Goddard Space Flight Center (GSFC) is presently developing an X-ray spectrometer (XRS) to be flown aboard the Advanced X-ray Astrophysics Facility (AXAF). XRS uses an array of 32 microcalorimeters, capable of detecting X-rays in the energy range of 0.3-10 keV with a resolution of 12 eV. In order to accomplish this the detectors must be operated at a temperature of 0.065 K. In space, an adiabatic demagnetization refrigerator (ADR) must be used to cool the detectors to that temperature. A spaceworthy ADR has been under deverlopment at GSFC for almost a decade. When XRS because a viable AXAF candidate, the development effort intensified. A baseline model heat switch has been tested extensively with an on/off ratio of -10 000 and a parasitic heat leak of 10/zW. In the same time period various salt pills have been built employing different methods of thermal contact and of crystal growth. We are presently in the process of building the engineering model of the ADR. Using the data obtained from the breadboard models we have designed an ADR with improved structural integrity while, at the same time, improving on its efficiency and performance. The new ADR design will be reported in detail.