Gemini Digital Computer
The Gemini Digital Computer ( GDC ) was in the spaceships of the Gemini program used computer and thus the first used in a manned space vehicle digital computer . It was designed and built by the IBM Federal Systems Division .
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commitment
The spacecraft of the Mercury program - NASA's first manned space program - had no ability to maneuver independently. The accuracy of the earth's orbit was dependent on the control of the Atlas launcher , and the information needed to re-enter the earth's atmosphere was calculated by real-time systems on earth and transmitted to the spacecraft. In the Mercury program, for example, there was no need for a computer on board the spacecraft, since all functions required for the limited objectives of the Mercury program were provided by other systems.
The Gemini missions were used to test the critical rendezvous and coupling maneuvers of the lunar missions. Such maneuvers require precise entry into earth orbit , complex maneuvers when approaching the rendezvous target and precisely coordinated maneuvers during the rendezvous. In order to achieve greater safety during the launch with the Titan II launcher , a reserve system for the guidance system of the Titan II was also required. In addition, the aim was to achieve greater accuracy when re-entering the earth's atmosphere and to automate the system checks to be carried out before take-off. The Gemini Digital Computer was developed to fulfill these diverse tasks.
IBM received the order for the computer in April 1962 and delivered a total of 20 of these computers to NASA by December 1965.
The Gemini Digital Computer was used during the following six flight phases:
- before the start,
- as a reserve system during the ascent,
- when entering earth orbit,
- when approaching the rendezvous destination,
- at the rendezvous and
- upon re-entry into the earth's atmosphere.
description
hardware
The Gemini Digital Computer was 48.0 cm x 36.8 cm x 32.4 cm, weighed 26.75 kg, and consumed 94.54 watts on average. It was located to the left of the spacecraft commander's seat.
The computer was made up of discrete components , which means that no integrated circuits (ICs) were used. It also had no redundancy whatsoever . If functions failed, the corresponding phases had to be canceled (e.g. a rendezvous) or carried out manually (e.g. re-entry into the earth's atmosphere).
One command cycle of the Gemini Digital Computer - the duration of an addition - was 140 µs. Three instruction cycles (420 µs) were required for a multiplication and six instruction cycles (840 µs) were required for a division.
The primary storage consisted of a ferrite core storage . This consisted of 39 fields with 64 × 64 bits each. This resulted in 4096 addresses, each of which addressed a word with a length of 39 bits. These words, in turn, were divided into 13-bit syllables. The memory was also divided into 18 sectors. A complete address was thus formed from a sector and a syllable. A command was 13 bits long and a data word was 26 bits long. The data words were always stored in syllables 0 and 1 of a word, while a command could be stored in any syllable. This meant that up to three commands, but only one data word, could be stored in one word.
The arithmetic and logic circuits as well as the ferrite core memory were the main components of the Gemini Digital Computer. These components were connected to a plethora of other systems (primarily control and navigation systems) on the spacecraft.
From Gemini 8 the device was equipped with an additional tape storage . This tape storage increased the storage capacity to 1,170,000 bits. It was now possible to load individual programs from the tape drive into the syllables 0 and 1 of the ferrite core memory. It took about six minutes to load a single program.
software
The software running on the Gemini Digital Computer was written in machine language. Their command scope consisted of 16 different commands. The software was created from scratch for each of the Gemini missions. Programs have been changed or added over time. The individual software versions were called Gemini Math Flow . There were a total of nine different versions ( Math Flow 1 to Math Flow 9 ).
User interface
The Gemini Digital Computer user interface consisted of three parts:
- the computer control ,
- the Manual Data Insertion Unit ( MDIU , manual data entry unit) and
- the Incremental Velocity Indicator ( IVI , Incremental Velocity Indicator ).
Computer control
The computer control was used to start the various computer programs and was located on the lower dashboard. It consisted of a rotary switch , a start button, an indicator light for faults, an indicator light for computer operation and a reset switch.
The rotary switch had seven positions to select one of the available measurement or calculation programs. By pressing the start button, the program selected using the rotary switch was loaded into the memory and started. After a self-test, the computer was put into an operational state by pressing the reset switch.
Manual Data Insertion Unit (MDIU)
The MDIU was used for manual data entry and was attached to the instrument panel of the second astronaut (right side). It consisted of input keys for entering the digits 0 to 9 and a seven-digit number display, which consisted of a roller counter similar to an odometer . The memory address was indicated by the first two digits. Up to 99 addresses could be accessed. The remaining five digits indicated the dates. A negative value was indicated by the fact that a 9 was displayed in the first place. Errors were indicated by zeros.
Incremental Velocity Indicator (IVI)
The IVI was used to display the speed changes that were required for a steering maneuver or were the result of a steering maneuver. To do this, it had a three-digit display for each of the three axes (X-axis (forwards-backwards), Y-axis (left-right) and Z-axis (up-down)). The speed changes were displayed in ft / s. The IVI was on the commander's instrument panel (left side).
literature
- David A. Mindell: Digital Apollo: Human and Machine in Spaceflight . The MIT Press, Cambridge 2008, ISBN 978-0-262-13497-2 .
- Bernd Leitenberger: The Gemini program: technology and history . 3rd, expanded edition. Books on Demand, Norderstedt 2010, ISBN 978-3-8391-4798-6 .
Web links
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- Computers in Spaceflight: The NASA Experience - by James Tomayko (Chapter 1, Part 1, The Gemini Digital Computer: First Machine in Orbit )
- IBM Archives - IBM and the Gemini Program
- IBM Archives - Gemini Computer