How much bigger could Earth be before rockets wouldn't work?

https://space.stackexchange.com/questions/14383/how-much-bigger-could-earth-be-before-rockets-wouldnt-work

by trashtensor • 2 years ago

442 249 2 years ago

space.stackexchange.com

The post about the 1.55R⊕ planet made me curious and I thought this was an interesting discussion

bell-cot2• 2 years ago

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• 2 years ago

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. :)

defaultcompany2• 2 years ago

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• 2 years ago

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.

grecy• 2 years ago

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.

Taek• 2 years ago

You could just perch the first stage at the top of a sufficiently tall mountain.

cratermoon• 2 years ago

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• 2 years ago

** And assuming sufficient propellant exists on your planet :)

ivanjermakov• 2 years ago

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.

Sharlin2• 2 years ago

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• 2 years ago

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.

bloopernova• 2 years ago

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• 2 years ago

Few lightyears is something like a hundred stars. Probably zero chance to nurture more than one unprobable life.

Retric4• 2 years ago

The upper limit on the size of the universe is infinite. So we can’t exactly rules stuff out for simply being improbable.

HeatrayEnjoyer• 2 years ago

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• 2 years ago

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.

bell-cot2• 2 years ago

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...

boringuser23• 2 years ago

fascist

This word is wild. Very interesting.

It basically means "evil bad guy".

somewhereoutth• 2 years ago

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

wongarsu3• 2 years ago

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

idlewords4• 2 years ago

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.

eru20• 2 years ago

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• 2 years ago

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.

eru20• 2 years ago

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• 2 years ago

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• 2 years ago

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• 2 years ago

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• 2 years ago

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• 2 years ago

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.

hermitcrab4• 2 years ago

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

jowea2• 2 years ago

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• 2 years ago

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• 2 years ago

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• 2 years ago

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• 2 years ago

Wonderful book! One of my all time favorites. Though just cause they did in a book doesn't validate the science of doing it.

PaulDavisThe1st2• 2 years ago

You're suggesting that Andy Weir did not science the shit out of that?

patrickwalton• 2 years ago

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• 2 years ago

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.

datameta2• 2 years ago

Buoyancy is indeed not affected by gravity, you're correct.

wolfram742• 2 years ago

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.

hermitcrab4• 2 years ago

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.

unsupp0rted5• 2 years ago

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.

exe34• 2 years ago

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• 2 years ago

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

scotty795• 2 years ago

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.

brucethemoose22• 2 years ago

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_k3• 2 years ago

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?

eru20• 2 years ago

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• 2 years ago

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.

eru20• 2 years ago

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• 2 years ago

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.

eru20• 2 years ago

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_propuls...

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.

Intralexical9• 2 years ago

[…] 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• 2 years ago

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.

njarboe2• 2 years ago

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• 2 years ago

This was a delightfully weird question! I'm sure it makes sense to calculate this before landing on another planet, though.

simne• 2 years ago

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

hermitcrab4• 2 years ago

Vacuum-dwelling spherical cows are immune to shockwaves.

seiferteric• 2 years ago

I wonder if air breathing rockets would change this much.

le-mark• 2 years ago

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.

jowea2• 2 years ago

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• 2 years ago

The idea is that you can pick up cargo, fuel, and rocket-building minerals directly from Mars.

jokoon• 2 years ago

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• 2 years ago

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.

eru20• 2 years ago

You can use active support to make a space elevator without super-materials. See https://en.wikipedia.org/wiki/Space_fountain

mr_toad3• 2 years ago

and even if it did exist, I have no idea how that thing would be put in place

You put a platform in geosynchronous orbit and then lower a cable while raising a counterweight. The orbit of the entire structure is then balanced. The tricky part is lowering the bottom part through the atmosphere and securing the base.

nntwozz• 2 years ago

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• 2 years ago

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• 2 years ago

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.

eru20• 2 years ago

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• 2 years ago

Unless you are in a black hole you can get off any planet theoraatally

PaulDavisThe1st2• 2 years ago

If Randall Munroe's name is not on the answer, it's not the answer.

zuminator2• 2 years ago

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• 2 years ago

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.

zuminator2• 2 years ago

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.

pottspotts• 2 years ago

How do you "fill something with vacuum"?

mr_toad3• 2 years ago

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.

jasonwatkinspdx2• 2 years ago

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• 2 years ago

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_toad3• 2 years ago

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.

wongarsu3• 2 years ago

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.

foota2• 2 years ago

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?

eek21212• 2 years ago

Ooh I absolutely did not click on the link yet, but I love this question!

eek21212• 2 years ago

(and the answers were just as amazing as I expected!)

idlewords4• 2 years ago

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• 2 years ago

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...

idlewords4• 2 years ago

Seconded! That is such a wonderful book.

_xk9i• 2 years ago

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.)"

refurb• 2 years ago

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?

signalToNose• 2 years ago

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

sillywalk• 2 years ago

There was also a quote about chlorine trifluoride in Ignition!

”It is, of course, extremely toxic, but that's the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water-with which it reacts explosively. It can be kept in some of the ordinary structural metals-steel, copper, aluminium, etc.-because of the formation of a thin film of insoluble metal fluoride which protects the bulk of the metal, just as the invisible coat of oxide on aluminium keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes.”

https://www.science.org/content/blog-post/sand-won-t-save-yo...

scotty795• 2 years ago

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.

eru20• 2 years ago

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...

landryraccoon• 2 years ago

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?

godelski5• 2 years ago

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.

fy20• 2 years ago

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• 2 years ago

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• 2 years ago

This thread made me curious, what's the most amount of stages a rocket has been launched with?

idlewords4• 2 years ago

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.

wongarsu3• 2 years ago

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• 2 years ago

What technological advancements would be impossible for a civilization that can't go to space?

scotty795• 2 years ago

Satellites mostly.

tester457• 2 years ago

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• 2 years ago

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• 2 years ago

Internet is almost entirely terrestrial. GPS (for civilian use) could be replaced with a land-based network of transmitters (like GSM network).

corn13read2• 2 years ago

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• 2 years ago

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• 2 years ago

Now try being from a water planet, and getting to escape velocity!

mxkopy• 2 years ago

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• 2 years ago

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• 2 years ago

if reaching escape speed is impossible, building a work rocket is impossible.

wcoenen• 2 years ago

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• 2 years ago

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• 2 years ago

https://en.wikipedia.org/wiki/Missile_Gap by Charlie Stross features this kind of concept.

njarboe2• 2 years ago

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.

LeroyRaz• 2 years ago

I assume civilizations on heavier planets just use nuclear propelled rockets. As I understand it, the fuel density is so much higher that you could likely manage to build feasible nuclear rockets for far far higher g than you could chemical ones.

Giorgi• 2 years ago

Can't you like... put rocket on a rocket to shoot it out on orbit?

How much bigger could Earth be before rockets wouldn't work?

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