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.

Comments

No shit Sherlock

[Anonymous Coward]

Redundancy is always good

Re:No shit Sherlock

[Rei]

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.

Re:No shit Sherlock

[hackertourist]

The N1 failed through a combination of lack of money, lack of political will and losing the space race.

The plan was to skip building a test stand for the first stage (which would be large and expensive, to cope with 5000 tons of thrust, and take lots of time to buil). Instead, they'd do test flights, fully expecting a number of initial failures. 14 test flights were planned.

After the Apollo 11 landing, the urgency was lost and funding slowed. The last straw was appointing Valentin Glushko as head of the Soviet space program. He was a known opponent of the N-1, favoring his own design.

Re:No shit Sherlock

[Rei]

Actually, they had an engine control computer called KORD. While simple, it was an electronic computer. It was fed 4 types of measurements from each of the engines, and based on a simple algorithm would decide if they were out of acceptable operating parameters, and if so issue a command to shut down the offending engine (in theory, before a catastrophic failure) and its opposing counterpart. It would then ramp up the good engines to compensate for the loss of the dead engines (they defaulted to operating at 75% throttle to allow for this, as well as to reduce stress on the poorly tested engines).

Computer controls based on sensors was a new thing for the team, and the difficulties in filtering out bad data came back to haunt them in the first flight; it misinterpreted pyro noise as a turbopump spinning out of control and shut down a pair of engines, then interpreted pogo and a different engine failure as all of the engines going bad - and shut down the engines for all of the stages, so they couldn't even test the upper stages.

KORD also turned out to have too long of a response time to prevent catastrophic failures in engines, which was one of the things the design team was counting on to overcome the known poor reliability of the engines. KORD's rushed schedule also left it with poor debugging and too few safety checks. On flight two, in addition to still being too overaggressive on engine shutdowns in general (in response to very real engine failures), it caused its widespread shutdowns while the rocket was still over the pad, rather than trying to keep it going long enough to clear the pad. The pad explosion was one of the largest manmade non-nuclear explosions in history and set the N1 project back a year and a half.

Re:Big Falcon Rocket

[sjbe]

Friendly reminder, millenials aren't kids anymore.

Yes they are. Get off my lawn!

Re:Not if the fail catastrophically

[hackertourist]

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.

Re:what about the center core?

[bws111]

They said that failure was because of lack of fuel, so more engines wouldn't help that.

Re:what about the center core?

[Rxke]

IIRC it was a chemical that starts up the engines that ran out TEA-TEB (Triethylaluminum-Triethylborane) So they could not fire the others

Re:I call BS

[Anonymous Coward]

"pay a substantial weigh penalty for the buttressing and gimballing"
Which is why they're doing a lot to optimize the design, including the octoweb (I imagine they'll do something similar on BFR) & not gimballing all of the engines.

"Soviets went with smaller but more numerous engines"
No, the Soviets went with their design because they lacked the manufacturing infrastructure that the US had and building that infrastructure would have taken time. As this was a race time is something the Soviets didn't really want to waste so they went with the smaller numerous engine design.

As far as the "boom" factor, that's a bit up in the air. The mindset for the past few decades in the US has generally been to limit the engines to as few high quality engines as possible, but the Russians have had just as much luck with the numerous cheap engines method (Soyuz has ~20 plus engines in the first stage, 5 engine assemblies with 4 combustion chambers each). The Falcon 9 has even had an inflight failure of an engine with little effect to the rocket. And that has been the only known failure in over 500 engines. With modern sensors and designs the engine failure issue on modern rockets may be moot, only time and flight records will tell.