Apollo 13

Just over 3 hours and 40 minutes have passed since the oxygen tank exploded in Apollo 13's Service Module. In that time, the crew and Mission Control have been coming to terms with the parlous situation they find themselves in and have begun the task of effecting a rescue.

Command Module Pilot Jack Swigert has powered down his Command Module Odyssey. He has little to do now. His crewmates, veteran commander Jim Lovell and Lunar Module Pilot Fred Haise have quickly activated the Lunar Module Aquarius and it is now their lifeboat, providing propulsion, power, oxygen, cooling and other consumables. They are still nearly 20 hours away from the Moon, and only after swinging around it will they start to move back towards the Earth. Although the plan to use the LM as the lifeboat for the return home exists on paper, it has never been practised by the crews.

The crew has been passed the details of a burn of the DPS engine that will return them to Earth for a landing in the Atlantic Ocean at 133 hours. That burn would occur 20 hours from now at the so-called PC+2 (pericynthion plus two hours) point.

Houston is trying to determine their exact position and velocity by using the radio link between the ground station and the spacecraft.

The radio carrier signal to the spacecraft (the uplink) is at a precise frequency. Upon reception, the spacecraft synthesises a downlink carrier from the uplink whose frequency is at a relationship of 240/221. Were the spacecraft motionless with respect to the ground station, the downlink frequency would reflect that exact ratio. Any velocity away from or towards the ground station will alter that frequency according to the Doppler effect.

For distance measurement, a carefully selected large digital number is placed on this uplink. This is called pseudo-random noise and when it is received back on Earth, the delay between the sent and received version of the code yields the range to the spacecraft. Add to this, the precise pointing angle of the antenna and the spacecraft's position and velocity (its state vector) can be calculated. This Earth-based result is usually more accurate than what can be produced by the onboard navigational equipment.

Long comm break.

They are discussing the Command Module side hatch here.

Comm break.

Haise, from 1970 Technical debrief: "You might think that you have to stop overboard dump because of the loss of the electric power and the heaters on the urine dump, but Jack actually rigged up the AUX urine dump through the forward hatch and I think he tried it."

Swigert, from 1970 Technical debrief: "Jim did it once."

Lovell, from 1970 Technical debrief: "You should tell Houston, too, that dumping overboard was a bad thing to do."

Haise, from 1970 Technical debrief: "The point I was making was, other than the problems of tracking - I don’t think you need a heater for that overboard dump, and I think you can use it forever and ever without having a heater. With the tracking, though, that made the picture entirely different. When we couldn’t dump it anymore, I think we improvised some place to store all this good fluid."

Swigert, from 1970 Technical debrief: "One comment on using that auxiliary dump for urine is that it does complete cloud up the hatch window. If you ever use that, you might as well forget about photography. We used it for one urine dump, and there were particles on the window from then on."

Many interesting things in this long message from Lousma in Mission Control. Currently on their list of important things is to try to figure out how to fly the unusual configuration while using the Lunar Module’s RCS engines. They are also concerned about the temperature and want to try to reinitiate PTC or Passive Thermal Control by making the stack spin in order so as not to keep one side pointing at the Sun for too long.

Apollo 14's backup crew consisted of veteran astronaut Gene Cernan (CDR) as well as first timers CMP Ronald Evans and LMP Joe Engle. The latter had the interesting distinction of already being an astronaut before being selected for the Apollo program, having flown the X-15 rocket powered plane to over 50 miles, thus earning him his USAF astronaut wings.

Star sightings are used to configure their guidance system by providing a reference point out in space. By telling the computer where particular stars are located on the celestial sphere and comparing this with the computer's idea where they should be located, they can determine their attitude and position with high accuracy. This not only helps with navigation, but also allows them to calibrate the inertial platform. Optimally this is done with the onboard telescopic instruments, but now they are contemplating whether it’s possible to do it by eye through the ship’s windows, using backup methods.

The inertial guidance system and the LM computer consume 9.76 amps. Telemetry shows that they are consuming 31.7 amps overall, meaning that taking off the platform and the computer would cause a drop of a third of their full power consumption. Presently, their power is calculated to last till 124 hours - seven hours short of the projected 133 GET landing, should they stick to the current plan for the PC+2 burn.

Comm break.

John Young is the backup commander of the mission. He has already flown to the Moon on Apollo 10 and has practised much of Jim's role up to the explosion. After Jim himself, he is one of the most experienced of NASA's astronauts.

