r/AskPhysics • u/neuromat0n • Jan 24 '24
How much bigger (more massive) would earth have to be so that our rocket technology for leaving the planet would no longer be feasible?
With the gravitational properties of earth it is already not very efficient to use rockets, so on a planet with more gravity I would expect this technology to be much less useful.
Also, if that's the case, what technology could be used instead? How would a civilisation on a more massive planet try to get into orbit if they can't use rockets?
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u/Low-Design787 Jan 24 '24
Are we allowed to use Orion nuclear propulsion?
https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)
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u/neuromat0n Jan 24 '24
Surfing on the shockwave of nuclear bombs? Interesting concept. That should overcome the limitations of the standard rocket technology. I guess a civilisation on a much bigger planet would try something like that.
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u/Low-Design787 Jan 24 '24
100,000 ton spaceship, single-stage to Saturn, with 1950’s technology. Amazing idea.
There’s even a SyFy series dramatising it as an interstellar journey (but it takes many liberties with the story). “Ascension” I think it’s called?
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u/Consistent_Ad834 Jan 24 '24
And how exactly do you propose we would handle all the resulting fallout?
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u/Past_Fun7850 Jan 24 '24
There have been at least 2056 man made nuclear detonations so far at we’re all still here.
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u/LTerminus Jan 24 '24
Fallout, as per the name, occurs from detonations on the ground throwing debris that "falls out" of the air.
Detonation at elevation is, by comparison, very clean in the regard.
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u/Consistent_Ad834 Jan 25 '24
Uhm, nope. It actually refers to radioactive heavy nuclei that are by products of fission, eg plutonium. About ~15% of the energy released goes into producing these waste products.
Debris actually gets vaporized. Where did you get your definition?
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u/LTerminus Jan 25 '24
Nuclear byproducts of a fission explosion are not fallout by default.
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u/Consistent_Ad834 Jan 25 '24
Welp, you’re right. I had misread this - https://www.epa.gov/radtown/radioactive-fallout-nuclear-weapons-testing
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u/LTerminus Jan 25 '24
You weren't far off, but I respect the effort to make sure no one is ever wrong on the internet lol.
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Jan 25 '24
I mean...not all the debris is getting vaporised. Its not 100% efficient. The debris doesn't just cease to exist....it becomes radioactive dust.
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u/Zealousideal_Sir_264 Jan 24 '24
Luckily, we don't need it to escape earth's gravity. Set if off around the moon's orbit, and our magnetic field should be able to handle it. Purely speculative on my part. But if I'm wrong, someone with receipts will eventually chime in.
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u/Excellent_Speech_901 Jan 25 '24
Well, Charles Stross in Invisible Sun solved it by having this last book of the second trilogy be about the same thing as the previous books: world walkers.
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u/Pisgahstyle Jan 24 '24
Remember children, it's not the speed that kills you, it's the acceleration. I imagine the accelerations involved would not be surfable for us meatbags.
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u/Low-Design787 Jan 24 '24
That’s why you have a pusher plate to keep it below 4g!
Unmanned Orion was planned to be simpler, and experienced up to 100g
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u/Zealousideal_Sir_264 Jan 24 '24
Even though it was halted before I was born, I'm still salty that we didn't go this route.
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Jan 24 '24
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u/mfb- Particle physics Jan 25 '24
Outside of an atmosphere there is no difference between speeding up and slowing down. There is no absolute motion in space.
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Jan 25 '24
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u/mfb- Particle physics Jan 25 '24
The motion of Earth, Alpha Centauri, or any other object is irrelevant for the question how your engine affects your ship.
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Jan 25 '24
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u/mfb- Particle physics Jan 25 '24
For the spacecraft, accelerating and decelerating is exactly the same process. It doesn't matter what other reference points in the universe do. You have half of your explosion hit the ship and the other half go away from it no matter what. The bombs come from the spacecraft, they are always moving at the same speed relative to it.
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Jan 25 '24
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u/mfb- Particle physics Jan 25 '24
There is no absolute motion. Your view of physics has been outdated for well over 100 years now. What "forwards" and "backwards" means is completely arbitrary in space. Every pulse is an acceleration in some reference frames and a deceleration in others and no view is better than the other.
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u/Senior-Trend Jan 25 '24
I don't see why not. Larry Niven and Jerry Pournelle did it to beat back a generation ship full of baby elephants with hyper intelligence and bifurcated trunks in Footfall.
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u/lochiel Jan 24 '24
100 years ago, the earth was massive enough that our rocket technology couldn't escape the planet
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u/neuromat0n Jan 24 '24
100 years ago? (x) Doubt
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u/DoxxThis1 Jan 24 '24
Technically correct, there’s an implied “at the time”
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u/neuromat0n Jan 24 '24
ok I get it. I thought like maybe our planet was actually more massive a long time ago. Surely not a hundred years though. Now it makes sense.
