 Because hypergolic rockets do not need an ignition system, they can fire any number of times by simply opening and closing the propellant valves until the propellants are exhausted and are therefore uniquely suited for spacecraft maneuvering and well suited, though not uniquely so, as upper stages of such space launchers as the Delta II and Ariane 5, which must perform more than one burn.Restartable non-hypergolic rocket engines nevertheless exist, notably the cryogenic (oxygen/hydrogen) RL-10 on the Centaur and the J-2 on the Saturn V. The RP-1/LOX Merlin on the Falcon 9 can also be restarted.Relative to their mass, traditional hypergolic propellants are less energetic than such cryogenic propellant combinations as liquid hydrogen / liquid oxygen or liquid methane / liquid oxygen.