Technical Overview Of The Apollo Spacecraft

All of the primary spacecraft systems revolve around a few key concepts:

  • POWER – Generate electrical power and distribute it throughout the spacecraft.
  • ENVIRONMENT – Provide a livable environment for the astronauts, and maintain the integrity of the spacecraft.
  • GUIDANCE & CONTROL – Provide the spacecraft a means to know where it is in the now, where it is going, and predict and execute any maneuvers that are necessary to place the spacecraft in a specific orientation, with a specific velocity, at a specified time in the future.
  • COMMUNICATION/TELEMETRY – The spacecraft must be able to be monitored, and provide a means for the crew to talk to mission control in order to solve problems, and to support the mission (i.e. to double check the heck out of everything).
  • MONITORING – This is somewhat related to telemetry, but Apollo had some dedicated hardware that monitored the health of a number of systems to ensure they were operating within normal operating limits.

There are other lunar mission specific concepts and goals, like the ability for the LEM (Lunar Excursion Module) to dock/un-dock/land/takeoff/re-dock with the CSM (Command Service Module). First we will focus on the systems we must understand to get ourselves into earth orbit. The following is a breakdown of the major Apollo systems, their abbreviated name is included in parenthesis. NASA technical manuals, checklists, and ground to air communications often used these abbreviated (or acronym) forms to talk about these systems.

Electrical Power System (EPS)

Power is arguably the most important thing to have a lot of on a spacecraft. Without power, your astronauts are basically sitting in an over glorified tin-can. Everything on the Apollo spacecraft hinged on the safe and reliable generation of power. The way Apollo accomplished the production of power was by combining pure oxygen and hydrogen in a reaction chamber known as a power plant. The water and hydrogen would react, forming three critical things necessary for all life in space: electricity, heat, and water. The Apollo Service Module contained two oxygen tanks, two hydrogen tanks, and three fuel cells. The fuel cells each consisted of a separate set of individual power plants in series. The water produced by the EPS was supplied to the Environmental Control System (ECS) to be used for cooling as well as drinkable water for the astronauts. The EPS was also equipped with two rechargeable batteries which lasted for about two hours, and were used for re-entry, when the Command Module separated from the Service Module, leaving behind all of the power generation equipment.

The EPS supplied power to two main electrical buses, so called Main Bus A, and Main Bus B. Each bus was 28V DC, and the various other spacecraft systems were connected to these buses in a predefined way (although this was not hard-wired, the bus assignment was selectable by switches and circuit breakers). The two buses were electrically isolated from eachother to prevent damage or problems on one from effecting the other. The bus assignments balanced the electrical load of the spacecraft across the two buses.

The spacecraft also had two electrically isolated alternating current (AC) buses, named AC1 and AC2. These were run by solid state inverters which took the direct current from MNA and MNB (DC buses A and B) and converted it to AC power, which was used by any equipment that required alternating current.

The design of these elements is of interest to us. It’s commonly understood that you can improve the reliability of a system by reducing the number of things that can go wrong, which usually translates into reducing the number of parts that can fail or break. Because the generation of power was so critical, this methodology was implemented in some critical areas of the EPS.

The oxygen and hydrogen on Apollo was stored in a cryogenic state. This means they were very cold, and under high pressure. It was cold because gas contracts, or reduces its volume the colder it gets, so it was easier to compress effectively and more of it could be stored in a smaller container at safe pressures. Because the gas was under pressure, it naturally wanted to escape, and therefore there were no needs for pumps or complicated mechanisms to move the gas around. It naturally wanted to flow through the piping it was connected to. Gas under pressure added an extra level of reliability, since no moving parts would be involved in moving the gas from point A to point B.

This doesn’t mean that the tanks had no moving parts in them. The amount of gas in the tank needed to be measurable. Also when gas is stored in a cryogenic state it tends to stratify which you can basically think of as collecting on the bottom in a pseudo solid/liquid form. This means that it’s hard to measure the amount of remaining gas in the tank, and also can compromise the flow of gas out of the tank. The solution to this was the addition of a probe down the center of the tank for measuring the contents, as well as a combination thermostat/heater and a fan.

The heater and fan would function in tandem to keep the gas at a constant temperature and pressure, and were manually activated by the astronauts on the MDC (main display console) at the behest of mission control. The idea is that the fan would blow the gas around to keep it from stratifying, and as needed the tank would be heated to keep it at a constant pressure (the tank would require more and more heating as the tank was emptied). The heater could be engaged to function automatically as well.

