Category How Apollo Flew to the Moon

Charlie Duke had been Capcom on the White Team in mission control when Armstrong and Aldrin brought Eagle down onto Mare Tranquillitatis. He was Capcom again when Columbia was preparing to leave lunar orbit. "Apollo 11, Houston. Your friendly White Team has your coming-home information, if you’re ready to copy. Over.”

Aldrin was fulfilling the role of secretary: "Apollo 11. Ready to copy.”

"Roger, Eleven,” replied Duke as he prepared to send the mind-numbing sequence of numbers that crews accepted as the difference between getting home to the cool green hills of Earth, or staying in the Moon’s embrace. He had two of these PADs to send. The first was for the burn they all hoped the crew would use at the end of their thirtieth revolution around the Moon. The second was a contingency in case the SPS engine failed to light at the first opportunity, in the hope that it would light next time around.

"TEI-30.” started Duke. “SPS/G&N: 36691, minus 061, plus 066, 135234156. Noun 81: 32 – correction – plus 32011, plus 06818, minus 02650 181 054 014. Apogee is N/A, perigee plus 00230 3286 – correction – 32836; burn time 228 32628 24 1511

357. Next three lines are N/A. Noun 61: plus 1103, minus 17237 11806 36275 1950452. Set stars are Deneb and Vega, 242 172 012. We’d like ullage from two jets for 16 seconds, and the horizon is on the 10-degree line at Tig minus two minutes; and your sextant star is visible after 134 plus 50. Stand by on your readback.”

Aldrin wrote all this in the standard P30 form, and then read it back to Duke to ensure that he had copied it down correctly. They then repeated the process for the contingency PAD.

The flight controllers in the MOCR were polled by flight director Gene Kranz about whether, within their area of responsibility, they were happy for Apollo 11 to attempt the upcoming burn. When a unan­imously positive response was The PAD for Apollo ll’s TEI manoeuvre. gathered, Kranz directed Duke

(Redrawn from NASA source.) to inform the crew. “Apollo 11,

Houston.” called Duke. "You are Go for TEI." Eight minutes later. Columbia passed behind the Moon for what would hopefully be its final Lime.

In the following translation of this PAD. some details have been glossed over. Readers should consult the fuller explanation of the PAD for LOI given in Chapter 8.

TEI-30, SPSiG&N – As usual, the initial statement gave the purpose of the burn (to perform a trans-Earth injection manoeuvre, in this case on the 30th orbit), which propulsion system was to be used (the big rocket engine sticking out of the back end of the spacecraft) and the system that was to control it (the guidance and navigation system). The CSM was expected to weigh 36,691 pounds (16.643 kilograms) at ignition.

Prior to the burn, the engine nozzle was to be aimed to act through the spacecraft’s calculated centre of mass, in this ease, minus 0.61 degrees and plus 0.66 degrees. This was only an initial setting to minimise attitude excursions at ignition. Once the engine was burning, the computer would take control of the nozzle, swivelling it as necessary to keep it properly aimed while the spacecraft’s centre of mass shifted.

The nine-digit number – 135234136 – represented the ignition time. This was 135 hours, 23 minutes. 41.56 seconds into the mission.

For such an unwieldy collection of data, Noun 81: plus 32011, plus 06818, minus 02650 was pretty simple. It detailed the change in velocity that the burn would be expected to impart on the spacecraft, expressed in tenths of feet per second. As is normal in the spaceflight realm, the total velocity change was broken down into three orthogonal vectors given with respect to the local vertical/local horizontal frame of reference, and were entered into the computer under the name ‘Noun 8Г.

It is plain that the largest component of the burn was positive in the v axis. 3.201.1 feet per second (975.7 metres per second), indicating that the burn was to be largely along their direction of motion. There was also a substantial component in the y axis. 681.8 feet per second (207.8 metres per second), to push the spacecraft slightly south of their original orbital plane. The smallest component. 265 feet per second (80.8 metres per second), would act opposite the г axis and therefore away from the centre of the Moon.

The numbers 181. 054 and 014 represented the required attitude of the spacecraft. The figures are angles given with respect to the orientation of the guidance platform. It is interesting to note that these attitude numbers appear rather arbitrary a fact that illustrates the development of Apollo’s procedures. For all flights to the Moon, the burn to enter lunar orbit. LOI, was performed with the platform aligned to an orientation that coincided in some way with their expected attitude for the burn. This made the FDAI displays easier to interpret. For TEI on Apollo 11, the platform is still orientated according to the ‘lift-off REFSMMAT’. In other words, the attitude angles given represent the attitude that the spacecraft should adopt for TEI as expressed relative to the orientation of Eagle’s landing site at the time of its lift-off from the surface. Although later missions used coincident REFSMMATs for TEI in the same fashion as for LOI, the idea was not considered so important for the early missions in view of the fact that, at TEI, the spacecraft was increasing its speed, and thereby would be rising away from the surface and not be in danger of crashing.

Apogee is NjA, perigee plus 00230 gave the size of the orbit expected to result from the burn, stated in nautical miles. These were given with respect to Barth. Since the spacecraft was coming all the way from the Moon, the apogee figure would be meaningless. The figure of 23 nautical miles given for perigee represents about 43 kilometres and was actually theoretical. Any spacecraft that approaches Barth on an orbit with a 43-kilometre perigee is destined to enter the atmosphere, be slowed, and most likely burn up if not protected. This is therefore a very good figure.

The total velocity change, delta-it, of 32836 was to be imparted by the engine along the plus-.x direction, given in tenths of feet per second. As such, it is really the vector sum of the three component velocities given earlier. It represents almost exactly a speed increase of one kilometre per second.

The number. 228. was the expected duration of the burn: 2 minutes. 28 seconds. The crew would keep an eye on this and make sure that if the automatic systems failed to shut down the engine around this time, they would do it manually soon after.

Another figure for velocity change, known as delta-re, of 32628 was very much like delta-vt, the main difference being that it was for the BMS digital display that provided a backup method of shutting down the engine. The EMS was a less sophisticated method of ending the burn because it could not account for the tail-off thrust that an engine has after shutdown, whereas this could be taken into account by the primary system and the controllers handled this by reducing the figure appropriately.

The numbers 24. 1511 and 357 were to provide a check of their attitude. The star designated by the octal number 24 (Gienah, or Gamma Corvi) should be visible through the sextant when its shaft angle had been set to 151.1 degrees and its trunnion angle to 35.7 degrees.

