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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The only thing that would really help is finding a way to use something more energy dense as rocket fuel. That way we would need less fuel to lift the rest of our fuel.
Only by a little bit. Rocket engines are very efficient currently. There’s only so much you can do in terms of chemistry and material science. The best way to make them to get “more miles per gallon” is to make the whole thing lighter.
The “efficiency” of rocket engine is measured in something called “specific impulse”, which is largely controlled by the chemistry. So there’s not much we can do there. Rocket engines are already extremely efficient in terms of converting chemical energy to motion. Much more so than internal combustion engines are. There’s just way less wasted energy on superfluous motion and friction as there is with something with moving components.
This means that the only real way to improve the overall rocket efficiency is to decrease the amount of dead-mass you have to haul around in addition to your fuel. Better materials, better engineering methods, and better structural design can do this.
Check out projectrho.com , also known as atomicrockets . A massive site about rockets, engines, and everything you need to know to write decent science fiction set in space.
Start by looking up “tyranny of the rocket equation”
Chemical rockets? a little but, but not much. We’re pretty close to the best we’re gonna do there. There are plenty of other spacecraft propulsion methods, but currently existing and possible but not yet tested, that are *wayyy* more efficient than chemical rockets, but they’re also low-thrust, which means they’re only useful when already in space and not for the actual launch.
As other commenters have said for a normal rocket engine the efficiency is measured in “specific impulse”. I found [this link](https://thephysicsofspacex.files.wordpress.com/2016/07/isp-upper-limits.pdf) which gives the theoretical upper limit of specific impulse for some common rocket propellants.
Since you asked about the Saturn V rocket specifically, it used kerosine/oxygen for the first stage and hydrogen/oxygen for the other stages. For kerosine/oxygen the theoretical maximum would be around 470s whereas the F-1 engines on the Saturn V only achieved 260s at sea-level, so only 55% efficiency. The second and third stage used the hydrogen/oxygen J-2 engine which has a specific impulse of 420s vacuum, which is already about 80% efficiency compared to the theoretical maximum of about 530s.
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.
We measure the efficiency of rockets in Specific Impulse which is how much momentum change(impulse) you get from a kilogram of fuel, rocket efficiency is all about mass not volume
Why do we care about momentum change instead of energy in the fuel? Because rockets are momentum based engines, not energy based like your car
We have a variety of different fuels that have various efficiencies. Hydrogen and oxygen are generally the most efficient(up to 450) but since hydrogen isn’t very dense you end up with massive fuel tanks like on the Saturn V. Other fuels like RP-1(basically kerosene) and methane are denser and easier to work with but aren’t quite as efficient
There are some cheats that can get you wildly more efficient rockets.
Nuclear thermal rockets run a propellant (generally hydrogen) through a hot nuclear core which heats it up and shoots it out the back. Now the rocket is only carrying the propellant instead of a propellant and oxidizer so the efficiency can be a lot higher but the thrust-weight ratio is generally worse.
Ion thrusters use electricity to accelerate gas up to stupid high speeds and shoot it off into space. Since the gas particles are moving super fast they have a fair amount of momentum so you get really good efficiency. Unfortunately we’ve yet to make one big enough to even lift more than about 10kg off the surface of the Earth so its only good once you’re up there and just making tiny adjustments, they also need a stupid amount of power (3.75MW for 88 Newtons of thrust)
Basically you can make more efficient spacecraft propulsion, but most things that provide the stupid amount of thrust required to get out of this gravity well end up being only moderately efficient but we are equipping satellites with ion thrusters that are 10x as efficient to extend their life with their limited fuel supply
The #1 way to increase efficiency with space flight would be through increased specialization – if you have one craft that only takes you up into orbit or back down from orbit, and then a second craft constructed in orbit that never enters atmosphere, the first and especially the second craft can be built a bajillion miles more efficiently because if it’s always in a vacuum and always in microgravity it doesn’t have to deal with gravitational or atmospheric forces working against it structurally, the propulsion system can be purpose built to operate in vacuum & not have to be able to work in atmosphere (it’s not a totally hard rule, but *generally* the better a system functions in atmo, the less efficient it is in space, and vice versa) it doesn’t have to deal with packaging things to fit into the form of a rocket, shielding for reentry, aerodynamic elements etc.
which means significantly less dry weight and total freedom in what kind of propulsion system is used – you could have a mostly unshielded thermonuclear rocket/reactor mounted on 200 meters of carbon fiber girder to separate it from the crew/pax compartments, stuff like that
with something like that, the fuel wouldn’t be the limiting factor for reaching anything in our solar system in a timely manner, it’d be how much heat is generated and how you got rid of it
the doors open up really wide when the spaceship never has to take off or land – as long as they still do, it’s a big bottleneck for everything design-wise on both ends – the model of starship with the detatching atmo-launch booster base and orbital refueling capabilities is the first step in that direction, the next step is having a launched vessel be able to stay up there indefinitely, and then after that, having it be assembled up there to begin with
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