The S-II (pronounced "S-two") was the second stage of the

Saturn V The Saturn V is a retired American super heavy-lift launch vehicle developed by NASA under the Apollo program for human exploration of the Moon. The rocket was human-rated, had multistage rocket, three stages, and was powered by liquid-propel ...

rocket. It was built by

North American Aviation North American Aviation (NAA) was a major American aerospace manufacturer that designed and built several notable aircraft and spacecraft. Its products included the T-6 Texan trainer, the P-51 Mustang fighter, the B-25 Mitchell bomber, the F- ...

. Using

liquid hydrogen Liquid hydrogen () is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecule, molecular H2 form. To exist as a liquid, H2 must be cooled below its critical point (thermodynamics), critical point of 33 Kelvins, ...

(LH2) and

liquid oxygen Liquid oxygen, sometimes abbreviated as LOX or LOXygen, is a clear cyan liquid form of dioxygen . It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an application which is ongoing. Physical ...

(LOX) it had five J-2 engines in a

quincunx A quincunx ( ) is a geometry, geometric pattern consisting of five points arranged in a cross, with four of them forming a Square (geometry), square or rectangle and a fifth at its center. The same pattern has other names, including "in saltire" ...

pattern. The second stage accelerated the Saturn V through the upper atmosphere with 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,

fueled engine. The contract for this engine was given to

Rocketdyne Rocketdyne is an American rocket engine design and production company headquartered in Canoga Park, California, Canoga Park, in the western San Fernando Valley of suburban Los Angeles, California, Los Angeles, in southern California. 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 in length and in diameter. In 1961 the

Marshall Space Flight Center Marshall Space Flight Center (officially the George C. Marshall Space Flight Center; MSFC), located in Redstone Arsenal, Alabama (Huntsville, Alabama, Huntsville postal address), is the Federal government of the United States, U.S. government's ...

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 The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

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

(who were also awarded the contract for the

Apollo Command/Service Module The Apollo command and service module (CSM) was one of two principal components of the United States Apollo (spacecraft), Apollo spacecraft, used for the Apollo program, which landed astronauts on the Moon between 1969 and 1972. The CSM functi ...

), with the manufacturing plant built by the government at Seal Beach,

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

. 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

When fully loaded with propellant, the S-II had a mass of about . The hardware was only 7.6% of this—92.4% was liquid hydrogen and liquid oxygen. At the bottom was the thrust structure supporting five J-2 engines in a

arrangement. The center engine was fixed, while the other four were

gimbal A gimbal is a pivoted support that permits rotation of an object about an axis. A set of three gimbals, one mounted on the other with orthogonal pivot axes, may be used to allow an object mounted on the innermost gimbal to remain independent of ...

led, similar to the engines on the

S-IC The S-IC (pronounced S-one-C) was the first stage of the American Saturn V rocket. The S-IC stage was manufactured by the Boeing Company. Like the first stages of most rockets, more than 90% of the mass at launch was propellant, in this case RP ...

stage below. Instead of using an ''intertank'' (empty container between tanks) like the

, the S-II used a common bulkhead (similar to that of the

S-IV The S-IV was the Multistage rocket, second stage of the Saturn I rocket used by NASA for early flights in the Apollo program. The S-IV was manufactured by the Douglas Aircraft Company and later modified by them to the S-IVB, a similar but distinc ...

and

S-IVB The S-IVB (pronounced "S-four-B") was the third stage on the Saturn V and second stage on the Saturn IB launch vehicles. Built by the Douglas Aircraft Company, it had one J-2 rocket engine. For lunar missions it was fired twice: first for Earth ...

stages) that included both the top of the LOX tank and bottom of the LH2 tank. It consisted of two

aluminum Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

sheets separated by a honeycomb structure made of

phenolic resin Phenol formaldehyde resins (PF), also called phenolic resins or phenoplasts, are synthetic polymers obtained by the reaction of phenol or substituted phenol with formaldehyde. Used as the basis for Bakelite, PFs were the first commercial synthetic ...

. It insulated a 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

ellipsoid An ellipsoid is a surface that can be obtained from a sphere by deforming it by means of directional Scaling (geometry), scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface; that is, a Surface (mathemat ...

al container of 10 meters diameter and 6.7 meters high holding up to or 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 () 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 for storing of liquid hydrogen. The S-II was constructed vertically to aid welding and keep the large circular sections in the correct shape.

Stages built

References

*
Apollo Saturn Reference Page
{{DEFAULTSORT:S-Ii Apollo program Rocket stages Saturn V

History

Configuration

Stages built

See also

References