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Biggest Planet a Modern Rocket Can Launch From

Discussion in 'SpaceX' started by Skotty, May 6, 2016.

  1. Skotty

    Skotty 2014 Model S P85

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    SpaceX hopes to build rockets that can land and take off from almost anywhere. One interesting question, though it's not of much concern in our particular solar system, is how big can a terrestrial planet be before a modern rocket would no longer be able to launch off the surface?

    The bigger a planet gets, the more gravity it has, and if I'm not mistaken, at some size the gravity will exceed the capabilities of a modern rocket. You can use bigger rockets, but at some point the fuel can't even lift itself.

    I've heard or read something not too long ago that suggested if Earth was much bigger, we wouldn't be able to get into orbit. I know the margins is pretty slim for modern rockets on Earth. How much bigger could the planet be before we would be stuck down here?

    Anyway, I just though it was an interesting question to share that I have never seen any space shows answer.
     
  2. LargeHamCollider

    LargeHamCollider Battery cells != scalable

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    a better question would be "how massive would a black hole have to be before we couldn't escape it if we started at distance X in orbit Y"

    With current tech we can escape from the sun's gravity which is much greater than that of any planet.

    Neglecting corrections for general relativity escape velocity is SQRT(2*GM/r).
     
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  3. Ben W

    Ben W P85 #61, Roadster #108

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    A rocky planet much larger than Earth would probably have an atmosphere too thick to be able to penetrate with modern rockets. I doubt we could launch from the surface of Venus for instance, even from the poles, where the temperatures are apparently a lot more moderate than previously thought. You'd have to use a Virgin Galactic-style hybrid approach to launch in such an environment, and it's not clear if that could scale up to rockets the size of the F9.

    Out of curiosity, how much would the F9 payload decrease if they had to launch from the Dead Sea? (427m below sea level.) Or from Siberia, where the atmospheric pressure has reached as high as 1085mbar? I suspect that would be a fairly significant hit. On the flip side, how much would the payload increase if launched from the summit of Everest or Kilimanjaro? Or (in principle) from inside the eye of a Cat-5 hurricane...
     
  4. jkn

    jkn Member

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    Escape velocity from surface of the Sun is 617.7 km/s.

    Tsiolkovsky rocket equation - Wikipedia, the free encyclopedia
    delta v = v_exhaust * ln(m0/mf)

    If 95 % of rocket is fuel, then m0/mf = 20
    In practice v_exhaust is less than 4500 m/s, but it is theoretically possible.
    One stage would add 13.5 km/s.

    We would need 46 stages to escape from surface of the Sun.
    If cargo is 1% of mass (fuel 95%, everything else 4%), each stage is 100 larger than next one. Repeat 46 times... Not possible.

    Trying again with larger cargo fraction: Cargo 10 % of mass, fuel 85%
    delta v = 4500*ln(100/15) = 8537 m/s
    72 stages. Now stage is only 10 times heavier than next. Better, but not possible (not even theoretically)


    Escape v from Jupiter is 59.54 km/s
    7 stages. Start mass of the rocket is 10000000 times mass of cargo. Not practical.
     
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  5. Grendal

    Grendal Active Member

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    As the engineers were describing during the launch, altitude is less important than lateral speed. That is why launching at the equator is actually the very best place to launch on the planet if I remember correctly. You get the maximum amount of rotational assistance from the planet. And that is why Cape Canaveral is a southern location in the US and SpaceX is setting up their launch facility in Boca Chica, Texas.
     
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  6. bxr140

    bxr140 Member

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    And why ESA set up in French Guiana and why sea launch sails out to the equator...
     
  7. AWDtsla

    AWDtsla Active Member

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    About 3% bigger than earth, or whatever the payload fraction of any given rocket is. Space travel is almost impossible, at least without nuclear power.
     
  8. AWDtsla

    AWDtsla Active Member

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    You're forgetting gravity losses. You could have a million stages and still not get anywhere.
     
  9. AWDtsla

    AWDtsla Active Member

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    This is not a winning strategy, on any planet.
     
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  10. Ludus

    Ludus Member

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    Without an atmosphere an orbit can be arbitrarily close to the ground (as long as there aren't any obstacles to run into). On earth anything above 30,000 feet would clear all mountains.

    You could have a very low lunar orbit, which I think would be pretty thrilling because there would be a sense of speed.

    Planets earth sized and above of course are unlikely not to have atmospheres.
     
  11. jkn

    jkn Member

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    Yes. I stopped thinking after impossible. Gravity losses are reduced by faster acceleration. But that is not enough. Starting point should be above most of atmosphere.
     
  12. jkn

    jkn Member

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    It would be foolish (perhaps impossible) to try to lift with rocket from bottom of Venus like atmosphere. Some kind of hybrid would be easy. On surface of Venus density of atmosphere is roughly same as density of liquid hydrogen. Zeppelin on Venus would be easy, because O2 N2 mix provides lift.
     
  13. jkn

    jkn Member

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    I find hard to believe that temperatures on poles of Venus would be significantly lower.

    Solar power/m^2
    Earth: Incoming 1.4 kW, 30 % reflected -> 0.98 kW absorbed
    Venus: Incoming 2.8 kW, 90 % reflected -> 0.28 kW absorbed

    Earth absorbs 3.5 times as much solar energy as Venus. Venus is hot because of lot of CO2. So poles are not cool.
     
  14. Ben W

    Ben W P85 #61, Roadster #108

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    "Venus may be boiling hot, but its poles are very, very cold."

    ESA Finds a Frigid Surprise Hiding at Venus' Poles | Astronomy.com

    Of course, this measures the upper atmosphere, not the surface directly. But still, very interesting.
     
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  15. RDoc

    RDoc S85D

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    Interesting question. It got me thinking about alien civilizations, presumably many of whom would live on super-Earths. They would have to use some kind of nuclear rocket engine to get into space, which would be a pretty high barrier to entry. Once they did get it working though, they'd have a great system for interplanetary travel. As soon as they got into orbit, their entire solar system would be at hand.
     
  16. Ben W

    Ben W P85 #61, Roadster #108

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    From an article at nasa.gov:

    Link here: NASA - The Tyranny of the Rocket Equation
     
  17. AWDtsla

    AWDtsla Active Member

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    The 50% number is only from escape velocity. It doesn't consider atmosphere and gravity losses while trying to get out of atmosphere. If you plainly use the equation it assumes your rocket can be arbitrarily large. If you need rocket that's the mass of the moon to get off the planet, that's not a realistic scenario. Even if you could build it the shockwave would kill everything in a thousand mile radius.
     
  18. RDoc

    RDoc S85D

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    Presumably on a large planet, their Space Agency would look into high altitude launches, e.g. balloons, carrier aircraft.

    The question I have is what happens on a planet that's a lot bigger, say 2-4 times our mass? I don't know if even nuclear rockets would work.
     
  19. Johan

    Johan Took a TSLA bear test. Came back negative.

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  20. AWDtsla

    AWDtsla Active Member

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    You can't make the balloons or aircraft big enough to life the launch vehicle. SpaceX isn't doing thing simply because it doesn't scale. You'd be better off trying to construct a mountain at the equator, whatever many centuries it would take to move all that dirt.
     

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