4 ms·

the catalog says 6.38x mass in one place and 5.6x mass in another they must be able to calculate mass from orbital physics? so you'd need a rocket 6x the size

by ck2 3mo ago

the catalog says 6.38x mass in one place and 5.6x mass in another they must be able to calculate mass from orbital physics? so you'd need a rocket 6x the size of SaturnV or whatever they are using for Artemis to escape it and most of that rocket is to lift the weight of the fuel for said rocket so it might be physically impossible to build such a creature at current level of tech (might be yet another angle to "why no ETs" unless they are WAY more advanced)

deleted 3mo ago
[deleted]
inigyou 3mo ago
√(G × mass÷radius) [escape velocity] = v_e × ln(m_0 ÷ m_f) [Tsiolkovsky] Impossible to tell how much extra mass you need but it's exponential. Adding a unit of v_e [effective exhaust velocity] to escape velocity means you need 2.717 times as much fuel in an ideal rocket. Earth escape velocity is 11000m/s ignoring atmosphere (which is not ignorable). If the new planet is 6x mass and 2x radius then √3 times escape velocity (about 1.73) would be about 8000m/s extra velocity which is about 3 times a random v_e which means you need about a 25 times bigger rocket. Ignoring the denser atmosphere which makes it even worse.
- shagie 3mo ago
  From the archives ... How much bigger could Earth be before rockets wouldn't work? https://space.stackexchange.com/q/14383 https://space.stackexchange.com/q/14383 Feb 3, 2024 https://news.ycombinator.com/item?id=39243303 https://news.ycombinator.com/item?id=39243303 And related... https://worldbuilding.stackexchange.com/questions/178131/what-is-the-maximum-size-of-a-planet-that-a-chemical-rocket-can-leave-it https://worldbuilding.stackexchange.com/questions/178131/wha... https://physics.stackexchange.com/questions/117347/when-a-planet-has-a-high-gravity-is-it-impossible-to-build-and-launch-a-success https://physics.stackexchange.com/questions/117347/when-a-pl...
  - Cthulhu_ 3mo ago
    Man I love this. > Up above 10g, something really interesting happens that is kind of a theoretical limit. The mass of the rocket reaches a measurable fraction of the mass of the entire planet it's launching from.
    - ck2 3mo ago
      one thing about physics, particularly astro-physics, is it's NEVER boring been diving into PBS-Space-Time and DrBecky past few years as distraction from the endless nightmare of the world we are living in this has been a great educational thread on HN too, I knew rockets would have to be bigger but didn't realize exponential and the 10g upper-limit also puts the "Mars sample return" mission into perspective, very difficult
      - shagie 3mo ago
        Playing with the trajectory browser at NASA is kind of neat. https://trajbrowser.arc.nasa.gov/traj_browser.php?NEAs=on&NECs=on&chk_maxMag=on&maxMag=25&chk_maxOCC=on&maxOCC=4&chk_target_list=on&target_list=Mars&mission_class=roundtrip&mission_type=rendezvous&LD1=2020&LD2=2040&maxDT=4&DTunit=yrs&maxDV=6.0&min=DV&wdw_width=-1&submit=Search#a_load_results https://trajbrowser.arc.nasa.gov/traj_browser.php?NEAs=on&NE... The "view" at the bottom has an animation of the various routes. They're all EM-ME though... If you pick some other interesting targets you can get EVJS or something like that (launch from Earth, Gravity assist at Venus, Gravity assist at Jupiter, rendezvous with Saturn)... and then the trajectory animations are much more interesting.
- hgoel 3mo ago
  Don't these estimates assume launching from the surface, fully via rocket? On Earth, having air breathing stages to gradually build up speed, or using other launch mechanisms, isn't worthwhile because rockets are more cost effective here, but those tradeoffs change if you're on a planet with higher gravity and a denser atmosphere.
  - dogma1138 3mo ago
    You still need to get to escape velocity that doesn’t change the delta v required does but not by that much you are looking at 5-10%. Maybe a bit more if the atmosphere is really really thick. Unless you skip chemical rockets altogether there is a pretty hard cap on how much bigger a planet can be than earth before a space capable civilization becomes almost impossible.
- TheOtherHobbes 3mo ago
  Perhaps that's why they're attempting to build a starship out of helium.
  - ck2 3mo ago
    or it's all the "pollution" leaking from their fusion reactors ;-)

