A circuit board from the Saturn V rocket, reverse-engineered and explained

In the Apollo Moon missions, the Saturn V rocket was guided by an advanced onboard computer system built by IBM. This system was built from hybrid modules, similar to integrated circuits but containing individual components. I reverse-engineered a circuit board from this system and determined its function: Inside the computer's I/O unit, the board selected different data sources for the computer.

A circuit board from the Saturn V LVDA. (Click this image (or any others) for a larger version.) This board was partially disassembled when I received it and some chips are missing.

A circuit board from the Saturn V LVDA. (Click this image (or any others) for a larger version.) This board was partially disassembled when I received it and some chips are missing.

This post explains how the board worked, from the tiny silicon dies inside its hybrid modules to the board's circuitry and its wiring in the rocket. This board was first studied by Fran Blanch in The Apollo Saturn V LVDC Project. Then EEVblog made a video about it. Now it's my turn to analyze the board.

The Launch Vehicle Digital Computer (LVDC) and Launch Vehicle Data Adapter (LVDA)

The race to the Moon started on May 25, 1961, when President Kennedy stated that America would land a man on the Moon before the end of the decade. This mission required the three-stage Saturn V rocket, the most powerful rocket ever built. The Saturn V was guided and controlled by the Launch Vehicle Digital Computer (below), from liftoff into Earth orbit, and then on a trajectory towards the Moon.1 In an era when most computers ranged from refrigerator-sized to room-filling, the LVDC was very compact and weighed just 80 pounds since it was mounted inside the rocket. The downside was that it was very slow, performing 12,000 instructions a second.

The LVDC mounted in a support frame for testing. Behind the operator is a test system called ACME (Aerospace Computer Manual Exerciser). The ACME paper tape reader is visible at the back. Photo from IBM.

The LVDC mounted in a support frame for testing. Behind the operator is a test system called ACME (Aerospace Computer Manual Exerciser). The ACME paper tape reader is visible at the back. Photo from IBM.

The LVDC worked in conjunction with the Launch Vehicle Data Adapter (LVDA, below), which provided the input/output functions for the computer. All communication between the computer and the rocket went through the LVDA, which converted the rocket's analog signals and 28-volt control signals to the serial binary data the computer required. The LVDA contained buffers (implemented with glass delay lines) and control registers for its various functions. The LVDA had analog-to-digital converters to read data from the inertial measurement unit's gyroscopes and digital-to-analog converters to provide control signals to the rockets. It also processed telemetry signals that were sent to the ground and received ground-based commands for the computer. Finally, power to the LVDC was provided by redundant switching power supplies in the LVDA.

The Saturn V LVDA was a 176-pound box that provided I/O for the LVDA. It had 21 round connectors for cables to other parts of the rocket.  From System Description and Component Data.

The Saturn V LVDA was a 176-pound box that provided I/O for the LVDA. It had 21 round connectors for cables to other parts of the rocket. From System Description and Component Data.

Because the LVDA had so many different functions, it was almost twice the size of the LVDC computer. The diagram below shows the circuitry crammed into the 176-pound LVDA.2 It had two sections filled with circuit boards called "pages": the front logic section and the back logic section. (The board I examined was from the front logic section.) The power supplies and filters were in the central section. A methanol coolant solution flowed through channels in the LVDA to keep it cool. The LVDA was wired to the LVDC and other parts of the rocket through the 21 round connectors on the ends.

Exploded diagram of the LVDA, from NASA.

Exploded diagram of the LVDA, from NASA.

Diode-Transistor Logic

There are many different ways to build logic gates. The LVDC and LVDA used a technique called Diode-Transistor Logic (DTL) that builds a gate from diodes and a transistor. This was more advanced than the Resistor-Transistor Logic (RTL) used by the Apollo Guidance Computer, but inferior to Transistor-Transistor Logic (TTL), which became very popular in the 1970s.

The standard logic gate in the LVDC was an AND-OR-INVERT gate3 that implements a logic function such as (A·B + C·D)'. It gets its name because it ANDs together sets of inputs, ORs them, and finally inverts the results. The AND-OR-INVERT gate was powerful because it could be built with many inputs, e.g. (A·B + C·D·E + F·G·H)'. While the AND-OR-INVERT gate may seem complex, it only required one transistor which was important in an era when every transistor counted.

If you want to understand how the gate works internally, look at the diagram below. It shows a four-input AND-OR-INVERT gate with two AND terms. First consider inputs A and B, which are both set to 1 (high). The pull-up resistor4 pulls the AND value high (red, 1). In comparison, in the lower AND gate, input C is 0, so current flows through input C, pulling the AND value low (blue, 0). Thus, the diodes and the pull-up resistor implement an AND gate. Next, look at the OR stage. Current from the top AND (red) pulls the OR stage high (1). Finally, this current turns the transistor on, pulling the output low (blue, 0) and providing the inversion. If both AND stages were 0, the OR stage wouldn't be pulled high. Instead, the pull-down resistor would pull the OR value low (0), turning off the transistor and causing the output to be pulled high (1).

An AND-OR-INVERT gate computing (A·B + C·D)'. Since inputs A and B are both high, the output is pulled low.

An AND-OR-INVERT gate computing (A·B + C·D)'. Since inputs A and B are both high, the output is pulled low.

An AND-OR-INVERT gate could be built with more resistors or diodes to provide as many inputs as required, potentially many inputs to each AND, and many blocks ORed together. You might expect that AND-OR-INVERT gate would be implemented on a single chip, but the LVDC used multiple chips for each gate, as will be shown below. Different chips had various combinations of diodes, resistors, and transistors that were wired up in flexible ways to form the desired logic gate.

Unit Logic Devices (ULD)

The LVDC and LVDA were built with an interesting hybrid technology called ULD (Unit Logic Devices).5 Although they superficially resembled integrated circuits, ULD modules contained multiple components. They used simple silicon dies, each implementing just one transistor or two diodes. These dies, along with thick-film printed resistors, were mounted on a .3-inch-square ceramic wafer. These modules were a variant of the SLT (Solid Logic Technology) modules used in IBM's popular S/360 series of computers. IBM started developing SLT modules in 1961, before integrated circuits were commercially viable, and by 1966 IBM produced over 100 million SLT modules a year.

ULD modules were considerably smaller than SLT modules, as shown in the photo below, making them more suitable for a compact space computer. ULD modules used flat-pack ceramic packages instead of SLT's metal cans, and had metal contacts on the upper surface instead of pins. Clips on the circuit board held the ULD module in place and connected with these contacts. The LVDC and LVDA used more than 50 different types of ULDs.

ULD modules (right) are smaller than SLT modules or more modern DIP integrated circuits (left). An SLT module was about 0.5" on a side, while a ULD module was 0.3" on a side and much thinner.

ULD modules (right) are smaller than SLT modules or more modern DIP integrated circuits (left). An SLT module was about 0.5" on a side, while a ULD module was 0.3" on a side and much thinner.

Internally, a ULD module contained up to four tiny square silicon dies. Each die implemented either two diodes or one transistor. The photo below shows the internal components of a ULD module, next to an intact ULD module. On the left, the circuit traces are visible on the ceramic wafer, connected to four tiny square silicon dies. While this looks like a printed circuit board, keep in mind that it is much smaller than a fingernail. Thick-film resistors were printed on the underside of the module, so they are not visible.

A ULD of type "INV" opened to show the four silicon dies inside. The upper-right die is a transistor, while the other three dies are dual diodes. The module was protected by pink silicone, which has been removed to show the circuitry. Photo courtesy of Fran Blanche.

