Apollo 6
Launch of Apollo 6 as seen from the top of the launch tower
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| Mission type | Test flight |
|---|---|
| Operator | NASA |
| COSPAR ID | 1968-025A |
| SATCAT no. | 3170 |
| Mission duration | 9 hours, 57 minutes, 20 seconds |
| Orbits completed | 3 |
| Spacecraft properties | |
| Spacecraft |
Apollo CSM-020 Apollo LTA-2R |
| Manufacturer | North American Rockwell |
| Launch mass | Total: 36,930 kilograms (81,420 lb) CSM: 25,140 kilograms (55,420 lb) |
| Start of mission | |
| Launch date | April 4, 1968, 12:00:01 UTC |
| Rocket | Saturn V SA-502 |
| Launch site | Kennedy LC-39A |
| End of mission | |
| Recovered by | USS Okinawa |
| Landing date | April 4, 1968, 21:57:21 UTC |
| Landing site | 27°40′N 157°55′W / 27.667°N 157.917°W |
| Orbital parameters | |
| Reference system | Geocentric |
| Regime | Highly elliptical orbit |
| Perigee | 32 kilometers (17 nmi) |
| Apogee | 22,533 kilometers (12,167 nmi) |
| Inclination | 32.6 degrees |
| Period | 389.3 minutes |
| Epoch | April 4, 1968 |
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Apollo 6, launched on April 4, 1968, was the second A type mission of the United States Apollo program, an unmanned test of the Saturn V launch vehicle. It was also the final unmanned Apollo test mission.
Objectives were to demonstrate trans-lunar injection capability of the Saturn V with a simulated payload equal to about 80% of a full Apollo spacecraft, and to repeat demonstration of the Command Module's (CM) heat shield capability to withstand a lunar re-entry. The flight plan called for following trans-lunar injection with a direct return abort using the Command/Service Module's (CSM) main engine, with a total flight time of about 10 hours.
A phenomenon known as pogo oscillation damaged some of the Rocketdyne J-2 engines in the second and third stages by rupturing internal fuel lines, causing two second-stage engines to shut down early. The vehicle's onboard guidance system was able to compensate by burning the second and third stages longer, though the resulting parking orbit was more elliptical than planned. The damaged third stage engine also failed to restart for trans-lunar injection. Flight controllers elected to repeat the flight profile of the previous Apollo 4 test, achieving a high orbit and high-speed return using the Service Module (SM) engine. Despite the engine failures, the flight provided NASA with enough confidence to use the Saturn V for manned launches. Since Apollo 4 had already demonstrated S-IVB restart and tested the heat shield at full lunar re-entry velocity, a potential third unmanned flight was cancelled.
Apollo 6 was intended to send a Command and Service Module (CSM) plus a Lunar Module Test Article (LTA), a simulated Lunar Module (LM) with mounted structural vibration sensors, into a translunar trajectory. However, the Moon would not be in position for a translunar flight, and the Service Module engine would be fired about five minutes later to slow the craft, dropping its apogee to 11,989 nautical miles (22,204 km) and causing the CSM to return to Earth, simulating a "direct-return" abort. On the return leg, the engine would fire once more to accelerate the craft to simulate the nominal lunar return trajectory with a re-entry angle of -6.5 degrees and velocity of 36,500 feet per second (11,100 m/s). The entire mission would last about 10 hours.
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