In the last three days I’ve made a series of posts detailing in a general sense how a space mission can go from the Earth to the Moon and back.  On Monday I discussed how to get into orbit and how orbits work generally.  On Tuesday I discussed how to go from an Earth orbit to a Moon orbit, and how to go from orbit to landing on the surface.  And on Wednesday I discussed the return journey from the Moon to Earth and how atmospheric drag can be used to help land on Earth.

Today I’d like to touch on the things I didn’t mention, the things NASA spent a lot of time and money to achieve because they were crucial to mission success.  In particular, NASA spent a lot of time and money figuring out how they could get the greatest amount of mass to the moon using the least fuel and the smallest rockets they could.  Rockets and fuel are big, expensive, and difficult to handle so the less of them you have to use the better.

This weight-saving starts in the first ascent when the spaceship is getting into orbit.  The rocket that launched from Kennedy space center was 363 feet tall and looked like THIS while the orbiting modules that went to the moon was about 37 feet tall and looked like THIS.  Where did all the rockets go?  Well the Saturn V rocket itself was big and heavy, and once all its fuel was expended it was detached from the orbiting modules and fell back to earth, allowing the modules to get into orbit on their own.  This in turn made getting to the moon cheaper and more fuel efficient because getting those little modules to the moon costs way less fuel than getting a giant Saturn V PLUS those modules to the moon.  This idea of saving weight by detaching from expended rockets was used all over the Apollo and Soviet programs, and will be discussed again shortly.

Next, once the modules got into orbit around the moon, we can save weight again by having only 1 module descend to the lunar surface while the other remains in orbit.  This significantly reduces the amount of weight we need to get on and off the Moon, and that in turn reduces the fuel usage.  Finally, once on the Moon the Apollo module would detach from some of its rockets yet again, leaving them on the Moon and sending only a small part of the lunar lander back to orbit, similar to how booster rockets were jettisoned during Earth ascent.

In all these above cases, fuel can be saved by simply taking less mass from one place to the other. Detaching from the rockets to take less mass from Earth orbit to Moon orbit, detaching the lunar module to take less mass from Moon orbit to Moon Landing, and then detaching from some lunar module rockets to take less mass from Moon Landing back to Moon orbit. All of these save the weight you have to move and thus save fuel, and one of the biggest difficulties in going into space is you fuel usage so this is a big help. Originally NASA didn’t want to detach a lunar module to detach from the command module for lunar landing, they wanted to land the entire module on the moon. This was because detachment and landing would have to be followed by an in-orbit rendezvous to get the astronauts back together for the return-to-earth part of the mission, and they didn’t know if in-space rendezvous were feasible. But the fuel-savings from this method were obvious so several missions were launched to test our ability to perform rendezvous, and once successful the lunar-module version of the mission was given the go-ahead.

The last trick is something I’d like to make clear about the physics of getting into and out of an orbit.  When I watched the Giant-Bomb let’s play of Kerbal Space Program, one of the commenters posed the question: “It’s easier to get down from orbit than back into orbit, it must be easier because you have gravity helping you, right?”. This is in fact a misunderstanding, to get from in orbit around the body to being stationary on a body requires the same amount of force as to do the opposite. You can get down from orbit more cheaply if all you want to do is crash, in that case you can simply shrink your orbit and crash into the body at a few hundred meters per second, saving you a lot on fuel (this is called lithobraking and was used to land the NASA rovers Spirit and Opportunity, although to protect the robots their fall was cushioned by inflatable airbags). So it will always take the same amount of energy to get from the ground into orbit as it takes to get from orbit to the ground, however importantly this does not take into account the atmosphere of a planet. The atmosphere of a planet creates drag which will slow down down any craft moving through it, and we can use that to our advantage when we try to land on Earth by letting the atmosphere slow our descent instead of needing to use rockets to slow ourselves like we did on the Moon. This is the final big fuel-saving for our trip and is why the Apollo capsules landed without their rockets, because they didn’t need those rockets to slow themselves and it would only make descent harder as they’d need a bigger parachute to slow themselves upon final descent to the ground.

All in all, saving fuel and weight is of primary importance to any space mission, and many of the techniques we take for granted had to be calculated and figured out by NASA before they became standard. Everything the Apollo rockets did had hundreds of pages on data and savings behind them, even if they aren’t immediately obvious to us, but they were all necessary to get to the moon.