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I know that chemical rockets would allow for some minor loads to escape the gravity of a planet with higher gravity.

My question is, what kind of technology would possibly allow a dominant species to escape the gravity of a planet and become a somewhat spacefaring species?

Are there any possibilities in today's technology? Or what fictional technology might accomplish this that might be in our near future?

Edit: Thank you, guys. This has been a treasure-trove of help and information. I now have enough information for my later chapters as well. I honestly appreciate it.

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    $\begingroup$ Related question on Space Exploration: How much bigger could Earth be, before rockets would't work? $\endgroup$ – David Richerby Feb 24 at 19:24
  • $\begingroup$ And on here: worldbuilding.stackexchange.com/questions/40161/… $\endgroup$ – kingledion Feb 24 at 20:34
  • $\begingroup$ Also related: worldbuilding.stackexchange.com/questions/21582/… $\endgroup$ – kingledion Feb 24 at 20:34
  • $\begingroup$ For "rockets" velocity achievable relates to -- Isp or Exhaust velocity -- number of stages -- mass ratio (= Mfull/mempty). || Optimising each of these increases Vmax and increases 'hardness', || Nuclear rockets and "Orion" allow several times Isp of best chemical rockets - getting harder with gains. | Mass ratio eats into amount launched per starting mass. | More stages gets messy. Overall earth is surprising close to the "getting very hard" limit - maybe 1.5 x to 2x the current Earth Vorbital is doable. Vesc_earth is 7 miles/second and Vorbit_earth ~= 5 milew/second. $\endgroup$ – Russell McMahon Feb 25 at 3:38
  • $\begingroup$ The rocket equation may be tyrannical, but it's not some arbitrary barrier we cannot cross. See David's link; it just becomes a stupidly high amount of stages you need. $\endgroup$ – Mazura Feb 25 at 23:07
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If you don't want to go the nuclear rocket route, and stick with technologies that are known to be physically possible and not require nonexistent materials....

If the planet spins extremely rapidly, such that you get a decent speed boost from equatorial launches and synchronous orbit is really low, then regular rockets and even space elevators may be suitable even for very massive planets, although rockets will be restricted in the inclinations they can reach. Earth is marginal for space elevators because the cable would have to be so long, and support so much of its own weight that there's nothing left for safety margin and payload, even with the strongest theoretical materials we know of. A higher spin, however, will lower synchronous orbit, potentially making elevators practical even with much higher gravity.

If you don't want to be restricted to high spin planets, you're left with basically one option: dynamic actively supported structures like Lofstrom launch loops and space fountains, possibly followed up by orbital rings (from which one could hang skyhooks in place of traditional space elevators). These kinds of structures make use of accelerating recirculating rotor materials inside their external static structure such that the reaction forces of the rotor materials on the static structure counteract its weight. This gets around the compressive strength limits of static materials, allowing you to raise platforms above the atmosphere and use electromagnetic tracks to accelerate to orbital or escape speeds in vacuum.

Of course, if you somehow manage to develop civilization on a planet with little or no atmosphere, you can just use electromagnetic launch tracks directly from the ground, coupled with low-power circularization rockets.

