Science: Fresh Air in the Depths

When the submarine Nautilus in Jules Verne’s Twenty Thousand Leagues Under the Sea needed a new air supply, it came to the surface, blew noisily like a winded whale and filled its reservoirs with two days’ supply of compressed air. This simple system was good enough for the imaginary Nautilus of 1870, but the real SS(N) Nautilus and her nuclear sisters of the modern U.S. Navy need better air, and are designed to stay submerged for months on end. In Naval Research Reviews the Naval Research Laboratory tells how their little worlds are kept almost as fresh as a sloop tacking into the wind.

Cruising endlessly under water, the Navy’s subs have a private atmosphere all their own in which a single supply of air is breathed again and again. Whenever the oxygen level gets low, huge high-pressure cylinders of oxygen refresh the air, and there is also an electrolytic cell that turns sea water into oxygen and hydrogen, shooting the latter out of the submarine. For emergencies, the Naval Research Laboratory has provided ingenious “candles” made of sodium chlorate and powdered iron. When they are ignited, they emit oxygen, not the carbon dioxide that is given off by ordinary candles.

Gas & Dust. Oxygen alone is not enough. As the crew breathes, it contaminates the air with exhaled carbon dioxide. In older subs the way to get rid of it was to absorb most of it in a caustic such as lithium hydroxide. The nuclear subs must have a far more elaborate system: secondhand air is passed through a liquid containing monoethanolamine, which absorbs carbon dioxide at room temperature, is then heated, and releases the gas so that it can be piped out of the sub.

A more dangerous gas is odorless carbon monoxide, which is produced by tobacco smoking and could kill off the entire crew unless it is removed. Then there is hydrogen, which emanates from batteries, can form an explosive mixture if as little as 4% accumulates anywhere. The smelly organic vapors from garbage and human sources must also be removed. Most of this unwanted stuff is eliminated by a hot catalyst that oxidizes it to CO2 and water. All traces of organic matter that escape the catalyst are mopped up by a bed of activated carbon, and finally an electrostatic precipitator removes the last aerosols (dust or smoke particles) from the sub’s fresh, clean world. In case of an atmosphere-fouling emergency, the crew can plug gas masks into a piped air supply.

Even better equipment is in the labs. The Navy is working on a magical electrolytic cell containing a sodium sulphate solution. When an electric current passes through it, oxygen bubbles off from one electrode. An acid is formed in the solution at the same electrode, and a caustic accumulates at the other electrode. The caustic can be withdrawn and used to absorb carbon dioxide from the sub’s atmosphere. When it is then remixed with the acid from the other electrode, the carbon dioxide separates and can be pumped out of the submarine. What remains is the same sodium sulphate with which the cycle started. When this system is at work cleaning and oxygenating the air, a nuclear submarine will be able to stay under water as long as its crew can stand being cooped up together.

On to Space. What works in the ocean depths may also work far out in the airless reaches of space. The apparatus to provide breathable air for long journeys aloft will have to use the same oxygen many times. Lacking space for large oxygen tanks, the scientists will have to find a way to extract oxygen from carbon dioxide formed by the crewmen’s breathing. Algae can do the trick when exposed to light, and while giving off oxygen they grow, providing an unpalatable but fairly nutritious food. But algae are hard to keep growing without a plentiful supply of water, so the Navy is looking for something better. By working in present-day submarines, it hopes to find the answers for future spacecraft.