Overview

Getting to the Moon and home again involved a small number of broad, basic tasks:

• Propulsion: Getting to orbit in space, and changing that orbit to meet and subsequently orbit the Moon.
• Navigation: Understanding where the spacecraft was and which way it was pointed.
• Systems and Life Support: Keeping the spacecraft and astronauts alive, powered, and temperature-regulated.

While Apollo was, in aggregate, an immensely complex system comprised of countless components, miles of wiring and circuitry, and cutting-edge-for-the-time technology, we can vastly simplify our effort to understand the spacecraft by approaching it at a high level from these three fundamental problem areas. Each is covered in detail in the Overview section.

There are a few other elemental concepts to understand before diving in:

Redundancy 101

Sending humans into space is a big deal, and it's dangerous. While the basic principles and science behind many aspects of spaceflight may be independently simple, the engineering reality is that things fail. Nearly every NASA mission at this point had a major quirk or two to deal with.

The most robust way to deal with failure is to increase the safety factor by introducing redundant systems. Each Space Shuttle Main Engine, for example, had two computers with two processors each, and a system that would redelegate control if errant calculations were detected. But in the Apollo days, redundancy didn't always mean identical backups. Often, to save weight, money, or complexity, the redundant systems were actually entirely independent designs. Sometimes this could actually be an advantage, as different designs will have different strengths and weaknesses, and thus different failure points.

Both Apollo spacecraft, for example, had one primary navigational sensor: the Inertial Measurement Unit. The IMU was a big powered gyroscope that tracked orientation, much like your smartphone today can detect how it's being tilted and rotated around. The IMU gyroscope was mounted on three freely rotating gimbal axes; it's a lot like turning about while holding a compass, except instead of magnetic force it is the rotational inertia of the spinning gyro that holds the "needle" still, and instead of one axis around which to rotate there are three. This allowed it to read the spacecraft's absolute orientation relative to its static reference angle, much like you can always read your bearing relative to magnetic North on a compass. This allowed great precision and confidence, but it also created a problem state known as gimbal lock, as the three gimbals together couldn't quite follow the spacecraft across certain extreme angles.

The backups to the IMU, then, were known as Body Mounted Attitude Gyros, or BMAGs. There were two of them in the Command Module, and rather than turning about freely on gimbals, they were locked down rate gyros. This meant they could only measure the rate of change of orientation rather than measure the absolute orientation. This is a bit like telling how far you've driven down a highway by sampling your speedometer over time and doing some math: it ought to work, but tiny imprecisions in measurement can throw you off. A really long tape measure will give you an absolute answer with far more confidence, and so the IMU was the preferred instrument. But the BMAGs were lighter, and because no rotating gimbals were involved they were not susceptible to gimbal lock.

Another example is the Abort Guidance System (AGS) on board the Lunar Module. It served as the backup to the Apollo Guidance Computer and some of its associated sensor systems (collectively known as the Primary Guidance, Navigation, and Control System, or PGNCS), in case it failed during a landing. In that case, the landing would be called off and the focus moved to getting back to orbit and rendezvousing with the still-orbiting Command/Service Module for the trip home: a much simpler task. Thus, the AGS had far fewer features than the AGC—and simplicity is often a recipe for greater robustness. You'll hear discussion of using the less power-hungry AGS in lieu of the AGC for the trip around the Moon in the final stages of the audio recording.

Lastly, sometimes redundancy simply wasn't practical. The Command/Service Module had two main DC electrical buses (A and B) that circulated electricity around the spacecraft. With hundreds or thousands of total sensors and components to wire, you can imagine that sending two entire loops of wiring to every single piece of electronics was not only infeasible, it would mean a greater chance of mistakes or accidents, and a far heavier spacecraft. In some cases, this meant that some instruments or components were only available on one bus or the other. In other cases, a hybrid approach was taken: of the three Command Module inverters that converted DC to AC power, for example, the first could only pull electricity off the A bus, the second only the B bus, and the third inverter could draw from either A or B.

Orbital Mechanics 101

Much has been written about this popularly misunderstood and rather counterintuitive topic. We can avoid most of the details here, and instead focus on developing a basic-enough grasp to understand what is happening in these six hours.

The first thing to understand is that given any instantaneous velocity (speed and direction) and position of an object above Earth, we can definitively (with some handwaving) calculate its entire orbit. Somewhat simplistically, this follows from calculating basic Newtonian physics, such as the Universal Law of Gravitation, over time. This observation isn't terribly intuitive or informative on its own, but it helps us think about what happens when we change the two parameters involved: velocity and position. As an example, it is not possible to speed up or slow down without profoundly affecting the shape of one's orbit; this is commonly misunderstood and incorrectly depicted in popular films.

