For $150 million(1968) and 3 years of development, the payload capability of the entire Saturn family could have been enhanced by almost a third. Boeing engineers had to change the thrust structure by mounting the center engine on a cylinder, attached to a new conical thrust structure on the back of the stage, to transfer the thrust to the stages skin. The center engine would need to be able to gimbal to control the vehicle. The outboard engine thrust structure would have been cross beams with a hollow cylinder in the center large enough to clear the center engine, by removing the cruciform where the center engine was originally mounted. The outboard structure would made to separate from the new center structure. The propellant lines would have needed shutdown and separation joints on the outer engines. The IU (instrument unit) ring normally carried atop the third stage S-IVB would be reworked and mounted atop the S-ID.
The outer ring of booster engines could have been made recoverable. The payload would have been even greater had the stage's fuel tanks been enlarged as proposed for various Saturn V upgrades. The F-1A engine with its higher thrust and efficiency would have also boosted the payload numbers.
Studies were done with various upper stages, showing about 30% improvement. For example,the Int-20 Saturn, put a SIVB (third stage for the Saturn V) on top of the S-ID first stage. This could launch 180,000 lb (81,600 kg) into orbit, an increase of 48,000 lb (21,800 kg) over the standard S-1C stage version. The first version of the SLS is supposed to put 70 tons in orbit, this version of the Int-20 beats that.
The S-ID would have been operational in 1972, probably would have been used in the third order of Saturn Vs. But in 1968, budgets were already being cut. The second run of Saturn Vs was canceled before Apollo 11 landed on the moon. Plans for enhanced Saturn Vs, space stations, and moonbases were not to be. The saturn rockets were scrapped in favor of the space shuttle which promised cheaper flights and reuse.
The S-1D would have weighed 5,099,000 lb (2,300,000 kg) at takeoff.