Long comm break.

Long comm break.

Comm break.

The initial attempt to separate the Lunar Module and the Instrumental Unit’s communications frequencies has improved their communications somewhat, but the situation is still not perfect. A further procedure will be performed later that will rectify the issue properly.

This panel has rotating controls for adjusting the aim of the steerable antenna and selecting between the various antennae. There is also the signal strength indicator. Currently it reads 2.4 AGC.

Comm break.

They were told to use the ascent stage oxygen tank earlier due to rising pressure in the tank. This condition has now been rectified and they can return to the much larger reserve in the descent stage.

This communication problem is caused by the S-band radios used on the LM and on the IU atop the S-IVB stage using the same frequency. On previous flights, the S-IVB's batteries would be spent by this time but on Apollo 13, extra batteries had been added so that the stage could be tracked all the way to its impact on the Moon. This was part of a seismic experiment that would use a seismometer that had been placed by the Apollo 12 crew. It had been expected that this impact would have occurred by the time that Aquarius was activated. With the LM's early activation, both systems are interfering with each other. The engineers at Honeysuckle Creek are attempting to separate the two systems.

The Unified S-Band system works by having a radio carrier of precisely known frequency sent to the spacecraft. This is the uplink. The spacecraft uses it to synthesise a different transmission frequency according to the ratio of 240/221 and this is sent back to Earth as the downlink. By carefully moving the frequency of the uplink, it is possible to also move the frequency of the downlink. The trick in this situation is to discriminate between the S-IVB and the spacecraft, both of which are in the same field of view of the smaller antennae available to MSFN.

They currently have what they assume to be good alignment data, transferred over from the Command Module into the LM’s own guidance system. If they have to shut down the PGNS, they lose this data and in their current situation, they are unsure whether they are able to re-calibrate the inertial guidance using the equipment available to them in the LM. The AOT is only designed for aligning the platform in lunar orbit and on the surface, not in the cislunar space.

Comm break.

Long comm break

Long comm break.

Current calculations in Mission Control are showing that at their present level of water usage in the Lunar Module, their supplies will run out around 94:00 GET - only 31 hours from their present GET.

Missions before Apollo 13 used the free return trajectory, where the translunar injection would lead to a lunar flyby that would then swing the spacecraft back to Earth so that in case of SPS failure, the RCS engines could be used to fine-tune the approach for a safe landing. For the missions after Apollo 12, a midcourse correction changed this approach to one that did not create such a gravitational slingshot. This would enable the Apollo spacecraft to approach the Moon in such a manner so as to provide better landing options within the constraints of their fuel supply.

The plan for the moment is to use the DPS engine to do a quick burn to adjust their trajectory so that the lunar approach will put them back onto a free return trajectory. Once they go around the Moon, they plan to perform the PC+2 maneuver to speed up their return. This would be a different burn profile to the one read up to them earlier, which was calculated with a lunar approach from the non-free-return trajectory.

Should nothing else be done, and the crew be unable to perform any kind of a PC+2 burn or any other maneuver, the free return trajectory will take them to the Indian Ocean for an emergency landing.
Landing target: The Indian Ocean
Splashdown point: 20.37° south latitude, 60.10° east longitude.
Time of Entry Interface: 151 hours, 45 minutes GET.

The proposed burn to return to a free return trajectory is not particularly time constrained at the moment, but they of course will want to perform it as soon as possible.

Both the Rendezvous Radar and the Landing Radar are surplus to requirements, given Aquarius's new role as the lifeboat.

The COAS (Crew Optical Alignment Sight) is a piece of semi-reflective glass onto which a graticule or reticle is projected by a unit mounted to one side. It is normally used during docking operations to help line one spacecraft up with another. It maintains a single line of sight and so can be used as a calibrated aim to sight on stars and therefore provide a rough attitude check. This can permit a P51 (coarse platform alignment) to be achieved.

The Alignment Optical Telescope is a periscope-like device that protrudes out of the top of the LM.

The AOT consists of the eyepiece, used by the crew to make star sightings as well as the control box that has buttons for sending signals into the computer once the proper stars have been spotted in the telescope viewer reticle. It is basically a miniature version of the sizeable control console for the optics in the Command Module.

The Earth and the Moon can be used for a rough alignment of the platform, and this option will come up later on in the mission.

