#propellant__propellent  something that propels
  supertype:  #substance__matter  that which has mass and occupies space; "an atom is the smallest indivisible unit of matter"
  subtype:  #rocket_fuel__rocket_propellant__rocket_propellent  an explosive charge that propels a rocket
  subtype:  #compressed_gas  gas at a high pressure that can be used as a propellant
     subtype:  #compressed_air  air at a pressure greater than that of the atmosphere; "compressed air is often used to power machines"
        subtype:  #air_cushion  the trapped air that supports a hovercraft a short distance above the water or ground

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