How Did We Get To The Moon?

[Admin Cadet: SaintsCadet wrote this excellent article on how we performed the Apollo missions.  I requested him to post his article and he kindly concurred.]

HOW DID WE GET TO THE MOON?  by SaintsCadet

How do we get humans to the Moon? The answer is: by a whole lot of acceleration. How much? First off, to either increase or slow down your speed while in a spacecraft, you have to fire your engines. To get to the Moon and back you do a number of engine firings or “burns” that either increase your speed or decrease your speed. Adding up the total time and amount of propellant used in these burns is what NASA space jockeys call “delta velocity”. The larger the delta velocity, the more cost and more complexity is needed for the mission. Assuming your starting point is Earth’s orbit, the delta velocity needed to get to the Moon and back for the total mission is approx 22,000 miles/hour, a very large number. And this assumes you plan to use the Earth’s atmosphere to slow down your vehicle when you return from the Moon thus reducing your propellant needs. This is what Apollo had to do when we first went to the Moon. There are techniques that lower this amount, but not by much.

Our first unmanned lunar surveyor missions in the 60s used a direct approach to the Moon. They did not enter into orbit around the Moon. They headed directly to the surface from Earth which reduced the delta velocity number by only 200 miles/hour. Not that much of a reduction in the overall scheme of things.

Let’s get into more detail. I am going explain how Apollo went to Moon. First, remember that the scientists and engineers in the 60’s and 70’s had no one to copy or learn from when doing this amazing feat. However, they were competing with the Soviets and they had a healthy pot of money to work with. This allowed them to use a massive single launch vehicle approach in place of using multiple launches.

Of course we needed a launch vehicle to lift the astronauts and their lunar vehicles from the Earth, and we had a very heavy-capable launch vehicle in the Saturn V Rocket. Not only did this vehicle put you into low Earth orbit, but it also had the capability to boost the lunar vehicles toward the Moon. This took a delta velocity of over 7000 miles/hour, which is almost a 1/3 of the total 22,000 miles/hour stated above. The Saturn put the Apollo vehicles into Earth orbit and then provided the initial burn to get the vehicles headed toward the Moon.

The lunar portion of the Apollo mission was made up of three vehicles. The first, called the Service Module, was a smaller propulsion vehicle that would do the engine burns needed when the vehicles got to the Moon and then returned them from the Moon.

The second was the vehicle that would house all the astronauts while not on the Moon and also land them back on Earth in the ocean. This was the Apollo capsule (also known as the crew module or Command Module).  Since it gets very hot when you start your braking upon return to the Earth, it needed a shape and heat shield that made this possible.

The third and final vehicle was the Lunar Module (also known as the Lunar Lander) which was made up of two sections: the first section was the lunar descent stage and the second was the lunar ascent stage.

Okay this explains the energy side of the operation and the vehicles used. What about the path you take? Unfortunately in space you can’t just draw a straight line and press the accelerator. You have to contend with two dominant forces. The first we all know about is gravity. However, this is not constant. The further you are away from Earth, the gravity working on your vehicle gets less. Not a lot of change between the Earth and Moon, but you still need to address it.

The second is centrifugal force. If you ever have been on a merry-go-around or go around a sharp curve you should know what this force is. When you go around in a circle this is the force that tries to pull you back to a straight path out of the circle.

The second force only becomes dominant when you near a gravitation object such as the Earth or Moon and you go into an orbit. Simply put, a stable circular orbit is where the gravity equals the centrifugal force. You don’t fly away and you don’t fall down.

The Saturn rocket put the three lunar vehicles in Earth’s orbit first. To save weight, the three lunar vehicles were not fully connected when launched from Earth. Actually the lander had to rendezvous with the service module/capsule in Earth’s orbit. Trying to launch them connected from Earth would have cost too much weight, and when going to the Moon, weight is king.

The Saturn vehicle pushed these vehicles out of the Earth’s orbit and toward the Moon by putting them in an elliptical orbit with the Moon and Earth at the outer ends. Once you get to the Moon, the service module then fired it’s engines for a decrease in delta velocity of 2235 miles/hour to enter the Moon’s orbit. Without this, the vehicle would naturally return to Earth. This natural return is important because it is what allowed us to safely return the Apollo 13 astronauts after their service module was rendered non-functional by a tank explosion.

While in the Moon’s orbit, the lander then disconnected from the service module/capsule using its own propulsion system and landed on the Moon.

After the lunar excursion was over, a lunar ascent portion of the lander lifted from the surface of the Moon and returned to the service module/capsule. The service module then fired its engines to leave the lunar orbit and re-entered into the elliptical path to the Earth. Only the service module and capsule returned to Earth.

That’s it. I hope this explanation made sense. There are slightly different ways to get to the moon. In my next article I hope to explain one of these different approaches and how it can be used to quickly ramp up a US Moon Program for my vision of a permanent human base on the Moon.

Simplified Apollo Profile (Note: LOR = Lunar Orbit Rendezvous) [Image Credit: usually attributed to John Houbolt/NASA aerospace engineer/lunar orbit rendezvous team lead]:

Apollo Trajectory Simplified

Saturn V Rocket and Lunar Spacecraft (including the Command Module, Service Module, and Lunar Module) [Image Credit: Encyclopedia Britannica, Inc., 2002]:

Apollo Vehicles

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2 thoughts on “How Did We Get To The Moon?

    • SaintsCadet tells me: Good luck. It’s been very difficult to track down. My words: Folks at JSC have had a lot of difficulty even reproducing the Apollo design results in similar studies. One of the issues is we are constrained by several design ground rules that the early NASA folks did not have to abide to (example: protection against solar flare events).

      For example, check out this comparison between the Apollo capsule and Orion capsule I found on NASAspaceflight.com – it shows how much larger and more complex the Orion is due to many requirements unique to Orion, even though Orion was originally designed for the Constellation lunar program:
      http://www.nasaspaceflight.com/2012/07/nasa-esd-key-orion-requirement-lunar-missions/

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