Apollo 13

Just over 16 hours ago, at 055:54:53 GET, Apollo 13's Service Module was severely damaged when a fire within an oxygen tank caused it to catastrophically burst. The crew don't know the reason for their crippled craft but they are well aware of its implications and have moved into Aquarius, their Lunar Module and now their lifeboat. Ten hours ago, they burned the LM's descent engine for 31 seconds to place the spacecraft onto a trajectory that would swing them around the Moon and bring them back to Earth. Specifically, it would land them in the Indian Ocean at about 152 hours Ground Elapsed Time. MCC are now planning another burn that will occur two hours after they pass the Moon. This is intended to shorten their drop to Earth and have them land in the Pacific Ocean at about 142 hours GET, at a landing site near to where the US Navy's recovery fleet were planning to recover them anyway.

An additional reason for this timing concerns a graphite cask on the side of the LM. It carries and protects a fuel element containing plutonium which had been intended to power experiments on the lunar surface. NASA would like to safely dispose of this material and intend to do so by having the LM re-enter Earth's atmosphere in such a way as to allow the cask to sink to the bottom of the Tonga Trench which, at over 10 kilometres, is one of the deepest parts of the world's oceans. With the graphite cask designed to survive reentry, this would also alleviate the fears of the Atomic Energy Commission.

Long comm break.

Very long comm break.

The Air to Ground loop tape has captured some conversation between the MSFN land station operators. Specific channels allowed the stations to talk to one another to keep everything running smoothly.

Charlie Duke is the Backup LMP for the mission and hence the perfect person to relay information about Lunar Module procedures to his compatriot Fred Haise. They would have trained closely together throughout the preparation for Apollo 13.

They crank up the primary communications gear and the ranging system for the duration of the navigation talk with Duke.

What they begin here is not a typical P52 realignment, but instead they are using the software's capacity to aim their optics at any point, as long as they have the correct time and the coordinates available. This was the procedure they were using earlier during the mission to attempt to spot Comet Bennett. The P52 was selected for the Sun check due to it being more fuel efficient than the other automatic maneuver modes.

Fred Haise is not talking about the comm signal breaking, but instead comments on Charlie Duke's fateful German measles that put the whole Apollo program into temporary turmoil and caused Fred's prime crew crewmate Ken Mattingly to be replaced by the backup Jack Swigert, from Duke's crew.

Charlie confirms the situation.

Apollo 13 backup Lunar Module Pilot Charlie Duke flew in that role as prime LMP on board Apollo 16, along with Apollo 13 backup Commander John Young and Ken Mattingly, finally getting his chance at the role of Command Module Pilot. 35-year-old Duke was a test pilot before being chosen for astronaut training in 1966 in Astronaut Group 5.

Comm break.

The LM Guidance and Navigation (or G&N) Dictionary serves as an onboard reference for the crew on the various procedures for using the Lunar Module Guidance Computer (LGC) and the Abort Guidance System (AGS).

The IMU Align procedure in the checklist has multiple possibilities - different options and techniques call for different procedures, and hence the checklist may command you to jump from Step 1 to Step 2 or 6, for example.

To start the IMU realign, the first move is to make sure that the AOT telescope's internal lighting is enabled by putting in the circuit breaker. Afterwards, the LGC is selected to run the associated program in its memory. To do this, they key in Verb 37, which is the Change Program command, hit Enter, and then type in 52, for the IMU Align program.

P52, Step 1 starts with entering Verb 04 Noun 06 (Display Decimal - Option Code), which allows them to choose from the options presented to them as choices such as listed on the procedure here. They select option 3, REFSMMAT and hit Pro to move to Step 6. Naturally all they would see on their computer is numbers on the green display, hence the paper checklist would be crucial on interpreting the computer output, and selecting their preferred input.

P52, Step 6 begins with entering Verb 50 Noun 25 (Please Perform - Checklist), and here they are to choose the manner of acquiring the stars they desire for the sightings to align the inertial platform. They choose the Manual Acquisition mode by hitting Enter at this point. Another option would be to allow the computer to choose suitable stars with the PICAPAR routine, but this is not what they need right now, since they are looking for a specific target and do not require the shortcut that is PICAPAR. It selects two stars close to each other within the telescope's field of view, so as to save RCS fuel by not needing to turn the spacecraft around to look for suitable navigational stars.

