The Air Force was in the midst of their Dyna-Soar project, and were designing a new booster to launch it under their "SLV-4" requirement. Their primary answer to this requirement was a Titan II missile equipped with a new hydrogen-burning upper stage, the Titan C. The resulting design had a somewhat bulbous appearance; as the hydrogen fuel required large tanks, the upper stage was in diameter, compared to the of the Titan II. Other teams within the Air Force were also developing the Space Launcher System concept, which combined the same Titan II with a number of solid fuel rockets as a "zeroth stage". By combining different numbers and sizes of these rockets, the launch stack could be tuned to different payloads. The SLS team also outlined a development path for a manned lunar mission under their Lunex Project proposal, using the Titan with four solids to test the re-entry vehicle from Earth orbit, and entirely new solids and liquid stages for flights to the moon.