| S-IV | |
| Manufacturer: | Douglas Aircraft Company |
| Country: | United States |
| Rockets: | Saturn I (stage 2) |
| Height: | 12.19m (39.99feet) |
| Diameter: | 5.49m (18.01feet) |
| Mass: | 50576kg (111,501lb) |
| Propmass: | 45359sigfig=3NaNsigfig=3 |
| Empty: | 5217sigfig=3NaNsigfig=3 |
| Status: | Retired |
| Launches: | 6 |
| Success: | 6 |
| Fail: | 0 |
| First: | January 29, 1964 |
| Last: | July 30, 1965 |
The S-IV was the second stage of the Saturn I rocket used by NASA for early flights in the Apollo program.
The S-IV was manufactured by the Douglas Aircraft Company and later modified by them to the S-IVB, a similar but distinct stage used on the Saturn IB and Saturn V rockets.
The S-IV stage was a large LOX/LH2-fueled rocket stage used for the early test flights of the Saturn I rocket. It formed the second stage of the Saturn I and was powered by a cluster of six RL-10A-3 engines. Each one of the engines supplied of thrust for a total of about . The cryogenic LH2 (liquid hydrogen) and LOX (liquid oxygen) tanks were separated by a common bulkhead. The forward bulkhead of the LOX tank formed the aft bulkhead of the LH2 tank. This saved up to 20% of structural weight.