I have in my collection part of a study to modify the Saturn V's S-IC (first) stage to serve as a stage-and-a-half to orbit booster. Reminiscent of an Atlas, the four outboard F-1 engines would be mounted on a thrust structure which would be jettisoned. The center engine would continue to fire, serving as a sustainer engine. That center engine, fixed on an S-IC, would be modified to gimbal, providing pitch and yaw control (just like the single J-2 on an S-IVB). At the time this study was written, it was uncertain what would be used for roll control.

The S-ID could be used by itself, or with one or more upper stages (e.g., an S-IVB). Although it's not the topic of this study, one is left to wonder how the engines would be controlled without the Saturn V Instrument Unit.

There's relatively little information about the S-ID out on the Internet; the place with the most details about the S-ID which I've found is Encyclopedia Astronautica.

This particular document is entitled S-ID Propulsion/Mechanical. At 44 pages, it deals with changes needed to the LOX and fuel systems needed to accommodate disconnecting and jettisoning the four outboard engines.

This is a simple, type-written report with only a few simple diagrams. It appears to be a rough draft, as there are blank spaces were diagrams are referenced, and the names of the diagrams were filled in by hand. There are also a number of hand-written notes and corrections.

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