(Not a rocket scientist or mathematician, but I spent 100s of hours playing KSP RP-1)
Just doing some estimates using data from the wikipedia page:
The dV (delta-V) needed to get into low Earth orbit is around 9.4km/s.
The dV for K2-18b might be around 19km/s, more than double that of Earth’s.
It’s practically impossible I think, you would need such a massive launch vehicle. For double the dV, you would need exponentially more fuel assuming current rocketry tech (fuel+oxidizer tanks and engines). There wouldn’t be any single-stage or two-stage rockets that could do this. With a 3 or 4 stage rocket maybe? But you would be sending nearly 100% fuel off the launchpad with virtually zero payload.
Check out the “tyranny of the rocket equation”. The more propellant you need to lift heavier rockets, the more propellant you need to lift that extra propellant and so on and so on.
I tried to factor in:
spoiler
Atmospheric drag - K2-18b’s atmosphere is quite dense with a huge radius:
The density of K2-18b is about 2.67+0.52/−0.47 g/cm3—intermediate between that of Earth and Neptune—implying that the planet has a hydrogen-rich envelope. […] Atmosphere makes up at most 6.2% of the planet’s mass
Since the atmosphere is so thick and takes up a lot of mass, I’ve picked 500km as the low orbit altitude (comparing to Earth’s ~100km Karman line, it makes you appreciate how thin our atmosphere is ).
Rotational assist - I’m assuming it’s tidally locked since it orbits so closely to its star (33 day years), and so you wouldn’t get the assist from rotation like you do on Earth:
The planet is most likely tidally locked to the star, although considering its orbital eccentricity, a spin-orbit resonance like Mercury is also possible.
With a denser atmosphere, wouldn’t that mean that you could get more lift from a traditional aerofoil than on earth? And if so, wouldn’t that technically make it easier to start from a high enough altitude that at least some of the gravity is mitigated?
Let’s say you do the same on Earth. If you fly to the top of the atmosphere you are 100 km above the ground. That’s a 1/60 of the distance to the center of the Earth. You don’t have to fight air resistance but gravity is almost the same, if I’m not wrong, less than 1% of difference.
You don’t have to launch from the ground, there are many things that can be done to allow them to reach orbit. It’ll be an enormously bigger undertaking but the physics doesn’t make it impossible.
No reason to think of it in terms of our current situation either, what we are behind our current science when it comes to rocket science, due to * waves at everything else *
Pretty late response, but if the atmosphere is that dense and hydrogen rich, you could reduce the start amount of fuel by a lot and capture it directly during flight?
Needs some elaborated technology and low angle flight vector but could work, does it?
(Not a rocket scientist or mathematician, but I spent 100s of hours playing KSP RP-1)
Just doing some estimates using data from the wikipedia page:
The dV (delta-V) needed to get into low Earth orbit is around 9.4km/s.
The dV for K2-18b might be around 19km/s, more than double that of Earth’s.
It’s practically impossible I think, you would need such a massive launch vehicle. For double the dV, you would need exponentially more fuel assuming current rocketry tech (fuel+oxidizer tanks and engines). There wouldn’t be any single-stage or two-stage rockets that could do this. With a 3 or 4 stage rocket maybe? But you would be sending nearly 100% fuel off the launchpad with virtually zero payload.
Check out the “tyranny of the rocket equation”. The more propellant you need to lift heavier rockets, the more propellant you need to lift that extra propellant and so on and so on.
I tried to factor in:
spoiler
Since the atmosphere is so thick and takes up a lot of mass, I’ve picked 500km as the low orbit altitude (comparing to Earth’s ~100km Karman line, it makes you appreciate how thin our atmosphere is ).
Rotational assist - I’m assuming it’s tidally locked since it orbits so closely to its star (33 day years), and so you wouldn’t get the assist from rotation like you do on Earth:
Kerbal Space Program is such an amazing game that secretly teaches you physics.
those are the best!
I’m stealing this
Nice. It’s from XKCD if you want the source.
What about something like nuclear pulse propulsion, or some kind of massive spin launch?
Nuclear propulsion, like Project Orion, would probably make it more likely they’d manage to get out of orbit. No idea on the math here, tho
https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)
If it’s tidally locked, no spin assist.
Missed that part but that doesn’t preclude what I was saying, just requires “more” of it
Likely tidally locked
With a denser atmosphere, wouldn’t that mean that you could get more lift from a traditional aerofoil than on earth? And if so, wouldn’t that technically make it easier to start from a high enough altitude that at least some of the gravity is mitigated?
Let’s say you do the same on Earth. If you fly to the top of the atmosphere you are 100 km above the ground. That’s a 1/60 of the distance to the center of the Earth. You don’t have to fight air resistance but gravity is almost the same, if I’m not wrong, less than 1% of difference.
You don’t have to launch from the ground, there are many things that can be done to allow them to reach orbit. It’ll be an enormously bigger undertaking but the physics doesn’t make it impossible. No reason to think of it in terms of our current situation either, what we are behind our current science when it comes to rocket science, due to * waves at everything else *
Pretty late response, but if the atmosphere is that dense and hydrogen rich, you could reduce the start amount of fuel by a lot and capture it directly during flight? Needs some elaborated technology and low angle flight vector but could work, does it?