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    $\begingroup$ Rats, I wanted to write myself about space-elevators on a high-spin planet - and you beat me to it. +1 for being faster :-) Anyway, high spin would help a space elevator much more than equatorial launches: Rotational speed grows linearly with your height on a space elevator, the energy that you take away from the planets rotation by the square; and equatorial launches are restricted to the rotational speed near the planets surface. $\endgroup$ – cmaster Feb 24 at 19:03
  • $\begingroup$ Although, you know, if gravity is higher, the space elevator apparatus will be heavier. $\endgroup$ – Harper - Reinstate Monica Feb 24 at 20:38
  • $\begingroup$ @Harper Yes, but that just means that the relation between gravity and minimum spin is not linear, not that higher spin can't make up for higher gravity at all. Consider a limiting case: if the spin is high enough that the planet is on the verge of disintegration, and the ground at the equator is in synchronous orbit--then the elevator length is zero, and any material can hold up over zero length under arbitrarily high gravity. Say the spin is a little less, so the elevator length is a few feet--even boring steel cable can support itself for a few feet, even under hundreds of gees. $\endgroup$ – Logan R. Kearsley Feb 24 at 22:12
  • $\begingroup$ Given that there is quite a lot of room for a planet to count as "higher gravity" before we get to "hundreds of gees", and we have materials with more tensile strength than steel cable, intuitively evident even before getting into the concrete mathematical analysis that there is a wide range of non-extremal spin rates under which a planet with 2, 3, 5, or even 10g surface gravity could support a space elevator made of, e.g., Kevlar, or even just steel cable. $\endgroup$ – Logan R. Kearsley Feb 24 at 22:15
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    $\begingroup$ You could have an EM launch track inside a vacuum tube, exiting as high in the atmosphere as possible. The main limit on exit velocity would be the acceleration that the spacecraft contents could endure. As a way of delivering basic payloads to LEO, it would be a good bet on Earth. en.wikipedia.org/wiki/Space_gun $\endgroup$ – Andrew Dodds Feb 26 at 16:27
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There is always the possibility of using very high powered rockets to escape from the planet, including nuclear rockets and even Project Orion type rockets or nuclear pulse rockets using a series of atomic explosions to propel the ship.

https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion1

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    $\begingroup$ Of course, there is the small problem of dealing with the resulting fallout. Multiplied by the (presumably large) number of launches. Left as an exercise for the reader. $\endgroup$ – WhatRoughBeast Feb 24 at 18:17
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    $\begingroup$ "Very high powered" chemical rockets are basically doomed by the rocket equation. You can't just keep making your rocket more and more powerful because then it needs more and more fuel to generate that power. But then it needs even more power, because you have a load of fuel to lift. And that means you need more fuel, and more power and... $\endgroup$ – David Richerby Feb 24 at 19:17
  • $\begingroup$ @DavidRicherby: But is there a point where a chemical rocket truly becomes impossible? I guess one real limit would be when its mass is a significant portion of the planet’s mass (at which point launching the rocket would affect the whole planet and you might as well move the whole planet)… $\endgroup$ – Michael Feb 24 at 20:37
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    $\begingroup$ @Michael It's not clear what's "truly impossible" but the rocket getting (literally) exponentially heavier makes the engineering quickly get infeasible, even before you get to significant fractions of the planet. $\endgroup$ – David Richerby Feb 24 at 20:51
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In theory Space Elevators are possible regardless of the surface gravity of the world on which they are built, material tensile strength is an as yet unsolved issue with building these structures. Wil McCarthy's Hacking Matter proposes some options in this area but the practical applications are still some years away.

Alternatively any form of em drive, field propulsion system or gravimetric drive, would work but none of these systems is more than highly theoretical at this time.

That's all if you want rocket-like spaceships whizzing about; if all you care about is spreading a species to the stars then the Einstein–Rosen bridge is also an option. Wormholes allow you to send people to other worlds with or without spacecraft and with any technological or magical embellishments you want to use.

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  • $\begingroup$ That's actually massively helpful. Most of what I needed was in that answer. Thanks for answering. I greatly appreciate it. $\endgroup$ – GaryS Feb 24 at 13:54
  • $\begingroup$ @GaryS All good, I've almost certainly missed some interesting options in the area of alternative drives but I'm glad the ones I've given are useful. $\endgroup$ – Ash Feb 24 at 13:57
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    $\begingroup$ The problem with space elevators is how to put them in orbit the first time. $\endgroup$ – vsz Feb 24 at 18:38
  • $\begingroup$ I agree, but there is the possibility of remote-drones being sent up first to help with the assembly of the space elevator. The space elevator can then be sent up one part at a time. Yes, this might be error-prone and lengthy, but I believe it might be possible, no? $\endgroup$ – GaryS Feb 25 at 11:40
  • $\begingroup$ Please stop pretending the EMDrive is not a hoax, everyone. It took far too many years to get rid of the Dean Drive hoax as it is, no need to do it all over again. $\endgroup$ – Eth Feb 25 at 12:07
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1. Gravitational anomalies.

gravitational anomalies

https://en.wikipedia.org/wiki/Gravity_of_Earth

A planet is not a ball bearing. It is inhomogeneous. Your planet can take that farther. There are areas where gravity is less and areas where it is more. Perhaps there is an accumulation of lightweight material in the crust, or an ancient huge chondritic lightweight meteorite incorporated from a long ago impact. Your spacefarers take advantage of the light areas.

