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
| 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 |
Eight memory modules, simplex (maximum capacity)
| 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 |
Figures for the whole machine
| Metric | Value | Evidence | Basis | Method | Source |
|---|---|---|---|---|---|
| Memory capacity (words) whole-system | 32 768 words | Confirmed | Vendor-rated | Design capacity Stated by the vendorEight 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 vendorComputer 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
| 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
| Metric | Value | Evidence | Basis | Method | Source |
|---|---|---|---|---|---|
| Clock frequency cpu | 2.048 MHz | Confirmed | Vendor-rated | Nominal clock Stated by the vendorFour 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 vendorAdd-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 vendor24 bit accuracy. | |
| Instruction execution time cpu | 328 µs | Confirmed | Vendor-rated | Multiplication instruction Stated by the vendor24 bit accuracy. |
2 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 |
| Metric | Value | Evidence | Basis | Method | Source |
|---|---|---|---|---|---|
| Memory capacity (words) memory | 4096 words | Confirmed | Vendor-rated | Design capacity Stated by the vendorOne 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. |
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.
A yardstick from the commercial world
NASA’s own description of the guidance system makes a commercial comparison when it gets to memory. A fully populated computer, the report says, has a capacity “equivalent to the basic memory capability of the IBM 7090 commercial machine”. The 7090 was what businesses and universities of the period were buying.
The comparison belongs to the report’s authors rather than to a later retelling. It sits in the paragraph introducing the memory modules, and it records what a large memory looked like in 1970 to the people building one.