S-II

• 1. Introduction

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  The S-II generated one million pounds of thrust as the second stage of the Saturn V rocket. Five J-2 engines in a quincunx formation burned liquid hydrogen and liquid oxygen to produce that force. Its job was to take over from the first stage and push the vehicle through the upper atmosphere toward orbital speed. The engineering decisions behind that role were as remarkable as the missions it served.

• 2. Winning the Contract

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  In December 1959, a committee recommended designing a high-thrust engine fueled by liquid hydrogen. Rocketdyne received the contract for that engine, which would later be named the J-2. At the same time, engineers began drafting an outline for the stage that would use it. Early plans called for four J-2 engines and a stage 74 feet long and 260 inches in diameter.

  By 1961, the Marshall Space Flight Center began searching for a company to build the stage. Thirty aerospace companies were invited to a conference where the initial requirements were laid out. Only seven of those companies submitted proposals a month later. Three were eliminated after their proposals had been reviewed. Meanwhile, NASA concluded that the original specifications for the entire rocket were too small. The remaining four competitors had to adapt to a scale that NASA had not yet fully defined.

  On the 11th of September 1961, the contract went to North American Aviation. North American was already building the Apollo Command and Service Module. This made it the builder of two of the most critical components in the program. The government constructed the manufacturing plant at Seal Beach, California. The contract called for fifteen flight stages. NASA also developed plans for ten follow-on stages, S-II-16 through S-II-25, but funding for their assembly never materialized. They had been earmarked to support the Apollo Applications Program, a planned expansion of spaceflight that never reached full scale.

• 3. The Common Bulkhead

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  Placing two propellant tanks directly against each other, with no empty container between them, saved 3.6 tonnes in weight. Most rockets of the era used an intertank section to keep fuel and oxidizer separated by some distance. The S-II designers chose instead a common bulkhead, similar in approach to the S-IV and S-IVB stages. This shared wall consisted of two aluminum sheets separated by a honeycomb structure made of phenolic resin. That honeycomb layer had to insulate a temperature difference of 126 degrees Fahrenheit. It stood between the liquid oxygen tank below and the liquid hydrogen tank above.

  The subscale version of this design was tested in 1965 on a structure called the Common Bulkhead Test Tank. It was assembled from just two liquid hydrogen tank cylinders. At the base of the stage, the thrust structure held the engines in position. The center engine was fixed in place, while the outer four could be gimballed to steer the vehicle. This arrangement was similar to the engines on the S-IC first stage below. The S-II was assembled vertically to help welders work on the large circular sections while gravity kept them in the correct shape. That vertical orientation shaped the whole assembly process. But the fabrication of the oxidizer tank itself required a technique far outside the usual range of aerospace manufacturing.

• 4. Shaped by Explosion

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  The LOX tank was 10 meters in diameter and 6.7 meters high, and it held up to 83,000 US gallons of liquid oxidizer. That volume translated to 789,000 pounds of liquid oxygen when full. The body of the tank was formed by welding together 12 large triangular sections called gores. Two circular pieces capped the top and bottom.

  Each of the 12 gores was shaped inside a 211,000-liter tank of water. Engineers set off three carefully orchestrated sets of underwater explosions to press each gore into the required curvature. The force of the water transmitted the blast energy evenly across the metal. This formed a curved shape that a conventional press could not easily achieve at that scale. The liquid hydrogen tank above it presented a different kind of challenge altogether, one that engineers spent years trying to solve.

• 5. The Insulation Struggle

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  Liquid hydrogen had to be kept colder than 20 degrees Celsius above absolute zero, a temperature of minus 423 degrees Fahrenheit. The tank holding it was built from six cylindrical sections. Five stood 2.4 meters high and the sixth stood 0.69 meters high. Getting the insulation right was the defining manufacturing challenge of the stage.

