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saturn V

God has truly blessed my life! He has given me two occupations that I really
enjoyed. For the first 31 years of my work life I was an electrical design engineer in the
aerospace industry. The last 28 year I have been on staff at my church ministering to
seniors (old people like me!). During my aerospace days the most important project I
worked on was the Saturn 5 moon rocket that put men on the moon. The following is a
little write up of my part of the project.

In 1963, while working for Douglas Aircraft I was assigned to the design team
for the third stage of the Saturn 5 rocket One of the first design problems that we tackled
was how to chill down the rocket engine and its associated fuel lines to prevent gas
bubbles from forming during ignition. If this happened the main engine pump would
cavitate and keep the engine from starting. Fuel for this stage was liquid oxygen and
hydrogen. To accomplish this chilldown, two pumps were used to circulate the cryogenic
fuels through the engine prior to ignition. One horsepower motors were use to drive the
pumps. The motor used to drive the hydrogen pump was placed inside the hydrogen tank
in direct contact with the liquid hydrogen (Something that had never been done before).
The motor for the liquid oxygen pump was placed in a sealed container within the LOX
(Liquid Oxygen) tank.
It is obvious from the environments the motors experienced that a brush type DC
motor was not a viable candidate. A three phase AC motor was the only possible option
but the only power on the stage that could be used was 56 volts DC. My task was to take
the 56 volts DC and change it into 400 cycle three phase AC in order to drive the motors,
no small task in 1963. A 1.5 KVA inverter was designed with a modified square wave
output to drive a three phase wye connected induction motor. To meet the weight (The
flight unit weighted 12 pounds) and efficiency criteria, 50 amp. germanium transistors
were used.
Today the brushless motors used on our model airplanes are really three phase AC
motors. The only difference being that we can now control the speed of the motor,
something we did not need to do back then.
The design task was formidable. Special testing had to be applied to the 50 amp
germanium transistors in order to screen out the ones that could survive the harsh
environment. Only about 50% of the units tested survived. After the units were placed an
inverter, the entire assembly was tested with a simulated motor load at both high and low
temperatures. Strangely enough it was low temperature that gave the most problems. The
gains of the germanium transistors increased at low temperatures causing them to fail by
exceed their maximum current rating during motor startup. This resulted in a redesign,
late in the program, where current limiting was added. Three of us worked in shifts, seven
days a week, 24 hours a day, for three months to accomplish this redesign.
The operational testing of the pumps also had problems. The first time the pumps
were used to chill the engine down on the test stand the cryogenic fuels hitting the
relatively hot engine caused so much back pressure that the pumps were turned into
turbines and driven backwards at 11,000 RPM causing the inverters to fail. Adding check
valves in the lines between the pumps and the rocket engine solved that problem.

saturn V

The five years that I spent on the Saturn 5 program were some of the most
challenging of my life. I used to have nightmares before Saturn 5 launches because I
knew that if the inverters did not work properly the missions would not succeed. The
third stage engine had to fire twice during a lunar mission. Once during liftoff and then
again to move the Apollo capsule and its service module from earth orbit into a trajectory
to the moon. Fortunately however the inverters worked flawlessly during all the Saturn 5
flights. Looking back I am still amazed at the success of the Saturn 5 rocket. It is still the
most complicated piece of machinery ever built and it was designed by a bunch of us
young kids who didn’t know that it couldn’t be done.

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