At the start of Apollo operations, mission control standardised the software and associated forms for PADs like this one for P30. and it included two methods of checking their attitude. The remark that the next three lines are NjA, reflected the fact that the spacecraft’s windows were facing the Moon, and therefore the COAS, mounted in the left-hand rendezvous window, could not be used to sight on a star. Noun 61 in the computer held the latitude and longitude of the planned landing site on Earth in geodetic coordinates so Noun 61; plus 1103. minus 17237 indicated that the target was in the mid-Pacific Ocean at 11.03N. 172.37 W.

Given in tenths of a nautical mile, 11806 was the distance the command module was to travel between entering Earth’s atmosphere and landing. It is equivalent to 2.186.4 kilometres. At entry, the CM would be expected to be travelling at 36,275 feet per second or slightly over 11 kilometres per second. Mission control expected that, upon entering the atmosphere, the crew and spacecraft would sense one – iwenlicth of 1 g at 195 hours. 4 minutes, 52 seconds mission elapsed time.

The crew’s backup method of determining their attitude reference, should they lose the platform, required that they align the stars Deneb and Vega in the telescope eyepiece in a prescribed way. If they were to do this, their attitude would be given as: 242 degrees in roll, 772 degrees in pitch and 12 degrees in yaw.

The remainder of the PAD consisted of notes pertaining to the burn. An ullage burn prior to the TEI burn itself was required to settle the propellants to the bottom of the large tanks in the serviee module. This burn was to be made using two of the four rearward-facing RCS thrusters for 16 seconds. As a quick check prior to the burn, two minutes before ignition they should expect to see the Moon’s horizon aligned w-ith the 10-degree mark inscribed on the left-hand rendezvous window. Finally, mission control noted that the star they were to use for the sextant check would not rise above the Moon’s horizon until about half and hour before the burn.

The velocity of a homew’ard-bound Apollo spacecraft was quite low’ during much of its coast. It dipped to a minimum of about 850 metres per second at the point where Earth’s gravity overcame that of the Moon. As the spacecraft continued to approach, the increase in its velocity was painfully slow until the final few’ hours of the mission w’hen Earth’s increasing pull ramped it up markedly. For example, on a typical mission it w’ould take over two days for the velocity to rise to half its highest value, yet it took less than two hours to make up the other half. This steep increase simply reflected the fact that the spacecraft w’as falling into a deep gravity well.

It w’as during the final hours of the flight that mission control had the last of their seven planned opportunities to carefully track the spacecraft, refine their knowledge of its trajectory and have the crew adjust the approach velocity for a perfect entry. On some flights, guidance was so good that this mid-course correction was not required, while on others only a very minor firing of the RCS thrusters was needed to correct for earlier unbalanced thrusting or the tiny thrust imparted w’hen gases and liquids w’ere vented from the spacecraft.

Diagram of the approach flight path towards and through Barth’s atmosphere. Definitions: entry interface and the 0.05-g event

To aid their calculation of the spacecraft’s entry trajectory, mission planners adopted a height of 400,000 feet or 121.92 kilometres, at which the returning command module was deemed to have left space and begun re-entry. This was entry interface. The Retro flight controller’s task was to shape their approach trajectory to ensure that when they reached this altitude, the flight path would form an angle to the horizontal of 6.5 degrees, with a leeway of about 1 degree to help to cope with weather or unfavourable trajectory conditions.

Entry interface, while being handy for the trajectory analysts, was an entirely arbitrary point that had little to do with the real atmosphere and its properties. It was therefore of little use in the conduct of the re-entry itself because it did not take into account the variations that the outer atmosphere would present to the spacecraft. A means of referring to the physical atmosphere was required to aid coordination and timing, something that meant the spacecraft had truly entered the atmosphere. NASA chose the moment when the tenuous gases of the upper atmosphere exerted a drag equivalent to 0.05 g. When the spacecraft’s acceler­ometers detected this level of deceleration, they signalled to the relevant instrumentation that re-entry was underway. Most aspects of the entry were measured with respect to this 0.05-g event. For the sake of calculation prior to entry, just as for entry interface, it was taken to occur at an altitude of 90.66 kilometres. When it was reached, two important things occurred: the computer began to fly the re-entry, and the entry monitor system (EMS; of which more later) began to monitor the progress of the flight path.

At 18 kilometres altitude, the cabin pressure relief valve was set for entry. In this mode, the valve held the cabin pressure at its nominal value until the outside pressure rose to a slightly higher level, at which point outside air would begin to flow into the cabin. On the main display console in front of the CMP and commander was a small altimeter whose needle now responded to the rising outside air pressure, allowing the crew to use its information to check the progress of the descent.

In preparation for the intensive sequence of pyrotechnic events to come, the SECS was again armed which also gave the crew the option of dealing with a possibly unstable plummeting spacecraft by manually deploying the drogue chutes early.

Ten kilometres up and falling, they switched on the logic system that would automatically trigger the components of the Earth landing system. It used timers and barometric switches to orchestrate the deployment of a series of parachutes and other events to bring the command module to a safe meeting with the ocean’s surface. They also threw a switch to finally disable the RCS thrusters. Next, 7,300

metres up and descending at over 150 metres per second, a barometric switch operated to jettison the upper section of the conical hcatshield, commonly referred to as the apex cover. Four gas-operated pistons pushed it off and a small parachute attached to it slowed it down to take it away from the descending command module. This exposed two canisters containing the drogue chutes and the main parachutes packed around the tunnel. As with all the automatic events about to occur over the next few minutes, a guarded pushbutton allowed the crew to deploy the apex cover should the automatic system fail.

Once the apex cover had enough time to clear. 1.6 seconds to be exact, two drogue chutes were fired away from the spacecraft by pyrotechnic mortars to ensure that they avoided the turbulent airflow’ directly above the plummeting spacecraft.

“Stand by for the drogues.” called Young as Charlie Broun continued descending.

“Stand by. There went something,” said Stafford.

“God damn!” cried Cernan. "There’s the drogues! There they are, babe.”

“God damn, we’re on the drogues,” affirmed Young. “Rock, rock, old baby; rock. rock. This son of a gun.”

These parachutes were designed to reduce the CM’s descent speed to 80 metres per second. By now, the crew expected to see their cabin pressure rise as the planet’s air flowed in from outside via the cabin pressure relief valve. If not, the crew could set the valve to its dump position.

“There wasn’t much of a rotation as the drogue chutes deployed.” explained Aldrin after his flight. "They seemed to oscillate around a good bit. but did not transmit much of this oscillation to the spacecraft. The spacecraft seemed to stay on a pretty steady course.”

