How Apollo flew to the Moon

by Prasad Ganti

We were inundated with programs on Apollo during July, marking the 50th anniversary of Apollo 11’s landing on the moon. Plenty of information came to the fore. All filled with nostalgia. In addition to all the TV programs, I came across a book titled “How Apollo flew to the moon” by David Woods. Lot of details are explained in a very lucid way. Here are some of my learnings distilled from these sources.

The space program essentially evolved once President Kennedy made the declaration of landing a man on the moon by the end of the 1960s. Starting with the Mercury capsule, which could just seat one astronaut to go into space and return safely back to the Earth. The Gemini capsule could seat 2 astronauts to orbit the Earth. It tested the space walks and the docking capabilities. Docking of two spacecrafts in the space was very crucial for the overall mission of sending human beings to the moon. With each mission, a new capability was tested.

Next stage was the Apollo spacecraft, which could seat 3 astronauts, and consisted of the command module, service module and the lunar module. Command module is the cone shaped module was would travel to the moon and come back to the Earth to make a splash down into the Ocean. Service module contained the consumable supplies and would be mated with the command module almost all the time. It would be jettisoned just before the re-entry into the Earth’s atmosphere. The lunar module is separated from the command and service modules for landing on the moon. While the command and service modules are still orbiting the moon. Then taking off from the moon and docking with the command and service module to return to the Earth. The lunar module is immediately jettisoned after the docking and the transfer of men and material into the command module.

Saturn V is the most powerful rocket ever built. To carry the Apollo spacecraft into space and hurl towards the moon. Rockets are usually multi stage, with each stage doing some of the pushing towards the space. First stage of Saturn V, namely S-1C built by Boeing, had five F-1 engines mounted at the bottom. It contained two fuel tanks – one for kerosene called RP-1 which is like jet fuel, and one for LOX (liquid oxygen). It ran for 2.5 mins before being thrown into the Atlantic. The second stage S-II carried the cryogenic (super cooled) propellants, built by North American Aviation, had five J-2 engines. The third stage S-IVB had one J-2 engine.

Rocketdyne built the most powerful F-1 engine powered by liquid fuels. Lot of trial and error over five years led to dampening of the instability of the F-1 engines. J-2 is a smaller engine with less power and used liquid hydrogen to improve its efficiency. The Command module had its own engine (SPS – Service Propulsion System), while the Lunar module had its own ascent and descent engines. The SPS engine used hypergolic fuels like hydrazine as fuel and nitrogen tetroxide as oxidizer, which would combust as soon as they come in contact. No sparking is required.

The Apollo spacecraft sat on top of the multi-stage Saturn V rocket. The whole contraption is a complex assembly of different vehicles, each having a specific role and jettisoned after its role is over. It is like carrying a motorbike, a boat, a rugged vehicle inside or attached to a truck. Throwing each vehicle away after a certain part of the journey. Picture below shows the fully mated Saturn V rocket.

AGC (Apollo Guidance Computer) played a key role in taking the spacecraft towards the moon and bringing it safely back. Ever since the Gemini program involved docking, there was a need for a computer which could take data in real time from all the sensors and control the outputs required to manipulate the spacecraft. Like switching on the engines to change direction or speed or the trajectory. The computer is very primitive compared to the modern smart phone. But it was very rugged to withstand the rigors of space travel.

There was a design flaw lurking in Mercury and Gemini, which caused Apollo 1 to be blown up on the launch pad. All the three astronauts died. Pressurized by oxygen alone, a short circuit initiated spark inside the spacecraft led to a massive explosion. A mixture of oxygen and nitrogen (like it is found in nature) is better than pure oxygen. That is what prevents everything from burning up on the Earth. Supplying two gases from two different cylinders would increase the complexity. The resultant shortcut taken with a single cylinder, blew up Apollo 1. Apollo 4 (there was no 2 and 3) tested the Command module to go into space and re-enter the Earth’s atmosphere. Apollo 5 tested the Lunar module. Apollo 10 left the Earth’s neighborhood and ventured towards the moon for the first time. Apollo 11 landed on the moon.

The IMU (Inertial Measurement Unit) got the data from the various sensors to determine the position of the spacecraft in space. The INS (Inertial Navigation System) used the data from IMU to guide the spacecraft towards its destination. The trajectory used to go to moon and return back is shown below. The free-return trajectory works by using the gravitational effects of the moon like a slingshot to loop around towards the Earth. The landing site was along the equator of the moon. Landing anywhere else on the moon requires more fuel. Any change in speed or direction in space requires burning of fuel.

courtesy - wikipedia

Now we realize what kind of uncertainty the crew and the engineers were dealing with. The uncertainty of landing on the moon, the uncertainty of the lunar module not taking off from the moon forcing the command module (along with the third astronaut) to return back to the Earth. The fact that everything worked as planned and the astronauts came back home safely was a great achievement. It was indeed a giant leap for mankind.

This entry was posted in September 2019, Sidereal Times and tagged . Bookmark the permalink.

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