The early Apollo service modules carried two oxygen tanks that supplied feedstock for the fuel cells for the generation of electricity, and air for the crew to breathe. After the Apollo 13 incident, when the contents of both tanks were lost after an overpressured tank burst, a third was added. This was isolated from the other two, both physically and by the routeing of its plumbing. Many assume that this tank was added in view of Apollo 13's near catastrophe, but it had actually been planned earlier as part of the upgrades to the spacecraft to support the extended operations of the J-missions.

The decision on the type of air to use in an Apollo cabin was not arrived at easily, and was tied up with the tragedy of the Apollo 1 fire. The difficulty was not in choosing the air supply for space. The problem arose because the air supply on the ground, prior to flight, proved to be a lethal mix of high-pressure oxygen and excessively flammable materials spread throughout the cabin, including Velcro and nylon netting.

The reasoning behind the cabin atmosphere to use in space was simple enough. On Earth, we experience air pressure at about 1,000 millibars. Since about 20 per cent of that air is oxygen, we say that the partial pressure of oxygen is about 200 millibars. To simplify the design of the Apollo spacecraft and to save weight, NASA decided to use a single gas, oxygen, for all stages of the flight. By having pure oxygen, there was no need to engineer the spacecraft's hull to hold sea-level pressure against the vacuum of space, or to carry apparatus to store nitrogen and to regulate the gas mixture. Instead, the spacecraft designers set the cabin pressure so that the concentration of oxygen molecules presented within the lung, where gases are exchanged to and from the blood, was similar to what would be found on Earth. This was achieved by regulating the oxygen atmosphere within the cabin at around 350 millibars. By adopting this lower pressure, the hull could be lighter, since it only had to hold two-fifths of sea-level pressure at most.

The problem with this arrangement arose on the ground. The early version of the Apollo spacecraft, Block I, had no facilities whatsoever for a two-gas atmosphere, even at the launch pad. Once the crew were sealed in, the system supplying them with oxygen had no option but to maintain it at the full sea-level pressure of 1,000 millibars because the hull was not designed to withstand high pressure from the

The Apollo 1 crew; Roger Chaffee, Ed White and Gus Grissom; with Robert Gilruth, the director of the space centre at Houston.

outside. Worse, when the spacecraft was being tested for leaks, the internal pressure was pumped even higher, despite being pure oxygen. Right through the Mercury and Gemini programmes that preceded Apollo, spacecraft tests on the ground were carried out with the cabin pressurised at about 10 per cent above the ambient pressure. But on 27 January 1967, the complexity of the Apollo spacecraft and the rush to launch it caught up with this flawed policy. Three weeks before the planned launch of Apollo 1, during a practise countdown on top of an unfuelled Saturn IB, an unknown ignition source set the interior of spacecraft 012 alight. Fed by high-pressure oxygen, the cabin burned intensely with the resultant deaths of the three crewmen on board: Virgil I. Grissom, Edward H. White II and Roger B. Chaffee.

In the light of this tragedy, the Block II spacecraft was redesigned to have a two-gas atmosphere while on the ground, mixing oxygen and nitrogen at a 60/40 ratio with a pressure of 1,000 millibars. Although this ratio was relatively rich in oxygen when compared to normal air, it suppressed flammability while minimising the time required to flush nitrogen out of the cabin after launch. During ascent, the cabin pressure was maintained at sea-level pressure until the outside pressure had dropped by 400 millibars, at which point the pressure relief valve began bleeding the nitrogen/ oxygen air out of the spacecraft to maintain a 400-millibar difference across the hull. During this time, the crew were sealed in their space suits breathing only oxygen from the suit circuit. The pressure in their suits was kept slightly high so that the excess gas would help to flush the nitrogen out of the cabin air. Like passengers in an aeroplane, they could feel the drop in pressure make their ears pop.

Because the total reduction in pressure during the ascent was quite large and occurred over a relatively short space of time, the crew had to condition their blood beforehand. A diver who rises to the surface too quickly can consequently suffer from the bends - a debilitating and painful condition, so-named because it makes the victim curl up tightly. Similarly, an Apollo crewman who took no precautions would also get the bends as the nitrogen gas that was dissolved in his bloodstream would come out of solution in the form of bubbles as the pressure dropped, just like the bubbles a fizzy drink bottle produces when opened. To prevent this occurring, the crew breathed pure oxygen from the time they suited up three or more hours prior to launch in order to flush dissolved nitrogen out of their blood.

By the time they reached orbit, the cabin pressure had settled at around 350 millibars and most of the nitrogen was gone. The crew could break open their suits by removing their helmets and gloves and begin preparing their ship for the Moon. Later, they removed their suits completely and worked in a shirtsleeves environment until a situation arose that required the suits to be donned again.

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