The second thing to understand is just how fast orbital speed is. In Earth orbit roughly 100nm (115mi/185km) above the surface, the Apollo spacecraft would typically be travelling at over 28,000km/h (~17,500mph). Most of a rocket's launch efforts are not invested in pushing the spacecraft up, but rather in hurling it sideways.

The reason this is important to understand is because one must realize how little impact maneuvering or even full engine burns have on a spacecraft's immediate location or trajectory. A typical Apollo mid-course correction burn might modify its speed by somewhere between 20km/h and 150km/h: these changes are minuscule adjustments to the incredibly fast speeds described above, and have next to no impact on the spacecraft's immediate path.

{{figure:orbits-fail}}

Instead, what a burn has far more impact on is the overall shape of the orbit. Let's try this out (figure 1). On the left is a typical Apollo orbit just after launch: the numbers described just above. The orbit and planet are to scale, while the spacecraft is blown up so you can see which way it's burning. Notice how close the orbit is to the surface of the planet, despite the spacecraft being well into space. We want to go to the moon, so let's wait until we're as close as possible, point at it, and burn for a little bit. We will increase our speed by 5,400 km/h to 32,400 km/h.

That didn't work at all. In fact, despite going faster we are now destined to crash back into Earth. This is because we forgot about our second observation, that our burns are relatively small percentages of the spacecraft's base speed. We're not going to be able to overwhelm the momentary velocity taking us parallel to the Moon by adding a small percentage of it towards the Moon.

{{figure:orbits-prograde}}

Let's try working with the speed we already have, then. We'll burn exactly the same amount, just in our direction of travel (figure 2).

That was more interesting. By burning in the same direction as our travel, we modified the opposite side of our orbit by moving it outwards significantly. Remember again: we can change the shape of our orbit by changing our speed, but most changes we can make don't really have much effect on our immediate local path. So all that change goes elsewhere in the orbit: in this case, to the opposite side.

It doesn't matter much today, but this is known as a prograde burn. Of course, if we wanted to go to the Moon, we'd want to do exactly the same thing but on the other side of Earth.

{{figure:orbits-retrograde}}

We can invert this effect by turning around so that instead our engines point in the direction of our travel, and burning. This is called a retrograde burn (figure 3).

{{figure:orbits-subtle}}

The final example (figure 4) demonstrates just how profoundly small changes in velocity can affect the orbit. From an orbit that just reaches the Moon, we'll decelerate by a mere 20m/s—not even highway car speed. It may not look like much on this extreme scale, but that small change is the difference between passing comfortably over the surface of the Earth by a margin of ~1,200,000km (~740,000mi) and smashing into the Earth while trying to pass ~270,000km (~168,000mi) below its surface.

When you listen to the astronauts worry about small details like the particular set of thrusters they are using to point the spacecraft in different directions, it is this effect that they are keenly aware of—particularly with a close approach to the Moon coming up.

There are other types of burns that do different things—we'll cover just one more. Prograde and retrograde burns modify the orbit, but only within the 2D plane in 3D space that the spacecraft is already travelling within. If that plane itself needs to change, this can be done by burning orthogonally to it. But recall again our second observation, that changing our immediate path is really difficult. Changing our plane necessarily involves changing our entire path, including our immediate path. So this is really expensive to do, fuel-wise.

These burn examples illustrate the third thing to understand, which is that much like orbits, burns can be described by two basic parameters: the point in orbit of the burn and the change in velocity (commonly referred to as ΔV). Because rocket engines do not instantaneously modify a spacecraft's speed by tens or hundreds of km/h and because it is easier to keep an eye on a clock than a position in the universe, this is more practically delivered to the astronauts and executed upon with three primary datapoints: clock time of burn, attitude of the spacecraft during the burn, and how long to burn for. (In reality, there are additional parameters for the computer's use, for checking star alignments, and other details.) These figures would be read up and written down on a pre-printed PAD form.

Our last note will be that nothing is ever so clean as presented here. In reality, any object orbiting anywhere in the Solar System is affected by a multitude of gravitational forces, the Earth is not actually spherical nor is its weight distributed evenly, and no burn is ever purely prograde or retrograde or plane change. Many, many factors go into actual orbital and burn calculation. For Apollo, these factors were all calculated on massive computers on the ground over the course of the mission.

Rocketry 101

There is one very important thing to understand about rocketry itself: the fuel is most of the weight you carry into orbit. If you want to go a bit further, you have to add some fuel. But then you also have to carry that fuel to the new, further point you wish to burn it, so you have to add even more fuel just to carry that added fuel—and so on. This game hits diminishing returns pretty quickly, and it's why the Saturn V is as incredibly gigantic as it is.

