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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

#31

Post by sebazzz »

jballanc wrote: post #7
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.

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

It is not an explosion. From the first comment:

Engine didn't explode, it detected an anomaly and shut itself down. Here's spacex's statement concerning the event-

"Approximately one minute and 19 seconds into last night's launch, the Falcon 9 rocket detected an anomaly on one first stage engine. Initial data suggests that one of the rocket's nine Merlin engines, Engine 1, lost pressure suddenly and an engine shutdown command was issued. We know the engine did not explode, because we continued to receive data from it. Panels designed to relieve pressure within the engine bay were ejected to protect the stage and other engines."

Re: Musk explains why SpaceX prefers clusters of small engines

#32

Post by shmerl »

orbital-decay wrote: post #15
nine_k wrote: post #5

Earlier quoted context omitted.

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 grou…

> engines were reliable enough for the time.

Engines alone could be, but the their combination wasn't reliable because of interactions between them and need for synchronization. As Musk points out, it was an avionics problem.

Re: Musk explains why SpaceX prefers clusters of small engines

#33

Post by marktangotango »

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.…

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.

Re: Musk explains why SpaceX prefers clusters of small engines

#34

Post by marktangotango »

jlebrech wrote: post #21

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.

In the post Falcon Heavy press conference, he specifically mentioned scaling up to a Super Heavy with two additional side boosters, four in all. Sounded like they had designed for that, from the way he just threw it out there.

Seems odd to me, since FH is an interim vehicle until BFR comes on line in 5-10 years.

Re: Musk explains why SpaceX prefers clusters of small engines

#35

Post by Diederich »

jlebrech wrote: post #21

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.

I believe that Musk has made it abundantly clear that the BFR is their goal: https://en.wikipedia.org/wiki/BFR_(rocket)

I read that Musk wanted to scrap the Heavy development to free resources to focus on BFR, but there was sufficient immediate customer demand to justify continuing.

Most of us are quite glad that he did.

Re: Musk explains why SpaceX prefers clusters of small engines

#36

Post by bunderbunder »

vannevar wrote: post #22
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.…

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…

How does one quantify the risk of cascading failure?

My initial instinct, informed by computing, is to say that it's easier to avoid cascading failures in the system that is composed of more, smaller parts. All other things being equal, in a rocket with 5 engines, if one of them fails, then each of the remaining ones needs to pick up 1/4 of the slack to compensate. In a rocket with 9 engines, not only would each of the remaining engines need to pick up only 1/8 of the slack, but the total size of the gap would be a little over half as much, too.

(This is obvs ignoring the possibility that the failure in question is catastrophic.)

Re: Musk explains why SpaceX prefers clusters of small engines

#37

Post by ygra »

marktangotango wrote: post #33
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.…

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.

Thrust isn't that much of an issue. Yes, Falcon 9 cannot hover. But they don't want it to anyway, as it's too wasteful on fuel.

Re: Musk explains why SpaceX prefers clusters of small engines

#38

Post by aurizon »

A large portion of the efficiency of small engines comes from the reduction of "hoop stress". This is the linear tension in the wall of a pressure vessel(rocket engine) which varies as the square of the diameter. Twice the diameter = 4 times the pressure (hoop stress) the walls must take. A rocket is a special case of a balloon - with an expansion nozzle attached to couple the impedance of the combustion chamber to the outside environment. This means that 9 smaller rocket engines will weigh less than 1 large rocket engine of the same thrust. On top of this is the small engine basic throttling capability with the overlay of cutting out engines to add greater capability.

Re: Musk explains why SpaceX prefers clusters of small engines

#39

Post by [deleted] »

bunderbunder wrote: post #36
vannevar wrote: post #22

Earlier quoted context omitted.

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…

How does one quantify the risk of cascading failure? My initial instinct, informed by computing, is to say that it's easier to avoid cascading failures in the system that is composed of more, smaller parts. All other things being equal, in a rocket with 5 engines, if one of them fails, then each of the remaining ones needs to pick up 1/4 of the slack to compensate. In a rocket with 9 engines, not only would each of t…

[deleted]

Re: Musk explains why SpaceX prefers clusters of small engines

#40

Post by gizmo385 »

sebazzz wrote: post #31
jballanc wrote: post #7

Earlier quoted context omitted.

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

It is not an explosion. From the first comment: Engine didn't explode, it detected an anomaly and shut itself down. Here's spacex's statement concerning the event- "Approximately one minute and 19 seconds into last night's launch, the Falcon 9 rocket detected an anomaly on one first stage engine. Initial data suggests that one of the rocket's nine Merlin engines, Engine 1, lost pressure suddenly and an engine shutdow…

> Panels designed to relieve pressure within the engine bay were ejected to protect the stage and other engines.

Does anyone have details on this? I'm just curious how these work because it sounds fascinating.

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