Comment on Lmao

turdas@suppo.fi ⁨8⁩ ⁨hours⁩ ago

• gami@piefed.social ⁨7⁩ ⁨hours⁩ ago

  (Not a rocket scientist or mathematician, but I spent 100s of hours playing KSP RP-1)

  Just doing some estimates using data from the wikipedia page:

  The dV (delta-V) needed to get into low Earth orbit is around 9.4km/s.
  The dV for K2-18b might be around 19km/s, more than double that of Earth’s.

  It’s practically impossible I think, you would need such a massive launch vehicle. For double the dV, you would need exponentially more fuel assuming current rocketry tech (fuel+oxidizer tanks and engines). There wouldn’t be any single-stage or two-stage rockets that could do this. With a 3 or 4 stage rocket maybe? But you would be sending nearly 100% fuel off the launchpad with virtually zero payload.

  I tried to factor in:

  spoiler

  • Atmospheric drag - K2-18b’s atmosphere is quite dense with a huge radius:

  The density of K2-18b is about 2.67+0.52/−0.47 g/cm3—intermediate between that of Earth and Neptune—implying that the planet has a hydrogen-rich envelope. […] Atmosphere makes up at most 6.2% of the planet’s mass

  • Since the atmosphere is so thick and takes up a lot of mass, I’ve picked 500km as the low orbit altitude (comparing to Earth’s ~100km Karman line, it makes you appreciate how thin our atmosphere is ).

  • Rotational assist - I’m assuming it’s tidally locked since it orbits so closely to its star (33 day years), and so you wouldn’t get the assist from rotation like you do on Earth:

  The planet is most likely tidally locked to the star, although considering its orbital eccentricity, a spin-orbit resonance like Mercury is also possible.

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  • jballs@sh.itjust.works ⁨4⁩ ⁨hours⁩ ago

    Kerbal Space Program is such an amazing game that secretly teaches you physics.

    Image

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  • PabloSexcrowbar@piefed.social ⁨4⁩ ⁨hours⁩ ago

    With a denser atmosphere, wouldn’t that mean that you could get more lift from a traditional aerofoil than on earth? And if so, wouldn’t that technically make it easier to start from a high enough altitude that at least some of the gravity is mitigated?

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    • Wildmimic@anarchist.nexus ⁨2⁩ ⁨hours⁩ ago

      That’s what i was thinking - the dense atmosphere might even allow for platforms which are permanently suspended in the air like an inverse submarine, offsetting a large amount of needed fuel for a space launch

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  • matsdis@piefed.social ⁨5⁩ ⁨hours⁩ ago

    Check out the “tyranny of the rocket equation”.

    Or ask Randall Munroe How many model rocket engines would it take to launch a real rocket into space?

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  • Not_mikey@lemmy.dbzer0.com ⁨3⁩ ⁨hours⁩ ago

    tidally locked

    Wouldn’t that be a non starter for life? One side would be perpetually baked and the other would be frozen.

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    • makyo@lemmy.world ⁨1⁩ ⁨hour⁩ ago

      I guess there could be a planetary Goldilocks Zone in the dusk area

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      • Not_mikey@lemmy.dbzer0.com ⁨51⁩ ⁨minutes⁩ ago

        I figured that area would be full of extremely violent megastorms due to the heat differential.

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  • ColeSloth@discuss.tchncs.de ⁨5⁩ ⁨hours⁩ ago

    Build a large enough magnetic rail launcher and you could save shit tons of fuel. Get a ship doing 2000 mph before it leaves the ground and needs its rockets and you’ll have a pretty good head start.

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    • wonderingwanderer@sopuli.xyz ⁨4⁩ ⁨hours⁩ ago

      Could even take a scramjet to the upper layers of the atmosphere before kicking in the chemical propulsion

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• sylveon@piefed.blahaj.zone ⁨7⁩ ⁨hours⁩ ago

  It’s probably still a lot harder though. You’re not just heavier, but also slower which means you’ll spend more time fighting gravity. And all the extra fuel you bring for that makes the rocket heavier which means you need even more fuel to launch the fuel. Higher surface gravity likely means a thicker atmosphere too which is a big issue and a more massive body also has a faster orbital velocity. Although in this case the larger diameter might counteract that a bit because higher orbits have slower velocities.

  My point is that this would probably still be a lot harder than just building a 50% bigger rocket. If you’ve ever tried launching from Eve in Kerbal Space Program you know the pain. Although in that case you also have to fly the entire rocket there first which is its own challenge.

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  • crank0271@lemmy.world ⁨6⁩ ⁨hours⁩ ago

    you’ll spend more time fighting gravity

    Aw man. This is already a significant portion of my day.

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• Speculater@lemmy.world ⁨6⁩ ⁨hours⁩ ago

  It would actually be impossible for them to get to orbit using chemical rocketry, like we use. The could theoretically do it with nukes.

