While viewing this week's press release for the taxi test of the Stratolaunch I noticed how closely the engines seemed to be spaced. It would seem the ideal position would be near the fuselage to minimize asymmetric thrust.
The B-52 places engines directly next to each other, while the 747 spaces them far apart. Is there a rule of thumb for ideal engine spacing?
[This question deliberately excludes propeller aircraft due to the many mechanical and aerodynamic differences.]
The Stratolaunch engines are approximately one engine diameter apart and very close to the fuselage.
The 747 has much larger spacing between engines (~3 engine diameters) and further from the fuselage.
Engine spacing on the Antonov 225 compared to the 747 is much closer to fuselage but about the same spacing.