S-II
In December 1959, a committee recommended the design and construction of a high-thrust liquid hydrogen fueled engine. The contract for this engine went to Rocketdyne and it would later be called the J-2. At that same time, the S-II stage design began to take shape. Initially, engineers planned for four engines and specific dimensions for length and diameter. In 1961, the Marshall Space Flight Center started looking for a contractor to build the stage. Thirty aerospace companies received invitations to a conference where initial requirements were laid out. Only seven submitted proposals just one month later. Three proposals were eliminated after investigation. NASA then decided the initial specifications for the entire rocket were too small. They increased the size of the stages used in the program. This change raised difficulties for the remaining four companies since NASA had not yet decided on various aspects including size. On the 11th of September 1961, the contract was awarded to North American Aviation. The manufacturing plant was built by the government at Seal Beach, California. Fifteen flight stages were originally scheduled for production.
When fully loaded with propellant, the S-II had a mass of about 480,000 kilograms. The hardware itself accounted for only 7.6% of this total weight. The remaining 92.4% consisted of liquid hydrogen and liquid oxygen. Five J-2 engines sat at the bottom in a quincunx arrangement. The center engine remained fixed while the other four gimballed during operation. Engineers avoided using an intertank between tanks like the S-IC stage below. Instead they utilized a common bulkhead that included both the top of the LOX tank and bottom of the LH2 tank. Two aluminum sheets separated by a honeycomb structure made of phenolic resin formed this component. It insulated a temperature differential between the two tanks. The use of a common bulkhead saved 3.6 tonnes in weight. Testing occurred in 1965 on the subscale Common Bulkhead Test Tank. The LOX tank measured 10 meters in diameter and 6.7 meters high. Welding twelve gores and two circular pieces created its shape. Three carefully orchestrated sets of underwater explosions shaped each gore inside a 211,000-liter water tank. The LH2 tank used six cylinders to store fuel. Initial insulation attempts failed due to bonding issues and air pockets. The final method involved spraying insulation on by hand and trimming excess.
The Common Bulkhead Test Tank demonstrated the design on a subscale assembly. It consisted of two LH2 tank cylinders and a modified aft bulkhead. This test happened in 1965. The S-II-F served as a Dynamic Test Stage replacement after destruction of other vehicles. It sat at the U.S. Space & Rocket Center in Huntsville, Alabama. Completed facilities checkouts and propellant load tests took place at Kennedy Space Center in 1966. The S-II-T was an all-systems test vehicle for engine firings. Assembly between 1963 and 1965 produced this first all-up stage. Several engine tests occurred at the Mississippi Test Facility. On the 28th of May 1966, pressure testing caused accidental LH2 tank overpressurization that destroyed the unit. The S-II-DDynamic test vehicle had its assembly canceled in 1965. Testing requirements transferred to the S-II-S which was renamed S-II-S/D. That structural and dynamic test vehicle met destruction on the 29th of September 1965 during a test stand failure. Plans also developed to build ten follow-on stages numbered S-II-16 through -25. Funding to assemble them never materialized.
S-II-1 flew on Apollo 4 on the 9th of November 1967. It carried camera targets spaced around the forward skirt. These cameras recorded the first stage separation event. S-II-2 launched with Apollo 6 on the 4th of April 1968. Two engines failed during ascent due to damage from pogo oscillation. Incorrect engine control wiring contributed to these failures. S-II-3 supported Apollo 8 which lifted off the 21st of December 1968. S-II-4 flew on Apollo 9 on the 3rd of March 1969. This version weighed 1800 kilograms less than previous models. More powerful engines allowed for 600 kg more payload capacity. S-II-5 operated during Apollo 10 on the 18th of May 1969. S-II-6 handled the historic Apollo 11 launch on the 16th of July 1969. S-II-7 supported Apollo 12 on the 14th of November 1969. An inboard engine failed during ascent on S-II-8 when Apollo 13 launched on the 11th of April 1970. The remaining stages numbered through S-II-12 flew missions up to Apollo 17 on the 7th of December 1972.
S-II-13 modified to act as a terminal stage flew with Skylab 1 on the 14th of May 1973. It became the only S-II unit to enter Earth orbit. An uncontrolled reentry into the Atlantic Ocean occurred on the 11th of January 1975. Interstage failure prevented separation due to payload damage during launch. S-II-14 came from the cancelled Apollo 18 mission and now sits at the Apollo-Saturn V Center. S-II-15 originated from SA-515 which served as the Skylab backup vehicle. NASA did not use this specific unit for flight. It was also earmarked for Apollo 19 before cancellation. Johnson Space Center currently holds this final airframe.
Engineers studied a four engine version intended as the Saturn C-4 second stage in 1960. Another eight engine version planned as the Saturn C-8 second stage appeared in that same year study. The S-II-C3 stage version received study in 1960 for the Saturn C-3 program. It consisted of four J-2 engines and measured 21.30 meters in height. Planned thrust reached 3,557.31 kN with a fueled mass of gross mass 204,044 kg. A five engine common second stage planned for the Saturn C-5 emerged in November 1961. This eventually developed into the standard Saturn V second stage configuration. A five engine version with a stretched fuel tank appeared in a 1965 study. This concept targeted the Saturn MLV-V-1, Saturn MLV-V-2 and Saturn MLV-V-4(S)-A programs. Engineers explored uprated J-2T 200k engines paired with stretched tanks in another 1965 study. A seven HG-3-SL engine version was considered in 1965 for the Saturn INT-17. Five engine versions using J-2-SL appeared in a 1966 study intended for the Saturn INT-19. Seven engine concepts with stretched propellant tanks were studied in 1966. These targeted the Saturn MLV-V-1A and Saturn V-ELV programs.
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Common questions
Who built the S-II second stage of the Saturn V rocket?
North American Aviation received the contract to build the S-II second stage on the 11th of September 1961. The manufacturing plant was constructed by the government at Seal Beach, California.
When did the S-II stage first fly in space and on which mission?
S-II-1 flew on Apollo 4 on the 9th of November 1967. This launch marked the first flight of an S-II stage with camera targets spaced around the forward skirt.
What is the mass of the fully loaded S-II stage including propellant?
The S-II had a mass of about 480,000 kilograms when fully loaded with propellant. Hardware accounted for only 7.6% of this total weight while liquid hydrogen and liquid oxygen made up the remaining 92.4%.
Where is the S-II-14 unit located today after being part of the cancelled Apollo 18 mission?
S-II-14 now sits at the Apollo-Saturn V Center following its origin from the cancelled Apollo 18 mission. It remains there as a static display piece.
How many J-2 engines were installed on the standard S-II second stage configuration?
Five J-2 engines sat at the bottom of the S-II stage in a quincunx arrangement. The center engine remained fixed while the other four gimballed during operation to steer the rocket.