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Musk explains why SpaceX prefers clusters of small engines

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Enlarge/ The Falcon Heavy rocket proved that 27 engines can fly together and not go all explode-y.SpaceX

One of the most striking aspects of this week's launch of the Falcon Heavy rocket is the number of engines the triple-core booster used to reach orbit. Each of the cores had nine Merlin rocket engines, making for a total of 27 engines.

Prior to this launch, no rocket had ever successfully ascended into orbit with more than nine engines—a feat accomplished previously by SpaceX's Falcon 9 rocket and Rocket Lab's Electron rocket. (The Russian Soyuz rocket has five engines, each of which has six thrust chambers.)

Launching a rocket with 27 engines, therefore, represents a notable step forward in rocket complexity. It is all the more so, considering the Soviet N-1 rocket. Four times, from 1969 to 1972, the Russians attempted to launch their titanic “Moon rocket,” and it failed spectacularly each time. Its 30 engines were just too many to fire, throttle, and steer at the same time.

During an interview with SpaceX founder Elon Musk this week prior to launch, Ars asked Musk if the history of the N-1 rocket concerned him. "No," he replied. "I think with the N-1 failure it was mostly avionics failure. They had engine to engine fire issues." Five decades later, SpaceX could do better.

Like a computer

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."

  • On Monday, Musk discussed the clustering of rocket engines with Ars. Trevor Mahlmann for Ars Technica
  • Each of the three cores has 9 engines. Trevor Mahlmann for Ars Technica
  • A view of the Falcon Heavy rocket on Monday, from one-quarter of a mile away. Trevor Mahlmann for Ars Technica
  • Musk seemed optimistic about the flight, with his gut saying there was a two-thirds chance of success. Trevor Mahlmann for Ars Technica
  • But in reality, the odds are probably closer to 50-50, he said. Trevor Mahlmann for Ars Technica
  • At its maximum capability, the Falcon Heavy could lift 64 tons to low Earth orbit. Trevor Mahlmann for Ars Technica
  • The Falcon Heavy rocket took off at 3:45pm ET Tuesday, with all 27 engines firing. Trevor Mahlmann for Ars Technica

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.

“It gives me a lot of faith for our next architecture," Musk said Tuesday night, after the Falcon Heavy's launch. "It gives me confidence that BFR is really quite workable.”

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