11 ms·
The tyranny of the rocket equation (2012)
- TheDudeMan 4y agoIf only Earth was a little bit smaller.
- bombcar 4y agoIf we launch enough rockets, it will be. Self-solving problem, NEXT!
- matheusmoreira 4y agoI wonder what the ideal gravity is. Weak gravity makes rockets harder to control but strong gravity consumes absurd amounts of fuel. Is there a sweet spot?
- inamberclad 4y agoWhy would a rocket be harder to control in lower gravity?
- matheusmoreira 4y agoLower gravity means less energy is needed to maneuver. There's less room for error, so the pilot must maneuver more precisely. Maybe this isn't a problem outside all the little simulators I've tried. Computer control exists, after all. Maybe these simulators all suck and I'm wrong. All I know is when I tried pilot the ship near a low gravity planet, the slightest mistake would put me in an escape trajectory. So I developed this intuition that I need to be much more careful in lower gravity, like a space version of a therapeutic index.
- adgjlsfhk1 4y agoThat's just a consequence of having an oversized rocket. If you scale down the rocket, it becomes controllable again.
- GoOnThenDoTell 4y agoThe issue with a smaller planet is in keeping hold of an atmosphere
- m4rtink 4y agoWorldhouse can solve this: https://www.orionsarm.com/eg-article/484746e824a3a https://www.orionsarm.com/eg-article/484746e824a3a Doesn't need super advanced technology, can be build incrementally, avoids most catastrophic failure modes and allows you to settle essentially any random rock in space.
- deleted 4y ago[deleted]
- grumblepeet 4y agoSpelling? Equation has an ‘I’ in it. Easy to miss if you’re in a hurry.
- taylorius 4y agoThey removed the i to save weight.
- smilespray 4y agoHow much weight? An equaton.
- AtlasBarfed 4y agoPulse nuclear rocket! Project Orion (NOT the boondoggle rocket) FTW! I honestly wonder if they could do more bomb design wiht modern simulation/supercomputers that will have near-zero bad isotopes / fallout. The #1 reason I think we should have a moon base is that we can use it as a launch site for a Project Orion craft that would economically move a couple million tons around the solar system. You have a million ton ship that can get to Jupiter in a month, that means practical asteroid mining.
- rocqua 4y agoI think nuclear salt-water rockets might be more tenable. They produce consistent thrust, and do not involve sending nuclear weapons into orbit, hence not breaking any treaties. Their thrust is lower, but the complexity is lower aswell. No weird damping system to 'absorb' the explosion, and crucially, much less radiation. You are dumping the reaction products away from you, rather than shooting the reaction products at yourself as with Orion. Hence you need much less radiation shielding.
- AtlasBarfed 4y agoYeah, save the excess nuke bombs for a nuclear orbital cannon to get stuff up there.
- dehrmann 4y agoNice to see other fields have problems with ~exponential scaling.
- midoridensha 4y agoWe just need to develop anti-gravity technology.
- betwixthewires 4y agoYou still wouldn't get around the energy requirements to propel you, no matter what technology you use for the propulsion.
- function_seven 4y agoSure, but the fuel or batteries or whatever won't have to lift themselves out of the gravity well. The only fuel needed would be for inertial changes. It's a free G of acceleration! "Just" need to plug in the anti-grav module and efficiency will soar.
- midoridensha 4y agoHow do you figure? With anti-gravity, you would need very little power for propulsion since gravity would not be a factor (or less, depending on how effective your anti-gravity tech is). The question is how much power the anti-gravity requires.
- mr_mitm 4y agoI suppose "we just need anti-gravity" is equivalent with "we just need to violate energy and momentum conservation".
- moring 4y agoIt could also mean: We only have to feed the escape energy of the rocket itself into the rocket while it is on the ground, and without also carrying fuel upwards.
- quickthrower2 4y agoThe “anti gravity” was just zero altitude momentum all along. Just use that big washing machine thing. Can’t stick humans in it though :-(
- seibelj 4y agoIf you had a ladder stretching all the way into space, you could climb it without needing any rocket propellant
- kurthr 4y agoOr a sky hook that rotated at the rate of the earths rotation. On the moon it's just about doable with current technology and realistic levels of mass.
- ncmncm 4y agoOr, better, a sky ring. Weighs a bit more, but is an easier target. And the structural stresses work out better.
- BenjiWiebe 4y agoIf you are talking a rigid ring circling the earth, don't forget your attitude jets. Paging Larry Niven...
