 Complete the following phrase:  DARPA studied modifying the current external tank design to be able to carry low density payloads in a 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 need to 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. The proposal was reconfigured as an Aft Cargo Carrier ACC to be positioned towards the bottom of the tank rather on top. This idea made it far into development with Martin Marietta contracted to design and fabricate the container. First flights of the ACC were expected in 1986. After the 