Apollo 11

When behind the Moon, Apollo 11 fires the large engine on the Service Module to accelerate out of lunar orbit and enter its homeward-bound trajectory towards Earth. The crew photograph the receding Moon, and receive a call from the Head of the Astronaut Office, Deke Slayton.

Being in a higher orbit, the LM's velocity around the Moon is slower and so the CSM is getting ahead of it.

Comm break.

The first task, to stir the cryos, is to activate fans in the cryogenic storage tanks for hydrogen and oxygen. This disturbs density layers that form in the contents as heat leaks into the tanks. These layers of differing density would otherwise affect quantity measurements. Purging of the fuel cells is required periodically to remove contaminants in the reactants that build up on the cell surfaces. It merely requires increasing the flow of reactant gas to flush the contaminants away.

Long comm break.

The PAO announcer is about to explain Duke's last comment.

Flight Plan, page 3-100.

The correct time for sextant star visibility was 134:50. It is later corrected by Retro at 134:08:18. This PAD represents all the information the crew need to burn their engine and begin their return path to Earth. A fuller interpretation of the PAD follows:

• Purpose: This PAD will be used for TEI-30 Return-to-Earth burn.
• Systems: The burn would be made using the SPS (Service Propulsion System) engine under the control of the Guidance and Navigation system.
• CSM Weight (Noun 47): 36,691 pounds (16,643 kg).
• Pitch and yaw trim (Noun 48): -0.61° and +0.66°. These are the initial angles for pointing the SPS nozzle so that the thrust acts through the calculated centre of mass of the spacecraft. As the burn proceeds, automatic control will slowly correct and maintain the thrust vector's direction to compensate for changes in the centre of mass.
• Time of ignition, T_IG (Noun 33): 135 hours, 23 minutes, 41.56 seconds.
• Change in velocity (Noun 81), fps (m/s): x, +3,201.1 (+975.7); y, +681.8 (+207.8); z, -265.0 (-80.8). These velocities are expressed with respect to the Local Vertical/Local Horizontal frame of reference of the Moon. The major component is prograde, which is in the direction of the CSM's travel and which will accelerate it out of lunar orbit.
• Spacecraft attitude: Roll, 181°; Pitch, 54°; Yaw, 14°. The desired spacecraft attitude is measured relative to the alignment of the guidance platform which itself has been aligned to the lunar lift-off REFSMMAT.
• H_A, expected apogee of resulting orbit (Noun 44): Not applicable. The spacecraft will be on a trajectory coming from the Moon so any apogee figure would be meaningless.
• H_P, expected perigee of resulting orbit (Noun 44): +23.0 nautical miles (+42.6 km). The perigee distance is so low, it intersects Earth's atmosphere. In other words, the spacecraft will re-enter.
• Delta-V_T: 3,283.6 fps (1,000.8 m/s). This is the total change in velocity the spacecraft would experience. (It is a vector sum of the three components given above.)
• Burn duration or burn time: 2 minutes, 28 seconds.
• Delta-V_C: 3,262.8 fps. The crew enter this figure into their EMS Delta-V counter display. The EMS can shut down the engine using this data if the G&N system fails to do so. Its value is lower to allow for the extra thrust imparted by the engine after shutdown, a quantity allowed for the the G&N software but not by the EMS.
• Sextant star: Star 24 (Gienah, Gamma Corvi) visible in sextant when shaft and trunnion angles are 151.1° and 35.7° respectively. This is part of an attitude check.

The next three items refer to viewing a star through a window using the COAS. However, this is not applicable because the windows would be facing the Moon. The next five parameters all relate to re-entry, during which an important milestone is "Entry Interface," defined as being 400,000 feet (121.92 km) altitude. In this context, a more important milestone is when atmospheric drag on the spacecraft imparts a deceleration of 0.05g.

• Expected splashdown point (Noun 61): 11.03° north, 172.37° west; in the mid-Pacific.
• Range to go at the 0.05g event: 1,180.6 nautical miles. To set up their EMS (Entry Monitor System) before re-entry, the crew need to know the expected distance the CM would travel from the 0.05g event to landing. This figure will be decremented by the EMS based on signals from its own accelerometer.
• Expected velocity at the 0.05g event: 36,275 fps. This is another entry for the EMS. It is entered into the unit's Delta-V counter and will be decremented based on signals from its own accelerometer.
• Predicted GET of 0.05g event: 195 hours, 4 minutes and 52 seconds GET.
• GDC Align stars: Stars to be used for GDC Align purposes are Deneb and Vega.
• GDC Align angles: Roll, 242°; pitch, 172°; yaw, 12°.