Environmental Control Systems (ECS)

The primary function of the Apollo ECS was to provide a livable, breathable atmosphere for the astronauts. This included the cabin atmosphere, providing pressurized suit connections (which were redundant to the cabin atmosphere), hot and cold drinkable water, cabin heating, as well as what is called a water-glycol loop which was a fluid pumped around the innards of the vehicle to dissipate heat (just like a car radiator). In the event that the glycol radiators were not effective enough at cooling the system could dissipate heat by evaporating excess water-gylcol.

Guidance & Control

The guidance and control abilities of the Apollo spacecraft were provided by a number of systems working in tandem, including inertial measurement equipment, thrusters, a guidance computer, a mission programmer, and the astronauts themselves. It can take time to gain a complete understanding of how these systems interoperated.

Primary Guidance and Navigation Control System (PGNCS), which consisted of:

  • An Inertial Measurement Unit (IMU).
  • The Apollo Guidance Computer (AGC)
  • An Alignment Optical Telescope(AOT)
  • Five Coupling Display Units (CDU’s), three were part of the Inertial Subsystem (ISS), and two were part of the Optical Subsystem (OSS).
  • Flight Directors Attitude Indicator (FDAI).

The first part of the puzzle starts with what is called an inertial platform. If you know the starting orientation of the spacecraft, and can measure changes in its attitude (the direction it is pointing) then you can keep constant track of where it is pointed in space. These changes in attitude, are measured on each axis, called yaw, pitch, and roll. These terms apply to the three space axis you are familiar with in everyday life, coined X, Y, and Z. If you imagined the X axis pointing straight out of your nose, the Y axis running through your ears, and the Z axis coming out of the top of your head, then you can relate yaw pitch and roll. Looking side to side would change your heads yaw, looking up and down would change its pitch, and rotating your head side to side would change its roll.

This analogy is directly applicable to the Apollo spacecraft. When sitting strapped in your seat in the CSM, you are facing the X axis, which points out the nose/cone of the command module. The top of your head points towards the Z axis, and a line drawn through your ears would be the Y axis. If the spacecraft were to rotate, like your head in the above example, its yaw, pitch, roll would change as well.

Before liftoff the Apollo inertial platform (also called the guidance platform) was aligned (set to coincide ) to a predefined known orientation on the launch pad. This programmed what is known as a Reference To Stable Member Matrix (REFSMMAT) in the Apollo spacecraft. The meaning of this acronym is not terribly important, but it is important to remember it and know what it does. When the guidance platform in Apollo was aligned, it programmed a given orientation of the spacecraft into the inertial platforms REFSMMAT. All changes to the spacecraft orientation from that point forward would be measured as changes relative to the REFSMMAT. The absolute orientation of the spacecraft in space could be determined by knowing the REFSMMAT of the vehicle, and adding changes in orientation that it had measured.

You may ask yourself what the point of a REFSMMAT is, and why it can’t just be chosen at launch and left alone. The chosen reference orientation to be programmed into Apollo’s REFSMMAT played an important role for the astronaut because it allowed them to view changes to their attitude relative to something that was convenient and easy to understand. For example, at launch you may want to watch that your attitude stays within a certain range to make sure that you are heading in the right direction and will make it into orbit. But, when you want to fly to the moon, you also may want to make sure that you are pointed in the right direction relative to the moon. For example consider aligning your guidance platform to face directly at the moon, meaning that when you are facing it exactly, you will read all zero’s for your change in attitude. As you fly towards it, you know you are heading towards your destination simply by making sure you attitude changes are very close to zero. If you were unable to align your guidance platform to point at the moon as its basis for measurement, but instead were stuck with whatever was programmed at launch, then you would have to figure out what sets of yaw, pitch, and roll would point you at the moon relative to your launch attitude, and then, when you are burning your engines, you’d have to watch to make sure you stayed within this range. What you would be looking at would be much harder to discern and understand quickly than just making sure things were close to zero’s.

