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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.]

Stratolaunch

The Stratolaunch engines are approximately one engine diameter apart and very close to the fuselage.

747

The 747 has much larger spacing between engines (~3 engine diameters) and further from the fuselage.

Antonov 225

Engine spacing on the Antonov 225 compared to the 747 is much closer to fuselage but about the same spacing.

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    $\begingroup$ Part of the constraint was they had to get three engines on one wing, because the center section is reserved for the space ship. $\endgroup$ – zeta-band Feb 28 '18 at 17:50
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    $\begingroup$ The B52 has two engines in the same nacelle, so that would seem to be a logical minimum. $\endgroup$ – selectstriker2 Feb 28 '18 at 19:10
  • $\begingroup$ @zeta-band: They couldn't have put them on top of the center section (given that the rocket goes under there)? Or spaced the engines further apart on the outer sections? $\endgroup$ – Sean Dec 31 '18 at 19:38
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    $\begingroup$ @selectstriker2: The B-52's engines are low-bypass JT3Ds; trying to do that with the high-bypass engines used on the Stratolaunch (or, for that matter, on the 747 and An-225 used as examples) might pose problems. $\endgroup$ – Sean Dec 31 '18 at 19:44
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Placing the engines farther out is mainly done for bending relief in the wing. Given that the Stratolaunch needs to keep its center wing clear of fuselages and engine nacelles, the engines are out enough already. Moving them farther out would create more thrust asymmetry once one engine fails.

Airliners with a central fuselage are different. We have an answer here discussing optimum engine placement. Why they should not be combined in one nacelle is explained here.

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