There is a clutch of additional notes associated with this PAD. The ullage burn to settle the contents of the propellant tanks is to fire two RCS (Reaction Control System) jets for 16 seconds. At the correct attitude for the burn, and two minutes to ignition, they should expect the Moon's horizon to line up with the 10° marks on the left rendezvous window. Finally, the sextant star will be hidden by the Moon until 134 hours, 50 minute.

In case their intended TEI burn does not occur for any reason, Duke reads up another PAD that modifies the first to support a burn one orbit later. This PAD is interpreted as follows:

• Purpose: This PAD will be used for a contingency TEI-31 Return-to-Earth burn.
• Systems: The burn would be made using the SPS (Service Propulsion System) engine under the control of the Guidance and Navigation system.
• CSM Weight (Noun 47): 36,691 pounds (16,643 kg).
• Pitch and yaw trim (Noun 48): -0.61° and +0.66°.
• Time of ignition, T_IG (Noun 33): 137 hours, 22 minutes, 39.85 seconds.
• Change in velocity (Noun 81), fps (m/s): x, +3,283.8 (+1,000.9); y, +684.5 (+208.6); z, -248.7 (-75.8). These velocities are expressed with respect to the Local Vertical/Local Horizontal frame of reference of the Moon.
• Spacecraft attitude: The value for their attitude in pitch changes to 52°.

All other items in the PAD are NA as they will already have been entered into the computer if this PAD were to be used.

Long comm break.

Comm break.

Very long comm break.

This P52 realignment of the guidance platform was achieved by Mike sighting on stars 01 (Alpheratz, Alpha Andromedae) and 11 (Aldebaran, Alpha Tauri). The angles through which the platform had to be rotated to restore its ideal orientation were +0.166° in X, +0.212° in Y and -0.019° in Z. The difference in the actual angle between these two stars and Mike's measured angle is 0.01°, a good result.

Very long comm break.

Long comm break.

Very long comm break.

Verb 64 is 'Start S-band antenna routine'.

Apollo 11 now passes behind the Moon for the last time.

Flight Plan, page 3-101.

Based on information read up as part of the TEI PAD, Neil expects that when they are in the correct attitude for the burn, star 24 (Gienah, Gamma Corvi) will be visible in the sextant when the instrument's shaft and trunnion angles are 151.1° and 35.7° respectively.

Verb 37 is for changing the program that is running as the major program in the computer, in this case Program 40. This is the program that will control the SPS engine through the TEI burn.

The IMU was properly aligned at 134:34:00 and now the backup system, the Stabilization and Control System (SCS) can be aligned to match. The GDC Align push button transfers the alignment data from the IMU-driven primary system to the BMAG-driven SCS.

The BMAGs (Body Mounted Attitude Gyros) are gyroscopes which are affixed to the structure of the spacecraft. This is different to the IMU where its gyros are mounted on a gimbal-supported platform around which the spacecraft can freely rotate (at least to an extent). If the IMU gyros detect a change in the platform's orientation, they send a signal to the gimbal motors to counter that change and restore the correct orientation. Since a BMAG's orientation is not isolated from the spacecraft's attitude, it must work in a different fashion. Being body mounted, any change in the spacecraft's attitude will cause this gyro to place a force on its mountings. This is because a gyro wants to keep its spin axis orientated in one direction. The size of that force and the information it reveals can be processed to reveal the magnitude of any change in the spacecraft's attitude and this is the job of the Gyro Display Couplers (GDCs). However, the GDCs must be given a valid starting point and this is the purpose of the GDC Align button. It uses attitude information from the primary system (essentially the IMU) to give the GDCs a starting point. Additionally, the BMAG and GDC combination are much more prone to drift which is why they form part of a secondary attitude control system.

The crew now settle into the typical challenge-and-response method of working through the checklist. They are on page F5-3 of the CSM operations checklist. Mike also has a cue card with an abbreviated version of the checklist.

Panel 8 is to the left of the Main Display console and populated mostly with circuit breakers. Though there are two rows of breakers for the SCS, the checklist line is referring to a single breaker at the top left.

These twelve SPS circuit breakers are also on panel 8, second row from the bottom. They feed power to the gauging system, the helium pressurisation valve, the engine gimbal motors and the two pilot valves that control the flow of propellant to the engine.

