01	Air Breathing Engines	Unconventional engines which use ingested air to oxidize their fuel, for example, the (Unconventional) liquid air cycle engine (LACE). For conventional air breathing engines, See 21/05, Jet and Gas Turbine Engines, and 21/07, Reciprocating and Rotating Engines.
02	Combustion and Ignition	Combustion and flame studies of fuels; Ignition and ignition systems. For combustion studies of nonfuels, See 07/04, Physical Chemistry. For combustion devices, see the group where the application is treated.
03	Electric and Ion Propulsion	All types of engines deriving power from free ions or electrons; Ion, plasma and arc-jet engines; Photopropulsion. For vehicles propelled by electric motors, see the group where the vehicle is treated.
04	Fuels	Production, performance and storage of all types of fuels except those used in rocket engines; Fuel thickeners and their additives. Includes fuel tanks and fuel storage tanks. For rocket fuels, See 21/09, Rocket Propellants; 21/09/01, Liquid Rocket Propellants; and 21/09/02, Solid Rocket Propellants.
05	Jet and Gas Turbine Engines	Design, performance and testing of all types of jet and gas turbine engines and their components, such as engine nozzles. Includes hydroduct, ramjet and turboprop engines.
06	Nuclear Propulsion	Nuclear systems for marine, ground, air and space propulsion.
07	Reciprocating and Rotating Engines	Design, performance and testing of reciprocating and rotating engines of various configurations for all types of propulsion. Includes internal and external combustion engines. For turbine engines, See 21/05, Jet and Gas Turbine Engines.
08	Rocket Engines	Design, performance and testing of rocket engines and propulsion hardware; Gaseous, thixotropic and hybrid rocket motors. See also 21/08/01, Liquid Propellant Rocket Engines, and 21/08/02, Solid Propellant Rocket Engines.
08/01	Liquid Propellant Rocket Engines	Studies of liquid propellant rocket engines and propulsion hardware.
08/02	Solid Propellant Rocket Engines	Studies of solid propellant rocket engines and propulsion hardware.
09	Rocket Propellants	Production, handling, stability and performance of chemical rocket propellants and propellant combinations other than all liquid or all solid propellants, including fuels, oxidizers, additives and binders. Includes propellant tanks.
09/01	Liquid Rocket Propellants	Production, handling, stability and performance of all liquid rocket propellants, including fuels, oxidizers and additives. Includes propellant tanks.
09/02	Solid Rocket Propellants	Production, handling, stability and performance of all solid rocket propellants, including fuels, oxidizers, additives and binders. Includes propellant cases.