What amazes (and saddens) me is that we don't seem to have not even a far fetched theoretical approach to dramatically improve rockets within the next few decades.
Nuclear rockets are a dramatic improvement on chemical rockets. We've already built those. Ion engines are a dramatic improvement on chemical rockets. We are currently using those. Robert Forward also calculated that almost any conceivable mission in the solar system could be accomplished by an antimatter rocket with only a 3/5ths fuel fraction. Fusion rockets would be a somewhat watered-down version of that. (Though, nowhere near what's shown on The Expanse with Epstein Drives.)
Beamed propulsion throws out the rocket equation altogether, as do light sails.
The problem with beamed propulsion is that you still need a rocket, or some equivalent thereof to stop going, whenever you get to your destination.
False. Robert Forward has a technique for deceleration for interstellar lightsail craft, which is described in Rocheworld and one of his science books. Magsails using beamed particle propulsion can also exploit the interstellar medium to decelerate without onboard fuel.
Not an issue for flybys, but an issue if you want to get into an orbit.
Carrying a small rocket for a circularization burn is vastly cheaper than a 1st stage. This is covered in Jordin Kare's modular laser launch proposal, and this technique was also considered for Bull's light gas space gun. Those can produce launch economics approaching those of space elevators.
Sorry, but you're citing a science fiction author as proposing a credible method, yet don't even state what that method is.
Yes, I know Robert Forward used to work in aerospace, but his fiction writings are... fiction. He's been out of the industry since 2002 and a hand-wavy "he has a technique" doesn't strike me as being particularly credible.
I think Venus might have an economic edge in the early days of the trans-Solar System economy, because the whole of its civilization will be floating at high altitude. Even without orbital rings and space elevators, everything will have very cheap access to space through skyhooks.
Lots of solar power, too, and lightsails and magsails have more to work with.
(I was thinking of free-space skyhooks, exchanging momentum between inbound and outbound trips, but the ones for planets are also cool. Just harder when the planet is heavy like Earth and Venus.)
That's true. I wonder what the energy use would be in comparison to chemical rockets. And if it would be inherently safer since it wouldn't be moving via a controlled explosion.
It's a bit sad, I do long for the stars and planets.
However, rramatically improving rockets might be like trying to improve the carriage to get a much faster personal transport, while keeping the horse.
It seems to be the case, that the cost of launching commercial payloads are still "low" enough compared to the cost of the payloads since there are few big budget ventures that tries to make lifting much cheaper through alternative methods.
I'm not saying that space elevators or other alternative lift devices are feasible, but we tend to get huge investments even into tech that seems absolutely implausible if there is a big market. The lack of these makes investments makes it plausible that the current launch costs are actually acceptable for all current, and expected near future commercial actors.
The nice thing about rockets is that you can start as small as you'd like and build progressively larger and larger rockets and get the engineering details right before you have to worry about getting into space.
Space elevators, skyhooks, and launch loops are all big enough that it's an all-or-nothing deal. You can test individual components, but a whole end-to-end space elevator has to be exactly the distance between ground and geosync or else it just doesn't work. At best, you could build smaller versions of this stuff on the moon first and then scale up to Earth-size later, taking into account such factors as water, atmosphere, and the fact that the Earth is not tidally locked with the sun. But that requires hauling metric fucktons of parts and equipment to the moon, even if you're using lunar resources to build the space elevator itself, so there's no guarantee you end up ahead in the end.
The basic physics behind rockets is simple as all hell, and it still required von Braun to spend his entire adult life building progressively bigger and bigger rockets and refining the design details. The V2 only needed to be able to land nose-first 200 miles from where it was launched, but without that knowledge and experience, the Saturn V would have been impossible. That's the kind of iteration that's fundamentally impossible with these other launch systems.
Yeah, it's not the type of thing you can iterate on and get usable results for without the whole thing. It's not like you can build a tiny space elevator and use it to bomb London, for example.
