S-II
Template:Short descriptionTemplate:For Template:Refimprove Template:Infobox rocket stage
The S-II (pronounced "S-two") 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 quincunx pattern. The second stage accelerated the Saturn V through the upper atmosphere with Template:Convert of thrust.
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 Template:Convert in length and Template:Convert 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.<ref>Template:Cite book</ref> 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.<ref name="16-25">Template:Cite web</ref> These stages would have supported later Apollo missions, including those of the Apollo Applications Program.
Configuration
When fully loaded with propellant, the S-II had a mass of about Template:Cvt. The hardware was only 7.6% of this—92.4% was liquid hydrogen and liquid oxygen.<ref>Apollo 18-19 Ground Ignition Weights (NASA)</ref>
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 Template:Convert 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.<ref name=CBTT />
The LOX tank was an ellipsoidal container of 10 meters diameter and 6.7 meters high holding up to Template:Convert or Template:Convert of oxidizer.<ref>Template:Cite web</ref> 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 (Template:Convert) 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 Template:Convert for storing Template:Convert of liquid hydrogen.
The S-II was constructed vertically to aid welding and keep the large circular sections in the correct shape.
Stages built
| Serial number | Use | Launch date | Current location | Notes |
|---|---|---|---|---|
| Common Bulkhead Test Tank (CBTT) | Demonstrated S-II's common bulkhead on a subscale tank | Unknown | Subscale S-II tank assembly composed of two LH2 tank cylinders, a standard forward bulkhead, common dome, and aft skirt with a modified aft bulkhead. Tested in 1965.<ref name=CBTT>Template:Cite web</ref> | |
| S-II-F | Used as Dynamic Test Stage replacement after destruction of S-II-S/D and S-II-T | At the U.S. Space & Rocket Center, Huntsville, Alabama Template:Coord |
Completed facilities checkouts and propellant load tests at Kennedy Space Center in 1966 as part of the SA-500F stack.<ref name=slr /> | |
| S-II-T | "All-systems" test vehicle for engine firings | First all-up S-II stage, assembled between 1963 and 1965. Completed several engine tests at the Mississippi Test Facility (now the Stennis Space Center). Destroyed by accidental LH2 tank overpressurization during pressure testing May 28, 1966<ref name="MHR-5p7">Template:Cite web</ref><ref name=slr /> | ||
| S-II-D | Dynamic test vehicle | Assembly canceled in 1965 to prioritize work on the first flight stage, S-II-1. Testing requirements transferred to S-II-S, which was renamed S-II-S/D.<ref name=slr /> | ||
| S-II-S/D | Structural and Dynamic Test Vehicle | Destroyed in test stand September 29, 1965 | ||
| S-II-1 | Apollo 4 | November 9, 1967 | Template:CoordTemplate:Citation needed | Carried "Camera Targets" spaced around the forward skirt and carried cameras to record first stage separation |
| S-II-2 | Apollo 6 | April 4, 1968 | Carried cameras to record first stage separation, similar to Apollo 4. Two engines failed during ascent due to damage from first stage pogo oscillation and incorrect engine control wiring. | |
| S-II-3 | Apollo 8 | December 21, 1968 | Template:CoordTemplate:Citation needed | |
| S-II-4 | Apollo 9 | March 3, 1969 | Template:CoordTemplate:Citation needed | 1800 kg lighter allowing 600 kg more payload, more powerful engines and carried more LOX |
| S-II-5 | Apollo 10 | May 18, 1969 | Template:CoordTemplate:Citation needed | |
| S-II-6 | Apollo 11 | July 16, 1969 | Template:CoordTemplate:Citation needed | |
| S-II-7 | Apollo 12 | November 14, 1969 | Template:CoordTemplate:Citation needed | |
| S-II-8 | Apollo 13 | April 11, 1970 | Template:CoordTemplate:Citation needed | Inboard engine failed during ascent due to pogo oscillation. |
| S-II-9 | Apollo 14 | January 31, 1971 | ||
| S-II-10 | Apollo 15 | July 26, 1971 | ||
| S-II-11 | Apollo 16 | April 16, 1972 | ||
| S-II-12 | Apollo 17 | December 7, 1972 | ||
| S-II-13 | Skylab 1 | May 14, 1973 | Template:CoordTemplate:Citation needed | Modified to act as the terminal stage. The only S-II to enter earth orbit, made an uncontrolled reentry into the Atlantic on January 11, 1975.<ref>Template:Cite news</ref> Interstage failed to separate due to payload damage during launch. |
| S-II-14 | Apollo 18 (cancelled) | N/A | Apollo-Saturn V Center, Kennedy Space Center Template:Coord |
From the cancelled Apollo 18 mission.<ref name=slr>Template:Cite web</ref> |
| S-II-15 | Apollo 19, later Skylab 1 backup (not flown) | N/A | Johnson Space Center Template:Coord |
From SA-515 the Skylab backup vehicle which NASA did not use. Also earmarked for Apollo 19.<ref name=slr /> |
Proposed variants
Besides the early four engine version version intended as a Saturn I stage,<ref>Template:Cite web</ref> 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).<ref>Template:Cite web</ref>
Saturn S-II-8
Eight engine version planned as the Saturn C-8 second stage (1960 study).<ref>Template:Cite web</ref>
Saturn S-II-C3
The S-II-C3 stage version was studied in 1960 for the Saturn C-3,<ref>Template:Cite web</ref> consisted of four J-2 engines and had a height of 21.30 m and a diameter of 8.25 m.<ref name=":0">Template:Cite web</ref> Planned thrust was 3,557.31 kN with a fueled mass of gross mass 204,044 kg.<ref name=":0" />
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.<ref>Template:Cite web</ref>
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.<ref>Template:Cite web</ref>
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.<ref>Template:Cite web</ref>
Saturn II-INT-17
A seven HG-3-SL engine version (1965 study), intended for the Saturn INT-17.<ref>Template:Cite web</ref>
Saturn II-SL
A five engine version using J-2-SL (1966 study), intended for the Saturn INT-19.<ref>Template:Cite web</ref>
Saturn MS-II-1A
Seven engine concept with stretched propellant tanks (1966 study), intended for the Saturn MLV-V-1A and Saturn V-ELV.<ref>Template:Cite web</ref>
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.<ref>Template:Cite web</ref>
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.<ref>Template:Cite web</ref>
Saturn MS-II-3B
Five Toroidal 400k engine concept with stretched propellant tanks (1967 study), intended for the Saturn V-3B.<ref>Template:Cite web</ref>
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.<ref>Template:Cite web</ref>