The proper operation of the Farth landing system was of particular concern to the Apollo 12 crew’, given that their spacecraft had sustained two lightning strikes during its launch ten days earlier. They had no means of knowing w’hether the surges of electricity had damaged the pyrotechnic devices that would bring the crew to a safe landing, except by firing them.

"Stand by for drogues,” called Gordon. They w’ere keenly watching the event timer to see if events occurred on time.

"What’s the time on the clock? 8:04?" asked Conrad.

The entry PAD had predicted that they should expect deployment of the drogue chutes 8 minutes 4 seconds after entry interface. That time had just passed and there was no deployment. Gordon began to count out loud.

"8:12, 13. 8:14.” Ten seconds had passed, the spacecraft had fallen a further kilometre and a half and still there were no drogue chutes.

“8:15, 16.” continued Gordon.

Unlike the predictions of trajectories in space, wdiere New’tonian mechanics affords great precision, re-entry predictions are subject to the vagaries of the atmosphere and how its density acts to slow the spacecraft. Moreover, entry interface w’as an entirely arbitrary point with little relationship to the atmosphere. But it was all the crew had to go on. Gordon kept counting.

‘’8:18. There go the drogues!’-

“’There it goes,– confirmed Conrad in relief.

“Good show,’- added Bean who passed on the news to mission control. “We got drogues, Houston.– He added for his crewmates. “Man. This is hauling it. isn’t it?”

Having established the spacecraft in its initial trajectory around the Moon. FIDO in mission control could begin working on his next move: a short burn by the SPS that would be carried out about four hours later, after two complete orbits, in order to get the Apollo stack into its operational orbit. This burn would be very carefully monitored to ensure that it had exactly the required effect.

The details of this burn depended on the flight in question. For Apollos 8 to 12. a relatively short burn, known as LOI-2, brought the apolune down from 300 kilometres to 110 kilometres and made the orbit circular. Apollo 8. without the mass of a lunar module, required only a 9-second burn to achieve this. On the next three flights, the extra 16 tonnes or more of the LM meant that their burns had to be somewhat longer. On Apollo 10 the lunar module lacked a full propellant load and the LOI-2 burn was 14 seconds, but the full LM tanks on the next two flights extended the burn to 17 seconds. On these early flights, the CSM never left its 110- kilometre circular orbit, and come landing day, the LM would have to do all the work of manoeuvring down to the surface. The first part of this would be the descent orbit insertion (DOI) burn to place the LM in an orbit with a low point of only 15.000 metres. This was the descent orbit, so called because this perilune was the point from which the LM would begin its final descent to the surface.

After Apollo 12 the strategy changed. Planners were keen to increase the capability of the Apollo system, and analysis had shown that the LM’s payload capacity to the lunar surface would be maximised by having the CSM make the DOI burn to take the LM into the descent orbit, thereby saving its propellant for the Final descent. Later, once the LM was released and inspected, the CSM would make another burn to circularise its orbit at 110 kilometres to undertake a programme of lunar reconnaissance while the LM was on the surface. This also placed the CSM in a suitable orbit for the rendezvous when the LM returned.

Owing to its unforeseen circumstances, Apollo 13 never got as far as carrying out this burn. On Apollo 14, which was first to perform this manoeuvre, the burn Look 21 seconds, which was four seconds longer than Apollo 12’s LOI-2 burn, reflecting the fact that, in this case, the near-side altitude was being dropped all the way down to 17 kilometres. As the mass of the stack increased for the final three J-missions. the duration of the DOI burn stretched to 24 seconds.

Much of the checkout of the DBS was the responsibility of mission control because telemetry gave the flight controllers access to far more data than was available in the LM cockpit. Therefore, they preferred to use the LM’s steerable antenna to carry the required high-bit-rate data.

Not on Apollo 16. however. The steerable antenna was the LM’s equivalent of the CSM’s high-gain antenna. Mounted on the right side of the LM’s roof, its dish could be moved under manual or automatic control to aim at Earth. When Charlie Duke tried to move Orion’s steerable antenna, he discovered it would only steer in one axis. “Well, we’re not gonna have TV from the LM, unless we get that high – gain up," he said glumly to John Young as they battled to get their spacecraft checked out for the descent. IJp to this point in the checkout everything had gone smoothly, but the failure of the mechanism to steer the antenna was the First of a series of problems that sprang out of nowhere and which threatened to jeopardise the surface mission. They continued the checkout as best as they could using their low-gain antennae, but when a failure in the RCS pressurisation system threatened to burst its safety devices. Young opined, “’fhis is the worst jam I was ever in.” He didn’t know the half of it.

One consequence of the loss of the steerable antenna was that mission control could no longer directly access the computer’s memory. Duke had therefore to copy down two lists of numbers, 179 digits long in total, which represented an updated state vector and the RLFSMMAT for the landing. Capeom Jim Irwin read them up,

and Duke read them back as a check. Young and Duke then laboriously entered them into the correct addresses in the computer’s memory, checking to ensure that they did not make a mistake. Spacecraft communications were normally handled by 26-metre antennae, but they managed to overcome many of the problems caused by the loss of the steerable antenna by using the extra sensitivity of the 64-metre dish at Goldstone in California. Also, by optimising the LM’s attitude, the less capable omnidirectional antennae were operated through a favourable lobe in their reception pattern.

During the final far-side pass before the landing, CMP Ken Mattingly prepared for his circularisation burn by testing the systems associated with his SPS engine.

After about nine minutes, when P63 had delivered the LM to high gate, typically only 2,200 metres up and 7.5 kilometres from landing, control was passed to Program 64, whose role it was to guide the LM through the approach phase to a point just above the landing site. Many aspects of the descent changed at this point. In particular. P64 did not continue the effort by P63 to reach a point below the surface. Prior to high gate, the crew’s windows had been facing into space, so one of P64’s first actions was to give the crew a chance to see where it was taking them. It fired the RCS thrusters to pitch the spacecraft forward sufficiently to enable the crew to view the horizon ahead – a manoeuvre called pilchover. This change in attitude with respect to the ground meant that the antenna for the landing radar had to rotate to its second position so as to continue to face roughly downwards. Meanwhile, P64 continually rode the engine’s throttle setting to aim for a point 30 metres above and five metres short of where it thought the final landing site was located.

As Pete Conrad waited for P64 to begin, he strained at his window to look for a familiar pattern of craters towards which he had trained to fly. Careful correlation of photographs taken two years earlier by a Lunar Orbiter mission with surface pictures taken by the Surveyor 3 unmanned spacecraft had shown that it had landed within a 200-metrc-diameLer crater that formed the torso of a distinctive pattern of five craters known as the Snowman. Planners had decided that this would make a good target to prove the pinpoint landing capabilities of the Apollo system.

