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It's thought that popped up a while back when I was looking for habitable exoplanets, and found out that the majority of discovered potentially habitable exoplanets are 3-4 Earth-mass super-earths.

Then it dawned on me: is it possible that there're countless alien civilizations out there, but most of them gave up on space exploration because of, well, too much gravity?

I mean, look at us: the majority of weight our rockets carry, in order to send basic satellites into space, is its own fuel. How much more difficult it would be to send a rocket into space on a life-bearing world with 4G? Would that even be technically achievable or economically viable? What if those alien civilizations simply gave up on space-exploration because it's too difficult? If I was better at math I could probably come up with a formula like Drake equation to model how long it would take life evolve to the stage of being space-capable on a life-bearing world, with gravity as one of the independent variables.

Anyway, what else could gravity affect a civilization's technological progress to becoming space-capable? (I know I'm asking a broad question here and I'm not looking for solid answers, and the best answer will be the one that helps me clear this train of thought and/or gives me new ideas.)

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closed as primarily opinion-based by Xandar The Zenon, WhataTiberius, T.J.L., Aify, JDługosz May 16 '16 at 7:33

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ So the best answer is the one that best matches your private thoughts? I think this is the definition of "Opinion Based". You nailed it as far as it being more difficult to get off the ground (as long as their tech level is similar to ours in the 50's) $\endgroup$ – AndreiROM May 10 '16 at 23:16
  • $\begingroup$ @AndreiROM lol "match my private thoughts" requires I have "private thoughts" on the subjects to begin with, whatever the heck that is. $\endgroup$ – hexpallett May 10 '16 at 23:31
  • $\begingroup$ Maybe I didn't express myself in the best possible way. Allow me to quote you: "the best answer will be the one that helps me clear this train of thought" <- so we just keep rambling until you're happy? How do we know when that is? Your question offers no real way to determine what the best (or even a good) answer might be. Aka opinion based. $\endgroup$ – AndreiROM May 10 '16 at 23:34
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    $\begingroup$ Just get rid of the parenthetical and the end and this is perfectly on topic $\endgroup$ – Azor Ahai May 12 '16 at 18:09
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    $\begingroup$ Whether or not someone could shoot off a rocket from a 4g world seems perfectly calculable without any opinion required. $\endgroup$ – Brythan May 16 '16 at 4:07
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You would find you need more and more exotic fuels as you look at higher gravity levels.

Fuels are rated with ISP: specific impulse. Specific impulse is the amount of impulse you generate per kilogram of fuel mass. Typically it is in units of N-s/kg (except in English units, where it is measured in seconds...). Wikipedia has a list of these, from which we can pick some examples:

  • LOX+H2 (liquid oxygen and hydrogen) - 3816 N-s/kg
  • LOX+RP1 (liquid oxygen and kerosene) - 2941 N-s/kg
  • N2O4+N2H4 ( nitrous oxide and hydrazine) - 2862 N-s/kg

That list is for liquid fuels, the ISP for solid boosters is typically around 2400 N-s/kg

Escape velocity is proportional to the square root of the mass of the planet. This means your planet with 4Gs of gravity at the surface will require vehicles to reach twice the speed to escape. As you notice, the gains from using exotic chemicals to fuel your rocket were limited. There were benefits, but only so much. This means the only real way to increase your velocity is to pack more fuel on. Moving to more exotic fuels just wont cut it (and trust me, hydrazine is exotic)

There's an equation for this, known as the Tsiolkovsky rocket equation: $$\Delta v=I_{sp}ln(\frac{m_0}{m_f})$$

Where $m_0$ is the dry mass of the rocket after the maneuver and $m_f$ is the wet mass before the maneuver. We can use this to figure out what this does to the mass of our fuel by rearanging for wetmass: $$m_f=\frac{m_0}{e^{\frac{\Delta v}{I_sp}}}$$ If we then define $\Delta v^\prime$ to be the new escape velocity we need ($\Delta v\prime = 2\Delta v$) and $m_f^\prime$ to be the new wet mass we need to reach escape velocity, we can find out just how much more wet mass we need to do the job: $$m_f^\prime=\frac{m_0}{e^{\frac{\Delta v^\prime}{I_sp}}}$$ $$\frac{m_f^\prime}{m_f}=\frac{e^{\frac{\Delta v^\prime}{I_sp}}}{e^{\frac{\Delta v}{I_sp}}}$$ $$\frac{m_f^\prime}{m_f}=e^{\frac{\Delta v^\prime}{I_sp}-\frac{\Delta v}{I_sp}}$$ $$\frac{m_f^\prime}{m_f}=e^{\frac{\Delta v}{I_sp}}$$

Since we are comparing against earth gravities, where $\Delta v$ for escape velocity is 11200m/s and using rockets which top out at an ISP of somewhere around 3000N-s/kg (which if you do a dimensional analysis, is identical to 3000m/s, you get a ratio of 41.8! This means reaching escape velocity requires a whopping 41.8 times more fuel than it did on Earth!

