Apollo 15

Although they have travelled for only one of the three days that it takes to coast to the Moon, they have passed the halfway point in terms of distance.

There has been a change of shift in the Mission Operations Control Room and as part of that, Karl Henize has taken over the CapCom console from Joe Allen. Meanwhile, the crew of Apollo 15 are having their midday meal and preparing for the next activities on the Flight Plan.

Flight Plan page 3-36.

Before a series of photography experiments, they are to check the pressure difference across the forward hatch and if it is reading not greater than 2.7 psi (18.6 kPa), they will place the Tunnel Vent valve to its Vent position. This will dump the bulk of the LM's atmosphere prior to its repressurisation from the CM.

Very long comm break.

Flight Plan page 3-39.

The next hour is dominated by the sextant photo test and, afterwards, UV photography of Earth. Page 3-38 of the Flight Plan is given over to the checklist for these two activities.

For the Sextant Photo Test, the Command Module's DAC (Data Acquisition Camera) will be attached to the sextant eyepiece and will use a magazine of very high speed black and white (6,000 ASA) 16-mm film. The DAC was attached to the sextant in much the same way as any other lens might be attached to a camera; the regular lens would be removed, and the camera would instead use the sextant's optics. A cable is carried in the CM to provide the DAC with power from the spacecraft's 28V DC supply. The spacecraft is maneuvered to the required attitude for the first of two tests. Using Program 52, the sextant is pointed at the required star and the cabin lights are dimmed. Two seconds of film are run through the camera at 24 frames per second, using a high shutter speed, in order to bring a fresh frame into the gate; one which would not have any fogging through inadvertent light leakage in the magazine. Four frames are then exposed for the test, one for 60 seconds, then 20, then 5 and 1 second. Finally, another 2 seconds are run through at 24 frames per second to ensure the test frames are not on the surface of the take-up pack. This procedure is repeated for a second target.

There ought to be a cable which connects between the camera and the spacecraft to carry data to permit controllers on the ground to see when the camera is operating. This PCM (Pulse Code Modulation, essentially digital coding of information) cable ought to be 2.74 metres (108 inches) in length, but is not long enough to reach from the DAC, mounted on the sextant eyepiece, to one of two receptacles on panels on the spacecraft walls. These small panels, number 227 on the right-hand equipment bay, and 162, below the lower equipment bay, have only a socket for the cable with a screw-down cover, and a switch to apply power to it. It is unclear what the exact length of the power cable onboard is.

The crew have tried all the cables they can find, including the one for the Hasselblad camera.

Long comm break.

The crew will record cues on the voice track of the DSE recorder in lieu of using the cable. However, Al Worden expresses his opinion on this recorder that the experiment is not going to work very well because of a large number of bright particles visible outside the spacecraft. He is aware that the exposures are long and the film is very sensitive.

Very long comm break.

Panel 162 should be nearer to the sextant than 227, yet they cannot reach that either.

Very long comm break.

With the HGA (High Gain Antenna) stowed behind the Service Module, communication is through one of the four omni-directional antennas mounted around the Command Module. Mission Control have limited control over which antenna is currently in use. They can switch between antenna D and another. In this case, it is unclear why they have asked the crew to switch for them. Perhaps low signal strength means they are not able to get the switching signal through.

Once the sextant photo test is complete, the crew will take a sequence of photographs of Earth. The procedure is similar to that employed at 010:10:07, including the change to the second pair of photographs taken through filter 2. To summarise, the UV transmitting window, number 5, is brought to face Earth. Magazine N, loaded with UV sensitive film, is mounted on the Hasselblad along with a UV transmitting lens. Two photographs are taken through each of four filters.

The first four images appear grossly overexposed with cloud detail only appearing in the fourth. Multiple reflections, presumably from the quartz panes, are visible. The pair taken through filter 3 are rather dark while the last two, taken through filter 4, are good, clear images of a half-Earth.

Afterwards, magazine M is used on the camera to take a photograph using 64 ASA Ektachrome film.

Flight Plan page 3-40.

This query refers back to a comment in the Flight Plan at 032:00:00. This instructed the crew to increase the pressure difference between the two craft, if it was less 2.7 psi (18.6 kPa), by using the Tunnel Vent Valve. As CapCom Karl Henize is about to explain, this is to allow the atmosphere in the LM to be replaced as, in an hour's time, they will be opening up the tunnel to enter the lander.

