[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 : 410 points
Date : 2024-02-03 18:36 UTC (1 days 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
| 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.
| amelius wrote:
| So an object of zero volume will arrive in New York.
| eru wrote:
| Well, at that level, you are talking about clouds of
| probability or structures in the wave function or
| something like that.
| evilduck wrote:
| On your long bar, the push propagates at the speed of
| sound in the material. Look up Slinky drops on YouTube.
| jasonwatkinspdx wrote:
| When you tap on the bar it creates an acoustic wave that
| will propagate at the speed of sound in the material.
|
| For subatomic particles, the most intuitive way to think
| about things is to adopt the "fields are real" mindset.
| Here fields are the underlying reality, and particles are
| just a pattern of waves excited in the fields.
| Disturbances in all fundamental fields we've discovered
| propagate at the speed of light, and we have pretty solid
| reasons for believing no future discovery will contradict
| that, as it would break causality in a fundamental way.
| AlotOfReading wrote:
| The other answers are correct that it's an acoustic wave,
| but sometimes it helps to see a demo "proving" it:
|
| https://youtu.be/DqhXsEgLMJ0
| meindnoch wrote:
| There's no such thing as instantenously "pushing" on a
| particle. E.g. electrons can be accelerated by
| electromagnetic fields. If the field changes, the
| electron feels a force and is accelerated according to a
| = F/m (handwaving away relativity). When you
| macroscopically push against a rigid body, what happens
| at the particle level is your constituent atoms'
| electrons (and protons) interact with each other through
| the electromagnetic field.
| superposeur wrote:
| Interactions between particles such as quarks are
| mediated by _fields_ filling the space between them (such
| as electromagnetic field and gluon field). Ripples in
| these fields propagate at speed less than or equal to c.
|
| This is a classical picture, but the quantum picture is
| similar: evolution is generated by a local Hamiltonian
| constructed out of _field_ operators attached to every
| point of space.
|
| So, both classically and quantumly, relativity demands
| the existence of fields filling space to propagate causal
| influences at finite speed.
| JoeCortopassi wrote:
| "Speed of light" makes more intuitive sense when you
| think of it more as the speed of causality i.e. the
| fastest physical speed a cause can have an effect
|
| The speed of light in a vacuum happens to be the best
| representation of the maximum speed of causality
|
| Which also makes more sense why you can't do things like
| travel faster than light (your effect would precede the
| cause), and why two protons going past each other in
| opposite directions don't violate this law
| grecy wrote:
| Surely for a train to "travel" from LA to NY, it "starts"
| when the front of the vehicle passes a line in LA, and
| "finishes" when the front of the vehicle crosses a line in
| NY.
| imtringued wrote:
| Just build two fronts. The Germans did it with their ICE
| trains.
| 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.
| usrusr wrote:
| If you really have a lot of time for the project
| (starting early in the star's burn?), you might try using
| photovoltaics to move a lot of mass across the surface to
| ahead of where tides would accumulate, slowly speeding up
| the day/night cycle. The faster you spin, the flatter
| your geoid and you should probably stop accelerating
| before your entire equator region goes interplanetary.
| eru wrote:
| On earth, you could use active support to build your tall
| structures, then you don't need exotic super-materials.
| See eg https://en.wikipedia.org/wiki/Space_fountain
| 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.
| 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.
| kuchenbecker wrote:
| You can make a fairly strong statistical argument there are
| no spacefaring species in our galaxy. Even at 1% the speed
| of light a species could fully colonize the galaxy in 10
| million years, 0.1% the age of the universe.
|
| That we see nothing implies intellgent life is rare, short
| lived, or we're early in the age of the universe. For
| example, red dwarfs will last trillions of years compared
| to the sun's 5B lifespan.
| Retric wrote:
| 1% the speed of light is stupidly fast to colonize a
| galaxy. It only sounds reasonable in the same way startup
| claim if we can just capture 1% of the X market, while
| forgetting numbers much smaller exist.
|
| Let's assume super tech they can build that somehow
| allows vastly faster speeds than we can today ~120 km/s
| worth of DeltaV. Half that is spent slowing down so we're
| talking 0.02% c.
|
| Now let's assume half the time is spent in flight and
| half the time is spent colonizing stars before launching
| ships. So now we're down to 0.01% C. Suddenly 1 Billion
| years is a more reasonable estimate and even that takes
| super tech we don't have any idea how to build and
| assumes nothing fails.
|
| Several more advanced civilizations could be colonizing
| the galaxy today that are still 10 billion years from
| finishing.
| DennisP wrote:
| Fusion rockets would be able to get over 1% c. I think
| the theoretical maximum for fusion is around 10%.
| Retric wrote:
| Fusion rockets _might be able_ to get _a sufficiently
| large spacecraft_ over 1% c. If you're instead sending
| multiple generations of craft and waiting until each
| succeeds then it's 1 /n %c etc.
|
| Colonization using a massive habitat capable of extreme
| redundancy and asteroid mining could be a completely
| viable solution to colonization. But such a structure
| wouldn't be light.
|
| Hitting 1% c could very well take megastructures that a
| civilization would rather spend on redundant craft etc.
| We can dream, but we're nowhere close to being able to
| say what's actually viable.
| Intralexical wrote:
| > That we see nothing implies intellgent life is rare,
| short lived, or we're early in the age of the universe.
|
| Or it implies that not everybody's first instinct on
| seeing a vast galaxy is to try to take it over ASAP.
|
| If you want resources for quality of life-- Gas giants
| are a thing. If you're an explorer driven by curiosity,
| then take only samples, leave only memories, right.
|
| If you want money-- Century-long shipping times with
| civilization-scaled fuel costs tend to eat into profit
| margins.
|
| If you're worried about survival-- _genuinely_ worried
| about survival, on a level personal enough to motivate
| action, not just academically or for fun-- then the focus
| is on people you know and care about; all of those are
| here.
|
| In fact, I suspect the ones that see other stars and
| immediately think "Mine mine all mine!" probably have a
| higher chance of nuking themselves before they even get
| out of their star system.
| perilunar wrote:
| > That we see nothing implies intellgent life is rare,
| short lived, or we're early in the age of the universe.
|
| > > Or it implies that not everybody's first instinct on
| seeing a vast galaxy is to try to take it over ASAP.
|
| Or it may simply imply that intelligent life is good at
| hiding and does not want to be seen e.g. the dark forest
| hypothesis.
| eru wrote:
| That's why we typically only talk about the observable
| universe, which is very much finite.
| 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.
| the__alchemist wrote:
| Vacuum inside the tube?
| 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...
| boringuser2 wrote:
| >fascist
|
| This word is wild. Very interesting.
|
| It basically means "evil bad guy".
| somewhereoutth wrote:
| For the record (and the GP is using the term somewhat out of
| bounds), there are definitions of fascism that are rather
| tighter than 'evil bad guy':
| https://en.wikipedia.org/wiki/Ur-Fascism
| boringuser2 wrote:
| That's a stronger definition, but it's still weak because
| it isn't primary source material. This guy isn't a
| disinterested political historian. Imagine taking as
| definition the views of Giovanni Gentile on Marxism.
|
| I wouldn't call any of these groups "fascist" except in the
| loose pejorative sense
|
| The closest governments to actual Fascism that I can think
| of are governments like modern China or even Singapore, and
| I don't mean that in a pejorative sense. They're just very
| fascist in character, i.e. national interest, social
| welfare, strong-arming of capital, etc.
| wongarsu wrote:
| If I had to describe fascism in one sentence it would rather
| be "take social darwinism 'the strong rule the weak' and add
| a pragmatic leader who helps your group crush those who hold
| you back and rule everyone else"
|
| You know, exactly the kind of ideology you don't want a
| neighboring nation to have, no matter how you judge their
| actions morally
| boringuser2 wrote:
| That's definitely not even close to reality.