P52, Step 7 moves on by entering Verb 01 Noun 70. (Display Octal Component - AOT Star Code/Detent) This is where the crew specifies what viewing device and what sighting star they wish to use for the alignment. The choice is coded onto Register 1 of the LGC with the 00CDE code, where the letters are codes for the various numeric values the string can have. The letters on the checklist are there to help with interpreting the printed checklist. Duke's command of entering '200' here means that they pick the number 2 option, which is the F for Forward detent on the AOT, and '00' is for the star code of the star to be sighted, which here signifies planet. Starcode 00 is used as placeholder, of sorts, and requires them to input the vector values of the location where this object might be found. It could also be a star not in the in-built catalogue, too.

It is of interest to note that should the AOT not be available, the P52 alignment could also be done by using the COAS monocular on the forward window or the overhead docking window.

Step 7 continues with another Pro from the operator that is followed by entering the command Verb 06 Noun 88 - (Display Decimal in R1, R2, R3 - Celestial Body Vector X, Y, Z) as per their earlier choice of 'option 00' on the star code selection, and this allows them to enter the expected vector values for the sighting at the time of the AOT check. This way the computer will know where they should be looking at at the time of the Sun check - although it is up to the crew to identify and confirm it by looking into the telescope! This time, their target being the Sun, there ought to be no difficulty in interpreting the results.

By halving the unit vectors, the values can be stored in the computer with a higher number of significant numbers, and hence more precision.

At Step 8 it is time to maneuver the ship to an attitude as specified by the FDAI. Entering the command Verb 50 Noun 18 (Please Perform - Desired Maneuver To FDAI RPY Angles) allows them to program their desired attitude into the computer. They choose to use the Auto mode on the PGNS computer instead of going manual so that the LGC will then try to fly them into this attitude. It is important to note here that Verb 50 Noun 18 requires them to input roll, pitch and yaw angles to be read from the 8-ball, which means that these values are not identical to the alignment on these axes for the inertial measuring platform.

Apollo 11's Neil Armstrong and Buzz Aldrin performed a Sun check to confirm their LM IMU alignment as well, before heading for the surface.

Verb 62 commands the computer to Display Total Attitude Error. In this context error refers to the difference between their desired attitude and their present attitude. The FDAI 8-ball has mechanical needles that the computer can drive to give a visual and intuitive reference for the pilot on how to make their attitude as close as possible to that they wish to move to or maintain. During the automatic maneuver, they would see these error needles move towards the zero position.

Comm break.

The DAP, or Digital AutoPilot, is a routine in the computer that runs in the background to automatically keep control of the spacecraft's attitude. Various parameters concerning its operation can be set by altering the contents of a register. For example, whether its controlling just the CSM or whether there is another vehicle docked to it, which thrusters to use, what size of dead band and how fast it should correct any error. '31120' reads as docked configuration (CSM and LM), 2 jets from system B, normal scaling at 2 degrees per second, 5-degree dead band, and .2 degrees per second rates allowed.

If the Sun is found at the correct time, they can simply stop the P52 without doing any changes to their platform. If the Sun check does not pass, they plan to do a P52 realignment using sightings of the Sun and the Earth instead of stars. For this they require another set of three-dimensional coordinates for each spatial body.

Charlie is referring to the apparent size of Earth in the field of view of the AOT, as well as how much of its surface is lit by the Sun at the time.

On Step 8 they use Verb 50 Noun 18 (Please Perform - Desired Maneuver To FDAI RPY Angles) to enter the FDAI angles. Each angle occupies one register (or one line) on their computer display. The input RPY or Roll/Pitch/Yaw angles will take the spacecraft to the desired position for the Sun check, manually or automatically using computer control.

Charlie Duke caused a brief confusion by giving the angles in the wrong format to support them being input into the computer in the form required by the procedure.

R1, R2 and R3 refers to each register, or display lines on the computer's three lines of numeric display.

The Rendezvous Radar dish is currently blocking the AOT and they need to drive it to a new position using the computer.

Verb 49 maneuvers utilize movement in three axes, while the P52 is a two-axis maneuver.

So far this procedure gets up Inverter 2 to provide alternating current for the power-hungry radar, and enables primary computer control of the radar antenna.

Next step is to use Verb 41 Noun 72 to input the desired shaft and trunnion angles to position the radar antenna. The antenna has two degrees of motion, hence they input values for both of them.

