" wrote in
oups.com:

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? How is the asymmetry of the SRB thrust compensated for?
By gimballing the RD-180s? By offsetting the RD-180s? In the former
case, isn't this inefficient? In the latter case, how does a 40x or
50x model manage to fly straight?

The asymmetry IS interesting, but apparently the offset
isn't severe and the RD-180 has excellent control
authority. The Atlas V comes with any number of solids
from 0 to 5, but you'll note that the offset is never
more than one motor unit. Possibly the solids' nozzles
could also be individually adjusted, although they are
non-steerable.

--Damon