Long comm break.

At launch, the LM is filled with ambient air at sea level pressure. This is vented away via the open overhead Pressure Equalization Valve (also referred to as a 'depress valve') as the LM ascends through the atmosphere. After the CSM docks with the LM, the LM cabin is pressurized to about 34 kPa (5 psi) using the CM's air, again through the open depress valve. No replenishing is done after this, and the LM's atmosphere is allowed to leak away. Valuable information about leak rates are obtained in this way, which is essential for calculating LM's oxygen usage later in the flight. However, this atmosphere may have a significant amount of nitrogen in it as the CM was still flushing out the nitrogen/oxygen mix it had from the moment of launch. Nitrogen is undesirable in the Apollo cabin because it can build up in the suit circuit, not being metabolised by the body. Therefore, to flush out any remaining nitrogen in the LM's atmosphere, the air it gained earlier in the mission is dumped and replenished with the purer air now in the CM cabin.

Comm break.

The absolute pressure of the cabin is a little higher than normal. They don't, at present, have a measurement of the absolute LM cabin pressure so they rely on reading differences in pressure across the tunnel hatch. To bring the LM cabin pressure down to the required value, they must take account of the CM cabin pressure.

Note that Houston are asking them to switch between antennas that are 180° apart on the skin of the spacecraft. They have been using Omni Delta for the last half hour and Mission Control have not been able to switch it. When the crew switch to omni B, Mission Control ought to be able to switch between it and D.

Comm break.

As happened soon after the crew's awakening today, the read-outs from the PRDs (Passive Radiation Dosimeter) are low on the crew's priorities.

One of the last items on the checklist for the UV photography was to comment on the condition of window 5, the right-hand window which is made from quartz panes to transmit UV light.

Venting of the LM is slower than Dave would like and seems to be settling at a figure just below what Mission Control requested.

Comm break.

Long comm break.

Pressure drop during a vent is relatively fast at first but it slows as the pressure, which is driving the gas out through the valve, also drops. This effect was noted during depressurisation of the LM before the moonwalks on the lunar surface. The inward-opening door of the LM could not be opened against more than about 0.7 kPa (0.1 psi) of pressure and crews got impatient waiting for the final few wisps of air to escape through the dump valve.

Very long comm break.

The crew is on page 3-40 of the Flight Plan. From there, they are instructed to follow page 2-1 on the CSM Systems Checklist. This deals with the steps required to prepare the LM to be entered by pressurising it, removing the forward hatch, docking probe and drogue, and reading off the tunnel index angle. This angle indicates by how much the CSM is rotationally aligned with respect to the LM in the current docked configuration.

The Docking Tunnel Index marks are painted on the inside of the tunnel connecting the CSM and LM. Measured in degrees, they have complementary scales on the CM and the LM. When perfectly aligned, the indexes are lined up, with a zero degree difference. Aside from a simple indication of how accurate the docking was performed, this angle is used as part of the initial calculations necessary to perform a coarse alignment of the LM's inertial platform.

This coarse alignment is performed using a surprisingly low-tech procedure: To simplify the arithmetic to come, the spacecraft is oriented at right angles to the present platform alignment. Thus the crew will only have to deal with multiples of 90°. The IMU gimbal angles are then recorded from the CM's computer display. Next, the angular difference, 90°, between the orientation of the CM's and LM's IMU is added in. Finally, the angular difference along the X-axis, as indicated by the Docking Tunnel Index Angle is added in. The resulting set of angles is a good first order approximation to how the LM's platform should be oriented. These angles are entered into the LM's computer, and the platform is then torqued to that orientation. Following this "coarse alignment," it becomes easier to perform a "fine alignment", a standard P52 IMU realignment, as the appropriate stars will already be in the sextant's field of view.

The LM is pressurised from the oxygen in the CM. The CM pressure is brought up prior to this and when filling of the LM has brought it down to a predetermined figure, it is repressurised before recommencing LM pressurisation. Once the two spacecraft are at the same pressure, the hatch and docking equipment can be removed.

Irwin, from 1971 Technical debrief: "Removing the tunnel hatch and the probe and the drogue was an order of magnitude easier than it's ever been in practise. It went exactly as we'd seen on the mockup over here. I thought it was a very easy operation. We put the hatch underneath the left-hand couch. We put the probe in the center couch, and lashed it down. We put the drogue underneath the left-hand couch and tied it down so that we had good clear access to the tunnel area. I thought the whole operation was very easy. No problem."

