The U.S. Navy is getting a little nervous about the growing popularity of Air Independent Power plants (AIP) for non-nuclear subs. Russia and Germany began working on AIPs in the 1930s. For the next thirty years, there were many attempts to build diesel engines that were modified to operate underwater as a closed system. Germany actually built a class of submarines at the end of World War II with an AIP system. But the AIPs then were too big, too unreliable and too dangerous for regular use. The Russians continued working on AIP into the 1950s, but stopped work when they realized that nuclear powered subs were a more practical way to go. But work did not completely stop on AIP systems. By the 1980s, Sweden and Russia both developed more reliable systems. Russia ran out of money to build subs with its system, but Sweden did produce subs with a practical AIP power plant. The Swedish system enabled a sub to stay under water for up to 20 days at a time. Only one problem remained; cost. The Swedish AIP added about $100 million to the cost of each sub using it. But work continues. In the late 1990s, Germany developed a cheaper system based on new fuel cell technology. This system did not operate a closed system engine, but used an electrochemical process to generate power for the subs electrical motors. This didn't produce as much energy as the older Swedish design, but was simpler and cheaper. It now appears that in the next few years there will be AIP systems that will have the best of the German and Swedish systems and will only add about $50 million to the cost of a non-nuclear sub (which currently cost $200-400 million each). The AIP subs are quieter and smaller than nuclear subs (which need to run noisy water pumps all the time to keep the reactor cool). The latest classes of non-nuclear subs are being equipped with excellent sensors and weapons (missiles and torpedoes), making them as lethal as nuclear boats. But in battles between subs, the quieter boat tends to get the first shot in, and that's usually the end of the battle.