The stages of the Saturn V
The Saturn V had three stages: the S-IC (built by Boeing, five kerosene-burning F-1 engines), the S-II (North American Aviation, five hydrogen-fueled J-2 engines) and the S-IVB (Douglas, a single restartable J-2), topped by an IBM-built Instrument Unit that guided the whole stack. The Saturn V stages fired in turn for roughly 2 min 40 s, 6 min 30 s and, over two burns, about 8 min 30 s.
Contents
Stage-by-stage specs
| Element | Builder | Height | Diameter | Fueled mass (approx.) | Engines | Burn time (approx.) |
|---|---|---|---|---|---|---|
| S-IC | Boeing | 42 m (138 ft) | 10.1 m (33 ft) | 2,280 t | 5 F-1 | 150–165 s |
| S-II | North American Aviation | 24.9 m (81.5 ft) | 10.1 m (33 ft) | 480 t | 5 J-2 | 360–390 s |
| S-IVB | Douglas Aircraft | 17.8 m (58.6 ft) | 6.6 m (21.7 ft) | 120 t | 1 J-2 | about 2.5 min, then 5.8 min |
| Instrument Unit | IBM | 0.9 m (3 ft) | 6.6 m (21.7 ft) | 2 t | none | n/a |
Masses varied between vehicles: the late-program rockets for Apollo 15 to 17 carried more propellant and slightly uprated engines. Above the Instrument Unit sat the lunar module adapter, the Apollo command and service modules and the launch escape tower.
The S-IC first stage
The S-IC was the giant of the stack: 42 m tall, 10.1 m across and close to 80% of the vehicle's liftoff mass. Its two huge tanks held about 770,000 liters (203,000 gal) of RP-1 kerosene and about 1.3 million liters (330,000 gal) of liquid oxygen. The five F-1 engines sat in a cross pattern: a fixed center engine and four outboard engines that gimbaled to steer.
Boeing assembled the stage at the Michoud Assembly Facility near New Orleans, and test firings took place at the Mississippi Test Facility (today's Stennis Space Center) or in Huntsville. In flight, the center engine shut down early to cap acceleration; the stage separated at about 65 km (40 miles) altitude and fell into the Atlantic. Several of its engines were recovered from the ocean floor in 2013.
The S-II second stage
The S-II was the riskiest technological leap in the program. Liquid hydrogen, stored at -253 °C (-423 °F), is extremely low density, so large tanks are needed for modest mass. North American Aviation used a common bulkhead between the hydrogen and oxygen tanks, an insulated double wall that saved several meters of height and many tons, but caused serious delays and test failures. The stage was built in Seal Beach, California.
The S-II's five J-2 engines ran for about six and a half minutes, carrying the vehicle almost to orbital velocity. On Apollo 6 and again on Apollo 13, early S-II engine shutdowns tested the rocket's ability to compensate for a failure. For Skylab, the S-II was the final powered stage and reached orbit itself.
The S-IVB third stage
Built by Douglas in Huntington Beach, California, the S-IVB was the only stage shared with another launcher: a closely related version served as the second stage of the Saturn IB. Its single J-2 engine could restart in space, giving it two jobs. A first burn of about two and a half minutes put the stack into a parking orbit near 190 km (118 miles); one or two orbits later, a second burn of just under six minutes, translunar injection, sent the crew toward the Moon.
After the spacecraft separated, early S-IVBs were sent into solar orbit; from Apollo 13 onward they were deliberately crashed into the Moon so the seismometers left by earlier crews could record the impacts. For the Skylab station, an S-IVB was converted into a habitable orbital workshop.
The Instrument Unit: the rocket's brain
A ring less than a meter tall, the Instrument Unit housed the Launch Vehicle Digital Computer (LVDC), the ST-124 inertial platform and telemetry systems. IBM built it in Huntsville. It commanded stage separations, engine steering and S-IVB ignition. When Apollo 12 was struck by lightning, the Instrument Unit kept guiding the rocket normally while the Apollo spacecraft's instruments briefly dropped out.
How staging worked
S-IC/S-II separation used retrorockets on the first stage and small ullage motors on the second, which settled propellant at the bottom of the tanks before the J-2s lit. About 30 seconds later the interstage ring dropped away, followed by the escape tower. The minute-by-minute Apollo 11 launch shows the full sequence.
FAQ
How many stages did the Saturn V have?
Three powered stages (S-IC, S-II and S-IVB) plus the unpowered Instrument Unit that housed guidance. For the Skylab launch it flew with only two powered stages, because the station took the place of the S-IVB.
Which stage was the heaviest?
The S-IC first stage, at about 2,280 t fueled, close to 80% of the rocket's liftoff mass. It emptied its tanks in roughly two and a half minutes.
Why did the upper stages burn hydrogen?
Liquid hydrogen delivers much higher specific impulse than kerosene, which matters most once the rocket is out of the dense atmosphere. At liftoff, the denser kerosene allowed more powerful engines and more compact tanks.
What happened to the stages after use?
The S-IC and S-II fell into the Atlantic. The S-IVB went into orbit around the Sun (Apollo 8 to 12) or was crashed into the Moon (Apollo 13 to 17).
Sources
- NASA Marshall Space Flight Center, Saturn V Flight Manual SA-503, 1968.
- Roger E. Bilstein, Stages to Saturn, NASA SP-4206, 1980.
- NASA History Office