S-II

History

The beginning of the S-II came in December 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 74 ft in length and 260 in 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 seven 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.

On September 11, 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. 15 flight stages were to be produced.

Plans were also developed to build 10 follow-on stages, S-II-16 through -25, but funding to assemble them never materialized. These stages would have supported later Apollo missions, including those of the Apollo Applications Program.

Configuration

At the bottom was the thrust structure supporting five J-2 engines in a quincunx arrangement. The center engine was fixed, while the other four were gimballed, similar to the engines on the S-IC stage below.

Instead of using an intertank (empty container between tanks) like the S-IC, the S-II used a common bulkhead (similar to that of the S-IV and S-IVB stages) that included both the top of the LOX tank and bottom of the LH2 tank. It consisted of two aluminum sheets separated by a honeycomb structure made of phenolic resin. It insulated a 126 F-change temperature differential between the two tanks. The use of a common bulkhead saved 3.6 tonnes in weight, both by eliminating one bulkhead and by reducing the overall length of the stage. The S-II's common bulkhead design was tested in 1965 on the subscale Common Bulkhead Test Tank (CBTT), made of only 2 LH2 tank cylinders.

The LOX tank was an ellipsoidal container of 10 meters diameter and 6.7 meters high holding up to 83,000 USgal or 789,000 lb of oxidizer. It was formed by welding 12 gores (large triangular sections) and two circular pieces for the top and bottom. The gores were shaped by positioning in a 211,000-liter tank of water with three carefully orchestrated sets of underwater explosions to shape each gore.

The LH2 tank was constructed of six cylinders: five were 2.4 meters high and the sixth was 0.69 meters high. The biggest challenge was the insulation. Liquid hydrogen must be kept colder than about 20 °C above absolute zero (-423 F) so good insulation is very important. Initial attempts did not work well: there were bonding issues and air pockets. Initially, the stage was insulated with a honeycomb material. These panels had grooves milled in the back which were purged with helium during filling. The final method was to spray insulation on by hand and trim the excess. This change saved both weight and time and avoided the issues with air pockets entirely. The LH2 tank volume was 260,000 USgal for storing 153,000 lb of liquid hydrogen.

The S-II was constructed vertically to aid welding and keep the large circular sections in the correct shape.

Stages built

Proposed variants

Besides the early four engine version version intended as a Saturn I stage, other versions were proposed for several vehicle concepts:

Saturn S-II-4

Four engine version planned as the Saturn C-4 second stage (1960 study).

Saturn S-II-8

Eight engine version planned as the Saturn C-8 second stage (1960 study).

Saturn S-II-C3

The S-II-C3 stage version was studied in 1960 for the Saturn C-3, consisted of four J-2 engines and had a height of 21.30 m and a diameter of 8.25 m. Planned thrust was 3,557.31 kN with a fueled mass of gross mass 204,044 kg.

Saturn II C-5A

A five engine common second stage planned for the Saturn C-5, Saturn C-3B, Saturn C-4B, Saturn C-3BN and Saturn C-5N (November 1961). Eventually developed into the Saturn V second stage.

Saturn MS-II-1

A five engine version with a stretched fuel tank (1965 study), intended for the Saturn MLV-V-1, Saturn MLV-V-2 and Saturn MLV-V-4(S)-A.

Saturn MS-II-1-J-2T-200K

A five engine version using the uprated J-2T 200k and a stretched fuel tank (1965 study), intended for the Saturn MLV-V-1/J-2T/200K.

Saturn II-INT-17

A seven HG-3-SL engine version (1965 study), intended for the Saturn INT-17.

Saturn II-SL

A five engine version using J-2-SL (1966 study), intended for the Saturn INT-19.

Saturn MS-II-1A

Seven engine concept with stretched propellant tanks (1966 study), intended for the Saturn MLV-V-1A and Saturn V-ELV.

Saturn MS-II-2

Five HG-3 engine concept with stretched propellant tanks (1966 study), intended for the Saturn MLV-V-3 and Saturn V/4-260.

Saturn MS-II-1-J-2T-250K

Five J-2T 250k engine concept with stretched propellant tanks (1966 study), intended for the Saturn MLV-V-1/J-2T/250K and Saturn MLV-V-4(S)-B.

Saturn MS-II-3B

Five Toroidal 400k engine concept with stretched propellant tanks (1967 study), intended for the Saturn V-3B.

Saturn MS-II-4(S)B

Standard five engine S-II stage with structural strength increase, resulting in weight reduction (1968 study). Intended for the Saturn MLV-V-4(S), Saturn V-23(L), Saturn V-24(L), Saturn V-25(S)B, Saturn V-4X(U) and Saturn V-25(S)U.

References

• Apollo Saturn Reference Page

References

1. Aikens, David. (15 May 1965). ""Saturn Illustrated Chronology - Part 2: January 1961 through December 1961"". NASA-Marshall Space Flight Center.
2. (14 June 1967). "Manufacturing plan for Saturn S-II, Stages 16-25".
3. [https://history.nasa.gov/SP-4029/Apollo_18-19_Ground_Ignition_Weights.htm Apollo 18-19 Ground Ignition Weights] ([[NASA]])
4. "Second Stage Fact Sheet".
5. "The Tortoise Steps of Saturn S-II".
6. (15 May 1965). "Saturn Illustrated Chronology - Part 7: January 1966 through December 1966". NASA-Marshall Space Flight Center.
7. (January 11, 1975). "Skylab rocket debris falls in Indian Ocean". Chicago Tribune.
8. "Saturn Vehicle History".
9. "Saturn II".
10. "Saturn S-II-4".
11. "Saturn S-II-8".
12. "Saturn C-3".
13. "Saturn S-II-C3".
14. "Saturn II C-5A".
15. "Saturn MS-II-1".
16. "Saturn MS-II-1-J-2T-200K".
17. "Saturn II-INT-17".
18. "Saturn II-SL".
19. "Saturn MS-II-1A".
20. "Saturn MS-II-2".
21. "Saturn MS-II-1-J-2T-250K".
22. "Saturn MS-II-3B".
23. "Saturn MS-II-4(S)B".