Category How Apollo Flew to the Moon

The next two items in the PAD – 01696, phis 00584 – gave the crew information about the orbit that was expected to result from the burn. They indicated, in tenths of a nautical mile, the expected altitudes of the orbit’s apolune and perilune using the 5-digit format of the computer’s display. Therefore, they showed that the initial elliptical orbit should measure 169.6 by 58.4 nautical miles, or 314.1 by 108.2 kilometres. The plus sign was a heart-warming confirmation that the perilune was above ground. A negative value here would have been a cause for some worry, as it would mean that the spacecraft was headed for a point below the surface and was therefore doomed!

If the three delta-r components given earlier in the PAD were added together using vector addition, the result would be the total velocity change given in the next

5-digit number 30001. This vector sunt totalled 3,000.1 feet per second (914.4 metres per second) and was labelled ‘dclta-iT. It represented the total velocity change that the spacecraft should experience along its longitudinal axis. When it was time to execute this burn, the crew were able to watch as the computer’s display showed this number descend to zero as the engine worked on the dclta-v it had to achieve in all three components.

Next in this PAD was the expected duration of the burn – 641 – in this case, 6 minutes 41 seconds. The eventual duration would depend on the actual weight of the spacecraft and whatever thrust was actually achieved by the engine. Within limits, the engine was not shut down until the required dclta-v had been achieved.

Following on was another 5-digit number 29939 This also represented delta-v and was of a remarkably similar magnitude to the total dclta-v. but slightly lower. Known to the engineers as ’delta-vc’ – the letter ‘c’ stood for counter – this was related to one of two automatic mechanisms for shutting down the engine in a normal burn. The primary means was the computer in association with the accelerometers mounted on the guidance platform. As soon as the desired delta-v had been achieved, it sent a command for engine shutdown. The secondary means was the EMS and its delta-v counter.[4] Prior to the burn, the crew entered this delta – vc value into the EMS display. As with the primary system, this number represented the dclta-v the EMS should experience as the burn progressed. During the burn, a dedicated accelerometer in the EMS measured the resultant change in velocity and its output caused the displayed dclta-v to count down to zero. When it reached zero, the EMS generated a signal to shut down the SPS engine.

At first glance, it might appear that the value for the velocity change that was entered into both the primary and secondary systems should have been equal, but in fact delta-re was always slightly smaller where the SPS was involved. This reflected the difference in the sophistication of the two control systems. When a rocket engine is commanded to shut down, the thrust never falls to zero instantly; there is always an appreciable continuation of thrust that tails off over a short span of time. The Apollo SPS was no different. The primary guidance and control system was sophisticated enough to take this tail-off thrust into account, its magnitude having been measured during ground tests and perhaps confirmed by earlier short firings of the engine en route to the Moon. The secondary system within the EMS was a much simpler affair since it was not designed for propulsive manoeuvres, but for aerodynamic braking in re-entry. It ignored the tail-off thrust, so it was left to the flight controllers to adjust down the value that they issued to the crew. If the EMS did have to shut the engine down, it would do so early enough to allow the extra thrust to complete the desired manoeuvre.

Immediately after the Apollo programme was announced, there was the question of where to land. Based on the limitations imposed by flight dynamics, especially the free-rciurn trajectory, NASA narrowed their search for a site to an equatorial zone 10 degrees wide across the Moon’s near side that ranged no more than 45 degrees east and west of the central meridian. Within this area, planners looked for an open area within which an ellipse could be drawn that represented their best guess of the LM’s landing accuracy and where a crew could probably find a level spot without having to hover for an excessive time. Additionally, they wanted a relatively smooth ground track on the approach so that rugged terrain would not fool the LM’s landing radar. The site chosen for the first landing attempt was in the southwestern portion of Mare Tranquilliiatis. In the event, Neil Armstrong and Buzz. Aldrin found that tiny errors in their descent orbit resulted in their landing six kilometres beyond the planned point.

If future crews were to undertake meaningful science on the Moon, it was essential that they be able to land at a predetermined spot on the surface to provide access to specific geological structures identified on pre-mission photography – and this was in an age before the invention of satellite navigation. To show’ that such a point landing could be made, and ostensibly to sample an unmanned probe that landed 31 months earlier, Apollo 12 was assigned Surveyor 3 as its target.