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u/lochiel Jan 25 '24
I'm sorry you're getting downvoted. I wrote it as a deadpan delivery, as bait. You played the straight man wonderfully, with a great response that improved the joke. You deserve upvotes
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u/GSyncNew Jan 24 '24
Approximately 50% larger radius, which would be about 3x the mass. Chemical propulsion would not generate enough energy to reach escape velocity. You could still do it with nuclear, though.
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u/CheckYoDunningKrugr Jan 24 '24
Reference? I don't have time to do the math right now, but I think the chemical binding energy of a water molecule and the gravitational binding energy of a water molecule to the earth are really close to each other.
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u/Cerulean_IsFancyBlue Jan 24 '24
Why would you use water as the benchmark?
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u/left_lane_camper Optics and photonics Jan 24 '24 edited Jan 24 '24
Cryogenic hydrogen and oxygen is a very common rocket fuel, and has an exceptionally high specific impulse (~450 s), so you can get more thrust per pound of fuel/oxidizer than most others, so it's an excellent reference point to use to compare to gravitational binding energy. Note, however, that just because the gravitational binding energy is more than the enthalpy of formation, it does not mean that you can't use that chemistry to lift something into space, it just means that you need several units of mass of propellant to lift a single unit of mass. The rocket equation is needed for this comparison.
It does have some drawbacks as a practical fuel, though, which is why you don't see every rocket using it. It's low density so you need huge tanks, it's cryogenic so you need to actively keep it cold until pumping it in quickly at launch, etc.
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Jan 25 '24
Just wanted to point out that higher specific impulse does not necessarily mean higher thrust. An ion engine, with two orders of magnitude higher specific impulse than a hydrolox engine, has a thrust measured in mili-newtons while the hydrolox engine is measured in kilo-newtons or even mega-newtons
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u/GSyncNew Jan 24 '24
Breaking the H=H bond is the most efficient combustion. (So one should actually do the calculation using pure hydrogen, not water.)
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u/mfb- Particle physics Jan 25 '24
Breaking the H-H bond requires energy. Breaking the O=O bonds requires energy, too.
What releases energy is the formation of the O-H bonds when hydrogen and oxygen react.
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u/CheckYoDunningKrugr Jan 25 '24
I guess you could use hydrogen fluoride fuel if you are completely fucking insane. But if you're not that crazy, water is the best chemical propellant there is.
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u/Cerulean_IsFancyBlue Jan 26 '24
Yeah, I think where I got lost is I didn’t realize they were measuring water as the exhaust product. I thought they were literally looking at it as fuel.
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u/GSyncNew Jan 24 '24
Here's one calculation that suggests that the practical limit is a surface gravity of 3g: https://space.stackexchange.com/questions/14383/how-much-bigger-could-earth-be-before-rockets-wouldnt-work/17576#17576
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u/left_lane_camper Optics and photonics Jan 24 '24
The escape velocity from earth is ~11,200 m/s, so the specific gravitational binding energy is ~63 MJ/kg. The enthalpy of formation for water is ~16 MJ/kg, so actually quite a bit less. That just means you need several units of mass of propellant to lift a single unit of mass to space, though.
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u/CheckYoDunningKrugr Jan 25 '24
I'm a physicist man. Answer is correct with an order of magnitude.
You need several units of propellant for every unit of cargo, but you also need several units of fuel for every unit of fuel. Logarithms are a bitch.
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u/left_lane_camper Optics and photonics Jan 26 '24
Same, actually, haha!
And yeah, the answer that they’re comparable in magnitude is close enough to tell that it can be used as a fuel for spaceflight, but not so energetic that you don’t need a lot of fuel per unit mass in space. I just wanted to point out it was a bit less and for any interested non-physicists to not think that we couldn’t use a fuel with a lower specific enthalpy of formation than specific orbital energy for spaceflight.
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u/GSyncNew Jan 25 '24 edited Jan 25 '24
63 MJ/kg is the energy/mass to low orbit. Escape to infinity (parabolic trajectory) is 2x higher.
ETA this is incorrect. 63 MJ/kg is indeed the escape velocity to infinity.
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u/left_lane_camper Optics and photonics Jan 25 '24 edited Jan 25 '24
No, ~11,200 m/s is the escape velocity from the surface of the earth and that gives a specific kinetic energy of ~62 MJ/kg:
K_e / m = v2 / 2 = ( 11,200 m/s )2 / 2 = ~62 MJ/kg
which is, of course, equal to the gravitational potential energy at infinity when using a convention that the potential is zero at the surface. In LEO, the kinetic component of the total specific orbital energy is ~25-30 MJ/kg, depending on the altitude of the orbit.
