Propellant level remaining at Apollo 11 landing: separating truthfrom myth

Well, might as well get cracking with a tech post now that this NG is
revived...

There's a lot of "mythology" about how close Apollo 11 was to propellant
depletion at the time of landing. Rather than having between 7-20
seconds of propellant remaining, as some sources have derived, there
were actually 45-50 seconds remaining. The following is an excerpt from
the Apollo 11 Mission Report (note that I did not bother to clean up OCR
errors in the PDF):

http://history.nasa.gov/alsj/a11/A11_MissionReport.pdf

9.8.3 Gaging System Performance

During the descent orbit insertion maneuver and the early portion of
powered descent, the two oxidizer propellant gages were indicating
off-scale (greater than the maximum 95-percent indication), as expected.
The fuel probes on the other hand were indicating approximately 94.5
percent instead of reading off-scale. The propellant loaded was
equivalent to approximately 97.3 and 96.4 percent for oxidizer and fuel,
respectively. An initial low fuel reading also had occurred on Apollo
i0. As the firing continued, the propellant gages began to indicate
consumption correctly. The tank I and tank 2 fuel probe measurements
agreed throughout the firing. The tank i and tank 2 oxidizer probe
measurements agreed initially, but they began to diverge until the
difference was approximately 3 percent midway through the firing. For
the remainder of the firing, the difference remained constant. The
divergence was probably caused by oxidizer flowing from tank 2 to tank i
through the propellant crossover line _- as a result of an offset in
vehicle center of gravity.

The low level light came on at 102:44:30.4, indicating approximately 116
seconds of total firing time remaining, based on the sensor location.
The propellant remaining timeline from the low level light indication to
calculated propellant depletion is as follows.

(figure not attached, see report)

The indicated 45 seconds to propellant depletion compares favorably with
the postflight calculated value of 50 seconds to oxidizer tank 2
depletion. The 5-second difference is within the measurement accuracy of
the system. The low level signal was triggered by the point sensor in
either the oxidizer tank 2 or fuel tank 2.