The Apollo 11 Mission Sequence

Preparation started weeks before launch, but the final count down started 13 hours before lift off. The propellant and oxidiser tanks were filled and the crew ran through the mission checklist.The first stage ignition sequence started 8.9 seconds before the launch.

During the final stages of count down, the crew were sitting on 2500 tons of propellant with a potential explosive yield of just over half a kiloton of TNT, equivalent to a small nuclear bomb*, waiting for ignition and blast off into the unknown in a complex space vehicle constructed from 7 million components which had to perform flawlessly to get them back home safely.

Perhaps the Apollo crew reflected on the comments of John Glenn, America's first astronaut to orbit the Earth, about his thoughts as he waited in his Mercury capsule for blast off into space. He reportedly said "I look around at all the complex equipment and think that every piece was bought from the lowest cost vendor".

The five F-1 kerosene fuelled rocket engines of the Saturn V launch vehicle, each with a thrust of over 1.5 million pounds, fired 0.3 seconds apart to lessen resonance effects in the fuel system which could cause "pogo stick" like vibrations on the rocket's structure and systems as well as on the astronauts, and Apollo 11 slowly rose from the launch pad taking 11 seconds to clear the launch tower.

S-1C (First Stage)

S-1C powered flight with a thrust of 7.6 million pounds from the five F-1 engines gulping 13 tons of kerosene (RP-1) and liquid oxygen (LOX) per second, pinning the astronauts into their seats with a "g" force of 4.5 g.

Pitch and Roll

Two miles off the launch pad and travelling at 1500 mph, Saturn's guidance computer set Apollo 11 on the planned trajectory with directional adjustments being made by means of hydraulic actuators on the gimbal mounts of the four outer F-1 engines.

Loss of control during the critical launch phase would have resulted in the destruction of the rocket. Such an event would however have triggered the Launch Escape System to eject the crew capsule and deploy its parachutes to bring the crew safely back to the ground.

Mission Control Center (MCC) in Houston tracked the spacecraft throughout its mission, except when it was behind the Moon during which time communications were not possible. The timings and duration of the engine burns at key points were controlled by Houston Mission Control and were programmed into the Guidance Computers in the Instrument Unit, the Command Module and the Lunar Module.

S-1C (First Stage) / S-11 (Second Stage) Separation

After 2.5 minutes the 4.5 million pounds (over 2,000 tons) of propellant in S-1C were used up and the S-IC's eight retro rockets were fired to separate it from the S-11. The S-1C was jettisoned and fell into the Atlantic Ocean. By that time, Saturn V had reached an altitude of 45 miles, 350 miles downrange and was travelling at 6,300 m.p.h.

Four seconds later the five J-2 hydrogen fuelled engines of the S-11 started up.

S-II (Second Stage)

S-11 powered flight with a total thrust of 1.125 million pounds from the five J-2 engines.

Launch Escape Tower Jettison

Once the second stage was safely on its way, at a height of 60 miles, the Launch Escape Tower was jettisoned.

S-11 (Second Stage) / S-1VB (Third Stage) Separation

After 6.5 minutes the 942,000 pounds (over 400 tons) of propellant in Stage 2 were exhausted. The S-1VB Stage 3 took over and the S-11 was jettisoned.

By that time Saturn V had reached an altitude of 110 miles, 1,400 miles down range travelling at 15,000 m.p.h.

S-1VB (Third Stage)

S-1VB powered flight with a single J-2 re-startable engine with a thrust of 225,000 pounds.

Earth Parking Orbit

The J-2 engine burned for 2.75 minutes to put the Saturn V into a circular parking orbit 1,640 miles downrange at an altitude of 118.8 miles (191.2 km) with an orbital velocity of 17,432 mph and an orbital period of 1 hour, 28 minutes, 16 seconds. The third stage remained attached to the Apollo 11 spacecraft while it orbited the Earth one and a half times during which time the astronauts and mission controllers checked that all systems were functioning correctly and positioned the rocket precisely for initiating translunar injection.

Trans Lunar Injection - Accelerating to the Escape Velocity

After one and a half Earth orbits, 2 hours and 44 minutes after launch, the Saturn's third-stage J-2 engine was re-started to produce a second burn lasting 5.2 minutes which accelerated the Apollo 11 to an altitude of 190 miles with a velocity of 24,500 mph (10.9 kms/sec) to escape from its Earth parking orbit and enter into a "free return trajectory" to the Moon. This meant that when the spacecraft eventually passed by the Moon its velocity, direction and radial distance from the Moon would be such that the Moon's gravity would deflect it into a "slingshot" path around the Moon and back in the direction it came from towards the Earth. Too close or too slow would cause the spacecraft to crash into the Moon. Too far away or too fast would send the spacecraft on a one way journey into space.

