to create an unmanned heavy lift vehicle since the early 1980s. One of the earliest proposals was called Shuttle C. There have been several other similar proposals since then, such as Zubrin's Ares idea. The Soviet Energia rocket was designed from the beginning with this sort of flexibility in mind.

Think about it - a loaded Shuttle orbiter weighs over 200,000 pounds and can carry a payload of about 50,000 pounds. Remove the wings, life support systems, most of the structure, etc, and you could launch well over 100,000 pounds into orbit. You'd use the Shuttle's external tank and solid rocket boosters and create a cargo module on the side. You might use the Shuttle Main Engines on the bottom of the cargo pod to keep the launch pad and flight configuration similar to the Shuttle to minimize required changes. It's even possible to design the vehicle so that the cargo pod and main engines can be recovered for reuse.