Stirring the tanks: genesis of a failure 209

“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 to stir all four tanks sequentially. Suddenly, the data stream to Earth began to drop out, interrupting the flow of information about the spacecraft to the controllers’ displays. Something had disturbed the spacecraft’s attitude and caused its 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 board the space­craft. “We’ve had a main bus В 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 several centimetres. Although the tank appeared to be unda­maged, a tube to allow 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 during a routine test, and technicians found that it was slow to empty afterwards. 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 designed to prevent 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 all electrical items for 65 volts instead, as this voltage 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 wann 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. Since power was necessary for the operation of the SPS engine, the catastrophe also deprived the CSM of its propulsion.

ft 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 and its engines fresh. NASA had even studied the possibility that one day the LM might be used as a lifeboat, and had tested a burn of a LM main engine while docked to a CSM during Apollo 9. Although it was far from
ideal and could not re-enter Earth’s atmosphere, the lunar module had plentiful oxygen, a working RCS and two reasonably powerful engines. The CM was the only part of the spacecraft that could bring the crew safely through the atmosphere. If they could use the LM to bring them to Earth, the CM’s remaining consumables would be preserved so that it could take them to the ocean.

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 liaise had to deal with. The range and depth of hazards they faced cannot be overstated, 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-Lonnc dead weight hanging off the end of it. There w-crc problems of guidance, of communication and tracking, of excess carbon dioxide, oflack of food, 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 IS and its successful return to Earth has become a by-word for the never-say-die. failure-is – not-an-option doggedness that turned this flight into the successful failure of the Apollo programme.