Pinpoint landings such as this were achieved using two techniques. The first was a series of sightings through the CSM’s optics of a feature at the landing site that helped navigational engineers to determine the site’s exact position, not only in terms of its lunar coordinates but also its distance from the lunar centre, a value known as its radius of landing site (RLS), there being no ‘sea level’ against which to measure height. Then, as the LM came around from the Moon’s far side for the final time before landing, engineers measured how the Doppler effect changed its radio signal and compared this with w’hat was predicted for a perfect landing. This yielded how’ far the predicted landing site was offset from the intended site. It was then simply a case of fooling the LM’s computer in moving its aim point by this offset, and then let it alter its descent profile and land at the desired position.

As Apollo matured and scientists increasingly took charge of the programme’s goals, they sought to explore more scientifically interesting locations. Landing sites for the later missions were nestled within mountain ranges that promised to provide clues to the Moon’s history. By doing so. Apollo’s planners had to face the fact that, with the exception of the equatorial belt, the Moon had not been w’ell mapped. The Lunar Orbiicr missions had been tasked to support Apollo, and so had photographed selected parts of the equatorial zone in great detail. Once this task was completed, the Lunar Orbiter programme was released to the scientists to garner wider photographic coverage at the expense of resolution.

Relatively poor imaging meant that Apollo 15, the first mission to leave the equatorial zone for a more northerly site, had to contend with significant uncertainty in the position of its landing site, not only in terms of its latitude and longitude, but also its RLS value. Additionally, the crew’ had to contend with landing at a site surrounded on three sides by mountains, and literally thread their way between peaks that rose more than four kilometres above the surrounding landscape. Planners designed a steeper approach trajectory that dealt with the mountains, and

careful interpolation of the available Lunar Orbiter imagery allowed the site at Hadley Rille to be certified as safe for landing.

The ability to track a feature at the landing site, first practised on Apollo 8, gave the trajectory team the confidence to make a successful landing precisely where they desired. It was A1 Worden’s task as CMP on board Endeavour, to make repeated sightings of a selected feature close by Falcon s planned point of touchdown, a crater appropriately named Index sited at the end of a line of craters named after three of the books of the New Testament.

Prior to the landmark tracking exercise, mission control read up a PAD that would help Worden to coordinate his activities. These were a set of timings and an attitude: T1 was when the landmark appeared on the horizon; T2, which came soon after Tl, was an appropriate time to begin pitching the spacecraft nose down to ensure that the landmark kept within the articulation range of the optics; TCA was the time of closest approach of the spacecraft to the landmark; and T3 marked the end of the tracking exercise. Additional information often included a suitable initial attitude for the spacecraft at which to begin to pitch down, and a note of where the landmark was expected to be, in both position and altitude. If that information was not forthcoming, approximate values could be obtained from the flight plan.

It is interesting to note how longitude was handled by the software in the computers in the CSM and LM. Conventionally, longitude around a body would be expressed in the range 180 to —180 degrees. Using the 5-digit display of the DSKY, a large longitude value could only be represented to two decimal places, e. g. +178.62 degrees. Remembering that in this primitive computer there was no provision for the decimal point to float, we can see that all longitudes would have had to be expressed to two decimal places, and around the equator, 0.01 degree represented a third of a kilometre, an uncertainty that was much too large for Apollo. An elegant solution arrived at by the programmers was to stipulate that all longitudes would be handled by the computer after they had been divided by 2. As the largest specified value was now 90 degrees, longitudes could be expressed to three decimal places. This brought the inherent resolution of the value down to a mere 60 metres.

Worden used Program 24, the so-called rate-aided optics tracking program, for his task. Upon entering the landmark’s assumed position, the computer would drive the optics to aim them at where it thought the landmark should be. Peering through the eyepiece. Worden then used a little joystick to finely adjust the aim and place the graticule precisely on the landmark, taking marks at regular intervals as he passed overhead. If the spacecraft’s orbit was well understood, this data could be used to refine the position and altitude of the landmark. The sightings were carried out with the unity-powered telescope, an instrument with a very wide field of view, instead of the 28-power sextant. "1 would have felt much wanner about the landmark tracking if I had done it with the sextant, rather than with the telescope.” said Worden after his flight. "The telescope presents a pretty large field of view, and you’re trying to track a very small object down there. Apparently the numbers don’t show7 that to be true that there is a great deal of difference betw een the tw o. 1 think my own personal feelings would have been that I would have felt much better about it if I had done it w ith a sextant, because then I know7 I’m really on the target.”