Deadband refers to the range of attitudes around the ideal that the spacecraft can drift through before active steps are taken to correct it. There are two settings for this: maximum is ±5° and minimum is ±0.5°.

When the RCS system does need to correct the attitude of the spacecraft, it can be set to do so at a high (7° per second) or low (0.7° per second) rate.

The Limit Cycle switch enables a subtle aspect of automatic attitude control. With the switch set to On, then during active attitude correction, when the spacecraft approaches the deadband, the RCS jets are made to pulse on and off quickly rather than making long firings. This helps to reduce the overshoot effects of liquids sloshing about in the tanks.

Three switches, one each for roll, pitch and yaw, set the mode that the spacecraft's attitude is controlled in. The Rate Command position gives control to the SCS and allows rotation rates that are proportional to the deflection.

This refers to another three switches, one each for roll, pitch and yaw, which set how the BMAGs are used. In this configuration, BMAG 1 is ignored while BMAG 2 supplies rate of change of attitude information for the SCS.

The rotational hand controllers have two modes of operation. In Normal, they operate via the computer so that commands can be interpreted into different ways of firing the RCS thrusters. In Direct mode, the controllers directly signal the thrusters to fire. This is a mode that must be used carefully as it can cause prodigious amounts of manoeuvring fuel to be consumed.

Thrust Vector Control is normally handled by the Guidance and Navigation system. As a backup, the commander can fly the engine manually. In case he has to resort to it, this switch setting readies the manual control system in a rate-damped mode which is the easiest way to control the engine.

At the bottom of Panel 1, two switches set how the pitch and yaw gimbals that move the engine bell are controlled. In auto, the control is automatic. However, control signals can be sent to the primary (1) or secondary (2) gimbal system.

The crew are ahead of the timeline given in the checklist. Its next item, connecting the electrical busses together, is due to occur at 6 minutes before ignition so they will wait until then to continue.

On page F5-1 of the checklist towards the bottom is a line that says 'DET Set'. This is the Digital Event Timer, essentially the precursor of the digital kitchen timer. It shows only minutes and seconds and can be set to a desired starting point and set to count up. When it reaches past 59:59, it goes to 00:00 and continues to count up. The crew will use it to coordinate their activities around the lighting of the engine. The checklist does not define what the starting point of the DET should be so it is assumed that this is a crew option. Nevertheless, the timings on the left side of page F5-3 (+54:00 and 55:00) refer to the DET.

If the crew had set the DET to 50:00, then this may be the point when they start the timer counting up.

Buzz is referring to the spacecraft's evaporator of which there is a primary and secondary unit. The evaporator's function is to lose heat by evaporating water in the vacuum of space. This is essentially boiling the water though it occurs at a very low temperature. Boiling is normally associated with a temperature of 100° Celsius but this is only because it is usually seen to happen in the atmospheric pressure we live in. Take that atmosphere away and water will readily boil from a liquid to a gas at any temperature and will carry heat away in the process. In normal circumstances, automatic systems are designed to use the evaporator only when the spacecraft's radiators are unable to sufficiently lose heat from the coolant. In other words, it handles the peaks in the spacecraft's thermal load.

The spacecraft is orbiting in an inertial attitude, having already reached the correct orientation for the TEI burn.

At TEI, they will be travelling pointy-end first. This means that at LOS, when they went around the western limb of the Moon, the engine nozzle was more or less pointed away from the ground below. As they approach the point in their orbit when they ignite the engine, the Moon's horizon will be seen to gradually move down their windows. Mike's rendezvous window has angle marks on it and it had been calculated that the Moon's horizon should appear at the 10° mark two minutes before lighting the SPS.

It is possible to orient a spacecraft along the correct axis for an engine firing, yet manage to get the alignment 180 degrees out - in effect pointing backwards instead of forwards, or vice versa. This happened inadvertently during Apollo 7's alignment for retro-fire in Earth orbit. Armstrong's reference here may be to his own experience on Gemini 8, where his dangerously tumbling craft had to be stabilized before a premature retro-fire and emergency return to Earth. Journal contributor, Phil Karn, elucidates.

Journal contributor, Phil Karn:- "Readers who aren't totally up on orbital mechanics might not get the point of the crew's very dark humor. In fact, it's one of the best examples I've seen of this unique kind of humor among pilots in general as well as astronauts. I think it's one of the ways they deal with the stress of knowing that so many mistakes could be fatal.

