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