(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.
Yeah I realized that right after I made that comment. If the gravity is strong enough to hold a gas on the planet, it’ll definitely have a prominent effect on something denser like a solid.
That’s what i was thinking - the dense atmosphere might even allow for platforms which are permanently suspended in the air like an inverse submarine, offsetting a large amount of needed fuel for a space launch
yeah there’s also antimatter drives which give an even greater effective exhaust velocity (which is the speed of light). the highest possible achievable.
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, and we are behind our current level of possibly when it comes to rocket science, due to * waves at everything else *
Build a large enough magnetic rail launcher and you could save shit tons of fuel. Get a ship doing 2000 mph before it leaves the ground and needs its rockets and you’ll have a pretty good head start.
I thought scramjets were supposed to be really fuel-efficient? Just launch them with your gauss cannon idea so that they don’t need much fuel to get up to speed.
Maybe you’re right about the weight though. I’m not an engineer.
They’re efficient for what they are but think of it more like a gas pickup truck getting 30 mpg would be considered very efficient. But that would be terrible for a compact car.
Also, scram jets only get efficient once they’re going fast enough.
Yeah that’s why you do multi-stage like conventional jet -> ramjet -> scramjet
But again, yeah if it needs to carry a rocket then it might be unfeasible. We could try your gauss cannon idea, that sounds fun. Like a maglev train, but shaped like those rides from roller coaster tycoon where you could launch people to their deaths. Except instead of crashing, the rocket kicks on mid-flight. It could work.
So you want a giant ass jet that can carry millions of pounds to carry a giant ass ram jet, scram jet, and rocket ship with boosters and rocket fuel?
Also, the thrust required from that jet is completely impossible. Let alone how wildly inificient all that would be. Every step of the way. Like, this is truly a God awful horrible idea. Like, the worst. Also, it’s not good.
(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.
I’m stealing this
Nice. It’s from XKCD if you want the source.
those are the best!
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.
Yeah I realized that right after I made that comment. If the gravity is strong enough to hold a gas on the planet, it’ll definitely have a prominent effect on something denser like a solid.
That’s what i was thinking - the dense atmosphere might even allow for platforms which are permanently suspended in the air like an inverse submarine, offsetting a large amount of needed fuel for a space launch
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)
yeah there’s also antimatter drives which give an even greater effective exhaust velocity (which is the speed of light). the highest possible achievable.
none have been built, so far
If it’s tidally locked, no spin assist.
Likely tidally locked
Missed that part but that doesn’t preclude what I was saying, just requires “more” of it
Or ask Randall Munroe How many model rocket engines would it take to launch a real rocket into space?
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, and we are behind our current level of possibly when it comes to rocket science, due to * waves at everything else *
Build a large enough magnetic rail launcher and you could save shit tons of fuel. Get a ship doing 2000 mph before it leaves the ground and needs its rockets and you’ll have a pretty good head start.
Could even take a scramjet to the upper layers of the atmosphere before kicking in the chemical propulsion
I mean, that’s kinda still just adding on weight and another “stage” to the rocket. A scram jet hauling a rocket ship will use tons of fuel.
I thought scramjets were supposed to be really fuel-efficient? Just launch them with your gauss cannon idea so that they don’t need much fuel to get up to speed.
Maybe you’re right about the weight though. I’m not an engineer.
They’re efficient for what they are but think of it more like a gas pickup truck getting 30 mpg would be considered very efficient. But that would be terrible for a compact car.
Also, scram jets only get efficient once they’re going fast enough.
Yeah that’s why you do multi-stage like conventional jet -> ramjet -> scramjet
But again, yeah if it needs to carry a rocket then it might be unfeasible. We could try your gauss cannon idea, that sounds fun. Like a maglev train, but shaped like those rides from roller coaster tycoon where you could launch people to their deaths. Except instead of crashing, the rocket kicks on mid-flight. It could work.
So you want a giant ass jet that can carry millions of pounds to carry a giant ass ram jet, scram jet, and rocket ship with boosters and rocket fuel?
Also, the thrust required from that jet is completely impossible. Let alone how wildly inificient all that would be. Every step of the way. Like, this is truly a God awful horrible idea. Like, the worst. Also, it’s not good.
I already agreed that it’s unfeasible, I don’t know why you’re getting so upset. I literally said your gauss cannon idea is better, chill out.
Wouldn’t that be a non starter for life? One side would be perpetually baked and the other would be frozen.
I guess there could be a planetary Goldilocks Zone in the dusk area
I figured that area would be full of extremely violent megastorms due to the heat differential.
Oh interesting that is a good point