But the flip-side of this is that the spacecraft gets vastly lighter as it burns that fuel, while the engines remain just as powerful. This means that by the time the spacecraft is on its way to the Moon, for example, and has burned most of its fuel, that little bit of fuel left can cause, percentage-wise, an immense amount of change in spacecraft velocity.

This nonlinearity is why rockets and spacecraft are typically not accounted in terms of liters or gallons of fuel remaining, but rather in ΔV remaining: how much more change in velocity do we have in reserve?

It is also a big factor in the design of the Apollo architecture, as we shall cover briefly.

{{figure:mocr}}

There are many fantastic, complete references on the various flight controllers, their roles, and the people involved. The Ars Technica article is a great, free place to start; the book Go, Flight! goes much deeper into the controllers' history throughout the early years of NASA. Here, we seek to provide enough information such that the real-time audio experience may be fully appreciated.

It is important to note that the flight controllers in the main Mission Operations Control Room (MOCR) were far from the sole engineers and problem-solvers on hand during this or any other mission. There were the Staff Support Rooms (SSRs) in which internal teams backed up each flight controller, each on their own private team audio loop, as well as the Spacecraft Analysis (SPAN) room, in which engineers and other representatives from the various private contractors that actually designed and built the components of the spacecraft were on hand to offer data and analysis. In addition, there were stations all over the world, on sea and on land, which tracked the spacecraft and in doing so provided valuable information and analysis on top of the telemetry they received. Before the time of Apollo, much of this data was not networked back to Houston, and so flight controllers would actually have to be deployed to the various remote stations to directly perform their duties on-site.

Below, we review the controller positions you'll hear from most in the audio. For further information on the other equally important roles, please refer to one of the references linked above.

Flight Director (FLIGHT)

The Flight Director was the ultimate authority during an active mission, charged with ensuring, in order, crew survival and mission success. There were actually multiple teams of flight controllers during any mission, each of which was led by its own Flight Director. You will hear from two today: Gene Kranz and Glynn Lunney. As the center of the room, both figuratively and literally, each Flight Director defined their own style of leadership and set the tone of the room. The differences between Kranz's and Lunney's styles are immediately apparent in the loop audio presented here.

One of the tools Flight had in managing their relatively sizable team was a set of manually toggled status lights, one for each flight controller. In addition to their use during go/no go calls, Flight would often call for their use as a status indicator for various critical room-wide issues, such as shift changes and status updates.

Electrical, Environmental, and Consumables Manager (EECOM)

As the poor sap in charge of the electrical and cryogenic gas systems that got literally blown up during Apollo 13, EECOM is the flight controller from whom you will hear the most in the early stages of diagnosis. EECOM's purview included both electrical power and life support systems, as these systems were intertwined in their reliance upon the same cryogenic gases. These systems included gas storage and distribution, the fuel cells, batteries, alternators, and electrical buses, and the various life support and regulator valves that maintained a habitable atmosphere within the spacecraft. As implied by the name, monitoring and managing the consumption of the consumable gas (and therefore electrical) resources were also EECOM's charge.

During Kranz's shift and the initial incident, you will hear EECOM Sy Liebergot, who at one early point was described as staring at his telemetry readouts with a two-handed death grip on the handles of his display. Clint Burton takes over as part of Lunney's team—he oversees most of the power-down procedure of the CSM.

Guidance, Navigation, and Controls Systems Engineer (GNC)

GNC managed all of the go-there systems: the propulsion hardware, the RCS attitude thrusters, the propellant to be burned, and the navigation instruments and software, including the guidance computer. The explosion broke many things loose, jarred many valves open or shut, and knocked electrical buses offline, and so GNC makes frequent appearances in the first hour or so following the incident to try to find a combination of RCS thrusters that are trustworthy to put on the working electrical bus, and to shut down errant thrusters which were pushing the craft about or wasting power while failing to fire.

Said appearances are made by Bill Strable, who is then relieved by Jack Kamman with Lunney's Black team.

Guidance Officer (GUIDO)

The Guidance Officer was in charge of the spacecraft position and attitude at all times, and the appropriate use of the attitude adjustment and propulsion systems to ensure smooth operations. As an example, you will hear GUIDO crop up when the spacecraft nears gimbal lock, a condition which would invalidate all available understandings of spacecraft orientation until a lengthy recalibration could be performed. You'll also hear him talk about some of the various control modes of the spacecraft that translate crew input into thruster firings.

Bill Fenner and Ray Teague serve as White team GUIDO, while Gary Renick and Will Presley take over as part of Black team.

