D-17B | 1962

 

The D-17B (D17B) computer was used in the Minuteman I NS-1OQ missile guidance system. The complete guidance system contained a D-17B computer, the associated stable platform, and power supplies.

The D-17B weighed approximately 62 pounds (28 kg), contained 1,521 transistors, 6,282 diodes, 1,116 capacitors, and 5,094 resistors. These components were mounted on double copper-clad, engraved, gold-plated, glass fiber laminate circuit boards. There were 75 of these circuit boards, and each one was coated with a flexible polyurethane compound for moisture and vibration protection. The high degree of reliability and ruggedness of the computer was driven by the strict requirements of the weapons system.

Design constraints

High reliability was required of the D-17B. It controlled a key weapon that would have just one chance to execute its mission. Reliability of the D-17B was achieved through the use of solid-state electronics and a relatively simple design. Simpler DRL (diode–resistor) logic was used extensively, while less-reliable DTL (diode–transistor) logic (which provides gain and inversion) was used only where needed. In the late 1950s and early 1960s, when the D-17B was designed, transistors lacked today's reliability. Reliability was also enhanced by the rotating-disk memory with non-destructive readout (NDRO). In actual real-time situations, Minuteman missiles achieved a mean time between failures (MTBF) of over 5.5 years.

The Soviets had much larger rockets and could use vacuum tubes in their guidance systems. The Minuteman I, II, and III weighed 29,500 kg, 31,746 kg, and 35,000 kg respectively, versus the Soviet R-7 missile (1959) at 280,000 kg. The US planners had to choose either to develop solid-state guidance systems (which weigh less) or consider the additional cost and time delay of developing larger rockets.