- ncmncm 4y agoNo, that idea was just dumb. A ring with a diameter matching your skyhook's length might seem to need 22/7 times the mass lofted to orbit, but the stresses on it are much less complicated, so it may be lighter than that. You may imagine one rolling around Earth, dipping to 100 km and flinging off whatever grabs on at up to 10 mi/s. BTW, I calculated that the tensile strength of the material of any rotating ring, whether girdling the sun like Niven's, or just orbiting the regular way like a Halo or Culture "orbital", must be enough to support against its inside surface "gravity" a full radius's length of the material. So, a ring rotating to provide 1G and taking 24hrs to do it (so days are the right length), would have to be about 2M km in radius. It would therefore need to be made out of something that could support a 2M km long constant-thickness cable against 1G, or more than 100x as strong as what you would need for an earthly space elevator. A ring of 100 km radius rotating so that its tangential velocity matches earth orbital velocity less earth's rotation, 7.5 km/s, accelerates anything that latches onto it at 560 m/s^2, or 55 G. One at 1000 km radius would pull 5.2G, a little less strenuous for puny humans; they would need to endure it for a few minutes until flung off. That one would need to be made of stuff that could hold up 5200 km of itself, which might be just within range of what we can make. Methods for dodging satellites are left as an exercise for the reader.
- JadeNB 4y agoTypo in the title: "Equaton" for "Equation".
- dang 4y agoArgh! Belatedly fixed. Thanks.
- pfortuny 4y agoI am dumbfounded: did they really not write the rocket equation? It is good to be accessible to everybody but, honestly, this seems like a silly omission.
- deleted 4y ago[deleted]
- moffkalast 4y agoThe Tyranny of the Layman
- brudgers 4y agoHere is a link explaining the rocket equation, https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation
- ec109685 4y agoWhy isn’t it possible for a bunch of rockets with a little propellant left already in space to join forces with a rocket (rendezvous with it) to propel it further and faster than it could achieve by launching from earth with no additional assists?
- sigmoid10 4y agoYou'd need extra docking mechanisms, which would add a lot of complexity and negatively impact payload performance. By launching 2 rockets you're also doubling the launch cost. With today's prices, it's much more reasonable to design staged rockets and compact payloads than it is to use several rockets so that one of them may fly further. SpaceX aims to overcome this with starship, which is designed to be be refilled in orbit, but we have yet to see if and how it works.
- ChuckMcM 4y agoThis is SpaceX's plan for Starship (and one of the reasons I think it will take longer than expected[1]). ULA posited an orbital tanker for cryogenic fuels[2] which was most notable for having an internal combustion engine that ran off the fuel it would hold to keep the fuel chilled. [1] If NASA were to plan a mission that needed on orbit refueling they would not even start building a spacecraft until they had tested out all of the elements of on orbit refueling before hand. SpaceX is "assuming" they can make it work and proceeding with Starship. To date NASA has awarded some contracts (https://spaceflightnow.com/2020/10/16/nasa-selects-companies-to-demonstrate-in-space-refueling-and-propellant-depot-tech/ https://spaceflightnow.com/2020/10/16/nasa-selects-companies...) [2] https://sciences.ucf.edu/class/wp-content/uploads/sites/23/2017/04/ULA-UCF.pdf https://sciences.ucf.edu/class/wp-content/uploads/sites/23/2...
- ncmncm 4y agoI like that he doesn't suggest a stopover in lunar orbit. I wonder if a SpaceX SuperHeavy with no Starship mounted could get itself to orbit. Probably its tanks would not start out full, for that. I have taken to calling Starship the "can". With on-orbit refueling you can send cans to lots of places. First paid for seems to be a can to the moon. AFAICT there is no actual use for Orion or SLS; they could launch the "spam" in a Crew Dragon and transfer it to the can in LEO for trans-lunar, and back again to land in. Seems like getting the booster and can crew-rated would be a good thing to do early.
- philistine 4y agoSpaceX has a great track record of safety that rests on a very simple precept: the rocket will fly so much that it will get very safe before it gets human-rated. Compare with the SLS getting one empty flight before they can’t get anything wrong because they’ll kill people.
- cousin_it 4y agoHere's a random idea. The rocket equation is a problem because the propellant you throw backwards becomes just useless mass, right? What if instead you had two spacecraft, and one accelerated by throwing the other one backward, then the second one changed direction via a gravity assist or something, and caught up with the first one again? Maybe they could even subdivide recursively. Or the ones thrown backward could link up with the next rocket passing the same way, and become reaction mass for it in turn, and so on.
- blueflow 4y agoAre you aware of https://en.wikipedia.org/wiki/Conservation_of_energy https://en.wikipedia.org/wiki/Conservation_of_energy ?
