Atlas 5-411 Astra 1KR, Flt#8, with single strapon.
Note the vectoring main motors.
Atlas 5-511-AV010, New Horizons, Flt#7
All up with five strapons.
This illustration speaks for itself.
A special thanks to Gunter Krebs who made comment on my Atlas? 5 post. He also posted a link to the Atlas 5 info on his most excellent website; Gunter's Space Page
I had visited Gunter's sites in the dim and distant past and since then lost his address.
So it turns out that the Atlas 5 can be flown with up to five strap-ons, or as few as one [or zero of course]. This is still practically unique in the space launch industry. Aside from the two shuttle systems which are asymetric by nature, the augmented Atlas 5 is the only other one that I know of. This of course requires a robust thrust vector system and an equally capable flight control computer able to adapt to, or be programmed for, the asymetry from the moment of liftoff in addition to the usual trajectory programming, weather, and any other possible anomolies.
While the original Atlas balloon tankage was welded stainless steel, I learned that the Atlas 5 tankage is isogrid [machined] aluminum. The main motor[s] are/is a Russian RD-180. Actually a single assembly with two chambers. One more very useful link: Atlas 5 Data Sheet
