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Guidance computer

Saturn Launch Vehicle Digital Computer

The computer in the Saturn instrument unit: a liquid-cooled, triple-redundant serial machine that navigated, guided and controlled the launch vehicle from lift-off through orbital insertion and translunar injection.

Manufacturer
IBM Federal Systems Division
Released
1966
Region
United States
Record status
approved

About this machine

Where the Apollo Guidance Computer flew the spacecraft, the LVDC flew the rocket. It is a serial fixed-point machine clocked at 2.048 MHz, taking four clocks per bit for a 512 kilobit per second rate, with ultrasonic glass delay lines used as serial arithmetic registers and as dynamic storage for the instruction counter. Reliability came from redundancy rather than from margin: triple modular redundancy runs three identical logic channels divided into seven voted modules each, at the cost of roughly 3.4 times as many components as a simplex system, and memory modules could be run in duplex. Magnesium-lithium frames are liquid-cooled to keep the electronics at a low operating temperature. Each toroidal-core module stores 4096 words; each word is 28 bit wide.

Configuration

Four memory modules

United States

Component Role Count Clock Capacity
Saturn Launch Vehicle Digital Computer Main CPU 1 2.048 MHz Nominal clock —
Saturn LVDC toroidal core memory module Capacity is per module. System memory 4 — 4096 words

The configuration the report's weight and power figures describe: 35 kg and 137.0 W.

Eight memory modules, simplex (maximum capacity)

United States

Component Role Count Clock Capacity
Saturn Launch Vehicle Digital Computer Main CPU 1 2.048 MHz Nominal clock —
Saturn LVDC toroidal core memory module Capacity is per module. Eight in simplex give the stated 32 768 words maximum. System memory 8 — 32 768 words

The report gives no weight or power figure for this configuration, so neither is recorded against it.

Figures for the whole machine

Saturn Launch Vehicle Digital Computer: system figures
Metric Value Evidence Basis Method Source
Memory capacity (words) whole-system 32 768 words Confirmed Vendor-rated Design capacity Stated by the vendor Eight memory modules operating in simplex; the maximum the design allows. A maximum, not the capacity of every flight article. Running the modules in duplex for redundancy halves the words available.
Rated power consumption whole-system 137.0 W Confirmed Vendor-rated Rated power Stated by the vendor Computer with four memory modules. The report gives no figure for other module counts. A design figure for the computer alone, excluding the data adapter that connects it to the rest of the instrument unit. It is not a measured draw and not a cooling-design target.
1 documented absence
Saturn Launch Vehicle Digital Computer: system figures: documented absences
Metric Value Evidence Basis Method Source
Memory bandwidth memory unknown The report specifies the memory by module capacity, word length and cycle time, and states no transfer rate. Not stated Vendor-rated Theoretical peak Stated by the vendor Unknown for the same reason as the Apollo Guidance Computer’s, and the pair is worth seeing together: two machines of the same program and the same year, both described in microseconds and words, neither in bytes per second.

Figures for its components

Saturn Launch Vehicle Digital Computer
Metric Value Evidence Basis Method Source
Clock frequency cpu 2.048 MHz Confirmed Vendor-rated Nominal clock Stated by the vendor Four clocks per bit, giving 512 kilobits per second in serial operation. The same clock rate as the Apollo Guidance Computer, and the coincidence says nothing about their speed: the LVDC is a serial machine that spends four clocks per bit, so it adds in 82 µs against the AGC's 23.4 µs.
Instruction execution time cpu 82 µs Confirmed Vendor-rated Addition instruction Stated by the vendor Add-subtract, 26 bit accuracy. Add-subtract and multiply-divide run simultaneously.
Instruction execution time cpu 656 µs Confirmed Vendor-rated Division instruction Stated by the vendor 24 bit accuracy.
Instruction execution time cpu 328 µs Confirmed Vendor-rated Multiplication instruction Stated by the vendor 24 bit accuracy.
2 documented absences
Saturn Launch Vehicle Digital Computer: documented absences
Metric Value Evidence Basis Method Source
Peak FP32 rate cpu not applicable The LVDC is a fixed-point machine, as its own characteristics table states in the type row. There is no floating-point rate to give. Not stated Vendor-rated Theoretical peak Stated by the vendor
Peak FP64 rate cpu not applicable Fixed-point machine. The 26 bit data word carries 25 magnitude bits and a sign bit, not an exponent. Not stated Vendor-rated Theoretical peak Stated by the vendor
Saturn LVDC toroidal core memory module
Metric Value Evidence Basis Method Source
Memory capacity (words) memory 4096 words Confirmed Vendor-rated Design capacity Stated by the vendor One basic module: fourteen 128 by 64 core planes, 4096 non-redundant 28 bit words. A word here is 28 bit, against the Apollo Guidance Computer's 16. Word counts from machines with different word widths are not a like-for-like capacity comparison.

Derived figures and comparisons

Computed by this project rather than read from a source, and recomputed from their inputs on every build.

3.5×

Formula
a ÷ b ratio v1
Inputs
  • lvdc Instruction execution time: 82 µs
  • agc-block-ii Instruction execution time: 23.4 µs
Rounding
half-up , 2 significant digits

Compares instruction-execution-time at cpu scope, obtained by add-instruction. Valid only within this comparability group, and reported to 2 significant digits — the precision of the least precise input.

Editorial notes

The release date is editorial context. The LVDC first flew on a Saturn IB in 1966, but the cited report does not carry that figure; it documents the computer as flown for Apollo. Every measurement attached to this system comes from the report.

Sources