|
| National Socialism and Italian Fascism were all about the
| strong taking responsibility for the nation, in a
| philosophical sense.
|
| These were social welfare states that provided for the
| "people" far more than modern American liberalism (the
| standard operating produre for free economies of scale in
| the modern world), for example.
|
| Now, obviously I'm not saying this was a good system, but
| you're so far off base that it's ridiculous.
|
| The discourse around this complex historical movement is
| profoundly anti-intellectual. I know this because I grew up
| in the same society you did, the one where I also learned
| and used fascist as a pejorative without any further
| information.
|
| Now, as someone interested in having sophisticated ideas
| about systems, I'm not really in a mental state where I'm
| going to take appeals or emotion or shortcuts that shut
| down thought seriously. Regardless of the context, I still
| want to try and reflect reality as closely as possible in
| my minds eye.
|
| How many liberal, or even communist thinkers have you read?
| Personally, countless. As for fascists, almost none. It's a
| taboo, the works aren't translated, etc -- but it's there,
| and has intellectual underpinning that is more complex than
| mindlessly calling people you don't like "fascists".
|
| This can't be conscionable to any earnest intellectual.
| Imagine sitting here and tolerating people pejoratively
| calling people "communists". It's so stupid.
|
| Edit: I get in trouble here because there's something
| really interesting going on with controversial topics: all
| you need to do is make a choice and your model of reality
| is much more accurate than the presented model of reality.
| It's the easiest way to take Ws out of discourse, nobody
| has good ideas when the id has the reins.
| thriftwy wrote:
| > Imagine sitting here and tolerating people pejoratively
| calling people "communists".
|
| Also tolerated - by calling them "Russians".
|
| The logic goes, if there's nothing innately wrong with
| Communism then Russians (and, to extent, Chinese) must be
| up to blame.
| defrost wrote:
| That's pretty much it - if you check your history it's
| clear that neither Russia nor China adopted Communism in
| practice - they both went for authoritarian committe rule
| with power struggles as some kind of "neccessary" middle
| state while they work their way towards _Twue Communism_.
| thriftwy wrote:
| It was still way more socialist than what California or
| South European left wants.
|
| Soviet Union had socialized assigned housing, affirmative
| action policy and equalized wages right from the start.
| Still, it is largely ignored by socialist LARPers of
| today.
| defrost wrote:
| Australia was and likely still is more socialist than any
| US centralists wants.
|
| The 1900's Harvester agreement indexed the minimum wage
| to am eight hour work day with a week sufficient to feed,
| house, and clothe a worker and their family.
|
| The Whitlam years saw free university education for
| anyone that merited by high school (and equivilency)
| exams, health care has been universal - now with a split
| of both public and private, pharmacy companies are capped
| on their generics so that costs are reasonable,
| differences are picked up for those that can't afford
| medication (for almost all prescriptions), etc.
| idlewords wrote:
| I think mostly the Drake equation shows a lack of imagination
| about the forms life might take. Every time you add a term to
| it, you're baking in additional assumptions.
| eru wrote:
| Maybe. But you still have to explain the observation that the
| night sky is empty of signs of life.
|
| Keep in mind that human technology is pretty close to being
| good enough to detect not just foreign civilisations (via eg
| radio waves), but signs of life itself: studying the spectra
| of light reflected by exoplanets can tell you what chemical
| elements are in their atmosphere, so you can detect
| atmospheres that are far from chemical equilibrium, like
| earth's oxygen rich one.
|
| We emitted radio waves for only a few decades. But earth had
| oxygen for billions of years. So that widens the window of
| time of development that we could detect.
| mcmoor wrote:
| Both lack of imagination of what life can be, and supporting
| factors we take for granted. I dismiss drake equation as
| fully useless as it doesn't provide useful upper nor lower
| bound.
| eru wrote:
| > 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.
|
| Fascism already barely works on earth, and gets out-competed.
| See
| https://tvtropes.org/pmwiki/pmwiki.php/Main/FascistButIneffi...
| Similarly with slavery. (See
| https://www.econlib.org/library/Columns/LevyPeartdismal.html to
| go off an slight tangent.)
|
| In space, slavery is even less useful. That's mostly because
| humans are even less useful: we are already doing pretty much
| all of our useful space exploration with robots, and sending
| humans is just for bragging rights. Keeping space slaves alive
| costs you more than they ever could conceivably do for you.
|
| Of course, aliens might have biologies that are much better
| adapted to surviving in space, maybe?
| 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.
| hermitcrab wrote:
| I once got to briefly discuss project Orion with Freeman
| Dyson at a book signing. IIRC (it was a long time ago) he
| said that :
|
| - he thought it could be made to work
|
| - all big engineering projects (dams, skyscrapers etc) kill
| people
|
| - putting all that radiation into the earth's atmosphere
| couldn't be justified
| jowea wrote:
| I can't help but think that any species insane enough to use
| Orion drives in the first stage probably already found a way
| to blow itself up before it gets to that point.
|
| And maybe I'm taking Terra Invicta too seriously but maybe
| they would wait until they figure out nuclear fusion and have
| more options.
| SigmundA wrote:
| NTR have high specific impulse but relatively low power to
| weight, this makes them good in space and poor for getting out
| of the gravity well as discussed here. They are efficient at
| using reaction mass but not for power to weight.
|
| From the article:
|
| Early publications were doubtful of space applications for
| nuclear engines. In 1947, a complete nuclear reactor was so
| heavy that solid core nuclear thermal engines would be entirely
| unable[23] to achieve a thrust-to-weight ratio of 1:1, which is
| needed to overcome the gravity of the Earth at launch. Over the
| next twenty-five years, U.S. nuclear thermal rocket designs
| eventually reached thrust-to-weight ratios of approximately
| 7:1. This is still a much lower thrust-to-weight ratio than
| what is achievable with chemical rockets, which have thrust-to-
| weight ratios on the order of 70:1.
| jaywee wrote:
| NTR is a very inefficient use of nuclear fuel. What you want is
| a NSWR: https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket
|
| A true nuclear rocket. Just like a chemical rocket is a
| controlled explosion, NSWR is a controlled (cough) nuclear
| explosion.
| 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.
| PaulDavisThe1st wrote:
| You're suggesting that Andy Weir did not _science the shit_
| out of that?
| eru wrote:
| Well, he gave the aliens some hand-wave-y super-material
| called Xenonite.
| jonathanpglick wrote:
| Jazz hands!
| 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.
| antod wrote:
| Depends how compressible the fluid you're floating in is
| right? Note that's 'fluid' rather than just liquid.
|
| As you increase gravity, with fully compressible fluids the
| buoyancy scales the same as weight, you wouldn't sink
| lower. But with any incompressibilty you'd need to displace
| proportionally more (ie sink down more) to counter the
| increasing weight.
|
| (I think)
| Intralexical wrote:
| > But with any incompressibilty you'd need to displace
| proportionally more (ie sink down more) to counter the
| increasing weight.
|
| The water would also weigh more. Buoyancy is the force of
| the water around the volume you displaced being pulled
| down into that space, exerting pressure that pushes you
| up. So you'd float just as well.
|
| Actually, the compressible fluids would become denser,
| and make it easier for you to float (assuming you're
| relatively incompressible). At the extreme end, you could
| swim in pressure-liquified air (assuming you survive
| being crushed, of course).
| datameta wrote:
| Replies are accurate (more precise?), it is more true to
| say that buoyancy in a virtually incompressible liquid is
| not affected by gravity because even if gravity is
| increased, the water experiences it the same as the buoyant
| mass.
| 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.
| nine_k wrote:
| I would argue that situation is usually more stable with
| less secrecy, so a lack of spy satellites would not be
| beneficial.
