A modular expandable cooling system for use on aircraft having an on-board evaporator through which refrigerant circulates to cool a portion of the aircraft, the cooling system comprising: a rack sized for receiving a plurality of modular cooling units and having a ram air inlet for being connected to a ram air source; and a modular cooling unit including: a housing sized for receipt in the rack; a ram air inlet located on the housing for receiving ram air from the ram air inlet when the unit is received in the rack; a ram air outlet positioned downstream from the ram air inlet for exhausting spent ram air from the unit; a refrigerant inlet connectable to an outlet of the evaporator when the unit is received in the rack; a refrigerant outlet connectable to an inlet of the evaporator when the unit is received in the rack; a condenser mounted in the housing in fluid communication with the ram air inlet having an outlet in fluid communication with the refrigerant outlet and an inlet; a compressor mounted in the housing downstream from the condenser having an outlet in fluid communication with the condenser inlet and an inlet; and an accumulator having an outlet in fluid communication with the compressor inlet and an inlet in fluid communication with the unit refrigerant inlet. a rack sized for receiving a plurality of modular cooling units and having a ram air inlet for being connected to a ram air source; and a modular cooling unit including: a housing sized for receipt in the rack; a ram air inlet located on the housing for receiving ram air from the ram air inlet when the unit is received in the rack; a ram air outlet positioned downstream from the ram air inlet for exhausting spent ram air from the unit; a refrigerant inlet connectable to an outlet of the evaporator when the unit is received in the rack; a refrigerant outlet connectable to an inlet of the evaporator when the unit is received in the rack; a condenser mounted in the housing in fluid communication with the ram air inlet having an outlet in fluid communication with the refrigerant outlet and an inlet; a compressor mounted in the housing downstream from the condenser having an outlet in fluid communication with the condenser inlet and an inlet; and an accumulator having an outlet in fluid communication with the compressor inlet and an inlet in fluid communication with the unit refrigerant inlet. a housing sized for receipt in the rack; a ram air inlet located on the housing for receiving ram air from the ram air inlet when the unit is received in the rack; a ram air outlet positioned downstream from the ram air inlet for exhausting spent ram air from the unit; a refrigerant inlet connectable to an outlet of the evaporator when the unit is received in the rack; a refrigerant outlet connectable to an inlet of the evaporator when the unit is received in the rack; a condenser mounted in the housing in fluid communication with the ram air inlet having an outlet in fluid communication with the refrigerant outlet and an inlet; a compressor mounted in the housing downstream from the condenser having an outlet in fluid communication with the condenser inlet and an inlet; and an accumulator having an outlet in fluid communication with the compressor inlet and an inlet in fluid communication with the unit refrigerant inlet.