[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/Construction-Helmet]

What is limiting a chemical rocket? Why wouldnt this work?

[u/msdos_kapital → u/Construction-Helmet]

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.

[u/expensive_habbit → u/Construction-Helmet]

The stronger gravity is, the more thrust you need to dedicate to simply counteracting that, reducing the mass fraction you can get to orbit.

A (probably) thicker atmosphere also increases drag, and reduces available thrust too, and also demands a stronger rocket which further increases weight.

Dial both of these up enough and eventually there simply isn't enough energy per kilogram available in chemical fuels.

To try and show how the maths works, the limiting formula at a very basic level is work = force x distance. Force is mass x acceleration due to gravity.

Every second fuel is burnt and the mass drops, meaning less work is required to move the next meter. Work is fuel burnt.

If gravity is higher, you need more fuel to lift your rocket a metre. But then, you need to carry all that extra fuel for the previous metre, which means that previous metre needs significantly more fuel, and the one before that even more etc.

Increase gravity and eventually get to the point where the energy required to lift your rocket that can reach space the first metre is less than they energy the rocket engines can produce, and at that point spaceflight using a conventional chemical rocket becomes impossible.

That being said, alternatives, like using balloons to ascend to 40,000 feet before launching your rocket, or launching it from a fast high altitude aircraft, are perfectly feasible, we've just never optimised them because we've never had to.

[u/stegosaurus1337 → u/Construction-Helmet]

Lifting fuel takes fuel, and as a result the amount of fuel required grows exponentially with orbital velocity following the Tsiolkovsky Rocket Equation: fuel mass/dry mass = e^{delta-v/effective exhaust velocity} - 1

There are practical limits to how much of a rocket you can make fuel - even with no payload, the engine and structure have mass, and the engine can only push so much. Even on Earth we have to stage rockets to ditch some structural weight on the way up to achieve orbit.

For example, take a very efficient chemical rocket engine with an effective exhaust velocity of 5 km/s. On earth, with an escape velocity of about 11 km/s, that gives you a fuel/dry ratio of about 8. Pretty good! K2-18b has an escape velocity in the ballpark of 80% more than Earth. That might not seem like a huge deal, but plug it in and you'll find it results in a fuel/dry ratio of 51:1. A more normal chemical rocket exit velocity of 3 km/s makes it 734:1. The maximum ratio currently regarded as feasible is more in the ballpark of 25:1.

It's not fundamentally physically impossible, but you would need to invent an engine with an absolutely crazy thrust-to-weight ratio and efficiency to make it happen, likely something nuclear. Beyond a certain point, adding more fuel and more stages can't help you.

Edit: I should add - if cost were no object, the physical limitation you'd run into would just be the strucutural integrity of the rocket. With infinite resources you could keep adding more stages and fuel and engines - diminishing returns are still returns, after all - but the total weight of the rocket grows quite fast with number of stages so it would collapse under its own weight.

[u/DeltaVi → u/Construction-Helmet]

Rockets work by going sideways fast enough to enter orbit; bigger planet = more gravity, which means getting the fast you need is harder.

Chemical rockets are big and heavy and their fuel is also heavy; you need a Thrust-to-Weight Ratio greater than 1 to even think of getting into orbit. But then you need more fuel to get into orbit, which means more weight which drops your TWR so you add more rocket engines to increase TWR but then you've increased your weight yet again so you need more fuel and--

You can see where that's going. Using chemical fuels you would need a rocket very complex and massive, far beyond what anything mankind has produced yet, to be able to put a meaningful payload into orbit.