[HN Gopher] How much bigger could Earth be before rockets wouldn...
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How much bigger could Earth be before rockets wouldn't work?
Author : trashtensor
Score : 146 points
Date : 2024-02-03 18:36 UTC (4 hours ago)
(HTM) web link (space.stackexchange.com)
(TXT) w3m dump (space.stackexchange.com)
| trashtensor wrote:
| The post about the 1.55R[?] planet made me curious and I thought
| this was an interesting discussion
| exe34 wrote:
| That's very good to hear, what specifically did you find
| interesting?
| trashtensor wrote:
| Honestly the whole thing - this is a topic I don't know very
| much about and the question and answers were fascinating to
| read.
| starttoaster wrote:
| Please don't take this as rude, but speaking more plain and
| directly than the previous person you replied to: I think
| they were politely telling you to come up with more
| substance in your comment.
|
| The culture on Hacker News has historically been along the
| lines of, "if your comment is not more insightful than
| providing an upvote on the post, withhold it and leave an
| upvote instead." This was on the "welcome" page when
| signing up for a Hacker News account, and can be found
| here: https://news.ycombinator.com/newswelcome.html
|
| See:
|
| > The most important principle on HN, though, is to make
| thoughtful comments. Thoughtful in both senses: civil and
| substantial. ... The test for substance is a lot like it is
| for links. Does your comment teach us anything?
|
| I think Hacker News comments tend to miss the mark on this
| test more often than not, to be fair. But it's perhaps
| worth at least politely pointing out when the substance
| test falls as far as, "this was essentially my approval of
| the more substantial parts of the post."
|
| The irony of my comment is that I'm adding nothing to the
| discussion, myself. But hopefully there's some small value
| in reinforcing Hacker News' social ideals.
| trashtensor wrote:
| I mean I made the comment because the field is there when
| submitting the post, I didn't realize it was going to end
| up as an independent comment. Maybe the HN devs need to
| make it clearer what will happen if you fill out that
| field when a link is provided.
| Dylan16807 wrote:
| Oh definitely. I didn't even realize you were the person
| that submitted it.
| bell-cot wrote:
| If you have a sufficiently tall* first stage, and use hot
| staging, then you can make it work on even on an extremely large
| Earth.**
|
| *First stage may need to extend well above the atmosphere.
|
| **No, that's for-sure not a Randall Munroe book in my hand.
| amluto wrote:
| I'm trying to decide whether you're describing a rocket or a
| space elevator. If you build a tower that extends to somewhere
| near geostationary orbit, you can pretend it's a rocket stage
| delivering a delta V of zero, and you can "hot stage" a tiny
| little stage off the top, and voila, you're in orbit.
|
| Of course, once you've managed to build this, the rockets are
| basically optional. :)
| defaultcompany wrote:
| This is the same way you make a train capable of traveling from
| Los Angeles to New York in 1 second. A sufficiently long train.
| mike_hock wrote:
| I think the speed of sound in steel (or whatever the train is
| made out of) is slower than that.
|
| But I guess you could cheat by having multiple engines along
| the way that all accelerate in lockstep.
| contravariant wrote:
| Or you could just put the engine in the front. There's no
| rule a train can't arrive before leaving after all.
| defaultcompany wrote:
| Related theoretical question for those who are of the
| physics mindset - if I had a long (very long like 1 light
| minute long) bar of metal and I pushed on one end, I'm
| assuming the other end would not move instantaneously
| because that would imply some part somewhere inside the bar
| was moving faster than the speed of light. So I'm assuming
| that the bar would just compress slightly and for a period
| of time in between when I pushed on one end and when the
| other end moved the bar would be slightly shorter. That's
| fine if that's the case.
|
| But what if the thing I push on is a quantum particle? Does
| this same thing happen at the smallest scales? If one end
| of a quark is pushed on does the other end move
| instantaneously or is there a small(!) delay?
|
| Probably the answer is just "that's not how quarks work"
| but I've always been curious.
| addaon wrote:
| > If one end of a quark is pushed on does the other end
| move instantaneously
|
| Quarks don't have an "other end." To the best of our
| knowledge, particles are points.
| evilduck wrote:
| On your long bar, the push propagates at the speed of
| sound in the material. Look up Slinky drops on YouTube.
| Taek wrote:
| You could just perch the first stage at the top of a
| sufficiently tall mountain.
