Tracking-space mission Apollo 6  (AS-502) 
Launched - 04/04/1968

This cacheted cover commemorates the Launch  of the AS-502 (Apollo 6) Saturn 1-B flight with the LEM aboard, launched on 01/22/1968 from Kennedy Space Center, FL .   
The Apollo 6 was to show that the Saturn V could carry the weight of the Apollo fast enough and far enough into space to make a flight to the Moon possible. It was also designed to show that the Apollo Command Module's heat shield could survive the high speed re-entry into the Earth's atmosphere. The project was designed to fly into space at the necessary speeds, turn around, and return in about 10 hours.

However, problems in the fuel lines of several Saturn V second and third stage engines stopped it from reaching the speeds Apollo would need to reach the Moon. It was able to test the return speeds by using the Apollo spacecraft's engine. This had been done with Apollo 4, the first Saturn V test. Despite the engine failures, the flight gave NASA enough confidence in the Saturn V to use it for manned launches. Since Apollo 4 had also tested the heat shield at a full speed re-entry, a third unmanned flight was cancelled.

Apollo 6 had problems right from the start. Two minutes into the flight, the rocket experienced severe Pogo oscillations for about 30 seconds. Various parts of the rocket began to vibrate like a tuning fork. This included the fuel pipes. These vibrations caused damage to the part that joined the Command Module and Lunar Module to the Saturn V. Several pieces fell off during the launch. 

Orbit

Because of the launch problems, the rocket did not reach the planned height. After two orbits, the third stage, S-IVB would not restart. It was decided to use the Apollo Service Module engines to get the spacecraft into a higher orbit, as had been done in Apollo 4. It burned for 442 seconds (longer than it would ever fire on a lunar mission) to get to the planned 11,989-nautical-mile (22,204 km) height. There was not enough fuel to speed up the atmospheric reentry. The spacecraft only entered the atmosphere at a speed of 33,000 feet per second (10,000 m/s) instead of the planned 37,000 feet per second (11,000 m/s) of a lunar return. However, this had been demonstrated on Apollo 4.


Ten hours after launch, it landed 43 nautical miles (80 km) from the planned touchdown point in the North Pacific Ocean north of Hawaii, and was lifted on board the 

USS Okinawa.

 This cover was hand cancelled at the Edwards AFB in CA.
(The NASA Flight Research Center) tracking station on 04/04/1968.