The spaceborne navigators have encountered a problem that has plagued everyone who has ever tried to navigate based on star sightings - they are unable to see them at the moment.

Normal rotation maneuvers are made by firing a pair of opposing thrusters positioned either side of a vehicle's centre of mass. This arrangement means that as much of the thrust as possible goes into making the rotation and very little will cause the ship to translate. Only if there is a slight mismatch in the thrusters would there be a translation component. However, Jim is not making normal rotation maneuvers because the thrusters he has to play with, those on the LM, are at one end of a docked pair of spacecraft. At the other end is a 29-ton, fully loaded CSM and there are no opposing thrusters on the opposite side of that. The centre of mass of the stack is well away from his opposed pairs. A roll maneuver of the stack is straightforward enough but to achieve a worthwhile pitch or yaw of the whole stack, he has to use sideways translation maneuvers. The thrust vector for these translations is sufficiently far from the stack's centre of mass that it imparts a rotation. The problem that Jim is aware of is that these maneuvers will also impart a significant translation thrust. Over the long term, such uncoupled thrust will profoundly affect their trajectory, especially important if they intend to miss the Moon by only 60 nautical miles (110 km).

This 127-page document has special procedures for emergency situations, including systems malfunctions, power failures, and other eventualities. Right now they are to go through the 2-hour Activation Sequence and Descent Propulsion System startup to prepare for the burn to change their trajectory. This backup procedure would normally be planned for the situation where the SPS engine cannot be used to brake them to enter lunar orbit, resulting in the need to use the DPS in a short notice. The whole document and the plan they are to undertake is a good show of NASA’s insistence on having the crew ready for most emergency situations that might arise. Although the unprecedented difficulties of Apollo 13 demand some improvisation, for most part our crew on Earth and in space are relying on previously thought out, planned, calculated operations that are in there just for the unlikely situation that they are needed.

Comm break.

Page 1 covers entering the LM, turning on the power, and activating the onboard lights and the Environmental Control System. Fred foregoes doing much, considering they only recently started up the LM.

The 2-hour activation continues with turning on the Electric Power System, and these steps have also been taken already.

The inverter generates 115V AC from the LM's 28V DC supply. They don't want to activate inverter 1 since they are using inverter 2 to power their AC equipment.

The power system is already running and requires no more attention. They skip step 6, which would be a checkup of both of their 2 inverters.

Fred reads out the circuit breakers that are enabled, hence also listing the systems they have powered. Currently they have battery power with AC enabled, the GASTA and FDAI systems, and the telemetry and control systems enabled.

The rest of the circuit breakers control power to the RCS heaters, the EPS fan, the glycol cooling system, and the computer with its associated inertial platform.

DECA is the Descent Engine Control Assembly. It mediates between the control inputs coming from the computer or the crew and the descent engine. This breaker lets the DECA control the gimbals that steer the engine.

Comm break.

They need to power up more electronics to control the DPS engine during the burn.

The main structure of the Descent Stage is like a plus sign (+). The four arms each carry propellant tanks and the center section is a box with the Descent Engine suspended within on motorized arms. These will be used to vector the engine - to point the nozzle as required.

The checklist has several pages that show the desired circuit breaker configuration in a diagrammatic form. Each page has one of the circuit breaker panels (the left-sided one is panel 11 and the right-sided is panel 16) with the open and closed breakers color-coded into it.

A pushed in, closed, circuit breaker appears black. An open breaker appears as white.

The TCAs refer to the RCS quads.

The LM has its own tape recorder for data and voice storage, named DSEA. They will not power it now.

Despite Lousma’s earlier order to configure the circuit breakers as per the pre-printed plan, they are now deciding what kind of changes to do to better suit their current situation.

Jim keeps power in the air circulating fan as well as the RCS quads and the manual control to them.

The VHF radio won't be powered up at any point of the mission.

They will remain using only one S-band transceiver.

Comm break.

The Up Data Link electronics consume only a little amount of power, but they need to save everything they can.

The PAO transcript contains this remarkable announcement from Mission Control at this moment. Although partially in jest, it is also an appropriate comment. It is 2 am in Houston and a lot of people are up past their bedtime - and duty shifts - trying to figure out ways to help Apollo 13.

They will not power the propellant measuring system, nor the Ascent Engine’s helium regulator or the System Engineer’s Crosspointer, which is an instrument only useful for landing on the Moon, not their present activity.

