16 July 1969: Apollo 11/Saturn V AS-506 accelerates with all five Rocketdyne F-1 engines burning. As the rocket climbs through thinner atmosphere, the engines become more efficient and the total thrust for the S-IC first stage increases from 7,648,000 pounds of thrust to 9,180,000 pounds of thrust at about T+1:23.0.
In order to limit acceleration, a pre-planned signal to cut off the center engine is sent at T+2:15.2 (Center Engine Cut-Off, “CECO”). As the first stage burns fuel at a rate of 13 tons per second, the rapidly deceasing weight of the Saturn V and the increasing efficiency of the F-1 engines could cause the limits of vehicle acceleration to be exceeded.
By T+2:30, the Saturn V has reached an altitude of 39 miles (62.8 kilometers) and is 55 miles (88.5 kilometers) downrange.
The photograph above was taken by a 70mm telescopic camera aboard a USAF/Boeing EC-135N A/RIA (Apollo Range Instrumentation Aircraft) serial number 60-374. The airplane is in the collection of the National Museum of the United States Air Force.
© 2019, Bryan R. Swopes
I’m wondering if the shutdown to limit vehicle acceleration was due to structural limitations or to maintain proper trajectory? Maybe something else?
Brian, I am uncertain of the answer, but have a vague recollection that it was a crew limitation. According to the Saturn V flight manual, CECO occurs well beyond the point of Maximum Aerodynamic Pressure (MAX Q). With fuel burn, the Saturn V’s weight has dropped to 2.6 million pounds. Engine thrust drops from 9.2 to 7.2 millions pounds, and longitudinal acceleration drops from 120 ft/sec/sec to 98 ft/sec/sec. The vehicle is approaching maximum aerodynamic heating, however.