View source for S-II

{{Short description|Second stage of the Saturn V, built by North American Aviation}}{{for|the functionally-defined brain region|Secondary somatosensory cortex}}
{{Refimprove|date=July 2009}}
{{Infobox rocket stage
| image         = ap6-MSFC-6758331.jpg
| caption       = The [[Apollo 6]] S-II stage during stacking operations in the [[Vehicle Assembly Building|VAB]]
| name          = S-II
| manufacturer  = [[North American Aviation|North American]]
| country       = United States
| rockets       = [[Saturn V]]
| height        = {{cvt|24.9|m|ftin|order=flip}}
| diameter      = {{cvt|10|m|ftin|order=flip}}
| mass          = {{cvt|1058000|lbs}}
| propmass      = {{cvt|977000|lbs}}
| empty         = {{cvt|79700|lbs}}
| status        = Retired
| launches      = 13
| success       = 12
| other_outcome = Partial failure ([[Apollo 6]])
| first         = November 9, 1967 ([[Apollo 4]])
| last          = May 14, 1973 ([[Skylab 1]])
| stagedata     =
{{Infobox rocket/stage
| name          = Specifications 
| engines       = 5{{nnbsp}}×{{nnbsp}}[[Rocketdyne J-2|J-2]]
| thrust        = {{cvt|1000000|lbf|kN}}
| SI            = {{cvt|421|isp}}
| burntime      = 367 seconds
| fuel          = [[LH2|LH<sub>2</sub>]] / [[Liquid oxygen|LOX]]
}}
}}

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]] (LH<sub>2</sub>) and [[liquid oxygen]] (LOX) it had five [[J-2 (rocket engine)|J-2 engines]] in a [[quincunx]] pattern. The second stage accelerated the Saturn V through the upper atmosphere with {{convert|1000000|lbf|MN}} of thrust.

==History==
[[Image:Saturn v building seal beach.jpg|left|thumb|200px|S-II assembly building in Seal Beach, CA]]
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 (rocket engine)|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 {{convert|74|ft|m}} in length and {{convert|260|in|m}} 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>{{Cite book|last=Aikens|first=David|url=https://history.nasa.gov/MHR-5/part-2.htm|title="Saturn Illustrated Chronology - Part 2: January 1961 through December 1961"|date=15 May 1965 |publisher=NASA-Marshall Space Flight Center|pages=28}}</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">{{Cite web|title=Manufacturing plan for Saturn S-II, Stages 16-25|url=http://libarchstor2.uah.edu/digitalcollections/items/show/11131|access-date=2023-03-18|website=uah.edu|date=14 June 1967 }}</ref> These stages would have supported later Apollo missions, including those of the [[Apollo Applications Program]].

==Configuration==
[[Image:SaturnV S-II.jpg|left|thumb|Cutaway illustration of the S-II (second) stage]] When fully loaded with propellant, the S-II had a mass of about {{cvt|480|t|lb}}. The hardware was only 7.6% of this—92.4% was liquid hydrogen and liquid oxygen.<ref>[https://history.nasa.gov/SP-4029/Apollo_18-19_Ground_Ignition_Weights.htm 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 [[gimbal]]led, 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 {{convert|126|F-change|adj=on}} 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 [[Liquid oxygen|LOX]] tank was an [[ellipsoid]]al container of 10 meters diameter and 6.7 meters high holding up to {{convert|83,000|USgal|m3}} or {{convert|789,000|lb|t}} of oxidizer.<ref>{{cite web|url=http://history.msfc.nasa.gov/saturn_apollo/documents/Second_Stage.pdf|title=Second Stage Fact Sheet|access-date=2014-09-23|archive-url=https://web.archive.org/web/20150326211327/http://history.msfc.nasa.gov/saturn_apollo/documents/Second_Stage.pdf|archive-date=2015-03-26|url-status=dead}}</ref> It was formed by welding 12 [[Gore (segment)|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 [[explosive forming|three carefully orchestrated sets of underwater explosions]] to shape each gore.

The [[liquid hydrogen|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&nbsp;°C above absolute zero ({{convert|-423|F|K C|disp=or}}) 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 {{convert|260,000|USgal|m3}} for storing {{convert|153,000|lb|t}} of liquid hydrogen.