The Caution and Warning system is not powered on. Albeit important, it is not absolutely necessary for the operation of the spacecraft. The telemetry systems are functional and hence ground control can monitor the onboard systems status and alert the crew if they need to intervene.

They will not power up the Abort Guidance System (AGS) at this time. The ATCA breaker provides power to the control electronics.

ATCS AGS breaker provides a backup power supply to the ATCA when being used with the AGS.

They will not power up the steerable antenna, but leave its heater elements working to ensure that the antenna could be brought online should it be needed.

Ascent ECA refers to the Ascent Stage Electric Controller Assembly, and is not powered up. Since they are not using the batteries in the Asecent Stage, their control electronics are not needed either.

Mission Control has determined their velocity and position and want to update it in the Lunar Guidance Computer over the radio.

ORDEAL is only used for orbital operations, and is not powered up.

A small army of technicians, engineers and astronauts are in the simulators, trying out procedures that might be useful for the stricken crew.

P30 refers to 'Crew Defined Maneuver'. In this computer routine, the guidance computer calculates a desired engine burn based on data input by the crew into the computer. The data is provided in the PAD, to be read to them from Mission Control.

The PAD is interpreted as follows:
Purpose: This PAD is for a short burn to return Apollo 13 to a free-return trajectory. This ensures that the spacecraft will swing around the Moon and be on a trajectory for Earth.
Time of ignition (Noun 33): 61 hours, 29 minutes, 42.84 seconds.
Change in velocity, fps (m/s) (Noun 81): x, -21.3 (-6.5); y, +4.1 (+1.2); z, -31.2 (-9.5). The change in velocity is resolved into three components which are quoted relative to the LVLH (Local Vertical/Local Horizontal).
H_A and H_P are both not applicable to this burn as it is placing the spacecraft on a looping trajectory, not an orbit in the conventional sense.
Delta-V_R: 38.0 fps (11.6 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: 31 seconds.
Spacecraft attitude: Roll, 120°; Pitch, 298°. 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): x, -21.3 (-6.5); y, +4.1 (+1.2); z, -31.2 (-9.5). The change in velocity is resolved into three components which are quoted relative to the LVLH (Local Vertical/Local Horizontal).
GDA angles: Pitch, 5.86°; roll, 6.75°. These are the angles to which the nozzle of the descent engine will be aimed to place its thrust axis through the stack's estimated centre of mass.

Additional notes include the throttle settings that should be used during the burn. Five seconds at 10 per cent thrust will give the computer time to sense if the GDA angles ought to be trimmed. The rest of the burn will be carried out at 40% thrust. There is also a note to settle the propellants at the bottom of their tanks by burning two of their RCS thrusters for ten seconds, a so-called 'ullage' burn. However, since the system is already configured to use four thrusters, they will soon choose to use that set up.

Several pages are simply skipped since they brought these systems online as part of their start up earlier.

Comm break.

The landing gear is stowed to save space in the SLA (Spacecraft/Lunar Module Adapter) section on top of the S-IVB stage. To deploy them, the crew must energize the associated pyrotechnic devices, their control electronics, and the fire the charges that are used to push the gear into position.

The pyrotechnics push the gear into position and lock them into place.

Their guidance system has been updated earlier to set up the computer to navigate in cislunar space using the PTC REFSMMAT, and hence no further updates are needed at this point before the engine burn.

This refers to the T-shaped TTCA hand controller on Jim's (left) side of the cockpit.

They are concerned that their recently attained stable attitude will be lost if the computer starts firing the RCS jets automatically.

Repeated simulations before the mission throw a wide range of possible failure scenarios at the crew and flight controllers in order to test their response to the situation. Lousma's joke is that this is like a particularly difficult simulation. Of course, it is no simulation and the lives of the crew really are in the balance if the team does not get this right.

They decide to use the automatic maneuver mode after all.

Comm break.

Comm break.

The DAP is the Digital AutoPilot, a routine in the computer that works to maintain the spacecraft in a desired attitude. It is configured by placing digits into a register in the computer.

They are changing from a DAP load of '30120' into '32021'. As per the configuration guide, they are changing to a 4-jet RCS configuration, fine scale, and a rate selection of .5 degrees per second.

