Saturn S-2 Geneology Saturn S-II Geneology Credit: © Mark Wade. 12,205 bytes. 639 x 416 pixels.

There was a large payload gap between the Saturn IB's 19,000 kg low-earth orbit capacity and the two-stage Saturn V's 100,000 kg capability. Marshall considered the best way to fill the gap was to use the Saturn V's second stage, the S-II, as the first stage of an intermediate launch vehicle.

Using the S-II had several advantages. It could be mounted atop a 'milk stool' and use the existing Saturn V launch gantry arms and plumbing for fueling and preparations (this approach was actually used later for Saturn IB launches for Skylab and ASTP). Discontinuing use of the Saturn IB would eliminate one rocket stage production line together with associated configuration and quality control headaches.

A dazzling array of combinations of S-II stages, S-IVB stages, and a variety of solid rocket motor strapons were considered. In most cases the S-II would have to be fitted with 'sea-level' versions of its J-2 engines, which were designed only for operation in near-vacuum conditions. This resulted in a decrease in engine performance. Since the S-II stage did not have enough thrust to get off the ground by itself, various combinations of solid rocket motor augmentation and propellant off-loads had to be used. The resulting configurations would have provided a payload range of between 13,000 kg and 66,000 kg to low earth orbit, thereby filling the payload gap and replacing the S-IB.

Launch Vehicle: Saturn INT-17. North American study, 1966. Saturn variant with a modified S-II first stage with seven high-performance HG-3 engines; S-IVB second stage. Poor performance and cost-effectiveness and not studied further.

Launch Vehicle: Saturn INT-18. North American study, 1966. Saturn variant with Titan UA1205 or 1207 motors as boosters, Saturn II stage as core, and Saturn IVB upper stage. Various combinations of numbers of strap-ons, propellant loading of the two core stages, and sea-level versus altitude ignition were studied. Liftoff Mass - kg Strapon Motor Type Number of Strapons S-II Propellant - kg S-IVB Propellant - kg Payload - kg 1,132,000 Titan UA1204 4 215,400 80,300 21,300 1,132,000 Titan UA1204 4 215,400 78,500 23,100 1,030,400 Titan UA1207 2 254,000 80,700 27,400 1,132,400 Titan UA1207 2 349,200 79,800 35,400 1,083,000 Titan UA1205 2 431,600 77,400 40,500 1,570,500 Titan UA1205 4 440,400 74,400 51,700 1,895,200 Titan UA1207 4 446,700 75,700 66,400 1,476,200 Titan UA1205 4 446,700 No S-IVB 39,000 1,779,600 Titan UA1207 4 446,700 No S-IVB 44,000

Launch Vehicle: Saturn INT-19. North American study, 1966. Saturn variant with 4 to 12 Minuteman motors as boosters, Saturn II stage as core, and Saturn IVB upper stage. Saturn II stage would be fitted with lower expansion ratio engines and would ignite at sea level. Various combinations of numbers of strap-ons, propellant loading of the two core stages were studied.