A ULD of type "INV" opened to show the four silicon dies inside. The upper-right die is a transistor, while the other three dies are dual diodes. The module was protected by pink silicone, which has been removed to show the circuitry. Photo courtesy of Fran Blanche.

The microscope photo below shows a silicon die from a ULD module that implements two diodes. The die is very small; for comparison, grains of sugar are displayed next to the die. The die had three external connections through copper balls soldered to the three circles. The two lower circles were doped (darker regions) to form the anodes of the two diodes, while the upper circle was the cathode, connected to the substrate. Note that this die is much less complex than even a basic integrated circuit.

Photo of a two-diode silicon die next to sugar crystals. This photo is a composite of top-lighting to show the die details, with back-lighting to show the sugar.

Photo of a two-diode silicon die next to sugar crystals. This photo is a composite of top-lighting to show the die details, with back-lighting to show the sugar.

The schematic below shows the circuitry inside the "INV" module shown earlier.7 The left side forms an AND-OR-INVERT gate with a single input. A gate with a single input may seem pointless, but additional AND inputs can be attached to pin 1 and additional OR gates can be attached to pin 3. The right side of the schematic provides components that can be used as additional inputs.

Schematic of the "INV" inverter module. Based on  Saturn V Guidance Computer, Semiannual Progress Report, page 2-37. Pins 7 and 14 switched from original, which didn't match the actual circuitry.

Schematic of the "INV" inverter module. Based on Saturn V Guidance Computer, Semiannual Progress Report, page 2-37. Pins 7 and 14 switched from original, which didn't match the actual circuitry.

The board also uses AND gate modules (types "AA" and "AB"), shown below. Keep in mind that these aren't independent gates, but components that can be wired to an INV chip to provide more AND or OR inputs.6 These modules can be wired up in many flexible ways; there are no specific inputs and outputs. One common configuration is to use half of an AA chip as a three-input AND gate. Part of an AB chip can provide two more inputs if needed.

Internal schematics of the type "AA" and type "AB" AND gates. From Laboratory Maintenance Instructions for LVDA, Vol 1.

Internal schematics of the type "AA" and type "AB" AND gates. From Laboratory Maintenance Instructions for LVDA, Vol 1.

The photo below shows the semiconductors (dual diodes) inside an AA gate. You can match up the components with the schematic above if you wish; pins 1 and 5, the common pins, are most interesting. Note that the pin numbering does not match the standard IC scheme.

A ULD of type "AA" opened to show the four silicon dies inside. The four dies are dual diodes with the cathodes connected. Original photo courtesy of Fran Blanche.

A ULD of type "AA" opened to show the four silicon dies inside. The four dies are dual diodes with the cathodes connected. Original photo courtesy of Fran Blanche.

The board's circuitry

To determine what the board did, I tediously beeped out the connections between chips with a multimeter to create wiring diagrams. (Shortly after I finished, LVDA manuals with schematics turned up8 making my reverse-engineering effort unnecessary.) The board forms a 7-input multiplexer, selecting one of 7 input lines and storing the value in a latch. With 1960s technology, this simple function required a whole board of chips.

The schematic below is a simplified diagram of the board. At the left, the board receives 7 inputs; six of them are 28-volt signals that need to be buffered to generate logic signals, while the seventh is already a 6-volt logic signal. One of the seven select lines is energized to select the corresponding input, which is then stored in the latch.9 (The main simplification is that there are multiple select lines for each input. The full schematic is in the footnotes.10) When the "reset multiplexer" signal and the "multiplexer address" are energized, the latch is reset.

Simplified schematic of the board. It is a multiplexer that selects one of the six inputs and stores the value in the latch.

Simplified schematic of the board. It is a multiplexer that selects one of the six inputs and stores the value in the latch.

While the schematic shows many logic gates, it is implemented with just two AND-OR-INVERT gates. The yellow gates form one large AND-OR-INVERT gate, while the blue gates form a second. (The two yellow OR gates merge into one.) The two gates are implemented across eight chips: two chips of type INV, four AA, and two AB. This illustrates the flexibility and expandability of the AND-OR-INVERT logic model, but it also shows that circuits use many chips. Note that there are only two transistors in the logic circuit (one in each INV chip); almost all of the logic is implemented with diodes.

The buffer circuitry

Of the 26 chips on the board, 18 of them were analog chips that buffered and processed the input signals. The inputs were 28-volt signals, while the logic requires 6-volt signals. Each input (except #7) passes through a "Discrete Interface Circuit" that converts the input to a logic signal. The diagram below shows the circuit, built from chips of types 321, 322, and 323.11 The photos show the contents of each chip. Since the 321 chip only consists of resistors (on the underside), the chip appears empty from the top. The 322 chip contains a single diode, while the 323 chip contains two transistors. (The dies are missing from the 323 photo; they are small squares as in the 322.)

Discrete Input Circuit, type A (DIA). The published "322" pinout is wrong, showing two pins 5. From Laboratory Maintenance Instructions for LVDA, Vol 1, Figure A-15.
321 and 322 photos courtesy of Fran Blanche.

Discrete Input Circuit, type A (DIA). The published "322" pinout is wrong, showing two pins 5. From Laboratory Maintenance Instructions for LVDA, Vol 1, Figure A-15. 321 and 322 photos courtesy of Fran Blanche.

The diagram below summarizes the structure of the board. The eight logic chips in the middle are outlined in green. Each of the six input buffers consists of three chips (321, 322, and 323). The signal flow through these chips is shown with the blue arrows. The board has 35 spots for chips, of which 26 were used. By putting chips in the empty locations, the same circuit board could be reused for slightly different functions.13

The circuit board with input paths in blue and logic circuitry in green. Original photo courtesy of Fran Blanche.

The circuit board with input paths in blue and logic circuitry in green. Original photo courtesy of Fran Blanche.

The board's role in the LVDA

This board was part of the multiplexer in an LVDA subsystem called the "System Data Sampler" that selects signals and sends them either to the computer or to the ground for telemetry. The System Data Sampler consists of a multiplexer that selects one of eight signals, and the Serializer-Selector that converts the 14-bit data to serial form. The multiplexer has several data sources: the RCA-110 ground computer that was connected to the rocket before launch;14 the "command receiver" that received computer commands from the ground after the rocket had launched; the "control distributor" box that provided various discrete signals;12 "spare discrete inputs"; feedback from the "switch selector", a relay box that the computer used to control the rocket; telemetry from the Digital Data Acquisition System (DDAS); and real-time data.

Physically, many of these data sources were large boxes in the Instrument Unit. For instance, the "control distributor" was a 35-pound box next to the LVDA, connected by a thick cable. The LVDA's "command receiver" input came from the "command decoder", a 7.5-pound box connected to other boxes that provided radio input and output. Because the LVDA was cabled to many different devices in the Instrumentation Unit, it required 21 connectors.

The locations of the LVDA, LVDC, Command Decoder, and Control Distributor in the Instrument Unit. Also shows the electronic assembly (ST-124-M3) that interfaces the inertial measurement unit to the LVDA. From the Saturn V Flight Manual page 7-8.

The locations of the LVDA, LVDC, Command Decoder, and Control Distributor in the Instrument Unit. Also shows the electronic assembly (ST-124-M3) that interfaces the inertial measurement unit to the LVDA. From the Saturn V Flight Manual page 7-8.

