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.
source.image: Hazegrayart
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.
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These studies were eventually abandoned due to the fact that the new aerodynamic profile would make a Return to Launch Site (RTLS) maneuver impossible.