The crew are also to prepare an assemblage of items which will be transferred to the LM and a list of these is given in the LM Activation Checklist. The list goes over a second page. Among these items are 13 magazines for the Hasselblad cameras that Dave and Jim will use on the Moon, and 10 magazines of 16-mm movie film for the LDAC (Lunar Data Acquisition Camera). Use of the LDAC would be largely unsuccessful during the lunar EVAs.

The spacecraft is now maneuvered to the preferred attitude for the transfer to the Lunar Module. Angles are also given in the Flight Plan for pointing the HGA.

Comm break.

The probe sits between the CM and LM hatches. By the operation of a lever, the probe folds to allow it to be removed from either the LM or the CM side.

A narrow beamwidth on the HGA is required for the upcoming TV transmission, as well as telemetry from the spacecraft during LM activation.

Comm break.

Television coverage of the ingress to the LM is due to be transmitted from 33:45 to 34:30.

All the 'extra gear' is the items they will take over to the LM once they get it opened up.

Comm break.

Apollo 15 has just experienced a glitch in its electricity supply. There is a disturbance in the communications at 033:47:26.

Journal reader, George Giusti noticed the similarity between Dave's read-out to Mission Control, and the comm from Apollo 13 immediately after it was rocked by an explosion in one of its oxygen tanks. He wondered if the Caution and Warning light caused the Apollo 15 crew any moments of "deja vu".

Scott, from 1999 correspondence: "The Main Bus B Undervolt was no big deal at the time - we had seen so many of those during sims due to so many causes, that, at least for me, the thought of another A-13 situation did not even occur - there are just too many other loops to go thru to begin isolating the cause. Besides, the probability of the same fault occurring on two missions is really remote, especially when the first one was so well exercised - it's usually the new things that will get you; so its best to look for new trails rather than going back over plowed ground."

The glitch has caused the High Gain Antenna to cease pointing at Earth and must be aimed correctly again.

Dave Scott is actually asking "How is your received signal from our High Gain Antenna?

Long comm break.

Dave and Jim have moved into the LM. Al has configured the TV system and is transmitting a signal. He floats through the tunnel with the camera to join them. The first image we see is of Jim working at his station to the right of the LM.

Comm break.

For a while, we can see Jim checking the positions of the circuit breakers against a diagram in the checklist. Al them moves the camera to the instrument panel and Dave comes into shot.

The ASA is a set of accelerometers and gyros in the LM which provide inertial information for that spacecraft's alternate guidance electronics.

Long comm break.

The camera is giving a good shot of the instrument panel. The Sun is coming in on Dave's side and small particles can be seen floating in the middle of the cabin. After a while, Al aims the camera to the left of the panel where Dave is inspecting one of spacecraft's displays, the tapemeter. Bright sunlight illuminating objects in the camera's field of view causes the automatic exposure to darken the image. When the camera's exposure rises again, we can see quite a few particles floating around Dave.]

Al then takes the camera towards the instrument panel and the tapemeter. As he does so, many particles become visible.

Flight Plan page 3-41.

The LM Activation Checklist is being followed by the crew. The initial status of the LM circuit breakers is checked by reference to page 1-3 and page 1-4 which show nine diagrammatic lines which illustrate each breaker in its position with respect to the others. If the breaker is white in the diagram, it should be open; black, and it should be closed. Pilots will be familiar with the white ring found on aircraft circuit breakers which appears when the button is pulled out and the breaker is opened.

This begins the first of two LM inspection and housekeeping tasks that Dave and Jim will perform on the way to the Moon. The second will be relatively brief as it is primarily to allow flight controllers to gather data on the health of the lander's batteries. During this first visit, film and equipment are transferred to the LM from the CSM's stowage cabinets and initial checks of various LM systems are performed. A check is made to see that all the circuit breakers and switches haven't changed since launch. Next, two major housekeeping tasks are performed: Unstowing equipment and setting up cameras. After that, only a few systems are truly exercised. The LM batteries are brought online, and the electrical system is checked. All the various communications modes are checked, the Environmental Control System is activated and the Oxygen Purge Systems are checked out.

