Category How Apollo Flew to the Moon

Descent propulsion system

Most of the LM’s descent stage was taken up with the descent propulsion system (DPS). In the parlance of Apollo, the DPS was always pronounced dips’. The designers had come up with a simple cruciform structure for the descent stage which held a propellant tank in each of the four box-shaped bays around a central space where the engine was mounted.

This engine was remarkable for its time because it could be throttled, i. e. its thrust was variable, which was a major technical achievement. The basic idea of the throttle mechanism was to alter the area of the injector plate in use at any one time – similar


“The Eagle has wings.” Apollo ll’s LM flies free after undocking. (NASA)


The cruciform structure of a LM descent stage. (NASA)

to an adjustable shower head. At its theoretical full power, the DPS could generate a thrust of 47 kilonewtons, about half of one engine on a Boeing 727 airliner. In operation, it could be throttled smoothly between 10 and 65 per cent full thrust or run at a steady 92.5 per cent. This ability to be throttled was essential to enable the computer to optimise the vehicle’s descent to the surface and to hover in the final moments before touchdown.

As in the SPS engine, propellants were forced into the combustion chamber by pressure alone. There were no pumps to fail. This pressure was provided by two helium tanks, one of which stored the gas at ambient temperatures; the other tank stored it as supercritical helium (SHe), a strange phase of the gas brought about by a combination of very high pressure and extremely cold temperatures. By using SHe, more of the gas could be crammed into a much smaller tank, thereby eliminating over 100 kilograms of weight from the vehicle. Care had to be taken, however, to carefully handle the heat in the overall system. In steady operation, the warmth in the fuel would be used to heat the SHc via a heat exchanger. But at engine start, before the fuel got flowing, there was a possibility of it being frozen by the SHe. This was the main reason for using the ambient Lank, which would not freeze the fuel while it provided the pre-start pressurisation. First, three explosively operated valves were opened by command from the crew7 to release the ambient helium into the tanks. This was part of the preparatory checklist. Later, once the engine was actually running, another explosively operated valve would automatically open to release SHc into the propellant tanks.

Gauging the propellant

In view’ of the severe weight restrictions imposed on the LM. and given that propellant w-as a major fraction of the spacecraft’s weight, typically about 70 per cent, it wras vital that the tanks, which were somew hat larger than they wrere required to be, were loaded with only as much fuel and oxidiser as would ensure a safe landing. It was equally vital that a system be in place that would allow’ the crew and flight controllers to monitor the remaining quantity, especially because the levels would get low just when the commander w-as likely to be hovering, looking for a safe place to set down.

The tanks of the DPS had two independent systems for measuring propellant quantity, either of which could be monitored by a gauge in the cabin and both of which could be monitored from Earth. As the descent progressed, flight controllers closely watched how each system responded to the falling propellant levels and decided which one seemed to be more trustworthy and appropriate for the crew to monitor.

”Eagle, Houston,” said Duke from his Capcom seat. “It’s ‘Descent two’ fuel to monitor. Over."

Recharging the PLSS

Except for Apollo 11, surface crews had an important item of housekeeping to perfonn once they had returned to the relative safety of the LM’s cabin, namely to replenish their PLSSs if they had another EVA scheduled. Oxygen and water tanks had to be refilled – oxygen for breathing and water for cooling. Batteries for power and lithium hydroxide canisters for carbon dioxide removal had to be replaced. The Apollo 12 crew were the first to carry out these procedures and the engineers saw a chance for a data point.

The early missions tended to be pathfinders for the later extended missions and it was important to know just how– long the consumables in the PLSS had lasted in true lunar conditions as opposed to rehearsals on Earth. The crew could monitor oxygen consumption on their RCU displays and indications of a crewman’s metabolic output could be inferred from the biomedical data that was telemetered to Earth. What was missing was an indication of how much water had been consumed by the sublimator during the EVA to keep a crewman cool, so Conrad and Bean were tasked with weighing the remaining water after their first moonwalk. However, Conrad was none too enamoured with the equipment supplied for the job.

"Houston, you’ll never believe what wfe have been doing for the last 35 minutes.”

