The S-II stage

The S-II in the transfer aisle of the Vertical Assembly Building (VAB). This particular stage is waiting to be installed on top of the first stage of the SA-501. SA-501 is the serial number of the Saturn V launch vehicle, which was used for the Apollo 4 mission. July 3, 1967

credit to NASA
Scanning credit to Kipp Teague

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. The required 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 unpractical for the first stage. It would have required a very large fuel tank because of the much lower density of hydrogen compared to kerosene. 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.

The Saturn V second stage (S-II) had to be newly developed and a lot of new design and construction challenges had to be faced. The S-II stage had to hold liquid oxygen and hydrogen in unprecedented quantities for its five cryogenic J-2 engines. As the project went on, the required payload mass for moon missions increased for a number of reasons. Much effort went into keeping the launch vehicle as light as possible. According to calculations much was to be gained with the S-II stage. Its design requirements on mass and tank isolation were increased. 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. Typical flight profile
    • Ignition of the five J-2 engines at an altitude of 70 km
    • S-II stage burn out at an altitude of 160 km
    • Downrange distance at S-II stage burn out: 1700 km
    • End velocity: 6 690 m/sec (24 000 km/hr; 22.2 Mach)
    • Time of operation : 370 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-503 (Apollo 8) during ascent. The engines of the S-II stage were ignited at about 2½ min. after lift-off at an altitude of 70 km. It delivered thrust for about 6 minutes and was then disposed of.

Apollo 8 ground track during ascent

This picture shows the ground track of the SA-503 (Apollo 8) during the first 17 minutes into the flight. Earth orbit insertion was achieved after about 12 minutes.
The S-II provided thrust for about 6 minutes and was then separated. After a 10 minutes ballistic flight, the S-II stage plunged into the Atlantic Ocean about 4200 km off the east coast of Florida.

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 engines

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Copyright 2005 by   Sander Panhuyzen
All pictures and drawings contained on and through these pages are the author's, unless otherwise noted. No unauthorized reproduction without permission.