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  • $\begingroup$ That's not what your math is saying. Mf is the final mass (I.e. Just your payload), and M0 is the initial mass (payload + fuel). As it turns out, your math in the second fraction was wrong (the primes should be diagonally opposite each other, not at the same level for Mf) but was write if you rename Mf to M0 from that point forward. What your equation actually says (if you invert Mf'/Mf) is that for a given initial mass (payload+wet mass), the payload can be 41.8x as massive on Earth as it could on the other planet. $\endgroup$ – iAdjunct May 11 '16 at 13:04
  • $\begingroup$ More and more exotic fuels?! Amongst the stuff that's sane to use LH2 + LO2 is the most energetic there is! If LH2 + LO2 can't get you to orbit you'll have to do something other than chemical rockets. $\endgroup$ – Loren Pechtel May 15 '16 at 21:30
  • $\begingroup$ The most energetic fuel is LH2 + LF2, but it's less than 10% better, the LF2 is trouble looking for a place to happen and the exhaust is HF acid--incredibly nasty. $\endgroup$ – Loren Pechtel May 15 '16 at 21:50
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Absolutely, while atmospheric density would have a very strong effect on spaceflight development, gravity on its own would slow or even prevent any development if it was strong enough.

Furthermore, increased gravity often coincides with increased atmospheric density, which increases drag on a spacecraft and forces even more fuel to be burned. This fuel weighs more thanks to the gravity, which means even more is burned. There is likely to be a cutoff point where gravity is strong enough that no conventional rocket could ever hope to escape.

Spaceflight that did develop on such a world would be a prime candidate for some divergent development. Increased atmospheric density could be used to make spaceplanes more viable, if I were writing a spacefaring race from such a planet I'd have them make heavy use of spaceplanes and zeppelins, the latter of which are staggeringly effective even on Earth.

I suggest doing some research on terminal velocity and ways to reduce drag in aircraft, and see how plausible a spaceplane hitting escape or orbital velocity inside the upper atmosphere is. If things looks promising, then there's your answer.

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Your biggest problem actually lies in fuel. With a higher gravity, the rocket will need to burn more fuel, meaning less rockets. They will still be able to go into space, but they will be unlikely to waste time trying to colonize planets and will more likely use that valuable space time to do more productive things like place satellites.

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Here are a few points that might help.

  1. They might develop space flight later in development after they have a higher level of technology, better miniaturization for instance.
  2. Better fuels, as already mentioned.
  3. More fuel, less payload. Perhaps for them, some kind of carbon nanotube lattice technology would be the only way to get materials strong enough to make the rockets big enough to hold the fuel needed, even if it's more expensive. Also, perhaps they would launch more robotic payloads since they can be smaller. Imagine an Apollo rocket carrying a payload the size of a grapefruit. This, of course, would make their 'spacefaring' vicarious.
  4. Once they get off of the planet, they might have to rely on alternate ways of getting a population into space (assuming that's what you want). For instance, maybe just one or two scientist make it into space, but they carry with them thousands of eggs and sperm (assuming they even breed that way) and the grow or clone a population.
  5. Some of the effect might be mitigated by a faster spin. Space vehicles get an assist when they launch near the equator. I didn't calculate anything, so this might not add much.

I had a couple of other ideas, but I forgot them as I typed.

Anyway, I don't have a problem with "opinion based," so I hope what I've typed as given you some ideas to mull around. Good luck.

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The rocket equation is brutal, it doesn't take a world too much bigger than Earth before chemical rockets simply aren't going to do it. However, that doesn't mean they will be forever planetbound.

Nuclear thermal rockets can approach doubling the ISP of chemical rockets, thus permitting launch with twice the gravity well.

There's also the gas-core version of nuclear thermal--your reactor is gaseous uranium rather than solid. This more than doubles what you can get from nuclear thermal, again this doubles the velocity you can attain. The fairly narrow operating limits of a nuclear reactor make this design seem awfully scary, though!