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To complicate the manoeuvre even further, the Moon is a moving target travelling at 2286 m.p.h. orbiting the earth in 27 days so that the direction of the spacecraft as it left its Earth orbit was not to where the Moon was at the instant of departure, but to where it would be in three day's time when the spacecraft actually reached it. This required very precise control over the point and timing at which the spacecraft left the Earth orbit and the duration of the engine burn.

Note that the velocity necessary to overcome from the Earth's gravitational pull depends on the spacecraft's orbital altitude. At the Earth's surface, the escape velocity is slightly higher at 25,000 mph (11.2 kms/sec). (See Entering Space).

Initial Translunar Coast

Apollo 11 began its three-day, 240,000 mile un-powered flight through the vacuum of space during which the spacecraft's momentum alone kept it moving towards the Moon.

Service Module Orientation, Docking and Extraction

During its transit through the atmosphere the flimsy Lunar Module (LM) sat on top of the Saturn V third stage inside the LM Adapter which protected it from damage caused by aerodynamic pressures and friction. Then during the first 10 minutes of the translunar coast, when Apollo11 was above the Earth's atmosphere, the attitude of the spacecraft was adjusted in readiness for extracting the Lunar Module from the LM Adapter and coupling it to the Command and Service Module (CSM).

The next three steps were the Lunar Module Transposition and Docking

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Lunar Module Transposition and Docking

Step 1 - CSM separation from the LM Adapter

The panels of the LM Adapter were jettisoned and the reaction control thrusters of the CSM mother ship were fired to separate it from the Saturn S-1VB third stage, leaving the Lunar Module behind and stopping 50 to 75 feet away.

Step 2 - CSM docking with LM / S-IVB (Third Stage)

The CSM thrusters then rotated the CSM by 180 degrees and docked it with the Lunar Module which was still attached to the third stage. All this happened with the Saturn S-1VB and the CSM hurtling separately through space at over 24,000 miles per hour.

Step 3 - CSM /LM separation from S-IVB

The LM now attached to the CSM was extracted from the LM Adapter and the third stage S-IVB, together with its attached Instrument Unit was jettisoned, and went into orbit around the Sun.

After 4 hours and 40 minutes into the flight, the LM now docked with the CSM mother ship was ready for its journey to the moon.

The CSM and LM continued their three day journey to the Moon tracked by mission control in Houston. Travelling through the vacuum of space, the flimsy Lunar Module no longer needed the protection of the LM Adapter. During this phase of the journey the spacecraft was put into a slow roll of two revolutions per hour to provide uniform solar heating, however this was stopped during navigation sightings and course corrections.

Still influenced by the Earth's gravitational pull, the spacecraft gradually slowed down to about 2040 mph at 39,000 miles from the Moon but as the Earth's gravitational effect continued to weaken, the Moon's gravity began to dominate and the spacecraft speeded up again to about 5600 mph until it was eventually pulled into a "slingshot" trajectory around the back of the Moon.

Mid-Course Correction

Mid-course corrections to ensure that the spacecraft did not stray from its "free return trajectory" were implemented by either, or both, the Service Module's reaction control thrusters or by its main propulsion engine.

Besides the mid-course correction, during the flight, the crew periodically checked their position with respect to the stars using a sextant and this celestial reference was used to correct for any drift of the gyroscopes in the inertial navigation system.

Loss of Signal

Although the Apollo 11 had autonomous navigation and control systems, it still needed constant communications with Mission Control in Houston since it did not have sufficient computing power at all times to monitor the spacecraft's speed and position in space and to program the timing and duration of the main engine and control thruster burns to make any course corrections necessary to initiate lunar orbit insertion or to get the spacecraft back to Earth in an emergency.

But communications. between the Earth and the spacecraft need a "line of sight" link between the transmitters and the receivers and this raised potential problems at both ends of the link.

While in lunar orbit, the Apollo 11 spent 45 minutes behind the Moon during each two hour orbit, and while it was behind the Moon, communications with the Earth were not possible. At the other end of the link the Earth is rotating and part of the time the Mission Control Center in Houston was facing away from the Moon and unable to "see" the spacecraft.