  The first approach used honeycomb panels mounted to the outside of the tank, with grooves milled into the back of each panel. During filling, those grooves were purged with helium to keep moisture from condensing inside. Bonding problems and air pockets made the system unreliable. Engineers eventually switched to a method far simpler in concept: spraying insulation directly onto the tank by hand, then trimming the excess. That approach eliminated the air pocket problem entirely and saved both weight and manufacturing time. The LH2 tank held 260,000 US gallons of liquid hydrogen, weighing 153,000 pounds. Those insulation decisions, made after significant trial and error, meant the stage was ready for the test stand and, eventually, the launchpad.

• 6. Test Stands and Launches

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  The first complete S-II stage, designated S-II-T, was assembled between 1963 and 1965 for engine testing. It completed several engine firings at the Mississippi Test Facility, a site now known as Stennis Space Center. On the 28th of May 1966, an accidental overpressurization of the liquid hydrogen tank destroyed it during a pressure test. A second structural and dynamic test vehicle, S-II-S/D, had already been destroyed in the test stand on the 29th of September 1965. S-II-D, a separate dynamic test vehicle, had its assembly cancelled in 1965 so resources could be directed toward the first flight stage. A replacement test stage, S-II-F, completed facilities checkouts and propellant load tests at Kennedy Space Center in 1966. It was part of the SA-500F stack. S-II-F is now displayed at the U.S. Space and Rocket Center in Huntsville, Alabama.

  The first flight stage, S-II-1, launched on Apollo 4 on the 9th of November 1967. It carried camera targets spaced around the forward skirt and cameras to record the separation from the first stage. S-II-2 flew on Apollo 6 on the 4th of April 1968 and suffered two engine failures during ascent. Pogo oscillation from the first stage damaged the engines, and incorrect wiring in the engine control system compounded the problem.

  Apollo 9 flew with S-II-4, which engineers had made 1,800 kilograms lighter than earlier stages. That weight reduction allowed 600 kilograms of additional payload. The stage also carried more LOX and used more powerful engines. S-II-8, which launched on Apollo 13 on the 11th of April 1970, lost its inboard engine during ascent due to pogo oscillation.

  The final flight of an S-II came on the 14th of May 1973, when S-II-13 launched Skylab 1. It was the only S-II stage to enter Earth orbit. The interstage failed to separate because of payload damage during launch. The stage made an uncontrolled reentry into the Atlantic Ocean on the 11th of January 1975. Two stages were never used in flight. S-II-14, from the cancelled Apollo 18 mission, is preserved at the Apollo-Saturn V Center at Kennedy Space Center. S-II-15, earmarked for Apollo 19, stands at Johnson Space Center. But the physical hardware tells only part of the story. Engineers were simultaneously studying far more ambitious versions of the stage than any that flew.

• 7. Proposed Variants

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  As early as 1960, engineers studied a four-engine version of the S-II intended as the second stage of the Saturn C-4. A separate eight-engine version was studied the same year for the Saturn C-8. Neither reached production.

  The most detailed of the 1960 studies involved the S-II-C3 configuration, planned for the Saturn C-3. That variant would have carried four J-2 engines in a stage 21.30 meters tall and 8.25 meters in diameter. Its planned thrust was 3,557.31 kilonewtons and its gross mass 204,044 kilograms. By November 1961, engineers had settled on a five-engine common second stage. It was planned to serve the Saturn C-5, Saturn C-3B, and several related vehicles. That concept eventually became the Saturn V second stage.

  Later studies from 1965 and 1966 explored stretched propellant tanks and upgraded engines. These were developed for a series of vehicles designated Saturn MLV, or Modified Launch Vehicle. They included versions using the uprated J-2T engine in both 200,000-pound and 250,000-pound thrust variants. A separate configuration used a Toroidal 400,000-pound engine and was studied in 1967. A seven-engine version using the HG-3-SL engine was studied in 1965 for the Saturn INT-17. The last of the major variant studies was completed in 1968. It aimed to increase structural strength while reducing weight, and was targeted at six different Saturn vehicle configurations.