Apollo 6 (also known as AS-502), launched on April 4, 1968, was the final unmanned Apollo test mission. Its 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 and service module's (CSM) main engine, with a total flight time of about 10 hours.
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| An Apollo 6 launch cover featuring the "KSC Officials" stamped cachet, postmarked at Kennedy Space Center with a machine cancel on April 4, 1968. |
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| An Apollo 6 launch cover from "SpaceCraft Swanson", postmarked at Patrick Air Force Base with a machine cancel on April 4, 1968. |
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| An Apollo 6 recovery cover with a printed “Morris Beck B747” cachet, postmarked aboard USS Okinawa with a hand cancel on April 4, 1968. |
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| An Apollo 6 recovery cover featuring a magenta “USS Okinawa” ship’s cachet, postmarked aboard USS Okinawa with a hand cancel on April 4, 1968. |
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| An Apollo 6 recovery cover from "Orbit Covers", featuring a magenta “USS Okinawa” ship’s cachet on the reverse, postmarked aboard USS Okinawa with a hand cancel on April 4, 1968. |