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

==Stages built==
{| width=100% class="wikitable"
|-
! Serial&nbsp;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>{{cite web|url=http://heroicrelics.org/info/s-ii/s-ii-tortoise-steps.html#download-links|title=The Tortoise Steps of Saturn S-II|access-date=2023-03-20}}</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]] <br>{{coord|34.710544|-86.657185
|format=dms |type:landmark_region:US-AL |name=S-II-F}} 
|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">{{cite web |url=https://history.nasa.gov/MHR-5/part-7.htm |title=Saturn Illustrated Chronology - Part 7: January 1966 through December 1966 |last1=Aikens | first1=David |date=15 May 1965 |publisher=NASA-Marshall Space Flight Center |accessdate=February 17, 2011}}</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
|{{coord|32|12|N|39|40|W |name=S-II-1}}{{citation needed|date=January 2012}}
|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
|{{coord|31|50|N|38|0|W |name=S-II-3}}{{citation needed|date=January 2012}}
|
|-
|S-II-4
|[[Apollo 9]]
|March 3, 1969
|{{coord|31|28|N|34|2|W|name=S-II-4}}{{citation needed|date=January 2012}}
|1800&nbsp;kg lighter allowing 600&nbsp;kg more payload, more powerful engines and carried more LOX
|-
|S-II-5
|[[Apollo 10]]
|May 18, 1969
|{{coord|31|31|N|34|31|W |name=S-II-5}}{{citation needed|date=January 2012}}
|
|-
|S-II-6
|[[Apollo 11]]
|July 16, 1969
|{{coord|31|32|N|34|51|W |name=S-II-6}}{{citation needed|date=January 2012}}
|
|-
|S-II-7
|[[Apollo 12]]
|November 14, 1969
|{{coord|31|28|N|34|13|W |name=S-II-7}}{{citation needed|date=January 2012}}
|
|-
|S-II-8
|[[Apollo 13]]
|April 11, 1970
|{{coord|32|19|N|33|17|W |name=S-II-8}}{{citation needed|date=January 2012}}
|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
|{{coord|34|00|N|19|00|W |name=S-II-13}}{{citation needed|date=January 2012}}
|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>{{Cite news|url = http://archives.chicagotribune.com/1975/01/11/page/6/article/skylab-rocket-debris-falls-in-indian-ocean|title = Skylab rocket debris falls in Indian Ocean|date = January 11, 1975|work = Chicago Tribune|accessdate = October 22, 2014}}</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]] <br>{{coord|28.52385|-80.68345 |format=dms |type:landmark_region:US-FL |name=S-II-14}}
|From the cancelled Apollo 18 mission.<ref name=slr>{{cite web |last1=Kyle |first1=Ed |title=Saturn Vehicle History |url=https://www.spacelaunchreport.com/satstg5.html |archive-url=https://web.archive.org/web/20220321061519/https://www.spacelaunchreport.com/satstg5.html |url-status=dead |archive-date=March 21, 2022 |website=spacelaunchreport.com}}</ref>
|-
|S-II-15
|Apollo 19, later Skylab 1 backup (not flown)
|N/A
|[[Johnson Space Center]]  <br>{{coord|29.554051|-95.094266 |format=dms |type:landmark_region:US-TX |name=S-II-15}}
|From SA-515 the Skylab backup vehicle which NASA did not use. Also earmarked for Apollo 19.<ref name=slr />
|}
{{kml}}

== Proposed variants ==
Besides the early four engine version version intended as a Saturn I stage,<ref>{{Cite web |title=Saturn II |url=http://astronautix.com/s/saturnii.html |access-date=2025-11-18 |website=astronautix.com}}</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>{{Cite web |title=Saturn S-II-4 |url=http://astronautix.com/s/saturns-ii-4.html |access-date=2025-11-18 |website=astronautix.com}}</ref>

=== Saturn S-II-8 ===
Eight engine version planned as the [[Saturn C-8]] second stage (1960 study).<ref>{{Cite web |title=Saturn S-II-8 |url=http://astronautix.com/s/saturns-ii-8.html |access-date=2025-11-18 |website=astronautix.com}}</ref>

