STIRRING THE TANKS: GENESIS OF A FAILURE

One of the regular tasks for the CMP was the perfectly routine stirring of the service module’s tanks that contained the cryogenic oxygen and hydrogen reactants for the fuel cells. Each tank was essentially an efficient vacuum flask whose contents were best described as being a very dense fog rather than a liquid. As the gas was drawn off for the fuel cells or for the cabin air, the pressure in the tanks reduced slightly. If gas pressure falls but the volume stays the same, then according to the gas law that shows how pressure, volume and temperature are related, the temperature will also fall. Therefore, electrical heaters, w’hich could be switched on automatically or manually as required, were installed to help to maintain the tanks at their operating pressure.

Two long devices ran through the middle of each tank. One was a set of heating elements wrapped around a supporting tube. Two fans w’ere mounted, one at either end of the tube, to stir the tank’s contents. The other was a probe that determined the quantity of gas remaining in the tank. It consisted of a tube within a Lube and it measured the electrical characteristics across the gap between – a quantity known as capacitance. The capacitance of the probe depended on the density of the gas between the tubes, and this could be calibrated to infer how’ much gas was present in the tank. However, in the weightless environment of space, the gas tended to gather in layers of differing densities against the probe, which skewed the readings. This was where the fans came in. At regular intervals, they were switched on to stir the contents of the tanks in order to homogenise its density and allow’ an accurate reading. When EECOM Sy Liebergot asked Capcom Jack Lousma to ask CMP Jack Swigert on Apollo 13 to stir the tanks in Odyssey s service module almost 56 hours into the mission, the result became part of popular culture.