The S-II-13 stage

General information

To keep the ratio between payload mass and total mass of the Saturn V stack within the desired limits, the two upper stages had to be propelled by high performance thrust engines to provide maximum thrust with a minimum of propellant mass. Such performance could only be achieved if liquid oxygen and liquid hydrogen were to be used as propellants. (Hydrogen and the extremely hazardous fluoride is a more powerful mix but was not considered as a real option)

The use of liquid hydrogen as a fuel was benefitial for the upper stages, but was impractical for the first stage. Because of its very low density compared to kerosene, the required size of the fuel tank therefore would have been very large. Such a large S-IC stage would have been heavier and much more difficult to handle.

At that time the use of these cryogenic highly combustible propellants had been a development topic for a while. The Saturn Vís third stage for instance had been developed within the Saturn I development program. This S-IV stage acted as a second stage for the Saturn I booster and performed well. The origin of the name S-IV dates back to the time that a moon mission called for a four stage rocket design. The S-IV could also act as a third stage for the Saturn V booster after some modifications to meet the new mission profile. The modified version was named S-IVB.

During develeopment of the Saturn V second stage (S-II), a lot of new design and construction challenges had to be faced. The S-II stage was expected to hold liquid oxygen and hydrogen in unprecedented quantities for its five cryogenic J-2 engines. As the project went on it turned out to be that the required payload mass to conduct a moon mission gradually increased for a number of reasons. Design requirements on the S-II stage became therefore high with respect to mass and tank isolation. The development of this stage proceeded with great difficulties, making reliable welds between complicated curved tank parts made out of a newly developed aluminium compound was one of them. The project was also plagued by managerial problems which could seriously jeopardize the progress of the Apollo program. Around January 1966 a project management change at North American Aviation became necessary to bring the project back on track.

S-II stage main parts

From bottom up:
  1. The five J-2 engines
  2. A conical shaped thrust structure
  3. The aft skirt
  4. The fuel tank and the liquid oxygen tank with their common bulkhead
  5. The forward skirt
  6. The interstage ring


  1. Height      : 24.45 meter
    Diameter  : 9.90 meter
    Mass empty stage           : 35.40 Ton
    Mass of propellants        : 451.74 Ton
    Mass of loaded stage      : 487.14 Ton
    Amount of fuel:
    Liquid Oxygen    : 379.26 Ton, 333 920 litres (Mass dens. LOx: 1.137 kg/ltr)
    Liquid Hydrogen : 72.48 Ton, 1 030 889 litres (Mass dens. LH: 0.799 kg/ltr).
  2. The S-II-13 Flight profile
    • Ignition of the five J-2 engines at an altitude of 90 km
    • S-II stage burn out at an altitude of 433 km. The J-2 engines were shut down by the guidance computer in the Instrument Unit, and not until propellant depletion like in the Apollo missions.
    • Downrange distance at S-II stage burn out: 1800 km
    • End velocity: 7 330 m/sec (26 400 km/hr; 22.3 Mach)
    • Time of operation : 427 sec.
  3. J-2 engines:
    J-2 engine thrust : 104 Tons (metric)
    The five J-2 engines provided a total thrust of about 520 Ton.
    Height of J-2 engine      : 3.38 meter
    Diameter of J-2 engine  : 2.07 meter
    Mass of J-2 engine       : 1.58 Ton
    Combustion pressure inside an J-2 chamber : 53 bar
    Combustion chamber diameter probably around 0.47 meter.
    Fuel consumption rate per J-2 engine : 0.23 Ton/sec.
    Specific Impulse at ground level : 440 sec.
  4. Manufacturer of the J-2 engines: Pratt & Whitney (initially)
    North American Aviation, Rocket dyne Division (eventually)
    Manufacturer of the S-II stage: North American Aviation

This picture shows the flight path of the SA-513 (Skylab 1) during ascent. The engines of the S-II stage were ignited at about 2½ min. after lift-off at an altitude of 90 km. It delivered thrust for about 7 minutes to bring Skylab into a 433 km high nearly circular orbit. After separation with its payload, the S-II-13 was manoeuvred into a slightly lower orbit, but its initial altititude of around 400 km was two times higher than the altitude of 180 km at which the S-II stage was released during an Apollo mission . It took nearly 2 years before the S-II-13 reentered Earth's atmosphere and came down in the middle of the Indian Ocean.

Skylab 1 ground track during ascent

This picture shows the ground track of the SA-513 (Skylab 1) during the first 17 minutes into the flight. Earth orbit insertion was achieved after about 10 minutes from lift-off.
The S-II provided thrust for about 7 minutes and was then separated.

Plan View S-II stage Forward section, Looking down

The S-II stage

From top to bottom: the forward skirt, the combined oxygen & hydrogen tank, the aft skirt and the conical thrust structure

Plan View S-II stage Aft section, Looking up

Five J-2 eingines

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Copyright 2005 by   Sander Panhuyzen
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