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Could someone explain why Burt Rutan designed Stratolaunch with two separate tail structures instead of joining them with a center section like the wing? It seems like the center wing could be subject to a lot of stress, especially in turbulence or with an engine out.

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Control surfaces behind the payload would be aerodynamically shadowed, and, depending on payload shape, either unable to develop significant control moments or subject to chaotic airflow that'd make them more of a liability than an asset.

But wouldn't a center tail section still help with overall structural strength even if it didn't have control surfaces?

Could be that it provides more flexiblity to the airframe. Or that too much horizontal stabilizer would limit pitch ability. Other aerodynamic effects may be in play as well, since it's carrying the payload in the center.

Single engine out isn't probably much of a concern as there are two redundant engines per side.

I'm just speculating though.

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