The second and third stage used liquid hydrogen for fuel, but the blastoff started with kerosene. What was the advantage, or why would hydrogen have been impractical?
I don’t have an answer for the question, but I do know that the Saturn V was built upon previous rockets. The first stage (which used kerosene) was already completely designed and functional before the second and third stages were completely developed. I’m guessing that they figured out how to get better performance out of hydrogen (more energy and less weight compared to kerosene) but they didn’t go back and redesign the first stage at that point since it would have taken too much time to do it over.
I’ve heard it said that one of the reasons the Saturn rockets worked so well and their soviet N1 counterparts didn’t work so well was because the Saturns used a lot of testing and re-used a lot of components, where the N1 rockets didn’t have so much testing and reliability behind them (and therefore tended to explode at a much more alarming frequency).
I found this:
Hydrogen contains the most units of Energy per unit of Mass, while JP Fuel, Kerosene Diesel, and other heavy species hydrocarbons have high units of energy per units of Volume.
Using Kerosene in the first stage would make the fuel tanks small enough to fit.
I would like to add the liquid hydrogen engines on the second and third stages were highly advanced and much more efficient then the first stage kerosene. This is why the Saturn 5 which had considerably less thrust than the Russian N-1 booster which did not use LH fuel could lift a heavier payload to earth orbit or to the moon.
Plus, kerosene is stable at room temperature. Hydrogen tends to gasify and has to be bled off lest it overpressurize the tank, it is highly flammable, and you spend a lot of energy trying to turn it into a liquid slurry suitable for storage. Liquid kerosene you just pump out of the ground and distill to a specified level of content.
In terms of the quality as a fuel, diatomic hydrogen has a specific heat of 1.75 kJ/kg @ 20.4K, RP-1 (kerosene) has 0.45 kJ/kg @ 298K (or 77°C), and hydrazine us at 0.736 kJ/kg @ 293K. The specific gravity of these fuels is 0.071, 0.58, and 1.005, respectively, which makes it clearly why hydrazine is preferred in applications where space is at a premium (like liquid ballistic missiles) and kerosene is desirable when dealing with a large booster that needs a practical degree of fuel density. Another fuel that is not currently seeing much use but has been discussed for future operations is methane (CH[sub]4[/sub]), which has a specific heat of 0.835 kJ/kg and specific gravity of 0.424, making it comparable in terms of performance versus kerosene.
So S-IC used kerosene because there was just such a large amount of energy required that a less dense fuel would dictate substantially more structure (with a concomitant reduction in payload fraction) and make it difficult to fuel. There are also some issues with the (slightly) greater effectiveness of a high molecular weight propellant while burning in the lower atmosphere; since you not only push against the rocket chamber and nozzle but also against the ambient pressure of the atmosphere, a higher momentum per particle propellant gives a little bit better actual performance (as measured by specific impulse). Once you get well above ground level and the ambient pressure drops, this becomes less of an issue and a low molecular weight propellant is far more effective, hence why S-II (stage II) and the S-IVB (orbital insertion and lunar injection stage) used LOX and LH2. The drop in ambient pressure also makes the rocket more effective to the tune of about 10% to 30%, depending on propellant selection and nozzle configuration.
While the monstrous F-1 was certainly under development about a decade before the J-2 used in the S-II and the S-IVB, I doubt this was the driving factor. An offshoot of F-1 development was the hydrogen-powered M-1 motor, which developed a whomping 1.5Mt of thrust. The M-1 was originally intended as a much larger second stage for the Nova-class evolution of the Saturn family, but some concepts were also considering it for first stage, albeit with a significantly downrated specific impulse.
This just isn’t true. The NK-33/-43 motors were noted for high reliability and extremely good performace, and derivatives of this engine are being used today on the Zenits used by SeaLaunch and the American Atlas V. However, their concept with the N1 rocket was to gang 30 of these engines together in a ring for the first stage, where they experienced vibrational resonance that inhibited fuel flow and overstressed the structure.