The Atlas V 411 configuration is interesting because there is a single SRB on one side of the first stage, requiring the main engines to vector substantially to keep the thrust mostly axial.
The series of beautiful images in Space Flight Insider Photo Gallery: Launch of NASA’S OSIRIS-REx show the night launch, and details of the single sided booster of the Atlas V 411 configuration.
In one image I noticed that the exhaust plumes are expanding differently - the SRB exhaust is expanding nicely, but the exhaust from the main engines appears to contract after it exits. Does this mean that the pressure in the exhaust is actually sub-atmospheric (if I put a rugged pressure meter in there, moving at the same speed as the exhaust would it read below 15 psi?) , or is it moving so fast with respect to the atmosphere that the "Bernoulli effect" causes a pressure drop across the interface, causing it to contract?
I'm looking for more than the simple answer about compromises in expansion ratio - I'd like to know if the pressure in the first stage exhaust is really sub-atmospheric in a frame moving with the exhaust itself.
above: cropped detail of photo below