- michaellee8 4y agoActually I am thinking of the EMDrive thing now. Would that be possible?
- Rayhem 4y agoConservation of energy doesn't torpedo this idea a priori. The "exhaust" mass would steal energy from the massive body/planet in the gravity assist. If you do this a million million times you might slow the planet down to where it has no more energy to usefully donate, but a one-off maneuver is negligible.
- rocqua 4y ago> Although a momentum balance, these variables can be cast as energies. They are the energy expenditure against gravity (often called delta V or the change in rocket velocity), the energy available in your rocket propellant (often called exhaust velocity or specific impulse), and the propellant mass fraction (how much propellant you need compared to the total rocket mass). This is false, and in a meaningful way. Energy and momentum are not the same. Momentum scales linearly with speed, whilst momentum scales quadratically. One case where this difference matters a lot is the Oberth effect, where the fact that Delta V is a measure of speed rather than energy is of key importance. The Oberth effect states: "the most energy-efficient method for a spacecraft to burn its fuel is at the lowest possible orbital periapsis". Or, more accurately "the use of a reaction engine at higher speeds generates a greater change in mechanical energy than its use at lower speeds". It just turns happens to be that, in orbit, you are going fastest at the lowest point. This effect is a direct result of the scaling differences. Adding 1m/s will always require the same change in momentum, but the faster you are going, the more it will increase your kinetic energy. For a 2kg object, going from 0m/s to 1m/s increases kinetic energy by 1 joules. Going from 1000m/s to 1001m/s increases kinetic energy by 2001 joules. It is key here that, when expending reaction mass at a given exhaust velocity, it will always result in the same change in momentum, whilst resulting in a different change in kinetic energy. Delta-V is just the total speed change that can be achieved by all momentum generated through reaction mass. The rocket equation is about converting this momentum into a speed by compensating for the fact that a rocket gets lighter as it expends reaction mass, to get to Delta-V. If Delta V was a measure of energy, then it shouldn't matter where you use your delta V, you always get the same kinetic energy back. Luckily Delta V is a measure of momentum, which allows us to exploit the Oberth effect for more efficient orbital changes. As for exhaust velocity, _if_ your fuel is your energy source, the exhaust velocity and the energy density are indeed related. But consider an ion engine, where the reaction mass is just mass, and the energy comes from solar panels (or another source of energy).
- msandford 4y agoThat's all true, but it's not the whole story either. The most efficient way to drop into the sun is to shoot a big orbit way out, then when going super slow out halfway to Pluto to burn just a little bit more to slow down to nudge your orbit and now you're headed to to sun.
- JumpCrisscross 4y agoAdjunct: the tyranny of the wagon equation [1]. [1] https://maximumeffort.substack.com/p/the-tyranny-of-the-wagon-equation https://maximumeffort.substack.com/p/the-tyranny-of-the-wago...
- xani_ 4y agoThey kinda forgot donkeys can just eat local grass and other plants, so if you're going thru area where they can graze it's not exactly eating up the wagon load.
- dr_orpheus 4y agoThe pack animals typically can't eat enough to entirely subsist off of local plants. Plus any time that you have to let the animals graze is time that you aren't marching. ACOUP had a good writeup on it https://acoup.blog/2022/07/15/collections-logistics-how-did-they-do-it-part-i-the-problem/ https://acoup.blog/2022/07/15/collections-logistics-how-did-... "Wagons are more promising. A big wagon pulled by two horses can carry perhaps a ton (1000kg) at maximum (in practice many medieval wagons capped out well below this and were pulled by four horses), but now we have two horses and a driver to consider...Their nutrition requirements are too high and so they require feed, at least some 4.5kg of it per day assuming local grass is available along with time to let the horses graze it (during which the wagon is, of course, stopped). The Romans seem to have allocated around 7kg of barley per day per cavalryman for their cavalry" EDIT: Ah, I did not realize before linking the acoup blog that the article is actually in response to that same article.
- suoduandao2 4y agoWhy isn't there more interest in the dirigible launch/landing platform idea? Heck of an engineering feat to construct one of course, but nothing obviously defying the laws of physics and it would get us out of the thicker part of the atmosphere pretty effectively. And a self-landing rocket wouldn't need as much fuel for the return trip because the buoyancy of the landing platform would absorb some of the shock.
- thrtythreeforty 4y agoTo paraphrase xkcd, space isn't about going up. It's about going sideways fast enough that you miss the ground.
- effie 4y agoIt's much easier to go sideways fast at height of 10km. That's why rockets start vertically (go up), and only later in height they tilt to "go sideways".