Now we are into the dirty engines:

The open cycle nuclear thermal rocket--you maintain a chain reaction in uranium gas (probably UF6 to make it easier to handle), this is 2x to 6x the ISP over the closed cycle engine as you don't have to contain it. However, your used fuel becomes your exhaust--nasty for the environment and the crew picks up a substantial dose also.

Orion starts out at in the middle of this range. Again, your exhaust is hot. Bigger rockets have better performance, if they're big enough you can use boosted bombs and up your ISP still further. I can't find any estimates of the radiation exposure to the crew but given the numbers for the open cycle nuclear gas rocket I find this worrisome.

Finally, there are the non-rocket approaches:

First, the launch loop. You build two stations thousands of miles apart but at the same latitude. They throw iron bars back and forth. The bars never stop, when a station catches them they're whipped around with a huge magnet and tossed back, retaining their velocity. Note that the bars chase each other as close as possible, each will be drafting behind the one in front, the air drag isn't anything like what you would expect.

Once you have these bars flying back and forth you build a track on top of them--think of a maglev train but upside-down, the track is levitated on the bars, not the other way around. Of course this exerts a downward force on the bars but this is countered by speeding them up. The objective here is to get a good chunk of track basically out of the atmosphere. You can then mount a linear motor on it, your spacecraft goes up the track at sane atmospheric speeds, once it's clear of the atmosphere it hits the high power track and is boosted to orbit. The track must be massive enough to absorb the recoil without being knocked out of operation.

Huge, expensive and since it falls apart if any of the active systems fail it would be dangerous. However, the ISP is infinite as no craft resources are being used for the boost.

Finally, a solution that makes the launch loop look like a kids toy:

Build a maglev track around the world at the equator. Build a train on the track that has a maglev setup on top as well as the bottom. Make the track into an evacuated tunnel and boost the train to above orbital velocity. It's now exerting an upward force on the track which can counter the mass of the system. Build higher, build another train. Each train supports the mass of the section below it so the normal limits of how tall you can build do not apply--you can build out of the atmosphere and then resort to linear motors to push your spacecraft.

While again this is a dynamic system that collapses if it stops moving I don't think it's as dangerous as so long as you are using superconductors it's self-contained in the short run. If the power goes out it will continue to operate until the magnets quench.

Neither system could reasonably be built in a society with terrorists or madmen prone to spectacular suicides.

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  • $\begingroup$ This answer makes more sense than all the others, and I wonder why something like this is not presently done on Earth, where the technology would be much easier to implement. $\endgroup$ – frank May 16 '16 at 3:54
  • $\begingroup$ Presumably because building a maglev train on top of another maglev train while in motion is effectively impossible, not to mention the lack of a superconductor needed to power it. On top of that, I doubt such a structure can actually support it's own weight. $\endgroup$ – Space Ostrich May 16 '16 at 4:30
  • $\begingroup$ @SpaceOstrich You misunderstand the geometry. No train is on top of another train. The tracks are stacked vertically but that's nothing more complex than a double-deck bridge is today. And if the trains are moving fast enough and are massive enough it would work. This is mega-engineering but certainly possible. We have the superconductors now--the holy grail is ones that don't need extreme cooling to work, but if you'll cool them enough there are plenty of materials to build them from. $\endgroup$ – Loren Pechtel May 16 '16 at 6:09
  • $\begingroup$ I used train interchangeably with train track. Either way, the point remains. Building a maglev rail on top of a moving train, then building a train on that track, all while in motion, is infeasable. $\endgroup$ – Space Ostrich May 17 '16 at 1:50
  • $\begingroup$ @SpaceOstrich You misunderstand---the rail on the top is because the train will be moving above orbital velocity--it will be pushing up, not down. $\endgroup$ – Loren Pechtel May 17 '16 at 2:21
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If the gravity was high enough to make liftoff from the ground uneconomical but spaceflight is something your species wants, they can try out methods that sci-fi authors have already described to start from a higher altitude.

Gravity's effect is lower the further you're away from the center of mass - starting from a mountain would take slightly less fuel than starting from sea level. Far more relevant though: starting from a plane (which could use the higher atmospheric density to achieve lift for the mass of your rocket more easily) or a blimp would likely mitigate the high-G enough to make spaceflight feasable.

What's more, some say that even here on earth it would be more effective to start like that - but because we have a working ground-based system and new methods carry risks and development costs, we're unlikely to try it anytime soon. So a high-G civilization might, after a slower start, actually be able to progress faster because they have two vectors along which to apply improvements (better blimps able to give them a higher starting point, maybe make the blimp travel faster and faster with earth's rotation for some added velocity meaning even less delta-V needed from the rocket, and of course the rocket itself and its fuel).

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