Merlin engines are arranged in an Octaweb Structure. ‘Octa’ means eight and ‘web’ structure as it depicts radiating lines from center. The Octaweb is a metal structure that supports eight engines surrounding a center engine at the base of the launch vehicle. 

Falcon 9 v1.0 and v1.1 engine
Falcon 9 v1.0 and v1.1 engine. Credits: Craigboy/Wikipedia

What is its purpose?

The short answer is: stability. The Octaweb also reduces the length and weight of the Falcon 9 thrust structure, simplifying the rocket’s design and assembly. Streamlining the manufacturing process ultimately keeps launch costs down. Further Octaweb helps in isolating the engine damage. Each engine is in its own compartment and a malfunction in one engine won’t make a detrimental impact on other engines. So, no cascading of damage from one engine to all other will happen.

 Mating the Octaweb of a Falcon Heavy centre core to the tank © SpaceX
Mating the Octaweb of a Falcon Heavy centre core to the tank © SpaceX

The engines are placed in offset

All the Falcon 9 engines apart from the center, are placed in offset with respect to the compartment. This is to make additional space for the exhaust of the gas generator. (Merlin engine runs on a gas generator engine cycle.)

Engines are placed at offset. The smaller black holes is for the gas generator exhaust.
Engines are placed at offset. The smaller black holes is for the gas generator exhaust. Credits: SpaceX

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