Apollo Expeditions to the Moon
REPROGRAMMING IN FLIGHT
The next Apollo 14 problem occurred just prior to the final descent for landing
at Fra Mauro. An abort command was received by the lunar module's guidance
computer. Had the abort command been initiated, it would have separated the ascent
stage from the descent stage and terminated the landing. The descent had to be delayed;
and, as Al Shepard and Ed Mitchell orbited the Moon, the ground valiantly tried to
determine the cause of the problem. It was isolated to one set of contacts of the abort
switch on the instrument panel. Recycling the switch or tapping on the instrument
panel removed the signal from the computer. A computer program was developed
and verified within two hours by the Operations Team and inserted manually into the
computer, allowing the computer to disregard the abort command. The unexpected
came again within minutes. As the crew started the descent to the Moon, the altitude
and velocity lights of the computer display indicated that the landing radar data were
not valid. This information provided essential updates to the computer. Flight Controller
Dick Thorson made a call to recycle the landing radar circuit breaker. The
crew complied. The lights were extinguished and the necessary computer entry update
was made at an altitude of about 21,000 feet. Apollo 14 and Al Shepard's and Ed
Mitchell's climb almost to the top of Cone Crater are now history.
There were occasions when the problems that came up did not require an
instant decision but rather resulted in long hours in conference in Mission Control.
For example, on Apollo 15, the flight of Endeavour and Falcon, as the spacecraft
traveled from the Earth to the Moon, the service propulsion system developed a
problem. This is the system that is required to place the spacecraft in orbit around
the Moon and on its trajectory back to Earth. Needless to say, this was a critical system.
A light had illuminated showing that the engine was firing while it obviously was
off. This had to be caused by a short in the ignition circuitry. Had this circuit been
armed while the short was present, the service propulsion engine would have fired. The
Operations Team, working with Don Arabian, a legend in his own time, and Gary
Johnson, an excellent young electrical engineer, isolated the short to one of two
systems. A test firing was initiated by the crew to verify that the short existed on the
ground side of one of two sets of valves. Procedures were then developed by the ground,
working with the flight crew, and the mission continued.