The DPS has a dual system for pressurization. For the first start, they use the pyrotechnic detonator to open the line to a small bottle of gaseous helium - known as the ambient helium tank. This high pressure gas is used for the initial pressurization of the system. This is done to prevent fuel lines from being frozen by the sudden introduction of the extremely cold supercritical helium (SHe) which will be the main source of system pressurization. Some 1.3 seconds after engine start, once a flow of fuel has been established through a heat exchanger, the engine controller will automatically send a signal to the pyrotechnics to fire the explosive isolation valve to open the SHe line. This begins the primary pressurization. The SHe passes through the same heat exchanger to be warmed by the passing fuel prior to it being introduced into the tanks.

Comm break.

Since the RCS checkout procedure assumes that they have newly started the maneuvering jets, there's no point to do test firings on them now because they already went through a lot of use while they attempted to stabilize the LM-CSM stack.

Long comm break

Comm break.

Verb 65 inhibits the up and down-facing RCS jets from firing during a DPS engine burn. This is desirable, since otherwise they might be either giving them extra acceleration, or fighting against the thrust of the main engine.

Throughout this exchange, Fred's intimate knowledge of the Lunar Module, its systems and how they are meant to work is very apparent. Fred was not wrong in his assumption that the abort system should be left unpowered, lest they experienced an accidental separation of the two halves of the Lunar Module - with the loss of their consumables needed for the journey home.

Page 17 of the 2-hour checklist shows various procedures to configure the systems for engine firing. Here they modify the plan to use manual throttle instead of having the computer do so automatically.

Register value 203 indicates that the engine firing control has been transferred to the PGNS computer.

This procedure would lock the engine nozzle so as to prevent the computer from turning it, using Attitude Hold to stabilize the spacecraft, and then use the TTCA hand controller for engine throttling and thrusting.

Albeit the throttle control was set to manual, it was the computer that started and stopped the burn during this short maneuver.

Verb 16 Noun 85 has been entered on the computer. This displays the velocity components still to be gained. In essence, it displays the difference between the velocity change they wanted and what they got, known as the residuals. With the knowledge of what the residual velocities are, they have a choice whether to leave them as they are or to use their RCS thrusters to bring the values to zero, known as 'nulling the residuals', or trimming.

Comm break.

Jim wants them to get the LM's power usage down as soon as possible, to save on their precious supplies of battery power and cooling water. He knows that the computer and the IMU are high demand items and will likely need to be turned off, but he is also aware that they might be unable to get proper navigational alignment again if they lose the one they currently have.

Comm break.

Long comm break.

To start with a power-down after the burn, they have turned off the DPS control electronics that are not needed anymore, immediately saving about 2 amps.

Comm break.

The Command Module telescope and sextant are usually electrically operated, but they can be hand cranked into position with one of the tools stowed onboard, if necessary.

These switches allow them to choose between the various communications electronics and modes. These will become frequently operated once they begin to experiment with ways to save power while using the radio. Right now they will simply flick the XMTR/RCVR switch to the central Off position.

Very long comm break

Many interesting points here in this long read up from Jack Lousma.

At the moment, water is the critical consumable. Water is used for cooling and of course for drinking by the human element of the spacecraft. The Lunar Module has a non-renewable supply, unlike the Command Module which gets its water from the waste products of the fuel cells. The water and electric power consumption are intimately tied together - the more equipment they have running, the more heat is generated, and more water is required to remove that heat. Hence, reducing the power load also reduces the water usage, and trying to minimize both of these is something the crew and Mission Control desire a great deal.

Mission Control is suggesting that the crew takes turns sleeping with one up at all times to answer to the communications or to react to any emergencies. This was behind the thinking during the early Apollo flights as well where a similar arrangement was used to ensure that a crewmember was always ready. The well-meaning plan backfired when it became more than apparent that it was impossible to sleep in the Command Module if anyone was awake and Mission Control kept calling them on the radio. The realization prompted NASA mission planners to decide that it was easier to let everyone sleep at once and have Mission Control monitor systems telemetry in case anything out of place showed up. This arrangement worked very well throughout the rest of the program.

Lovell, from 1970 Technical debrief: "We had urine all over the place, stacked in places we never even thought about. The nice thing about it, though, is that we found enough quick disconnects and rigged up lines to get urine into things that normally we were putting other stuff into or taking stuff out of. So it worked out that we could store a lot more urine than we thought we could."

Comm break.

The inertial platform will maintain its alignment without the computer, hence the LGC can be turned off but the computer would be practically useless without the IMU - hence this procedure would make sense at the moment.