| cratermoon wrote:
| Could, but at least on Earth the difficulties outweigh the
| gains enough to make it too expensive.
| https://youtu.be/4m75t4x1V2o?si=6FzbQYrLtl7Zfe0J&t=157
| chipweinberger wrote:
| ** And assuming sufficient propellant exists on your planet :)
| ivanjermakov wrote:
| You just need to launch off of the highest point on the planet!
| (Kerbal Space Program approved).
|
| We don't talk about ground logistics though.
| Sharlin wrote:
| You're going to have a hell of a difficult time trying to
| construct anything that tall on a high-g planet. The taper
| ratio between the base and the top would have to be enormous -
| likely a sizeable fraction of the radius of the planet! Though
| I guess it would have to be anyway so you have somewhere to
| attach all those first-stage engines...
| jstanley wrote:
| If you're allowed to build it tall enough, just don't even
| light the first stages. Launch the final stage directly from
| a high enough altitude that it can escape on its own.
| Sharlin wrote:
| Well, yeah, but building something tall enough to reach the
| synchronous orbit is impossible even on Earth, there's no
| material with even a thousandth of the compressive strength
| required. Space elevators are only possible because they're
| tensile structures and the "bottom" that supports the
| weight of the entire structure is up there in a low gee
| environment.
|
| Remember that just getting outside the atmosphere is the
| almost trivial part of rocketry compared to the problem of
| having to then accelerate to >= orbital speed fast enough
| to not fall down!
|
| And anyway you'd have to dismantle the planet to build your
| launch tower, which I guess would solve your problem, in a
| fashion. Though - whatever you turned your planet into
| would just have an annoying tendency to rapidly collapse
| back into a ball.
| bloopernova wrote:
| Is there an equivalent to the Drake equation that includes a
| factor that describes planets small enough to escape?
|
| Very depressing to me to think about how vanishingly rare smart,
| spacefaring life might be. But on the flipside of that, there may
| be a little corner of the universe where multiple spacefarers
| contemporaneously live within a few light years of each other.
| That might be cool from a space opera point of view but it'd
| probably end up being dominated by a space fascist enslaving
| everyone.
| nathanaldensr wrote:
| Not all authoritarians are fascist.
| jurynulifcation wrote:
| Such an odd correction, can we get this dude on a watchlist?
| Terr_ wrote:
| If anything, it should be the opposite: The people who _don
| 't_ know history--such as the difference between an
| absolute monarchy or a tyrannical theocracy or a modern
| fascist state--are _more_ likely to blithely want something
| terrible in the near future.
| Terr_ wrote:
| P.S.: I realize this thread already wandered quite far
| from the rocket equation, but I'm going to lean on the
| principle that quoting Deus Ex (2000) is always nice:
|
| > [Be Safe: Be Suspicious] How can you tell who might be
| a terrorist? Look for the following characteristics:
|
| > * A stranger or foreigner.
|
| > * Argumentative, especially about politics or
| philosophy.
|
| > * Probing questions about your work, particularly high-
| tech.
|
| > * Spends a greater than average amount of time on the
| Net.
|
| > * Interests in chemistry, electronics, or computers.
|
| > * Large numbers of mail-order deliveries.
|
| > * Taking photographs of major landmarks.
|
| > And those are just a few. If you're suspicious, then
| turn them in to your local law enforcement for a thorough
| background check. Better safe than sorry. You and your
| neighbors will sleep more securely knowing that you're
| watching each other's back.
| dTal wrote:
| There was nothing to imply that the original comment held
| any misconceptions about the difference between fascism
| and authoritarianism (nor is it especially relevant in
| context) so the rush to "correct" it signals a somewhat
| disquieting obsession, hinting perhaps at an unusually
| _personal_ investment in the fine distinction between
| fascism and authoritarianism.
| black_puppydog wrote:
| As unexpected this was to read for me as well, it's
| actually good to keep in mind.
|
| Because someone saying "but I'm not fascist" in a political
| debate may well be _technically_ right, and still be
| authoritarian.
| HeatrayEnjoyer wrote:
| They didn't say space authoritarians, they said space
| fascists.
| szundi wrote:
| Few lightyears is something like a hundred stars. Probably zero
| chance to nurture more than one unprobable life.
| Retric wrote:
| The upper limit on the size of the universe is infinite. So
| we can't exactly rules stuff out for simply being improbable.