“Standing by for P64.” he told Л1 Bean standing beside him. “I’m trying to cheat and look out there. I think 1 see my crater.” He was the shortest of the astronauts, and was straining against the harness restraint to see the lunar surface in the bottom corner of his triangular window. He had not yet seen his crater.

“Coming through 7,” said Bean as they passed 7,000 feet or 2,150 metres. “P64 Pete.”

“P64,” confirmed Conrad.

“Pitching over,” said Bean as the LM began to tip forward.

“That’s it; there’s LPD,” said Conrad as he brought up the angle display of the landing point designator.

The lunar day. as measured from sunrise to sunset, lasts for about 14 Earth days and therefore, the crew’’s circadian rhythm had to be maintained artificially. When it was time to sleep, the best that could be done was to put shades up over the LM’s three windows while the unfiltered Sun, rising slowly in the east, beamed down on the spacecraft’s exterior.

Unfortunately, the first three crews on the surface had to remain in their suits for the duration of their stay. The LM’s operational lifetime w? as very short, there was concern about the effect of dust on the zippers and the additional time required to get out of and back into their suits was considered too expensive. They simply had to endure an uncomfortable rest period. Once Armstrong and Aldrin had discarded their PLSSs out of the cabin, they could arrange things for the night. Aldrin lay across the floor in front of the hatch, his knees bent to allow for the confined space. Armstrong perched himself on top of the ascent engine. His head was towards the back of the cabin near a noisy coolant pump while his legs dangled above Aldrin supported by a lash-up he had fashioned from a cord in the spacecraft.

Their attempt to put the lights out was less than successful. Armstrong noted after the flight that, on top of the fact that the shades turned out to be not as opaque as they w’ould have liked, there w’ere several warning lights and luminous switches that could not be dimmed. And there was a final, more troublesome source of light. "After I got into my sleep stage and all settled down, I realised that there was something else shining in my eye. It turned out to he that the Earth was shining through the [telescope] right into my eye. It was just like a light bulb.”

They had elected to sleep with their helmets on in an attempt to limit the noise from the spacecraft and to keep front breathing the pervasive dust. But now that they had settled down and were no longer active, they began a battle with the environmental control system to stay wann. “We were very comfortable when we completed our activities and were bedded down.’’ continued Armstrong. "After a while, I started to get awfully cold, so I reached in front of the fan and turned the water temperature to full up. It still got colder and colder. Finally, Buzz suggested that wc disconnect the water, which 1 did. I still got colder. Then. I guess, Buzz changed the temperature of the air flow’ in the suit.”

The next two crews fared only a little better, even though they didn’t get cold, thanks to procedural changes after Apollo 11, and they had hammocks across the cabin to make their rest more comfortable. Conrad had made a small error with the sizing of his suit shortly before the flight and he paid for it during his rest period with the resultant pressure on his shoulders. Bean then spent an hour making adjustments to the legs of Conrad’s suit to relieve the pressure.

Bean also struggled to get some proper rest as he explained after the flight. "I think I didn’t sleep well because I was just nervous and excited.” How’ever, there was a solution in the medical kit, if he chose to use it. "If 1 did it over again, 1 would take a sleeping pill on the Moon." Bean’s explanation hinted at the issues NASA would have to deal with before the J-missions began, each involving three 7- hour EVAs. "I felt like I was tired towards the end of the second EVA and I felt like it w’asn’t from the physical effort. It was from the lack of good sleep. I didn’t take the pill because it was not a macho thing to do, [but] 1 felt like 1 was really running out of gas.”

The problem for the Apollo 14 crew’ was that the LM had settled with a 7” tilt that was enough to upset their sense of up and down in the dark, as Mitchell described after the flight: "We both had the feeling throughout the night that the blasted thing was trying to tip over on us. Actually, w’C got up and looked out the window a couple of times to see if our checkpoints w ere still right w’here they were supposed to be.”

Whereas the first three landings got away w’ith a single rest period on the Moon, it was NASA’s intention that the. І-mission crews w’ould stay on the surface for up to 72 hours. If a crew’ were to be expected to work at a high level of physical effort and mental concentration; to make a landing, spend three days on the surface and then guide their ascent stage to rendezvous with the CSM then, during three rest periods on the Moon, the suits w’ere going to have to come off.

To practise for this, Scott and Irwin even stayed a full night in the LM simulator in Florida starting w’ith a simulation of a landing. "We had it as high a fidelity as wn could possibly get it. So we had everybody put everything in the simulator down to the last detail.” He continued, "We got a terrible night’s sleep. I mean, boy, that’s crummy, trying to sleep in those hammocks in one g in that little thing. We did the suit doffing and everything. Of course, the suit doffing was such a pain, anyway, especially in one g. But it really paid off because, w’hen we got to the Moon we were very comfortable in doing that sort of thing.”

Doffing the suits made all the difference to sleeping in the LM. Crews found one – sixth-g to be very comfortable as they had enough weight to lie normally in the hammocks but not enough to cause pressure points. ‘‘I slept much better on the lunar surface than I did in orbit," remembered Jack Schmitt. "One-sixth gravity is a very pleasant sleeping environment with just enough pressure on your back in those hammocks to feel like you’re on something but not enough to ever get uncomfortable. 1 slept but my impression was that I only needed about five hours sleep to feel rested whereas ordinarily on Earth at that time I usually felt that I could use seven. But I think that’s related mainly to the lower gravity environment. You just don’t get physically as fatigued as you would on Earth. You get as fatigued mentally obviously you’re working just as hard with your neurons but physically you don’t work as hard.”

All subsequent Apollo landings included time to deploy a full ALSEP, each consisting of a varying set of instruments cabled to a central station, all of it powered by a radioisotope thermoelectric generator (RTG). This was an early example of the type of power supply that would energise a generation of probes to the outer planets.

The Apollo RTGs used the radioactive decay of plutonium-238 to generate heat which was directly converted to electricity by an array of thermocouples. The presence of plutonium on the spacecraft had certain repercussions. It could not travel to the Moon in the RTG for fear of contamination if there were to be an accident near Barth. Instead, it was packaged into a fuel element or capsule which was transported to the Moon inside a graphite cask mounted vertically on the outside of the descent stage where it could radiate its heat. This cask was strong enough to withstand re-entry through Barth’s atmosphere and, thanks to this ability, the Apollo 13 plutonium now lies at the bottom of the Tonga trench in the Pacific Ocean.

Once on the Moon, it was the LMP’s task to remove the plutonium fuel capsule from its cask and insert it into the body of the generator. Alan Bean w as the first to try this and ran into problems when reality failed to match any Barth-bound trials. First he hinged the cask down to gain access to the removable dome at one end. then he removed it with a special tool.

