Stirring the tanks genesis of a failure

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 the pressure falls and the volume stays the same, then according to the gas law that shows how pressure, volume and temperature are related, the temperature must also fall. Therefore, electrical heaters, which could be switched on automatically or manually as required, were installed to help the tanks to maintain their operating pressure.

Two long devices ran the length of each tank. One was a spiral fan. The other was a probe that determined the quantity of gas remaining in the tank. It consisted of a tube within a tube and measured the electrical characteristics across the gap between them - 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 yield how much gas there was in the tank. However, in the zero-g environment of space, the gas tended to gather in layers of differing densities against the probe, which skewed the readings. This was where the fan came in. At regular intervals, it was switched on to stir the tank's contents, 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 as they neared the Moon, the result became part of popular culture.

''13, we've got one more item for you, when you get a chance,'' said Lousma. Liebergot had been getting poor data from the quantity sensors and had been calling for more frequent stirs. ''We'd like you to stir up your cryo tanks.'' ''Okay. Stand by,'' replied Swigert.

A minute or so passed as Swigert began stirring all four tanks sequentially. Suddenly, the data stream to Earth began dropping out, interrupting the flow of information about the spacecraft to the controllers' displays. Something had disturbed the spacecraft's attitude and caused the dish antenna to lose lock. Then a call came from Swigert. ''I believe we've had a problem here.''

''This is Houston,'' said Lousma, his voice suddenly taking a more authoritative tone. ''Say again, please?''

Lovell immediately took over. ''Houston, we've had a problem.'' He then launched into a technical discussion of what was happening on the spacecraft. ''We've had a main bus B undervolt.'' The CSM was losing power.

So began a 4-day drama that gripped the world and seriously threatened the lives of the crew. The story was traced back 18 months, to when an oxygen tank originally intended for Apollo 10 was dropped a small distance. The tank seemed to be undamaged but a tube that allowed it to be filled and emptied may have worked loose. It was then installed as the number two oxygen tank in Apollo 13's service module. Three weeks before launch, the tank was filled as part of a routine test, but technicians found that it was slow to empty. Their solution was to switch the tank's heaters on and boil the gas out. The second major thread in the story then kicked in.

The heater circuits included thermostatic switches that should have stopped the tank from overheating. When originally designed in the early 1960s, NASA's engineers had specified that spacecraft systems should run on 28 volts, but they later instructed their contractors to rate everything for 65 volts instead, as this was to be used at the launch site. Unfortunately, the message was not passed to the sub-subcontractor who supplied the switches. When the tank became too warm during the attempt to empty it, the thermostat tried to open the circuit, became welded shut by an arc of electricity that it could not handle, and continued to feed power to the heaters until the temperature within the tank exceeded 500°C. As a result, the insulation on the wiring was baked and became brittle.

At 328,300 kilometres from Earth, as Apollo 13 coasted towards the Moon, the agitation caused by tank 2 being stirred brought exposed wires into contact, and the short circuit ignited their insulation. A vigorous fire ensued within the tank, fed by the extremely dense oxygen and the combustible materials that constituted the tank's innards. The pressure rose rapidly until the tank wall ruptured with such a force that the entire panel from that side of the service module was blown off. The consequential disruption to the plumbing allowed the oxygen in the undamaged tank 1 to leak out into space as well, thereby depriving the command module of its source of power and air, and therefore its propulsion.

It might have ended there had the blast occurred on Apollo 8 - four days away from home, heading away from Earth with the crew slowly dying of asphyxiation in a dead ship - except for Apollo 13's lunar module Aquarius. Luckily, it was still attached with its supplies unused. NASA had even studied the possibility that one day, the LM might be used as a lifeboat. Although it was far from ideal and could not re-enter Earth's atmosphere, it had plentiful oxygen, a working RCS and reasonably powerful engines, and it enabled the remaining consumables in the command module to be preserved so that, once the spacecraft returned to Earth, the command module would take them to the surface.

More than at any other time, the toughness and competence of mission control and the huge array of supporting staff behind them came to the fore to overcome the almost intractable problems that Lovell, Swigert and Haise had to deal with. The range of hazards they faced cannot be understated, and each was handled with a creativity and tenacity beyond expectations. The LM seemed to lack sufficient battery power for the return. Its RCS thrusters were never intended to steer a ship that had a 30-tonne dead weight hanging off the end of it. There were problems of guidance, of communication and tracking, of excess carbon dioxide, of sleep deprivation, of cold and discomfort. In addition, in the command module there was the problem of condensation over a mass of electronics that had to work on re-entry.

Thanks to a successful Hollywood movie in the 1990s, the story of Apollo 13 and its successful return has become a by-word for the never-say-die, failure-is-not-an-option doggedness that turned the flight into the successful failure of the Apollo programme.

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