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I was playing with a rocket design simulator, and I noticed that if I set Earth's gravity to be much higher than it is today, it became impossible to design a rocket that could reach escape velocity.

Is this a real phenomenon? Like if Earth was a larger planet, we might not have reached the moon by now?

Or am I just horrible at rocket design? :)

Nathan Tuggy
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Beaker
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1 Answers1

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Yes, if the planet is sufficiently large we would not be able to reach orbit with our current available materials and engineering techniques / technologies, this is partial due to the tyranny of the rocket equation:

If the radius of our planet were larger, there could be a point at which an Earth escaping rocket could not be built. Let us assume that building a rocket at 96% propellant (4% rocket), currently the limit for just the Shuttle External Tank, is the practical limit for launch vehicle engineering. Let us also choose hydrogen-oxygen, the most energetic chemical propellant known and currently capable of use in a human rated rocket engine. By plugging these numbers into the rocket equation, we can transform the calculated escape velocity into its equivalent planetary radius. That radius would be about 9680 kilometers (Earth is 6670 km). If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport.

Source: http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html

In the above quote, a greater radius is used, but the greater radius means more surface gravity, so it talks about the same thing.

Obviuosly there are other technologies that could still allow for space travel. The most simple way to increase delta-v on a rocket is staging. However, there are still practical limits for staging. One problem is thrust, The F1 is the biggest rocket engine ever built, and some studies suggest that there is a practical limit of 1.5x the size of an F1 above which engines can't be constructed. Morever you can't stack vertically forever, since the lowest stage could not support the weight of the whole stack. Adding stages radial is also limit, the sheer stress would at some point break the whole rocket apart. So while this might shift the numbers a bit, I don't think it has significant influence on the spirit of the question. It doesn't really matter wether Earth needs to be 1.5x or 2x as big, at some point we can't reach orbit anymore with what we currently have.

There are other speculative technologies - rail launchers, launch loops etc. that have been research, but those aren't technologies that are currently in use. And they have mainly been researched for application on Earth, so I would be hesistant to deem them appropriate for higher gravity / radius without researching their limits, first (there is a good possibility that the materials can't handle the incresed velocities / stresses).

Polygnome
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  • Wow, thanks! That's very cool! I can imagine that somewhere in the Universe there might be intelligent species that are stuck on their large planets. And how can they make a technological leap to escape high gravity when they can't even learn anything about space!

    Add "modest gravity" to the list of desirable factors in the "Goldilocks zone" for planets. :)

     – Beaker Jul 22 '16 at 20:39
  • They wouldn't necessarily be stuck on their planet forever. The could eventually build something like Project Orion or a launch loop. – Austin Jul 22 '16 at 22:28
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    This answer is incorrect, as it does not take into account staging. See this question @Michael_Kjorling quoted above, especially Russell Borogrove's answer. – Jerard Puckett Jul 22 '16 at 23:02
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    From my answer to the linked question: "Second, you can just add stages to your chemical rocket to get off of a larger planet. It will simply cost more, so you will need to be more determined. Third, you don't need to use chemical rockets. You could use electromagnetic rail launchers, nuclear fission high Isp engines, or thermonuclear bomb propulsion (see Orion). All within our current technological capability." – Mark Adler Jul 22 '16 at 23:27
  • Yes, there are technologies that could help. But I have explicitly written with our current materials and technologis. launch loops, rail launcher etc. are still theoretical at this point, and the NERVA has never been used. Staging allows for better results, but even with staging there is a point where the whole stack is so heavy that the lowest stages will simply not support the weight. So yes, the answer is correct, even if the numbers are derived only for one case. – Polygnome Jul 23 '16 at 08:43
  • The passage I have quoted explicitly states at the end "[...] at least using rockets for transport". – Polygnome Jul 23 '16 at 08:44