|
| Absence of comm satellites would also help fragment the
| world and make the idea of a surprise attacks more
| enticing.
| Aeolun wrote:
| Not beneficial, but I imagine you'd just see a lot more
| spyplanes.
| nine_k wrote:
| Spy planes can be shot from the ground, or with another
| plane. This is closer to an act of hot war :(
|
| Orbital space (around modern Earth) is ex-territorial, so
| killing a spy satellite would be seen as an act of
| aggression, not legitimate defense. This holds back
| "kinetic action" in near-Earth space.
| eru wrote:
| > Orbital space (around modern Earth) is ex-territorial,
| [...]
|
| That's a historical accident of arrangements on earth (so
| less useful in the Fermi-paradox / filter debate), and
| could easily have gone differently. Ie air space could
| also have been seen as ex-territorial.
| wolfram74 wrote:
| Assuming something like air space territories in these
| other cultures, extending them out to infinity seems...
| tricky. Wouldn't it imply that the ownership of various
| celestial bodies changes with time?
|
| My recollection of how it evolved on Earth was the
| soviet's more asked forgiveness than permission and
| eisenhower basically shrugged and said "whatever, at
| least our spy satellites can go over you too"
| 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.
| slavboj wrote:
| Magnetic field protects from solar wind that strips
| atmosphere.
| outofpaper wrote:
| It's often said but with heavy evidence to yhr contrary
| e.g. Venus. Venue's negligible magnetic field is almost
| non-existent yet its atmosphere is many many times
| thicker than ours.
| idiotsecant wrote:
| In fact, earth actively loses material to space _because_
| is has a magnetosphere, polar outflow of oxygen for
| example.
| kijin wrote:
| Venus loses a lot of material, too.
|
| One of the reasons Venus still has a dense atmosphere is
| because its atmosphere is mostly composed of a relatively
| heavy compound, CO2, which is harder to lose than lighter
| gases like H2, N2, and O2.
| hermitcrab wrote:
| If you turned off the earth's magnetic field today, then
| presumably the atmosphere would be gradually stripped,
| away over millions of years. Similar to what happened to
| Mars. But it would not make any immediate difference to
| the density.
| AnimalMuppet wrote:
| Not immediate, no.
|
| But magnetic fields don't usually just switch off. If the
| planet didn't have one to begin with, then it probably
| doesn't have much of an atmosphere for long enough for
| advanced life.
| unsupp0rted wrote:
| War may be the exception, not the rule.
|
| Just because we're built for it doesn't mean other species
| will be.
|
| If evolving in a different environment, they might be built
| for cooperation. That is, in a certain environment the only
| species that can evolve enough to go interplanetary might be
| a species that learned to co-exist internally and externally,
| otherwise the environment would have kept them down.
| Intralexical wrote:
| > War may be the exception, not the rule.
|
| > Just because we're built for it doesn't mean other
| species will be.
|
| You heard of what chimps get up to? Ants? Microbes? They
| don't just have wars; They have raiding parties, take
| slaves, serve as battlefield medics, compete in
| intrafactional and interfactional rivalries that slowly
| boil over... Hell, even trees actively release toxins to
| try to kill other nearby plants.
|
| On a long enough timescale, war is almost certainly highly
| (and lethally) maladaptive.
|
| But in a non-post-scarcity environment with social contact,
| creatures whose bodies disagree with entropy tend to learn
| that violence is an effective tactic for taking others'
| calories/oil and nutrients/minerals.
|
| Maybe there's exceptions. I hope so, anyway.
| unsupp0rted wrote:
| All those species evolved on the same planet, with the
| same constraints.
|
| War works well for them, so they evolved to get better
| and better at it.
|
| But war is resource-intensive and costs lives. Lives are
| easily replaced on Earth.
|
| It's not hard to imagine a planet where going to war
| would be mutually assured destruction on a species level,
| even for ants and microbes.
| Jensson wrote:
| > It's not hard to imagine a planet where going to war
| would be mutually assured destruction on a species level,
| even for ants and microbes.
|
| That is very hard to imagine, how do you reckon that
| would be possible? Does the planet only support a couple
| of anthills and then all resources are consumed? How
| would ants even appear on such a planet?
| kolinko wrote:
| Interesting thought, but how would that work?
| Intralexical wrote:
| ....Could be something like extremophilic archaea here on
| Earth? Not sure how they treat each other, but they're
| usually quite friendly (beneficial, or harmless-- never
| pathogenic or parasitic) to us mammals- Something about
| branched separation in biochemistry and highly diverse
| ecological niches making resource competition less of a
| thing, I'd guess.
|
| But that's not really "mutually assured destruction on a
| species level", so much as more to gain by working
| together- Which honestly is better.
| unsupp0rted wrote:
| If the environment is so hostile that life keeps
| appearing, failing to find a foothold, then getting
| crushed by statistics, then if some resilient life does
| eventually develop, it might be able to survive in such a
| hostile environment only through internal/external
| cooperation or symbiosis.
|
| For instance, if two or more extremophiles evolved
| together but remained separate species. They might even
| require one another's contribution to successfully
| procreate. And successful procreation might be rare.
|
| That sort of life, if it evolved to consciousness, would
| be averse to any form of damaging competition.
|
| One poorly timed selfish move and the hostile environment
| wins: everybody dies.
|
| This cooperation imperative would be built into their
| biochemistry, same as war is built into ours.
|
| You'd probably still find insane or outlier members of
| their society, who are radically uncooperative or
| individualistic. But they would be rare and containable,
| otherwise their species couldn't exist.
| Intralexical wrote:
| > For instance, if two or more extremophiles evolved
| together but remained separate species. They might even
| require one another's contribution to successfully
| procreate. And successful procreation might be rare.
|
| > That sort of life, if it evolved to consciousness,
| would be averse to any form of damaging competition.
|
| Uh. That sounds like us. Our dependence on our
| mitochondria and chloroplasts to survive as microbial
| life, it turns out, did not translate into an aversion to
| war after we grew up as macroscopic life and everybody
| around us had their own endosymbionts too.
|
| Honestly I think you're going in the wrong direction with
| this. A crueler world results in crueler people; scarcity
| begets conflict. Maybe you could _technically_ create
| peace by simpy isolating everybody in some kind of
| desert-like environment, but if you want a Nash
| equilibrium and selection pressures favouring active
| prosocial cooperation, then I think what our own history
| of war, domestication, self-domestication,
| democratization, etc. shows is that you effectively need
| (amongst other things) an almost post-scarcity
| environment, where basic physical resources are no longer
| a constantly urgent limiting factor on life-- A techno-
| utopia with nuclear weapons and additive manufacturing
| has both much more to gain from cooperation and much more
| to lose from war than their less fortunate equivalent
| struggling just to survive.
|
| But then that goes back to my original reply to you:
| Anybody who evolves through that initial awkward phase of
| competition in fear of entropy is probably going to have
| violence as a part of that history, and part of
| themselves.
|
| ---
|
| Note that effectively post-scarcity environments _do_
| actually appear in nature now and then, and when they do
| appear, they _do_ sometimes result in apparently utopic,
| peaceful, and more empathetic societies. E.g. for a
| particularly stark example, see bonobos versus chimps.
|
| But as with many good things, it seems to usually be
| highly spacially/socially local, and temporally
| transient.
| exe34 wrote:
| If you can't shoot them, surely you can keep stabbing them
| until you develop bio-warfare? Then you can both go extinct
| and the filter keeps working.
| BuyMyBitcoins wrote:
| You may find the concept of Superhabitabilty interesting.
|
| The hypothesis suggests that larger planets with more mass and
| gravity than Earth would be more favorable to life. It's
| certainly possible that there is a lot more life out there on
| planets where getting into space is nearly impossible with
| conventional chemical rockets.