’’There you go,” said Pete Conrad encouragingly.

”It came off beautifully.” said Bean. ‘ [I’ll] put the tool and the dome aside.”

This had started well. Next he had to engage the capsule removal tool. “Go ahead,” said Conrad.

“There you go.” commented Bean. “Sliding right in there. Okay. [I’ll] tighten up the lock.”

With the tool firmly engaged, Bean pulled on the capsule, only to discover that it wasn’t going anywhere. “You got to be kidding,” he exclaimed.

“Make sure it’s screwed all the way down,’’ suggested Conrad.

Bean was caught between wanting to give it a good yank but not wanting to break the mechanism that attached the tool to the capsule. Gear that w? ent to the Moon was built as light as possible. There wouldn’t be much strength in reserve.

“Thai could make a guy mad. you know it?” moaned Bean.

“Yup,” replied his commander.

“Let me undo it a minute, and try it a different way.”

“Yup.”

“It can really get you mad."

Bean reinserted the tool with its prongs rotated to use different slots.

"You guys got any suggestions?” asked Conrad of the folks in Hous­ton.

"I just get the feeling that it’s hot and swelled in there or something,” Bean said as he tried again to extract the capsule. "Doesn’t want to come out. I can sure feel the heat, though, on my hands. Come out of there! Rascal.”

The capsule was seated within two steel rings that held it away from the graphite cask. It seemed that, with it giving off 1.5 kilowatts of heat, the expansion of the arrangement was holding it snug on the rings.

"You know, everything operates just exactly like it does in the training mock-ups and up at GE (General

Electric Corporation). The only problem is, it just won’t come out of the cask. I am suspicious that it’s just swollen in there or something and friction’s holding it in. But it’s such a delicate tool, I really hate to pull on it too hard.”

Unfortunately engineers had not fully taken account of the length of time that the capsule would sit in its cask from Florida to the Moon, its 700°C heat soaking the steel mounts.

Bean piped up. “Go get that hammer and bang on the side of it.”

“No. I got a better idea,” said Conrad. “Where’s the hammer?”

“That’s what I said.”

“No, no. But I want to try and put the back end in under that lip there and pry her out. Let me go get the hammer. Be right back.” Conrad’s idea was to use the hammer’s blade to lever the capsule out.

“Let me get the tool off,” said Bean as he felt the capsule’s heat move along the handle. “It’s starting to warm up.” He disengaged the tool as Conrad went around the LM. They were not unduly bothered by the radiation from the capsule. It was alpha radiation and as such, was stopped by a small amount of material. They would not be exposed to it for long anyway. Their real concern was that the thermally hot capsule might damage their suits. They had to use the tool to extract it. Once Conrad had retrieved the hammer. Bean re-engaged the tool with the element. They were both leaning towards a little percussive persuasion.

Bean spoke first. "Now, my recommendation would be pound on the cask.” He preferred that Conrad not use the hammer’s blade on the capsule. As Bean pulled on the tool, Conrad began to repeatedly hit the side of the cask with the hammer.

”Hey, that’s doing it!” yelled Bean excitedly as the cask began to yield its contents. "Give it a few more pounds. Got to beat harder than that. Keep going. It’s coming out. It’s coming out! Pound harder/’

“Keep going/’ commanded Conrad to the balky capsule.

‘’Come on, Conrad!” laughed Bean.

"Keep going, baby."

“Thai hammer’s a universal tool.”

“You better believe it/’ cheered Conrad.

With every thump, the capsule edged out until, after a few centimetres, it came away easily. To Conrad’s giggles. Bean swung it over to the RTG unit.

"That’s beautiful. That’s Loo much.” said Bean.

“Well done, troops/’ congratulated Ed Gibson, Capeom in Houston.

“We got it, babe!" explained Bean. “It fits in the RTG real well! It’s just the cask was holding in on the side. Don’t come to the Moon without a hammer.” He brought the hammer home to Earth and now uses it to texture his paintings in his post-Apollo life as an artist.

Deployment of the ALSEP required a reasonably flat site a few hundred metres from the LM. The complete kit was mounted on two pallets and stored on rails in the LM’s descent stage. Once lowered to the ground by pulleys, the packages were hung on a bar bell and carried to the site. The layout was roughly star-shaped with ribbon cables that radiated out from the central station to the various instruments. Each cable was on a reel which fed out both ends simultaneously. There were often stringent constraints on the placement of each instrument, requiring care to avoid interference between instruments and to minimise heat conduction with the ground. For example, the magnetometer had to be clear of other instruments that contained magnets. Also, the seismometer was mounted on a stool surrounded by a reflective Mylar skirt that kept the Sun from heating the ground because the expansion and contraction from the heating cycles w’ould have added noise to the instrument’s output. However, the skirt itself routinely added unwanted signals each morning when the Sun first hit it and caused it to flex and buckle in the heat.

Prior to deployment, the various instruments were attached to their pallet by ‘Boyd bolts’, spring-loaded fasteners that required a crewman to insert the end of their universal hand tool (UHT) and give a fifth of a turn to release them. To help the crew7, each bolt had a collar that guided the tool to the bolt. In general, the bolts worked well but on some occasions, what had seemed simple on Earth became much more difficult in the dust and light gravity of the Moon.

“Another one of those beautiful Boyd bolts is all full of dust,” muttered Shepard

Assembled panorama of Apollo 17’s ALSEP after deployment. RTG is on the left, central station to the right with its antenna aimed E <i+hward. (NASA)

sardonically as he tried to release a small instrument, the supra thermal ion detector, from its pallet.

‘’Yep,” agreed Mitchell. "’Everything else is going to be full of dust before long. Be filthy as pigs."

Shepard first tried the obvious solution. "Tm going to have to lift it up and shake the dust out of that Boyd bolt; I can’t get it otherwise. Let’s just turn it upside down and shake it.’’

As they lifted it. parts fell away. "Well, there’s a lot of Boyd bolts falling off,’’ said Mitchell, referring to the parts of the bolts that Shepard had already unfastened.

"Yeah, but them’s not the ones we’ve got the problems with. Okay, flop it over a minute."

‘"That’ll do it?" asked Mitchell hopefully.

"No, it’s still not clear.’’