To address the first problem the spacecraft received targeting information from Houston before each loss of signal as it passed behind the Moon. If acquisition of the signal failed when it emerged from behind the Moon, the landing would be abandoned and the crew would use that information to target a contingency burn home.

To maintain communications with the spacecraft as the Earth rotated, at least three satellite ground stations separated by approximately 120 degrees of longitude and connected by a terrestrial network to Mission Control were required so that as the Earth turned the spacecraft was always above the horizon of at least one station so that there was no loss of signal due to the Earth's rotation..

Lunar Orbit Insertion

To breakout from its slingshot track around the Moon and enter into lunar orbit, a retro-thrust was required to slow the spacecraft down from its velocity of around 5,600 mph, with respect to the Moon's velocity, to 3,600 mph allowing it to be captured by the Moon's gravity. Due to the ballistics of the slingshot trajectory around the Moon, this manoeuvre had to take place while the spacecraft was behind the Moon and out of communication with the Earth. Thus after loss of signal, as Apollo 11 passed behind the Moon, the Service Module fired its main propulsion engine in two burns in the direction of travel to slow its momentum and enter lunar orbit. The first burn lasted 6 minutes and placed the craft in an initial elliptical lunar orbit of 196 by 69 miles. The second burn lasted just 17 seconds and eased Apollo 11 into a circular orbit of 69 miles, in preparation for Lunar Module separation and powered descent. Using two shorter burns reduced the chance of over-burn, which would slow the spacecraft too much causing it to crash into the lunar surface.

Lunar Orbit

The CSM docked with the LM, travelling at 3,600 mph at an altitude of 69 miles, continued orbiting the Moon once every two hours during which time it was out of communication with the Earth for 45 minutes. During 13 orbits the crew checked that all systems were functioning correctly, made observations of their planned landing site in The Sea of Tranquility and also spent some time resting.

Crew Transfer to LM

The LM pilots, Neil Armstrong and Buzz Aldrin transfered from the Command Module to the crew compartment in the Ascent Stage of the LM and checked that all systems were performing correctly.

Lunar Module Separation

Using its reaction control thrusters the LM was undocked from CSM to prepare for descent while Michael Collins in the Command Module made a visual check that the LM legs of the landing gear had opened from their stowed position and were latched in place.

LM Descent

Because the Moon has no atmosphere it was not possible to use parachutes to make a soft landing nor wings to glide down to the surface. A rocket powered descent to the lunar surface was therfore needed. Balancing a rocket on its exhaust jet and preventing it from toppling over while simultaneously reducing its thrust to make a gentle, upright landing is no mean feat. The LM's Digital AutoPilot (DAP) and gimbals kept the direction of the rocket's thrust in line with the centre of mass of the LM to ensure stability during the descent.

The descent to, and ascent from, the Moon were both controlled from the LM crew compartment in the Ascent Stage which contained the necessary status displays and control systems. A single set of 16 reaction control thrusters, identical to those used on the Service Module, to control both the descent and ascent was also mounted on the Ascent Stage of the LM. However the LM Ascent and Descent stages each had their own main engines, the descent engine with a thrust of 10,125 pounds being much larger since it had to support a much greater weight, and the ascent engine with a thrust of 3,600 pounds.

In a series of pre-programmed, controlled burns by the descent engine to slow the Lunar Module, the LM was first put into an elliptical orbit of 70 miles by 9.5 miles (50,000 feet), high enough to clear the Moon's mountains, and to bring it over the planned landing point at its pericynthion (the orbit's nearest point to the Moon), then a further braking burn took the LM down to 9000 feet and oriented so that the crew could see and begin to evaluate the suitability of the landing zone. A further pre-programmed burn then took the LM down to 500 feet at which point the crew had to take over manual control of the descent.

Final Landing Phase - Touchdown - 20 July 1969 at the Moon's Sea of Tranquility

Before landing, the crew were unaware of the precise nature of the Moon's surface and whether they would sink into a deep layer of dust. Depending on the nature of the terrain below, the pilot had to guide the LM away from any observed hazards such as boulders, craters, fissures and rock outcrops.

Real time feedback control from Mission Control of the final landing manoeuvres was not possible, even if it was desirable, because the two-way delay of a radio signals, travelling at the speed of light, carrying sensing inputs to Houston and responses back to the Moon was too long at 2.6 seconds and would have resulted in instability and loss of control. (To this delay would have to be added the further delays introduced by the terrestrial signal networks).