If I understand the article, that would require either a new both radically strong and radically lightweight material for the non-propellant parts the rocket (assuming H/O propellant), or using non-chemical physics for propellant that exceeds the payment energy of H/O, and that is stable enough to be relatively safe. Or both.
Well, rockets sure, but we have other propulsion technologies that aren't governed by the rocket equation: solar sails, magnetic sails, and laser propulsion all come to mind. And even with momentum rockets, ion drives, fission rockets, and fusion rockets are massive improvements over the standard chemical rockets we use today.
Nuclear has amazing power ratios but detonating a nuclear warhead to get moving is somewhat of a problem when you're close to earth.
Fission Heat Engines are quite good but not good enough for takeoff. Ion engines suffer the same fate.
Almost no alternative propulsion method can replace takeoff boosters, thusly limiting the mass you can put into space. With limited mass you can only make engines that big. Or bigger if you assemble them in space which opens other cans of worms.
> Nuclear has amazing power ratios but detonating a nuclear warhead to get moving is somewhat of a problem when you're close to earth.
It's not really a 'near earth' issue. Fallout concerns can probably be mitigated (and Freeman Dyson came up with a few ideas in that direction). Research in this direction, however, got killed as part of the various test ban treaties.
From the perspective of trying to reduce any country's desire for having a nuclear bomb program, Project Orion, and other nuclear solutions, is a no go. So even if you were to promise "We'll only start the nuclear component when we hit the moon" the proliferation concerns would still kill NASA's interest in pursuing it.
So yeah; it's not really a technical issue at it's core (though you definitely have technical issues to solve), but rather a political one (which certainly doesn't invalidate it).
Nuclear rockets do not have to be about detonating nuclear warheads. Nuclear thermal rockets have no detonation, no fallout and no wide-scale meltdown risk. They've been demonstrated[1] to have efficiencies 3-4x greater than conventional engines without sacrificing thrust.
There’s a lot of reason why they haven’t been used yet, but I think most the compelling argument against them is that they won’t be used until fuel costs become the competitive factor in launch prices.
Aerospikes are not a dramatic improvement in anything. They just buy you a bit more nozzle efficiency, and that only really comes into its own for ssto.
Maybe not as dramatic as we would all like, but it would be nice to see some work put into trying to revive the idea of the air-augmented rocket. Apparently the Russians worked on the idea in the 1960's[1]. But the PR-90 look like its as far as they got with the idea[2].
But maybe there won't be one dramatic change that makes rocket travel cheaper. Maybe we keep iterating on rocket and rocket engine designs and eventually looking back it seems revolutionary. As composites get better and better more and more of rockets will be made from composites instead of the current material of choice, lithium aluminum alloy. Plus the increased use of oxygen rich combustion[3] with it's improved efficiency. Also it looks like Ch4/LOX might be more common in the future[4][5][6], which is good because Ch4/LOX has a higher ISP then RP-1 and also is significantly more dense then H2 propellant. There's also so called "slush" or "super chilled" propellants[7], Spacex has even been using it for awhile[8]. So maybe the future will be extremely light, reusable, oxygen rich, staged combustion, methane burning rockets with super chilled propellants?
It pushes the limits of materials and design. But I think a reusable 2-stage rocket—as boring as it sounds—will end up working better in the near term. It can be done with safe margins.
Honestly, we do. The rocket equation is not just exhaust velocity, it's ALSO mass fraction. And material advances can make a huge difference there.
Chemical rockets are also much more efficient devices than is given credit for. A good, very optimized two stage rocket like SpaceX BFR (and the ITS concept before it) can get payloads to orbit using just ~300 Megajoules per kg (250MJ/kg in the more efficient tanker configuration). The specific kinetic energy of orbit is about 25MJ/kg (40MJ/kg if we include gravity and aero losses and changes in altitude), so rockets are total 10% efficient at converting chemical energy directly into orbital energy, and with an optimized, high pressure and variable mixture ratio hydrogen rocket engine architecture with some improvements in materials science, we could improve that to 20% or more. That beats the chemical to mechanical efficiency of the typical car.
