In the novels of Arthur C. Clarke, a spacecraft reaction drive system invented by Russian physicist Andrei Sakharov, in use around 2010. The Sakharov Drive employed a pulsed thermonuclear reaction to heat and expel propellant material. A variety of propellants could be used; the most efficient was hydrogen, but methane, ammonia, and even water could be used as well.

The Russian spacecraft Leonov, the second manned spacecraft to visit Jupiter, was equipped with a Sakharov Drive unit, only the fourth to be built. As its propellant, Leonov used hydrogen in four large, discardable external tanks to launch from Earth orbit, and ammonia for its later braking and Earth-return maneuvers.

(The Sakharov Drive may or may not be related to the later muon-catalyzed fusion power plants, which were perfected about 2040.)

Source: Arthur C. Clarke, 2010: odyssey two. (Final parenthetical note based on an addendum to 2061: odyssey three.)