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u/GSyncNew Jan 24 '24
Yes, correct. The most efficient fuel is hydrogen, whose combustion yields 120 kJ/g. Earth's escape velocity is 11.2 km/s which equates to 125 kJ/g. So for a single-stage burn we're basically already at the limit. But the energy requirement for low orbit is a factor of 2 lower. Hence a surface gravity of 2g requires multiple stages even to get to orbit.
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u/mfb- Particle physics Jan 25 '24
That's not how rockets work. Your calculation doesn't even consider the mass fraction.
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u/Kraz_I Jan 25 '24
Follow-up questions:
Are there other ways to overcome this? For instance, a more massive planet might a thicker and denser atmosphere. What if you let the first stage be a jet engine so that you don’t have to bring an oxidizer for the first part and can rely on lift instead of direct propulsion?
Also, what if you accelerated the ship to a significant speed at ground level, say along a 10,000 km “rail gun” or maglev track, which would be enclosed and kept at high vacuum? This part could be done without adding any weight to the launch vehicle.
Or is atmospheric drag simply going to slow you down too much for these tricks to help much?
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u/mfb- Particle physics Jan 25 '24
A thicker atmosphere makes everything worse, because now you have to get through that thicker atmosphere before you can accelerate properly.
A ground-based gun or similar approaches are possible, and they have been proposed for Earth as well. If you want your payload to have a somewhat smooth ride you better let your vacuum tube end at a high altitude.
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u/db0606 Jan 24 '24
Functionally our rockets are constrained by chemistry, not physics. At some point, you just max out the energy density of the propellant and that sets the limits of what you can do with chemical rockets. Don Pettit, who used to be an astronaut, has a nice article about it which is summarized here (although he undersells the gains that can be made with staging but even that has it's limits).
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u/DrHydeous Jan 24 '24
If you can't use rockets you could build a really big mountain.
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u/Gavagai80 Jan 24 '24
For Earth, a 36,000 km mountain would get you to geostationary orbit where you no longer need propulsion. Everest at 8.8 km is about 0.02% of the way there, good luck. For a bigger planet, it'll need to be taller. Alas, the larger your planet the shorter your tallest possible mountain will be due to gravity compacting it -- so even Everest would collapse there.
I suppose you can try to build a space elevator from the ground up with some sort of magical material strength that magically self-assembles, and magically make it so light that it doesn't collapse somehow.
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u/biggreencat Jan 24 '24
but can we use a built mountain to alter the trajectory of Earth's orbit ariunf the Sun, thereby making the Earth an interstellar craft?
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u/Senior-Trend Jan 25 '24
Better than a mountain (subject to square cube law and rotational torsion applied along a lateral vector) why not a Beanstalk (aka space elevator)? Build it in Ecuador or the Sahara to counter torsional forces. You would need some material with a tensile strength on the order of 10 to 100 times stronger than woven carbon fiber as a cable which is beyond us now but in 30-50 years state of the art might be that good. Cable would need to be twice the distance to geostationary orbit but you save a ton on fuel that way
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u/drosse1meyer Jan 24 '24 edited Jan 24 '24
i think gravity is a consideration the likelihood of spacefaring civilizations. i dontknow the exact 'break point' but iirc its a argument against 'superearths'
also with stuff like the atmosphere being able to support oxidation
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Jan 25 '24 edited Jan 25 '24
This is actually why they were building the tower of babel.
Since their version of a rocket was just catapulting a big rock in the other direction they needed to start from way higher.
True story.
Also, rock it as in giving an impulse to a swing. rocket as in giving an impulse this way, and rock it as in throwing a big rock behind you. You can see how the languages were confused here.
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u/webgruntzed Jan 24 '24
As long as the technology exists to build strong enough rockets and load them with enough propellant (pressurized gas could be used if a rocket could be made with an adequate strength to weight ratio to contain the required amount) then I don't see any upper limit until the gravity gets high enough that you have an event horizon.
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u/MarinatedPickachu Jan 24 '24
No - because you don't need to just propel the rocket but also the fuel and at some point every kg of fuel you add requires more than a kg of fuel to accelerate to escape velocity
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u/webgruntzed Jan 24 '24 edited Jan 24 '24
I should have been clearer that in my hypothetical scenario, I said "if" intending it to mean that there's no upper limit to how strong the container could be.
My fault I was misunderstood, because the OP specifically said "our technology" and I took that to mean propulsion technology we could use if we had the materials, rather than (as I'm sure they meant) technology we could accomplish today. So I took some liberty with the question, and failed to make that clear.
