Stage 2 (S-II)

Saturn V Stage 2 (S-II)
The Apollo 6 S-II stage during stacking operations in the VAB The Apollo 6 S-II stage during stacking operations in the VAB
ManufacturerNorth American
Country of originUSA
RocketsSaturn V (stage 2)
Saturn INT-21 (stage 2)
Size
Height24.9 m (82 ft)
Diameter10 m (33 ft)
Mass480,900 kg (1,060,000 lb)
Engine details
Engines5 J-2 engine
Thrust5,115 kN (1,150,000 lbf)
Burn time367 seconds
FuelLOX/LH2

The S-II (pronounced "ess 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 cross pattern. The second stage accelerated the Saturn V through the upper atmosphere with 5 MN of thrust.

History

S-2 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. At the same time the S-II stage design began to take shape. Initially it was to have four J-2 engines and be 22.5 meters in length and 6.5 meters 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.

In the end on 11 September 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.

Configuration

Cutaway illustration of the S-II (second) stage

When fully loaded with fuel, the S-II had a mass of about 500,000 kg. The hardware was only 3% of this97% was liquid hydrogen and liquid oxygen.

At the bottom was the thrust structure supporting five J-2 engines. The center engine was fixed, while the other four were gimballed.

Instead of using an intertank (empty container between tanks) like the S-IC, the S-II used a common bulkhead that included both the top of the LOX tank and bottom of the LH2 tank. It consisted of two aluminium sheets separated by a honeycomb structure made of phenol. It insulated a 70 C (125 F) temperature differential between the two tanks. The use of a common bulkhead saved 3.6 tonnes in weight.

The LOX tank was an ellipsoidal container of 10 meters diameter and 6.7 meters high. 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 0.69 meter high. The biggest challenge was the insulation. Liquid hydrogen must be kept colder than about 20 C above absolute zero (20 K or -252 C or - 423 F) so good insulation is very important. Initial attempts did not work well: there were bonding issues and air pockets. The final method was to spray insulation on by hand and trim the excess.

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
S-II-FUsed as Dynamic Test Stage replacement after destruction of S-II-S/D and S-II-TAt the U.S. Space & Rocket Center, Huntsville, Alabama
S-II-TDestroyed in explosion May 28, 1966
S-II-DConstruction canceled
S-II-S/DStructural and Dynamic Test VehicleDestroyed in test stand September 29, 1965
S-II-1Apollo 4November 9, 1967Carried "Camera Targets" spaced around the forward skirt and carried cameras to record first stage separation
S-II-2Apollo 6April 4, 1968carried cameras to record first stage separation
S-II-3Apollo 8December 21, 1968
S-II-4Apollo 9March 3, 19691800 kg lighter allowing 600 kg more payload, more powerful engines and carried more LOX
S-II-5Apollo 10May 18, 1969
S-II-6Apollo 11July 16, 1969
S-II-7Apollo 12November 14, 1969
S-II-8Apollo 13April 11, 1970Inboard engine failed during ascent due to pogo oscillations.
S-II-9Apollo 14January 31, 1971
S-II-10Apollo 15July 26, 1971
S-II-11Apollo 16April 16, 1972
S-II-12Apollo 17December 7, 1972
S-II-13Skylab 1May 14, 1973Modified to act as the terminal stage
S-II-14Apollo 18 (cancelled)N/AApollo-Saturn V Center, Kennedy Space CenterFrom the cancelled Apollo 18 mission.
S-II-15Skylab 1 backup (not flown)N/AJohnson Space CenterFrom SA-515 the Skylab backup vehicle which NASA did not use.