"Collins mentions this exchange in 'Carrying the Fire'. He says Armstrong's remark, 'Shades of Gemini', refers to the care with which they did retrofire preparations on Gemini - all three were Gemini veterans.

"It occurs to me that this comment might also refer to how their previous Gemini training had conditioned them to make a retrograde burn to return home. This time, they had to remember that the burn to return home had to be a prograde burn.

"The delta-V of the upcoming TEI burn is about 1,000 m/s, about two thirds of their current 1,600 m/sec orbital velocity around the moon. A retrograde burn would remove 1,000 m/s from their velocity instead of adding it. Deprived of enough forward velocity to stay in orbit, the CSM would swiftly fall onto the far side of the Moon without ever having re-established contact with Houston.

"I can actually relate to all this somewhat as I was involved in the planning for the orbital maneuver of the AMSAT-OSCAR-10 spacecraft in 1983. I got VERY concerned about flipping a sign somewhere, doing the burn backwards and causing a swift re-entry on the following perigee.

"Since I wasn't on board (mostly a good thing) I couldn't just look out the window so I kept trying to think of all the additional independent checks we could make on our spacecraft attitude, like figuring out where the sun should be and checking that each solar panel was (or wasn't) generating current as expected."

Mike is in the left couch and Buzz is occupying the right. This leaves Neil with the middle seat and only the hatch window to see out. This is looking down towards the lunar surface passing below. Mike and Buzz have the forward-facing rendezvous windows which are looking in the direction that the spacecraft is pointed.

They pick up the checklist halfway down page F5-3. The Main Bus Tie switches on panel 5 operate motor-controlled switches to connect the CM's batteries across the DC busses. This helps share the load expected during the burn, particularly from the gimbal servos.

This step supplies power to the Thrust Vector Control system, including the gimbal servos.

Two hand controllers need to be operational for this manoeuvre. If the automatic control of the engine gimbals fails, then the rotational hand controller can be used to hold the spacecraft's attitude. Prior to SPS ignition, a ullage burn will be carried out which is essentially a plus-X translation manoeuvre, requiring the use of the translational hand controller.

Momentary switches on panel 1 activate relays that send power to the primary gimbal motors on the SPS.

With the primary gimbal motors powered, the THC is turned clockwise to try to control the engine's aim. If their configuration is correct, the engine should not respond to his commands as the primary gimbals are to be controlled by the G&C system and any manual control is to be via the rotational controller.

Their next check is to ensure the secondary gimbals do respond to Mike's commands, as they should.

Now the secondary gimbal motors are activated via relays.

GPI is Gimbal Position Indicator, a set of dual-use gauges on panel 1. During ascent from Earth, they showed the tank pressures in the launch vehicle using the 0 to 50 scales. Now they will show the angles to which the SPS engine is being aimed using the scales that run from +4.5°, through zero to -4.5°.

The crew have set the trim angles for the SPS nozzle on the thumbwheels. These only have relevance at the start of the burn. As the burn progresses, the TVC will slowly adjust these trim angles as it senses shifts in the spacecraft's centre of mass. The Manual TVC Mode is being tested now. The secondary motors are controlled by the Rotational Hand Controller. Mike tries to move the controller and confirms that the engine moves by watching the needles on the GPI.

The THC was previously turned clockwise to enable the secondary TVC mode. Now that the check is complete, the THC is returned to normal.

They are now at the top of page F5-4 in the checklist. The checklist actually says that the GPI meters should return to zero if they have the TVC under control of the G&N system, otherwise, if the SCS is in control, the trim angles would be displayed.

The spacecraft's engines, SPS and RCS, are under the control of the computer.

The two BMAGs are set so that BMAG 1 provides attitude information and BMAG2 provides rate of rotation information.

They now test that the computer can drive the SPS gimbals.

Power for the rotational hand controller is switched to come from both main DC power busses.

With the Helium Valves in Auto, helium is only allowed into the propellant tanks when the engine valves are energised. The Limit Cycle switch was set On during the TVC check.

Around the edge of the FDAI are three displays that show the rate of rotation of the spacecraft.

The FDAI Scale switch allows the crew to select what is meant by the full scale deflection of these needles. Its positions are ±1° per second, ±5° per second and a "50/10" position that means ±10° per second in pitch and yaw, and ±50° per second in roll.

