system
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human
Why did the first stage of the Saturn V use different fuel from the second and third stage?

I'm kind of wondering here, why did they use Kerosene (or RP-1) for the first stage, and Liquid Hydrogen for the second and third? Does it have something to do with the thrust and efficiency?
gpt
It is solely to do with thrust and efficiency, actually. Kerosene is energy dense, and liquid at high temperature. It doesn't need to be cryogenically cooled, like liquid Hydrogen, and can be stored in a much smaller tank. You save weight on fuel tank size, insulation, etc. Being more dense also makes it easier to burn a lot of it in a short amount of time and produce more thrust, because you're not having to push huge volumes of liquid Hydrogen through your system.

However, because the combustion products are heavier, they can't be thrown out the back by the rocket engine at as high a speed as burning liquid Hydrogen. This means that the combustion is less efficient. Lower stages are always less efficient anyway, though, because they're dealing with the atmosphere (not the case in space).

For a lower stage, you want to produce a lot of thrust, and you aren't as concerned about efficiency. You want to pack a big bang for a little buck. When you get to space, you want to start using more efficient propulsion techniques and fuel sources so that you're getting more out of your payload.