Hence they would input the following:

+00000 - the angle of the shaft (up and down motion) goes to zero

+28300 - the angle of the trunnion (left and right motion) is set to 283 degrees, which moves it to the left and outside the field of view of the AOT during the upcoming Sun check. It is interesting to note the nomenclature for the radar's axes. The word 'trunnion' is derived from the tilt axis of a cannon and its usage would imply the pitch axis (as occurs in the CM's sextant.

The Rendezvous Radar. It has two degrees of motion, along the shaft axis (for pitch, or up and down) and the trunnion axis (for yaw, for left and right).

Inputting Verb 16 Noun 72 displays the current angle of the Rendezvous Radar on the computer's R1 and R2 registers, i.e. the first and second lines on the DSKY.

Verb 44 is the command to terminate the Rendezvous Radar operation via the computer and release it for other functions. The last step is to take out the Inverter 2 circuit breaker to turn off the AC power.

Comm break.

The AOT has an internal lamp that illuminates the reticle inside. They have to enable the lamp's power supply and crank up the AC inverter to provide the lamp with AC power.

The FDAIs have been powered off to save battery power during the coast to the burn, and need to be brought back up for their maneuvers.

Gemini and Apollo astronaut David Scott is supporting the mission by running simulations on ways to control the spacecraft in its unusual state.

Dave Scott would later serve as a technical advisor on the set of the Ron Howard movie, Apollo 13.

Comm break.

Comm break.

The rings that support the guidance platform act as gimbals which let the spacecraft rotate as the platform maintains a constant orientation with respect to the stars. This is achieved because the hinges between each gimbal afford the arrangement three degrees of freedom. Unfortunately, thanks to the (deliberate) choice of using only three gimbals, there is an orientation which can be adopted whereby the middle gimbal moves in such a way that the axes for the outer and inner gimbals line up.

If this occurs, all the axes are now in the same plane and so the arrangement has essentially lost one of its degrees of freedom. It is no longer capable of isolating the rotation of the spacecraft from the platform. This condition is termed gimbal lock. If it is inadvertently adopted, it will require a realignment of the platform, something they don't want the crew to have to do. Hence, both the crew and Mission Control are keeping a careful eye on the middle gimbal. Should its angle exceed 70 degrees, the computer will light the Gimbal Lock warning light. 85 degrees is the absolute limit, after which a gimbal lock condition does exist and their attitude reference is lost.

The middle gimbal monitors the roll axis, (also known as R, or the Z axis). It is a conscious design choice to implement the inherent limit of the three-gimbal design to the middle gimbal, since rotations on the roll axis are the least required, and easiest to avoid in mission planning. The advantages gained are in the mass of the IMU and a lesser tendency to drift out of alignment.

They need DC and AC power to the FDAI as well as the GASTA electronics that interface between the IMU and the 8-ball.

Their return to the low power comm configuration has also returned the poor audio quality.

Long comm break.

Fred is of course referring to the Apollo 11 astronauts. Besides Tom Stafford and Gene Cernan of Apollo 10 and Apollo 12's Pete Conrad and Alan Bean, these six astronauts are the ones with experience operating the AOT in the actual near-lunar environment.

Astronaut Vance Brand is back for his first CapCom duty shift since the beginning of the emergency.

Jim thinks that the radar antenna could still cause trouble when they are trying to spot the Sun and the Earth for their P52 realignment.

Comm break.

It sounds like Jim has spotted some debris moving along with them.

Comm break.

Comm break.

Comm break.

Long comm break.

While they wait the upcoming Sun check, the astronauts go back to their common topic of using the PLSS backpacks for extra water supplies or for transferring water supplies.

Mission Control can't read their computer display or the gyroscopic information indicating their attitude due to the telemetry being on the Low Bit Rate mode.

Mission Control attempted to use a larger antenna to receive the weak signal from the spacecraft, with no success. They have to switch back to low bit rate to receive at least some telemetry.

Comm break.

Jim does not sound entirely convinced about the plan to use sightings of the Earth and the Sun to align the IMU, if it should come to that. As the Earth and the Sun are not only much larger targets to get a fix on, they are also extremely close in comparison to the stars they usually use for navigational purposes, hence there is further potential for errors.

