Icicles

This RL10 engine is fueled by a mixture uber-cold liquid oxygen and hydrogen. The rocket nozzle contains hundreds of small passages, much like a radiator. The cryogenic liquids used to power this engine are pumped through these passages before being injected into the combustion chamber, simultaneously warming the liquids and cooling the nozzle. As CECE (Common Extensible Cryogenic Engine) burns its frigid fuels, gas composed of hot steam is produced and propelled out the nozzle creating thrust. The steam is cooled by the cold engine nozzle, condensing and eventually freezing at the nozzle exit to form icicles.

Liquid Oxygen:     -182.8° Celsius = 90.35 Kelvin = -297.04° Fahrenheit
Liquid Hyrdrogen:  -252.8° Celsius = 20.35 Kelvin = -423.04° Fahrenheit

Picture and some of the text from Pratt & Whitney.