S-II

Key takeaways

S-II belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect S-II to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of S-II from memory before moving on to harder problems.

Reference excerpt

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 payload through the upper atmosphere with 1,000,000 pounds-force (4.4 MN) 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 74 feet (23 m) in length and 260 inches (6.6 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. 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 (70 °C) 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 US gallons (310 m3) or 789,000 pounds (358 t) 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 or 20.4 K or −252.8 °C) 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 US gallons (980 m3) for storing 153,000 pounds (69 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

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.

See also S-IC S-IVB Apollo (spacecraft) MS-II

References

Bilstein, Roger E. (1980). Stages to Saturn: A Technological History of the Apollo/Saturn Launch Vehicles. NASA History Series SP-4206. NASA. Apollo Saturn Reference Page

Illustrations

Worked examples

Example 1 — a first encounter with S-II

Start with the simplest possible case. Write down what S-II claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to S-II before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about S-II ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of S-II

In research: S-II appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses S-II in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: S-II is common in secondary-school and first-year university syllabi. It links to neighbouring topics Apollo program, Rocket stages, Saturn V, so understanding it makes those chapters shorter.
In everyday life: Look for S-II outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study S-II in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what S-II means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain S-II out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is S-II in simple terms?

The S-II (pronounced "S-two") was the second stage of the Saturn V rocket. It was built by North American Aviation.

Why does S-II matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study S-II?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on S-II.