Doppler here refers to the effect of their velocity on the frequency of the radio signal sent to Apollo and then bounced back to the land-based antennas. The change in frequency can be used to track their exact velocity in relation to the ground station on Earth.

Long comm break.

In the subject of orbital mechanics, pericynthion refers to the point in a trajectory of a spacecraft launched from Earth where it is closest to the Moon. This is distinct from perilune, the closest point of an orbit around the Moon. Fred picks up on the difference, a measure of his and everyone's desire for precision in language as much as in procedure and measurement.

In this case, a 252-km pericynthion implies that the spacecraft is now on a free return trajectory and is therefore headed for Earth once it coasts over the far side of the Moon, pulled around by lunar gravity.

Long comm break.

Long comm break.

Comm break.

Very long comm break

The term 'squelch' is familiar to radio hams but may be less so for those not used to the properties of analogue radio. When a radio signal weakens, an amplifier within the receiver increases its gain in order to compensate for the falling levels. As it does so, it also increases the noise inherent in the received signal. There comes a point when the noise overwhelms the signal and this is usually difficult to listen to. The noise is 'squelched' by having it cut out when the amplifier tries too hard to amplify the incoming signal. Usually there is an adjustment to set the level at which squelch occurs.

To preserve power, the LM's high-gain steerable antenna is not going to be used. Instead, their S-band communication will be via either of the two omnidirectional antennae mounted fore and aft of the cabin. The selection of the most appropriate antenna depends on the attitude of the stack and it has been decided that, rather than squelching out the noise when an antenna is no longer favoured, they will simply use the rising noise levels as a cue to switch over to the other antenna.

The TTCA needs the onboard computer for the automatic firing control of the RCS jets. Direct mode bypasses the automatic control and fires the jets manually.

Comm break.

By '06' Jim is referring to Program 06 in the computer, which idles down the LGC.

Comm break.

Haise, from 1970 Technical debrief: "The only shortage we had was what I mentioned on the air. What we really needed was a big, blank pad of paper for our unusual situation."

The plan at the moment includes trying to align the IMU platform while they are in the shadow of the Moon to improve visibility of the stars, and also to start up the LM's Abort Guidance System, to serve as a backup during the PC+2 burn.

Despite the good efforts of the ground control to set up a new duty cycle for the crew, it will become very difficult to stick to it during the rest of the mission.

Comm break.

Long comm break

Powering down their AC inverter and the FDAI and the associated electronics will save almost 3 amps in power expenditure. It will make it much more difficult to fly the spacecraft, however.

Long comm break

In the Verb 20 Noun 20 mode, the spacecraft's orientation on each of the three axes of motion - pitch, yaw and roll - can be read from the computer display.

Long comm break

Fred has made a mistake in his readback that Lousma will correct shortly. 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 just a unmodified free-return trajectory.
Time of ignition (Noun 33): 79 hours, 27 minutes, 40.13 seconds.
Change in velocity (Noun 81), fps (m/s): x, +814.4 (+248.2); y, -44.3 (-13.5); z, -222.6 (-67.8). 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, beyond the limit of the computer's display.
H_P, expected perigee of resulting orbit (Noun 42): 20.5 nautical miles (38.0 km). The perigee distance is so low, it will take the spacecraft well into Earth's atmosphere and thereby, the spacecraft will re-enter.
Delta-V_R: 845.5 fps (257.7 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, 268°; Pitch, 261°. 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, +815.5 (+248.6); y, -44.3 (-13.5); z, -218.7 (-66.7).
GDA angles: Pitch, 5.85°; roll, 6.74°. These are the angles to which the nozzle of the descent engine will be aimed to place its thrust axis through the stack's estimated centre of mass.

Additional notes include a so-called 'ullage' burn to settle the propellants at the bottom of their tanks by burning two of their RCS thrusters for ten seconds. 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. Retro expects that this burn should result in a splashdown time of 142:47 GET.

Three options for the PC+2 maneuver were presented on April the 14th by the Retro controllers. Each of the three options offered one return scenario for them using the Descent Propulsion System.

A PC+1 maneuver, performed one hour after passing pericynthion instead of 2, would have utilized the entire delta-V capacity of the DPS with a very large burn of 4,728 fps (1,441 m/s). This would have put them down in the Mid-Pacific Landing area at 118 hours GET, or just under 40 hours from the burn. In order to generate such thrust, it would be necessary to separate the Service Module, then only dead weight for the spacecraft stack.

