The Saturn V Third Stage: The Rocket That Sent Apollo to the Moon

The third stage of the Saturn V did two jobs: it put the Apollo crew into Earth orbit, and then it fired again to send the spacecraft toward the Moon. A NASA film in the Apollo Digest series showed how it was built and tested.

Super-cold fuel

Like the second stage, the third burned super-cold propellants: 20,000 gallons of liquid oxygen and 63,000 gallons of liquid hydrogen, the hydrogen stored at 423 °F below zero. If they are not kept that cold, both turn to gas and are useless as propellants.

A single J-2 engine, seen in the film during a full-throttle static test, produced 200,000 pounds of thrust. The stage stood more than 58 feet tall and 22 feet across, and weighed 130 tons fully fueled.

Two burns

• Into orbit. The third stage ignited after the second stage burned out, about 117 miles above Earth and traveling about 15,300 miles an hour — high enough for orbit, but too slow to stay there. It burned for just over two minutes to reach orbital speed, then shut down.
• Toward the Moon. After the spacecraft had circled long enough to pin down its exact position and path, the stage reignited on command. To escape Earth’s gravity it had to reach about 24,300 miles an hour, a burn of about six minutes. Once the spacecraft was on its way to the Moon, the stage shut down for good, though it still helped hold the spacecraft’s orientation.

How it was built

• An aluminum interstage, a cone, joined the second and third stages.
• A thrust structure spread the engine’s force evenly around the fuel tank.
• The aft skirt fit between the interstage and the propellant tank.
• The tank assembly was really two tanks — liquid oxygen at the bottom, liquid hydrogen on top — sharing a common bulkhead, which saved both weight and space.
• The forward skirt supported the instrument unit, the topmost section of the launch vehicle.

With the sections assembled, workers installed the J-2 engine and the stage’s other systems: hydraulics to swivel the engine; electrical power; telemetry to send flight data to the ground; environmental control; flight controls for pitch, roll and yaw; and a system to push the propellant to the bottom of the tanks for reignition — needed because the stage is weightless in orbit.

After checkout, each stage was static-fired at full thrust for its full duration at the Sacramento test area, then shipped to Kennedy Space Center to take its place on a Saturn V.

Sources

Based on "Apollo Digest Series: SATURN THIRD STAGE," a NASA film and its description, NASA Image and Video Library; a work of the United States government in the public domain. The film gives the stage’s speed after its first burn as 7,500 miles an hour, which cannot be right for a stage already moving at 15,300, and the liquid oxygen temperature as 273° below zero; neither figure is repeated here.