. Unrelated to the number of engines here, obviously; just another point to consider. And if they end up designing a new engine they could just as well apply what they have learned in the meantime. Merlin is a very conservative design, favouring…

> from which you can only get back if you can produce fuel on the ground

Is this actually realistic?

Re: Musk explains why SpaceX prefers clusters of small engines

#94

Post by greglindahl »

philipwhiuk wrote: post #92
mhandley wrote: post #12

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.…

It's slightly ironic that SpaceX has brought economies of scale to the space industry and Telsa is struggling with scale in the automotive industry.

Yes and no -- SpaceX has the greatest scale in the space industry, and Tesla has small scale in the car industry. That's mainly a statement that cars are a huge industry, and rocket launchers is a small one.

Re: Musk explains why SpaceX prefers clusters of small engines

#95

Post by marktangotango »

Robotbeat wrote: post #47
marktangotango wrote: post #33

Earlier quoted context omitted.

The landing case makes me wonder about the wisdom of Blue Origins choice of going with a larger engine, the BE-4 with 550,000 lbf for the New Glen vehicle. Much more thrust than a Falcon 9 landing on a single M1D. Surely they've run the numbers and find it viable though.

Blue Origin also has worse mass fraction on the booster stage than SpaceX does, which means they don't need to throttle down as much. If you look at New Shepard as a preview to New Glenn, there are a whole bunch of aerosurfaces which add dry mass, essentially ballast. New Glenn will add big side fins, which will help increase lift, drag, and provide more ballast. Also, the New Glenn booster will have 7 engines. Not s…

The thing about these reusable boosters that’s surprising to me when I think about is; they really don’t need very many of them. Like 10 or 50?

Re: Musk explains why SpaceX prefers clusters of small engines

#96

Post by Nomentatus »

allthenews wrote: post #69

There's something I don't understand about space travel. We made it to the moon in 1969, on the first computer to use ICs[1]. Since then we've seen monumental advances in computer science, material science, manufacturing, rocketry, and just about any other component of space flight. Why 50 years later is it still such a relatively difficult task to launch a rocket into space? Why is it still so expensive and failure…

True, we haven't launched THAT many rockets or kinds of rockets: but according to Musk, oligopolies and rent-seeking has been most of the problem keeping us stuck; with everything from sourcing Aluminum to finding a better way than just milling a huge chunk of it, and on and on. No real competition, so little real research or change, and providing that was his motivation to throw his hat into the ring.

Re: Musk explains why SpaceX prefers clusters of small engines

#97

Post by Nomentatus »

SpuriousSignals wrote: post #4

It 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.

I believe a leak in a fuel line (even a dissection) would need oxygen to cause an explosion rather than failure, and would not be enough force to cause a structural failure. The fuel isn't under extreme pressure, else those pumps wouldn't be needed, and Pogo effects wouldn't have been a problem.

Re: Musk explains why SpaceX prefers clusters of small engines

#98

Post by aurizon »

theothermkn wrote: post #56
aurizon wrote: post #55

Earlier quoted context omitted.

LOL, read and weep https://en.wikipedia.org/wiki/Cylinder_stress https://en.wikipedia.org/wiki/Rocket_engine the round nozzle gets bigger in two dimensions, while the walls do not, so the walls must get thicker as the nozzle gets larger in a square law manner

I read most of that, and did not weep, 20 years ago when I was getting my degree in aero/astro engineering. Neither of your links support what you're trying to say. The walls get linearly thicker, and your intuition about why the walls need to get thicker is incorrect.

The walls need to get thicker indeed, and that extra thickness gradually adds to the weight of the engine faster than adding extra small engines for the same thrust = less and less payload

Re: Musk explains why SpaceX prefers clusters of small engines

#99

Post by marktangotango »

allthenews wrote: post #69

There's something I don't understand about space travel. We made it to the moon in 1969, on the first computer to use ICs[1]. Since then we've seen monumental advances in computer science, material science, manufacturing, rocketry, and just about any other component of space flight. Why 50 years later is it still such a relatively difficult task to launch a rocket into space? Why is it still so expensive and failure…

I'll throw out another answer no has mentioned yet; those cats back in the 60's were good, damned good. They did an exceptional job with what they had. Recall the SR-71 is from that era as well.

Re: Musk explains why SpaceX prefers clusters of small engines

#100

Post by [deleted] »

philipwhiuk wrote: post #93
ygra wrote: post #41

Earlier quoted context omitted.

One goal of BFR is Mars, from which you can only get back if you can produce fuel on the ground. Which kinda rules out RP-1 as propellant. Hence the need for a methane engine. Unrelated to the number of engines here, obviously; just another point to consider. And if they end up designing a new engine they could just as well apply what they have learned in the meantime. Merlin is a very conservative design, favouring…

> from which you can only get back if you can produce fuel on the ground Is this actually realistic?

[deleted]
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