The board's physical structure

The circuit boards in the LVDA and LVDC used interesting construction techniques to withstand the high accelerations and vibrations of the rocket and to keep the circuitry cool. The board I examined was damaged and missing its mounting frame but the photo below shows an intact unit called a "page". The page's frame is made from a magnesium-lithium alloy that combines light weight, strength, and good heat transfer properties. Heat from a board flowed through the frame to the LVDA or LVDC's chassis, which was liquid-cooled via methanol flowing through channels drilled in the chassis.

A page including the metal frame. This board implemented voting circuitry in the LDVC. Photo from Dmitris Vitoris via Virtual AGC.

A page including the metal frame. This board implemented voting circuitry in the LDVC. Photo from Dmitris Vitoris via Virtual AGC.

Each page could hold two circuit boards, one on the front and one on the back. The printed circuit board has 12 layers, which is a remarkably high number for the 1960s. (Even in the 1970s, commercial PCBs typically had just two layers.) The page has a 98-pin connector, with 49 connections to each PCB. The two boards were connected by 30 "thru pins" at the top of the board. The top of each board also has 18 test connections; these allowed signals to be probed while the boards were installed. (IBM reused this page construction in its System/4 Pi aerospace computers.15)

The board I examined had been forcibly separated from the other board in the page. The photo below shows the back of the board. The thru-pins are visible at the top; they would have been connected to the other board. At the bottom, the 49 connections from the connector to the missing board are visible. Some of the board's insulation has been removed, showing the 12 vias at each ULD module position. These provide a connection from a chip pin to any of the 12 layers of the circuit board.

Back of the LVDA board. A second board was mounted on this side originally, but has been removed.

Back of the LVDA board. A second board was mounted on this side originally, but has been removed.

Conclusion

This small circuit board illustrates several stories about computing in the 1960s.

The board used hybrid modules rather than still-new integrated circuits. While this technology may seem backward, it was a key to IBM's success with the IBM System/360 line. Introduced almost exactly 56 years ago (April 7, 1964), these computers used hybrid SLT modules with AND-OR-INVERT logic. These computers dominated the market for years, and the System/360 architecture is still supported by IBM's mainframes.

The LVDC and LVDA also led to IBM's System/4 Pi line of aerospace computers, announced in 1967. These computers used the same "page" design and connectors as this board, even though they abandoned ULD modules for flat-pack TTL integrated circuits. The System/4 Pi line of computers evolved into the AP-101S computers used on the Space Shuttle.

Finally, the board shows the remarkable improvements in technology since the 1960s. Each ULD module contained up to 4 transistors, so even a basic circuit like a multiplexer took a whole board of modules. Now, an iPhone processor has over 8 billion transistors. It's amazing that such simple technology was enough to get to the Moon.

I announce my latest blog posts on Twitter, so follow me @kenshirriff for future articles. I also have an RSS feed. This work builds on Fran Blanche's Apollo Saturn V LVDC Project. Thanks to Fran for providing photos, Ben Krasnow for passing the board along to me, and Mike Stewart for documentation. For more information on the LVDC, see the Virtual AGC project's LVDC page. I recently wrote about the core memory stack in the Saturn V LVDC.

Notes and references

  1. The LVDC was one of several computers onboard the Apollo mission. The better-known Apollo Guidance Computer (AGC) guided the spacecraft to the Moon's surface. (I recently helped restore an Apollo Guidance Computer to running condition.) The Command Module had an AGC while the Lunar Module had a second AGC. The Lunar Module also contained the backup Abort Guidance System computer. The LVDC/LVDA was connected to the Flight Control Computer, a 100-pound analog computer mounted in the Instrument Unit.

    Multiple computers were onboard an Apollo mission. The Launch Vehicle Data Adapter (LVDA) is discussed in this blog post.

    Multiple computers were onboard an Apollo mission. The Launch Vehicle Data Adapter (LVDA) is discussed in this blog post.

    The LVDA and LVDC were mounted in the rocket's Instrument Unit, a ring between the rocket stages and the payload, the Apollo spacecraft. The Instrument Unit contained the guidance and control systems for the Saturn V rocket as well as extensive telemetry systems sending hundreds of parameters to the ground.

    The Saturn V Instrument Unit under construction. The LVDC (Launch Vehicle Digital Computer) and LVDA (Launch Vehicle Data Adapter) are silver boxes. For scale, note the engineer sitting on the left. Photo from NASA.

    The Saturn V Instrument Unit under construction. The LVDC (Launch Vehicle Digital Computer) and LVDA (Launch Vehicle Data Adapter) are silver boxes. For scale, note the engineer sitting on the left. Photo from NASA.

     

  2. The detailed block diagram of the LVDA below is from the IBM Study Report. (Click the image for a larger version.) This diagram shows that the LVDA has many different functions, registers, and circuits, with many connections to the LVDC (left) and the Instrument Unit (top and bottom). The board I examined is part of the "Digital Input Multiplexer", highlighted in yellow. Note the various data sources feeding into the multiplexer.

    Block diagram from IBM Study Report.

    Block diagram from IBM Study Report.

     

  3. IBM's use of diode-based AND-OR logic goes back to vacuum tube computers from the 1950s. The large 700-series computers primarily used AND-OR diode networks for their logic, with vacuum tubes for amplification instead of transistors. The photo below shows an 8-tube module. Note the large number of diodes (black components with white stripes) in the module below. I think the role of semiconductor diodes is largely ignored in the era of vacuum tube computers. The IBM 709, for instance, used 2000 vacuum tubes and 14,500 diodes in its arithmetic unit.

    Tube module from an IBM 700-series computer in the 1950s. Note the many diodes, especially in the lower left.

    Tube module from an IBM 700-series computer in the 1950s. Note the many diodes, especially in the lower left.

     

  4. One unusual feature of the LVDC's gates is that the pull-up resistor often isn't connected to the positive voltage source, as you'd expect. Instead, it is connected to a clock signal. When the clock is high, the AND gate functions normally, but when the clock is low, the AND gate is disabled. This has two benefits. First, the pull-up acts as an additional input, ANDing the clock into the result. Second, this reduces power consumption, since there is no current through the pull-up resistor when the clock is low. 

  5. Dr. Wernher von Braun wrote an interesting article about the use of ULD modules for Apollo: Tiny Computers Steer Mightiest Rockets (Popular Science, Oct 1965). 

  6. The ULD logic chips exist in a liminal space, a transition between individual components and integrated circuits. They are not arbitrary components, but neither are they logic gates with defined functions. Instead, they are sets of components that can be pieced together into gates in flexible ways. 

  7. While the ULD chips have 14 pins, the numbering doesn't match normal 14-pin integrated circuits. The top contacts are numbered 1 through 7 (left to right), and the bottom contacts are 8 through 14 (left to right). (Note that The Apollo Saturn V LVDC Project does not use the IBM numbering.) In addition, the circuit board can only use 12 of the pins because of the 12 vias at each position; contacts 4 and 11 (the middle ones) are not connected. 

  8. There is very little documentation available for the LVDC and even less for the LVDA. The Virtual AGC document library is the best source that I found. In particular, the strangely-named "Laboratory Maintenance Instructions for LVDC" volume 1 and volume 2 provide detailed explanations and schematics. The recently-uncovered "Laboratory Maintenance Instructions for LVDA" volume 1 and volume 2 provide similar detail for the LVDA. The System Description and Component Data has photos of the Instrument Unit components and brief descriptions. The Saturn V Flight Manual discusses the LVDC and LVDA at a high level. The IBM Apollo Study Report has more high-level information on the LVDC and LVDA and some nice diagrams. To get more information the LVDC and LVDA, I'll need to visit the US Space and Rocket Center in Huntsville, Alabama, but currently travel is off the table. 

