S-V
The S-V, pronounced "S-five," made its first appearance on the 27th of October 1961. A Saturn I rocket climbed to 136.5 kilometers with the stage loaded with water rather than fuel. Built by Convair Astronautics, the S-V was designed around cryogenic engines intended to push payloads to higher altitudes. But across all four of its flights, those engines never fired. What drove the decision to fly such an advanced stage as ballast is a question worth exploring. So is how the design managed to outlast the entire Saturn program.
Convair Astronautics won the contract to deliver S-V stages for the Saturn I. The design called for two RL-10A-1 engines running on liquid hydrogen and liquid oxygen. Convair separated those propellants using a common bulkhead inside the tanks. That single shared wall kept the two cryogenic fluids apart while reducing overall mass. The original rationale was military: the stage was meant to boost lift capacity for U.S. military launches planned for the 1960s. Convair delivered two S-V stages in February 1961. One was earmarked for SA-1; the other underwent dynamic testing of the complete Saturn I stack before it was also flown.
In May 1961, NASA removed the requirement for all Saturn I launches to include an S-V stage, and the agency flew a two-stage version instead. Even so, the S-V flew four times in total, always on suborbital test arcs. Because the missions never reached orbit, engineers loaded each stage with water as ballast. On SA-2, launched on the 25th of April 1962, that water paid a scientific dividend. The rocket reached 145 kilometers, then released 86,000 kilograms into space as part of Project Highwater. SA-3, on the 16th of November 1962, climbed to 167 kilometers and released a second 86,000-kilogram load. SA-4 lifted off on the 28th of March 1963, reached 129 kilometers, and ended the S-V's run with the Saturn I. With the Saturn IB entering service and military needs satisfied through other vehicles, no future rocket in the American inventory required the S-V.
A version of the S-V stage flew on the Atlas-LV3C rocket under the name Centaur. It carried forward the same liquid hydrogen and liquid oxygen approach that Convair had first developed for Saturn. As successive launch vehicles replaced one another, Centaur evolved alongside them. Modern derivatives of that design now fly on the Atlas V and the Vulcan Centaur. No other stage from any Saturn rocket has continued flying in any form. As the Atlas V and Vulcan Centaur continue to operate, the S-V remains the only Saturn-era stage still reaching space.
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Common questions
What was the S-V stage on the Saturn I rocket?
The S-V, pronounced "S-five," was the third stage of the Saturn I rocket. Built by Convair Astronautics, it used two RL-10A-1 engines designed to burn liquid hydrogen and liquid oxygen. It flew four times between 1961 and 1963, always on suborbital test missions.
Who built the S-V stage and what engines did it use?
Convair Astronautics built the S-V under contract to increase lift capacity for U.S. military launches in the 1960s. The stage used two RL-10A-1 engines fueled by liquid hydrogen and liquid oxygen, with the propellants separated by a common bulkhead inside the tanks.
How many times did the S-V stage fly on Saturn I?
The S-V flew four times on the Saturn I, on missions SA-1, SA-2, SA-3, and SA-4. Those flights spanned from the 27th of October 1961 through the 28th of March 1963, all on suborbital trajectories.
What was Project Highwater and how was the S-V stage involved?
Project Highwater was an experiment in which large quantities of water were released into space from the S-V stage at high altitude. SA-2 released 86,000 kilograms of water at an apogee of 145 kilometers on the 25th of April 1962; SA-3 released another 86,000 kilograms at 167 kilometers on the 16th of November 1962.
Why was the S-V stage flown with water ballast instead of propellant?
All four of the S-V's Saturn I flights were suborbital tests, so the stage was not needed as active propulsion. Engineers filled it with water to serve as ballast. NASA had already eliminated the requirement to fly an S-V on all Saturn I launches in May 1961.
What modern rockets use technology descended from the S-V stage?
The Atlas V and Vulcan Centaur both fly stages derived from the S-V. The connection runs through the Centaur upper stage, originally flown on the Atlas-LV3C, which carried forward the S-V's liquid hydrogen and liquid oxygen design. The S-V is the only Saturn-era stage still operating in any form.
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2 references cited across the entry
- 1Saturn Illustrated Chronology - Part 215 May 1965