The future of rapid cooldown cryogenic IR systems lies in lightweight, compact and inexpensive designs that use state-of-the-art configurations, materials and mass-production manufacturing processes. Miniature JT cryocoolers are ideally suited for cooling applications that require long storage periods (10 - 15 yr), fast cooldown times (< 30 s) and short operating missions (< 3 min). Most of the applications cool seeker sensors on missiles, projectiles, or `smart' bombs where environments conditions of temperature, acceleration, shock and vibration are harsh, where volume and weight must be minimized, and where low cost is required for large quantities. The Lockheed Palo Alto Research Laboratories are currently developing the Low Cost Sensor Integrated Dewar Assembly (LCS IDA) using an APD Cryogenics Inc. JT cryocooler. This paper presents the design fundamentals of rapid cooldown dewars and JT cryocoolers with the LCS IDA as a case study. Specific topics include: the performance trade-offs of the many thermodynamic parameters; the systems approach to the design process, and; recent test results.