In practice even for a planet with as thin an atmosphere as Earth, getting past the atmosphere is more difficult than actually reaching escape velocity. One of the most common times for a rocket to break up is near Max Q which is where maximum aerodynamic stress occurs. This is generally in the range of about 10 km to 20 km up.
In worlds too big to escape by propulsion, people may come up with the idea of the space elevator, but the extra gravity will require taking into account the structure’s weight.
Getting enough mass up there to build a space elevator is itself a very tough problem.
Some world out there may have a ridiculously tall mountain that extends into the upper atmosphere. Gravity at the top will be lower, and if a launch platform can be built there, takeoff will be easier. Of course, this is an “if” bigger than said mountain.
Whether gravity is stronger or weaker on top of a mountain is surprisingly complicated and depends a lot on the individual planet’s makeup. However, at least on Earth-like planets it is weaker. See here. Note though that if a planet is really massive it is less likely to have large mountains. You can more easily get large mountains when a planet is small. (e.g. Olympus Mons on Mars).
India has a huge coastline, but for mythical/cultural reasons, Hinduism used to have a taboo against sea travel. In the worst scenario, our heavy aliens may stay on ground, not because they can’t, but because they won’t; maybe their atmosphere looks too scary or their planet attracts too many meteorites or it has several ominous-looking moons or something.
This would require everyone on the planet to take this same attitude. This seems unlikely to be common.
You got me curious, and I read a bit more, and found this on Wikipedia:
A rocket moving out of a gravity well does not actually need to attain escape velocity to escape, but could achieve the same result (escape) at any speed with a suitable mode of propulsion and sufficient propellant to provide the accelerating force on the object to escape. Escape velocity is only required to send a ballistic object on a trajectory that will allow the object to escape the gravity well of the mass M.
In lay terms, I guess this means that, unlike a cannon ball, which only gets one initial “push”, a rocket is being “pushed” continually and thus doesn’t need to worry about escape velocity.
Because of the atmosphere it is not useful and hardly possible to give an object near the surface of the Earth a speed of 11.2 km/s (40,320 km/h), as these speeds are too far in the hypersonic regime for most practical propulsion systems and would cause most objects to burn up due to aerodynamic heating or be torn apart by atmospheric drag. For an actual escape orbit a spacecraft is first placed in low Earth orbit (160–2,000 km) and then accelerated to the escape velocity at that altitude, which is a little less — about 10.9 km/s. The required change in speed, however, is far less because from a low Earth orbit the spacecraft already has a speed of approximately 8 km/s (28,800 km/h).
So first they get the rocket high enough to be safe from the air, and then they speed it up.
Good analysis! A few remarks:
In practice even for a planet with as thin an atmosphere as Earth, getting past the atmosphere is more difficult than actually reaching escape velocity. One of the most common times for a rocket to break up is near Max Q which is where maximum aerodynamic stress occurs. This is generally in the range of about 10 km to 20 km up.
Getting enough mass up there to build a space elevator is itself a very tough problem.
Whether gravity is stronger or weaker on top of a mountain is surprisingly complicated and depends a lot on the individual planet’s makeup. However, at least on Earth-like planets it is weaker. See here. Note though that if a planet is really massive it is less likely to have large mountains. You can more easily get large mountains when a planet is small. (e.g. Olympus Mons on Mars).
This would require everyone on the planet to take this same attitude. This seems unlikely to be common.
You got me curious, and I read a bit more, and found this on Wikipedia:
In lay terms, I guess this means that, unlike a cannon ball, which only gets one initial “push”, a rocket is being “pushed” continually and thus doesn’t need to worry about escape velocity.
So first they get the rocket high enough to be safe from the air, and then they speed it up.