When FIDO, the flight dynamics officer, had planned the DOI manoeuvre, he arranged for the resultant descent orbit to have a perilune of about 15,000 metres altitude over a point on the Moon 500 kilometres uprange of the landing site, this being where the descent to the surface would begin. As the LM coasted towards this point, with all the required checks completed, the flight director spoke over his communication loop to all the flight controllers in the MOCR. briefly interrogating each relevant controller as to whether, as far as his area of responsibility was concerned, he was happy for the mission to proceed to the next stage – powered descent initiation (PDI).

Ten minutes prior to PDI, the commander started P63 running in the LM’s computer, which would handle the start of the burn and most of the subsequent descent.

“Okay. Master arm’s on,” said David Scott with less than a minute to go to PDI in Apollo 15’s LM Falcon. “1 have two lights.” Lxplosivcly operated valves were ready to fire and let supercritical helium enter the propellant tanks.

“Average g,” said Scott as the DSKY blanked, showing that the guidance system, the PGNS, had begun to measure the acceleration acting on the spacecraft and that it would average out the short-term transients that might be associated with engine startup. “Armed the deseent [engine]. We have guidance.”

“Standing by for ullage,” said Irwin.

“Standing by for ullage,” repeated Scott, in the conventional challenge-and – response manner of those steeped in aviation. The thrusters that pointed in the same direction as the main engine (against the direction of travel) were burned for a short period to settle the heavy propellants to the bottom of their tanks so that, on ignition, the light helium gas would be at the opposite end of the tank from the plumbing that led to the engine.

“Go for the Pro,” said Scott. Then, “Pro,” as he pressed the ’Proceed’ button on the computer to give P63 permission to proceed with ignition and the commence­ment of the braking phase of the deseent.

All the power required by the LM for the duration of its separate mission came from batteries. There were four mounted in the descent stage, two in the ascent stage and a fifth battery was added in the descent stage for the J-missions solely to extend the crew’s stay Lime on the surface. Known as the ’lunar battery, it was not used in flight. For the duration of the powered descent, all six major batteries were linked in parallel to power the spacecraft. The batteries for the ascent stage were included in order to power the LM in the event of an abort. Now. on the surface, fewer systems required power so the crew took the ascent batteries offline and reduced the number of online descent batteries. This was when the lunar battery, if present, was brought into use to share the load with combinations of the other four.

Many of the LM’s electronics were designed to run on AC power so a couple of inverters were provided. These electronic devices convert direct current to alternating current but unlike the 50-Н/ or 60-Н/ supplies that most people arc aware of in their homes, these ran at 400 Hz. Control of all these power systems was via two electrical control assemblies (fCAs), one each on the commander’s and LMP’s side of the cabin. Little talkback indicators showed the status of the ECAs to confirm that their switch settings had worked. For the next few’ minutes, the crew w’orked their w’ay through a checklist that set their systems either to a mode suitable for the surface, or pow’ered dowm altogether.

Other tasks at this time included dealing with the multitude of items, large and small, that had to be unstowed and prepared for the first EVA. From back packs to cameras and film magazines, even a brush to keep their lenses clean, they had to be placed in bags that would be taken to the surface or readied for when suiting up would begin.

Among the most remarkable items to be mounted on the rover were those associated with communication, at the centre of which was the lunar communications relay unit (LCRU). This went on the very front of the vehicle between a mast for the TV camera and another for an umbrella-type dish antenna. These turned what was already a uniquely capable vehicle into the most outlandish television outside broadcast unit ever seen.

The LCRU had a number of important functions. It acted as a relay for voice communication between the two crewmen on the surface and Houston, and it passed biomedical telemetry to the Surgeon. It sent telemetry about the state of the astronauts’ back packs and itself to the flight controllers. But its most visible role was to send TV pictures to Earth. It also allowed a flight controller in Houston to remotely operate the camera while the crew got on with their work. An S-band radio link with Earth was provided by either the high-gain dish if TV was to be sent, or a low-gain antenna if only voice and telemetry were required. When the rover halted at a geology station, one of the crew had to manually aim the high-gain antenna before television could be received on Earth and for this, the antenna included a simple sighting arrangement that could be used by a fully suited crewman.

