S-IV

• 1. Introduction

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  On the 27th of October 1961, at 15:06 UTC, the S-IV fired for the first time as the second stage of NASA's Saturn I rocket. Douglas Aircraft Company had built it for the early flights of the Apollo program. Over the next four years, NASA would launch ten of these stages. What allowed a rocket stage of that era to cut its structural weight by up to 20 percent? And when the Saturn I program closed, where did the design go from there?

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• 2. Thrust and a Common Wall

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  Six RL-10A-3 engines drove the S-IV from the base of the stage. Each produced 66.7 kilonewtons of thrust, and the six together delivered around 400 kilonewtons in total.

  Liquid hydrogen and liquid oxygen, the stage's two propellants, were stored in cryogenic tanks arranged end to end inside the stage body. Douglas engineers divided them with a single shared bulkhead rather than giving each tank its own pair of end caps. The forward face of the liquid oxygen tank served at the same time as the rear face of the liquid hydrogen tank. That single wall in place of two saved up to 20 percent of the stage's total structural weight. When the Saturn I program concluded, Douglas would modify the S-IV into a successor stage for other rockets in the Saturn family.

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• 3. The Flight Record

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  SA-2 lifted off on the 25th of April 1962, the second mission to carry an S-IV stage. SA-3 flew on the 16th of November 1962, and SA-4 followed on the 28th of March 1963. Three more missions, SA-5, SA-6, and SA-7, flew across 1964, launching in January, May, and September.

  SA-9 launched on the 16th of February 1965 before SA-8 had flown. SA-8 did not lift off until the 25th of May 1965. The numerical sequence and the actual launch order did not match. SA-10, the final S-IV mission, launched on the 30th of July 1965. Douglas built its successor, the S-IVB, as a modified but distinct stage that flew on both the Saturn IB and the Saturn V.

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