Shuttle, and its cousin Shuttle-C, are now history as the launch pads have been converted for use by other systems. Claims made for the Shuttle-C were that it would have taken 4 years and cost $6 billion to develop. Given the time/costs associated with the first generation of SLS units, exactly how are they an improvement on Shuttle-C?
The reality is that any estimate out of NASA on costs needs to be taken with a salt lick the size of LC-39B's former tower.
Had they done Shuttle-C, odds are likely it would cost as much, be as delayed as, SLS. They can screw up anything it would seem.
In theory, Shuttle-C said, replace JUST the orbiter with a non-winged payload fairing. Still needs Space Shuttle main engines (SSME), still needs Orbital Maneuvering System (OMS) pods. No heat shield or wings though.
This in theory allows for a much simpler design, fundamentally a fairing (which is well understood) on top of a boattail that holds the engine 'stuff'.
The Shuttle is on the order of 165,000 lbs, by itself empty. Then 65,000 lbs payload or so.
That means if you reduce the 'overhead' of the orbiter to only 40,000lbs (40%) potentially you could get a mighty fine payload to orbit. This article mentions "... anywhere from 100,000 to 170,000 pounds (45,360 to 77,112 kilograms)" to LEO.
Seems like the general shape for launch could be preserved, since it is just the deletion of the wings that affects aerodynamics.
This minimal change model did not seem to have any attraction, when the purpose of SLS is a jobs program. Without wings and heat shield you could probably reduce the people working on the Shuttle a fair bit as well.
SLS in theory, in later models should be able to launch more payload than Shuttle-C, but that is still many many years out, and I will believe it when I see it.