First-Hand:Description of the Error Angle Counter module

Submitted by Mark Birnbaum

I worked on the Apollo project at the MIT C. Stark Draper lab in the 60’s.

I designed a module used (in 3 places) in the “on-board” guidance system the lab was designing for the Apollo Spacecraft Command module.

When the project finished, they gave us mementos of our contributions – mine was a walnut board with a complete early or test module attached. It sat on a bookshelf in my den.

In 2008 I donated it to the San Diego Air & Space Museum, along with a several page simplified explanation of its purpose, and those of the on-board guidance system, and how they were made. (They also have a whole capsule there…)

If you think this would be appropriate (or not) for your anecdote project, please let me know, with any suggestions you have, re key elements, expectations, etc.

Below is a copy of the content of the artifact description.

Error Angle Counter module, which was part of Electronic Coupling Data Unit (ECDU) prototype, which was part of the Apollo Guidance System used in the Command Module and Lunar Excursion Module.

Apollo Program 1963-1967

Designed at Massachusetts Institute of Technology Instrumentation Laboratory by Mark D. Birnbaum

What is it?

An early model of a piece of the guidance system that steered the Apollo astronauts to the moon, the landing, and the return to earth. It was part of the Electronic Coupling Data Unit (ECDU or CDU), which worked with the Apollo Computer to steer the spacecraft.

What was the Guidance System used For?

The astronauts steered the Apollo spacecraft using position data radioed from earth, an on-board telescope and sextant that measured angles between the earth, sun, moon, and stars.

These angles were measured by precision instruments (resolvers) and converted into digital numbers (by the Coupling Data Unit) that the on-board computer could use.

The spacecraft could be pointed in any direction by a series of small compressed gas jets controlled by the computer through the Coupling Data Unit (CDU). This positioning is called “attitude control” and was used to steer the spacecraft toward the moon or the earth, and control the critical attitude during re-entry into the earth’s atmosphere.

What is the Module Made of?

The module frame (green) was made from a solid piece of magnesium (a strong, lightweight metal), and some 120 microchips in little gold plated packages called “flat-packs”. The flatpacks each contain a silicon microchip with two electronic circuits called 3-input NOR gates.

(All the logic in the CDU and the Apollo computer were made with only one type of gate, to ensure manufacturing reliability. Modern computers use many kinds of gates.)

Fine gold wires connect the chip contacts to the flatpack’s outside wires (leads). The leads were then welded (for reliability) to a thin stack of flat nickel wires separated by insulation under the flatpacks. (Similar to the printed circuit boards used today to interconnect circuit elements). The flat wires interconnect the flatpack circuits and also make connections to the 72 module pins at the bottom of the CDU module.

Some 22 similar modules plugged into a a large magnesium “backplane” full of connectors. Reliable twisted wire contacts (“wire-wrap”) made interconnections between the module connectors. A foam insulation filled the wired backplane. Two trays bolted together back-to-back (like a clamshell) to form the whole CDU.

The flat packs and the trays were hermetically sealed (airtight) so no gas or moisture could get in. Cables connected the CDU box to the Computer box, and to the instruments.

The Apollo computer and the CDU) were both made from these primitive gates. (In contrast, a modern laptop computer contains millions of gates in a package about 1.5 inches square.)

Continuous and careful attention to reliability led to the discovery and prevention of many problems. Aerospace companies flight-screened components lot by lot. Post-production hardware tests included vibration, shock, acceleration, temperature, vacuum, humidity, salt fog, and electronic noise. The Apollo program was responsible for many of the microchip manufacturing procedures and tests still in use today.

More about the Guidance System

The Apollo spacecraft had three major parts. The cone-shaped 3-man Command Module (CM) held the three astronauts and all their life support equipment. The Lunar Excursion Module (LEM) was designed to land on the moon, and looked like a giant bug. These two modules were connected by the tubular Service Module (SM) which held a rocket motor.

The flight to the moon and back had several phases.

The Apollo modules had to get off earth and into an orbit around the earth.

The Apollo flights were launched into earth orbit by a giant Saturn rocket. The Apollo assembly (CM, SM, LEM) then separated from the Saturn booster (which then fell to earth).

The earth was moving around the sun, and the moon and Apollo were moving around the earth at different speeds and distances. In order to intercept the moon a few days later, they had to fire the SM rocket at precise times to drive the spacecraft at the proper speed and direction.

Once close to the moon, they went into orbit around it. Two astronauts crawled into the LEM, separated from the CM/SM (with one astronaut left on board), and fired small LEM rockets to descend to the moon’s surface. After some exploration, they took off from the moon in the LEM and returned to moon orbit. Again, with precise timing of the rocket/jets fire they linked up again with the CM/SM, and got back in the CM.

Leaving the LEM behind, the CM/SM next set out to return earth. After reaching earth orbit, the CM disconnected from the SM. With precise timing and positioning the CM slowed itself with the heat shield end towards the earth, leaving earth orbit. After a fiery entry into the atmosphere, the CM popped out a parachute and landed in the ocean, hopefully near some waiting ships! Everyone breathed a sigh of relief.

The CM and LEM guidance systems used the same components and performed very well – they allowed midcourse corrections to be made with radio data from flight control on earth, and other data from an on-board inertial platform, telescope, and sextant. It had manual backup for some functions as well.

The cone-shaped Command Module (CM) was not aerodynamically stable, and the thick heat shield had to be kept pointed just right to avoid being burned up like a meteor during re-entry. One or two degrees off, and the CM would either burn up or skip away from earth’s atmosphere like a stone skipping on water!

Since space is a vacuum, the interior of the craft could not use fans to blow hot air away. So the attitude control kept one side of the spacecraft always kept facing away from the sun. The heat could be radiated out the dark side, which was way below zero. All the electronics heat was conducted to this outside radiator.

More about the CDU

The spacecraft measured its position with respect to a stable inertial platform (similar to the ship and submarine navigation systems from which it evolved).

High precision coarse and fine resolvers (angular transformers) sensed the angles within a few seconds of arc. (The fine resolution was about 13 bits – one part in 8092).

The CDU used an unusual fast response, high resolution continuous tracking analog-to-digital (A-D) converter to transform the low level alternating current angular sensor information into digital numbers for the computer. Part of the algorithm to control the stable platform involved computing an error angle, hence the name of this module.

The CDU involved many common mixed (digital and analog) signal system problem such as mixing low level AC signals with high level digital switching noise. Two grounding systems were necessary – a digital ground and an analog ground, connected only at one place.

Interestingly, the IBM designed Saturn rocket guidance computer system used triple redundancy to achieve reliability. The MIT designed computer system instead used exhaustive testing of a single universal component (the 3-input NOR gate) to achieve reliability with low weight and power. (Most of the power weight and physical space budget was alloted to the astronaut life support).

Most guidance system parts were generally NOT new cutting edge technology, but instead technologies with well known, non-catastrophic failure modes. Most operations had manual or radio communication backup methods.

Back to Oral Histories, Topic Articles, and Milestones related to the Human Space Travel: