wrote:

Watching the NH launch reminded me of a something that's been
bothering me for a while: why is the arrangement of SRBs on the
Atlas V asymmetric?

The arrangement is assymetric because there are two external fittings
on the Atlas V common booster core, an avionics pod and an LO2
feedline. These are at about the 10:30 and 1:30 positions. Two of the
solids are between these fittings at about the 11:30 and 12:30
positions. The other 3 are on the opposite side at the 5, 6, and 7
o'clock positions.

How is the asymmetry of the SRB thrust
compensated for? By gimballing the RD-180s?

Presumably.

...isn't this inefficient?

Not really. The shuttle does exactly this. Another possibility is
canting the solid booster nozzles differently to compensate for the
assymetry.

Jim Davis