|An LV-909 burning oxidizer and liquid fuel|
Oxidizer is a resource that fuels liquid fuelled rocket engines. An liquid fuel engine burns 11 parts oxidizer for every 9 parts liquid fuel. It was newly added with 0.18 and is prohibiting rocket engine to be fuelled by jet fuselages alone. It is stored in liquid fuel tanks together with the usually correct amount of liquid fuel. Planes using jet engines don't need oxidizer, because the intake air is the substitution for the oxidizer, but air intakes work on Kerbin and Laythe.