The INCO flight controller in mission control had the job of operating the camera in response to requests by others in the mission control team. In this way, many eyes in Houston could watch what the two crewmen were doing, and it enabled scientists in the mission support room to build up panoramic views of each such site and look
around for interesting rocks for the astronauts to inspect.

As he prepared the rover for their first traverse, David Scott noticed that the newly installed camera was following his every move. "Gee, you’re watching me flounder around out here.” He later recounted the moment he and frwin realised they were not really alone on the Moon.

"You’re suddenly aware of the third person. I remember that, at that moment, I realised for the first time that we were being watched by everybody behind that lens! It was almost like looking through the lens into the control room.”

Although the TV camera used on the rover was among the best flown on Apollo, engineers continued to improve the final picture quality throughout the J-missions. Apollo 15’s camera had no lens hood and as soon as Scott began to drive, significant dust was kicked up by the wheels onto the lens. Then, as INCO panned around their first geology stop, he found that when the Sun caught even a small amount of dust on the lens surface, the scattered light greatly degraded image quality. Though the crew brushed it regularly, even a small amount of residual dust scattered sunlight effectively and affected the picture which required the dust to be brushed off the lens every time they arrived at a new station. A more effective solution had to wait for Apollo 16 with the addition of a lens hood to exclude the Sun whenever possible. For the last two missions of the Apollo programme, further improvements were made by having the TV signals linked to California where a proprietary system enhanced the images before they were returned to Houston for distribution.

The cooling mechanism for the LCRU was similar to that for the batteries in that it used radiators, with a dust cover provided for when the rover was moving. But one interesting difference was that the LCRU contained a quantity of wax that absorbed large amounts of heat as it melted from solid to liquid form. When the covers came off, the heat radiated away and the wax solidified, ready for the next cycle.

Had the Moon been a smooth, featureless body with no variations in its composition

or surface structure, then the expected heating and cooling of its surface would be simple to predict. The temperature of any object in space that does not have its own heat source is a balance between the heat it absorbs from the Sun and any other sources, and the heat it radiates into space. These properties are strongly affected by the thermal conductivity of the surface, its structure, its reflectivity, or albedo and the angle of illumination. Under a vertical Sun, surface temperatures can reach well over 100°C while just before lunar dawn after the 2-week-long night they can fall as low as -180°C. In the permanently shadowed craters at the lunar poles, it can be -230°C. Since different materials in their various forms will heat and cool in different ways, important clues to its nature can be gained by studying the detailed temperature of the lunar surface.

Apollo 17’s infrared scanning radiometer was an early attempt to measure the Moon’s thennal profile by having an infrared sensor pass over the landscape, both night and day, as the spacecraft orbited overhead. The concept is similar to the thermal pictures taken of houses in cold climates to show where warmth from the building is being lost, except that Apollo’s sensor was only a single point, like a one – pixel camera. There was no multi-line or multi-pixel imaging sensor to create a ‘picture’, and images had to be processed from the results of the spot scanning a site over multiple passes, one line per orbit.

The instrument showed how varying rock types within craters could strongly affect the temperature profile of a landscape. For example, at night time, the central area of the crater Kepler proved to be over 30°C warmer than the surrounding mare, perhaps due to exposed rocks at the bottom of the bowl absorbing heat from the daytime and slowly radiating it at night while the surrounding dust chilled quickly. It was hoped that hot spots might be found over the night-time hemisphere indicating a source of volcanism, but none was found.

This radiometer was a forerunner of a later generation of instruments that have provided thermal images of many of the solar system’s worlds. Notable among such instruments was the thermal emission spectrograph which could analyse infrared light to deduce rock types. These were used at Mars, both in orbit and on the surface, to locate rocks that implied a history of running water on the red planet. Apollo was paving the way, but with a much cruder technology.

At first glance, this episode after Apollo 15’s docking might appear to be a comedy of errors by both the crew and MOCR. yet it indicates how. in an environment that is extremely unforgiving, a safe and successful outcome was achieved. Scott
recognised that the separation burn was unsafe (probably because of the extra revolution around the Moon), brought it to the MOCR’s attention and proceeded to carry on, trusting the people on the ground to assess the situation correctly.

