The Centaur upper stage is powered by one or two RL10 engines, which are manufactured by Aerojet Rocketdyne, and burn liquid hydrogen and liquid oxygen. The Centaur is the first rocket stage to use these fuels. On the Atlas V ULA typically only uses the once RL10 version of the Centaur upper stage. However, Aerojet Rocketdyne supplies two RL10A-4-2 engines for each Starliner mission flown by the Atlas V. The dual-engine Centaur configuration is used on human-rated launches for safety reasons. This configuration allows the rocket to fly on a shallower path to orbit, meaning horizontal velocity is emphasized over vertical velocity. This in turn reduces the maximum G-forces endured by the crew and allows for a safe abort at any time during the launch. The standard payload fairings are 4 or 5 meters (13 or 16 ft) in diameter with varying lengths. The dual-engine variant of the Centaur has flown more than 100 times on Atlas variant rockets. 166 of the Centaur's flights have used dual-engine variants. As of late 2019, Centaurs of all kinds had flown 251 times.