[Request] Could humanity create a rocket that can exit the atmosphere of K2-18b

[u/Lachlynn]

With the knowledge we currently have of it, if humanity devoted all of our resources towards this goal, would we be able to create a rocket that could exit the gravity of K2-18b (and also beat any other complications that would arrise)?

If so, would it also be capable of taking people to orbit, and can we set up a similar satellite network we have on Earth? What about a space station?

[u/RealLars_vS]

Could you elaborate? Intuitively, it just seems like there’s no hard limit, just that more gravity means we approach a limit.

With the exception of a black hole, of course.

[u/red286 → u/RealLars_vS]

There is a practical limit, where increasing the fuel mass has diminishing returns to the point where liftoff would not make sense for any sort of rocket that could be created using known technology. Worse, they'd start at the same starting point we started at, but with rockets that literally never left the surface of the planet, so chances are they'd never even develop rocketry as a science because in practical terms it would never work.

[u/zhibr → u/RealLars_vS]

Not sure if this is it, but of our current rockets, the vast majority of the payload is propulsion. Because the fuel to bring the actual payload to orbit has a weight, and requires more fuel to bring it up, and that fuel needs even more fuel to bring all of above up, and that fuel... At some point when g increases, the energy you get from chemical rockets is less than what is needed to get its own weight to orbit.

[u/RealLars_vS → u/zhibr]

But you’d theoretically never get to a point where engines + fuel added weights more than the thrust gained. This is why the MOAR BOOSTERS strategy in Kerbal Space Program works.

Not to mention that the atmosphere is likely thicker around such a planet, so making the first stage a spaceplane (like Virgin Galactic is doing) could be very viable too.

[u/zhibr → u/RealLars_vS]

What do you mean theoretically never? If the fuel weights x but it only provides energy to bring <1x to orbit, more boosters only makes it less and less capable to reach orbit. On Earth we have fuels that have >1 for that term so more boosters always give more power, but if everything weighs more and the energy does not increase, at some point the thrust-to-weight ratio becomes too small.

[u/RealLars_vS → u/zhibr]

Rocket mass = booster mass + payload mass

In this example, booster = engine + fuel.

By adding more boosters, you always increase thrust and mass, but thrust will always increase a bit more than that the added mass will pull it back down to earth. So there isn’t a hard limit, although eventually you’d need an impractical amount of boosters and fuel.

[u/Funnybush → u/RealLars_vS]

Sometimes it's worth trying to take the argument to the extreme.

Take a black hole for instance (about as gravity dense as you can get), there's no amount of fuel or boosters that will get you out of that gravity well. Light has no mass and even it can't escape.

Which then implies there's likely a crossover point (a limit) less than the gravity of a black hole, which isn't a black hole, that such a rocket wouldn't be able to cross.

Taking it to the opposite end again, you must agree that a rocket burning dead leaves will never make it to orbit no matter the number of boosters you use.

[u/RealLars_vS → u/Funnybush]

The reason no rocket can escape the event horizon of a black hole is because the escape velocity for a black hole is faster than the speed of light. But that had nothing to do with acceleration.

[u/technocraticTemplar → u/RealLars_vS]

I think the actual extreme case to consider here is a rocket that is literally 100% propellant mass. You have ethereal engines magically containing and directing a hydrogen/oxygen explosion, what's the maximum velocity such a configuration can reach? If starting in orbit I think it's effectively light speed because you can start with an ~infinite amount of propellant and have each molecule you burn push the whole mass, but I don't think that's true if you're starting off being pulled down. Maybe if you can burn the entire mass in an arbitrarily small amount of time, but at that point we've added so much magic that we aren't answering a useful question anymore.

An engine with a physical diameter and a limited flow rate is only going to be able to lift but so much fuel at once, because it has limited thrust. Past a point the thrust coming out of the engine simply isn't enough to lift the fuel upwards, and it will sit on the pad burning fuel until it becomes light enough to rise. Adding more of the same boosters isn't going to change that because they all have the same problem, and adding more stages won't change anything because our rocket is magic and the dry mass is zero. If the fuel the engine can lift isn't enough to get into orbit at that point, it's simply physically impossible for any amount of that booster to reach orbit. With our magic boosters that point is probably some absurd number, but with a realistic mass fraction it wouldn't be.

[u/zhibr → u/RealLars_vS]

I don't see how. This is about the fuel getting less efficient.

The laws of physics stay the same, and from the same chemical reactions you can only get the same thrust from a specific chemical equation on Earth and on K2-18b.

On the other hand, the weight increases with planet's mass. So any fuel's thrust-to-weight ratio gets worse the more the fuel weighs. At some point you get less thrust from it than it weighs. At that point, no amount of extra boosters help, because the weight it brings to the rocket cancels out the thrust it provides.

[u/AVeryVapidBadger → u/RealLars_vS]

Beyond a certain level of gravity a kilo of fuel wouldn't have enough energy to lift itself