Several important systems are not tested. The guidance system is not powered up and neither are the RCS, Descent and Ascent Propulsion Systems powered up or pressurized in order to minimize the stresses on these systems. The useful lifetime of the propulsion components are limited once they are pressurized. For example, once the supercritical helium system for the Descent Propulsion System is opened up through a series of explosively actuated valves, heat starts to leak into the system, raising the pressure of the entire system. Once the helium pressure rises above a certain point, the excess pressure is released through a "burst disk". Once broken open, all the helium in the system is vented out into space and descent engine capability is lost. The movie, Apollo 13, portrays the moment when the LM Aquarius' burst disk blew for just this reason, providing a small moment of drama in the film during the journey home, though fully expected in the real flight.

There has been a suspicion that the loss of S-band comm at about the same time as the electrical glitch meant the two were related. Mission Control are not so sure now and think that the HGA reached the limits of its articulation when a problem on Earth interrupted the signal to the spacecraft and caused the antenna to lose lock.

Long comm break.

The TV picture clearly shows Dave going through each switch and control as he works his way across the panel. Al goes to the right where Jim is checking the switches on panel 14 which deals with the LM's power systems. Between the two crewmen, we see one of the two backpacks or PLSS (Portable Life Support System) they will wear on the Moon. Further round to the right, behind Jim's station, we see the controls for the ECS.

It is actually Jim who is checking the ECS.

Comm break.

Jim removes a movie camera from its stowage position and mounts it on a bracket in his window.

Neither the LM Activation Checklist nor the Flight Plan explain why Al is requesting a time check from Mission Control at this moment but it is probable that he simply wishes to ensure his mission timer is correct before using it to help Dave set the clock in the LM.

Long comm break.

Al is giving us an excellent view of the LM instrument panel. Above it can be seen te AOT (Alignment Optical Telescope) with the yellow bars around it. This is the equivalent of the CM's optics. Both Jim and Dave are sometimes visible in the shot. Behind Dave's head is his window, lit by the Sun. When the camera's exposure drops, we can clearly see the lines inscribed on the window that Dave will use as a guidance aid when landing on the Moon. This is the LPD (Landing Point Designator). With the camera now being held by Dave while Al returns to the Command Module, the shot returns to Jim and we catch a glimpse of the keyboard for the AGS (Abort Guidance System), a small computer that provides redundancy.

Long comm break.

Two independent sets of VHF systems are tested. Both are used exclusively for communications between the LM and CSM. An additional function of the VHF system is that it supplied range and range-rate data to the CSM, which is used as a back-up to the LM's rendezvous radar. S-band communications with Earth via MSFN (Manned Space Flight Network) are also tested.

Dave turns the camera's view to look up the tunnel to the Command Module where we see Al prone across the cabin at the far end. Al then dives back into the LM towards us and takes the camera.

We see Dave at the circuit breaker panel at the left of the LM cabin with the overhead window at the top of the frame. Al then moves in to give a close-up of Panel 1, Dave's main instrument panel, where he stops momentarily on the tapemeter.

This frame is from the television pictures during the instant when we can clearly see that the glass face of the meter is broken. Just to the left is a small swatch of velcro the crew use to carry their cue cards. The camera moves to Dave's station looking over to Jim as Dave is holding it once again While Al return to the CM to begin a communications check.

Now that there are essentially two separate entities in communication with each other, as far as radio is concerned, the crew begin using the callsigns for each vehicle; 'Endeavour' for the Command and Service Module, 'Falcon' for the Lunar Module.

Comm break.

"5 square" means 5 by 5 or, as Karl Henize put it "loud and clear.

Comm break.

While Goldstone work to sort their problem, we see Jim working at panel 12 to his right, the comms panel.

The camera pans from Jim to Dave, implying that Al is being cameraman again, filming from above the ascent engine cover. For a few seconds, the TV disappears. When it returns, Dave is in shot.

On the TV, we can see Dave trying to speak to Mission Control but we cannot hear him.

Since the data link is working, the crew know the S-band system is up and running and only the voice requires patching through.

Telemetry data is relayed to Earth over the S-band system in a PCM (Pulse Code Modulation) format. Two data rates are available; 9,600 bits/second and 56,000 bits per second. Although contemporary readers will see these speeds as being perfectly ordinary modem speeds, it is important to remember that this hardware was designed in the mid-1960s; almost 30 years before commercial modems became widely available.

Comm break.

The camera returns to the instrument panel and particularly, the DSKY (Display and Keyboard).