Jim later remembered the Sun check in an official NASA history of the mission. Here he painted it in a rather more dramatic light than is immediately evident from the transcription.

Lovell, from Apollo Expeditions to the Moon (NASA SP-350, 1975): "We had many crises on Apollo 13, but the biggest heart-stopper has hardly been noticed, partly because the transcription released to the press was garbled, and partly because there wasn't much point in talking about a crisis that had been averted earlier."

Lovell, from Apollo Expeditions to the Moon (NASA SP-350, 1975): "We had transferred the CM platform alignment to the LM, but we had to make sure that this alignment was accurate before we made the long P.C.+2 burn."

Lovell, from Apollo Expeditions to the Moon (NASA SP-350, 1975): "So what to do? A genius in Mission Control came up with the idea of using the Sun to check accuracy of our alignment. No amount of debris could blot out that star! Its large diameter could result in considerable error, but nobody had a better plan."

Lovell, from Apollo Expeditions to the Moon (NASA SP-350, 1975): "When I looked through the AOT, the Sun just had to be there. It really had to be. And it was. At 73:46 hours, the air-to-ground transcript sounds like a song from 'My Fair Lady'. [...] If we raised our voices, I submit it was justified."

Jim is most likely referring to the song 'The Rain in Spain' from the popular 1956 musical and its 1964 film adaptation.

Lovell, from Apollo Expeditions to the Moon (NASA SP-350, 1975): "I'm told the cheer of the year went up in Mission Control. Flight Director Gerald Griffin, a man not easily shaken, recalls: 'Some years later I went back to the log and looked up that mission. My writing was almost illegible I was so damned nervous. And I remember the exhilaration running through me: My God, that's kinda the last hurdle—if we can do that, I know we can make it. It was funny, because only the people involved knew how important it was to have that platform properly aligned.' Yet Gerry Griffin barely mentioned the alignment in his change-of-shift briefing. 'That check turned out real well' is all he said an hour after his penmanship failed him. Neither did we, as crew members, refer to it as a crisis in our press conference nor in later articles."

There appears to be some confusion as to what kind of angles Vance is offering to the crew.

Comm break.

Verb 49 activates the computer routine that can be used to input a desired attitude into the computer, and then used as a reference for flying into that attitude.

A momentary confusion has ensued from Vance's attempt to read the crew the angles for their preferred attitude. It turns out that the reason was due to the different way the Command Module and the Lunar Module handles the input of the angles into the guidance computer. While the Command Module Computer takes them in the order of RPY - Roll-Pitch-Yaw, the Lunar Module uses YPR - Yaw-Pitch-Roll.

Another reason for this questioning is that the angles given are the ICDU - Inertial Coupling Data Unit angles, which refer to the angles of the IMU gimbals, not the FDAI 8-ball indicator, which would give its own readings.

Verb 50 Noun 18 is the Auto maneuver they want to take to the new attitude.

The middle gimbal senses their motion and acceleration on the roll axis, and is the one prone for the infamous gimbal lock.

The gimbal lock is closer than ever, resulting in a quick warning from John Young, who is monitoring in Mission Control again.

Verb 49's automatic maneuver is trying to fly them right into gimbal lock. They have to try to do it under crew control.

Mission Control photo from during the emergency shows many key players in returning the crew home. Seated are Deke Slayton, Director of Flight Crew Operations, Jack Lousma serving as the CapCom and backup Commander John Young, smoking a pipe. Grounded Command Module Pilot Ken Mattingly and CapCom astronaut Vance Brand are standing up.

A further confusion has ensued among the crew about whether the angles were for the IMU or the FDAI.

Long comm break.

They need improved signal strength to be able to transmit the high bit rate telemetry Mission Control requires to monitor the gimbal angles.

Long comm break.

Comm break.

Vance's cryptic-sounding instructions are used to change the dead band maintained by the DAP. To do so, they use the Verb 21 Noun 01 command to change information in the computer's 2 kilobytes of erasable memory. They first input the known memory location - or address - of the computer instruction they want to access, and then input a new value for the program to take so that the DAP will perfom as desired.

Comm break.

They go back to the low power comms configuration and also power off the FDAI to save some power.

Comm break.

Comm break.

Comm break.

Long comm break.

The original transcript transcribes Fred's question as 'contingency QR DPS?' but upon close listening of the tape it is clearly heard that he says 'two hour' instead.

