The advantages of an SSN over a conventionally powered SSK are much longer endurance (limited more by the crew than the boat, a nuclear submarine can stay submerged for months and does not need refueling in their 25-year lifespans), and higher speed. Unlike most SSKs, SSNs do not have to surface periodically for air, which would compromise their stealth. Some of the newest conventional submarines approach these advantages: Stirling engine powered vessels can cruise underwater for up to two weeks and, like diesel/electric vessels (and in theory LOX powered vessels), are significantly quieter than nuclear submarines, since they do not need to run the powerful (and noisy) pumps associated with the cooling circuits of pressurized water reactors.