|
| We may be living on a comparatively barren rock, but the
| tradeoff of that we are actually able to get into orbit.
|
| https://en.wikipedia.org/wiki/Superhabitable_planet
| scotty79 wrote:
| Imagine visiting such planet. They'd think you are gods
| because, how did you get up there? But you can't really land
| because it would be one way trip for your tech.
|
| So you just hang around and talk with radiowaves, sending
| them pictures of their world from above they could never see
| otherwise.
| eru wrote:
| Well, you could send remotely controlled probes. And there
| might be some volunteers for a one-way trip, too. There are
| certainly volunteers for one-way trips to Mars right now,
| and by the time humanity would be an interstellar species,
| our population size would have gone up by several orders of
| magnitude; so even if volunteers are rarer as a proportion,
| they would be much more numerous in absolute terms.
| p-e-w wrote:
| No advanced civilization would conclude they are dealing
| with "gods" just because they see someone with presumably
| better technology than themselves.
|
| In fact, if they are anything like humans, they have
| probably already realized that species that live on a
| lower-gravity planet could escape that planet using the
| same chemical reactions that are available to them also.
| unsupp0rted wrote:
| Rockets are out, but you could drop a space-elevator line
| down
| scotty79 wrote:
| Higher gravity means space elevator might be infeasible
| as well. It's barely feasible on Earth.
| Intralexical wrote:
| If you have the power to cross the stars, surely taking off
| from a steep gravity well wouldn't be a problem.
|
| But I do like the idea that you wouldn't be able to.
|
| So instead you slingshot your orbital craft past the
| planet, using its gravity well itself to build up speed--
| And you release a cable ahead of you, that swings down
| through the atmosphere to zero surface velocity at the
| point of your perigee, so your away team and their new
| friends can attach it to a glass elevator and be smoothly
| hoisted into space.
| Maxion wrote:
| > If you have the power to cross the stars, surely taking
| off from a steep gravity well wouldn't be a problem.
|
| Different problems entirely. You don't need a lot of
| thrust to get to high velocities. But you need a lot of
| thrust to leave a planets gravity well. On a planet, your
| thrust needs to win not only gravity, but also any
| atmospheric losses. E.g. on a 3g planet, you'd need
| thurst in excess of 3g's to leave.
|
| But to reach say 0.25c, a tiny ion engine over a long
| enough time would suffice. an engine that wouldn't even
| get you off of earth.
| Intralexical wrote:
| > But to reach say 0.25c, a tiny ion engine over a long
| enough time would suffice.
|
| Tsiolkovsky says otherwise, by a factor of over 10^663
| (not even counting relativity):
|
| https://www.wolframalpha.com/input?i=1%2Fe%5E%2874900km%2
| Fs%...
|
| ...Seriously. I tried to figure out just how much xenon
| you'd need to make that work. But you'd need to be able
| to store it in something like 15-dimensional space to
| even fit it within the diameter of the observable
| universe. And even if your ion engine and Hubble-scale
| fuel tank weighed less than the mass of the lightest
| quarks, the amount of propellant you'd need for it to
| reach 0.25c is still well over 10^600 times the combined
| mass of the entire observable universe, and would also
| collapse somewhere around 10^600 times the diameter of
| the observable universe into a single black hole:
|
| https://www.wolframalpha.com/input?i=2G%282.2MeV%2Fc%C2%B
| 2%2...
|
| Of course, this also shows the intent of my original
| comment: Energy density matters, and somebody packing
| enough to casually cross interstellar densities isn't
| going to struggle with a planetary gravity well unless
| Idk they're doing like a low-tech off-grid trend or
| something.
|
| Also, skyhooks!
| scotty79 wrote:
| Maybe light sail and beaming energy from the home planet?
| brucethemoose2 wrote:
| Is this really a big factor?
|
| Not being multiplanetary seems like the _least_ of our
| existential problems here on Earth, and will continue to be
| that way for awhile.
|
| At the same time, chemical rocket efficiency becomes totally
| irrelevant for a slightly more advanced civilization than us.
| nine_k wrote:
| You can't build a space elevator before getting to the orbit
| first.
|
| A jet engine capable of leaving a deep gravitational well
| must have a big ratio of thrust to weight. If a chemical
| rocket is too weak, a nuclear jet engine is the only
| remaining option. Would you be comfortable running it in the
| thick atmosphere of a densely inhabited planet?
| brucethemoose2 wrote:
| A civilization just a few decades ahead of us is
| (theoretically) almost unimaginable. Bio augmentation, true
| AIs, who knows what advances in fundamental physics
| knowledge... Just to start.
|
| What I'm saying is that whatever engineering and
| environmental limitations we currently perceive are
| probably irrelevant.
| eru wrote:
| > Not being multiplanetary seems like the least of our
| existential problems here on Earth, and will continue to be
| that way for awhile.
|
| That might be true, especially if your 'for awhile' talks
| about millennia at most.
|
| But it's an extremely relevant concern in the context of the
| Fermi paradox.
| awwaiid wrote:
| Yeah, but those evolved on high-gravity planets are smarter
| (and maybe stronger) since they too must calculate thrown-
| object trajectories but have to do so faster. Our brains use
| just enough energy, but no more, to keep us alive. Being
| smarter than we are would be a waste .... but if we HAD to
| think faster, we would evolve to match.
|
| So maybe they'd figure it out.
| eru wrote:
| Humans are (nearly?) the only animals that got really into
| throwing stuff, especially throwing stuff with heft and
| precision. (As a corollary: you can train seals to balance
| balls, and apes might through excrement; but only (some)
| humans can juggle.)
|
| If throwing things well had been much harder, perhaps no
| animal would have ever bothered?
| GuB-42 wrote:
| The article limits itself to chemical rockets. They work well
| enough on Earth so that's what we are using, but we can do
| better. Replace chemistry with nuclear, and use the air in the
| atmosphere as a reaction mass. On Earth, that would cause more
| problems than it would solve, that's why we don't do that
| despite having the tech to do it. But on a higher gravity
| planet it may be what we would do. Harder, but not impossible.
|
| It is interesting how we got nuclear technology that would
| allow for way more capable rockets at the same time we
| perfected chemical rockets enough to get to orbit. So much that
| we could have been able to escape a 10g planet almost as soon
| as we have escaped Earth.
| eru wrote:
| If you want to use nuclear technology to get to orbit on a
| planet with an atmosphere, you pretty much have to use bombs.
| See https://en.wikipedia.org/wiki/Project_Orion_(nuclear_prop
| uls...
|
| More conventional nuclear propulsion has similar trade-offs
| to an ion drive: great for long distance travel when you are
| already in space, but useless to get off a planet.
| Intralexical wrote:
| > [...] ability to become multiplanetary and survive great
| filters is limited.
|
| So, the known quantities that term refers to tend to be steps
| more like planetary habitability and abiogenesis, which might
| prevent complex life from getting established in the first
| place. But it sounds like you mean some kind of cataclysmic
| event which wipes out an already existing industrialized
| civilization.
|
| What, specifically, are the "Great Filter" scenarios which
| being multiplanetary is actually supposed to help with?
|
| Supernovas? GRBs? Simple asteroid impacts? You can usually see
| those coming from millions of years in advance. And surely
| building a couple layers of solar sail material to shield the
| planet, stockpiling ozone generators to repair the damage
| quickly, gently nudging the asteroid, or simply digging some
| holes/eating a gas giant and weathering the storm, would be
| easier _and_ save vastly more people than establishing a
| sizable population in another star system.