To prepare for the PC+2 burn, they are using the LM Contingency Activation Handbook's pre-written procedures and altering them as required.

This simply activates the EPS displays, which have been powered down to save up their batteries.

The only real procedures here are changing the desired inverter configuration.

Normally this procedure assumes a fully functioning Command Module, including its mission timer and onboard clock. While usually the time would be input by having someone in the CM communicate it to the LM by shouting it down the hatch, they will now use the LM computer to display the time and then input it into the mission timer.

Page 3 offers procedures for activating the Caution and Warning system for the burn. Their warning lights will light up if any trouble comes up with the engine pressures or the electric power delivered to the CES - the engine control electronics.

This circuit breaker configuration only leaves the RCS thrusters and their control systems powered.

This configuration leaves suit fan 1, the glycol pump transfer, the IMU and Inverter 1 powered.

A large diagram of LM Panel 11 can be viewed here. From heroicrelics.org.

Charlie moves on to talk about the configuration desired for the LMP's side of the circuit breaker panels. Again only the RCS thrusters remain powered, with the crew able to choose whether they want to use the internal lighting or not. A large diagram of LM Panel 16 can be viewed here, from heroicrelics.org.

No changes are made on the pages concerning the computer, the communications or the RCS systems, since these are up and running since they powered up the Lunar Module.

The AGS user interface may seem baffling at first, but is down to a simple operational logic. The Address Indicator is used to display the octal numeric address of the location in the computer memory either for inputting a new value into it or to reading out its current contents on the DEDA display.

All data input follows a simple procedure:

Press CLR (Clear) button to empty the display

Input the three-digit memory address

Press + or - sign

Input five-digit data in either decimal or octal numbers

Press ENTR (Enter)

To access data in the AGS computer memory, the procedure consists of pressing CLR to empty the display, then enter the three-digit address of the data to be viewed, followed by pressing the Readout button to display the stored value on the DEDA screen.

The Hold button can be used to keep the current readout on the screen where it will refresh twice a second, until CLR is pressed again to reset the DEDA and to allow the crewman to begin a new DEDA input or readout.

An abbreviated AGS startup follows on page 12. It begins by selecting the appropriate switches to power up the Abort Guidance System electronics. Once the AGS computer is hence turned on steps 2,3 and 4 perform a brief procedure to test it and then to input a few basic values the AGS needs to operate successfully.

All of the following commands are given with the normal procedure:

CLR 000 +888888 Enter - lights up all the digits on the DEDA electroluminescent display and ensures that they are working properly

CLR 123 -45679 Enter - tests the DEDA keyboard by pressing all the numeric keys on it

CLR 412 +00000 Enter - runs the AGS Self Test

A successful test is indicated by the DEDA displaying 412 +10000

The other possible displays indicate failures of the AGS electronics. Steps 5,6,7 conclude an initial setup of the AGS.

574 - Descent Stage Flag (indicates whether the LM has staged or not)

+00000 Enter is input

604 Lunar Surface Flag (tells the AGS whether the LM is landed on the Moon)

+00000 Enter is input

612 Staging Sequence Counter (status check)

+00000 Enter is input

The IMU align on page 13 is not done, since they have a properly aligned platform already.

For the AGS Align procedure, they input 400 +50000 into the DEDA. This aligns the AGS inertial reference frame with the XYZ body axes of the Lunar Module.

They change the gimbal/throttle setup only in that automatic computer control mode is used instead of manual firing.

The Verb 48 loads DAP data into the LGC. Noun 46 allows them to input the five-digit DAP configuration code into the register. In this case, it is 31021 instead of the 32021 suggested by the checklist. The difference is that they want to use two RCS jets instead of four and hence the second digit is changed to correspond to the DAP setup for this configuration.

Verb 47 is used to load the known mass (referred to as weight) of the CSM and the LM. While the LM mass is printed on the checklist, it tells the astronaut to ask the CSM mass from either the docked spacecraft or Mission Control. The LM would have been essentially inert until this point, hence its mass is known.

They insert a Verb 34 Enter after the mass input, which is the code to terminate the operation. They do not wish to proceed with the Noun 48 on the checklist, which would be used to change the orientation of the Descent Engine Gimbals that control the position of the engine nozzle.