In the LM, a switch has been thrown which puts the crew's voice circuit onto the S-band.

As Dave and Jim set up the S-band comm system in the LM for a voice check, they have switched it to Voice (or Vox) mode, as shown on page 1-15 of the LM Activation Checklist. The transmitter is controlled by a voice-operated switch and every time they speak, they can be heard in Houston. This time, it is fully intended, but on occasion, Apollo crews have forgotten Voice mode was on, or had accidentally been selected, requiring a gentle prod from the CapCom to prevent further embarrassment.

Apollo 15 is the first mission on which the LM is being entered on the day after TLI. Previous missions have had the crews perform these tasks the day before entering lunar orbit. The foresight in checking the LM early for any problems is confirmed when Dave and Jim find the glass cover to the Range/Range-Rate tapemeter broken with small pieces of glass scattered throughout the LM cabin.

This breakage raises some concerns. Floating shards of glass could be inhaled by the crew or interfere with the suits and associated hoses. The tapemeter displays crucial range and range-rate information during the landing and rendezvous and Mission Control must determine whether the loss of the glass impairs its operation. Two out-of-focus photographs are taken of the instrument panel, including the tapemeter.

Several of the critical instruments in the LM were implemented as tapemeters, an instrument which displays its values through a moving length of tape rather than a needle, gauge or digital display. A tape, with values such as altitude pre-printed on it, was stored on spools not unlike a video cassette tape cartridge. The tape moved forwards and backwards under control of the LM's computer, and was read using an indexing bar on the face of the instrument. Tapemeter instruments were popular in a number of high performance aircraft at the time, most notably the astronauts' favorite, the F-106. Further validating the utility of tapemeters, the Space Shuttle would incorporate several in the early versions of its cockpit.

Scott, from 1971 Technical debrief: "Having the LM housekeeping moved up a day, or the day after TLI, gave them a chance to do all the testing on the tapemeter. That gave me a warm feeling to know that they had checked the thing out and it would work with a broken outer pane of glass."

Scott, from 1971 Technical debrief: "I think it is a good idea to go take a look at the LM early and analyze your problems and get a good handle on them before you get too far down the road. If they want to take another look at the batteries, the second housekeeping day is no problem. It is nice to go back and take another look around anyway. We got another chance to clean up some more glass. We did find a number of pieces on the second day."

These are the checks in step 3 on page 1-15.

This is step 4 on page 1-16.

Comm break.

The TV from the LM is continuing but a disturbance in the picture upsets the colour system on Earth, making the colours wrong. The camera settles on Dave's hand where he is holding a checklist and a roll of grey tape with pieces of glass stuck to the end of it.

Where the crew and ground use the term "PCM", the checklist uses "TLM" though they both mean the same thing as the telemetry uses Pulse Code Modulation, an early term for what we now just call "digital".

Comm break.

Jim is relaying the voltage on two batteries that provide power for the explosive devices in the LM. There are quite a few of these on the LM, used for separating the stages, actuating helium valves and deploying the landing gear.

The checkout of the sequence camera, or the LDAC (Lunar Data Acquisition Camera), a battery powered version of a similar camera stowed in the CM, is step 10, the final step on page 1-16.

Page 1-17 of the LM Activation Checklist has a single step which is to read and record the source pressures on the OPS (Oxygen Purge System). The OPS attaches to the top of the PLSS (Portable Life Support System), the backpack worn by a crewman on the surface. The OPS is intended as an emergency supply of oxygen for up to an hour during Dave and Jim's forays onto the lunar surface, in case an excessive leak develops in a suit or if there is a failure in the PLSS. It consists of two high-pressure, non-rechargeable, spherical tanks which are fed into the suit through a regulator at one of two rates, 50 per cent or 100 per cent flow. Although the PLSSs are discarded on the Moon, the two OPS will be retained in case the LM crew have to transfer to the CM by embarking on a spacewalk. During the trans-Earth coast, one will be used by Al Worden when he goes out to the SIM bay to retrieve the films from the Panoramic and Mapping cameras. Then, on the journey home, the CM cabin will be topped up by emptying both units.

A gauge on the OPS gives a reading of the pressure in the tanks, This is called the "source pressure" in the checklist.

Comm break.

When Al docked the CSM, there was some swinging of the spacecraft in pitch and yaw after initial capture and soft-dock. However, Al's alignment in roll has essentially been spot on.