|
| The other "Great Filter" idea which seems to be memetically
| adapted for proliferating in modern discourse is the idea of a
| locust-like swarm of technologically advanced aliens that kill
| any industrial civilizations which do emerge. But in that case,
| presumably settling multiple star systems is the _opposite_ of
| what you 'd want to do; You'd be better off quieting your
| emissions to shrink your footprint than spreading even more
| biomarkers around at high blueshift.
|
| Frankly, I think this entire idea of needing to "become
| multiplanetary and survive great filters" is more mainstreamed
| now largely due to one specific individual fancying himself a
| savior of humanity. SpaceX builds interesting machines, but I
| liked it better when it was people like Sagan, Aldrin, and
| Zubrin getting excited about Mars.
|
| But even then, I'm not sure if the idea of colonizing more
| planets in order to survive planet-scale catastrophes really
| jives with how people think-- Plenty of us already live within
| splash radius of the Pacific Ring of Fire, Yellowstone Caldera,
| tornadoes, tropical cyclones, land below sea level... and yet
| there's no billion-dollar emergency backup cities in Antarctica
| to "make San Francisco into a multicontinental city and survive
| great quakings".
| ikari_pl wrote:
| > surely building a couple layers of solar sail material to
| shield the planet, stockpiling ozone generators to repair the
| damage quickly, gently nudging the asteroid, or simply
| digging some holes/eating a gas giant and weathering the
| storm, would be easier
|
| The assumption is that we have a problem getting _anything_
| off the planet. All these would require some good rocket
| engineering.
|
| I agree with everything else here a lot.
| njarboe wrote:
| Maybe if you can't launch rockets into orbit with chemical
| rockets then your path to becoming multiplanetary would
| probably be quicker rather than slower. You would have to
| develop nuclear rockets (quite doable) and then have a much
| better tech for exploring the solar system and beyond.
| 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
| hermitcrab wrote:
| Vacuum-dwelling spherical cows are immune to shockwaves.
| 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, or Mars' moons?
| tooltower wrote:
| The idea is that you can pick up cargo, fuel, and rocket-
| building minerals directly from Mars.
| 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.
| eru wrote:
| You can use active support to make a space elevator without
| super-materials. See
| https://en.wikipedia.org/wiki/Space_fountain
| 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".
| SilasX wrote:
| Um doesn't balloon assistance become increasingly effective in
| that case? Use your plentiful surface energy blow up a balloon
| and float it up past the upper atmosphere.
|
| But they explicitly exclude that from this question:
|
| >For our purposes, let's not explore alternative or hybrid launch
| systems or boost systems (such as balloons, planes, laser beams,
| space elevators etc.). Just stick to chemical propellant rockets.
| PeterisP wrote:
| Floating a balloon to the upper atmosphere doesn't make a
| meaningful difference in escaping the planet, it saves you a
| few percent of the energy but you still have to do most of the
| work to bring it up to escape velocity. Going to space isn't
| about getting high, it's about getting fast.
| eru wrote:
| Though you might be able to get past a substantial portion of
| the atmosphere, and that would help you get past a lot of
| sources of friction.
|
| Getting off a planet, even a heavy one, that doesn't have an
| atmosphere would be relatively easier, because you could
| 'just' build very long, flat rails to accelerate along.
| m3kw9 wrote:
| Unless you are in a black hole you can get off any planet
| theoraatally
| PaulDavisThe1st wrote:
| If Randall Munroe's name is not on the answer, it's not the
| answer.
| zuminator wrote:
| On a something like a gas giant with a hydrogen atmosphere
| surrounding a rocky core, would it be possible for the vessel to
| be hydrogen breathing until it reaches the edge of space and then
| ignite a stage to carry it out of the gravity well? Or if a
| nitrogen or CO2 atmosphere is thick enough, to fly
| aerodynamically or even float until it reaches a point where the
| gravity is appreciably lower than at surface level?
| sandworm101 wrote:
| No. To float you would need a gas lighter than hydrogen, which
| isnt a thing. And powered flight (wings) without oxygen would
| be trickey, requiring more of a rocket motor than an aeroplane
| engine.
|
| Yes, you could use a balloon filled with vacuum, but lifting
| something the size of an orbital rocket in a hydrogren
| atmosphere would require a vacuum chamber at least the size of
| a city, possibly the size of a small state. It would probably
| be easier to build a tower.
| zuminator wrote:
| Sorry if I wasn't clear, but I brought up the question of
| floating with respect to nitrogen/CO2 atmospheres (thinking
| Titanlike or Venuslike) not hydrogen.
| lisper wrote:
| It's a moot point, but I still want to point out that "a
| gas lighter than hydrogen" is a thing: it is simply
| hydrogen at a higher temperature, i.e. a hot-hydrogen
| balloon, analogous to a hot-air balloon.
| thfuran wrote:
| Well, maybe if there's no oxygen nearby.
| lisper wrote:
| If you have a planet with a hydrogen atmosphere, it's a
| pretty good bet there is no (free) oxygen nearby.
| thfuran wrote:
| Fair point
| simcop2387 wrote:
| I do not want to ride in a hot hydrogen balloon :)
| eru wrote:
| Why not? Hydrogen inside a hydrogen atmosphere is
| perfectly safe.
|
| It's hydrogen inside an oxygen atmosphere that's the
| problem.
|
| (So on Jupiter, you wouldn't want to ride in an oxygen
| balloon, ie you wouldn't want to ride in a hot air
| balloon there.)
| pottspotts wrote:
| How do you "fill something with vacuum"?
| xyzzy123 wrote:
| With a vacuum pump ;)
| mr_toad wrote:
| > No. To float you would need a gas lighter than hydrogen,
| which isnt a thing.
|
| The atmosphere gets denser further down. You just need a
| negative pressure vessel, or to heat the hydrogen, like a hot
| air balloon. At 1 (Earth) atmospheric pressure the gravity of
| most Gas giants is quite low.
| jasonwatkinspdx wrote:
| The basic physics of
| https://en.wikipedia.org/wiki/SABRE_(rocket_engine) have been
| vetted, which is an air breathing rocket engine. I don't know
| how much difference trying to liquify H2 vs O2 is though.
| jaggederest wrote:
| The oxygen is the majority of the mass (but not volume!) in a
| stoichiometric hydrogen engine, so the mass savings would be
| less I think. The RS-25 (space shuttle main engine) runs at a
| higher fuel ratio. Should work very similar to SABRE in
| general - the concept of a high speed atmosphere collector
| and precooler is pretty universal to any gas, and hydrogen
| has an extremely high heat transmission rate.
| mr_toad wrote:
| On a smaller gas giant you could build a floating platform
| (like a giant zeppelin) and launch from there. Because gas
| giants are so large the "surface" gravity at the altitude such
| a platform would be floating at is not as high as you might
| expect. On Uranus and Neptune it's actually lower than 1G.
|
| However, past Jupiter size the mass keeps increasing while the
| radius doesn't, so even from a floating platform you're
| contending with multiple G's.
| wongarsu wrote:
| On rocky planets gravity doesn't get much lower in the orbits
| we're concerned about. For example the ISS still experiences
| 90% of the gravity we experience at the surface. Reaching orbit
| is mostly about reaching a speed where the arc in which you are
| falling never intersects the surface.
|
| But you can absolutely use an aircraft to gain height and
| speed, and then launch a much smaller rocket from that aircraft
| (where the speed is the primary advantage, and is what rockets
| use most of their fuel for). This setup is used by Virgin
| Galactic's SpaceShipTwo. There is also Virgin Orbit's
| LauncherOne, which is a small rocket that launches from a
| modified Boeing 747. On Earth it's just about not worth the
| additional complexity, but on planets with stronger gravity but
| comparable access to powered flight this might be the preferred
| method of reaching space.