This illustrates the close relationship between a crew and mission control. The people in Houston had a very high visibility into the spacecraft, its systems and its trajectory by virtue of telemetry, available computing power and the knowledge and experience of the entire team in the control centre. The crew had a high situational awareness by virtue of having their eyes and ears in situ so to speak. The two sides then worked together to fulfil the mission’s objectives. This was an extension of the aviation model where the pilot in command of an aircraft works with air traffic control to ensure safe travel in what is a very unforgiving medium. In a sense, both are in collaborative control, linked in their common purpose by the air/ground communications loop. Rarely do the two get out of phase and when they do, it is usually down to the quality of communication on this loop.

Gerry Griffin, one of Apollo 15’s flight directors, and later director of the Johnson Space Center, made this very point. "In aviation, pilots don’t control what goes on in the airspace, they control their aircraft by a set of rules and by following instruc­tions from the various control facilities who also operate under certain rules. For sure, the aircraft commander can take any action he or she deems necessary to safely operate the aircraft, including disobeying an instruction from an air traffic control centre, approach control, or a control tower. As soon as the aircraft commander takes that overriding step, he or she will have a lot of explaining to do when they get on the ground, and if they can’t convince the powers-that-be that they took the proper course of action given the conditions, they won’t be flying anymore, or at least, they won’t be flying for a long while. It is no different in the American manned space flight environment. Like the aviation analogy, the commander in Mercury-Gemini-Apollo controlled the spacecraft and could take any step he felt necessary to operate the spacecraft safely and to finish the task at hand in accordance with the flight plan and mission rules. Those of us in the mission control centre [MCC] understood that fully and agreed with it. But when any out-of-the-ordinary situation reached a ‘safe harbour’ or stopping point, the commander or crew was expected to (and always did) work closely with the MCC to proceed on with the mission. Often the ‘next steps’ were ‘directive’ in nature and emanated from MCC.”

One of the important differences from the air traffic analogy is the visibility

mission control has into so many of the spacecraft’s systems. Air traffic control looks at the movements of many aircraft all at once but cannot diagnose an impending technical problem in a single aircraft. Apollo’s mission control, on the other hand, while having no other spacecraft to worry about, could see far more detail about its health than the crew could, and could bring much more brainpower to any situation. Throughout an Apollo flight, there were many times when a decision from mission control was vital to the progression of the mission, typified by the ‘Go/no-Go’ call which became a media catchphrase of the MOCR, for example, before ТІЛ, LOI and the final decision to land on the Moon (PDI), as Griffin explained:

"The MCC had to make sure the ground-based systems and the systems the crew couldn’t see (for example, the S-IVB before TLI, or the command module before PDI) were ready. Then, we unequivocally told them that we were Go, or no-Go. If the MCC and the crew were Go, the crew was expected to carry out the ‘next step’. Of course, similar to the aviation analogy, the commander or crew could hold off perfonning one of those milestone events even after we both had agreed it was Go if they didn’t like something they saw. If they were right, so be it – good catch – but they had better be right.”

The Mission Control Center evolved during the 1960s through the Mercury and Gemini programmes, beginning with Chris Kraft as the model for the flight director. He defined this role, and also that of mission control and the crew. Around him, the very best engineers and specialists were brought together, some in the MOCR, others in outlying rooms and buildings or at contractors’ premises, able to coordinate and run a normal mission, and to react to and troubleshoot an anomalous one.

During Apollo, international TV coverage of the astronauts at their work was often interspersed with a wide-angle shot of the MOCR during periods when no pictures were available from the spacecraft. Images of serious-looking people in shirt and tie, seated at high-tech consoles where screens flickered and lights blinked, became part of

the public mythology of American spaceflight. Л generation later, the imagery of astronauts and mission control turns up in such movies as Contact, Deep Impact and. of necessity, in Apollo 13. But, as is often the case, this is an incomplete image. Gerry Griffin advised on the making of some of these movies and even took a cameo role as a flight controller. But he believes this public imagery needs to be put into perspective.

“The astronauts didn’t run the Apollo programme, neither did the flight controllers, and neither of the two were totally responsible for the success of Apollo. The Apollo programme was run by a very capable bunch of guys-on-the-ground who managed the funding for. and the building of. the flight and ground hardware. These same guys also got the agency enough money to hire the best people in the world at NASA and its contractors to do the work including astronauts and flight controllers. The MCC included all of these guys-on-the-ground, not just the flight controllers from the Flight Control Division at [the Manned Spacecraft Center, later renamed as the Johnson Space Center]. When a decision was made by the entire team on the ground it was always discussed with the crew for their input, adjusted if necessary, then implemented. While the astronauts and flight controllers got most of the visibility in Apollo, we both were actually very small, albeit very important, parts of the programme.”