“Go ahead,” said Gibson, the Capcom in mission control.

“I am going to take this 35-cent scale that they sent out here to weigh these bags with and break it over somebody’s head!”

NASA had supplied an off-the-shelf spring scale designed for Earth’s gravity with the idea that a lunar measurement would require it merely to be scaled appropriately. But one feature had been overlooked, as Conrad explained after the flight. “The scale ought to have been set to zero. If anybody had thought about it. including myself, the spring tension in the scale itself was never zero in one-sixth g. As I unscrewed it to zero. I unscrewed it all the way, and the screw, the spring and everything disappeared into the bottom of the scale. We had some difficulty putting that baby back together again."


There was still enough momentum for science on Apollo 11 for NASA to begin a crash programme for tw’o simple experiments that the first crew’ could deploy in a few’ moments. This early Apollo scientific experiments package (EASEP) included a seismometer in a first attempt to investigate moonquakes. Solar panels powered the instrument throughout the lunar day and seismic signals were radioed to Earth in real time. It had small radioisotope heaters to provide warmth for the electronics during the intense cold of the long lunar night, but on the first attempt to reawaken it, engineers found it w’as malfunctioning and permanently switched off.

The second experiment still works today. It was the first of a series of laser retro



reflectors that used the fact that an internal corner of a cube mirror will reflect light directly back to its source; in this case, pulses of laser light from Earth. Apollo crews would eventually deploy two more reflectors at widely separated sites. Two further reflectors would be added by the Soviets mounted on their Lunokhod remote – controlled unmanned vehicles. This laser ranging retro-reflector (LRRR) allowed the distance between Moon and Earth to be measured to tens of centimetres accuracy, later to within millimetres. This elegant technique has become a powerful tool for understanding the relative motions of the Moon and Earth, and in turn, a wide range of subtle geophysical phenomena that include the tides, the structure of the Moon and even the move­ment of Earth’s tectonic plates.

A third experiment, not part of NASA’s EASEP, was the solar wind collector. This was essentially a sheet of foil that was left to bask in the sunlight and the solar wind.

It was designed by the University of Bern in Switzerland and jokingly referred to as the Swiss Flag. To­wards the end of the moonwalk, it
was rolled up and stored in a vacuum with the hope that it would contain embedded solar wind particles gathered from beyond Barth’s magnetosphere whose composition could be determined on Earth. Once returned to Earth, this experiment would provide direct information on the chemical composition of the Sun.


The various rescue plans for the CSM meant nothing if the LM could not get off the Moon and reach some kind of orbit. Should the ascent engine have underperformed for some reason and come up significantly short on velocity, the surface crew would be doomed within an hour of lift-off. If il failed to ignite, they w’ere doomed to expire on the surface. This was Michael Collins’s private fear as Neil Armstrong and Buzz Aldrin prepared to make their ascent in the top half of Eagle. “When the instant of lift-off does arrive, I am like a nervous bride. I have been flying for 17 years, by myself and with others; I have skimmed the Greenland ice cap in December and the Mexican border in August; I have circled the Earth 44 times on board Gemini 10. But I have never sweated out any flight like 1 am sweating out the LM now-. My secret terror for the last six months has been leaving them on the Moon and returning to Earth alone; now’ I am within minutes of finding out the truth of the matter. If they fail to rise from the surface, or crash back into it, I am not going to commit suicide; I am coming home, forthwith, but I will be a marked man for life and I know it.”

When compared lo the fuss and bother of a launch from Earth, with its enormous launch gantries, heavy conerete pads, ground support equipment and launch control facilities, it is almost amusing to consider the relative ease with which two Apollo crewmen harnessed themselves into position, configured their spacecraft for launch and pressed a button to get themselves off the surface of another planet. The difference, of course, is that Earth is at the bottom of a very deep gravity well and every last item required for an Apollo voyage had Lo be lifted through a thick atmosphere and hurled towards the Moon. Such a feat required a vehicle of immense power and complexity the Saturn V and a cast of hundreds to send il on its way. The LM ascent stage, on the other hand, was a far simpler machine, made as light as could be and it would launch from an airless world whose gravity was barely one-sixth that of Earth. And given that its task was merely to rendezvous, it would require to provide only several hours of life support.

