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Living inside a Spacesuit

Pressure in a Spacesuit

Even if an experienced astronaut is able to don a spacesuit quickly, preparations for spacewalks begin many hours in advance of the activity. The atmospheric pressure that is maintained artificially inside the cabin of an orbiter or of the International Space Station is the same as on Earth (101 kPa). The composition of the air we breathe is also about the same as it is at ground level (78% nitrogen and 20% oxygen). However, this is not exactly the case for the atmosphere created inside the suit. For technical reasons, the pressure is only 29.6 kPa, which means that the atmosphere must be composed of 100% pure oxygen. If an astronaut went from the atmosphere in the space station cabin to that in the spacesuit without an adjustment period, he would suffer from a potentially dangerous illness: decompression sickness, also called caisson disease.

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The forming of nitrogen bubbles in the body fluids and organic tissues causes decompression sickness. These bubbles (nitrogen saturation) result from the expansion of nitrogen that occurs when the body is is subject to an accelerated decompression (consistant with Boyle’s Law). The symptoms of this illness are usually cramps, choking and paralysis. When it occurs, decompression sickness must be treated quickly by having the individual undergo a gradual repressurization.

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To prevent decompression sickness, astronauts planning extravehicular activities_space outings, or EVA_undergo a gradual decompression. In some cases, 24 hours prior to the activity, the pressure in the shuttle’s cabin is lowered to approximately 70 kPa, and the percentage of oxygen in the air is slightly increased.

Not long before suiting up, astronauts put on an oxygen mask in order to breathe pure oxygen for 30 minutes. At this stage, most of the nitrogen in their bodies has been eliminated and they can begin putting on a maximum absorption garment and then move into the airlock. The airlock is a small chamber separating the shuttle or the space station from the outside and which, when both doors are closed airtight, can be pressurized and depressurized as required. The astronauts finish putting on their spacesuits and accessories in the airlock.

Once the suit assembly is complete and the helmet is well sealed, a series of tests used to detect air leaks are conducted while the atmosphere that surrounds the astronauts inside their suits is gradually replaced by pure oxygen, which, once again, astronauts must breathe for 30 to 40 minutes. Finally, the internal pressure of the suit is lowered to 29.6 kPa while the inside door of the airlock is sealed and the airlock is completely depressurized. The outside door can now be opened to enable the astronauts to step out into space.

Expansion Equilibrium

In space, where there is no atmospheric pressure, the suits become somewhat of a big inflated balloon inside which the astronauts are found. The suits are deliberately made so as to be relatively flexible. Stiff suits were rejected by engineers because they restrict the astronauts’ movements too much. Due to their flexibility, modern suits must be made so as to limit_or restrict_their expansion (caused by the difference in pressure inside and outside the suit).

Different solutions were studied. Engineers considered surrounding the flexible parts with nets, but this option would restrict movements too much. They also though about developing a girdle containing many small, partially inflated balloons. Their pressure would be opposed to that of the atmosphere in the suit and would still provide certain flexibility, since air can move freely inside each balloon.

At present, the pressure bladder, made of urethane-covered nylon, resides in one of the intermediate layers of the suit.

Flexibility

The pressure inside the suit is far below the pressure on Earth. The water seal and overall integrity of the suit would be difficult to guarantee if it were any higher. The flexibility of the suit required for astronauts to move freely is another practical reason to justify the relatively low pressure inside the spacesuit.

If the internal pressure were too high, the flexible sections of the suit would become difficult to bend. Astronauts could even have trouble moving their fingers. Any type of movement would be exhausting. Test this out using a thin inflated balloon. The more it is inflated, the stiffer it is and the harder it becomes to fold in two.

Articulation

Regardless of the relatively low pressure inside the suit movement still requires a level of effort. If astronauts were not conditioned to work effectively without getting exhausted easily, the extravehicular activities would not be very successful.

Additional effort is required to move the elbows, wrists, fingers, knees, ankles and waist. The suit must allow these parts of the body to move and flex as naturally as possible. Engineers have developed a series of solutions in response to this challenge. Breaking points can be placed strategically in the outer layers of the suit in order to create joints that bend much more easily. We can also sow series of “folds” in the pressure bladder, which can expand or contract easily, like the ribs on a vacuum cleaner hose.

In addition to making the astronauts’ job easier, these solutions remedy another problem. Without joints at the articulations, the internal volume of the suit would decrease when flexing a body part. This would increase the internal pressure, which is the exact opposite of the desired effect. If the internal pressure increases, the suit becomes increasingly stiff, making it harder to move.

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