Nuclear submarines are divided into generations. The first-generation submarines, built in the 1950s, are characterized by relatively high noise and imperfect hydroacoustic systems. The second generation was built in the 60's - 70's. The shape of the hull was optimized to increase speed. The third generation appeared in the early eighties and is characterized primarily by a significantly increased displacement, increased autonomy, and improved crew life, as well as the unification of submarines and their classes. American boats like "Ohio" and "Los Angeles" received reactors that work without recharging for up to 11 years and do not require major repairs during the entire life cycle - up to 30 years.