By the time the advanced Apollo missions got into their stride, NASA had enough confidence to partially power the LM down during the lunar stay, conserving battery power and permitting three days of exploration. In particular, the primary guidance and navigation system (PCNS) was turned off. Turning it back on involved a complete realignment of its guidance platform. As with platform alignments in space, Lite crew used the alignment optical telescope (AOT) mounted into the Lop of the LM to sight on a star. A major difference in the procedure came from the use of the direction of gravity as their second reference.

Earlier, the crew had temporarily depressurised the LM cabin to throw out any items not needed for the journey to orbit, especially the back packs that had kept them alive on Lite surface. They would keep their suits on until they returned to the CSM but would be connected directly to the LM’s supply of oxygen and coolant. Other equipment and samples had to be carefully stowed in predetermined positions around the cabin to ensure that the centre of mass of the ascent stage remained as near to ideal as possible the further it was from ideal, the more the RCS thrusters had to work during the ascent to maintain the stage’s attitude.

If the pressure of time allowed, the surface crew would try to test their rendezvous radar on the CSM as the mothership passed over the landing site one orbit prior to lift-off. The rendezvous radar worked with a transponder on the CSM to provide range, range-rate and direction to its quarry. Its dish was mounted above the LM’s front face and could move up and down or side Lo side as it tracked the CSM from a distance of up to 750 kilometres. At the same time, the CMP carried out a tracking program in his computer to help aim the 28-power sextant at the landing site. By taking marks on the LM centred in his viewfinder, he helped mission control to improve their reckoning of the LM’s state vector information that was loaded into the LM computer shortly before lift-off.

Mission control then read up a lift-off PAD to both the LM crew and the CMP that gave details of their rendezvous. On later missions, two PADs were sent covering two types of rendezvous – one as a fallback in case the other had to be aborted. With less than an hour to lift-off. the commander gave his RCS thrusters a cheek by firing them while still sitting on the surface. When Pete Conrad did this, he managed to blow over an umbrella-like dish antenna that he and Л1 Bean had deployed on the surface and the resultant loss of communications meant they had to switch over to the high-gain dish on the roof. Power was then switched away from batteries in the descent stage to a pair of batteries in the ascent stage. Flight control displays were set up for flight and the abort guidance system (AGS) was initialised to back up the PGNS for rendezvous guidance.

Proceeding on through the launch checklist, the surface crew donned their helmets and gloves. They were about to ignite the ascent stage’s engine while it sat on top of the discarded descent stage, which raised the possibility of the pressure wave from combustion compromising the LM hull. It was therefore wise to be fully suited for the ascent. Then with all checks completed and only a few’ minutes remaining to lift-off, the crew could make their final preparation to leave the surface.

‘ Stand by. You ready to watch the APS pressurise?’- Apollo 15’s David Scott was checking to make sure that mission control was going to watch the vital signs from the ascent stage’s propulsion system, the APS. Its tanks had remained unpressurised until this point.

“Okay, let’s let her go,” replied Ed Mitchell. It had become customary for the LMP from the previous mission to serve as Capcom for ascent because his awareness of what the LM crew were trying to do made him particularly suited to this role.

To pressurise the tanks, explosively actuated valves from two very-high-pressure helium tanks were operated to release the gas into the propellant tanks and bring them up to their working pressure. Mission control checked each tank in turn, fuel and oxidiser, to ensure that if there were any signs of a leak, lift-off could be carried out as soon as possible in order to minimise propellant loss. “Okay, here comes tank 1.” announced Scott. “And w’e‘11 stand by for your call for tank 2.”

“Roger.” said Mitchell. After a brief pause for flight controllers to monitor tank I’s pressure, Mitchell gave the go-ahead for the second Lank. “Okay. Go with tank 2. looks good.”

“Okay. Tank 2 coming now.”

After another pause, Mitchell confirmed that all wtis well. “Looks good down here.-’

“Okay, thank you. Looks good up here,” replied Scott.

