In chapter 6 of SP-4221 "The Space Shuttle Decision" they are talking about the fall of the "Two-Stage Fully Reusable Shuttle".
In one paragraph they talk about how this Two-Stage Shuttle would work:
Fueled and cleared for launch, the Shuttle would thunder into the air with the thrust of 12 SSMEs in the base of the booster. The orbiter would separate and fly onward, propelled by its own SSMEs. The booster, empty of fuel, would come down through the atmosphere and return to a runway at the Cape, with power from up to 12 jet engines. After completing its mission in space, the orbiter would reenter and land on the same runway. Preparations for the next flight, covering both the booster and orbiter, would take as little as two weeks.
From what I can understand here it meant that there were plans to create a booster with 12 engines and 12 jet engines as well?
Were they talking about a booster with SSMEs as engines? What were the jet engines they were talking about? I've never heard anything about using jet engines in a booster before.
Also are there any pictures or prototypes ever created of such design?