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How much bigger could Earth be before rockets wouldn't work?
- trashtensor 3y agoThe post about the 1.55R⊕ planet made me curious and I thought this was an interesting discussion
- exe34 3y ago[flagged]
- trashtensor 3y agoHonestly the whole thing - this is a topic I don't know very much about and the question and answers were fascinating to read.
- starttoaster 3y agoPlease don't take this as rude, but speaking more plain and directly than the previous person you replied to: I think they were politely telling you to come up with more substance in your comment. The culture on Hacker News has historically been along the lines of, "if your comment is not more insightful than providing an upvote on the post, withhold it and leave an upvote instead." This was on the "welcome" page when signing up for a Hacker News account, and can be found here: https://news.ycombinator.com/newswelcome.html https://news.ycombinator.com/newswelcome.html See: > The most important principle on HN, though, is to make thoughtful comments. Thoughtful in both senses: civil and substantial. ... The test for substance is a lot like it is for links. Does your comment teach us anything? I think Hacker News comments tend to miss the mark on this test more often than not, to be fair. But it's perhaps worth at least politely pointing out when the substance test falls as far as, "this was essentially my approval of the more substantial parts of the post." The irony of my comment is that I'm adding nothing to the discussion, myself. But hopefully there's some small value in reinforcing Hacker News' social ideals.
- trashtensor 3y agoI mean I made the comment because the field is there when submitting the post, I didn't realize it was going to end up as an independent comment. Maybe the HN devs need to make it clearer what will happen if you fill out that field when a link is provided.
- Dylan16807 3y agoOh definitely. I didn't even realize you were the person that submitted it.
- deleted 3y ago[deleted]
- AnimalMuppet 3y agoWait, what? I keep seeing these posts on the "New" page, with only one comment, by the submitter, with some summary comment that doesn't add much, and I always assumed that it was a sketchy way to try to attract attention to the post. So I downvoted such comments when I saw them, because such games should be discouraged. Now I feel like a real jerk. I've been downvoting innocent people who were led astray by the forum software. I don't remember any specific names, but I owe a number of you an apology...
- noizejoy 3y ago> The irony of my comment is that I'm adding nothing to the discussion, myself. Arguably by now the percentage of genuinely new information or arguments on HN is pretty low and falling. Which in a place where discussion > new knowledge is only natural. Pretty much like any place I can think of, HN is increasingly a virtual water-cooler social hangout where the participants cycle through. Just reading the thread about encryption at the top of HN as I’m writing this comment was a bit depressing, because I didn’t find any remotely novel questions, answers or lines of reasoning before giving up on that thread. The intellectual and emotional downer for me wasn’t any better than reading the shallow/empty comment that you responded to. So while I wholeheartedly concur with the sentiment you expressed, I’m also pretty much resigned to the fact, that I should expect less from HN and just move on to other parts of the net. Thank goodness, that RSS seems to be making a quiet comeback.
- bell-cot 3y agoIf 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 3y agoI’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 3y agoThis 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 3y agoI 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 3y agoOr you could just put the engine in the front. There's no rule a train can't arrive before leaving after all.
- defaultcompany 3y agoRelated 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.
- bloopernova 3y agoIs there an equivalent to the Drake equation that includes a factor that describes planets small enough to escape? Very depressing to me to think about how vanishingly rare smart, spacefaring life might be. But on the flipside of that, there may be a little corner of the universe where multiple spacefarers contemporaneously live within a few light years of each other. That might be cool from a space opera point of view but it'd probably end up being dominated by a space fascist enslaving everyone.
- nathanaldensr 3y ago[flagged]
- HeatrayEnjoyer 3y agoThey didn't say space authoritarians, they said space fascists.
- jurynulifcation 3y ago[flagged]
- Terr_ 3y agoIf anything, it should be the opposite: The people who don't know history--such as the difference between an absolute monarchy or a tyrannical theocracy or a modern fascist state--are more likely to blithely want something terrible in the near future.