Shepard was having problems on three levels. Lunar dust gets everywhere and having found its way into the Boyd bolt, its cohesive nature in the vacuum helped it to stay there. Additionally, the weakness of the lunar gravity gave little assistance in clearing the bolt’s sleeve, even when Shepard turned it upside down. The situation was exacerbated by the bolt being relatively inaccessible and, as Mitchell would later explain, it was difficult to see what was happening. “On the lunar surface, there’s no air to refract the light in there. So, it’s either shadow or it’s light and, unless you’ve got a direct sunlight on it, there’s no way in hell you can see anything. That’s an amazing phenomenon on an airless planet. It’s amazing how much we count on reflected and refracted light here. But there, unless you had it directly in sunlight, it was just pitch black. And that’s what he was wrestling with, there. The dirt was packed in around it and, besides that, he couldn’t see dowm in there unless we picked it up, physically, and twisted it and held it so we could get it in the sunlight."

That one bolt cost Shepard nearly ten minutes before he finally got it loose. David Scott later discussed how important it was for something that small to work correctly. "In a training context, especially [as Apollo 12 backup commander]. I remember trying to get the Boyd bolts to work, and they would hang up. One would hang up, and you couldn’t deploy the ALSEP. Or, the UIIT’s hexagonal probe that goes into the socket w’ould sort of strip and get w’orn and you couldn’t turn it. And, if you turned it too hard, you’d strip the edges. The Boyd bolts were challenges. I think all ours w’orked just fine. But the UIIT and the Boyd bolts w’ere a big deal; because, if it didn’t work, then you didn’t get that piece of the ALSEP up. And there were a lot of Boyd bolts."

Across the six missions that landed, crew’s deployed a large number of instruments as part of the ALSLP or as standalone experiments. Seismology was a popular topic, with passive seismometers being carried on most missions. Some missions included active seismometry. with small explosive charges being set up to provide calibrated shockwraves that w’ould help to profile the local subsurface. Magnetometers sensed the local magnetic held which, on the Moon, was dominated by its monthly passage through Earth’s magnetotail. However, because some of the Apollo 11 rocks proved

to be magnetic, some later missions included a portable magnetometer to measure remanent magnetic fields at points along a traverse. Many experiments tried to sense and characterise the various particles that comprised what little atmosphere the Moon possessed. Most of these were from the solar wind or from the rocket exhaust of the spacecraft, but there was also the question of whether the change from night to day, and the resulting rise in ultraviolet exposure caused tiny particles of charged dust to levitate for a while.

There were experiments on the mechanics of the lunar soil which acted in ways that were not foreseen. Though the top layer was extremely loose and powdery, its characteristics changed markedly just a few centimetres below the surface. Millions of years of slow settlement had caused it to become extremely tightly packed and crews sometimes found it difficult to drive in items like flagpoles and the solar wind collector. Trenching experiments showed that the vacuum and the very finely ground nature of the powder made it remarkably cohesive and able to support steep sides. Even Buzz Aldrin’s famous bootprint photograph showed how well the powder could hold an impression.

There was an ultraviolet telescope on Apollo 16, a device for measuring the local gravitational field mounted on Apollo 17’s rover, and experiments to determine the electrical properties of the lunar surface. Add to all this the intense expeditions to photograph, document, sample and generally geologise across their site, this feast of science kept all the Apollo surface crews extremely busy for their precious hours walking on the Moon. One particular ill-starred experiment served to teach everyone about the difficulties of trying to carry out science in a vacuum, under an unfiltered Sun into a poorly understood, extremely dusty and abrasive soil while wearing in an awkward pressure suit with limited visibility. This was the heat-flow experiment.

Drill problems and the heat-flow experiment

Geophysicist Marcus Langseth was good at taking Earth’s temperature and now he had a chance to take the Moon’s. More specifically, he wanted to accurately

measure the temperature of the lunar soil at various depths. From these measurements, he hoped to calculate how much heat was flowing out of the Moon’s interior and infer whether its core is still molten. He designed an experiment for Apollo 13’s ALSEP that would place temperature probes into drill holes. The equipment burned up in Earth’s atmosphere after that mission’s LM had served as a lifeboat for its crew.

Langseth had to wait over a year for the Apollo 15 crew to try again. Using the father of the cordless drill, the experiment required two holes, each nearly three metres deep into which the probes would be inserted. Separate from the experiment, the drill would also be used to extract a deep core of material that would give geologists a record of the depositional layering of the soil potentially going back hundreds of millions of years.

Scott quickly found the drilling to be hard going and could only get the first hole down to 1.6 metres. He tried putting his weight on the drill but in the Moon’s weak gravity, this provided little push and, if anything actually worked against the design of the drill stems and their helical external flutes. At any rate, the designers had not taken sufficient account of the nature of the Moon’s surface. Although the Moon is draped in a blanket of finely ground-up rock – the regolith – which is many metres deep at the landing sites, the highly compacted nature of all but the uppermost few centimetres made it more like hard rock. Worse, the flutes at the joins of the drill stems had been narrowed to strengthen those joins but, with the dust unable to go anywhere else, it caused the stems to jam. Mission control agreed that Scott should place the probes in the existing hole even though it compromised the quality of the experiment. The second hole fared worse and at one metre, they decided to revisit it the next day. On returning to the site, Scott tried to lift the drill and rotate it to help clear the flutes but unbeknownst to him this actually caused the bit to disengage

from the stem above. Subsequent drilling with the hollow upper stem merely created a core running down alongside and when Scott inserted the probes they penetrated no further than about one metre.

With the heat-flow probes less than ideally placed, Scott began to drill a deep core sample using hollow stems which meant the material in the hole would be kept rather than pushed aside. Also, the flutes were of uniform depth and much faster progress was made. Scott readily reached 2.4 metres in depth, much deeper than the other two holes but by this time, they needed to return to the LM and leave the extraction to the final day. Much time and frustration had already been expended on the drilling.

On the final day, Scott learned that extraction of the deep core was to take precedence over their final drive to Hadley Rille and a feature known as the North Complex, a possibly volcanic site of some interest to Scott and the geologists. Usually rover drives came first so the crews would have more consumables available in case they had to walk back from a stalled vehicle. The decision to favour the drill meant that there would be no visit to the North Complex. However, when they tried to remove the core, it proved difficult to budge. Both Scott and Irwin had to work to extract the core, first by pulling hard on the drill’s handles, and eventually putting their shoulders under the handles and shoving so hard that Scott managed to injure his shoulder. It was a measure of the confidence that the crews had in their suits that they felt they could expend maximum physical effort without fear of a rip dumping their air.

"It shows how tough and durable the things were,” remembered Scott when reviewing the incident years later. "I’m really surprised that somebody in the back row in Houston didn’t get real squeamish about all of this. I’m surprised some boss didn’t just say. ’Hey, just knock that off.’ because they could hear us grunting and groaning – two guys on the Moon in pressure suits doing this kind of stuff. In retrospect, not smart, from a safety point of view.”

