A Mass Memory Unit for the Space Shuttle Orbiter

Abstract

A high-capacity, medium access time data storage system has been developed to interface with the five General Purpose Computers (GPC) in the Space Shuttle System. Termed a Mass Memory Unit (MMU), this system uses magnetic tape as the storage medium. There are two basic functions for the MMU in the Space Shuttle System. First, the MMU will be used to provide display format storage. These displays will be used for vehicle operational procedures and on-line status. The second function is as an auxiliary memory to be used to store and load/reload all phases of flight/ground software. The stored data is catalogued by using eight longitudinal channels (tracks) and eight transverse channels (files). Each file is divided into eight subfiles, and each subfile is divided into 32 blocks. Any one or more blocks or any subfile can be addressed by the GPC. The MMU is capable of storing 1.31 X 10⁸ bits and has a nominal access time (to the nearest block of data) of 600 milliseconds. The data transfer rate is 1 X 10⁶ bits per second and recording is at a packing density of 5 X 10³ bits per inch. The MMU is a hermetically sealed unit which occupies approximately 1,000 cubic inches and weighs approximately 25 pounds. The wear-related items have been designed to ensure over 20,000 tape passes without maintenance. As an advanced form of data storage, the MMU fills a void between slow-access tape drives of high-storage capacity and disks or drums with fast access time and relatively low-storage capacity. In contrast with disk or drum memories, the MMU consumes operating power only when active, which yields a very low average mission power.