The most important lesson to take from how Apollo managed to operate so well, was that it represented the epitome of teamwork. The commander and his crew have the situational awareness at the sharp end of the operation, while the personnel at mission control have a far wider knowledge of the context within which the flight is flown. David Scott commented on this in later years:

“Mission control doesn’t have any control over the spacecraft, so it’s “mission advisory’. It’s like air traffic control; they control the airspace but they don’t control the airplane. They advise the airplane and the pilot is accountable for all of his actions and that’s basically the way the system works. Who’s in command and who’s in control and who’s advisory – it’s a team kind of thing. Everybody has to work. You have to balance the situational awareness with the advice from MCC because they have much more data to look at so their advice is invaluable to the situational decision. But it’s not a command. Some people in management and MCC would consider it a command, but that’s OK. The commander, in a situation, sometimes has to override the words that he gets from MCC in order to complete the objectives for w’hich he’s responsible."

Having crews that were mostly derived from military and test flying affected the melding of the crew/controller relationship. These w’ere people who w’ere used to fulfilling orders and getting the job done in association with controllers, yet able to cope at the sharp end of aircraft command in sometimes difficult situations, as Scott explained: “Another mindset of those of us who were flying during those days was that wc had a lot of flight experience alone in airplanes where we had to make decisions. If you don’t have that, then you probably have a more open mindset to MCC’s instructions or advice. In other words, if you haven’t been in these situations where you got bad advice or had to make decisions on your own in flight, then you would rely more totally on MCC because you don’t have this experience of needing to do your own decisions on all the data.”

We will leave it to Gerry Griffin to sum up why NASA’s mission control worked so well: “Simply stated, it was the best flight operations team ever assembled. It was built on respect, trust and teamwork. The flight crews and flight controllers were a tight bunch who trusted each other to do the right thing. The only time it didn’t work, and work extremely well, was Apollo 7; and even at that, the mission turned out to be a huge success.”

Although the Apollo spacecraft was a truly amazing machine for its time, it was. in many ways, extremely marginal. Only by the very careful husbanding of all the on­board consumables could a mission be completed. There was little room for error or excess. However, it was sometimes difficult to measure how much of a resource was being consumed. Hydrogen and oxygen were relatively easy. The tanks had sensors to directly measure quantity, the flow of these two elements to the fuel cells was w ell understood and the pow er produced w as related to their consumption. If power was being produced as expected, then the amount of w’ater available for cooling, food and drinking would be well known. Oxygen flow – for the cabin air supply w as also directly measured.

The propellant for the RCS thrusters presented a greater problem and illustrated some of the indirect techniques used by flight controllers to understand how much remained – for like so many systems on Apollo, the functioning of these little engines was crucial to the correct execution of the flight. Propellant quantity was not directly measured because this is always difficult to do in weightlessness.

Instead, controllers used every form of data available to them to make estimations of usage. The tanks contained the fluids within bladders to ensure that only liquid was expelled to the thrusters. As propellant was consumed, the space between the bladder and the tank walls w’as filled with helium that applied pressure for expulsion, and as this volume increased, the helium pressure reduced. Also, every thruster firing was telemetered to Earth to enable the usage for each quad cluster to be summed. However, techniques such as these gradually accrued errors winch became greatest towards the end of the mission, just when controllers wanted to know the quantities remaining most accurately.

Controllers made use of the fact that each quad cluster had a primary and a secondary pair of tanks – four tanks in all – and that the secondary pair constituted

39.5 per cent of the total available. Therefore, by switching across to the smaller tanks at some point, they got a data point which they knew’ was a precise value for how much propellant remained available to that quad. In order to avoid running the primary tanks empty, this ‘crossover’ was carried out when the total remaining had been calculated to have reached 43 per cent. It provided an accurate measure to work from in continuing to plot the quantities for the remainder of the mission.

Timing their tasks up to and beyond re-entry was important to help the crew’ to coordinate their progress through an increasingly busy checklist. To this end, they set their digital event timer to count up to, and beyond the moment of entry interface.