Instead of the procedure that would be used to test fire newly started RCS jets, they use this part of the checklist to make sure that the attitude control options and the manual controllers are properly configured.

Jim is probably referring to the COAS monocular.

The long procedure on the top of the page is the Program 30 or Crew-defined Maneuver, which allows them to input the engine burn data they have written down on the PC+2 PAD into the LGC so that the computer can calculate the burn commands.

The CSM Maneuver is removed, since they will use the LM to maneuver into the burn attitude, having no operational CSM.

Again Fred shows that he is following Charlie Duke's instructions carefully, and that he is intimately knowledgeable of all the procedures and systems involved with the LM.

The checklist makes preparations for using the Ascent Propulsion System in case the DPS fails or is not sufficient to put them on a return trajectory. This is not the plan for the moment and hence this procedure is axed.

The top of the page 17 of the checklist contains instructions on how to set up the Abort Guidance System for the upcoming engine burn. These will be used to monitor the PC+2 burn as well as provide backup in case of primary system failure. The first four instructions are input with the CLR/DEDA Address/Plus Or Minus/Five-Digit Code/Enter procedure.

400 +00000 - sets up AGS Attitude Hold

404 +00000 - Delta-V on the X axis

405 +00000 - Delta-V on the Y axis

406 +00000 - Delta-V on the Z axis

These four AGS commands order the Abort Guidance System to monitor the LM's acceleration on all three axes of motion during the engine burn.

The last thing input is the address 470 which is the AGS address for Monitor Delta-V X. By pressing Readout they instruct the AGS to display the change in the spacecraft acceleration on the DEDA along the X-axis. This way the AGS produces another way for them to display the amount of velocity change generated by the firing of the DPS engine during the PC+2 burn.

Fred is curious about what they should do if the PC+2 burn fails. Duke's advice is to shut down the engine in case of trouble and they'll handle any required changes to their trajectory later on.

Master Arm is required to be On only during the first DPS engine burn so that the supercritical helium can be released via pyrotechnic valve. Since they have already burned the engine once, it is not required here at this time for PC+2.

Here comes a rare disagreement between Fred's view of the procedure to take, and Mission Control's.

Fred comes to a quick realization of the logic behind Mission Control's thinking, and has no qualms about it.

In the run-up to the engine start, they will enable Engine Arm and then perform the ullage by firing the RCS thrusters to settle the fuel in the Descent Engine tanks.

At minus 5 seconds, the computer flashes Verb 99 Noun 40 (Enable Engine Ignition - Time From Ignition). Once the ullage burn is complete, the crew hit Pro to let the computer continue the engine firing under automatic control.

Charlie offers not one but two different ways to start the DPS should the automatic computer start not occur.

Panel 5 has the protected manual engine start button, to be used in case the automatic system won't work.

Duke suggests that in case the manual engine start fails, they can use the DES ENG CMD OVRD (Descent Engine Command Override) switch to fire the engine.

They add the command Verb 76 (Set Minimum Impulse Mode In DAP) here to go to such mode in the Digital Autopilot after their engine burn.

Charlie describes here, should the computer fail to perform the engine stop and start, how to do it manually using the buttons and the manual throttle.

Charlie Duke wants to give Fred advice on how to modify the onboard DPS burn cue card to accommodate the changes made to the procedures for the PC+2 burn. The card is meant to be used for normal Powered Descent operations. These kind of cue cards could be Velcro'd to various parts of the LM cabin to provide a quick reference to crucial procedures and information at times of high activity.

This in-training photo shows several sticky cue cards attached to the LM console. There are further cards on the DSKY platform as well as what appears to be a checklist clipped onto the yellow handles around the AOT.

Jim suggests that the changes to the card are perhaps superfluous, considering that he does not plan to use the card.

The G&N Dictionary contains a list of the navigational sighting stars. Jim has identified 33 (Antares) and 37 (Nunki)

Mission Control thinks that they do not need to do any further refinements to the inertial platform. They want to save up RCS fuel by not making more maneuvers at the moment. Jim was enthusiastic about performing a proper P52 inertial platform alignment by star sightings.

