The Titan IIIL series was a Martin Marietta concept (late 1960’s into early 1970’s) for a heavy lift derivative of the Titan IIIC 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 IIIL2, Titan IIIL4, Titan IIIL6). The Titan IIIL6 concept was considered as a first stage booster for the Space Shuttle.
The Titan IIIL2 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.