The second option, which was also adopted as the one that was performed, was a PC+2 burn of 845 fps (258 m/s). This too was a burn to target them at the Mid-Pacific Landing area, with a splashdown timed around 142 hours GET. This would have used less than half of the remaining DPS capacity of 1,977 fps (603 m/s) of delta-V with the Service Module attached.

The final option considered was a PC+2 burn using all of the remaining DPS capacity and with the SM attached. A delta-V of 1,997 fps (603 m/s) would hurtle them to a landing in the Atlantic Ocean at 133 hours GET.

So why not to jettison the Service Module and utilize the DPS to its maximum capacity to get the crew home as quickly as possible? Wasn't this the most desirable action to take to ensure that the crew would make it back alive within the time they had left on the LM's supplies? Two issues in particular arise. Using the high delta-V options created larger potential for errors in their trajectory. A midcourse correction would also require more thrust, which they might not have if they expended all the DPS fuel. Once the DPS fuel was used up, their course correction capacity would be down more or less to the Lunar Module RCS. This was calculated at 44 ft/s. It was calculated that a course correction from the maximum velocity trajectory could take up to 200 ft/s of delta-V to accomplish. This could require the use of the Ascent Propulsion System, which could only be used after jettisoning the Descent Stage, and with it the majority of their remaining consumables. This would have put them at a considerable risk of running out of water, power and oxygen before landing.

Second, uncertainties existed in regards to the behaviour of the Lunar Module-Command Module docked configuration. This had never been flown before and it was not known how it would maneuver. Jettisoning the Service Module would also expose the heat shield at the bottom of the Command Module. Usually this took place minutes before re-entry, but should they have opted to eject the SM before PC+2, they could have been looking at 50 to 60 hours of being exposed to space. The heatshield had thermal limitations in regards to high and low temperatures that could become an issue during the long coast back home. It was also possible that the heat shield was damaged by the internal explosion, and these thermal issues as well as any potential micrometeroid collisions could create additional dangers.

Overall, the safest bet was to press on with the docked condition. It provided a known configuration and protected the heat shield. There were also calculated options to perform further DPS burns after the low-energy PC+2 in order to speed up their return should their consumables situation become worse. The math said that the oxygen, water and power would last long enough to support the slower, and most probably safer return. They simply were not in enough of a hurry to want to face even more extra risks, as detailed above. Saving DPS fuel was essential.

Comm break.

Comm break.

In Verb 16 Noun 20, the gimbal angles are displayed on the three lines of the DSKY in the order of yaw (outer gimbal), pitch (inner gimbal) and roll (middle gimbal). Jim's remark about the third register means that he is observing the roll angle. It is the middle gimbal that has the unwanted tendency of locking up.

The Pyro sequence breakers connect the two pyrotechnic power system batteries into the sequential system and allow them to fire the explosive devices in the Command Service Module.

Although the computer DSKY can be used to display the gyro angles, it is not very intuitive to use and causes plentiful confusion for the crew. Usually the 8-ball would show them the deviation and which way to move to correct it, but without the display, it will be much more difficult. This operational mode is viable, but requires them to forget their usual procedure and adopt a new way of thinking while under extreme pressure. The 8-ball is in the spacecraft because the astronauts who have to pilot it wanted it to be there.

This movement takes place on the X axis and has the spacecraft begin to rotate to the right, as if standing on a turn table.

Comm break.

Lovell, from 1970 Technical debrief: "We have our Contingency checklist; our method of doing a DPS burn; and how to control using the TTCAs for attitude control, pitch, and roll, and ACA for yaw control. This technique did work and was adequate. In fact, that is the way we flew the vehicle all the time. Our only big problem was when we shut down the FDAI to save power and went to the computer flashing 16 20 , which gave us yaw, pitch, and roll, actually outer, inner, middle gimbal angles. We wanted to keep the middle gimbal angle out of gimbal lock. The technique that is in the contingency checklist is not valid . You can't use the TTCA and fly the computer the way we fly the 8-ball."

Haise, from 1970 Technical debrief: "That's right."

Lovell, from 1970 Technical debrief: "As a matter of fact, we spent hours trying to do it. I still don't have the technique. You just have to try to figure out by experimenting which way to hit the thruster. It changes depending on where you are, what quadrant you are in, and what the angles are as to which way to throw that translation controller to stop the angle from going toward gimbal lock. We were trying to keep it at a gimbal angle as close to zero as possible. I wasn't too worried about the other two gimbal angles. It was a continual battle to find it."

