Launch Vehicle Digital Computer

The Launch Vehicle Digital Computer (LVDC) was a computer that provided the autopilot for the Saturn V rocket from launch, through Earth orbit insertion, and the trans-lunar injection burn that would send the Apollo spacecraft to the Moon. Designed and manufactured by IBM's Electronics Systems Center in Owego, New York, it was one of the major components of the instrument unit, fitted to the S-IVB stage of the Saturn V and Saturn IB rockets. The LVDC also supported pre- and post-launch checkout of the Saturn hardware. It was used in conjunction with the Launch Vehicle Data Adaptor (LVDA) which performed signal conditioning from the sensor inputs to the computer from the launch vehicle.

Hardware The LVDC was capable of executing 12190 instructions per second. For comparison, as of 2022, researchers at the University of California created a chip capable of running at 1.78 trillion instructions per second, 146 million times faster.

Its master clock ran at 2.048 MHz, but operations were performed bit-serially, with 4 cycles required to process each bit, 14 bits per instruction phase, and 3 phases per instruction, for a basic instruction cycle time of 82 μs (168 clock cycles) for a simple add. A few instructions (such as multiply or divide) took several multiples of the basic instruction cycle to execute. Memory was in the form of 13-bit syllables, each with a 14th parity bit. Instructions were one syllable in size, while data words were two syllables (26 bits). Main memory was random access magnetic core, in the form of 4,096-word memory modules. Up to 8 modules provided a maximum of 32,768 words of memory. Ultrasonic delay lines provided 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 16,384 words, the computer weighed 72.5 lb (32.9 kg), was 29.5 by 12.5 by 10.5 inches (750 mm × 320 mm × 270 mm) in size and consumed 137W.

The LVDC communicated digitally with a Launch Vehicle Data adapter (LVDA). The LVDA converted analog-to-digital and digital-to-analog with a Flight Control Computer (FCC). The FCC was an analog computer.

Software architecture and algorithms LVDC instruction words were split into a 4-bit opcode field (least-significant bits) and a 9-bit operand address field (most-significant bits). This left it with sixteen possible opcode values when there were eighteen different instructions: consequently, three of the instructions used the same opcode value, and used two bits of the address value to determine which instruction was executed. Memory was broken into 256-word "sectors". 8 bits of the address specified a word within a sector, and the 9th bit selected between the software-selectable "current sector" or a global sector called "residual memory". The eighteen possible LVDC instructions were:

Programs and algorithms In flight the LVDC ran a major computation loop every 2 seconds for vehicle guidance, and a minor loop 25 times a second for attitude control. The minor loop is triggered by a dedicated interrupt every 40 ms and takes 18 ms to run. Unlike the Apollo Guidance Computer software, the software which ran on the LVDC seems to have vanished. While the hardware would be fairly simple to emulate, the only remaining copies of the software are probably in the core rope memory of the Instrument Unit LVDCs of the remaining Saturn V rockets on display at NASA sites.

Interrupts The LVDC could also respond to a number of interrupts triggered by external events. For a Saturn IB these interrupts were:

For a Saturn V these interrupts were:

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