- Terr_ 3y agoP.S.: I realize this thread already wandered quite far from the rocket equation, but I'm going to lean on the principle that quoting Deus Ex (2000) is always nice: > [Be Safe: Be Suspicious] How can you tell who might be a terrorist? Look for the following characteristics: > * A stranger or foreigner. > * Argumentative, especially about politics or philosophy. > * Probing questions about your work, particularly high-tech. > * Spends a greater than average amount of time on the Net. > * Interests in chemistry, electronics, or computers. > * Large numbers of mail-order deliveries. > * Taking photographs of major landmarks. > And those are just a few. If you're suspicious, then turn them in to your local law enforcement for a thorough background check. Better safe than sorry. You and your neighbors will sleep more securely knowing that you're watching each other's back.
- hinkley 3y agoIt’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 3y agoSince 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 https://en.wikipedia.org/wiki/Nuclear_thermal_rocket
- ravi-delia 3y agoNot 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 3y agoWhen 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…
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- adolph 3y agoIn 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/eridian_biology_from_project_hail_mary_canon_the/ https://www.reddit.com/r/ProjectHailMary/comments/s5n7j4/eri...
- namrog84 3y agoWonderful book! One of my all time favorites. Though just cause they did in a book doesn't validate the science of doing it.
- PaulDavisThe1st 3y agoYou're suggesting that Andy Weir did not science the shit out of that?
- eru 3y agoWell, he gave the aliens some hand-wave-y super-material called Xenonite.
- jonathanpglick 3y agoJazz hands!
- patrickwalton 3y agoFascinating. 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 3y agoOne 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 3y agoBuoyancy is indeed not affected by gravity, you're correct.
- bugbuddy 3y agoThis 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 3y agoDepends 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)
- kebman 3y agoThis was a delightfully weird question! I'm sure it makes sense to calculate this before landing on another planet, though.
- simne 3y agoInterest 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/Northrop_Grumman_Pegasus https://en.wikipedia.org/wiki/LauncherOne https://en.wikipedia.org/wiki/LauncherOne
- hermitcrab 3y agoVacuum-dwelling spherical cows are immune to shockwaves.
- seiferteric 3y agoI wonder if air breathing rockets would change this much.
- le-mark 3y agoBy 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 3y agoSpacefaring 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?
- deleted 3y ago[deleted]
- tooltower 3y agoThe idea is that you can pick up cargo, fuel, and rocket-building minerals directly from Mars.
- jokoon 3y agomore 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 3y agoI 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 3y agoYou can use active support to make a space elevator without super-materials. See https://en.wikipedia.org/wiki/Space_fountain https://en.wikipedia.org/wiki/Space_fountain
- mr_toad 3y 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 3y agoYou 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 3y agoUm 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 3y agoFloating 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 3y agoThough 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 3y agoUnless you are in a black hole you can get off any planet theoraatally
- PaulDavisThe1st 3y agoIf Randall Munroe's name is not on the answer, it's not the answer.
- zuminator 3y agoOn 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 3y agoNo. 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 3y agoSorry 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 3y agoIt'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.
- eek2121 3y agoOoh I absolutely did not click on the link yet, but I love this question!
- eek2121 3y ago(and the answers were just as amazing as I expected!)
- idlewords 3y agoThe 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 3y agoFor 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.pdf https://library.sciencemadness.org/library/books/ignition.pd...
- idlewords 3y agoSeconded! That is such a wonderful book.
- _xk9i 3y agoJust 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.)"
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- dmix 3y agoThis thread made me curious, what's the most amount of stages a rocket has been launched with?
- idlewords 3y agoI'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 3y agoThere 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 3y agoWhat technological advancements would be impossible for a civilization that can't go to space?
- scotty79 3y agoSatellites mostly.
- tester457 3y agoThose 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 3y 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 3y agoInternet is almost entirely terrestrial. GPS (for civilian use) could be replaced with a land-based network of transmitters (like GSM network).
- corn13read2 3y agoAnd 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 3y agoDo 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 3y agoNow try being from a water planet, and getting to escape velocity!
- mxkopy 3y agoI 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 3y agoCan 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 3y agoif reaching escape speed is impossible, building a work rocket is impossible.
- wcoenen 3y 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 3y agoPlanes 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 3y agohttps://en.wikipedia.org/wiki/Missile_Gap https://en.wikipedia.org/wiki/Missile_Gap by Charlie Stross features this kind of concept.
- njarboe 3y agoBy 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 3y agoI 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 3y agoCan't you like... put rocket on a rocket to shoot it out on orbit?