  9. The latch is a circuit to store a single bit; it is a standard SR NOR latch, built by cross-coupling two NOR gates. 

  10. The schematic for the board is below. (Click for full-size.) Each box corresponds to a logic element, part of a chip. The top line "A", "I" shows the element type (AND, INVERT) while the bottom line ("A31") shows the chip position on the board. ("NU" indicates "Not Used"; the board is wired with the circuitry but the chip is not installed.) The left side of the schematic is the input buffers, while the right side is the logic.

    Schematic of the board. From Laboratory Maintenance Instructions for LVDA, Volume II, page 10-114.

    Schematic of the board. From Laboratory Maintenance Instructions for LVDA, Volume II, page 10-114.

     

  11. Most of the chips in the LVDA/LVDC have descriptive alphabetic codes such as INV (invert), DLD (delay line driver), or ED (error detector). However, the analog chips on the board have numbers instead: 321, 322, 323, and 324. It looks like instead of coming up with descriptive names for these chips, they just took the last three digits of the part number, e.g. "323" has part number "6000323". I also noticed that on the 6000322 parts, the last "2" has been retouched on the chips; I'm not sure what significance that has. 

  12. The "discretes", the binary inputs to the LVDA/LVDC, consisted of high-level signals such as "Liftoff", "S-IB Outboard Engine Out", "S-IVB Engine Manual Cutoff", or "S-IB Stage Separation". I was surprised that the hundreds of measurements throughout the rocket are ignored by the computer; it only cares about the major state transitions such as the engine stopping and a stage separating. (As well as the inertial guidance data, which was key to the computer's navigation.) 

  13. The board has nine empty positions where modules aren't installed, but these positions are wired into the circuitry. The purpose of this is that the same circuit board can be used for multiple functions based on which chips are installed. Specifically, the multiplexer used 13 boards of which 4 were identical to the one I examined, 8 had a few different chips, and 1 was entirely different. The reason for this is that the multiplexer was 14 bits wide, while the inputs were of varying widths. For instance, there were 8 Discrete Input Spares and 10 Telemetry Scanner bits. Thus, some of the boards didn't use some of the inputs and those chips could be omitted, saving a small amount of weight and cost. The diagram below shows the missing chips that can be added.13

    The circuit board with the missing chips filled in. The chip with an X could be replaced by the 321 below it. Original photo courtesy of Fran Blanche.

    The circuit board with the missing chips filled in. The chip with an X could be replaced by the 321 below it. Original photo courtesy of Fran Blanche.

    The board had two unused inputs; to use these, additional 321/322/323 chips were installed. The board also had one input wired up so it could use either a 324 input chip (as in the board I examined) or a 321 input chip. The 321 chip was used for a discrete input that used standard 28-volt signaling, while the 324 chip was used for a signal that was either grounded or floating. The 324 chip included a diode and pull-up resistors. By putting the necessary chip in the appropriate spot, the same PCB could be used for either type of input.

    Two of the boards included an extra logic gate separate from the multiplexer (the INV and AA chips). These gates generated the signals to switch the command input between the RCA-110 mainframe when on the ground, and the radio command decoder after liftoff. In other words, when the umbilical cable pulled out of the Instrument Unit during launch, the signal ("ICS") from the ground computer was lost. Through these two gates, the multiplexer switched the command input from the ground computer to the command decoder, enabling radio commands for the LVDC. 

  14. The RCA-110A computer that communicated with the rocket was in the mobile launch platform, complete with card reader, keypunch, and line printer. In other words, they were moving a whole computer room on the crawler out to the launch pad, with the rocket mounted on top. (In the photo below, the computer room is at the front left of the blue launch platform, under the launcher-umbilical tower.) It communicated with a second RCA-110A computer in the firing room. For details on the mobile launcher and swing arms, see Apollo Maniacs or the book Rocket Ranch. To summarize the wiring, cables went from the RCA-110A computer room near the rocket nozzles, up the tower and across swing arm 7, through the umbilical panel, and to the LVDA. One bit of these signals went to the multiplexer board I examined.

    Apollo 11 Saturn V on the mobile platform, July 1, 1969. Swing arm #7 (marked with arrow) is connected to the Instrument Unit and the top of the S-IVB stage. Photo from NASA.

    Apollo 11 Saturn V on the mobile platform, July 1, 1969. Swing arm #7 (marked with arrow) is connected to the Instrument Unit and the top of the S-IVB stage. Photo from NASA.

     

  15. IBM's 4 Pi series aerospace computers in the 1960s used the same mechanical board structure as the LVDC, with two multi-layer boards mounted on a "page" mounted in a metal frame. The 4 Pi boards were also double-width or triple-wide compared to the LVDC boards, using two or three of the same 98-pin connections. (Compare the board below with the board that I examined.) The circuitry was entirely different though; the 4 Pi boards used flat-pack TTL integrated circuits instead of ULD modules. The 4 Pi architectures and instruction sets were also entirely different from the LVDC. These early 4 Pi systems were used in aircraft such as the A-7E, F-111 and space missions such as Skylab. The 4 Pi series led to the AP-101 computer used on the Space Shuttle.

    An IBM 4 Pi page. From Technical Description of IBM System 4 Pi Computers (1967).

     

Repairing a vintage 40-kilovolt xenon lamp igniter

What do xenon lamps and the invention of radio have in common? The box below is a 1960s German high voltage unit that CuriousMarc obtained as part of an auction. After some research, we determined that it is an Osram1 igniter2, which generates a 40-kilovolt pulse3 to ignite a xenon arc lamp. The unit didn't work, so I opened it up, figured out its circuitry, and fixed it, so we could generate some sparks. The circuit turned out to be very similar to a Tesla coil, although the sparks are much smaller.

The igniter, producing a nice 40 kV spark.

The igniter, producing a nice 40 kV spark.

A xenon arc lamp generates light by producing a high-temperature plasma of ionized xenon between two electrodes. It produces bright white light that has a spectrum similar to daylight and is useful for movie projectors, searchlights, and laboratory uses. Although the lamp is powered by a low-voltage, high-current DC power supply, a high-voltage spark is required to start the arc, and that is the role of this 40 kV igniter.

Closeup of a 4 kW Osram xenon arc lamp for a movie theater. Image by Hyperlight, CC BY-SA 2.5.

Closeup of a 4 kW Osram xenon arc lamp for a movie theater. Image by Hyperlight, CC BY-SA 2.5.

I searched for information on this ignitor. The only thing I found was a 1964 paper titled A Spectrofluorophosphorimeter that described an experimental setup for measuring fluorescence and phosphorescence spectra. The experiment used a 450-W Osram xenon arc lamp, ignited by a Z2201 igniter, the same as this one. The research was done at SRI (Stanford Research Institute), just a few miles away, so there's a good chance that Marc obtained the exact unit that was used in this research.

The igniter's output is on a cone sticking out of the box. It also has five screw terminals for the 220V input, ballast, and ground. Photo courtesy of Marc Verdiell.

The igniter's output is on a cone sticking out of the box. It also has five screw terminals for the 220V input, ballast, and ground. Photo courtesy of Marc Verdiell.