Al takes the camera back up the tunnel, looking down towards the ascent engine cover.

Very long comm break.

TV coverage ends.

Flight Plan page 3-42.

Very long comm break.

Al's query is based on the note for Mission Control listed on the left-hand column of the Flight Plan.

The PTC (Passive Thermal Control) was stopped at around 026:55:00. It is desirable to restart it before too long to keep temperatures in the Service Module's systems under control and to protect the Command Module's heatshield. See commentary at 005:29:22, 010:40:06, 011:44:58 and at 013:03:55 for discussion of the PTC maneuver.

Verb 49 commands the spacecraft to maneuver to an attitude defined by the crew. In this case, Al is requesting the initial attitude for the PTC, and he will then wait for rotation rates to settle.

Mission Control believed that the loss of the S-band link at about 033:48:00 was due to a problem at the ground receiving station. However, the crew's report of a glitch in the spacecraft's power buses is causing them to rethink their theories.

Comm break.

Page 2-15 of the Flight Plan carries a table listing a schedule of block data read-ups, or PADs which are to be given to the crew throughout the mission. These PADs give the necessary information to allow the spacecraft to return to Earth in an emergency. The table lists what the purpose of each PAD is, when it is due to be read up, approximately when it would be used, and what type of PAD it is. A complete PAD for use in Program 30 was read up at 001:30:00 GET and the following PADs gave the data to be used in Program 37 where they are factored in with already available data if an abort curtails the mission. These P37 PADs are designed to return the spacecraft to Earth immediately, without continuing around the Moon, and are effective until 60 hours GET.

CapCom Karl Henize has just read up the eighth PAD in this table, the LOI minus 5, or flyby PAD, and it is a complete P30 PAD.

An interpretation of the PAD follows.

• Purpose: Apollo 15's current path is designed to allow it to reach its planned landing site, but it is not on a trajectory that would bring it back to a safe landing on Earth. This burn is calculated to set them onto such a homeward trajectory and so would only be used in the event of an emergency. It would be fired 5 hours before the planned insertion into lunar orbit. At that point, Apollo 15 would be so near the Moon that it would be easier to allow Apollo 15 to continue and fly around the Moon.
• System: The burn would be made with the SPS engine under the control of the Guidance and Navigation System.
• CSM Weight (Noun 47): 66,655 pounds (30,234 kg).
• Pitch and yaw trim (Noun 48): +1.24° and -0.11°.
• Time of ignition, T_IG (Noun 33): 73 hours, 30 minutes, 56.8 seconds, Ground Elapsed Time; about 5 hours before the planned Lunar Orbit Insertion would have occurred.
• Change in velocity (Noun 81), fps (m/s): x, +257.2 (+78.4); y, +230.9 (+70.4); z, -314.9 (-96). These velocities are all with respect to the Moon and the local vertical that the spacecraft makes with that body.
• Spacecraft attitude: Roll, 331°; Pitch, 109°; Yaw, 79°. This is with respect to the attitude of the guidance platform, itself aligned to a preferred REFSMMAT.
• H_A (Noun 44): As with the previous P30 PADs for returning to Earth, there is no value for H_A (height at apogee); the apogee of the expected orbit is greater than the 9999.9 nautical miles value that the computer will display.
• H_P (Noun 44): Expected perigee of resulting orbit is 20.8 nautical miles (38.5 km). This height is theoretical because the spacecraft would encounter the atmosphere and re-enter before reaching this altitude. However, this perigee value will bring the spacecraft in at the correct angle for entry.
• Delta-V_T (total velocity change): 467.6 fps (142.5 m/s). This is a vector addition of the three Delta-V components given above.
• Burn duration or burn time: 1 minute, 11 seconds.
• Delta-V_C: 463.0 fps. This is the value to be entered into the EMS Delta-V counter. It is a lower value, to take into account the extra thrust the engine imparts as it tails off. The EMS will decrement this value to zero and stop the burn if the G&N system fails to do so.
• Sextant star: Star 02 (Diphda, or Beta Ceti) visible in sextant when shaft and trunnion angles are 148.5° and 26.4° respectively.

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, however, a more important milestone is when atmospheric drag on the spacecraft imparts a deceleration of 0.05g.