Their next task was to prepare the EMS by setting it to the starting conditions for re-entry, as read up in the PAD. The CMP moved the scroll to the start of the relevant monitor pattern, aligning the scribe to the velocity that would be expected when 0.05-g was sensed. The total distance for re-entry was entered into the digital display at the bottom of the panel and the dial of the roll indicator was aligned to ensure that its reference matched the GDC and that it would accurately show whether they were orientated feet-up or feet-down.

Then attitude control was transferred to the CM and the thrusters on both rings of the RCS were operated for a briefly to verify their operation. Once complete, attitude control was returned to the SM thrusters.

With just 25 minutes remaining to entry interface, the crew began to prepare the service module for jettison. A valve was closed to cut off the supply of oxygen from the SM tanks which left the CM reliant on the contents of a small tank mounted in its periphery. One of the three fuel cells was shut down to force the batteries to take more of the load and so help to warm them up. ‘I’he service module’s radio systems were switched off. Circuit breakers were pulled to remove power from a number of heaters that kept the radiators, the dump nozzles and the potable water tank warm. Once all these small tasks had been completed, a check was made to ensure that the spacecraft was still in the correct attitude.

Use of the computer was quite intensive during re-entry, especially since, if all was working wrell. it would be the computer that would fly the spacecraft all the way to deployment of the parachutes. This began with Program 61 (P61) which started re­entry navigation by measuring the acceleration acting on the spacecraft. Ii also accepted relevant information from the PAD to allow subsequent programs to control the re-entry. This included their planned impact latitude and longitude and whether they would be entering heads up or down, information that went into Noun 61; their maximum deceleration, their velocity and flight path angle at entry interface went into Noun 60. They then checked the contents of Noun 63, which held their range from the 0.05-g point to the landing site, their velocity at the 0.05-у event and the total duration of re-entry.

When they were happy with the numbers to this point, they pressed ‘Proceed’ and the computer moved to P62 which handled the jettisoning of the service module and placed the command module in the correct attitude for re-eniry. First the CMP had to carry out a horizon check at 17 minutes to go. All he had to do was look out to see where Earth’s horizon appeared with respect to a series of angle markings on the edge of the rendezvous window. It was expected that it would be coincident with a line draw’ll at the 31.7-degree mark. However, because the line of sight was dependent on exactly where the CMP placed his head, a tolerance of 5 degrees was allowed. If the horizon was outside these limits, the rules said that they should assume the G&N to be faulty and steer the ship manually.

Apollo 12 CMP Dick Gordon couldn’t even see the horizon, as he related after the mission: "It was dark and I never was certain that there was a horizon out there.” he told his debriefers. He was not concerned as they had already made so many attitude checks. "It really didn’t make any difference. We had already checked the alignment. We were satisfied with the IMU. We had a boresight star. We had a sextant star check. We knew where we were, and the DAP [digital autopilot] was working properly. We were confident the w:hole time, and I didn’t care whether I made that check or not.”

His commander Pete Conrad pointed out the inconsistency in this approach. ‘We ought to change the rule, because we actually violated the rule."

"Well, we actually picked up the horizon check later on during the entry.” pointed out Gordon. In fact, there was no real necessity for the check to be done 17 minutes prior to entry as the checklist included a graph. "You’ve got the chart of the Harth horizon angles versus time from entry interface and you can check that any time prior to entry interface. It’s a nothing cheek, and you can either do it or not do it. I couldn’t care less."

Continuing to the bottom of the LOI PAD form. Capeom Karl Hcni/c read, "25, 2671, 228; the rest is NA.” This refers to six lines on the form that were concerned with two methods of double checking to ensure that the spacecraft was in the proper attitude for the burn. On this PAD for Apollo 15, only the first was brought into play; it exploited the fact that the spacecraft’s sextant could be aimed precisely at the stars. If the spacecraft had been placed in the correct attitude for the burn, then flight controllers had calculated that a particular star should be visible through the sextant when its shaft and trunnion angles were set to specific values. In this case, the crew’ were to expect Star 25, which is commonly known as Acrux, in the constellation Crux, to be visible in the sextant when the shaft and trunnion angles were preset to 267.1 and 22.8 degrees respectively.

Ilenize indicated that nothing else need be entered on the form by pronouncing it as LNA’ for ‘not applicable’. What was skipped was the boresight star method. This used the crewman optical alignment sight (COAS) – a unit with an illuminated graticule similar to a gunsight that could be mounted in a window and whose aim could be calibrated. It was not required for LOI because the windows would be facing the Moon or the LM.