The same switch also selects the full scale deflection for the attitude error needles (the yellow needles within the ball display).The first two positions of the switch represent ±5° error while the bottom position selects ±15° in pitch and yaw, and ±50° in roll. The crew have therefore selected the position that has the coarsest resolution for the error and rate displays.

The next item (near the bottom of page F5-4) is to occur at 2 minutes to ignition (58:00 on the event timer).

Two guarded switches in the middle of panel 1 enable the main and backup Pilot valves in the SPS engine. Mike throws the A switch which enables the main valves, the 'A' bank, so that they will open upon command from the computer. Set B will be brought into play soon after ignition.

The two hand controllers are ready to be used if called upon if the primary automatic systems fail.

The Tape Recorder referred to here is the Data Storage Equipment (DSE) that records the data being generated by the sensors around the spacecraft and the voices of the crew. It is through this recorder that we have a voice transcript of the crew's discussions around the far side of the Moon. For the burn, they want this recorder to operate in its high speed mode of 15 inches per seconds in order to record high bit-rate data of the engine's operation.

Since the ullage burn is to be 16 seconds, they will begin it when the DET reaches 59:44.

A conversation occurs on the air-ground circuit between Houston and two ground stations on the other side of Earth; Guam and Honeysuckle Creek, Australia.

We return to the onboard conversation 30 seconds prior to the burn.

The accelerometers in the IMU are now actively measuring any change in velocity and their data are being used to update the state vector. This is different to the normal state of affairs where they are in freefall (weightless) and the state vector is calculated based on the mechanics of their trajectory.

They are now at the top of page F5-5. They select Verb 06 Noun 40 which displays the following on the three registers of the DSKY: R1 - Time from ignition and the time from cut-off; R2 - the velocity to be gained, a number that will start at 3,283.6 (fps, 1,000.8 m/s) and decrement towards zero; R3 - the accumulated velocity, a number that will start at zero and increment towards the desired Delta-V.

Two of the rear-facing RCS thrusters are fired for 16 seconds to settle the propellants to the bottom of their tanks where the outlets are.

Flight Plan, page 3-100a.

The Apollo 11 Mission Report gives the time for ignition of the TEI burn as 135:23:42.3.

Five seconds prior to ignition, the DSKY flashes '99' in the Verb display which acts as a request to the crew to enable the burn. Mike is required to press 'Proceed' before the count reaches zero, essentially saying, 'Yes, please go ahead with the burn.'

Having allowed the computer to start the engine using the A bank of valves, the crew manually open the B set of valves. This allows the engine to run at its maximum thrust due to the slightly greater flow of propellant to the injector.

These are the gimbals upon which the SPS engine is mounted, not those that support the guidance platform.

The valves that allow propellant into the combustion chamber are operated pneumatically by nitrogen gas, stored in two tanks, one for each engine control bank. Mike is particularly interested in the tank for bank B which had displayed a slight leakage during the insertion into lunar orbit.

To the crew, the burn appears to have been two seconds longer than predicted and Mike has attempted to manually shut it down, fearing an overburn. This is per the TEI Burn Chart on page 3-100a of the Flight Plan which gives a manual shutdown time of 2 seconds over the expected time and a reading of -40 on the EMS Delta-V counter. However, as reported by the PAO announcer at about 136:49:40, the burn was almost perfect. It seems likely that Mike's effort to shut the engine down was simultaneous with the computer's.

Collins, from the 1969 Technical debrief: "The EMS counter moves out pretty swiftly and it was difficult for me to estimate exactly when I might have minus 40 on the counter. The I_SP of the engine must have decreased or something; at any rate, the burn duration was longer than predicted and when burn time plus 2 seconds had elapsed, I had thought that I would have minus 40 on the EMS counter by the time I could get the thing shut down. There was some doubt in my mind as to whether it was shutting itself down automatically or not; so, at burn time plus 2 seconds and some small fraction, I turned both EMS Delta-V - or both Delta-V - Normal switches off. I think just a fraction of a second prior to this we got a good automatic shutdown. At any rate, our residuals were very small; so either we got a good automatic shutdown followed immediately by my turning the switches off or else I shut the thing down manually and was just extremely lucky in that it coincided with the PGNS residuals. For some reason, that burn duration was a little bit longer than I would have expected. LOI, you remember, was shorter than we had predicted and this was the next burn to follow LOI, so I was sort of surprised that it did take longer than normal."

On completion of the burn, the spacecraft has to return to its quiescent mode with the SPS equipment turned off. They are now halfway down page F5-6 and begin by disabling the computer's ability to control the engine valves.