We opened up the unit and I examined the unusual components inside. A large 220V to 7kV transformer is at the right of the photo below. The output transformer is the reddish flat cylinder at the back left; this transformer's output is the connection pillar on the front of the unit. In front of this transformer is a dark yellowish disk, a 1000pF 20kV capacitor. The most unusual component is the ceramic cylinder in the front.

Inside the igniter, showing the transformers, capacitors, and spark gap.

Inside the igniter, showing the transformers, capacitors, and spark gap.

I traced out the circuitry of the unit6. It is a high-voltage circuit that is also sometimes used in Tesla coils (details). The way it works is that the high voltage transformer raises the 220 V input to 7 kV. This charges the high-voltage "tank" capacitor until it has enough voltage to break down the spark gap, causing a spark across it. When the spark gap fires it conducts at low resistance. This creates a high-frequency resonant circuit between the tank capacitor and the output transformer's primary. Energy is transferred to the secondary, at a much higher voltage, producing the 40 kV output. As energy shifts back and forth between the primary and secondary, it is dissipated, until the spark gap stops conducting and the process repeats, thousands of times a second.5

Schematic of a Tesla coil circuit. This is a less popular topology for a Tesla coil, but is the circuit used in the igniter. (The igniter has an output, not a torus, of course.) Schematic from Omegatron.

Schematic of a Tesla coil circuit. This is a less popular topology for a Tesla coil, but is the circuit used in the igniter. (The igniter has an output, not a torus, of course.) Schematic from Omegatron.

So where is the spark gap in this unit? It turns out to be the ceramic cylinder. I opened up the cylinder and found a stack of eight metal disks with (maybe) carbon electrodes in the center. The disks are separated by mica washers to leave 0.33 mm gaps between each pair. This forms a series of 7 tiny spark gaps.

The spark gap disassembled, showing the stack of contact disks and mica insulators inside the ceramic tube.

The spark gap disassembled, showing the stack of contact disks and mica insulators inside the ceramic tube.

This type of spark gap is known as a "quenched spark gap". Spark gap transmitters were the first form of radio transmitter, used from 1887 to 1920. They used a spark to transmit Morse code via radio waves (details). The quenched spark gap was one type of spark gap used in these transmitters, as shown in the diagram below. By combining multiple small gaps, the quenched spark gap could cool off efficiently.

Diagram of a quenched gap, from Telegraph Office.

Diagram of a quenched gap, from Telegraph Office.

Repair

We cautiously hooked the igniter to 220V to test it, but nothing happened. I checked various parts of the circuit and everything seemed fine. In the photo below, notice the pink block at the left that looks like a Lego piece. This is a safety interlock that disconnects the 220 V input if the case is removed; the case has prongs that mesh with the interlock to close the circuit. Eventually, we figured out that the safety interlock had some loose screws that weren't making contact. This was tricky to find because when the case was open, the safety interlock was (of course) open.

Inside the igniter. The output transformer (reddish round unit) is at the top with the yellowish tank capacitor above it.
The ceramic spark gap is the cylinder in the middle. The pink Lego-link block is the safety interlock.
The HV power transformer is at the bottom (label visible).
T.

Inside the igniter. The output transformer (reddish round unit) is at the top with the yellowish tank capacitor above it. The ceramic spark gap is the cylinder in the middle. The pink Lego-link block is the safety interlock. The HV power transformer is at the bottom (label visible). T.

After tightening all the screws, the igniter worked. Since we didn't have a xenon arc lamp, we used the unit to generate sparks instead. Marc attached a strip of copper to the center output and a white wire to the ground, bending them to form a small gap. He pulsed the power switch to produce brief sparks, as seen in the video below. (Since the text on the unit indicates the unit should be powered for under 0.5 seconds, we kept the sparks brief to prevent overheating.) Although the repair was anticlimactic, at least we got some nice sparks.

Conclusion

Spark gaps generate radio waves across a wide spectrum;5 inventor David Hughes first noticed this interference in 1878. Marconi experimented with spark-gap transmitters in the 1890s, discovering how to transmit telegraph signals across short distances and then between continents. This work won Marconi the Nobel Prize for inventing radio. The CuriousMarc video below explains in more detail how the spark gap generator led to radio. Vacuum tubes made spark-gap transmitters obsolete by the 1920s, but these spark-gap circuits live on, igniting xenon arcs in modern headlights.

I announce my latest blog posts on Twitter, so follow me @kenshirriff for future articles. I also have an RSS feed.

Notes and references

  1. You might know Osram as the maker of headlights4 and other lights. The story starts with the Austrian chemist Carl Auer von Welsbach, who discovered four elements as well as inventing the gas mantle (used in Coleman lamps) and the metal flint used in lighters. He registered Osram as a trademark in 1906; the name was a combination of osmium and wolfram (tungsten), two elements he used in incandescent lamp filaments. In 1919, the Osram company was formed in Germany. 

  2. The document Osram guidelines for control gear and igniters discusses the properties of xenon arc lamps, how to power them, and the characteristics of igniters. 

  3. The front of the unit is shown below. Siemens-Schuckertweke AG is a German engineering company that I think owned Osram at the time. Under that are the warnings "Vorsicht! Hochspannung" (Danger! High voltage) and a circle labeled "In diesen Zone keine Metallteile" (No metal parts in this zone). At the center of the circled zone is a pillar with a screw terminal; this is the connection for the 40 kV output. At the bottom are connections for 220V / 50 Hz, which can be applied for a maximum of 0.5 s, as well as "zum Vorschaltgerät" (to the ballast).

    Front view of the igniter. The black text is hard to read under the brown front.

    Front view of the igniter. The black text is hard to read under the brown front.

    The label on the back of the unit (below) says ZX 501, Höchstzulässiger Lampenstrom 25 A (Maximum lamp current 25 A), Zündkreis (Ignition circuit) 220V/50Hz, Zündsp. ca. 40 kV (Ignition voltage approximately 40 kV), OSRAM - Best. - Nr. (Order number) Z2201. "

    The label on the back of the unit. Photo courtesy of Marc Verdiell.

    The label on the back of the unit. Photo courtesy of Marc Verdiell.

  4. Xenon headlights are also known as HID (high-intensity discharge) headlights. These headlights produce most of their light from an arc through vaporized metal halides, such as scandium iodide. However, it takes seconds to minutes for the light to heat up enough to vaporize these halides. During this startup time, a xenon arc provides the headlight's illumination. In other words, the xenon arc is just to provide light temporarily until the metal halides kick in. HID headlights require an igniter/ballast circuit to provide the high voltage (25 kV) for ignition and the regulated voltage (e.g. .41A, 85V) to power the light. These automotive circuits use modern switching power supply techniques and are much smaller than our igniter. 

  5. We measured the output from the igniter and found that it produces 2000-4000 very short spikes a second. The spikes decay very rapidly so they are about 1µs long, and are random noise in the tens of megahertz. This random noise has a very wide bandwidth showing that spark gap generators produce radio noise across a wide spectrum.

    Oscilloscope trace pickingup electrical noise from the igniter over the air. Image from CuriousMarc's video.

    Oscilloscope trace pickingup electrical noise from the igniter over the air. Image from CuriousMarc's video.

     

  6. I traced out the circuitry of the unit and made the rough schematic below. The unlabeled rectangle is the ceramic spark gap cylinder. The circuit is essentially the same as the Tesla coil schematic earlier, except there are two capacitors and an external ballast resistor on the output side to limit current. (We did not use a ballast resistor, but shorted the two connections.)

