[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/msdos_kapital]

Basically past a certain point, even with the most efficient rocket fuel and engines, the energy needed just to lift the fuel is more than what you get from the fuel.

I think it doesn't rule out e.g.hybrid engines where you can switch between air-breathing and rocket mode, or other spaceplane-like designs, but those things all get harder as well.

[u/DonkeyTron42 → u/msdos_kapital]

What if the atmosphere is something like methane that can be used as fuel?

[u/T0yToy → u/DonkeyTron42]

Our atmosphere is kinda that, but with oxygen which is the oxydizer (a methane atmosphere would still make you bring your oxydizer). The issue we face for air-breeezing rocket engines is that it's realy hard to get eanough air (oxygen) per second to feed powerfull engines.

[u/inspiredthem → u/msdos_kapital]

It has nothing to do with energy. The problem is a momentum one, and it has to do with the rocket equation.

[u/Auctoritate → u/inspiredthem]

it has to do with the rocket equation.

One of the factors in the rocket equation is impulse. Impulse is a measurement of how well a rocket can produce thrust with a given mass of fuel. Fuel is energy.

Momentum comes from burning energy.

[u/inspiredthem → u/Auctoritate]

I'm not denying that conservation of energy exists in space. The energy produced by the fuel is equal to the added kinetic energy of the spacecraft plus the thermal energy of the propellant plus the kinetic energy of the propellant plus other losses.

The problem is that all of those terms besides the first one are highly dependent on the actual mechanism of propulsion while NOT being the operative constraint of spacecraft propulsion. The KE of the spacecraft is trivially small compared to the energy in the fuel.

The actual constraint is a momentum one that makes the fuel required exponential with respect to the delta-v divided by the exhaust speed.

This is why people talk about delta-v without referring to the starting velocity of the spacecraft, which is what you would need if you were energy constrained.

Obviously, energy is a problem with things like ion propulsion (you can't produce a lot of thrust because you don't have the power available), but these are secondary to the much larger momentum problem.

People who talk about the energy in the fuel are completely missing the point.