Saturn Launch Vehicle Digital Computer

The Saturn Launch Vehicle Digital Computer (LVDC) was one of the major components of the Instrument Unit fitted to the S-IVB stage of the Saturn V and Saturn IB rockets. Its primary role was to provide an autopilot for the Saturn from launch to orbit insertion, but it also supported pre- and post-launch checkout of the Saturn hardware.

Hardware

By today's standards the LVDC was extremely slow, with a 2.048 MHz clock cycle, add operations taking 82 microseconds (vs a fraction of a nanosecond on a Pentium 4) and memory holding a maximum of 32,768 28-bit words in memory modules of 4096 words each, each equivalent to less than sixteen kilobytes. However, for the 1960s it was a sophisticated system, and easily capable of flying a three-thousand ton rocket into a hundred mile high orbit.

The computer processed 26-bit data (25 bits of magnitude and one sign bit), with two extra parity bits for error detection, and instructions were 13 bits in size with one parity bit. This meant that two instructions could fit in one data word, making the limited memory size less constricting for software. Main memory was random access magnetic core, with ultrasonic delay lines for temporary storage.

For reliability the LVDC used triple-redundant logic and a voting system. The computer included three identical logic systems. Each logic system was split into a seven stage pipeline. At each stage in the pipeline, a voting system would take a majority vote on the results, with the most popular result being passed on to the next stage in all pipelines. This meant that, for each of the seven stages, one module in any one of the three pipelines could fail, and the LVDC would still produce the correct results. The result was an estimated reliability of 99.6% over 250 hours of operation, which was far more than the few hours required for an Apollo mission.

With four memory modules, giving a total capacity of 16384 words, the computer weighed 72.5 pounds (approx 35 kilograms), was 29.5"x12.5"x10.5" in size (74x32x27 cm) and consumed 137 watts of power, the latter at least beating a modern PC.

oftware

LVDC instruction words were split into a 4-bit operand field and a 9-bit address field. This left it with sixteen possible operand values when there were eighteen different instructions: consequently, three of the instructions used the same operand value, and used two bits of the address value to determine which instruction was executed.

The eighteen possible LVDC instructions were:

Construction

The LVDC was approximately 30 inches wide, 12.5 inches high, and 10.5 inches deep and weighed 80 pounds. [Apollo Study Report, Volume 2, pages 3-36 to 3-37. The log book for the LVDC at National Air and Space Museum says the dimensions were 31x13.1x13 inches and the weight was 90 pounds.] The chassis was made of magnesium-lithium alloy LA 141, chosen for its high stiffness, low weight, and good vibration damping characteristics. [ M.M. Dickimon, J.B. Jackson, G.C. Randa. IBM Space Guidance Center, Owego, NY. "Saturn V Launch Vehicle Digital Computer and Data Adapter." Proceedings of the Fall Joint Computer Conference, 1964, page 511.] The chassis was divided into a 3 x 5 matrix of cells separated by walls through which coolant was circulated to remove the 138 Watts [Apollo Study Report, Volume 2, page 3-4.] of power generated by the computer. Slots in the cell walls held “pages” of electronics.

A page consisted of two 2.5 x 3-inch boards back to back and a magnesium-lithium frame to conduct heat to the chassis. The 12-layer boards contained signal, power, and ground layers and connections between layers were made by plated-through holes.

Up to 35 alumina squares 0.3 x 0.3 x 0.070 inch [ Apollo Study Report, Volume 2, page 2-37] could be reflow soldered to a board. These alumina squares had conductors silk screened to the top side and resistors silk-screened to the bottom side. Semiconductor chips 0.025 x 0.025 inch, each containing either one transistor or two diodes, were reflow soldered to the top side. Copper balls were used for contacts between the chips and the conductive patterns. [ M.M. Dickimon, J.B. Jackson, G.C. Randa. IBM Space Guidance Center, Owego, NY. "Saturn V Launch Vehicle Digital Computer and Data Adapter." Proceedings of the Fall Joint Computer Conference, 1964, page 509.]

The hierarchy of the electronic structure is shown in the following table.

Gallery

References

* IBM, " [http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730063841_1973063841.pdf Saturn V Launch Vehicle Digital Computer, Volume One: General Description and Theory] ", 30th November 1964
* Bellcomm, Inc, " [http://klabs.org/history/history_docs/s5_lvdc/mem_rocchio_67/index.htm Memory Requirements for the Launch Vehicle Digital Computer (LVDC)] ", April 25, 1967
* Boeing, " [http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940004379_1994004379.pdf Saturn V Launch Vehicle Guidance Equations, SA-504] ", 15th July 1967
* Walter Haeussermann, " [http://klabs.org/history/reports/tn_d-5869_1970023342.pdf Description and Performance Of The Saturn Launch Vehicle's Navigation, Guidance And Control System] ", July 1970
* NASA Marshall Spaceflight Center, "Saturn V Flight Manual SA-503", 1st November 1968
* NASA Marshall Spaceflight Center, " [http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19740021163_1974021163.pdf Skylab Saturn IB Flight Manual] ", 30th September 1972
* M.M. Dickimon, J.B. Jackson, G.C. Randa. IBM Space Guidance Center, Owego, NY. "Saturn V Launch Vehicle Digital Computer and Data Adapter." Proceedings of the Fall Joint Computer Conference, 1964, pages 501-516.
* S. Bonis, R. Jackson, and B. Pagnani. IBM Space Guidance Center, Owego, NY. “Mechanical and Electronic Packaging for a Launch-Vehicle Guidance Computer.” International Electronic Circuit Packaging Symposium 21-24 August 1964. Pages 226-241.
* IBM, [http://hdl.handle.net/2060/19720067238 Apollo Study Report, Volume 2.] IBM Space Guidance Center, Owego, NY, 1 October 1963. 133 pages.
* NASA MSFC, [http://hdl.handle.net/2060/19700070300 Astrionics System Handbook Saturn Launch Vehicles] NASA Marshall Space Flight Center, 1 Nov 1968. MSFC No. IV-4-401-1. IBM No. 68-966-0002. 419 pages. Chapter 15 is about the LVDC and Launch Vehicle Data Adapter.

External links

* [http://www-03.ibm.com/ibm/history/exhibits/space/space_saturn.html IBM Archives: Saturn Guidance Computer]