Martin Marietta Titan III-L series
"The Titan III-L series was a Martin Marietta concept (late 1960’s into early 1970’s) for a heavy lift derivative of the Titan III-C launch vehicle. The core would be increased in diameter from 10 feet to 15, and the number of liquid propellant rocket engines increased from two to four. Additionally, the vehicle could be given two, four or six solid rocket motors (Titan III-L2, Titan III-L4, Titan III-L6). The Titan III-L6 concept was considered as a first stage booster for the Space Shuttle.
The Titan III-L2 had enough lift capacity to launch an Apollo-derived capsule and service module, providing an alternative to the Saturn IB for space station logistics and crew transfer."
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