That translates to between $1 and $10 per kg to orbit. That's 3 orders of magnitude less cost than today, if we can master reuse. So arguably, chemical rockets are perfectly fine for achieving orbit cheaply.
In six years nobody has corrected the fact that one of the propellants listed is not "Methane-Oxygen" but "Earth orbit to near-Earth asteroids", and the following is not "Methane-Oxygen" but "Hydrogen-Oxygen."
The tyranny of the rocket equation has done an order of magnitude less damage to the progress of human space exploration than the mind-blowing waste of the Shuttle program.
Nice that we live on a planet with low enough gravity to get off of it. The article states that if the Earth was about 50% bigger, chemical rockets wouldn't be able to achieve escape velocity. Any species on a planet like that has a much much harder time getting into space.
We almost had a nuclear space program. It got cancelled for non-technical reasons. On a planet with harder take-off, that program would get all the funds it could possibly need.
built on the success of a rocket-based program! no way would a country fund a vehicle that spews out a constant stream of nuclear bombs as their species' first foray into orbit!
I once mentioned this to an old NASA engineer who worked on the moon landings. He corrected me and pointed out that it's still possible to launch - you would just need more staging. Obviously at very high levels of gravity, the staging requirements would be extreme. I'm not sure what it would be if Earth was 50% bigger. Maybe someone can do the math or share a link to a page that's done it already.
There is a point where surface gravity is sufficiently high that in order to put a few kilograms in orbit the "rocket" would have to weigh as much as the entire planet, even if you use the most efficient chemical fuel we've invented. Physics doesn't lie.
there are more tyrannical equations if you are promoting space colonization. I mean if you compare earth to a petri dish, what do you think a small domed shelter on some other planet will be?!?
I like space, lots of neat things to learn, but the discussion always seems to get a little religious and doomsday-ish, and I find it less than genuine. Like have people thought about where the energy to get massive numbers of people off the planet will come from, and how that will leave the planet in even worse shape for those that can't leave?
It is folly to think technology can fix everything, and I sometimes wonder if it has actually "fixed" anything, aside from helping to enable overpopulation.
The energy to get a human off the planet is relatively small. Let's call it an MWh. (It's 9.1 KWh/kg)
A billion people is a PWh. That's 0.5% of the earth's annual energy use. It's a blip on the radar, especially since it will subsequently reduce annual consumption in the 10% range.
factoring in Tsiolkovsky's equations, it seems more like 30mwh, and that is just one person, and just the fuel, not the processing or the preparation or even building a rocket, without any life support, or landing plan.
plus where are you going to send %10 of the population where they will be immune to human nature, whatever that is.
Since I first got interested in science, I've been presented a number of times with this idea (that lifting things in orbit is subject to very simple physics that have harsh economic implications).
I've always felt uneasy about this, as it seems to imply that the only way to put things in orbit is via rockets.
For example, there is the idea of a space crane (assuming it can be built) that is simply a very, very tall building, assembled one chunk at a time, and that once built can simply lift things in orbit.
Would a space crane be slave to the same equations?
Most of the energy of getting a rocket to orbit is in achieving orbital velocity, not orbital altitude :/. Unless you accelerate your satellite really really fast, it'll just fall back to the earth.
If you could build a proper space elevator, a payload could be lifted to the geostationary point and then released. At that point it's already at orbital velocity, and can use much less fuel (compared to an Earth launched rocket) to get back down to low earth orbit.
But again, this is still a materials science problem.
Yes, I do understand that we have an unsolved material science problem for the space elevator, but my question was more theoretical (hence the "assuming that") and boiled down to:
"doesn't the hypothetical space elevator break the tyranny of the rocket equation?" (which you did answer).
> Would a space crane be slave to the same equations?
No it would not, it has its own cross to bear, tensile strength.
If you start from the ground, then as the crane gets taller, the forces on its outer edges are the torque applied when a lateral force pushes against it (think wind). As with a stubborn nut, the longer the wrench handle the more force you can apply to the nut. If you think of the crane as your wrench handle, the taller it gets, the more force it will exert on its mounts when air moves past it. (this it true of sky scrapers as well).