Lovell, from 1970 Technical debrief: "Maybe we ought to do some more research into using that technique. In the future in event of such a contingency, we ought to look at keeping the 8-ball powered up and powering down the DSKY, or something like that. I think our PTC mode was finally the AGS ATT HOLD, which held the vehicle once it was in position."

Haise, from 1970 Technical debrief: "The problem could be handled the same way that we did it. We taped over each ball top and side and wrote in what the representative TTCA gave in terms of pitchup, pitchdown, roll right, and roll left. This is a nice handy reference. You didn't want to think about the geometry of things if you could just look at this piece of tape and tell you which way to do it."

Long comm break.

What Jack proposes is that instead of the graceful roll of the PTC mode, they will turn the spacecraft around in increments. This was euphemistically called "dynamic PTC" in the post-mission reports.

Comm break.

Verb 77 is 'Set Rate Command/Attitude Hold Mode in DAP' and will utilize the Digital Autopilot to maintain their attitude.

In Attitude Hold, the computer will attempt to maintain their present attitude.

One of the most high-power items on the top row are the LM window heaters, which alone could consume about as much power as their guidance system.

They keep the ATCA control electronics operational, as well as the AOT's heater and parts of the telemetry system.

This configuration keeps their PGNS computer and inertial platform still operational despite the earlier speculation that they should be powered down.

These circuit breakers govern power to the onboard sensors, the electronics that convert the signals into digital data, and then transmits them to Mission Control as telemetry data.

ASA refers to Abort Sensor Assembly, the package of gyroscopes and accelerometers that produce the backup attitude and inertial data for the AGS guidance system. They want to keep the heater powered to maintain it at an operational temperature.

They turn off all the displays but keep the S-band radio operational. The carbon dioxide sensor remains powered as well, being a critical safety system.

SE Audio refers to 'System Engineer's Audio' and this is the LMP's communications panel on his side of the cockpit. In Grumman's parlance, the LMP was referred to as the Systems Engineer and hence the acronym SE pops up occasionally.

This configuration keeps the RCS heaters operational, and powers down unnecessary displays. The EPS configuration maintains load sharing between the batteries in the Descent Stage. They will not tap into the Ascent Stage batteries at this time.

The ASA breaker maintains heating into the Abort Sensor Assembly which contains the backup attitude and acceleration measurement electronics used by the Abort Guidance System, or AGS.

Comm break.

Instead of the graceful, slow roll of the PTC barbeque mode, they will now instead lurch the spacecraft about one quarter of a full circle, let it sit in place and do the next move. This way they will accomplish the same as the more carefully controlled PTC mode, in preventing one-sided cooling or heating of the spacecraft stack structures.

Long comm break.

Dead band refers to an option in the DAP on how much the spacecraft is allowed to drift from its desired attitude before the DAP will start firing the RCS jets to attempt to return to that particular attitude. A wider dead band allows for more drift and hence saves fuel.

Jim is referring to the fact that the CM water system is normally operated with the oxygen pressure from the Service Module tanks.

This valve is used to control whether oxygen is drawn into the Command Module from the Service Module or not.

The alarm is not unlike a 1960's equivalent to an USB flash drive warning about being removed too quickly!

Long comm break.

Planning and speculating ways to transfer water from the PLSS backpacks into the LM's systems will be a favorite pasttime throughout the rest of the mission for the astronauts and ground crew alike.

Comm break.

Comm break.

The problematic ascent oxygen tank's valves still have a reverse leak, hence its pressure keeps rising. They want to use some of the oxygen again to reduce the pressure.

Comm break.

Although the LM can provide plentiful oxygen and scrub the atmosphere of carbon dioxide, dead air is a genuine problem. Without a constant air flow to move the air, pockets of CO₂ enrichened air may form.

Comm break.

Lousma orders Jack to go back to the Command Module to check the state of several valves in the Environmental Control System. This mostly amounts to making sure that all oxygen valves are closed to ensure none of the O₂ supply in the Command Module is accidentally used. The accumulators are water-glycol coolant tanks.

Oxygen from the life support system enters into a plastic bladder within the tank, hence applying pressure to the contents of the tank and pushing the water out to be used by the crew. Jack reads them a procedure that allows them to draw water from the drinking water tank by using oxygen pressure from the surge tank.