    Schematic of the spark generator.

    Schematic of the spark generator.

     

Inside a Titan missile guidance computer

I've been studying the guidance computer from a Titan II nuclear missile. This compact computer was used in the 1970s to guide a Titan II nuclear missile towards its target or send a Titan IIIC rocket into the proper orbit. The computer worked in conjunction with an Inertial Measurement Unit (IMU), a system of gyroscopes and accelerometers that tracked the rocket's position and velocity.1

The guidance computer, from Steve Jurvetson's collection.
Multiple connectors on top link the computer to the IMU and the rest of the rocket. The cover panels are protected by anti-tamper stickers so I probably voided the warranty by opening it.
(Click any photo for a larger image.)

The guidance computer, from Steve Jurvetson's collection. Multiple connectors on top link the computer to the IMU and the rest of the rocket. The cover panels are protected by anti-tamper stickers so I probably voided the warranty by opening it. (Click any photo for a larger image.)

This computer, called the Magic 352, is a 20"×16"×9" black box2 weighing 80 pounds, surprisingly heavy for something used in a rocket.4 Its sturdy aluminum case alone weighs 20 pounds. Internally, the computer is divided into thirds. The front section holds the processor and the core memory storage. There is no microprocessor in this computer; the processor is built from hundreds of simple integrated circuits. The back section of the computer holds the interface boards, mostly analog circuitry to connect to the rest of the rocket.5 Unexpectedly, the middle section is mostly empty space.6 The computer was made by Delco, a division of General Motors3 that built a whole line of "Magic" aerospace computers.

The digital side

The computer's front cover is held on by 18 screws. Removing them reveals the computer's processor boards and core memory. On the left are seven circuit boards with TTL digital logic. In the middle are two core memory modules, each holding 8192 words of 24 bits. Two memory electronics boards are next to the memory. At the right is the computer's switching power supply.

The front side of the computer, showing the circuit boards, core memory modules, and the power supply. The boards are identified with the code that is printed on each board.

The front side of the computer, showing the circuit boards, core memory modules, and the power supply. The boards are identified with the code that is printed on each board.

The circuit boards have alphanumeric codes on them; PR1 through PR6 are probably processor boards 1 through 6. It's unclear what "IOC" stands for; the IOC board looks like the other digital logic boards, but also has a circuit that's probably the computer's clock. The "ME" and "CME" boards appear to have high-current driver circuitry for the core memory modules, so "ME" could be "memory electronics".

Information on the Magic 352 computer is hard to obtain7 but it uses 24-bit words (plus a parity bit), and it uses 2's complement fixed point. It has 57 instructions (probably two per word) and can do an add/subtract in 6 microseconds. The processor has six index registers.

The photo below shows one of the digital logic boards; the other digital boards are similar. Each board has integrated circuits on both sides, so the back looks about the same. (My photo album of all the boards is here.) Each side of the board has space for 5 rows of 13 chips, for up to 130 chips per board. The printed circuit board appears to have six layers; two wiring layers and a ground plane for the chips on each side. Connections between the two sides are done through the 99 connections at the top of the board rather than vias. The boards are covered with conformal coating to protect the circuitry; decades later, the coating still smells strongly of turpentine. The edges of the boards are metalized and slide tightly into card guides, providing a path for heat to escape since there is no fan. The digital boards have a 198-pin connector at the bottom that plugs into the backplane, while the interface boards (discussed later) have a smaller 128-pin connector.

Processor board PR1.

Processor board PR1.

The boards are filled with TTL chips, probably MSI (medium-scale integration) chips such as counters, adders, or shift registers. Note that this computer does not contain a microprocessor chip, but has a processor built from simple building blocks. (In the 1970s, minicomputers were commonly built from boards of TTL chips.) From the part numbers on the chips, they appear to be manufactured by Signetics, in a CC2100 series. Unfortunately, even after extensive searching I couldn't find any documentation on these part numbers. (Please let me know if you have information on them.)

Some of the chips used by the computer.  The PCB traces are visible in between the chips.  The 7802 date code indicates they were manufactured the second week of 1978.

Some of the chips used by the computer. The PCB traces are visible in between the chips. The 7802 date code indicates they were manufactured the second week of 1978.

One interesting feature of the boards is they are keyed to ensure that a board can't be plugged into the wrong slot. The keying is implemented by splitting a hex nut in half. The circuit board and the backplane connector have matching halves, so the board can only be inserted into the right slot. There are six ways to split a hex nut corner-to-corner, and two hex nuts (one on the top and one on the bottom), making 36 possible keying combinations. The photo below shows part of the backplane with the boards removed so the connectors and half hex nuts are visible. Note that each connector has hex nuts at a different angle for the keying.

The half hex nuts fixed to the top and bottom of each connector are used to ensure each board is plugged into the right slot. Also note the cable of white and colored wires connecting the backplane to the external connectors on top of the computer. These slots are on the interface side of the computer.

The half hex nuts fixed to the top and bottom of each connector are used to ensure each board is plugged into the right slot. Also note the cable of white and colored wires connecting the backplane to the external connectors on top of the computer. These slots are on the interface side of the computer.

Core memory8

This computer uses magnetic core memory for storage (in contrast to the earlier Titan ASC-15 computer, which used a rotating magnetic drum). Core memory was the dominant form of computer storage from the 1950s until it was replaced by semiconductor memory chips in the 1970s. Core memory was built from thousands of tiny ferrite rings called cores, with one bit stored in each core. A core was magnetized either clockwise or counterclockwise to store a value. Cores were arranged in a grid called a core plane; energizing a specific row wire and column wire selected the particular core where the two wires crossed.

The photo below shows a closeup of the tiny magnetic cores in the Titan computer. There are four wires through each core: the vertical and horizontal red wires form the grid to select a core. Two colorful horizontal wires pass through each core in the plane: the sense line (used for reading) and the inhibit line (used for writing). You can see these wires looping from row to row at the right.

Closeup of the cores in a core plane. The cores appear glossy because they are covered in conformal coating.

Closeup of the cores in a core plane. The cores appear glossy because they are covered in conformal coating.

In a core memory, multiple planes are stacked together, one plane for each bit in a word. In most computers, the core planes were welded or soldered together into a block, but the Titan computer's core memory was built with an unusual patented technique: the cores and the circuitry were mounted on a long flexible printed circuit board that was folded accordion-style. This construction technique allows a core memory module to be opened like a book to access the cores and circuitry.

The core module unfolds like a book. The circuitry and core planes are on a flexible printed circuit board that is folded accordion-style and wrapped around metal carriers.

The core module unfolds like a book. The circuitry and core planes are on a flexible printed circuit board that is folded accordion-style and wrapped around metal carriers.

If you view the core memory module as a book, each "page" is constructed from a metal plate with the flexible printed circuit board wrapped over both sides. There are 6 of these "pages", so there are 12 core memory planes similar to the one below. Careful counting shows there are 128 horizontal wires and 128 vertical wires through the core plane, so there are 16,384 cores below. The 128 vertical wires are visible at the top and bottom, running loosely from plane to plane. Note that these are the delicate wires through the cores, passing continuously and unprotected through the entire set of core planes. The 128 horizontal core wires are gathered into bundles to run from plane to plane; the left bundle proceeds downward, and the right bundle proceeds upward.

One plane in the core memory has 16,384 cores. It consists of eight smaller regions ("mats"); each mat has 32×64 cores.

One plane in the core memory has 16,384 cores. It consists of eight smaller regions ("mats"); each mat has 32×64 cores.