• Expected splashdown point (Noun 61): The landing on Earth is calculated to occur at a position of 13.32° North, 174.04° west, in the Pacific Ocean, northeast of the Marshall Islands.
• Range to go at the 0.05g event: 1,098.7 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 0.05 g 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: 36,170 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: 170 hours, 59 minutes and 57 seconds GET.
• GDC Align stars: Stars 36, Vega (in Lyra) and 43, Deneb (in Cygnus) are to be used to align the gyro assemblies if it is not possible to use the guidance platform for this purpose.
• GDC Align angles: x, 209°; y, 009°; z, 349°.

With the SPS propellant tanks nearly full, there is no need for an ullage burn to settle their contents. The burn is made with the LM docked as its consumables would be useful to have in an emergency. The attitude reference is the preferred REFSMMAT so as to avoid the possibility of reaching gimbal lock. The mass of the LM is 36,220 pounds (16,429 kg), a figure which is required to allow the attitude control system to calculate the moment of inertia of the stack, and thence the required RCS thrusting durations.

Long comm break.

Talkbacks are indicator windows which, by putting a gray flag in front of a striped, or barber pole background, annunciate the status of various systems aboard the spacecraft.

Comm break.

The crew may be trying to work out Mission Control's thinking when they asked this question.

Panel 5 is a panel of mostly circuit breakers adjacent to the right of the Main Display Console.

Comm break.

The button type circuit breakers are closed by pushing the button in. If the button is pulled out, or if the breaker should 'blow', this is highlighted by a white ring around the button being exposed.

Panel 226 has five rows of seven or eight breakers per row. It is sited on the spacecraft wall, well to the right of the Main Display Console.

Very long comm break.

Scott, from 1971 Technical debrief: "I think the only consequence of that was [that] it sort of changed your pattern of operation down in the LEB because you didn't have the timer down there."

Worden, from 1971 Technical debrief: "We didn't have the mission timer down there, and the other thing that I missed in the LEB, which I found, I stubbed my toe on a few times, was that all of the program lights in the DSKY [Display and Keyboard, the computer's keypad] were out. The only thing we had was caution and warning panel down there. That was operational, but all of the status lights on the DSKY were out. They went with that circuit breaker. The backlighting in the EMS [Entry Monitor System] scroll was out. That was also off that circuit breaker."

From the 1971 Mission Report: "Postflight testing [showed that a] short circuit was in the mission timer. ... Teardown analysis revealed a shorted input filter capacitor. The capacitor is rated for 200-volt d-c applications and is being used in an a-c application at voltages up to 115 volts. Since the dielectric in the ceramic capacitor is a piezoelectric material (barium titanate), the 400-cycle [per second] a-c voltage actually causes the materials in the capacitor to mechanically vibrate at that frequency. Over a period of time, the unit could break down because of mechanical fatigue. This may have been the cause of failure of this capacitor."

The popped circuit breaker will not be reset throughout the mission. The circuit it was protecting supplied integral lighting to the EMS (Entry Monitor System) scroll, some lights in the DSKY (Display and Keyboard) and various other backlighting as well as the now defunct mission timer. The loss of these facilities is not serious but will have subtle repercussions on spacecraft operations in lunar orbit when Al will try to work around the lack of a timer in the LEB by using one of the programs in the computer to make the LEB DSKY into a timer. This then impairs the updating of the spacecraft's state vector over a period of time.

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Mission Control are hoping that by re-enacting the failure from their end, they can reproduce the same consequences.

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Flight Plan page 3-44.

Only two quads are needed for the gentle spin up. Uncoupled single quads can be used to make the tiny corrections required to damp the spacecraft's rates. The crew are going to use the slightly faster spin rate that they resorted to when the first attempt at PTC failed at 013:37:48. The PTC is about an hour late in getting started.

Mission Control will gain the best control of the omni-directional antennas by having the crew switch to antenna B. They will then be able to switch between B and D, as the PTC mode rotates the two opposite-facing antennae towards and away from Earth.

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Actually, it is a bit faster than that at 0.375° per second, as discussed at 036:58:04 above.

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Although there was no visible damage to the instrument itself, it is important to ensure that such a vital instrument will operate properly with an internal environment it was not designed for as the LM cabin will spend extended periods as a vacuum while on the lunar surface. For example, lubricants between the moving parts of the mechanism may outgas and dry up.

