way the temperature of the air which has been compressed by the air compressor 21 and heated up is preliminarily lowered by the heat exchanger 26 in this second preferred embodiment due to its use in the heat exchanger 26 the coolant consisting of liquefied air fed from the boost pump 24 can be used to absorb the latent heat of liquefication of the air within the mixer 25 39 therefore the amount of liquefaction can be further increased a third preferred embodiment of the present invention is shown in a schematic diagram in fig3 this preferred embodiment is similar to the embodiment in fig2 except that its air precooler 7 has associated therewith a defrosting means 30 and except that its air condensor 5 has associated therewith a sprayer 13 for spraying liquid oxygen thereinto the defrosting means can be a defroster which mechanically vibrates the air precooler e g a hammer or a vibrator or a defroster which blows high pressure fluid the inclusion of the defrosting means 30 can prevent moisture and or carbon dioxide contained in the air from adhering to heat exchange surfaces thereby increasing the heat transfer efficiency in addition the liquid oxygen sprayer 13 sprays liquid oxygen into the air condensor 5 after it has been fed from a liquid oxygen tank 9 fed through a pipe 10 and pressurized by a pump 11 in this manner the liquid oxygen acts as a refrigerant for liquefying air and is subsequently burnt in the combustor 19 as an oxidizing agent for oxidizing the liquid hydrogen so as to increase the propelling force of.