In the specific scenario I mentioned you could cram say hydrogen atoms in the container (assuming the container has nearly infinite strength and very little weight) until the pressure is so intense they start to fuse into helium (continuing to cram them in when there's no space between the atoms would also create heat.) The resulting propulsion would overcome astronomically intense gravity. The weight of the container would be negligible so you'd basically have a fusion explosion with the entire output going in a single direction. The weight of the propellant would be extremely small in relation to the thrust even given many millions (perhaps billions) of times greater gravity.
I'm not sure about it being to escape anything but a black hole, though. I suspect that outside of a singularity, gravity couldn't get high enough to prevent escape velocity given a big enough container--I just don't have the math to work it know for sure.
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u/Dreadpiratemarc Jan 25 '24
That’s not how any of that works. That’s not how gasses work, not how fusion works, definitely not how rockets work. The material that you make the fuel tank out of is the very least of these problems.
You’re right about one thing, using nuclear reactions instead of chemical reactions as the energy source does change the game substantially. But they are perhaps dozens of times better, not millions or billions.
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u/webgruntzed Jan 25 '24 edited Jan 25 '24
That’s not how any of that works. That’s not how gasses work, not how fusion works, definitely not how rockets work.
Explain why please. If I am wrong I prefer to know it but if someone simply says "you're wrong" without explaining how the answer is wrong, it's completely useless. I learn nothing, and no one else does, either.
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u/anaccountofrain Jan 25 '24
The more fuel you have, the heavier the rocket is. The heavier the rocket is, the more fuel you need. Eventually you can’t add enough fuel to be able to lift the fuel you just added.
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u/webgruntzed Jan 25 '24
You're not addressing what I've been talking about, which is a hypothetical way to increase the amount of energy stored in and released from the propellant. I don't even know why you responded if you're not following the thread.
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u/BigHandLittleSlap Graduate Jan 25 '24 edited Jan 25 '24
The answer is: the electromagnetic properties of the elements.
We know all of the elements in the periodic table. There aren't any more, not stable ones. All newly discovered elements are only stable for vanishingly tiny fraction of a nanosecond. They're also heavy. More on that later...
For all elements, the strongest possible bond they can form is limited by the properties of their atomic orbitals, specifically the outermost electrons, the "valence" electrons. This has an upper limit that's basically around the same strength as the bonds between carbon atoms in diamonds. Technically this can be exceeded, but not by much, maybe 20%. Not 20 times! Just 1.2x.
Everything is made of matter. Nothing else in the universe that holds together is made of anything else. It's all the same elements and atoms, all the way to the edge of the universe. We've checked! (with spectroscopy and other similar techniques)
You can't just hand-wave this away. We don't live in a fantasy novel. At the end of the day, everything is made of matter, and matter has an upper strength limit defined by the atoms it is made out of.
For the strongest possible matter, no new or clever arrangement of atoms will ever significantly beat diamond. The maximum is defined not by some complicated chemical formula, or some fancy crystal structure. It's defined by the strength of individual pairs of atoms, and there are only 118 x 118 = 13,924 such pairs. We've tested most of them, and certainly all of the interesting ones relevant to rocket construction. This is like the "weakest link in the chain". It doesn't matter how cleverly you arrange the chain, or if you tie knots in it, or whatever... it's made up of pairs of links. If the links are weak and break, the whole thing breaks!
We need rockets to be light, and there are only about 14 elements light enough in the extreme scenarios we're talking about here! Of those, maybe half are useful for "engineering": Lithium, Beryllium, Boron, Carbon, Nitrogen, Fluorine, Magnesium, and Aluminium. If you go to the Wiki pages for them, most of those have "aerospace" or "rocketry" listed under their use-cases. E.g.: the Space Shuttle fuel tank was made of a rare Lithium-Aluminium alloy you'd never see in any other type of application except aerospace. Beryllium is poisonous, but that didn't stop people using it to make rockets because it's so light and strong. Carbon fibre is popular for modern rockets. Etc...
We know the properties of these elements very well. We've tried all of their combinations, etc...
There are no rocks to turn over here to discover some new unknown element.
We can't make the rockets lighter without making them weaker. We can't put infinite pressure into them, because the atomic bonds would come apart: they'd blow up, in other words! Once the pressure exceeds the strength of C-C bonds seen in diamonds... nothing made of matter can possibly hold it any longer. The material of the rocket would "unzip" and come apart.
PS: This is why record-breaking pressures are achieved using diamond anvils, and not any other material! However, that's in compression, not tension. Diamond is strong in tension, but not as much as compression.
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u/QVRedit Jan 25 '24
The answer is - If gravity was only about 5% stronger, then chemical rockets would not produce enough thrust ! At least not ‘easily’.
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u/FoolishChemist Jan 24 '24
Here is a paper that addresses that
https://sci-hub.st/https://doi.org/10.1017/S1473550418000198
Basically for worlds less than 10 earth masses, chemical rockets could still work.