Mobile Base Crawler

It is necessary to go inside the "Enclave Mobile Base" and launch an airstrike to complete Broken Steel, as part of the last quest given for the DLC. Once inside you have the option to launch the airstrike on the Andrews Air Force Base or against the Citadel. There are three other locations mentioned as orbital strike options: Rivet City, Megaton, and Project Purity. Attempting to select these options, however, will simply elect an error message.

The Enclave Mobile Base concept seems to come from the Mobile Launcher Platform and Crawler-Transporter of the NASA, both very big vehicles used to ferry rockets and Space Shuttles.

By calling in the airstrike on either location, Andrews or the Citadel, you will end Broken Steel.