 Although they are preferred in space launchers, the difficulties of storing a cryogen like liquid oxygen in a missile that had to be kept launch ready for months or years at a time led to a switch to hypergolic propellants in the U.S. Titan II and in most Soviet ICBMs such as the R-36.But the difficulties of such corrosive and toxic materials, including leaks and explosions in Titan-II silos, led to their near universal replacement with solid-fuel boosters, first in Western submarine-launched ballistic missiles and then in land-based U.S. and Soviet ICBMs.The Apollo Lunar Module, used in the Moon landings, employed hypergolic fuels in both the descent and ascent rocket engines.