The PAD is interpreted as follows:
Purpose: This PAD is for a burn of the LM's main engine two hours after pericynthion (their closest approach to the Moon) in order to return them to a specific place on Earth; in this case the Mid-Pacific Landing site or MPL, and to do so faster than would have been the case with their current return trajectory.
Time of ignition (Noun 33): 79 hours, 27 minutes, 40.07 seconds.
Change in velocity (Noun 81), fps (m/s): x, +819.2 (+ 249,7 m/s); y, -20.0 (- 6.0 m/s); z, -218.9 (-66,7 m/s). The change in velocity is resolved into three components which are quoted relative to the LVLH (Local Vertical/Local Horizontal).
H_A, expected apogee of resulting orbit (Noun 42): Not applicable. This burn is to place Apollo 13 on a Earth-bound trajectory, the apogee of which would be over 9999.9 nautical miles above the local body (the Moon), beyond the limit of the computer's display.
H_P, expected perigee of resulting orbit (Noun 42): 20.9 nautical miles (38.7 km). The perigee distance is so low, it will take the spacecraft well into Earth's atmosphere and thereby cause the spacecraft to re-enter.
Delta-V_R: 848.2 fps (258.53 m/s). This is the total resultant change in velocity the spacecraft would experience and is a vector sum of the three components given above.
Burn duration or burn time: 4 minutes, 20 seconds.
Spacecraft attitude: Roll, 274°; Pitch, 82°. The desired spacecraft attitude is measured relative to the alignment of the guidance platform and is therefore set using the FDAI display in its inertial mode. Unlike a CSM PAD, yaw is not stated for the LM. Procedures for a lunar landing allowed the spacecraft to yaw around the thrust axis as required for operational reasons.
Change in velocity for the AGS (Noun 86), fps (m/s): x, +820.3 (+); y, -20.0 (-); z, -215.0 (-).
GDA angles: Not applicable. These would normally be the angles to which the nozzle of the descent engine would be aimed to place its thrust axis through the stack's estimated centre of mass.

The throttle settings that should be used during the burn are; 5 seconds at 10 per cent thrust to give the computer time to sense if the GDA angles ought to be trimmed. The next 21 seconds will be at a 40 per cent setting with the rest at full throttle.

Conventionally, a return-to-Earth PAD would be for the CSM and it would include information pertaining to re-entry. However, the PAD given above was for the LM and those PADs don't include re-entry information so Brand is reading up that part of a CSM PAD that is relevant in this case. Many of the items relate to the moment when the EMS accelerometer detects a deceleration of 0.05g (one-twentieth of a g). The precise timing of this moment will depend on atmospheric conditions at the time but it is estimated for the purposes of the PAD. It is usually about 30 seconds after the CM reaches Entry Interface, an arbitrary moment on their trajectory when they are 400,000 feet [121.92 km] above Earth.
Expected splashdown point (Noun 61): 21.47° south, 165.0° west; in the mid-Pacific.
Range to go at the 0.05 g event: 1,166.0 nautical miles (2,159.4 km). To set up their EMS (Entry Monitor System) before re-entry, the crew need to know the expected distance the CM would travel after the 0.05g event is triggered. This would be decremented later based on the readings from the EMS' own accelerometer measuring the change in their velocity and hence ability to fly downrange.
Expected velocity at the 0.05 g event: 36,291 fps (11,062 m/s).
Predicted GET of 0.05 g event: 142 hours, 39 minutes and 20 seconds GET.

They are cranking up the normal communications mode and select the power amplifier to ensure a good transfer of the computer update via the Up Data Link.

Very long comm break.

They are reversing the comms configuration back to the low power mode.

PCM refers to Pulse Code Modulation. It is the contemporary term for digitally coded information. The telemetry system gathers the analogue electric signals from the onboard monitoring equipment, converts it into digital data to form the PCM data stream, and then transmits it to Mission Control.

Comm break.

Comm break.

Long comm break.

In zero gravity, any loose particles float endlessly around, propelled by the flow of air within the cabin. The suction created by the ECS air flow can pull these particles into the system and damage it. They could also be accidentally inhaled by the crew. The crew is using simple screens in front of the hose openings to catch the dirt.

Comm break.

Vance Brand is referring to center of gravity, more accurately called 'center of mass'. It is essential to know the center of mass for the spacecraft, considering all of its maneuverability - and the predictability of it thereof - is dependent on it. A moment earlier, Fred asked if the Mission Control personnel were keeping book on everything that was removed from Command Module into the Lunar Module. The crew thinks that once they are about to return to Earth, they must know exactly what equipment has been taken back to the CM and what is left in the LM, so as to know the exact mass distribution of the Command Module. This has ramifications for their orientation during re-entry. Right now Vance suggests that the negligible mass of the hose caps will not have an effect.

