I don't know of any shuttle study that envisioned it being SSTO. The original shuttle configurations did contain a liquid-powered 'flyback booster', which was a first stage with the ability to fly back to the launch site. Many of these configurations also had retractable jet engines mounted on the shuttle orbiter, as well as fuel and/or oxidizer tanks in the orbiter, but all these options were nixed because of payload, cost, and schedule requirements. The quick turn-around was originally viewed as desirable, but other things were made higher priorities, and they gradually moved away from that objective.