The amazing way astronauts returned from the Moon to Earth

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The Saturn V rocket, more than 110 meters high and weighing almost 3,000 tons, was responsible for taking the astronauts of the Apollo program to Earth orbit and towards the Moon; Once they arrived, they would use the lunar module to descend to the surface.
Next, we tell you how such a relatively small ship would help the crew return to Earth from the Moon.

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Photo of the last lunar mission, Apollo 17, with an astronaut, the flag of the United States and the Rover and lunar module in the background.

Road to the Moon: Saturn V and the Lunar Module

The entire journey begins in the kennedy space center in Cape Canaveral, Florida. The rocket would be prepared on the launch pad while being loaded with fuel. At its tip were the command and service module and the lunar module, which from now on we will call CSM and LEM, respectively, by their acronym in English.

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Diagram of the location of the lunar module, command, service and escape tower of the Saturn V rocket. At launch, the astronauts travel in the “Command module”.

He Saturn V It had three stages: two of them were used to reach Earth orbit, using about 89% of the total available fuel, this being the most expensive part (energetically speaking) of the entire trip; the third was in charge of giving the last push to orbit and taking the CSM and LEM in the direction of the Moon.

Once on the way to the Moon, the CSM would extract the LEM and they would remain united as a single ship. The astronauts could not enter the LEM until they had escaped the Van Allen belts, due to the high radiation.

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Already in lunar orbit, two of the three astronauts (the commander and pilot of the lunar module) would go to the LEM for the final preparations. Subsequently, both ships would undock and move away due to orbital drift, and once in the periselenium (lowest point in the orbit), the descent engine would ignite at low thrust to allow the computer to correctly calculate the center of mass and make corresponding corrections for the lunar landing.

Once the checks are completed, proceed with full thrust and an automatic landing until the last second, when the commander could take manual control and land gently as if flying a helicopter.

During this maneuver the tanks of the descent stage would be almost completely empty and useless and their only purpose at this time would be to hold the structure together and serve as storage for instruments and equipment taken to the Moon, such as the Lunar Rover, the flag, among others.

Houston, the Eagle has wings

The lunar module is a ship divided into two stages: the first (descent stage), performs the task of descending from orbit to the surface and has a little more than the fuel necessary to perform this task, and once the engine hits the surface, it can become completely unusable.

The second (promotion stage), has its own engine and has enough fuel in independent tanks to return them to lunar orbit and perform ““Rendezvous” and coupling.

Before leaving Earth, the ascent stage already has everything necessary to function correctly, as it would be used if it were necessary to abort a moon landing due to an emergency. Since it would activate at any moment, the engineers decided to make sure they had an engine whose reliability was that of the 100%, opting for one powered by hypergolic fuels, an oxidizer-fuel pair that react strongly when they come into contact. This would allow for the elimination of complex ignition mechanisms by having a fixed shaft, eliminating moving parts that could jam and/or fail, and an extensive multi-engine testing program on the ground to ensure all possible failure points are covered.

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View of the camera of the Lunar Rover in charge of recording the takeoff of the lunar module of the Apollo 17 mission.

Using the descent stage as a launch platform, the ascent stage could hold firm before takeoff, waiting for the perfect opportunity to return to lunar orbit.

Before returning to the LEM, in the missions Apollo 15, 16 and 17, the astronauts parked the Lunar Rover in such a location that the camera, controlled remotely from Houston, could record and broadcast the launch to Earth.

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Granvil A. Pennington in the mission operations control room watches through the television monitor the liftoff of the Apollo 15 lunar module "Falcon"

Considering how huge and heavy the original rocket was, It may be strange to think that to take off from the moon it was enough with a small ship where you can barely see some fuel tanks. This is because, unlike the Earth, on the Moon there is no atmosphere that slows you down when you try to reach space, in addition to having one-sixth the gravity of Earth.

Therefore, not you require so much speed to achieve a stable orbit, and in a vacuum the engines are more efficient. All this allows the small and not at all aerodynamic design of the LEM to complete its task without major inconveniences.

Rendezvous and coupling

Once the LEM had taken off, it would have to go to a very specific orbit on a very strict schedule in order to be able to dock with the CSM, where the third astronaut would wait for them. In a series of maneuvers known as Rendezvous (encounter), the two ships could approach each other in a slow orbital dance.

The next step would be to join both ships to transfer all the cargo and crew from the LEM to the CSM, the latter being the only part of the entire rocket with the ability to survive atmospheric reentry. Once all elements have been moved, the LEM would be expelled to collide with the Moon in the near future.

Atmospheric reentry and ditching

The CSM would start its engine for the last time in the side of the Moon completely opposite to the Earth. This boost would give it enough speed to escape the Moon's gravity and head back home on a trip that would take about 3 days.

Throughout the return, multiple checks of the trajectory would be carried out. In order to survive the atmospheric reentry The ship would have to enter the atmosphere through a very narrow “corridor.”

If the angle of entry was too great, they would not be able to brake completely and would be left in an elliptical orbit from which they would not return to Earth in time before supplies ran out; If they have a very small angle, they could suffer accelerations large enough to render the crew unconscious or generate too much heat, thus destroying the ship by exceeding the tolerance of the thermal shield that the command module has.

Once atmospheric re-entry was over, the worst part would have already been completed. Now it would simply be missing deployment of the drag parachutes and the main ones, which would let them fall gently into the water to be recovered and conclude the mission.

Francisco Andrés Forero Daza
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