=== Saturn S-II-C3 ===
The S-II-C3 stage version was studied in 1960 for the [[Saturn C-3]],<ref>{{Cite web |title=Saturn C-3 |url=http://astronautix.com/s/saturnc-3.html |access-date=2025-11-17 |website=astronautix.com}}</ref> consisted of four [[Rocketdyne J-2|J-2]] engines and had a height of 21.30 m and a diameter of 8.25 m.<ref name=":0">{{Cite web |title=Saturn S-II-C3 |url=http://astronautix.com/s/saturns-ii-c3.html |access-date=2025-11-17 |website=astronautix.com}}</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 V|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>{{Cite web |title=Saturn II C-5A |url=http://astronautix.com/s/saturniic-5a.html |access-date=2025-11-18 |website=astronautix.com}}</ref>

=== Saturn MS-II-1 ===
A five engine version with a stretched fuel tank (1965 study), intended for the [[Saturn MLV|Saturn MLV-V-1]], Saturn MLV-V-2 and Saturn MLV-V-4(S)-A.<ref>{{Cite web |title=Saturn MS-II-1 |url=http://astronautix.com/s/saturnms-ii-1.html |access-date=2025-11-18 |website=astronautix.com}}</ref>

=== Saturn MS-II-1-J-2T-200K ===
A five engine version using the uprated [[J-2T|J-2T 200k]] and a stretched fuel tank (1965 study), intended for the Saturn MLV-V-1/J-2T/200K.<ref>{{Cite web |title=Saturn MS-II-1-J-2T-200K |url=http://astronautix.com/s/saturnms-ii-1-j-2t-200k.html |access-date=2025-11-18 |website=astronautix.com}}</ref>

=== Saturn II-INT-17 ===
A seven [[HG-3 (rocket engine)|HG-3-SL]] engine version (1965 study), intended for the Saturn INT-17.<ref>{{Cite web |title=Saturn II-INT-17 |url=http://astronautix.com/s/saturnii-int-17.html |access-date=2025-11-18 |website=astronautix.com}}</ref>

=== Saturn II-SL ===
A five engine version using J-2-SL (1966 study), intended for the Saturn INT-19.<ref>{{Cite web |title=Saturn II-SL |url=http://astronautix.com/s/saturnii-sl.html |access-date=2025-11-18 |website=astronautix.com}}</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|Saturn V-ELV]].<ref>{{Cite web |title=Saturn MS-II-1A |url=http://astronautix.com/s/saturnms-ii-1a.html |access-date=2025-11-18 |website=astronautix.com}}</ref>

=== Saturn MS-II-2 ===
Five [[HG-3 (rocket engine)|HG-3]] engine concept with stretched propellant tanks (1966 study), intended for the Saturn MLV-V-3 and Saturn V/4-260.<ref>{{Cite web |title=Saturn MS-II-2 |url=http://astronautix.com/s/saturnms-ii-2.html |access-date=2025-11-18 |website=astronautix.com}}</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>{{Cite web |title=Saturn MS-II-1-J-2T-250K |url=http://astronautix.com/s/saturnms-ii-1-j-2t-250k.html |access-date=2025-11-18 |website=astronautix.com}}</ref>

=== Saturn MS-II-3B ===
Five Toroidal 400k engine concept with stretched propellant tanks (1967 study), intended for the Saturn V-3B.<ref>{{Cite web |title=Saturn MS-II-3B |url=http://astronautix.com/s/saturnms-ii-3b.html |access-date=2025-11-18 |website=astronautix.com}}</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>{{Cite web |title=Saturn MS-II-4(S)B |url=http://astronautix.com/s/saturnms-ii-4sb.html |access-date=2025-11-18 |website=astronautix.com}}</ref>

==See also==
* [[S-IC]]
* [[S-IVB]]
* [[Apollo (spacecraft)]]
* [[MS-II]]

==References==

{{reflist}}
* {{cite book|first=Roger E.|last=Bilstein|year=1980|url=https://history.nasa.gov/SP-4206/sp4206.htm |title=Stages to Saturn: A Technological History of the Apollo/Saturn Launch Vehicles|publisher=[[NASA]]|series=NASA History Series SP-4206}}
* [http://www.apollosaturn.com/ Apollo Saturn Reference Page]

{{J-2 (rocket engine)}}

{{DEFAULTSORT:S-Ii}}
[[Category:Apollo program]]
[[Category:Rocket stages]]
[[Category:Saturn V]]