Comm break.

It sounds like the crew is doing some cleaning up while they are waiting for their next maneuver.

Comm break.

It sounds like Jim wants to take the PLSS backpacks into the Command Module once they begin re-entry.

They are debating whether to wear their spacesuits for the Earth landing. No crew has landed in suits since Apollo 7.

Jim might be planning ways on how to hook up their spacesuits to the PLSS while in the Command Module.

Comm break.

Jim has located the navigational stars again.

They want to know when they should start powering up the LM's systems for the PC+2 burn.

Comm break.

The distance of Apollo 13 from the Moon is now less than the distance between New York and London (3,008 nautical miles)

Long comm break.

Mission Control appears to have been using the computerized image processor to create simulations on what the expected view is through the LM's optical instruments and the windows, hence they are able to tell the crew what they should see to confirm their attitude.

Comm break.

They can view the navigational stars to ensure that they are maintaining proper attitude during the engine burn.

Through the eyepiece of the AOT, a crewman sees a graticule or reticle that shows a radial line and a spiral.

The pattern can be rotated so that, first, the radial line coincides with a star, yielding a 'shaft' angle, then it it rotated again to make the spiral coincide with the star which yields the 'reticle' angle. The computer can combine these to derive an accurate vector to the star. An alternative means of deriving a vector to the star is to manoeuvre the spacecraft so that the star crosses the X and Y lines, marking each time it does so. It is this method that Mission Control are not wanting the crew to use as it means taking the spacecraft out of its current attitude.

Comm break.

The LM windows have lines painted on both the inside and outside. If the commander positions his head in just the right place, the two sets of lines line up for him. In this position, they become relevant to the operation of the Landing Point Designator (LPD). This is a routine in the computer that is run during the approach phase of a landing and it allows the computer to indicate the spot on the ground to which it is flying the LM. If the commander doesn't like this spot, he has the ability to redesignate by moving his controller. The LPD lines are formed from a horizontal line that matches the view directly forward, and a vertical line calibrated in degrees down from the horizontal.

This refers to the Time of Ignition.

Comm break.

They are drawing from pre-existing abort plans to decide the conditions under which the PC+2 maneuver should be stopped. These include abnormal fuel pressure, movement outside their set limits for the attitude, as well as failures in the power system, the guidance computer, or the control electronics.

Lunar Orbit Insertion Mode I abort scenario assumes that instead of a LOI maneuver behind the Moon, they continue past the LOI point and perform the abort burn at PC+2 point.

The various Lunar Orbit Insertion (LOI) abort scenarios are presented in a table in the Flight Plan, should they be needed to cancel the maneuver to enter lunar orbit. These were designated for an otherwise fully functional pair of spacecraft, rather than a LM hauling an inert CSM.

They are to use manual start should the engine not start automatically. If the PC+2 does not succeed, they have an opportunity to try again two hours later.

If for some reason the PC+2 burn fails to happen completely, they are still on the free return trajectory they established earlier, and would end up splashing down into the Indian Ocean at 152 hours GET.

Fred's Freudian slip can surely be excused here.

Each crewman has a personal flashlight, and there are two spares in the Command Module.

The long-awaited lunar shadow is making the false stars caused by the surrounding debris to disappear, leaving the real stars to become visible.

Vance Brand is reminding them that since they have modified the DAP programming by accessing the code in the computer memory, they will have to re-program it to return to the usual values for the DAP dead band.

Comm break.

Jim is reminding his crewmates of the high stakes of the PC+2 maneuver. Those 10 hours off their travel time back home could be the difference between survival and being lost in space.

Long comm break.

Jim is still itching to do some star sightings for a platform alignment.

Comm break.

Very long comm break.

The last topic caught on the comm loop before Loss of Signal is Jim talking about food.

Apollo 13 has slipped beyond radio contact. Normally this would be the time of feverish waiting in Mission Control, to learn of the news whether the spacecraft was able to burn its SPS engine behind the Moon to put them into a lunar orbit. This time they are only swinging around, already on the free return trajectory. The flight controllers already know to the second when Apollo 13 will reappear around the opposite limb of the Moon.