posted 09-14-2008 03:16 PM               

The Saturn was initially (or at very early on) envisioned as a five-stage vehicle. Using a "build block" approach, some of the upper stages would be test-flown before the intermediate stages.

So, the very first Saturn (SA-1) consisted of an S-I stage and dummy S-IV and S-V stages. Before a live S-IV was ever flown (as the second stage of SA-5, the first of the Block II series), the idea of an S-V stage was dropped.

The Saturn I series gave way to the Saturn IB. The first stage of this rocket was modified and renamed to the S-IB (although it seems to me that there were more differences between the Block I and Block II S-I stages than between the Block II S-I and the S-IB). The second stage of this rocket was an uprated version of the S-IV, the S-IVB; this stage was considered sufficiently similar to the S-IV stage that it was sole-sourced to Douglas without the usual bidding process.

Of course, the S-IVB would also be used as the third stage of the Saturn V. Its first stage was the S-IC. Since no stage initially designed to be a second stage had ever been manufactured (remember, the second stage of the Saturn IB was still an evolved version of the initially-envisioned fourth stage), it received the S-II designation.

With the S-IC, S-II, and S-IVB, there was no need for another stage to be designed as a third stage.

posted 10-31-2008 03:29 AM            

Quite a number of Saturn stage configurations were considered between 1958 and 1960. One early version was the Saturn A-1, which had the familiar S-I stage plus a Titan I 1st stage as a 2nd stage and a Centaur as a 3rd stage. The Saturn A-2 was the same except that the 2nd stage consisted of a cluster of 4 Jupiters rather than a Titan 1st stage.

In the Saturn B-1, the 2nd stage was a custom-developed lox/RP-1 stage powered by 4 H-1 engines. Again, the Centaur was to be the 3rd stage.

Then there was the C-1: S-I plus S-IV plus Centaur, known in this application as the S-V. This is the Saturn I which actually flew, except that the S-V stage was never live.

The original C-2 design would have been S-I plus S-III plus S-IV plus, for some missions, S-V. The S-III was to have been a lox-hydrogen stage powered by two J-2 engines.

The original C-3 configuration was S-I plus S-II plus S-III plus, for some missions, the S-IV and even S-V. The S-II was to have had four J-2 engines and the S-I would have been upgraded either by increasing the thrust of its eight H-1 engines to 250,000 lb or by replacing the center four H-1 engines with a single F-1.

In December 1959, the Silverstein committee (formally known as the "Saturn Vehicle Evaluation Committee" or something like that) decided to skip development of the Saturn A and B configurations and go straight to the C configurations, which gave the most lifting power.

NASA HQ accepted the recommendations of the Committee and forwarded them to Hunstville. Huntsville concurred, but dropped the S-III stage. In the C-2 it was replaced by the S-II stage, with 4 J-2 engines. The upgraded S-I in the C-3 was replaced by a new stage powered by two F-1s. Eventually, as we all know, the C-2 and C-3 were dropped entirely. The C-4 was briefly considered (4xF-1 plus 4xJ-2 plus 1xJ-2) before being replaced by the C-5. The S-II designation was now applied to the 2nd stage of the C-5, of course.