Long-term cryogenic propellant storage requires mechanical cryocoolers to maintainzero or very low cryogen boil-off rates. Very large cryogen tanks such as those proposedfor orbital fuel depots may require cryocoolers with very high cooling capacity. In-situresource generation and storage of oxygen and methane on Mars also requires highcapacity cryocoolers, and low mass is extremely desirable for planetary missions becauseof the cost associated with landing mass on the surface of another planet. LockheedMartin's Advanced Technology Center has developed a high capacity low mass aerospacecryocooler with very high power density. This 7 kg pulse tube cryocooler can provide20 W of cooling at 70 K while rejecting heat at 300 K. This large cooling capability couldalso be used to cool large optical structures or other devices with high heat loads. Testingof the cooler with a secondary heat exchanger attached to the pulse tube was alsoconducted, and results are discussed.