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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.
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?
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.
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.