“And, Dave, you’re Go for the direct rendezvous. Both guidance systems look good; PGNS is our recommendation.” Mitchell w’as letting Scott know’ that, of their two practised methods for rendezvous, the flight controllers recommended that they use the planned-for quick technique called direct rendezvous, and that, of their two guidance systems, they should rely on the primary.

“Roger. Go for direct on the PGNS.”

“Okay, loud and clear, Dave, and you’re Go for lift-off.” Then Mitchell reminded the crew’ that it was time for another transition in their roles: “And I assume you’ve taken your explorer hats off, and put on your pilot hats.-’

“Yes sir, we sure have. We’re ready to do some flying.” replied Scott.

“Standing by for one-minute,” prompted Jim Irwin whose primary task was to

look after the AGS and see that its knowledge of the ascent matched that of the PGNS. “Guidance steering is in." was his next call as he commanded the AGS to take its guidance information and generate steering commands for the RCS to use in case of an emergency. With a normal ascent, the guidance mode control switch would route steering commands only from the PGNS, blocking those from the AGS. "

Prcloadcd with the data from the ascent PAD. the PGNS was nearly ready to ignite the engine. “Okay, Master Arm is On; I have two lights," called Scott, as he armed the pyrotechnic system that would sever the two halves of the LM. and saw an indication that the circuits were good.

“Average g is on…” The DSKY display had blanked to show that the PGNS was now calculating the average acceleration the LM would experience as it flew. In other words, it was now guiding the LM. It just had not ignited the engine yet. On the right side of the cabin, Irwin started a 16-mm movie camera aimed out of the window to film the view of the ascent.

Scott continued with his steps prior to ignition.

“Abort stage." Pressing the ‘abort stage’ button caused the ascent and descent stages to separate, using explosive bolts to sever the four attachment points holding them together. At the same Lime, explosive charges drove guillotine blades through the bundles of wiring and plumbing to sever those connections also.

“Engine arm to ;ascent’." Arming the ascent engine allowed the engine controller to open the valves on the engine. Then at T minus five seconds, the DSKY displayed Verb 99, which was its way of asking the crew if they washed it to proceed with engine ignition.

“99, Pro." intoned Scott, at which point he pressed the ‘Proceed’ button on the DSKY, essentially replying, “Yes, please."

Television viewers on Larth were given a ringside seat at the launch of the last three LM ascent stages. Before he entered the LM for the last time, the commander parked his rover 100 metres east of the LM. From this vantage point, this miniature interplanetary outside broadcast station, remotely controlled from Larth via its own radio link, provided coverage of the lift-off itself, and the quiet, still and desolate scene that followed for as long as the rover’s batteries and equipment continued to operate. When Apollo 17’s ascent stage lifted off. Ed Fendell, who operated the TV camera, managed to follow the early stages of Challenger s ascent to orbit, despite a З-second delay between his command to Lilt and seeing the result on his monitor. It showed how7 the ascent stage went straight up for just 10 seconds – yawing a little as it did so to aim the vehicle towards the flight azimuth then promptly pitched nose dow’n by a little over 50 degrees in order to start adding horizontal speed. This was very different to a launch on Earth, where a streamlined rocket has to rise essentially vertically during the first few minutes to escape the bulk of the atmosphere before it can ramp up its horizontal speed to reach orbit. The lack of an atmosphere on the Moon allowed the LM to start to gain horizontal velocity almost as soon as it left the ground w hich permitted a more efficient flight profile.

As the LM pitched over, the crew’ gained a view7 of the landscape they had previously been exploring. As Aldrin related during his debrief, they were also able


The lift-off of Challenger, Apollo 17’s LM, from coverage by the rover TV camera. (NASA) "

to watch the after-effects of their lift-off. “I could see radiating out, many, many particles of Kapton and pieces of thermal coating from the descent stage. It seemed almost to be going out with a slow-motion-type view. It didn’t seem to be dropping much in the near vicinity of the LM. I’m sure many of them were. They seemed to be going enormous distances from the initial pyro firing and the ascent engine impinging upon the top of the descent stage.”

“I observed one sizeable piece of the spacecraft flying along below us for a very long period of time after lift-off,” noted Armstrong. “I saw it hit the ground below us somewhere between one and two minutes into the trajectory.”

