Ok - serious rocket science question. Saturn V, and other rockets to my understanding, start with a first stage of kerosene, and switch to liquid hydrogen in the second stage. LO is always the oxidizer. From the wikipedia article on “Tripopellant rockets” which I’ve quotes from below, kerosene has higher energy density, and has more “thrust-to-mass”, but hydrogen has more “specific impulse”, which means it has more “impulse” (change in momentum) per unit of propellant. The higher the specific impulse, the less propellant is needed to gain a given amount of momentum. So kerosene has more thrust? But Hydrogen causes a greater change in momentum? I would think they would be about the same thing. If your basic force of thrust doesn’t cause a change in momentum -
then how is it different?
So I’m fuzzy on a few things.
How can one fuel have “more thrust”, and the other “increase momentum” more. I thought perhaps it was because Liquid oxygen and liquid hydrogen burn “faster” and send fewer molecules out at a faster rate than kerosene. I do not
know. The other point made in this article is that containing liquid hydrogen
requires a very serious container since it is under a huge amount of pressure.
So, if we made the entire rocket using only liquid hydrogen, the container might
weigh too much - requiring a bigger container, more weight… ad infinitum. But then this makes me wonder, what would happen if we made the rocket stages to use nothing but kerosene - it would take up too much room?
How’s that if it has more “thrust”. Any insights appreciated.
– quote from article
The other kind of tripropellant rocket is one that uses one oxidizer but two fuels, switching between the two in mid-flight. In this way the motor can combine the high thrust-to-mass of a dense fuel like kerosene early in flight with the high specific impulse of a lighter fuel like liquid hydrogen (LH2) later in flight. The result is a single engine providing some of the benefits of staging.
Although liquid hydrogen delivers the largest specific impulse of the plausible rocket fuels, it also requires huge structures to hold it due to its low density. These structures can weigh a lot, offsetting the light weight of the fuel itself to some degree, and also result in higher drag while in the atmosphere. While kerosene has lower specific impulse, its higher density results in smaller structures, which implies less loss to atmospheric drag. In addition, kerosene based engines generally provide higher thrust, which is important for takeoff, reducing gravity drag. So in general terms there is a “sweet spot” in altitude where one type of fuel becomes more practical than the other.