Abstract There is a need for reliable, space-qualified mechanical coolers for temperatures of 10 Kelvin and below for use in both X-ray and infrared systems. Our analysis shows that hybrids consisting of a Joule-Thomson (J-T) cooler coupled with a mechanical cooler are the most efficient when used in a low Earth orbit. We are developing a hybrid cryocooler consisting of a helium J-T system coupled to a Stirling cycle mechanical cooler for these applications. We plan to use the relatively mature Stirling cycle mechanical cooler to provide all of the precooling, and use the J-T recuperative system for refrigeration below 20 K, a region that has historically been difficult for regenerative coolers. In this paper we discuss our work in developing this precooler on our NASA Explorer 6 K program. A discussion of the J-T portion of this system will occur elsewhere in this conference1. Our precooler work is a continuance of our previous work on linear Stirling coolers. We have developed one-, two-, and three-stage Stirling coolers for various NASA and Department of Defense programs. For the Explorer 6 K, we have focused on adapting our three-stage Stirling cooler for operation at lower temperatures. In this program we are using breadboard tests to acquire both an empirical knowledge and an analytical understanding of the factors that affect the low-temperature performance of our cooler. This information will be used to develop a space-qualified precooler during a later phase of the system development. In our initial tests we have demonstrated improved performance when we substituted a lead shot regenerator for the previous regenerator constructed from phosphor bronze screens. Although the data suggests future directions for more cooling, at present the performance falls short of our analytical expectations.