S-II
The Apollo 6 S-II stage during stacking operations in the VAB |
| Fact sheet
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| Height | 24.9 m (82 ft)
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| Diameter | 10 m (33 ft)
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| Mass | 500,000 kg
(1,100,000 lb)
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| Engines | 5 J-2 engines
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| Thrust | 5,115 kN
(1,150,000 lb)
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| Burn time | 390 s
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| Fuel | liquid hydrogen and liquid oxygen
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The S-II was the second stage of the Saturn V rocket. It was built by North American Aviation. Using liquid hydrogen (LH2) and liquid oxygen (LOX) it had five J-2 engines in a cross pattern. The second stage accelerated the Saturn V through the upper atmosphere with 5 MN of thrust.
History
The beginning of the S-II came in December of 1959 when a committee recommended the design and construction of a high-thrust, liquid hydrogen fueled engine. The contract for this engine was given to Rocketdyne and it would be later called the J-2. At the same time the S-II stage design began to take shape. Initially it was to have four J-2 engines and be 22.5 meters in length and 6.5 meters in diameter.
In 1961 the Marshall Space Flight Center began the process to find the contractor to build the stage. Out of the 30 aerospace companies invited to a conference where the initial requirements were laid out, only 7 submitted proposals a month later. Three of these were eliminated after their proposals had been investigated. However it was then decided that the initial specifications for the entire rocket were too small and so it was decided to increase the size of the stages used. This raised difficulties for the four remaining companies as NASA had still not yet decided on various aspects of the stage including size, and the upper stages that would be placed on top.
In the end on 11 September 1961 the contract was awarded to North American Aviation (who were also awarded the contract for the Apollo Command/Service Module), with the manufacturing plant built by the government at Seal Beach, California.
Configuration
 Cutaway illustration of the S-II (second) stage At the bottom of the stage was the thrust structure that supported the five J-2 engines. The center engine was fixed, while the other four were gimballed.
Instead of using a intertank structure like the S-IC, the S-II used a common bulkhead that included both the top of the LOX tank and bottom of the LH2 tank. It consisted of two aluminium sheets separated by a honeycomb structure made of phenol. This had to insulate against the 70°C temperature difference between the two tanks. The use a common bulkhead saved 3.6 tonnes in weight.
The LOX tank was an ellipsoidal container 10 meter diameter and 6.7 meter height. It was formed by welding 12 gores, large triangular sections, together along with two circular pieces at the top and bottom. The gores were formed using a method of underwater explosions. The gore was positioned in a 211000 liter tank of water and this specially placed explosions would shape the metal. Three sets of explosions were required per gore.
The LH2 tank was a made of six cylinders, five of which were 2.4 meters high and the sixth only 0.69 meters high. The main issue that was found in designing and contructing the LH2 tank was the insulation. As liquid hydrogen in only a about 20°C above absolute zero it was necessary for the insulation to work extremely well. The initial ideas did not work well with bonding issues and air pockets forming between the tank and the insulation. In the end it was decided to simply spray the insulation on by hand and then cut of the excess.
The S-II was constructed vertically in Seal Beach as this made the welding easier and insured that the large circular sections remained in the correct shape.
Stages Built
References
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