The (GRAU index) 8K82K version is now usually called "Proton K". It is fuelled by very toxic unsymmetrical dimethyl hydrazine and nitrogen tetroxide. These are hypergolic fuels which ignite on contact, avoiding the need for an ignition system, and can be stored at ambient temperatures. This avoids the need for components that are tolerant of low temperatures, and allows the rocket to remain on the pad indefinitely (other launchers with such capability include the U.S. Titan II GLV, Titan III, and Titan IV, the Chinese Long March 2 and Long March 4, the Soviet/Ukrainian Tsyklon launchers, the Soviet/Russian Kosmos-3 and Kosmos-3M launchers and the European Ariane 1 to Ariane 4 launchers). In contrast, cryogenic fuels need periodic replenishment as they boil off.