|
| One important factor might be the speed of sound. Subsonic
| flight is much easier for aircraft than supersonic flight. In
| an atmosphere with a much higher speed of sound, like say
| hydrogen, aircraft could reach much higher speeds and thus
| would be a much more advantageous launch platform for rockets.
| Assuming you already solved the issue of powering those planes
| of course.
| foota wrote:
| I've never really understood this, why is it easier for a
| plane to reach that speed than a rocket? Is it sort of just
| another rocket stage?
| orost wrote:
| An air-breathing jet engine doesn't need to carry oxidizer,
| which in a rocket is most of the propellant weight. It also
| has access to unlimited reaction mass, so it can be much
| more energy-efficient in producing thrust (it is more
| efficient to produce thrust by accelerating a lot of mass
| by a little, than by accelerating a little mass by a lot,
| but a rocket can't take advantage of this because it would
| need to carry all that extra mass. A plane can use ambient
| air for this purpose)
|
| This all adds up to a plane needing to carry many times
| less mass to gain the same altitude and speed as a rocket,
| at least within relatively dense atmosphere.
| eek2121 wrote:
| Ooh I absolutely did not click on the link yet, but I love this
| question!
| eek2121 wrote:
| (and the answers were just as amazing as I expected!)
| idlewords wrote:
| The converse of this was kind of an open problem in the early
| days of rocketry. Given the theoretical rocket concept, was there
| a propellant combination with sufficient exhaust velocity to make
| an orbital rocket practical? The answer was not immediately
| obvious, and there's a Goddard paper where he talks about just
| how big the rocket has to grow as you lower the propellant
| velocity to get equivalent performance. Eventually you're burning
| entire mountains of gunpowder just to get a few dozen miles up.
|
| It was a nice surprise (and a relief) to the early rocket
| pioneers to realize that we lived on a planet where gravity and
| chemistry would make orbital rockets possible. The rest was just
| engineering.
| thedanbob wrote:
| For anyone interested in the history of rocket propellants, I
| highly recommend "Ignition!" by John D. Clark[0]. It has plenty
| of chemistry if you're into that, but even if you're not (like
| me) it's an enjoyable read.
|
| [0]
| https://library.sciencemadness.org/library/books/ignition.pd...
| idlewords wrote:
| Seconded! That is such a wonderful book.
| borlanco wrote:
| Just one of dozens of amazing passages in this book (page
| 48):
|
| > " _... its density was a little better than that of the
| other acid, and it was magnificently hypergolic with many
| fuels. (I used to take advantage of this property when
| somebody came into my lab looking for a job. At an
| inconspicuous signal, one of my henchmen would drop the
| finger of an old rubber glove into a flask containing about
| 100 cc of mixed acid -and then stand back. The rubber would
| swell and squirm for a moment, and then a magnificent rocket-
| like jet of flame would rise from the flask, with appropriate
| hissing noises. I could usually tell from the candidate 's
| demeanor whether he had the sort of nervous system desirable
| in a propellant chemist.)_"
| ctrw wrote:
| I think it's a bimodal distribution. On the one hand you
| have the unflappable who just calmly watch what happens. On
| the other you have the far too flappable who is already out
| of the lab and making good time out of the building and the
| state.
| borlanco wrote:
| Exactly this. Either they have the right stuff, or they
| don't.
| ctrw wrote:
| Both are the right stuff.
| borlanco wrote:
| I meant what was shown in this movie [0]. In a nutshell,
| the ability to remain calm when the unexpected happens,
| to try to solve the problem, or at least to not make it
| worse.
|
| [0]
| https://en.m.wikipedia.org/wiki/The_Right_Stuff_(film)
| DennisP wrote:
| Although for some reactions, the latter is the
| appropriate reaction.
| refurb wrote:
| As a former chemist, I thought this book was a great example
| of "applied chemistry".
|
| The theoretical aspects are challenging enough. But then you
| realize just how difficult the practical application of the
| theory can be. Sure, a mixture of fuming nitric acid and
| hydrazine will produce enough propulsion, but how do you dump
| tons of it into an engine without it just exploding?
| ethbr1 wrote:
| The section on building high-precision detonation speed
| timing apparatuses (and occasional explosive deconstruction
| of same) made me realize how uncomfortably close
| "information we require" and "catastrophic consequences of
| collection that information" are in the field.
| graphe wrote:
| https://youtu.be/K0FLy2nI13E turbopump
| https://en.wikipedia.org/wiki/Turbopump
| lazide wrote:
| There are few things that get certain types of chemists and
| engineers excited like being able to find out - and not
| being in trouble with 'the bosses' if it explodes a few
| times along the way.
| signalToNose wrote:
| There is an excellent YouTube channel that explains the V2
| rocket in detail. Almost down to the last screw. Highly
| recommend.
| https://youtube.com/@RocketPlanet?si=DdgyQ8HFnswrZgZr
| rwmj wrote:
| His videos about the Steam Pot [0] and the mechanical
| system that scheduled each stage of the rocket [1] are
| extremely good:
|
| [0] https://www.youtube.com/watch?v=5C9xipCTe8I
|
| [1] https://www.youtube.com/watch?v=lAFMl5bkP5Q
| scotty79 wrote:
| > we lived on a planet where gravity and chemistry would make
| orbital rockets possible
|
| It's kind of insane luck. Bit heavier planet and we wouldn't be
| able to have a single satellite before building nuclear
| engines.
| eru wrote:
| I'm not sure how nuclear engines would help?
|
| A nuclear reactor is a bit like an ion drive: great for long
| distance space travel, but not great for getting off a
| planet.
|
| Unless you mean the kind of nuclear engine that consists of
| detonating atomic bombs behind you? See https://en.wikipedia.
| org/wiki/Project_Orion_(nuclear_propuls...
| jccooper wrote:
| Thrust to weight of a nuclear engine is fairly poor, so
| they are best suited for upper stage or in-space work. A
| heavy-planet rocket might use chemical propulsion in a
| lower stage just like we do and a high-energy nuclear upper
| stage (or two) where the really high Isp would be quite
| useful.
| adrianN wrote:
| Maybe something like NERVA
| liamwire wrote:
| For non-manned launches and those that can be hardened to
| withstand extraordinary g-force, something akin to setup
| that resulted in the missing (900 kg) borehole cap of
| Operation Plumbbob may do the trick.
|
| Acceleration to 66 km/s is probably a little bit overkill,
| even.
|
| https://en.m.wikipedia.org/wiki/Operation_Plumbbob
| m4rtink wrote:
| Orion drive should work fine.
| jessriedel wrote:
| > A nuclear reactor is a bit like an ion drive: great for
| long distance space travel, but not great for getting off a
| planet.
|
| What are you basing this on? NERVA was for getting off the
| planet. It had a thrust of ~250 kN. In comparison, a SpaceX
| Merlin engine has a thrust of ~900 kN, while ion drives
| have <1 N of thrust.
| lazide wrote:
| High fixed (non propellant) drive weight compared to
| chemical rockets makes it pretty inefficient due to the
| gravity well - thrust/weight ratio vs time matters _a
| lot_ when you're quickly climbing out of the well. And it
| is very difficult to do that quickly with nuclear without
| exceeding our materials science abilities and causing a
| nuclear accident. Additionally,
| atmospheric density and friction matter a lot in these
| situations, and getting out of high density atmosphere
| and 'up' as quickly as possible pays large dividends.
|
| Once you're in a very low friction environment and
| ideally already moving near orbital or extra orbital
| velocities, taking your time is all good, and maximum
| end-to-end efficiency and power density matters more -
| you can have as much time as you want.
| landryraccoon wrote:
| Wouldn't it be fascinating if there were an advanced
| civilization on a planet with gravity that was much higher than
| earth that couldn't build chemical rockets and was therefore
| forced to build nuclear rockets?