Next they remove power from the primary and secondary motors that move the engine from side to side in its gimbals.

By pressing Proceed, the DSKY is made to display the velocity that is still to be gained along the three orthogonal axes. If it is a negative value, then the value has been overshot. The TEI Burn Chart in the Flight Plan instructs that if this value for the X-axis is greater than 0.2 fps, then Mike should use the X-axis RCS thrusters to bring it into that range. This is the direction of thrust that has the most effect on the accuracy of their fall to Earth.

What the SPS has just done can be visualised in two ways. The first is with respect to the Moon. From that perspective, it has accelerated out of lunar orbit, placing itself on an open ended hyperbolic trajectory that, to all intents and purposes, will not return.

Columbia's new trajectory can be viewed another way, one that is seen from an Earth-centred perspective. From this viewpoint, while the spacecraft was in orbit around the Moon, its velocity included the Moon's orbital velocity around Earth. This velocity happens to be right around 1 kilometre per second. In other words, the Moon is travelling around Earth at around 1 kilometre per second. It is then worth noting that the total Delta-V imparted by the TEI burn was 3,283.6 fps, or 1,000.8 metres per second - almost exactly 1 kilometre per second. Therefore this magnitude of burn effectively removes the Moon's orbital motion from the spacecraft. It is as if the spacecraft has brought itself to a dead stop and allowed the Moon to slide away from underneath it. It then falls back down to Earth.

The crew want to be able to view the receding Moon and take some photographs.

Verb 48 allows the DAP's settings to be altered.

Verb 83 means 'Start REND Param Display'. Neil elucidated after the flight.

Armstrong, from the 1969 Technical debrief: "We tried something different on this flight. The ground computed a postburn state vector, a predicted postburn state vector and put it in the LM slot. After the end of the burn, we could call up Verb 83 and get an R and R-dot [essentially position and velocity which is the rate of change of position] from our state vector over to the predicted state vector. It came out real close - 0.7 mile and 0.8 ft/sec - indicating (it's kind of another double check) that we really did get the burn that we thought we were going to get. That's not really any kind of requirement if everything works. It is a nice kind of thing if you have an SPS problem or if you take over with the SCS in the middle of the burn when your computer is working okay, but the guidance isn't working. You can use that vector in your hip pocket to find out how good of a switchover you did and how close your SCS burn came out."

Apollo 11 reappears around the limb of the Moon for the last time.

Post-TEI photography appears to begin around this time on Magazine V.

The camera with Magazine V is fitted with a 250-mm lens.

During the burn to insert Apollo 11 into lunar orbit, it was noted that the pressure in one of the nitrogen tanks that operates the SPS propellant valves had unexpectedly dropped, hence the interest in it performance on this burn. This is discussed more fully earlier in the journal.

Long comm break.

Photography continues on Magazine V with the 80-mm lens having been refitted.

Comm break.

REFSMMAT is REFerence to a Stable Member MATrix. This is a definition of an orientation in space to which the guidance platform (the 'stable member' in this case) will be orientated to. A particular REFSMMAT's orientation is fixed with respect to the stars but different REFSMMATs have different orientations. While in lunar orbit, the platform was aligned per the Landing Site REFSMMAT, one that represented the orientation in space of the landing site at the moment of landing. Now that they are returning to Earth, they wish to use a new REFSMMAT, one that suits the need to rotate the spacecraft side-on to the Sun for thermal control.

The reason for having to change the platform's orientation is to allow spacecraft operations while avoiding the gimbal lock phenomenon, a side effect of the IMU having only three gimbals.

Long comm break.

Long comm break.

During a long SPS burn, it is desirable to balance the consumption of the two propellants for a nominal ratio of 1.6:1, oxidiser to fuel. This was the function of the Propellant Utilization Gauging System or PUGS which helped the LMP to manually correct for any imbalance.

Panel 3 has the controls and displays relating to PUGS.

Two systems in the SPS work together to produce a measure of the propellant quantities and rates of depletion. One uses long capacitance probes, the other relies on point sensors. Their signals are processed together to drive the two digital displays at the top of the panel to indicate percentage propellant remaining. The meter on the left indicates any deviation from the desired consumption ratio and it was Buzz's job during the burn to operate a flow control valve in the oxidiser supply to achieve balance (left switch).