| HeatrayEnjoyer wrote:
| Rockets are not the only way off a planet. If humans had spent
| space program amounts of resources on railguns or another
| method of locomotion there's real possibility it would have
| been successful too.
|
| Rockets are most convenient for Earth's variables so engineers
| optimized for them.
| choilive wrote:
| Railguns also become much harder in a larger gravity well.
| Bigger planets generally have thicker atmospheres as well.
| Your payload will end up disintegrating at the velocities
| required even on Earth.
| avar wrote:
| The Earth is larger than Venus, but its atmosphere is 90
| times denser than ours.
|
| There's a lot more variables that just gravity.
| contravariant wrote:
| Balloon stage followed by a rail gun might work.
| bell-cot wrote:
| The Galactic Emperor is a _monarchist_ , thankyouverymuch! And
| It treats all of Its loyal subjects quite well, with no
| discrimination against the water-based ones. Vs. if you have
| the misfortune to visit the Andromeda Galaxy...
| hinkley wrote:
| It's easier to build a space elevator for a low gravity planet as
| well. So if some day we find a species living on a heavy earth,
| even throwing them a rope may be difficult.
|
| Though I don't suppose we'll be visiting any aliens with chemical
| rockets regardless. We don't have that kind of patience.
| crazygringo wrote:
| Since it's barely mentioned in the answers, and was my first
| thought -- nuclear thermal rockets are something to think about
| too, at least in theory:
|
| https://en.wikipedia.org/wiki/Nuclear_thermal_rocket
| ravi-delia wrote:
| Not so much for takeoff! Most rocket designs better than
| chemical rockets trade off thrust for specific impulse. That's
| an improvement in orbit, since delta-v is delta-v. But imagine
| a 10kg rocket- it's receiving ~100N of gravity. If your engine
| doesn't put out 100N of thrust you'll just sit there on the
| launch pad. As you pick up speed you no longer have to deal
| with that (after all, LEO has basically the same gravity and
| doesn't have to burn against gravity at all) but when you're
| launching off something other than a point mass, some of your
| thrust has to go towards ensuring you don't hit the planet, or
| you will not into space today.
|
| The practical designs we have for NTRs are solid core, which
| after long effort got up to a thrust to weight ratio of 7:1,
| meaning they could in principle carry up to 6 times their
| weight and accelerate up in Earth's gravity rather than down.
| Chemical rockets can get 70:1. No one ever had plans to use
| NTRs in lift platforms- instead they could serve as more
| efficient upper stage engines, for orbit-orbit transfer burns
| and the like. In principle there are engines which are
| technically NTR and offer much better performance, but no one's
| ever gotten a working prototype. Also you probably wouldn't
| want to launch with an open cycle rocket, since the open part
| describes how the radioactive fuel is ejected out the rear.
| Unfortunately, with the technology we have, we have to make
| tradeoffs between efficiency and thrust. For the lift stages
| chemical rockets are, for now, unrivaled.
|
| (Unless of course your nuclear propulsion is of the more, shall
| we say, entertaining variety. Project Orion has its
| proponents...)
| lumost wrote:
| When discussing potential alien civilizations, one can't
| discount the existence of civilizations which exist on
| substantially more radioactive planets.
|
| If the background radiation of earth was 100x higher, would
| we care about an Orion launch? Or a small nuclear exchange...
| Dylan16807 wrote:
| The more fuel you have to pile onto the rocket, the less the
| weight of the engine matters.
|
| Using the chart in the accepted answer, launching with
| chemical engines takes 50 thousand tons at 3x gravity and 3
| million tons at 4x gravity.
|
| Now consider a theoretical engine that has a 7:1 thrust to
| weight ratio at 1G but sips fuel. Take a 25 ton engine, strap
| 10 tons of fuel to it and 1 ton of payload. Watch it go to
| orbit on a single stage.
|
| A real NTR doesn't save nearly as much fuel, but it can still
| be useful in certain ranges.
| adolph wrote:
| In _Project Hail Mary_ one of the exoplanets is 8.45 Earth masses
| and the residents are able to attain space flight.
|
| https://www.reddit.com/r/ProjectHailMary/comments/s5n7j4/eri...
| namrog84 wrote:
| Wonderful book! One of my all time favorites. Though just cause
| they did in a book doesn't validate the science of doing it.
| patrickwalton wrote:
| Fascinating. This may weigh down the Drake equation, particularly
| in reducing the average time civilizations survive on planets
| with high gravity because their ability to become multiplanetary
| and survive great filters is limited.