|
| What if that actually made the exploration of their solar
| system easier, since once they left the gravity well of their
| planet getting to other planets with nuclear rockets was
| comparatively trivial?
| godelski wrote:
| These things are fun to imagine, but the real fun gets to be
| when you start talking about all the downstream effects. For
| example, if you can't build rockets you can't build GPS.
| Building a global communication system is much harder, which
| means things like shipping and flying are much more
| difficult. Not to mention that the gravity is much higher in
| the first place so flying is going to require way more fuel
| so how long does it take for them to get to that stage of
| civilization and how does their technological path differ? It
| gets even trickier once you start thinking about how the
| atmospheric composition will be different as gases follow
| similar escape velocities (e.g. Earth loses 3kg H/s but only
| 50g He/s) and it also determines what can even stay aloft. In
| general much of the technological paths are fairly straight
| forward, always iterating off of the current state (leaps and
| bounds are not common as they're more often a lack of domain
| expertise or not properly contextualized around the
| historical knowledge). But I think people forget how
| connected a lot of these things are. Then again, people often
| question why it is important that we build rockets, while
| asking those questions on their handheld computer connected
| to a global communication network. It's quite incredible how
| complex these interaction chains actually are and I think
| make you only admire the beautify of it all that much more.
| AtlasBarfed wrote:
| Which actually leads me to thinking that a space-adapted
| race really doesn't want to bother with planets and their
| big ass gravity well.
|
| Resource extraction from asteroids or moons is a lot easier
| than carting it out of a big gravity well. Building
| stations in zero G rather than having to worry about
| orbital degradation and the like. Atmospheres get in the
| way of solar energy collection.
|
| Earth is probably only useful as a vacation destination.
| Unless of course all those UFO reports are actual physics-
| defying antigrav drives with little green men.
| lazide wrote:
| The nice thing about gravity wells is they naturally
| concentrate things along density gradients.
|
| The bad thing about gravity wells is they naturally
| concentrate things along density gradients.
| necovek wrote:
| Interestingly, development of rockets has only made a bunch
| of the things you mention cheaper (to the multiple orders
| of magnitude), not impossible.
|
| Eg. determining location through radio signal triangulation
| can tell you a location pretty well, but would require
| placing a lot of signal stations throughout the world. Eg.
| remember the time-synchronisation mechanisms for watches
| through AM signals (including in hand watches).
|
| Similarly, we did build a global communications network by
| placing expensive undersea cables across the world, but
| systems like StarLink are much cheaper (once you get to
| economies of scale for launching satellites).
|
| So, like many things, rockets have accellerated discovery
| and progress, but are ultimately not the be-all solution:
| they work in tandem with the rest of science and
| engineering (including cultural development).
| lazide wrote:
| GPS done via land radio systems would be so flakey and
| expensive it would still likely not be implemented. Easy
| to jam too. And subject to control by terrestrial
| authorities.
|
| Putting a dozen satellites in orbit - and out of reach of
| local authorities - is so much cheaper and more reliable,
| it's not just a matter of cost - it's an entirely
| different product.
|
| Same with starlink. A big part of its advantage is
| someone can't just walk over and cut a cable. And no one
| needs planning approval to put a cable in.
|
| Line of sight to low orbit is about the only way to
| accomplish that - maybe some kind of high altitude
| ballon/plane could (loon?) but they're so comparatively
| easy to shoot down that it makes it a very different kind
| of situation.
| necovek wrote:
| It seems like you are looking at it only from one side.
|
| Undersea cables are probably more expensive than
| satellites today, but we'll still continue to put them
| in. And nope, someone can't just walk in and cut a cable
| sitting at 5000m under the surface.
|
| Detecting a StarLink terminal is relatively easy from the
| ground, and someone can just walk in and demolish it once
| they locate it.
|
| Basically, all tech has pros and cons.
| lazide wrote:
| Of course. But to Russias recent chagrin, blowing
| up/severing a fixed and very expensive cable (or
| underwater pipeline) is a generally far easier
| proposition than tracking down mobile and intermittent
| sources on the ground.
|
| Still possible. But orders of magnitude harder. Nothing
| says that starlink ground station needs to stay in one
| place, after all.
|
| Undersea cables get cut all the time, from shipping to
| nation states.
|
| Trains don't make cars obsolete, anymore than cars make
| trains obsolete. Taking out train tracks is much easier
| and more effective than taking out all possible roads
| though.
| dingaling wrote:
| You might be surprised to learn that Enhanced LORAN
| recently became operational around the UK's coast,
| specifically because _satellite-based PNT_ is so
| susceptible to interference and jamming.
| lazide wrote:
| Not at all. It's only being installed in specific, high
| value areas within a specific jurisdiction. And mainly as
| a backup. Notably by a party which doesn't control GPS
| (albeit a close ally).
|
| LORAN has also been used near airports in developed areas
| for a long time.
|
| That isn't the 'base case' though.
|
| The US military initially developed and launched GPS
| because of the reasons I stated, and it is still widely
| used as a base case for exactly those reasons.
| godelski wrote:
| Yeah and to add to this I think people are forgetting why
| Bell was given a sanctioned monopoly. Because there were
| just cables everywhere. When people say natural
| monopolies exist in markets with network effects, that
| can mean literal networks of cables that will block out
| the sun. Sure, this stuff will improve too, but I think
| people are also forgetting about the increased surface
| area and the increased gravity which makes each of these
| cables required to be thicker or require more support.
|
| https://history.stackexchange.com/questions/58872/did-
| they-r...
| kolinko wrote:
| Interesting thought. I think ground-based GPS wouldn't be
| too difficult though - we already have most of earth
| covered by GSM/3G/LTE, and with updated towers you could
| have something as precise, if not more, as GPS. Of course
| the coverage wouldn't be 100%, and navigating in ocean
| would be more difficult.
|
| Planes would be replaced by trains and aquaplanes for sure.
| Our modern fastest trains (TGV, Maglev) are only half as
| slow as the fastest commercial planes. Also, you might have
| rocketry on such a planet, just not for orbit, and for
| things that right now we use jets for.
|
| The biggest issue with be probably no detailed aerial maps,
| and in later stages - no space mining, so such civilisation
| would be limited to resources on their own planet.
|
| Also, I'm imagining that such a civilisation would send out
| more signals into space to encourage someone to come and
| visit them, and hopefully dropship resources from orbit :D
|
| Imagine two civilisations living like that in symbiosis -
| one on the orbit, able to drop things to the one that is
| lower, but being able to extract only information / art /
| mental labour / energy from below.
| onlyrealcuzzo wrote:
| If you could get to space with a fusion engine, then why
| would taking satellites into space on said rocket be any
| different then it is for us (to build GPS)?
|
| Wouldn't LTA blimps work BETTER in higher gravity for
| flying?
| JoeAltmaier wrote:
| A limit often ignored for high-gravity balloons is, the
| pressure gradient inside the balloon reaches a point
| where it tears it apart. When the atmospheric gradient is
| compressed to some point, the distance between the bottom
| of the balloon and the top can create enormous forces on
| the fabric.
|
| So depending on the gravity we're talking about, blimps
| are out!
| kevinmchugh wrote:
| All projectiles become much shorter range weapons. Maybe
| once they've got gunpowder they can finally fight at range,
| though each shot would require a lot more gunpowder
| relative to the same shot on earth. Maybe it sort of washes
| out if you figure the inhabitants are all stronger and more
| sturdy as a result of the gravity.
| godelski wrote:
| Such an effect may even highly discourage ranged combat
| in the first place. I'm sure you'd still have ballista
| but bows? Probably crossbows. But there's definitely a
| butterfly effect for sure.
