The Space Shuttle system was seemingly designed for modularity… the orbiter could be replaced, the boosters could be replaced. Payload shrouds could be placed alongside the tank in place of the orbiter, or on the nose of the external tank either replacing the orbiter or supplementing it.
Martin calculated that the payload for the standard Shuttle using lightweight fiber wound solid rocket boosters and a lightweight ET going to a 215 nautical mile orbit would be 60,800 pounds. Payload to the same orbit would increase to 69,700 pounds if the ET was taken all the way to orbit (the more efficient SSMEs would be used all the way to orbit rather than relying on the OMS engines). By using the ACC and dragging the ET and ACC all the way to orbit, total payload would be 60,100 pounds.
A brochure from Martin Marietta describing the ACC is available HERE.