| anonymouskimmer wrote:
| One of the biggest hypothetical great filters is massive war.
| Higher engineering requirements for rockets (or even simple
| projectiles such as cannons or arrows) would set limits on the
| rate of increase of warfare technology. It's possible other
| means of diplomacy would advance at sufficient speed to preempt
| population annihilation from global war.
|
| I'm curious what effect an increase of gravity may have on
| heavier-than-water displacement craft (canoes and other modern
| boats). I think probably none, since you're dealing with
| density, not weight. Except for any increase in density of
| early building materials and cargo/supercargo. But it's been
| long enough from physics I'm unsure.
|
| I think atmospheric density is more dependent on magnetic field
| than gravity.
| datameta wrote:
| Buoyancy is indeed not affected by gravity, you're correct.
| bugbuddy wrote:
| This is technically wrong. Increase in gravity does affect
| buoyancy because air density changes with gravity. The
| reason is that the column of air above you is compressed by
| gravity. With very large gravity, all the atmosphere could
| be compressed down to possibly a few km.
| wolfram74 wrote:
| The thing I often think about is while the demands for an
| orbital class vehicle quickly become untenable, ICBM's stay
| viable for a lot longer. I don't know if MAD is more or less
| stable without the prospect of space exploration.
| hermitcrab wrote:
| >I think atmospheric density is more dependent on magnetic
| field than gravity.
|
| Atmospheric density is very much affected by gravity. I'm not
| sure magnetic field has any appreciable effect at all on the
| density of the earth's atmosphere. Why would it? The vast
| majority of the atmosphere isn't charged, so doesn't interact
| directly with magnetism.
| kebman wrote:
| This was a delightfully weird question! I'm sure it makes sense
| to calculate this before landing on another planet, though.
| simne wrote:
| Interest thoughts, but forgot one very practical calculation,
| unfortunately not easy to calculate. I say about shock-wave,
| which is known from practice on Earth, and for Earth limit rocket
| starting mass about 10k metric tonnes at sea level _.
|
| What it mean, shockwave from supersonic engine exhaust creates
| literally powerful pressure on construction, so on mentioned
| scale, nothing will withstand it long enough.
|
| If it is possible to create much stronger materials, as I know at
| the moment, is unknown and we cannot forecast.
|
| _ Sea level is important, because, at the moment I only remember
| TWO space rockets, which started from much different position,
| and high altitude (air) launch have very different atmosphere
| properties, which could be solution to shockwave problem (but
| have other limitations).
|
| https://en.wikipedia.org/wiki/Northrop_Grumman_Pegasus
| https://en.wikipedia.org/wiki/LauncherOne
| seiferteric wrote:
| I wonder if air breathing rockets would change this much.
| le-mark wrote:
| By the same token, a space faring civilization based from the
| Moon or Mars is much more feasible, and a large argument for
| colonizing either imo, also rarely discussed nowadays.
| jowea wrote:
| Spacefaring would be much easier if we were Martians but going
| up, then down to colonize Mars just to start launching rockets
| back up from there seems mostly pointless? Isn't it much easier
| to colonize some asteroids, like Mars' moons?
| jokoon wrote:
| more depressing is that a space elevator might never see the day,
| since the material required for it is difficult to make
|
| and even if it did exist, I have no idea how that thing would be
| put in place
|
| if I remember, in the mars trilogy, it's assembled in high
| altitude, low gravity, and then put in place?
|
| but gravity is lower on mars so rockets work better?
|
| anyway, for earth, assembling a space elevator in space, meaning
| putting tough cable in orbit, would require so many launches and
| would emit a lot of CO2 in the process.
|
| also the cable might be progressively thicker starting maybe at
| 1/3 of the distance, to bear the entire weight of the lower cable
| that is the most affected by gravity, while the rest of the cable
| would have a progressively centrifugal force away from earth to
| compensate, so maybe the cable would not need to be thick
| everywhere.
|
| maybe that question was already asked
| dtaht wrote:
| I have been pointing out for years that space elevators are
| feasible from a class of asteroid called a "fast rotator". They
| do not need to be very big either.
| nntwozz wrote:
| You can escape any gravity with teleportation, but it's easier
| said than done.
|
| Or maybe we're just a dumb civilization/species? Maybe it's also
| dumb to assume our intelligence is "normal".
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