At 500 feet the LM pilot Neil Armstrong took control of the descent making any adjustments necessary. Finally, at 65 feet above the lunar surface, the LM was re-oriented and descended vertically to the Moon at 3 feet per second and the engine was shut off as soon as the landing gear touched the surface.

Dramas at the End of the Line

Descending to the surface, Apollo 11's Lunar module was about 6,000 feet above the surface and the descent engine was halfway through its final 12-minute burn that would land the crew safely on the moon, when a yellow caution light lit up on the computer control panel. It was coded 1202, an "executive overflow" alarm, which meant the computer was having trouble completing its work in the cycling time available, and the astronauts asked Mission Control for instructions.

As NASA legend has it, 26 year old Steve Bales, the guidance officer (GUIDO) who had the authority to issue a Go or No Go decision on the landing - continued to issue a confident "We're Go!" throughout the remaining seconds of the descent, even as the 1202 and a similar alarm, the 1201, sounded intermittently. When the lunar module made its landing, it had seconds of fuel remaining before it would have to abort. The icy calm of Bales is a dramatic, iconic moment in NASA history, but as you peel back the layers of preparation that led to those moments, the story becomes almost astounding.

Bales – who later accepted the NASA Group Achievement Award from President Nixon on behalf of the entire mission operations team – credited his quick decision to an even younger whiz kid, John R. "Jack" Garman, 24 years old, an expert in the guidance computer software. It was Garman who, a few months before Apollo 11, gave the simulation supervisor, Dick Koos, the idea of testing the reaction of flight controllers to computer error codes. He also supported flight controllers in Mission Control as a backroom advisor on computer systems. By the time the actual landing was being attempted by astronauts Neil Armstrong and Buzz Aldrin, Garman knew almost instinctively that a single 1202 or 1201 alarm did not mean the mission had to be aborted; it simply meant the computer was struggling to keep up. As long as the alarm did not become continuous, which would have meant that the computer was not getting any work done and vital tasks were being neglected, it would not prevent a landing.

And it was Garman supporting Bales from another console to whom Bales turned when the 1202 alarm went off. “Quite frankly,” Bales later recalled, “Jack, who had these things memorized, said, ‘That’s okay,’ before I could even remember which group [the alarm] was in.”

For his part, Garman gives credit for his memorisation of the alarm codes to Gene Kranz, the fiery Flight Director who brought the Apollo 13 astronauts back to Earth after the explosion in their spacecraft. “[Before the mission] Gene Kranz, who was the real hero of that whole episode, said, ‘No, no, no. I want you all to write down every possible computer alarm that can possibly go wrong.’” Garman did so, along with the correct reaction to those alarms – and kept this handwritten list under glass on his desk.

Searching for landing space in boulder field Apollo 11 Lunar Module touched down with only enough fuel and oxidiser in its tanks for a further 25 seconds of flight.

Exploration of Lunar Surface

Inside the controlled environment of the Lunar Module Armstrong and Aldrin changed from their pressurised flight suits into space suits with self-contained life support systems before depressurising the cabin and stepping down into the vacuum of space. Alighting from the LM they remained for two and a half hours of Extra Vehicular Activity (EVA) on the Moon during which they collected 22 Kg of samples of lunar rocks and soil as well as samples of the solar wind (charged particles emitted by the Sun) to bring back to Earth. They also took photographs and set up experiments to investigate soil mechanics and lunar seismic activity and they deployed a laser ranging retro-reflector to enable precise measurements of the distance between the Earth and the Moon.

LM Ascent Liftoff

After 21.5 hours parked on the lunar surface, the crew fired up the LM Ascent Stage to return to the CSM mother ship leaving the Descent Stage on the Moon.

The Lunar Module Ascent Stage didn’t need to accelerate to the Moon's escape velocity of 5,300 mph to leave the Moon. it only had to reach a lunar orbit with a velocity of 3,600 mph to rendezvous and dock with the CSM mother ship. The CSM provided the thrust to escape from the Moon's gravity. The main engine of the ascent stage took the LM up to over 11 miles and put it into an elliptical orbit.

There was no contingency plan if the Ascent Stage failed to get off the ground.