by ck2 3mo ago

deleted 3mo ago

[deleted]

inigyou 3mo ago

√(G × mass÷radius) [escape velocity] = v_e × ln(m_0 ÷ m_f) [Tsiolkovsky] Impossible to tell how much extra mass you need but it's exponential. Adding a unit of v_e [effective exhaust velocity] to escape velocity means you need 2.717 times as much fuel in an ideal rocket. Earth escape velocity is 11000m/s ignoring atmosphere (which is not ignorable). If the new planet is 6x mass and 2x radius then √3 times escape velocity (about 1.73) would be about 8000m/s extra velocity which is about 3 times a random v_e which means you need about a 25 times bigger rocket. Ignoring the denser atmosphere which makes it even worse.

shagie 3mo ago
From the archives ... How much bigger could Earth be before rockets wouldn't work? https://space.stackexchange.com/q/14383 https://space.stackexchange.com/q/14383 Feb 3, 2024 https://news.ycombinator.com/item?id=39243303 https://news.ycombinator.com/item?id=39243303 And related... https://worldbuilding.stackexchange.com/questions/178131/what-is-the-maximum-size-of-a-planet-that-a-chemical-rocket-can-leave-it https://worldbuilding.stackexchange.com/questions/178131/wha... https://physics.stackexchange.com/questions/117347/when-a-planet-has-a-high-gravity-is-it-impossible-to-build-and-launch-a-success https://physics.stackexchange.com/questions/117347/when-a-pl...
- Cthulhu_ 3mo ago
  Man I love this. > Up above 10g, something really interesting happens that is kind of a theoretical limit. The mass of the rocket reaches a measurable fraction of the mass of the entire planet it's launching from.
  - ck2 3mo ago
    one thing about physics, particularly astro-physics, is it's NEVER boring been diving into PBS-Space-Time and DrBecky past few years as distraction from the endless nightmare of the world we are living in this has been a great educational thread on HN too, I knew rockets would have to be bigger but didn't realize exponential and the 10g upper-limit also puts the "Mars sample return" mission into perspective, very difficult
    - shagie 3mo ago
      Playing with the trajectory browser at NASA is kind of neat. https://trajbrowser.arc.nasa.gov/traj_browser.php?NEAs=on&NECs=on&chk_maxMag=on&maxMag=25&chk_maxOCC=on&maxOCC=4&chk_target_list=on&target_list=Mars&mission_class=roundtrip&mission_type=rendezvous&LD1=2020&LD2=2040&maxDT=4&DTunit=yrs&maxDV=6.0&min=DV&wdw_width=-1&submit=Search#a_load_results https://trajbrowser.arc.nasa.gov/traj_browser.php?NEAs=on&NE... The "view" at the bottom has an animation of the various routes. They're all EM-ME though... If you pick some other interesting targets you can get EVJS or something like that (launch from Earth, Gravity assist at Venus, Gravity assist at Jupiter, rendezvous with Saturn)... and then the trajectory animations are much more interesting.
hgoel 3mo ago
Don't these estimates assume launching from the surface, fully via rocket? On Earth, having air breathing stages to gradually build up speed, or using other launch mechanisms, isn't worthwhile because rockets are more cost effective here, but those tradeoffs change if you're on a planet with higher gravity and a denser atmosphere.
- dogma1138 3mo ago
  You still need to get to escape velocity that doesn’t change the delta v required does but not by that much you are looking at 5-10%. Maybe a bit more if the atmosphere is really really thick. Unless you skip chemical rockets altogether there is a pretty hard cap on how much bigger a planet can be than earth before a space capable civilization becomes almost impossible.
TheOtherHobbes 3mo ago
Perhaps that's why they're attempting to build a starship out of helium.
- ck2 3mo ago
  or it's all the "pollution" leaking from their fusion reactors ;-)