Flight Dynamics Officer (FDO)

Down in the spirited Trench of the room, FDO managed spacecraft trajectory, both present and into the future. Where GNC and CONTROL managed the propulsion and attitude control hardware and software systems of the CSM and LM, respectively, and GUIDO was in charge of their actual, immediate use, FDO worked one level above that, formulating and overseeing the navigation and maneuver plans required to deliver the spacecraft where it needed to go. FDO collaborated with a variety of people, both on and off campus, to determine and calculate the necessary orbital maneuvers required to achive mission success.

Bill Stoval is FDO for Kranz's White team, and Bill Boone and Maurice Kennedy take over at shift change.

Integrated Communications Officer (INCO)

INCO was in change of communications for both spacecraft. These included the telemetry systems, and the various omnidirectional and steerable antennas upon each craft. In addition to managing the on-board equipment to ensure quality of signal and successful data operations, INCO worked with NETWORK and PROCEDURES to coordinate appropriate related reconfigurations of the ground-air link from both ends of the connection.

INCO pipes up quite a bit in the audio to ask for antenna changes as the spacecraft attitude is somewhat out of control following the accident; INCO is also a key player and communicator in identifying and resolving the LM/S-IVB communications issues that occur midway through the Black team shift.

The White team INCO was Alan Glines, and the Black team INCOs were Gary Scott and Ed Fendell.

Telemetry, Electrical, and EVA Mobility Unity Officer (TELMU)

TELMU was effectively the EECOM of the Lunar Module, responsible for many of the respective systems aboard the LM. As the Lunar Module lifeboat plan becomes increasingly real and correspondingly pressing, TELMU has to oversee an emergency startup of the LM, working with the crew on board to safely power up the spacecraft in literal record time. Later, TELMU is a key player in determining a low-power configuration that can bring the crew safely home while maintaining navigation.

Kranz's TELMU was Bob Heselmeyer, and Lunney's TELMU was Merlin Merritt.

Control Officer (CONTROL)

Where TELMU is effectively the EECOM of the Lunar Module, CONTROL is effectively the GNC. In addition to overseeing the navigation and propulsion systems during powerup and the lunar-bound correction burn following the accident, the heavy debate had by Lunney's team on maintaining platform control on a low electricity budget hinged on the navigation systems overseen by CONTROL.

During Kranz's shift, CONTROL was Larry Strimple. He was relieved by Hal Loden for Lunney's shift.

Capsule Communicator (CAPCOM)

CAPCOM was, with exceedingly rare exception, the only person allowed to talk directly to the crew. This allowed him to ensure the consistency, coherency, and accuracy of instructions and information transiting to and from the crew, and to translate to and from terms and descriptions more-familiar to the astronauts. The latter effort was aided by a convention that CAPCOMs were drawn from the astronaut pool.

Jack Lousma serves as CAPCOM throughout all available audio here. He later flew aboard the Skylab 3 mission, and commanded one of the very first Space Shuttle missions, STS-3.

End of tape.

After six hours of continual crisis, Apollo 13 is now safely back on a trajectory towards Earth, with a stable configuration and no immediate dangers. The Command Module is completely dark, and the crew is preparing to bring Aquarius down to a minimal state. It would eventually transpire, after some on-board experimentation with different solutions, that because of the added cost of the inverter required to keep the navigation eight-ball running, it was cheaper in total to leave the whole PGNS system running in Attitude Hold mode to prevent gimbal lock rather than having the crew watch the drift manually. For thermal control, the crew were simply instructed every so often to manually move the stack around to different attitudes.

Following the PC+2 past the far side of the Moon, even Aquarius would be powered down completely apart from COMM and life support, in order to conserve power. When a final midcourse correction was required after this, the infamous manual burn dramatized in the film was performed, as power could not be spared to power up the entire computer system.

Over the next many hours and days, many more problems need to be solved: the LiOH canisters mentioned briefly by Glynn Lunney deplete, and a makeshift adapter has to be fashioned for spares from the Command Module to be used in order to prevent a buildup of carbon dioxide; extensive resource planning is done to stretch consumables far past their designed limit; the crew deals with hypothermic conditions and dangerously scant water rations while demanding tasks continue to be required; an entirely new hours-long Command Module power-up procedure must be developed, tested, and successfully read over voice to an exhausted Jack Swigert to ensure a successful re-entry, a process that by all rights should take weeks rather than days.

Hopefully, you have not only enjoyed these six hours of audio, but as well gained a new appreciation and tactile understanding of what it was like to hurl Apollo through the heavens. If you wish to delve yet further into Apollo and spaceflight, please take a look at the further reading section.

If you have feedback or questions, please don't hesitate to send me a tweet or file a GitHub issue.

Photo Credit: NASA, with scans by Kipp Teague.