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.
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.
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.
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.
So you just hang around and talk with radiowaves, sending them pictures of their world from above they could never see otherwise.
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.
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?
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.
So maybe they'd figure it out.
If throwing things well had been much harder, perhaps no animal would have ever bothered?
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.
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.
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".
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.
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.
[0] https://library.sciencemadness.org/library/books/ignition.pd...
> "... 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.)"
”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...
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?
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?
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.
----
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
It's kind of insane luck. Bit heavier planet and we wouldn't be able to have a single satellite before building nuclear engines.
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...
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.
Rockets are most convenient for Earth's variables so engineers optimized for them.
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?
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.
This word is wild. Very interesting.
It basically means "evil bad guy".
You know, exactly the kind of ideology you don't want a neighboring nation to have, no matter how you judge their actions morally
*First stage may need to extend well above the atmosphere.
**No, that's for-sure not a Randall Munroe book in my hand.
But I guess you could cheat by having multiple engines along the way that all accelerate in lockstep.
Of course, once you’ve managed to build this, the rockets are basically optional. :)
We don't talk about ground logistics though.
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.
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.
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.
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.
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...)
If the background radiation of earth was 100x higher, would we care about an Orion launch? Or a small nuclear exchange…
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.
And maybe I'm taking Terra Invicta too seriously but maybe they would wait until they figure out nuclear fusion and have more options.
- 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
A true nuclear rocket. Just like a chemical rocket is a controlled explosion, NSWR is a controlled (cough) nuclear explosion.
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.
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.
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
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.
Or does it, it's just that this is space.SE so naturally they're asking about rockets specifically?
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.
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 .
https://www.reddit.com/r/ProjectHailMary/comments/s5n7j4/eri...
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.
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.
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.
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.
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
Though I don’t suppose we’ll be visiting any aliens with chemical rockets regardless. We don’t have that kind of patience.
Or maybe we're just a dumb civilization/species? Maybe it's also dumb to assume our intelligence is "normal".
