I was watching a video of a Saturn 5 Rocket launch and I was amazed at how much fuel was used and how massive the explosion at the beginning was (and how massive the continuous fuel burn was).
But it got me thinking can we, in the future, develop rockets which can lift more payload per gallon of fuel or are all of our rockets equally efficient in terms of the rocketry version of “Miles per Gallon” because of some law of physics which we already mastered?
And I know there are alternatives like Space Elevators, but I’m specifically curious about rockets.
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A rocket engine must have a fuel – a source of energy – and a propellant. The propellant is what gets thrown out the back to provide thrust.
In chemical rockets, the fuel *is* the propellant. So, for a given fuel, there is a fixed amount of propellant and energy available.
Given propellant and energy, there is a fundamental limit to how much dV (basically movement) your rocket can get. It’s actually pretty simple, and is based on conservation of energy and momentum. Our rockets get pretty close to this limit already, although there are some caveats.
For instance, rockets in an atmosphere have to be designed to operate at a specific pressure. As the rocket rises, the pressure changes, so a rocket designed to work at ground level will be significantly less efficient up near space.
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If we use a propellant and then a separate fuel, we can get much more “efficient” rockets. The downside is your rocket can either be propellant-efficient or energy-efficient. The faster the propellant flies out the back, the more energy you use with diminishing returns on dV. However, you get more dV from your propellant when it’s faster.
This is again fundamentally tied to physics. The same conservations as before.
So, to get a more propellant-efficient rocket, we need a better source of energy than the chemicals we use today. Finding better chemicals is an option, but all chemical energy has its limits. Nuclear power is *so* much better for this. Nuclear rockets are very appealing for this reason. An even denser energy source, like antimatter, would be even better but is way beyond anything we could produce right now.
One neat trick we use is getting our energy from outside of the rocket. Instead of hauling around a nuclear reactor, we can use solar panels and harvest energy from the sun. This allows a very propellant-efficient rocket which doesn’t need to carry fuel at all.
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