Aldrin was fascinated by the physics demonstration he was seeing: "It’s very difficult to conceive of such lightweight particles like that just taking off without any resistance at all,” he continued during his debrief. "It’s easy to think back and say

that they would do that. But it just seems so unnatural for such flimsy particles to keep moving at this constant velocity radially outward in every direction that I could see out the front window. I don’t recall seeing any impact with the ground, but there were sizeable pieces."

Just behind the two crewmen in the centre of the cabin was the cylindrical cover of the ascent engine. Many have remarked on how close two astronauts stood to an operating rocket engine. As he and Irwin flew over the meanders of Hadley Rille, Scott noted how the sound from the engine was audible through his helmet. “Both guidance systems are good, Dave." called Mitchell as the flight controllers closely watched the numbers coming up on the PGNS and the AGS.

"Okay, looks good up here." returned Scott. Then he remarked to Irwin. "It almost sounds like the wind whistling, doesn’t it?’’

Scott was impressed by the sound, and the ascent in general. “Truly amazing," he described decades later. "The LM launch and ascent were so quiet, especially when compared to Titans and Saturnsl It was almost peaceful – some vehicle oscillation, periodic at a couple of degrees, and the periodic sound of a slight wind, pulsing at about two to three seconds in frequency. And. of course, the view of the Rille as we flewr right along its course, face down. Just spectacular. Could not have been a better farewell. Most pleasant and certainly indelible.”

The gentle wobble imparted on Falcon as it soared through the lunar sky was caused by the periodic thrusting of the RCS jets. For simplicity, and to reduce the weight, the ascent engine was fixed. It could not aim its thrust anywhere but straight down along the spacecraft’s, v axis. As a result, the RCS had to do all the steering by turning the entire spacecraft and therefore aim the ascent engine in the right direction, which it did at each two-sccond cycle of the computer’s tasks. Only the downward-facing thrusters were used, there being no need to fire the upward-facing thrusters as they would be thrusting counter to the ascent engine.

The ascent was a critical event. If the APS engine were to underperform, which it never did, there was a real possibility that the LM would not achieve a stable orbit and instead would crash after less than half a revolution. The crew watched the velocity and altitude readings from both of their computers and compared them to charts, looking for any deviations that might indicate a problem. One remedy for an underperforming ascent engine was to augment its thrust with the four RCS jets that were aimed in the same direction. Because the ascent stage was relatively light and the Moon’s gravity relatively weak, the ascent engine’s rated thrust was only 15.6 kilonew tons. about the same as the first jet engines introduced during World War II. Four thrusters could provide more than one-tenth of that at 1.8 kilonewtons total a thrust that could make a difference in the later stages of a problematic ascent. With such emergency contingencies in mind, the LM’s designers had arranged the RCS plumbing so that, if their own tanks ran dry. they could draw propellant from the ascent engine’s tanks.

Once the PGNS had determined that the ascent engine had added enough velocity, it commanded a shutdown. Immediately, the crew quizzed the computer on the size of the orbit they had achieved.

“PGNS says it’s in a 40.6 by 8.9.’’ reported Scott as soon as Falcon had entered

orbit. The numbers showing on the DSKY represented the altitudes of the apolune and perilune respectively, given in nautical miles. ‘I’heir orbit appeared to be 75.2 by

16.5 kilometres, which was only slightly lower than desired.

“Roger, we copy," replied Mitchell in mission control; then reassuringly said, "the guidance still looks good to us.’’


Within a minute, mission control had their orbit, as determined by radio tracking, to hand. "Falcon, Houston. We have you at a 42 by 9,’’ announced Mitchell.

"You’re looking good."

"Okay. 42 by 9," confirmed Scott.

Although their tracking data put the orbit a little higher than Falcons at 77.8 by 16.7 kilometres, the trajectory experts were satisfied that the rendezvous should go ahead as planned.


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.

Rising speed

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

Diagram of the components of the Earth landing system at the apex of the CM. (NASA)

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.

Checkout nightmares

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,


The steerable antenna on Apollo 16’s LM Orion hangs uselessly. (NASA)

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.