S-II Overview

The S-II, the second stage of the Saturn V rocket, was the largest liquid hydrogen stage produced in the 1960s. Equipped with five J-2 rocket engines, it produced about 1,000,000 pounds of thrust. It was 33 feet in diameter and 81 ft. 7 in. high.

Saturn V Second S-II Stage cut-away diagram

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From the cover of the Propulsion and Fluid Systems Components Manual for the S-II Stage of the Saturn V, located in the Dunaway collection of the Dept. of Archives/Special Collections, M. Louis Salmon Library, University of Alabama in Huntsville.
Scan and restoration by heroicrelics.org.

Structurally, the S-II stage consisted of

Saturn V Second S-II Stage aft interstage

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From page 4-2 of the Saturn V News Reference, located in the Dept. of Archives/Special Collections, M. Louis Salmon Library, University of Alabama in Huntsville. Also available on the mirror as a 4.9 megabyte PDF.
Scan by heroicrelics.org.

This cut-away diagram shows the major structural elements assembled into the final stage, while at the same time showing the interior of stage subassemblies.

Saturn V Second S-II Stage cut-away diagram with callouts

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From an unnumbered page at the beginning of Section 4 of the Saturn V News Reference, located in the Dept. of Archives/Special Collections, M. Louis Salmon Library, University of Alabama in Huntsville. Also available on the mirror as a 4.9 megabyte PDF.
Scan by heroicrelics.org.

The diagram above shows 8 ullage motors, used to settle the S-II propellants after staging. Only SA-501 (Apollo 4) flew with 8 ullage motors; it was found that four ullage motors could provide adequate propellant seating and stage separation, so SA-502 (Apollo 6) deleted four of the ullage motors (but retained the empty fairings) and SA-503 (Apollo 8) through SA-509 Apollo 14) had only four ullage motors and fairings. It was later decided that the ullage motors were entirely unnecessary, so SA-510 (Apollo 15) and subsequent deleted the ullage motors and fairings entirely.

Of course, each of the S-II's structural elements themselves were gargantuan structures, comprised of subassemblies. This diagram breaks down several structures into their components, including the LH2 forward bulkhead, six LH2 cylinder subassemblies, the common bulkhead, the aft bulkhead, and the interstage. This diagram additionally calls out two pieces of ground support equipment, the forward hoisting frame and the static firing skirt:

Saturn V Second S-II Stage construction breakdown

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Taken from page 1-2 of the Manufacturing Plan for Saturn S-II, Stages 16-15, located in the Dept. of Archives/Special Collections, M. Louis Salmon Library, University of Alabama in Huntsville, which makes the document available as a 8.1 megabyte PDF.
Extraction and cleanup by heroicrelics.org.

This final diagram breaks each subassembly down into its component parts:

Saturn V Second S-II Stage construction assembly exploded view

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Taken from page 9 in the PDF of the Description of S-II Stage Structures
Extraction and cleanup by heroicrelics.org.

The liquid hydrogen tank was the largest subassembly and was also composed of the largest number of parts. The Description of S-II Stage Structures describes LH2 tank as follows:

The LH2 tank is made up of six cylindrical sections, an ellipsoidal forward bulk[sic] and the common bulkhead. The volume of the LH2 tank is 36,883 cu. ft. The forward bulkhead and the cylindrical sections are of 2014 aluminum covered with external insulation.

The cylindrical sections are made up of quarter panels welded together. The quarter panels have integral longitudinal and circumferential stiffeners. Riveted to the circumferential stiffeners are 7 inch wide rings of 2024 aluminum for additional support. The aft cylindrical section has external tapered longitudinal stiffeners 25 inches long to assist in transferring loads from the bolting ring to which it is fastened. The bolting ring is 15 inches long and has provision for attachment of the LOX tank, the aft skirt and the LH2 cylinder.

The forward cylindrical section has a boss at the forward end to which is bolted the forward skirt. The forward bulkhead is welded to this section.

The forward bulkhead is constructed of 12 welded gores and a circular section welded at the center. Access to the tank is through a manhole in the center of the forward bulkhead.

One fitting for pressurization and two for venting are provided in the forward bulkhead. Five fittings for LH2 engine lines, one for LH2 recirculation return line, and one for fill and drain are provided in the cylindrical section just forward of the bolting ring.

Installed in the interior of the tank is the continuous level probe and a pressurization gas diffuser.

Insulation on the cylindrical section and the forward bulkhead is external to the tank. It consists of glass-phenolic honeycomb core filled with isocyanate foam covered with a nylon phenolic skin and sealed with a plastic film. Provision is made in the insulation for purging and a leak detection system. Insulation on the bolting ring is machined blocks of plastic.

Note that the external insulation as described above was used on the various testing S-II stages and the first seven flight stages (the unmanned Apollos 4 and 6, plus Apollos 8 through 12). Starting with S-II-8 (used on SA-508, or Apollo 13), an external spray-on foam, not unlike the foam used on the Space Shuttle External Tank, was used instead.