| seanmcdirmid wrote:
| That civilization could have invested in rail transit and
| tunneling instead. Positioning isn't so hard on fixed
| roads, although fixing the, in the first place under oceans
| could be a problem. They might figure out triangulation
| using their planet's magnetic field or something. It's also
| completely possible that life isn't viable at all on non-
| Earth like planets.
| godelski wrote:
| Remember that the gravity is higher. Your mountains are
| more dense and your oceans have more pressure. You're not
| living on a world with 14psi. You're not living in a
| world with the same ground, air, or ocean composition.
| All these change. So all your drills have to be thicker
| and harder. All your cables need to be stronger.
| fy20 wrote:
| > once they left the gravity well of their planet getting to
| other planets with nuclear rockets was comparatively trivial?
|
| We are actually on that planet. Spacecraft have what is
| called delta-v, which is basically a measure of what orbit
| changes they can perform given the amount of fuel they have
| onboard. For example getting from the ground to LEO has one
| measure, and getting from LEO to moon orbit has another.
|
| It varies somewhat by the specific rocket to get into space
| (due to drag and effects of higher gravity), but once you are
| there it's basically the same for all spaceships.
|
| It takes around 9.6km/s (no relation to gravity, just a
| coincidence) of delta-v to get into LEO, however once you are
| there it's fairly cheap to get around the solar system. To
| get from Earth LEO to a captured orbit around Mars needs a
| delta-v of around 5km/s - yes, less than to get into Earth
| orbit. To get out further to Neptune would need around 12km/s
| of delta-v.
| somenameforme wrote:
| Oddly enough nuclear rockets aren't particularly powerful and
| tend to be extremely heavy. Their strength is that they're
| highly efficient, so they can just keep going with relatively
| little fuel. Chemical rockets, by contrast, tend to be
| extremely high power but also extremely inefficient. Here's a
| few comparisons:
|
| ----
|
| NERVA [1] / Nuclear / 1969 / 246kN thrust / 18,000 kg mass,
| 841s ISP (seconds of specific impulse - higher is better/more
| efficient, a little is a lot) / The only completed possibly
| launch viable nuclear rocket engine, as far as I know.
|
| F-1 [2] / Chemical / 1959 / 7,770kN thrust, 8,400 kg mass,
| 263s ISP / Powered the Apollo rockets
|
| Merlin [3] / Chemical / 2007 / 981kN thrust, 470 kg mass,
| 282s ISP / Powers the SpaceX Falcon 9 in a group of 9
|
| Raptor [4] / Chemical / ?? / 2,640kN thrust, 1,600 kg mass /
| 327s ISP / Powers the SpaceX Starship in a group of 33
|
| ----
|
| So what really matters in a rocket, for getting off Earth, is
| its thrust to weight ratio. NERVA isn't inefficient because
| it's dated (which was part of the reason I included the F-1),
| but simply because nuclear itself has an inherently poor
| thrust to weight ratio. However it just keeps going and going
| and going, which makes it absolutely awesome for travel once
| you're already in space.
|
| It's even "fast" in space, because of how travel in space
| works. You don't just keep thrusting in space; instead you
| make a limited burn and then coast to where you're going,
| making a final reversal burn towards the end. So even if it
| takes hundreds of times as as long to reach a higher cruising
| velocity, it'll end up getting to the destination long before
| a chemical rocket, for any sufficiently distant destination.
|
| ----
|
| [1] - https://en.wikipedia.org/wiki/NERVA
|
| [2] - https://en.wikipedia.org/wiki/Rocketdyne_F-1
|
| [3] - https://en.wikipedia.org/wiki/SpaceX_Merlin
|
| [4] - https://en.wikipedia.org/wiki/SpaceX_Starship
| dmix wrote:
| This thread made me curious, what's the most amount of stages a
| rocket has been launched with?
| idlewords wrote:
| I'm guessing you mean stages to LEO, but technically Apollo was
| a six-stage rocket.
|
| 1. Saturn V first stage
|
| 2. Saturn V second stage
|
| 3. Saturn V third stage
|
| 4. Lunar module descent stage
|
| 5. Lunar module ascent stage
|
| 6. Service module for Earth return.
| wongarsu wrote:
| There are a couple of four-stage rockets. For example the
| Proton has a couple of four-stage variants, and India's primary
| workhorse, the PSLV, has four stages in all configurations.
|
| Five stage rockets are a lot more exotic. There is the Minotaur
| V, which was launched exactly once, and India's ASLV, which
| they abandoned after a couple launches due to budget issues.
| blackoil wrote:
| What technological advancements would be impossible for a
| civilization that can't go to space?
| scotty79 wrote:
| Satellites mostly.
| tester457 wrote:
| Those aliens have no gps and worse internet. Flight travel and
| shipping is more expensive too. Weather forecasting is more
| difficult. They never create Starlink.
| Balgair wrote:
| > and shipping
|
| I want to mention that this would only be for heavier than
| air based airborne shipping. Liquid based shipping is
| unaffected by gravity. Archimedes' principle has the buoyancy
| force as the weight of the displaced liquid. The
| gravitational effects cancel out. Also, dirigibles would be
| possibly more useful here as, again, gravity cancels out.
|
| Something neat I remembered, great comment all the same,
| thank you.
| zajio1am wrote:
| Internet is almost entirely terrestrial. GPS (for civilian
| use) could be replaced with a land-based network of
| transmitters (like GSM network).
| corn13read2 wrote:
| And now let's break all the numbers by mentioning it's likely the
| aliens are not dumb enough to make rockets so inefficient for
| their task. Nuclear at minimum would be used.
| dalyons wrote:
| Do you have an explanation for how nuclear would actually be
| better? As far as I understand it's terrible for orbital
| insertion levels of thrust.
| Nevermark wrote:
| Now try being from a water planet, and getting to escape
| velocity!
| mxkopy wrote:
| I wonder if this was part of the inspiration for _Outer Wilds_ ,
| where the system's planets are so small that they could be
| explored with wooden spaceships.
| OJFord wrote:
| Can anyone ELI5 what the issue is; I understand/assume larger
| Earth increases gravity so more for rocket to overcome, but why
| doesn't that also affect jet aeroplanes?
|
| Or does it, it's just that this is space.SE so naturally they're
| asking about rockets specifically?
| melagonster wrote:
| if reaching escape speed is impossible, building a work rocket
| is impossible.
| wcoenen wrote:
| > _but why doesn 't that also affect jet aeroplanes?_
|
| Jet engines pull in air and expel it out the back, creating
| thrust. The energy to do so comes from fuel, but almost all of
| the reaction mass is air.
|
| Rockets don't have this luxury; they must bring all the
| reaction mass with them. This causes a big problem of
| diminishing returns. Adding more fuel means you can burn
| longer, but also makes the rocket heavier so it doesn't
| accelerate as much with the same thrust.
|
| The result is that the fuel required goes up exponentially with
| the desired delta-v, as expressed by the rocket equation .
| ben_w wrote:
| Planes get lift from the pressure _difference_ between top and
| bottom of the plane.
|
| A higher gravity planet pulls harder on air, increasing the
| pressure from any given mass over any given area, which IIRC
| doesn't affect this difference directly.
|
| Indirectly, a higher density atmosphere (which is technically a
| different question to pressure; look at Venus for example),
| will lead to higher drag, needing more engine thrust to
| maintain any given speed. Lift depends on speed, but is easier
| to design around.
| mxmilkiib wrote:
| https://en.wikipedia.org/wiki/Missile_Gap by Charlie Stross
| features this kind of concept.
| njarboe wrote:
| By the time of the first orbital rocket, we had already
| discovered nuclear power. If chemical rockets were not capable of
| reaching orbit, a nuclear rocket was not far behind. Humans would
| probably already be on other planets if chemical rockets could
| not launch ICBMs because nuclear rocket technology would have
| continued to be advanced instead of abandoned.
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