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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.
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.
The real reason they are taking this approach is larger engines are ruinously expensive and remain fundamentally outside the reach of private companies and the domain of nation states. The materials science research required to replace those 31 engines with say 3 or 5 engines would run up a bill pushing $100 billion. The Chinese and the Russians will remain in the game because at the end of the day that's exactly what they will do while SpaceX sits back and watches. With smaller engines you pay a substantial weigh penalty for the buttressing and gimballing that goes with each engine. All else equal a design with few engine is more efficient. And the redundancy argument is frankly nonsensical because the probability of failure goes up with the number of parts and smaller engines = more parts = a greater chance of your QC missing defects = a greater chance of BOOM. This is why the US always went with fewer but larger engines and the cash strapped Soviets went with smaller but more numerous engines. It was wholly a question of available funds.
The most extreme example of this sort of thing ever attempted was OTRAG.
John Carmack had some interesting things to say about that at his now-defunct Armadillo Aerospace website, some of which have been preserved at Wikipedia here.
Ceci n'est pas une signature.
The irony of the second flight was that KORD shut down every engine EXCEPT the one that was reporting a problem