To the right of the cores (above) is the circuitry to handle that plane. This circuitry includes sense amplifiers to read the signals from the core plane, and inhibit drivers for writing data to the plane. These integrated circuits are mounted on the same flexible PCB as the core planes.

The flexible printed circuit board is attached to standard rigid printed circuit boards at both ends; these boards form the outside of the module. The end boards also have connectors that plug into the backplane, providing the connection between the core modules and the computer. The photo below shows one of the end boards. Note that this board has just half the cores of a normal board.9 The reason is that this board holds the parity bit, while the other 12 planes each hold two bits. Thus, the complete module holds words of 24 bits plus one parity bit, with 8192 words in the module. The computer has two core modules, so it holds a total of 16K words.10

This board at the end of the core module has half of the regular core plane. Note the numerous connections to the left of the core; the 128 horizontal wires are connected to the circuit board here. The packages at the far left each hold 8 diodes.

This board at the end of the core module has half of the regular core plane. Note the numerous connections to the left of the core; the 128 horizontal wires are connected to the circuit board here. The packages at the far left each hold 8 diodes.

The interface circuitry

Turning the computer around reveals the circuit boards behind the back panel. These interface boards are wired to the connectors on top of the computer. Through these interfaces, the computer receives velocity and attitude pulses from the inertial measurement unit (IMU). The computer sends analog control signals to various actuators, as well as discrete (binary) signals to other parts of the rocket for thrusters, staging, and other functions. On the left is the power supply. The power supply receives power from the rocket through the connector on top of the computer and the cable to the power supply.

Cards in the back of the computer provide interfaces between the computer and external components. Each card has a three-letter code on it, but the meanings are unknown. The cables between the backplane and the connectors on top of the computer are behind the indicated supports.

Cards in the back of the computer provide interfaces between the computer and external components. Each card has a three-letter code on it, but the meanings are unknown. The cables between the backplane and the connectors on top of the computer are behind the indicated supports.

In contrast to the digital boards, which all appear similar, the interface boards have a wide variety of circuits. The CTL, MUI, and ADL boards are covered in TTL chips, similar to the boards in the digital section. The rest of the interface boards, however, are crammed with analog components such as transistors, capacitors, resistors, diodes, and hybrid modules, along with a few TTL chips. The interface boards have the analog components on the front only (probably because there isn't enough clearance on the back) and usually a few TTL integrated circuits on the back. I traced out some of the circuitry on the "AGO" board below and found 18 current-controlled outputs connected to TTL interface chips in the middle of the board. This board probably provides binary "discrete" outputs.

The AGO interface board; the "AGO" label is at the top left.
Note the different keying on the half-nuts on either side of the connector.

The AGO interface board; the "AGO" label is at the top left. Note the different keying on the half-nuts on either side of the connector.

The VMX board below has four mysterious 6-pin black hybrid modules along with numerous large capacitors. It's unclear what function this board has, or why it needs so many capacitors.

The VMX interface board. Like the other boards, it is covered with a thick conformal coating.  The connector at the bottom is much narrower than the connectors on the digital boards.

The VMX interface board. Like the other boards, it is covered with a thick conformal coating. The connector at the bottom is much narrower than the connectors on the digital boards.

The CON board uses hybrid modules including a large red "Angstrohm" module that has hand-lettered labeling on it.

The "Angstrohm" module has 11 numbered pins, 3 "Z" pins, and a "BAE" pin.

The "Angstrohm" module has 11 numbered pins, 3 "Z" pins, and a "BAE" pin.

Power supply

The computer uses a switching power supply to efficiently convert the missile's power (probably 28 volts) to the voltages required by the computer. The power supply is surprisingly heavy, about 15 pounds. Much of the weight is probably metal needed to dissipate heat since there is no fan.

The switching power supply used by the computer. The two cable connectors provide power to the digital and interface sides of the computer. The power supply receives electricity through the connector on the front.

The switching power supply used by the computer. The two cable connectors provide power to the digital and interface sides of the computer. The power supply receives electricity through the connector on the front.

Inside, the power supply is packed with inductors and transformers, power transistors, and circuit boards. A stack of filter capacitors in large metal cans is visible at the left in the photo below. The inductors and transformers don't look like the inductors in commercial power supplies, but are black blocks.

The switching power supply used by the computer.

The switching power supply used by the computer.

Several circuit boards control the power supply. They use metal-can integrated circuits, unlike the integrated circuits in commercial power supplies. The part numbers on these integrated circuits didn't turn up anything useful so they may be custom military parts. The boards are covered with a conformal coating to protect them against humidity and other threats. The conformal coating gives a shiny golden color to the integrated circuits.

Closeup of a board in the power supply.

Closeup of a board in the power supply.

The power supply probably generates 5 volts for the TTL chips, along with a higher voltage to drive the core memory, and multiple voltages for the interface circuits.

History and background

In this section, I summarize the complex history of the Titan missile and rocket, and its various guidance computers. The Titan missile, deployed from 1959 to 1987 was the largest ICBM deployed by the United States and delivered a 9 megaton nuclear bomb. To get a sense of how large the Titan was, the currently-deployed Minuteman missile weighs a third as much and its warhead has 1/25 the yield.

Test launch of a Titan II from a silo. U.S. Air Force photo.

Test launch of a Titan II from a silo. U.S. Air Force photo.

For much of its life, the Titan II's guidance computer was the IBM ASC-15 (Advance System Controller), dating to 1962. This was a 27-bit serial, transistor-based computer using discrete components in welded encapsulated modules. For storage, it used a rotating magnetic drum that held 3,840 words. This computer was used on the Titan II and Titan III, as well as the early Saturn I flights.11

The ASC-15 computer. It was emerald green in color. Photo from IBM Corporate Archives, via Saturn I Guidance and Control Systems.

The ASC-15 computer. It was emerald green in color. Photo from IBM Corporate Archives, via Saturn I Guidance and Control Systems.

Around 1964, the Titan II missile was modified for use as a satellite launcher called the Titan III. The most visible change was the addition of two solid rocket boosters for many Titan III launches. The first Titan III flights continued to use the ASC-15 guidance computer, but the project switched to the Univac 1824M Digital Flight Control System. This computer was more powerful and able to handle flight control as well as guidance and navigation. It first flew on Titan IIIC on Feb 9, 1969. However, the Univac 1824 project ended in 1969 due to cost and schedule over-runs.

Titan IIIC launch with an unmanned Gemini capsule, as part of the MOL project (1966).  Photo from NASA.

Titan IIIC launch with an unmanned Gemini capsule, as part of the MOL project (1966). Photo from NASA.

Meanwhile, the AC Spark Plug division of General Motors developed the Magic family of computers for airborne guidance starting in 1962; I wrote a detailed article on the Magic computers. Delco used some of these computers in an inertial measurement unit (IMU) guidance system called the Delco Carousel.12 The Carousel IV was a popular navigation system, used on commercial planes including the 747, 707, and DC-8. The Carousel IV used the Magic 311 computer (1967) and then the Magic 351 computer (1970).

The Carousel IV navigation system (with the Magic 351 computer) was turned into a military navigation system called the Carousel V, using the Magic 352 missile guidance computer (MGC). (This is the computer I examined in this blog post.) For space use, this system became the Universal Space Guidance System (USGS). The Titan IIIC rocket switched from the Univac computer to the USGS, first flying with it on December 13, 1973 (details). After its use on the Titan III, the USGS system was retrofitted onto the Titan II missile, replacing the obsolete ASC-15 (details) in a project called RIVET HAWK (1975-1976).