Eventually you move past the point where there is anything that can stay vertical (steels rip and bend, carbon fiber breaks, Etc.) There is a working solution, a pyramid, but to build a pyramid that reached low earth orbit requires a base that is larger than the state of Kansas and will sink down to bedrock. There is an entertaining discussion of this from 2011 here: https://www.wired.com/2011/07/does-the-slope-of-a-pyramid-re... We know from the Himalayas you can stack up a lot of rock into a really really tall mountain, but its really really a lot. This might occur naturally on some planet, Olympus Mons on Mars is 16 miles high (about 85,000 ft or almost 26 km) that is 25% of the way up to space (assuming the 100km is your line for 'space'). Eventually you hit the limit which is that gravity prevents you from having a feature stick out too far from a sphere.
Another way to do this is to put a weight in geosynchronous orbit so it stays over the same spot all the time. If you start building a cable downward and adding mass further out you can keep the "effective" mass at the geosyncronous point. Eventually, (materials science not withstanding) the part you are lowering down to the planet reaches the surface and voila you have your jack and the beanstalk type beanstalk.
As fantastical as that sounds, it should actually work, if, and it is a big if, you can make a material that can withstand the tensile force of being both pulled up from the counter weight, and down by gravity. Once built you could attach a vehicle that would ride up and down the cable moving things from ground level into geosynchronous orbit, with only the energy needed to climb up the cable. These "space elevators" are not precluded by physics so they show up in science fiction stories.
But once you get away from building structures you are left with either chemical rockets or some how pushing against the Earth. Creative ideas there are balloons that take you part of the way up, and electrostatic systems that use an electric field to push against the charge on the Earth. These ideas start off far fetched and move right into crazysauce fairly quickly.
To get into a circular orbit with a crane you have to build a space crane as tall as the circumference of earth.
The tallest structure ever built by humans is a little more than half a mile tall.
A space elevator to geostationary orbit (required for a circular orbit with no rockets) would have to be 22,000 miles tall.
Low earth orbit is a lot closer, what is considered LEO is pretty wide, but the ISS is at about 250 miles.
If you get an crane ride up 250 miles, you aren't orbiting. You haven't even done half of the work, to be in orbit at that altitude means you have to accelerate to several miles per second.
How orbital mechanics works is not very intuitive.
A space elevator that goes all the way from the surface of the Earth to orbit would be very hard to build on Earth because Earth's gravity well is so deep - though it's pretty easy on the Moon.
Something more feasible would be a Skyhook[1] which only provides part of the energy needed to get to space but which due to the exponential nature of the rocket equation could make a huge difference while being conceivable to build.
SpaceX’s BFR plans to overcome the propellant problem for getting to Mars by refueling in orbit. I am surprised the article didn't mention the refueling possibility. Maybe NASA didn't want to admit they are making a mistake putting all their cash into the SLS.
Also, the economic implications of the equation don't matter so much when you have a reusable rocket, and the BFR is supposed to be able to reuse both stages.
> As humans, we are powerless to change this number. We simply have to accept its consequences. I like to think of this as the travel cost.
This is selling rocket science a little short, I think. The availabilty of gravity assists and aerobraking make it possible to achieve large variations in delta V necessary for a trip, as long as you can make the right tradeoffs elsewhere (travel time, etc).
Where else would you get it? Unless it's beamed in the form of light, I don't know of many options.
The challenge is to get from the Earth's surface, to orbit. Yes, there's oxygen along the way, but the speeds you have to go will wreck you if you stay in the atmosphere at all. So you get into vacuum ASAP.
Once you're in orbit, you have a lot more options. For example, you don't need a high-thrust engine—instead, high-efficiency is fine, even if it's low thrust. Etc.
He's probably implying mining or synthesizing fuel on smaller gravity wells like Mars. Being able to synthesize fuel for the return trip could massively cut back on initial launch weight. Energizing propellant with a microwave beam (https://en.wikipedia.org/wiki/Beam-powered_propulsion) has been hypothesized, while a solar sail is much more realistic with modern technology.