Aldrin, from the 1969 Technical debrief: "The PUGS was a little bit unpredictable based upon performance during LOI. The fact that I couldn't catch up with the increase and it got ahead by about 0.4 or 0.5, something like that, plus the preflight briefing that that would be the case was why I left the switch in Increase. We lit off and got through the initial guidance and I looked at the meter and it was showing down in Decrease, which struck me as not being what it should do. I expected it to be in Increase, but I thought "Well, maybe this is a characteristic such that early in the burn it does this sort of thing." So I left the switch where it was to try to catch up. I guess in the meantime that the two numbers - where one had been bigger that the other - had changed positions, in addition to the fact that when it says Increase, you throw it in the Increase direction. It's not at all obvious during a burn if one is a little bigger than the other. You're not sure whether the needle is believable or not, so I left it in Increase and it seemed as though it was getting farther apart and the needle was staying down; so contrary to what we had been led to believe, I put the thing down to DECREASE just to see what was going to happen. Sure enough, it stopped the divergence of the two numbers. We didn't have a long enough burn for it to get right to zero, but it was within 0.2. Anyway, it was a little different than what we had expected. I guess, if you really want to play that game, you might need to write some cues or something on there so you don't misinterpret anything. It worked out well. But it was unusual and that might have something to do with burn time."

Flight Plan, page 3-102.

Very long comm break.

The Moon continues in its orbit at 1 km per second, leaving Apollo 11 behind to begin it three-day fall to Earth. A series of 14 photographs are taken over the next half hour. In the first seven, the Moon's image is too large to fit onto the Hasselblad frame. The next seven images do have a lunar diameter in view which allows the distance to the Moon to be calculated, as well as the approximate time.

Starting with AS11-38-5648 on magazine O, and on the assumption that the 80 mm lens is on the camera, the next six images of the Moon can have their diameters measured. This yields a value for the Moon's distance at the moment the photo was taken. Additionally, by interpolating between the PAO officer's announcements of distance, an approximate time for the exposures can be gained.

The photography continues on magazine V for another seven images, with the same lens on the camera.

Jay Greene was one of NASA's premier Flight Controllers and had earlier been FIDO (Flight Dynamics Officer, or FDO) for the descent and landing of Eagle. He was the Flight Director for the Space Shuttle Challenger during its disastrous ascent in 1986.

Comm break.

The Digital AutoPilot routine in the computer needs to know the mass of the CSM to properly control the spacecraft's rotation. 26,370 represents the mass in pounds [11,961 kg].

Long comm break.

The state vector currently within the computer is the result of modifying the state vector they had prior to TEI with the acceleration data measured by the IMU during the burn. The accuracy of this resultant state vector may not be as high as could be determined by radio tracking methods (or, for that matter, the cislunar navigation that Mike could carry out). However, it is sufficient for a first look at their trajectory.

Very long comm break.

Collins, from the 1969 Technical debrief: "At T_IG, I noticed more rate-needle activity that I had seen in previous burns. I had a start transient of probably 0.4-ft/sec activity on the rate needles in both pitch and yaw; there was very little attitude deviation. It was just a fairly rapid oscillation of both the gimbal position indicators and the rate needles and it damped itself down I'd say within the first 10 or 15 seconds of the burn. In roll, the vehicle was deadbanding. Instead of plus or minus 5 degrees, it appeared on my attitude indicator to be more like plus or minus 8-degree roll deadband and it was banging against the roll stops fairly crisply. It would cruise over, hit deadband and jets would fire, and it would go back the other way. This roll deadbanding was quite obvious during this burn as opposed to the other burns. I think all these indications are normal. They were just somewhat exaggerated during the first 20 seconds of the burn compared to the more damped case of having the LM attached."

The following animation has four frames from colour Magazine V (AS11-44-6664 to 6667) and 26 frames from black and white Magazine O (AS11-38-5654 to 5679).

The photography of the Moon switches from magazine V using colour film to magazine O which is loaded with black and white film. It appears that little time passed during this changeover as between images 6667 and 5654, the point of view of the Moon is very similar. Because the spacecraft is coming around from the far side of the Moon, the view of the globe is constantly changing and the lack of change between those two photos shows that they were taken within a short period of time. It is also clear that they are still using the 80mm lens. Had the 250mm lens been fitted, then the whole Moon would not have fitted into the frame and still maintain the same point of view. The spacecraft would have to be three times further away, by which time, the view would have changed radically. And anyway, by that time, the crew will be in their rest period. Another clue that these two photos were taken at a similar time is that the calculation of the distance from the spacecraft to the Moon (using image size and triangulation) gives a very similar result of 5,000 km in both cases.