How ever, their suits included systems to warn of problems with the air pressure or cooling. ‘’Only if a tone comes on do you do something.” continued Scott. “As long as there are no tones, you work as you would work on the Earth and you never really think about [the dangers], Houston, that’s their job. To sort of pace us and guide us, because once we’re out in the suits, boy, it’s very comfortable."

Eventually, Scott and Irwin extracted the deep core but Scott’s troubles were not over and he was getting frustrated at the time being spent on it. "Joe, I haven’t heard you say yet you really want this that bad.” Joe Allen was the tactful Capcom in mission control. As part of his job, he acted as a go-between for the crew and the geology team in the science back room. ‘’Tell me you really want it this bad.” implored Scott. “It’s hard for me to say. Dave," was Allen’s wistful reply.

The six-part core stem, including a treadle that had helped guide the stem into the soil, all had to be taken apart for return to Earth. To help, a simple wrench vice that gripped in one direction was on the rover. Scott was having difficulty getting it to work. ‘‘This vice just won’t hold. There’s something wrong with it." They needed that wrench because a suited hand does not have much grip. It is already working against the suit pressure trying to straighten it “My hand wrench works okay. The one on the back of the [rover] doesn’t seem to want to work for some reason. It may just be because of the threads on the stems. I just can’t get them broken apart!” As Scott struggled with the stem in the vice, it dawned on him what the problem was. "I hate to tell you. Jim, but that… Oh boy! This vice is on… I swear it’s on backwards.” In fact, a reversed diagram in the assembly manual had thwarted them. The wrench had indeed been mounted back to front.

With Irwin’s help. Scott managed to separate half of the segments, even though it meant gripping the stem’s sharp flutes, yet the final three refused to come apart.

“We might be able to return it just like that.” suggested Irwin. Although it was 1.5 metres long, they would be able to get it in the LM.

“I don’t know where we’re going to put it in the command module.” said Scott. "I guess we ought to take it back. There’s more time invested in that than anything we’ve done.”

When the deep core reached Earth, it was immediately x-rayed which revealed 58 distinct layers within the core. Grant Heiken, a scientist who painstakingly analysed each layer, grain by grain, described it as the most valuable sample returned from the Moon.

The Apollo 15 heat-flow experiment gave good results despite its problems and created enough interest in Langseth’s experiment for another to be taken on Apollo 16. All the lessons from Scott’s battle with the drill and wrench had been learned. Using redesigned drill stems, Duke had no problem drilling the holes and inserting the temperature probes.

“Mark has his first one. announced Duke. "All the way in to the red mark [on the rammer] on the Cayley Plain.”

“Outstanding!” replied Tony England. “The first one in the highlands.”

Moments later, John Young lifted a package away from the central station and as he walked, his feci got caught up with the cable leading to the heat-flow electronics package.

“Charlie."

“What?"

“Something happened here."

“What happened?"

“I don’t know." said Young. “Here’s a line that pulled loose."

“That’s the heat-flow," Duke informed. “You’ve pulled it off."

“God almighty." Young’s spirits dropped like a stone as he went to examine the damage closely. The wires at the end of the cable had been torn from its connector where it was plugged into the central station.

“Well, I’m wasting my time," said Duke as he realised there was no point in drilling the second hole for the heat-flow probes.

“I’m sorry. I didn’t even know, said Young. "Agh; it’s sure gone."

“Okay, we copy," said England in Houston as the engineering back rooms began to crank up in a futile effort to resurrect the experiment. “I guess we can forget the rest of that heat flow."

“Yeah," replied Duke. “I’ll go do the [deep core]. Oh. rats!"

In fact, it was surprising there were not more accidents like this. Having the RCU mounted on his chest afforded the astronaut almost no visibility of his feet and the numerous layers in the suit’s construction severely attenuated any sense of touch. He constantly had to work against the internal pressure and its tendency to return the suit to one stance. This made it difficult for Young to have seen or even to have felt the snagging cable. Additionally, in lunar gravity cables tended not to lie as flat as they would on Earth and they ’remembered’ their coiled-up shape to form numerous loops spiralling across the lunar dust.

After more than 2 ‘/■ years. Marcus Langseth finally triumphed when his heat – flow experiment was fully installed at Taurus-Littrow by the Apollo 17 surface crew. The measurements from the two sites where emplacement was successful showed that the Moon has little residual heat of its own. What heat it has is produced by radioactive decay in the topmost few hundred kilometres but it is insufficient to cause substantial melting of the lunar mantle.

Once NASA had accepted UOR as the Apollo mission mode, they had to work out how a rendezvous, whether in Earth or in lunar orbit, should be accomplished. The problem was far from straightforward, and the solution did not spring forth from the mind of some brilliant engineer. Rather, it evolved from 1964 right through to the first landing and eontinued to evolve throughout the programme. The problems were many. Some of the major factors with w hich they had to contend were:

• how accurately w’ould the engines perform?

• how would a crewman know his speed and the speed of the target spacecraft?

• w’hai should the lighting be during the delicate docking manoeuvre?

• what is the least amount of propellant required in the pursuing spacecraft?

• how high should the target spacecraft be orbiting?

• how long should a rendezvous take?

NASA first considered a direct ascent technique, but quickly dropped it. For the Gemini programme, the step-by-step approach of the coelliptic rendezvous was developed. As Apollo crews and engineers worked to improve performance, they devised the confusingly named direct rendezvous or short rendezvous.

"What’s the time?” asked Aldrin from Columbia’s right-hand couch. The crew of Apollo 11 had made their attitude checks prior to TEI. and were verifying that the engine bell was swivelling on its gimbal correctly in response to steering commands. Preparations were going smoothly and there was a light mood in the cabin as their incredible flight began to look as if it might actually come off.

“We have 12 minutes to go.” replied Collins, occupying the left couch.

Aldrin had been wondering what they should do once TEI w as completed: “You going to pitch up after the burn?”

“Sounds like a good idea,” agreed Collins. “Let’s look at the Moon after the burn. That’ll give us high-gain, right?”

“Cheek,” concurred Aldrin. Since they needed the spacecraft’s high-gain antenna to face Earth and it was positioned on the opposite side from their windows, it made sense to point the spacecraft down to have the high-gain in a favourable position for Earth and. meantime, watch the Moon recede.