2. Altitude. The higher you go, the less gravity is. You can go up on mountains for some benefit of this sort. Maybe your planet has some very high mountains - maybe one of them is that ancient chondritic meteorite? Or you can use rockoons. Capitalize on buoyancy to lift your spacecraft high above the ground. You can only get to about 100,000 feet on earth because the atmosphere thins out, but the amount of atmosphere a planet has does not depend on its size. You can give your heavy world an atmosphere way out, enabling a balloon to rise considerably farther before releasing its rocket cargo.

If rockoons bore you, maybe a rockeloonannon. Yes, there is a cannon involved. Usually that improves science.

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    $\begingroup$ The problem is, with higher total gravity both of these approaches would be made less believable, because there's greater force compacting the planet into a uniform sphere. Objects tend to become smoother and more homogeneous as they grow more massive. Earth is the largest rocky object in the solar system, but take a look at asteroids, comets, and some of the smaller moons to see the effects of reduced gravity on shape and surface properties. $\endgroup$ – ApproachingDarknessFish Feb 25 at 6:34
  • $\begingroup$ I just wanted to point out the units on that image of Earth's gravitational field. It's miligals. 1 gal is 1 cm/s^2, so a miligal is 10 μm/s^2 and the maximum of the scale shown is 500 μm/s^2 deviation from average. The average is approximately 9,806,650 μm/s^2. So, the maximum variation from the average on the scale in the image is approximately 0.0051%. Planets indeed aren't ball bearings, but they're closer than you might think. Also, gravity doesn't change that much with altitude, either. Even at the ISS's orbit, gravity is only about 12% less. $\endgroup$ – reirab Feb 25 at 13:38
  • $\begingroup$ At the top of Earth's highest mountains, the difference in gravity from sea level is around 0.25%. And, as ApproachingDarknessFish mentioned, a higher gravity planet would have a more smooth surface. $\endgroup$ – reirab Feb 25 at 13:45
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In addition to the other good ideas, here are two more:

  1. use aerodynamic lift to the edge of the atmosphere, then start the rockets at that point (similar to Virgin Galactic). As the atmosphere thins out, you can increase your velocity to maintain lift. At some point (Karman Line), the velocity you need to maintain lift will be enough to orbit your planet.
  2. laser propulsion uses very lightweight craft reflecting light from a ground based laser to overcome the tyranny of the rocket equation. You're going to be limited by your spacecraft's ability to reflect close to 100% of the photons, and to dissipate heat from those that are not reflected.
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With realistic technology, the solution would be the same as the one we're used to: a vessel of minimal weight and friction is subjected to a contained explosion strong enough to overcome the gravity well.

Some ideas that might make it work better (and generally more dangerously):

  • If it's a big planet that rotates very fast (has short days), centrifugal force in the Equator might help a great deal.

  • If the highest mountain is also in the Equator, that could help a little. If not, why not add to that centrifugal force by launching the vessel from a train that's moving Eastward very fast? (bonus points if the wind is blowing that way).

  • Aligning the rocket with a big moon that was close enough would counteract some of the planet's gravity: the vessel would be literally rising with the tides. For such a moon to not have crashed into the planet yet, it would need to have a fast orbit: maybe the aforementioned train would be following it?

  • Using the updraft of natural phenomena like cyclones or tornadoes to pull the vessel along, or even making artificial ones happen for that very purpose.

  • A bigger explosion, maybe big enough to make the entire region uninhabitable for a good while if they're not going out of their way to contain it -- maybe even as a Scorched Earth tactic?

More fictitious alternatives:

A really strong gravity would make it hard to not just leave the planet, but also move within it. This would be a perfect incentive to the development of technologies that bypassed the problem (like teleportation or wormholes) or even made it its own solution (antigravity).

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