posted 07-27-2016 12:39 PM               

The lunar module could do a lot of things independently, but it had to rely on people on the ground to tell the computer where it was starting on its landing position. On the backside of the moon there had been some venting — some perturbations unknown to us. The time we gave them to light the engine was about 4 or 5 seconds late.

posted 07-27-2016 02:50 PM               

The principal error induced by maneuvering of the spacecraft was, however, the incomplete vent of the tunnel propagated over one orbit after separation. We made a change in all future missions to get a MCC go-nogo on tunnel delta P before giving the crew a Go to undock.

posted 07-27-2016 02:54 PM               

In a 2002 e-mail, Kranz elaborates, "Floyd's note was correct on the velocity-induced position error at the start of the descent. There were several interrelated navigation problems, i.e. known deficiencies in the R2 lunar (gravitational) potential model, down-track (along the flight path toward the landing site) and cross-track (left or right, perpendicular to the flight path) propagation errors (errors that start out small but become larger as the flight proceeds), and errors induced by maneuvering of the spacecraft. The principal error induced by maneuvering of the spacecraft was, however, the incomplete vent of the tunnel propagated over one orbit after separation. We made a change in all future missions to get a MCC go-nogo on tunnel delta P before giving the crew a Go to undock. Page 82 of the Apollo 11 post mission report says ...'because of uncoupled attitude maneuvers such as hot fire tests, undocking impulse, station keeping, sublimator operation and possible tunnel and cabin venting. The net effect of these perturbations was a sizeable down-range miss.' To my recollection, the trajectory reconstruction determined that with the exception of the tunnel venting, most of the other perturbations were essentially self canceling. Further the post mission review indicated that the delta P gauge was too gross, the markings misleading and the tunnel had to be vented earlier in the timeline and the valve left in the tunnel vent position rather than returned to off.

posted 07-28-2016 01:26 AM               

As soon as Eagle had disconnected from Columbia, it was propelled faster than planned because of the residual oxygen expelled from the airlock. As a result, the LM overflew the planned landing site.

posted 10-07-2017 06:09 PM            

Because of Apollo Eleven's four-mile miss, the Apollo 12 flight planners have made some changes.

Apollo Eleven's computer information was out of date before its final descent. Armstrong and Aldrin were too busy preparing for the historic touchdown to work up on the computer the information obtained in 16 minutes of radio contact before the landing. Apollo 12 is to have more than 30 minutes of radio contact with the Earth before landing. This, it is said, is plenty of time to feed the latest tracking data into the lunar module's computer.

posted 04-15-2018 02:03 AM               

And does anyone know how later lunar landing missions (Apollos 12-17) compensated for these fluctuations?

posted 04-15-2018 07:39 AM               

As the amount of gravity acting on the lander varied then the rate that the lander fell towards the surface changed, i.e. more gravity equals fall faster. However, the mascon variation was only part of the reason Apollo 11 landed long.

Another contributing factor was the fact that the docking tunnel was not completely vented of all air prior to undocking. The additional push from the air pressure within the tunnel gave an additional push that added to the lander velocity.