Before the breakage, the interior of the instrument was hermetically sealed and pressurised with helium at 101 kPa (14.7 psi). Extreme temperatures and a vacuum in the LM cabin could increase this to 111 kPa (16.1 psi). The post-mission report concluded that the glass must have had a flaw which exceeded that which causes breakage at the stress levels encountered during the early stages of the flight. Subsequent flights had a second pane of glass to reduce the pressure differential across each pane and Velcro applied Teflon covers for similar instruments to retain broken glass particles.

The SPS can still be controlled by the computer using bank B, one of the two redundant control systems for the engine. The electrical short in bank A will cause the engine to ignite as soon as it is brought online, so brief burns can be carried out using the single, good control bank. However, the rated thrust of the engine is achieved when both banks are supplying propellant to the engine. By starting and stopping the engine automatically using bank B, the crew can bring bank A into use a few seconds after ignition, and disable it just prior to the automatic cutoff, simply by operating the pilot valve circuit breaker, thereby achieving full thrust for the majority of the burn.

Mission Control have been prompting for a read-out from each crewman's PRD (Passive Radiation Dosimeter) since they awoke nearly 12 hours ago.

Between 37:00 and 37:30, the crew are scheduled to take an exercise period. They have, however, got behind their time line and it is unclear whether they had a physical workout.

The AGS (pronounced "aggs") is the Abort Guidance System, which is a back-up to the LM's Primary Guidance and Navigation System (PGNS or "pings"). It consists of 2 sections. The ASA (Abort Sensor Assembly) makes its own measurements of attitude and acceleration using separate gyros and accelerometers, feeding this information to a computer (AEA or Abort Electronics Assembly) which has the simple brief to get the crew back into safe lunar orbit if the primary system should fail.

Both spacecraft use button type circuit breakers familiar to pilots, which are closed when the button is in but which reveal a white ring when it is out and open. Diagrams in the LM Activation Checklist use white or black circles to indicate which breakers should be closed or open in the required initial configuration and the pattern of circles allow a quick visual guide that the row of breakers in front of him are properly set.

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Flight Plan page 3-45.

At 38 hours, the crew are scheduled to begin their evening meal so Mission Control are leaving them alone for over an hour. The few tasks in the Flight Plan include Al donning his biomedical harness and Dave taking his off. It is Al's turn to be monitored overnight. They will also change one of their LiOH canisters, taking number 3 out of the A receptacle, stowing it in compartment B5, and replacing it with canister 5.

The Interim Stowage Assembly is a stowage bag that is spread across the forward instrument panels in the LM, including the Range/Range-Rate tapemeter.

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Battery B was charged yesterday, and is scheduled to be recharged tomorrow.

Panel 8 is adjacent to the left side of the Main Display Console. These two breakers, on the fourth from top row, route power to the gimbal motors that aim the SPS engine.

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The final act of the day is the presleep checklist which is on page 1-26 of the CSM Systems Checklist. As part of this task, the crew will give a status report on their condition, and a read-out of the onboard gauges. They will also stir the cryogenic hydrogen storage tanks, chlorinate their water supply and dump the contents of the E(erasable)-memory to Mission Control before configuring the communications system for the night.

One of the steps in the presleep checklist is to open the direct O₂ valve and raise the cabin pressure to 39.3 kPa (5.7 psi), if it is not already there.

Finally, at the end of the day, Mission Control gets its dosimeter readings!

Woods, from 1998 correspondence with Scott: "Judging from the transcript, I have the impression the three of you had little time for the radiation dosimeters during the translunar coast. Despite the Flight Plan calling for readouts to be given to Houston as part of the postsleep checklist on the morning of the second day, they are deferred until the evening of that day and Joe Allen has to specifically request them on the morning of the third day. Did you have an 'attitude' towards them?"

Scott, from 1998 correspondence: "Not really, but as I recall, they did take a fair amount of time to read since they were not in a convenient location, and the values changed very little - maybe we thought other activities and items were of higher priority at the time. However, we were clearly aware of the significance of radiation, especially during this particular period of high solar activity, i.e., during the A15 mission. You have probably read The Throne of Saturn, by Alen Drury? As I recall, radiation was the major contributor to suspense in this story."

According to information from the Greenwich Photoheliographic programme on sunspot area, the entire sequence of Apollo flights from Apollo 7 to 17 coincided just about perfectly with the high plateau of solar activity at that time.

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Turning off the power to the optics system is part of the presleep checklist.

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