Mike has been carrying out a realignment of the guidance platform using P52 in the computer. This realignment will also see the platform being aligned per the new REFSMMAT, one suitable for the Passive Thermal Control rotation that the spacecraft will soon adopt. The P52 exercise was achieved by Mike sighting on stars 01 (Alpheratz, Alpha Andromedae) and 43 (Deneb, Alpha Cygni). The angles through which the platform had to be rotated to compensate for drift in its orientation were +0.469° in X, -0.217° in Y and +0.383° in Z. The difference in the actual angle between these two stars and Mike's measured angle is 0.03°, an acceptable result.

The aircraft carrier USS Hornet is due to pick Apollo 11 up in the Pacific Ocean after splashdown.

As the crew left the LM's cabin, they stopped the cooling to the primary guidance system just to see how long it would last. The IMU lasted about 4 hours and now the LM's computer has also failed. The PAO announcer will further update this in about half an hour.

As outlined on page 1-18 of the Flight Plan, the CSM X-axis was pointed roughly north on the way out to the Moon and be pointed roughly south on the way home. The reason given is " to enable simultaneous viewing of the earth and moon through the side windows while maintaining a favorable high gain antenna position."

As a result of disabling quads C and D, the crew will use only A and B. This will mean that any thruster firing will be uncoupled (i.e. not matched by an opposite thruster) and thus a small amount of translation will result, which will perturb their trajectory very slightly. However, it does mean that rotational control will be more gentle as they look to settle the spacecraft's attitude prior to beginning the PTC roll. Since quad A has the least propellant, as Mike is about to report, balancing the propellant usage is not the reason.

Comm break.

Very long comm break.

More photos are taken of the receding Moon. Distances and times were calculated by measurement of the Moon's image on the scan and comparing it to the full width of the photo.

Armstrong's estimate is a little short, which is unsurprising due to the difficulty of judging distances on the lunar surface.

As indicated by this LROC photograph, the distance between the PSE and the LM body is half of the scale bar which shows that the seismometer was emplaced about 25 metres [82 feet] from the main body of the LM descent stage, at the 9.30 o'clock position, where the LM's straight forward orientation is 12 o'clock.

Comm break.

Unintended coverage by dust kicked up at lift-off could account for the seismometer absorbing more heat than intended. Apollo 11 did not have film coverage of the moment of lift-off but this film, taken two years later of the Apollo 15 lunar lift-off, does show that the descent stage formed an effective shield to keep the rocket blast from stirring up the surface dust. It also shows the Kapton foil insulation flying away form the descent stage on ballistic paths.

Comm break.

Long comm break.

The crew are preparing to impart a rolling motion to the spacecraft to even out temperatures across its surfaces. This is the Passive Thermal Control or PTC mode. Before they begin the roll, they want the CSM to be as still as possible, minimising as many unwanted rotations. This is thought to help the subsequent roll to be stable and allow the crew to sleep without having to be awakened to restart it.

Strictly speaking the LM hasn't completed 22 revolutions of the Moon because it spent 11 of the CSM's orbits sitting on the surface.

Long comm break.

Comm break.

Comm break.

Mike has evidently found more chlorine ampules for sterilising their water supply. 'Eureka' is derived from a Greek word meaning 'I have found it.' It is often attributed to the ancient Greek polymath Archimedes who is said to have run naked from his bath shouting it as he had realised he could use the displacement of water as a way of determining volume. This would be combined with a measurement of weight to check the purity of gold.

Comm break.

Very long comm break.

Flight Plan, page 3-103.

Owen Garriott of the Maroon Team has taken over as CapCom from Charlie Duke of the White Team.

Journal Reader Rob Bailey points out that Milt Windler named his team Maroon in honor of the colors of his Alma Mater, Virginia Tech. Mike Collins is having fun with them calling them 'purple people.'

Comm break.

Comm break.

Very long comm break.

Four more photographs are taken on magazine O of the receding Moon. Distance and time have been calculated based on image size measurements and triangulation. In these images, we are seeing more of the lunar near side and less of the far side.

Now safely coasting towards Earth, the exhausted crew of Apollo 11 can finally sleep soundly and will remain out of radio contact for almost 10 hours. Mission Control continues to monitor the spacecraft systems, of course, and the astronauts can be woken up if the situation requires it.