ecause "Zenit" is one of the most modern of the Soviet launch systems, its design shows many improvements compared with older Soviet vehicles. The "Zenit" uses only two stages, each fueled with kerosene and oxygen propellants that are safer and more environmentally friendly than the storable propellants used on launch systems such as "Proton" and "Tsiklon". The propulsion system is much more advanced than the previous LOX/kerosene engines, such as those used on the "Soyuz", for example. The RD-171 engine uses a high-pressure staged combustion cycle that results in 25% higher sea level specific impulse than the older RD-107 used on "Soyuz". "Zenit" also demonstrates improved technologies in its highly automated launch infrastructure. "Zenit" can launch within 90 min of reaching the launch pad.