# How much gravity is too much to (practicality) leave a planet

(inspired by this thread - but here, so as not to hijack)

I’m making the assumption that we built rockets to travel to the moon and space out of curiosity - we could look up and see them. We are fortunate enough to have three main things going for us (given our dexterity and intelligence):

1. Clear skies above to recognize the universe outside of us
2. Fossil fuels of a dense enough nature
3. And gravity low enough that those fossil fuels allow escaping the earth’s gravity

Suppose there is another planet out there that has dexterous, intelligent beings with energy dense fuel (aprox the same as ours) who are of an inquisitive nature that wish to travel to their moon, and then to adjacent planets. But lived on a much larger planet with much stronger gravity. How much gravity would be considered too much to practically leave their planet?

Tangent question related to number 1. If our skies were perennially cloud covered in some manner, would we still have wanted to leave earth?

Well, first of all, it should be pointed out that “how much gravity” can have multiple different meanings. Usually, when we’re referring to an amount of gravity, we mean the gravitational field, or acceleration, which on the surface of the Earth is 9.8 m/s^2. But for purposes of determining how hard it is to leave a planet, the more relevant quantity is the gravitational potential, which is not the same thing. If you had a planet that was larger than Earth, but also less dense, it could end up with the same gravitational field, but a higher gravitational potential (or likewise with a planet smaller but more dense, which could have a lower potential).

All that said, there are no hard cutoffs. For any given planet and any given rocket chemistry, it’s possible to calculate a fuel-to-payload ratio that’ll work. The stronger the gravity (potential), the higher the fuel-to-payload ratio will need to be. At some point, of course, the ratio gets impractically large… but then, it’s hard to use that as a cutoff, because we’re arguably already past that point here on Earth (i.e., space travel is possible but impractical for us).

That’s a question that’s better suited for IMHO than for FQ. Or possibly CS, given that it’s been part of the premise of a number of science fiction stories.

It’s amusing to note that some planet-like objects are even harder to leave - on the Ringworld, you face the situation that local gravity is about one gee - but a ship leaving from the inside inherits a tangential velocity of 770 miles per second, so simply pointing the rocket up won’t work (770 miles per second is also a lot higher than Earth’s escape velocity).

Or how about Mesklin? (Mission of Gravity).

Something like 700G at the poles, I seem to recall!

Or an even more extreme example, the world described in Robert Forward’s Dragon’s Egg, which has a surface gravity of 67 billion G. It is a neutron star rather than a planet per se, although it does have life on its surface. In the course of the book, the inhabitants do develop space travel despite the enormous gravitation of their world, but it involves technology to directly manipulate gravity, not anything like a rocket.

For more mundane worlds, the short story “Youth”, by Asimov, features a world with an Earthlike field but deeper potential well. As well as some plot twists.

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I think thae Earth has too much gravity to practically leave the planet. Yes, it happens, but there is nothing practical about it.

Here is a scientist’s take on the subject, which is a mix of serious analysis and tongue in cheek humor:

1803.11384 (arxiv.org)

It finds that chemical rockets are still possible on Super-Earths with up to 10x our planet’s mass.

Interesting, I would have thought at some point the fuel needed to lift the fuel would be a deal breaker. I guess I don’t know what “any given rocket chemistry” means.

Mostly, different fuels/combinations of fuels will have different nozzle velocities, which is one of the inputs in the rocket equation. And “the fuel needed to lift the fuel” is a very real concern, but it results in an exponential growth, not a vertical asymptote.

Actually, I’ll backpedal some… A real planet has both gravity drag and atmosphere. Gravity drag, I’m pretty sure you can deal with (at the expense of pushing out even further on the exponential rocket-equation curve), but atmosphere might be a deal-breaker: Dealing with gravity drag is likely to mean using more cross-sectional area for engines, but cross-sectional area will also increase drag, so there might be a hard cutoff there.

I will just point out that the advanced engines didn’t use fossil fuels. The Space Shuttle boosters used Ammonium perchlorate and aluminum powder. The main engines used hydrogen and oxygen. Of course along the way they burned lots of kerosene but that wouldn’t work for the Shuttle.

SpaceX Raptor engine uses liquid methane and liquid oxygen. Methane is the main constituent of natural gas, a fossil fuel.

Then they are not advanced engines with regards to their fuel at least. Nothing wrong with that as older fuels are certainly a well developed method. I remember some experiments with engines using polybutadiene rings with a hydrogen/oxygen flame up the center, initially lit with ethylene. I think that was a booster engine concept.

If a planet had a dense atmosphere (which a massive planet likely does), couldn’t you fly a rocket/jet up to high in the atmosphere at relatively low speed, then convert to a rocket engine up there. Drag would be higher, but lift would be as well. This would eliminate much of the drag and the escape velocity would be lower as well the higher you could get in “jet” mode. Presumably you could refuel at altitude as well.

I second this motion. IIRC it costs about half a billion dollars to run the Space Shuttle up there and back – and I’m not so sure that should count as leaving the planet anyway.