Slashdot Mirror
Elon Musk Explains Why SpaceX Prefers Clusters of Small Engines (arstechnica.com)
An anonymous reader quotes a report from Ars Technica: The company's development of the Falcon 9 rocket, with nine engines, had given Musk confidence that SpaceX could scale up to 27 engines in flight, and he believed this was a better overall solution for the thrust needed to escape Earth's gravity. To explain why, the former computer scientist used a computer metaphor. "It's sort of like the way modern computer systems are set up," Musk said. "With Google or Amazon they have large numbers of small computers, such that if one of the computers goes down it doesn't really affect your use of Google or Amazon. That's different from the old model of the mainframe approach, when you have one big mainframe and if it goes down, the whole system goes down."
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. So it was with rocket engines. "It's better to use a large number of small engines," Musk said. With the Falcon Heavy rocket, he added, up to half a dozen engines could fail and the rocket would still make it to orbit. The flight of the Falcon Heavy likely bodes well for SpaceX's next rocket, the much larger Big Falcon Rocket (or BFR), now being designed at the company's Hawthorne, California-based headquarters. This booster will use 31 engines, four more than the Falcon Heavy. But it will also use larger, more powerful engines. The proposed Raptor engine has 380,000 pounds of thrust at sea level, compared to 190,000 pounds of thrust for the Merlin 1-D engine.
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. So it was with rocket engines. "It's better to use a large number of small engines," Musk said. With the Falcon Heavy rocket, he added, up to half a dozen engines could fail and the rocket would still make it to orbit. The flight of the Falcon Heavy likely bodes well for SpaceX's next rocket, the much larger Big Falcon Rocket (or BFR), now being designed at the company's Hawthorne, California-based headquarters. This booster will use 31 engines, four more than the Falcon Heavy. But it will also use larger, more powerful engines. The proposed Raptor engine has 380,000 pounds of thrust at sea level, compared to 190,000 pounds of thrust for the Merlin 1-D engine.
Of course, it has to be actual redundancy. The Soviet N1 moon rocket had a problem that when its engines failed, they tended to take out adjacent engines. You have to be absolutely sure that failures aren't going to spread (pieces of shattering turbopumps, fires, backpressure, etc), or you're actually making the problem worse.
Of course, everyone working on rockets today knows the lessons of the N1 and it'd be incompetence not to exhaustively test for resilience against cascading failures.
Beyond redundancy, one neat thing about engine clusters is that you can create a virtual aerospike effect to some degree.
It's time for Operation Crazy Plan.
I thought the F in BFR stood for something else than Falcon...?
Yes but Falcon doesn't get censored in interviews
Pain is merely failure leaving the body
Yes. Redundancy is always good.
Obviously SpaceX has calculated this but Id like to see a graph of the probability of flight failure of a rocket with 5 big engines and a rocket of 31 small engines. The more engines the higher the chance one will not work but also the higher the redundancy. The fewer engines the less chance one will not work but also the greater the chance one going out dooms the flight.
I came to the datacenter drunk with a fake ID, don't you want to be just like me?
Yes. Redundancy is always good.
Yes. Redundancy is always good.
To a certain extent, SpaceX's architectural approach of many engines has arguably reduced costs. By making more copies of single engine design, the cost per engine has dropped significantly.. The manufacturing reliability is better, too. (What would the failure rate of a Model T have been if Ford was only building one per week? Building lots of something continuously brings you up the learning curve faster, reduces mistakes, and forces you to invest in tooling and fixturing that ensures each step is successful and repeatable.)
In this case, I think it is likely that the cost/kg and the reliability of a 9-engine rocket is better than a rocket that had a single engine of comparable power.
As you say - there are limits to this approach. (I'd call it modularity, rather than redundancy.) The efficiency of rocket engines doesn't scale down well and, as you point out, requires a build up of all the ancillary equipment.
Friendly reminder, millenials aren't kids anymore.
Yes they are. Get off my lawn!
The rocket is built to contain engine explosions. We don't know if that'll be effective for all engine failures, but they've already had at least one engine failure on a F9 flight without consequences for the mission.
Do you have to shut down computers opposite to the one that fails to maintain "computing balance"?
The Soviets did that on the N-1 because it allowed them to install the engines without gimbaling hardware, simplifying the design. The F9 does have gimbals, so it doesn't need to shut down the opposing engine.
Does a failing computer fundamentally alter your mission profile to the point that you have to change the computations for ALL other computers?
So what? That's what computers are really good at.
A lot of the Soviet plans were based around the expectation of failures. All of their (numerous) Venus missions, for example, were launched in pairs. The idea was that the incremental cost was low but the initial costs high, so you might as well send two. And if both work, you collect two separate datasets, from different locations. Usually when one failed they pretended it was an experimental or military launch - for example, Venera 4's twin was Kosmos-167, while Venera 7's was Kosmos-359.
It's hard to call one approach the right approach and one the wrong approach. The Soviet approach certainly paid dividends on Venus, but their Mars programme was a miserable failure compared to the US.
It's time for Operation Crazy Plan.
They said that failure was because of lack of fuel, so more engines wouldn't help that.
Damn straight! And while we're at it we should get back to single piston car and truck engines. Those Europeans are screwed with their finicky 12 cylinder sports cars or even (gasp!) 16 cylinders!!! It's madness I say!
Simplify it all to a more efficient single cylinder engine. And don't even get me started on all those crappy WWII airplane engine designs...
Has my point been made? No? Sometimes the cost and/or efficiency of the engine is not the biggest consideration in a project.