A study was carried out at the Jet Propulsion Laboratory during the first quarter of 1985 to develop a system concept for NASA's Large Deployable Reflector (LDR). This new system concept meets the primary scientific requirements and minimizes the cost and development time. The LDR requirements were investigated to determine whether or not the major cost drivers could be significantly relaxed without compromising the scientific utility of LDR. In particular, the telescope wavefront error is defined so as to maximize scientific return per dollar. Major features of the concept are a four-mirror, two-stage optical system; a lightweight structural composite segmented primary reflector; and a deployable truss backup structure with integral thermal shield. The two-stage optics uses active figure control at the quaternary reflector located at the primary reflector exit pupil, allowing the large primary to be passive. The lightweight composite reflector panels limit the short wavelength operation to approximately 30 pm but reduce the total primary reflector weight by a factor of 3 to 4 over competing technologies. System optical performance is calculated including aperture efficiency, Strehl ratio, and off-axis performance. On-orbit thermal analysis indicates a primary reflector equilibrium temperature of less than 200 K with a maximum gradient of =°C across the 20 m aperture. Weight and volume estimates are consistent with a single Shuttle launch and are based on Space Station assembly and checkout.