Rendezvous and Docking

To rendezvous with the CSM, the LM executed a series of burns by its reaction control thrusters, controlled by the LM computer on the basis of data supplied by Houston Mission Control, that initially put it into a circular orbit at an altitude of 69 miles concentric with the CSM, and then slowed it down to dock with the CSM. The LM commander took over control for the final docking manoeuvre.

Return docking was very critical and difficult.

Transfer of Crew and Equipment from LM to CSM

The crew returned to the Command Module and the hatch was sealed.

CSM / LM Separation and Lunar Module Jettison

The LM was detached from CSM which fired its reaction control thrusters to ensure complete separation. The LM Ascent Stage was left in lunar orbit.

Trans Earth-Injection

The CSM main engine fired for 2.5 minutes to increase its velocity from 3,600 mph to 5,500, just over the Moon's escape velocity, to break out of lunar orbit and send the Apollo 11 on a trajectory back to Earth. Like the Lunar Orbit Insertion, this manoeuvre took place when the CSM was behind the Moon and out of communication with Mission Control.

Trans-Earth Coast

Apollo 11's return voyage to Earth powered by the Earth's gravity. The gravitational effects of the Trans Lunar Coast were reversed. The velocity of the CSM initially slowed due to the gravitational pull from the Moon, but as the spacecraft moved away from the Moon, the Moon's gravitational effect diminished while and the Earth's gravitational pull increased. Once the Earth's gravitation became dominant the spacecraft was accelerated in a free fall all the way to the Earth reaching a velocity of 25,000 m.p.h. as it entered the Earth's atmosphere over three days later.

Mid Course Correction

Similar to the outward journey, the velocity and angle of approach to the Earth had to be very precisely controlled to ensure capture by the Earth's gravity and splashdown in the designated area.

Command Module / Service Module Separation

Shortly before entering the Earth's atmosphere at an altitude of 400,000 feet (around 75 miles) the Service Module was jettisoned by simultaneous firing of the reaction control thrusters in both the Service Module and the Command Module. The Command module was then rotated by 180 degrees to turn its blunt end towards the Earth.

Re-entry

Landing is just as dangerous as taking off and the precise re-entry trajectory is critical to making a safe landing. Once initiated there is no possibility of a second chance by "going around" and trying again if things go wrong.

The initial drag of 0.05 g experienced as the capsule entered the atmosphere triggered the Earth Landing Subsystem (ELS) which controlled the re-entry process.

The Command Module entered the atmosphere, blunt end first, at 400,000 feet with a velocity approaching 25,000 mph at an angle of 6.488 degrees to the horizontal and flew about 1240 miles around the Earth to its designated landing point in the Pacific Ocean. The velocity of entry and the angle of the flight-path had to be very closely controlled to achieve this.

At the very high entry velocity the compression of the air in front of the capsule heated its surface up to around 2760 °C (5000 °F), hot enough to vaporise most metals, turning the capsule into a shooting star. A 2.5 inch thick, sacrificial ablative heat shield which burns and erodes in a controlled way, carrying the heat away with its combustion products, protected the capsule from the heat of re-entry.

The only braking from the high re-entry velocity, down to the velocity at which parachutes could be deployed was by the drag of the atmosphere on the blunt shaped capsule. The reason for the 1240 mile trip half way around the world was mainly to provide adequate braking distance for the capsule, but also time for it to cool, by directing it along a sloping path through the atmosphere. Entering the atmosphere at too high an angle would either incinerate the crew or subject them to crushing g forces. In any case, in a normal landing the astronauts were typically subject to 6g. On the other hand, entering the atmosphere at too low an angle would not provide sufficient braking and the un-powered capsule would fly off into space with no possibility of return.

Communications Blackout Period

At the start of re-entry, the extreme heat of the shockwave generated by the compression of the air in front of the Command Module ionised the air creating a plasma which effectively blocked the transmission of radio signals to and from the spacecraft for about three minutes as it entered the atmosphere

Splashdown 24 July 1969

By the time the capsule had descended to 24,000 feet, it had slowed to around 325 mph and a barometric switch initiated the jettisoning of the forward heat shield and the deployment of drogue parachutes which stabilised the craft, slowing its descent even further. At 10,700 feet and travelling at 175 m.p.h. the three main parachutes were deployed reducing the Command Module's decent velocity to 22 mph for splashdown in the Pacific Ocean, just 15 miles from where the US Navy recovery ship Hornet was waiting for it.

Quarantine

The astronauts spent 21 days in quarantine as a precaution against the possibility of bringing back unknown pathogens from the lunar surface.