The guidance platforms ability to measure changes to its orientation was done using different kinds of gyros, and coupling display units (CDU’s). A gyro is something that spins at a high rate of speed and therefore resists changes to its direction. If you have ever played with a gyroscope before you’ll know what I mean. Some gryo’s in the Apollo spacecraft were mounted in such a way that they could freely rotate, as the spacecraft moved around them, generating electrical signals indicating how far the spacecraft had rotated in that direction. A different kind of gyro’s knows as Body Mounted Attitude Gryo’s (or BMAG’s) measured not the absolute orientation, but the rate at which the orientation was changed. This let the spacecraft directly measure its angular velocity. The IMU (inertial measurement unit) inside the spacecraft would measure the absolute orientation of the spacecraft with respect to the IMU’s inertially fixed gyro’s. The stable platform, or REFSMMAT, to my understanding, refers to the gryo’s inside the IMU. The IMU had the ability to torque its gryo’s into any orientation thereby storing a given orientation as the basis for IMU measurement (in other words its stored an orientation into its REFSMMAT).

The spacecraft also measured changes in velocity (which is measured as its speed in a given direction) using accelerometers. Change in velocity in Apollo is also known as Delta V. The iPhone, for example, has three accelerometers which measure the pull of gravity on the phone on three different axis, allowing it to calculate its orientation relative to the ground.

The Apollo Guidance Computer (AGC) performed a host of tasks to aid the astronaut in finding their way to the moon, including programming of the REFSMMAT. There are complexities involved in calculating the proper attitude and velocity needed to arrive in orbit around the moon which the Guidance Computer would assist with. The computer could also be programmed to perform precisely timed burns to speed up or slow down the spacecraft at critical junctures. Typically mission control would read up a PAD which is a bunch of vectors, orientations, and times to be programmed into the computer for execution of a burn or other maneuver.

The Coupling Display Unit’s (CDU’s) mentioned above were essentially digital to analog converters used to couple (or condition) the signals coming from the Inertial Subsystem (ISS) so they could be interfaced to the AGC, in a bidirectional manner. For example, the AGC could monitor and display attitude error at launch by simply playing back a preprogrammed perfect trajectory and driving its CDUs. The Flight Director Attitude Indicator (FDAI, more on this component in the Stabilization Control System) would display the difference between the perfect trajectory according to the AGC’s CDU’s, and the spacecrafts actual trajectory measured from the IMU. This difference represented error in the trajectory, and this was measured closely to make sure it fell within predefined tolerances or an abort would be triggered.

Reaction Control System (RCS)

The reaction control systems consisted of two separate subsystems the CM/RCS and the SM/RCS, so named because they are separate parts on the Command Module, and Service Module. The SM/RCS consisted of four RCS packages mounted on opposite sides on the spacecraft. Each RCS package contained four thrusters. These engines could be used to change the spacecrafts orientation in the three yaw, pitch, roll axis. The CM/RCS also had a set of RCS engines, but they differed from the SM/RCS engines. The CM/RCS consisted of 6 thrusters (two for each axis, yaw, pitch, roll). Typically the spacecraft would be operated to use a combination of all the RCS engines (dubbed CSM/RCS). The RCS could be controlled by a number of separate systems, like the Apollo Guidance Computer (AGC), the Stability and Control System (SCS), and manually by the astronauts.


A SM/RCS thruster package can be seen just right to the center of the image.

A couple SM/RCS thruster packages can be seen in this image (NOTE: they're not the cluster of four dish looking things by the big engine at the bottom of the craft, those are the S-BAND antennas)

Stabilization Control System (SCS)

The stabilization control system functioned to monitor, and control spacecraft orientation, and provide thrust vector control (TVC, i.e. controlling the big engine on the command module, also knows as the Service Propulsion System or SPS). The stabilization and control system could be made to operate in eight different modes, depending on what was needed. For example, one mode called Attitude Control Mode maintained a given attitude of the vehicle. The SCS operated with what is known as a deadband meaning that it would not produce thrust commands to the RCS as long as the error in attitude was less than the deadband. This is important because in reality it is impossible to absolutely eliminate drift, or produce an exactly perfect heading, not to mention any error introduced by the electronics on the spacecraft. The deadband served to eliminate oscillations and constant thruster firing of the attitude control jets (RCS) that would be caused because the SCS would always be sensing it was slightly off. The deadband operated between +-0.5 degrees and +-5 degrees.

