DARPA studied modifying the current external tank design to be able to carry low-density payloads in a 25 or 35 ft (7.6 or 10.6 m) diameter fairing where the oxygen tank was currently.
The Shuttle on average would only fly 66% of its payload capability but at nearly 100% of its payload volume. The external tank payload fairing would solve this problem.
The oxygen tank would be redesigned as a cylindrical tank rather than conical and the clamshell payload fairing would be mounted directly on it. In this configuration, the orbiter would launch without any payload.
These studies were eventually abandoned due to the fact that the new aerodynamic profile would make a Return to Launch Site (RTLS) maneuver impossible.