- themerone 4y agoAtmospheric drag is relatively insignificant compared to gravity.
- aaronblohowiak 4y agoRight but you can you bouyancy rather than ejection mass.
- ultra_nick 4y agoKeeping a floating platform up would be expensive. Gravity is the main force they're fighting. They have used planes in a similar way though: https://en.m.wikipedia.org/wiki/SpaceShipTwo https://en.m.wikipedia.org/wiki/SpaceShipTwo
- otikik 4y agoI really like the idea of using our own Moon for a fuel-saving gravity assist. http://svs.gsfc.nasa.gov/vis/a000000/a003300/a003364/ http://svs.gsfc.nasa.gov/vis/a000000/a003300/a003364/ Unfortunately it's not usable for a lot of targets because it closes the window of opportunity too much (a three week window for hitting Mars becomes a handful of days with a Luna assist). Main source for all this: https://space.stackexchange.com/questions/4027/has-luna-been-used-for-gravity-assist-for-interplanetary-probes https://space.stackexchange.com/questions/4027/has-luna-been...
- midasuni 4y agoDon’t all lunar journeys start with a low earth orbit? Couldn’t you launch into LEO a week or two before hand, then do the lunar assist at the proper time, eliminating the delay concerns of launching from the ground (weather etc) Or will the fuel needed to boost to escape velocity boil off over the wait?
- lmm 4y ago> Don’t all lunar journeys start with a low earth orbit? No, at least they don't have to. It's more efficient to boost directly into an orbit that's eccentric enough to bring you close to the moon, if you started from LEO you'd lose a lot of the gains. > Or will the fuel needed to boost to escape velocity boil off over the wait? No, but relighting an engine is a relatively high risk procedure, so it only makes sense if the gains are large.
- RcouF1uZ4gsC 4y agoI think in the past, we also used to have a "tyranny of the horse" equation. If you needed a horse to pull a wagon some distance over terrain unsuitable for grazing, you needed to carry a significant amount of fodder for the horse. The longer the distance, the more fodder you needed and thus the more load for the horse. If you were so heavy, you needed 2 horses, then you needed even more fodder. This was a huge limit to ground transportation in the past.
- qikInNdOutReply 4y agoWhich would be circumvented if the energy of the rocket actually came from a external source, like a microwave laser and the rocket only contained the medium. https://en.wikipedia.org/wiki/Laser_propulsion https://en.wikipedia.org/wiki/Laser_propulsion It might be dangerous for humans, but for material transport it seems comparable to spinlaunch.
- walnutclosefarm 4y agoI first became aware of Tsiolkovsky's equation 55 or so years ago, as an enthusiastic follower of the Apollo program, first venturing into the world of physics and the power of equations. At the time, I immediately wondered what it said about the suitability of various exo-planets (non of which were as yet "visible" or identified) for development of space-faring intelligent life. This wonderful article by Astronaut Pettit does the calculation I struggled with then: >If the radius of our planet were larger, there could be a point at which an Earth escaping rocket could not be built. Let us assume that building a rocket at 96% propellant (4% rocket), currently the limit for just the Shuttle External Tank, is the practical limit for launch vehicle engineering. Let us also choose hydrogen-oxygen, the most energetic chemical propellant known and currently capable of use in a human rated rocket engine. By plugging these numbers into the rocket equation, we can transform the calculated escape velocity into its equivalent planetary radius. That radius would be about 9680 kilometers (Earth is 6670 km). If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport. So, now, to the Fermi paradox: where are they (the intelligent extraterrestrial life we should have met by now)? Stuck, perhaps, in a gravity well?
- EvanAnderson 4y ago> Stuck, perhaps, in a gravity well? Chemical rockets aren't the only means of climbing out of a gravity well, though. Nuclear thermal and nuclear pulsed rockets both provide higher efficiency than chemical rockets.
- tuatoru 4y agoThere's a second tyranny, though: momentum change rises linearly with exhaust velocity, while energy expended rises quadratically. No engine is 100% efficient, so the spacecraft has to deal with quadratically more waste heat as "efficiency" (exhaust speed) rises.
- effie 4y agoYou can still imagine a gravity well that even a nuclear powered rocket would be unable to climb out of. Nuclear thermal rocket has greater specific impulse than chemical propellant rocket, but not by that much. But there are other ways to climb out of any gravity well (except black hole) than a usual mass-reaction rocket, not benefiting buoyancy. If the atmosphere is transparent enough, a plane or a ballon could carry a small craft high enough, and then it could be propelled by pressure of a strong laser below. Of course, in alien economics and sufficiently deep gravity well, this could be prohibitively expensive.