Some of the inertial systems were considered part of the SCS, as well as most of the display systems that the astronauts used. The interesting thing to note about the SCS was that it did not function as part of the AGC, rather the AGC could use the SCS to generate commands. The AGC would tell the SCS where it wanted to point by driving its CDU’s to the proper orientation. The difference between the CDU’s and the IMU would be measured by the SCS and the appropriate RCS commands would be generated to achieve the desired orientation. (This SCS mode is known as G&N Attitude Control Mode).

The Flight Directors Attitude Indicator was a round gimbal looking thing that was used to measure attitude and other things related to spacecraft trajectory. The FDAI would display different things depending on what mode the SCS was operating in. As in the above example, at launch, the FDAI was used to monitor error in the trajectory of the spacecraft during liftoff. You’ll learn more about what the FDAI does once we get onto the launch pad.


The communications system is comprised of a bunch of different radio equipment, telemetry circuitry, and antennas. The spacecraft could transmit over VHF/AM-FM frequencies, as well as S-BAND/C-BAND radio. Typically for near earth operations, like orbit, the VHF equipment would be used. The Manned Space Flight Network (MSFN) used these different types of radio transmitters to perform ranging operations, receive telemetry uplink/downlink (the way mission control monitored spacecraft systems; the uplink ability allowed mission control to transmit commands to the guidance computer), as well as voice communications. The S-BAND equipment was used during long range communications, and was packaged up into what is called the Unified S-BAND Equipment (USBE). The spacecraft also contained various types of VHF/HF transponders to aid in locating the command module capsule once it had landed in the ocean.

The S-BAND equipment could be placed in ranging mode during long distance communication. In this mode the spacecraft responded to a specially coded interrogation signal transmitted from the MSFN, by re-broadcasting a similar signal. This allowed the MSFN to accurately range the spacecraft. The S-BAND carrier wave was used to track velocity using doppler ranging. C-BAND was used during near earth operations for tracking.

The spacecraft had limited ability for the crew to record audio messages for later recovery by NASA during post mission evaluation. The last bit of equipment that is important is called the Signal Conditioning Equipment (SCE). This was the term applied to the system of electronics that was used to condition the electrical information collected by the 20 or so classes of transducers and sensors on board the vehicle. The SCE would condition the signals and send them to be displayed to the astronauts, as well as to the communications system that would transmit the information via the telemetry downlink. Some may recall the lightning bolt that struck the Apollo 12 launch vehicle during liftoff which fried the SCE’s main power-supply. John Aaron, one of the flight controllers, recognized the pattern of bad telemetry data (due to the damaged SCE power supply) and asked that the astronauts be told to “try SCE to AUX” to switch the SCE circuitry over to its auxiliary power supply.

Caution And Warning Systems (C&WS)

The caution and warning system monitors malfunctions and out of tolerance signals from the spacecraft systems. Each detected fault will light up the appropriate condition light on the MDC, and activate the master alarm. The master alarm is a blinking red light, and an audio tone (there are three identical master alarm light switches placed at convenient locations in the command module). The crew would extinguish the alarm by pressing one of the master alarm light switches. Settings for the C&WS system are determined by a couple switches. The CSM/CM switch determined what part of the spacecraft was monitored for faults: the Command Module or both the Command Module and the Service Module (CSM). During re-entry, when the SM is jettisoned, the C&WS is set to the CM position since there is no longer a Service Module to monitor. During normal flight the switch is set to CSM. The NORMAL-BOOST-ACK switch sets the C&WS to function normally when set to NORMAL (i.e. a fault condition causes the master alarm to go off, and illuminate the responsible system status light). The BOOST setting does the same thing as NORMAL with one exception: the Master Alarm Light on one of the MDC panels will not light up (the one near the abort light) so it can’t be accidentally confused with the abort light. The ACK position functions like the NORMAL mode, except that the C&WS status lights will not illuminate (the master alarm light and tone continue to function normally). In ACK mode in order to see what system caused the alarm, the crew would press one of the master alarm light switches.

Futher Reading (for those who are interested)

Learning the technical details of the Apollo Spacecraft can be done by reading books, and searching the web, but it can be time consuming to do. NASA has published numerous technical manuals on Apollo for public consumption. Here are downloads for the first volume of the Apollo Operations Handbook (this is not required reading for this walkthrough). These can be invaluable for getting a deeper understanding of how the spacecraft systems all fit together, and will be invaluable if you ever get truly stuck.

To The Launchpad

That wraps it up for the technical overview of the Apollo spacecraft. Next we will get onto the launchpad and start putting some of this to use.

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