To summarize, the Titan program used several different computers as techology advanced, ending up with the computer I examined in the 1970s.

Conclusion

Aerospace computers are mostly ignored in computer histories, even though they used a lot of innovative technologies. This Titan missile, for instance, computer used flexible PCBs in its core memories. It also had surface-mounted integrated circuits, years before they were common in commercial electronics. Building computers out of TTL chips became a technological dead end, however, as the capabilities of CMOS integrated circuits increased exponentially, following Moore's law.

You can see photos of the full set of boards here; the interface boards are worth examining due to their varied circuitry. I announce my latest blog posts on Twitter, so follow me @kenshirriff for future articles. I also have an RSS feed. Thanks to Steve Jurvetson. for supplying the computer.

Notes and references

  1. Guidance systems use a variety of algorithms, with earlier low-power computers using simple guidance algorithms, while later computers used more complex algorithms that provided increased accuracy and flexibility. The Titan II used "delta" guidance, a simple guidance algorithm for low-power computers. In this guidance system, the algorithm attempts to keep the missile on a pre-computed path, using a third-order polynomial to steer back to the correct path.

    The Titan IIIC required complex guidance software since the flight went through multiple stages. A typical Titan IIIC mission put a satellite into a geosynchronous orbit at an altitude of 19,323 nautical miles. To do this, the rocket launched and ascended to a parking orbit between 80 and 235 nautical miles, using Stage 0 (the boosters), Stage 1, and Stage 2. The rocket then used Stage 3 to move to an elliptical transfer orbit with an apogee of 19,323 nautical miles. Another rocket burn put the vehicle into a circular orbit at this altitude. Finally, the payload separated from the rocket, putting the satellite into geosynchronous orbit. The point is that the guidance computer needed to perform many different guidance tasks, as well as controlling the various rocket stages.

    The overall Titan IIIC guidance algorithm is called "explicit" guidance, where an explicit solution is computed during flight to reach the desired end result. (I haven't been able to determine if the Titan II switched to this guidance algorithm when the computer was upgraded.)

    For an overview of guidance algorithms, see this document (p225) as well as Titan IIIC Guidance. For a more humorous explanation, see "The Missile Knows Where It Is At All Times." 

  2. For more information on the physical characteristics of the Magic 352 computer, see Space Tug Equipment Data Bank page 58. 

  3. It's difficult to sort out the permutations of Delco, AC Spark Plug, AC Electronics, AC Delco, and so forth. AC Spark Plug started in 1908 and became a division of General Motors in 1927. It was named after Albert Champion who also started Champion spark plugs. AC Spark Plug's Milwaukee manufacturing facility became AC Electronics in 1965, with a focus on inertial navigation (details). Meanwhile, Dayton Engineering Laboratories (Delco) was founded in 1909, and acquired by General Motors in 1918. GM's defense systems laboratory was started in 1962 and merged into Delco Systems Operations in Goleta (where this Titan guidance computer was built). In 1970, the Delco Radio Division and AC Electronics Division of General Motors Corporation were consolidated into a new Delco Electronics Division. In 1985, GM purchased Hughes Aircraft and merged it with Delco to form Hughes Electronics, which was sold to Raytheon in 1997. 

  4. The photo below shows the label on the computer, serial number 69. The "CP-1331/DJW" designation is a military component designator. The "CP" indicates a computer unit and 1331 is the model number. The "DJW" is an "AN System" military designation for a guidance system, specifically "Missile/Drone Electromechanical Flight Control Equipment".

    The label from the Titan missile guidance computer.

    The label from the Titan missile guidance computer.

    The computer also has a repair label showing it was last repaired on March 14, 1986.

    The repair label on the computer.

    The repair label on the computer.

    Each removable panel was protected with tamper-proof seals:

    The sticker says "DO NOT BREAK SEAL". I broke the seals.

    The sticker says "DO NOT BREAK SEAL". I broke the seals.

    The computer also had an attached service tag. The penalty for removing the tag is up to a year in prison, so it's worse than a mattress tag.

    Serviceable Tag—Materiel.

    Serviceable Tag—Materiel.

     

  5. At the back left of the computer is a fill valve, used to pressurize the computer with nitrogen to 5 PSI above ambient. The valve appears to be a Schrader valve, the same as on an automobile tire. Before opening the computer, I vented the nitrogen and found that the computer was still pressurized decades later. 

  6. The underside of the computer has an access panel for the cables in the central section. The photo below shows the view looking up through this access panel, showing the connectors on top of the computer, as well as the cables attached to them. This part of the computer is almost entirely empty space. The backplane for the interface side of the computer is visible in the bottom of the photo; the boards plug into the other side.

    View into the central part of the computer showing the cabling.

    View into the central part of the computer showing the cabling.

    Most of the connectors on top of the computer are 61-pin circular MIL-Spec connectors. Note the keying pins sticking out of the circular shell below. Each connector has different keying to prevent attaching a cable to the wrong connector. The power input uses a 31-pin connector with larger pins that support higher current.

    One of the connectors on the computer, labeled "J5".

    One of the connectors on the computer, labeled "J5".

    Most of the connectors currently have yellow plastic caps, while two have metal screw caps. I think that the metal caps are for test connectors that would remain covered in flight, while the plastic caps are temporary covers for connectors that would be cabled up in flight. The test connectors are wired to the digital side of the computer. 

  7. I couldn't find many details on the Magic 352 computer, but there is some information in Guidance and controls for an Interim Upper Stage (IUS) page 339, and Titan IIIC Guidance page 15. 

  8. I'm a fan of core memory and have written about the core memory in the Saturn V LVDC, the Apollo Guidance Computer, the IBM 1401, and the IBM System/360, if you want to read more about core memory. 

  9. The wiring topology of the core memory module is worth noting. Because the parity end board has half of a regular core plane, it has 64 Y wires instead of 128. These 64 wires pass through the cores and then do a U-turn, returning to the next plane as the other half of the 128 wires. The 128 X wires, on the other hand, pass through the cores and then are terminated on the board. The board at the other end terminates the 128 Y wires (as two logical groups of 64) and the other end of the 128 X wires. Both boards have numerous diode packages for these wires. 

  10. I calculated that the computer's two core memory modules hold a total of 16K words of 24 bits plus parity. This matches the Magic 352 memory size specified in this article. However, another document says the Titan IIIC computer has 16K of memory with 2K erasable (it's unclear if these numbers are bytes or words). There's a patent related to the Titan computer describing a core memory that combines DRO (destructive read out, i.e. RAM) and NDRO (non-destructive read out, i.e. ROM). The ROM is implemented by omitting cores to store 0 bits. I believe the ROM was an optional feature, so you could get 14K of ROM and 2K of RAM, for instance. 

  11. The Gemini space flights (1964-1966) used a Titan II GLV missile, but the guidance system was entirely different. Gemini removed the Titan II inertial guidance and replaced it with a General Electric Mod IIIG radio guidance system, for guidance from the ground (details). The Gemini capsule contained the Gemini Guidance Computer (OBC), built by IBM. 

  12. The Carousel IMU got its name because the inertial platform rotated at 1 RPM (like a carousel) to reduce drift errors (details). Here is a photo of a commercial Delco Carousel. The Titan computer was connected to an IMU that was probably similar inside, but packaged in a black box that resembled the computer but more cubical.