  Chemical rocketry limits

  Nuking your way to orbit

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• expr@piefed.social ⁨8⁩ ⁨hours⁩ ago

  I assume it’s not just about the gravity, but also the much larger radius of the planet would mean much larger distance from the surface, and thus much more fuel needed.

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  • potatopotato@sh.itjust.works ⁨7⁩ ⁨hours⁩ ago

    That’s how how…what???

    F = G * (m1 * m2) / r^2

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    • lemmyman@lemmy.world ⁨7⁩ ⁨hours⁩ ago

      Escape velocity does scale with (square root of) radius so its not a dumb thought.

      And I’m not a rocket surgeon but I could imagine earth rockets might be operating near some physical limits that make 50% (or whatever) infrasible.

      en.wikipedia.org/wiki/Escape_velocity

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      • Lojcs@piefed.social ⁨6⁩ ⁨hours⁩ ago

        Wikipedia says energy = GMm/r.

        if g=GM/r²,
        energy = mgr, proportional to r given g is constant.

        PREVIEW_HERE

        My previous comment was wrong, I derivated while integrating.

        PREVIEW_HERE

        PREVIEW_HERE

        PREVIEW_HERE

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    • expr@piefed.social ⁨3⁩ ⁨hours⁩ ago

      I stated an assumption and was contributing to the conversation. Even if that assumption is incorrect, there’s no need to be a dick about it.

      It seems like a larger atmosphere would result in a longer duration exposed to atmospheric drag, thus requiring more fuel to overcome it.

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  • turdas@suppo.fi ⁨7⁩ ⁨hours⁩ ago

    That’s, uh, not really how that works. A taller atmosphere would mean you have to go through more of it, but unless it’s not a terrestrial then the atmosphere won’t be that much taller.

    If it is a non-terrestrial planet, it’s unlikely anyone would be building rockets on there to begin with.

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    • Pyr_Pressure@lemmy.ca ⁨7⁩ ⁨hours⁩ ago

      If it has a higher gravity would the atmosphere technically be lower since it will squish up closer to the planet?

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      • Crackhappy@lemmy.world ⁨7⁩ ⁨hours⁩ ago

        And your username would also be relevant.

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  • degenerate_neutron_matter@fedia.io ⁨7⁩ ⁨hours⁩ ago

    You're sort of right. The change in distance from the surface is insignificant, but a spacecraft orbiting a bigger planet has to travel further with each orbit so its speed must be faster to avoid falling out of orbit, even if the gravitational acceleration at its orbital height is the same.

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• cogitase@lemmy.dbzer0.com ⁨7⁩ ⁨hours⁩ ago

  I’ve been wondering what a hypothetical perfect habitable planet for spacefaring would look like. Could you have one where a plane line the SR-71 Blackbird or an even less capable aircraft could simply “fly” into orbit? Or what about something Earth-like but with a flat plateau at 15,000 m where you could launch rockets from?

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  • turdas@suppo.fi ⁨7⁩ ⁨hours⁩ ago

    I think Mars, assuming you terraform it, would be pretty close to that on both counts. Space planes might still be difficult, but the delta V is much lower and Olympus Mons would pretty much sit above the atmosphere.

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    • YellowParenti@lemmy.wtf ⁨7⁩ ⁨hours⁩ ago

      Holy shit, I hadn’t considered that you could use Olympus Mons as a launch site cause it sticks so high up.

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      • turdas@suppo.fi ⁨7⁩ ⁨hours⁩ ago

        The best part about it is that it’s an extremely gradual slope completely unlike the mountain ranges on Earth, so you could haul stuff up there on trucks or trains easily.

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  • wonderingwanderer@sopuli.xyz ⁨4⁩ ⁨hours⁩ ago

    Classic planes require an atmosphere to generate lift. There’s an outer limit where that would be a viable mechanism, and on Earth it’s still far below LEO. Still too deep in the gravity well for ion thrusters to ve viable.

    It requires chemical rocket fuels to bridge that gap. Maybe someday fusion propulsion will break that limitations, but for now the best you can do is reduce the amount of fuel needed by flying to the upper atmosphere and reaching hypersonic speeds before kicking into rocket fuel propulsion.

    Then after orbital injection, switching to ion thrusters to move around, and solar sails for exiting orbit into interplanetary/lunar routes.

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• suodrazah@lemmy.world ⁨7⁩ ⁨hours⁩ ago

  Orbital speeds would be very hard to reach compared to low Earth orbits.

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• JasonDJ@lemmy.zip ⁨7⁩ ⁨hours⁩ ago

  Or Uranus.

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  • AugustWest@lemmy.world ⁨7⁩ ⁨hours⁩ ago

    Or your mother’s.

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