RS-25 engines powering NASA's SLS (Space Launch System) rocket use liquid oxygen and liquid hydrogen as propellants – the same elements that combine to create water (H2O). For an RS-25 engine test, liquid hydrogen and liquid oxygen are mixed in the engine combustion chamber and ignited, which results in extremely high-temperature exhaust (up to 6,000 degrees F) exiting the nozzle into the flame deflector at a very high rate and pressure. Hundreds of thousands of gallons of water also are flowing