“Okay, 10 minutes until Tig,” called Armstrong. ‘Tig’ was the ‘Time of ignition’ and everything they did worked towards it being on time and as flawless as possible. As they were over the far side of the Moon, where it also happened to be lunar night, neither the Sun nor Earth was shining across the landscape, and the only way to see the Moon’s position was by looking at a huge void where there were no stars. The spacecraft had to be travelling with its apex forward to enable the engine at the rear to accelerate them out of lunar orbit, and Collins was straining at the window for some kind of confirmation of this fact.

“I see a horizon,” he laughed. “It looks like we are going forward.”

“Shades of Gemini.” reminded Armstrong.

“It is most important that we be going forward,” stated Collins.

Aldrin began gently mocking his crewmate. “Let’s see. The motors point this way and the gases escape that way. therefore imparting a thrust that-a-way.” They all laughed.

This was a chance to pause and reflect during their preparations, and to look for the horizon that they were supposed to check in a few; minutes.

“Beautiful looking horizon." said Armstrong. “It’s hard to describe."

“God, it has an eerie look to it,” added Aldrin. “It’s not a horizon, it’s just a band.”

Collins and Aldrin could sec directly forward through their rendezvous windows

along the plus-.v axis and towards the sunrise. Armstrong’s view from the middle couch was limited to the hatch window just above his head.

‘’It was really eerie when it first came/’ said Armstrong as the Sun rose and the terminator came into view. "And the way the terminator is, you don’t see the whole Moon at all."

"I know.” said Collins. "I was looking at it upside down for a while.”

"Yes, and then that scares you,” added Armstrong, "because that says you’re going retrograde, right? Well, let’s see. if it’s upside down, you’re going backwards/’ Collins brought them back to their checklist. "Alright, we’re coming up on bus tie time; we’ve got a little over 6 [minutes] 50 [seconds] until fig.”

The crew returned to the protocol of challenge and response, with Armstrong reading out a line from the checklist and Collins repealing it once he had carried out the instruction. Once they had dealt with the internal configuration of the spacecraft it was time for another external check.

"Two minutes to get our horizon check at 10 degrees.’’ Armstrong had little option but to have his head in the checklist.

"Yes, and sneaking up on there, looks pretty darn good.” said Aldrin. "Looks like we’re darn near right.” The spacecraft was holding a steady attitude with respect to the stars so, in a sense, the Moon appeared like a great, rounded hill and they were in a helicopter approaching the summit, ‘fhe Moon’s horizon crept down Collins’s window towards the 10-degree mark. Aldrin’s window did not have that mark but he could infer it. "Okay, coming up on two minutes.” he eallcd, "and this damn horizon check is going to be. would you believe, perfect?”

"I hope so,” said Armstrong.

"fantastic,” enthused Aldrin. "First lime we ever got a perfect horizon check. Spent too many hours in the simulator looking for an unreal horizon. Alright, horizon check passes.”

"Beautiful,” agreed Collins, who armed one of the engine’s control banks then proceeded with Armstrong through the final lines of the checklist.

"Okay, stand by for 35 seconds,” announced Collins. "Mark it. DSK. Y blanks; EMS is in Normal.” The guidance system had begun to measure their acceleration. Aldrin came back, "Check.”

"Coming up on 15 seconds,” said Collins.

Armstrong readied himself at the computer keyboard for when the display w ould start flashing ’99’ at him, asking for permission to light the engine. "Okay. I’ll get the 99.” – – –

"Okay,” said Collins. "Stand by for ullage. Ullage.’’

"Cot the ullage,” reported Aldrin. Two rearward-facing thrusters lit up, gently pushing the spacecraft forward and bringing the weightless propellant to the bottom of the tanks as the crew counted down.

"Burn!” shouted Collins as the SPS engine lit. "A good one. Nice.”

"I got two balls,” called Aldrin.

As planned, only two of the four ball valves on the propellant feed lines had been opened by the computer.

"Okay, here comes ihc other two.” said Collins as he threw the switch to bring in the second control bank and bring the engine to its maximum thrust. “Man, that feels like g, doesn’t it?"

When they fired the SPS engine on arrival at the Moon, the tanks in the service module had been full and the LM was attached to their nose. Now the CSM was by itself and its tanks were only one-third full, giving the SPS the ability to accelerate the spacecraft towards 1 g.

Collins was closely monitoring the displays in front of him. "Pressures are good. Busy in steering, but it’s holding right in there.’’

“Hotv is it. Mike’.’" asked Aldrin from the right.

"It’s really busy in roll," replied Collins, "but it’s holding in its dead band. Looks like it’s holding instead of plus or minus five, more like plus or minus eight [degrees]. It’s possible that we have a roll-thruster problem, but if we have, it’s taking it out. No point in worrying about it. Okay, coming up on one minute. Mark it, one minute. Chamber pressure’s holding right on 100 psi."

"Looks good," agreed Aldrin.

Collins continued with his commentary. "Gimbals look good: total attitude looks good. Rates are damped out. Still a little busy."

There was no problem with the roll thruster, but the sloshing propellant could have a significant effect on the spacecraft’s attitude which was corrected by the thrusters and the engine gimbals.

"Two minutes. Mark it," continued Collins. "When it hits the end of that roll dead band, it really comes crisply back." Collins was describing how well the computer was able to deal with the CSM’s tendency to drift off in attitude.

“Okay, chamber pressure’s falling off a little bit." Collins had one eye on the gauge that showed the pressure within the combustion chamber. "Now it’s going back up; chamber pressure’s oscillating just a tad."

Armstrong called out. “Ten seconds left.”

"We don’t care about the chamber pressure," said Collins. "Brace yourself. Standing by for engine off."

The 2 minutes 28 seconds that mission control had predicted for the burn came and went, but the engine was still firing.

“It should be shut down now’," said Armstrong.

Collins queried him, "Okay?"

“Shutdown," called out Armstrong.

Collins stopped the engine at the same time as the computer. It had burned for 3.4 seconds longer than predicted because its thrust during the LOI burn had been slightly high and mission control had used that data when planning TEI. In the event, a slight change in mixture ratio lowered the thrust and so it burned a little longer to achieve the same change in velocity.

"Let’s look at what we got," said Collins as they brought up the residual velocity components. "Beautiful," he commented, “,v and r, 0.2." A burn that had changed their velocity by 1,000 metres per second was showing an error of only six centimetres per second. "SPS, I love you," he exulted. "You are a jewel! Whoosh!"

As with the LOI burn, no one knew anything of this in the MOCR or anywhere else on planet Earth. Any communication with Columbia was blocked by a 3.476- kilometre ball of rock. What they did know in mission control, down to the second, were the limes when the CSM would come back into view if the burn had worked, and if it had not. The increase in velocity would dramatically shorten how long it was out of sight.