It's occurred to me that a rocket is perhaps the perfect metaphor for a pessimistic reading of the fermi paradox. It's a device that perfectly encapsulates how much easier it is to destroy society, than it is to pull society out of a gravity well. In the first instance, by blowing up on the launchpad. In the second, by providing the means for competing societies to nuke the whole world into a mass extinction.
The problem is the same for all of the ideas like space-elevators, or asteroid mining, or spacetravel as a normal thing - given our current politics, there's no way you'd avoid occasional rock-droppage with catastrophic consequences for people still stuck on earth. It would just be too easy. If you start thinking along the lines towards nuclear-fueled rockets, the problem just gets worse.
Non-weapon rockets are high-energy, fiddly devices - and their dumb purely-destructive siblings are far simpler.
If launching rockets became cheap enough to put people on Mars, that would also mean they'd be cheap enough so even small states could afford ICBMs. Because rockets are ICBMs.
Lots of modern technologies do that. Like a child growing up—at what age do you use sharp knives? a lawnmower? a car?—humanity has matured ethically alongside our technology.
I agree we have to be careful not to Great Filter ourselves.
>humanity has matured ethically alongside our technology
That's exactly the problem. I don't think we have at all. We do no serious study of ethics in our schooling, and frankly don't devote serious thought to the subject. It's very uncommon to meet somebody who has read even a single book about ethics. We're a society of meat-eaters who consider people who harm animals to be scum, a society that simultaneously considers the protection of children fundamental, and where the most likely age to die on a world scale is in childhood. These are reflections of a deep lack of ethical maturity.
Regardless of all of the inconsistency, I think it's hard to look at the times in which the world came to the brink of nuclear catastrophe in the cold war, and state confidently that this is a society with the maturity to hold nuclear weapons.
This is really fascinating. I still don’t have an intuitive understanding of why getting into space takes so much more propellant than flying an airplane, but it seems like it’s definitely a hugely more difficult thing to do. Thanks for posting.
It's about speed. Getting into space is relatively easy (it does take a lot more propellant than flying an airplane, but it's understandable since spaceships tend to fly a lot higher than airplanes). Staying there means you have to be flying so fast that you fall continuously around Earth instead of into it. Even the fastest airplanes in the world can't get that fast on their own (at least not yet). More speed requires more fuel (and since that fuel makes you heavier, you need even more fuel to counteract the added weight - that's the rocket equation).
Even more intuitively: it's about distance. Flying from Los Angeles to Salt Lake City requires less fuel than flying from Los Angeles to New York City. Flying from Los Angeles all the way around the world and landing again in Los Angeles takes even more fuel.
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[ 3.5 ms ] story [ 160 ms ] threadBeamed propulsion throws out the rocket equation altogether, as do light sails.
Not an issue for flybys, but an issue if you want to get into an orbit.
False. Robert Forward has a technique for deceleration for interstellar lightsail craft, which is described in Rocheworld and one of his science books. Magsails using beamed particle propulsion can also exploit the interstellar medium to decelerate without onboard fuel.
Not an issue for flybys, but an issue if you want to get into an orbit.
Carrying a small rocket for a circularization burn is vastly cheaper than a 1st stage. This is covered in Jordin Kare's modular laser launch proposal, and this technique was also considered for Bull's light gas space gun. Those can produce launch economics approaching those of space elevators.
Sorry, but you're citing a science fiction author as proposing a credible method, yet don't even state what that method is.
Yes, I know Robert Forward used to work in aerospace, but his fiction writings are... fiction. He's been out of the industry since 2002 and a hand-wavy "he has a technique" doesn't strike me as being particularly credible.
Sorry, but he was also a noted physicist.
yet don't even state what that method is.
http://lmgtfy.com/?q=Robert+Forward+laser+sail+deceleration
(I was thinking of free-space skyhooks, exchanging momentum between inbound and outbound trips, but the ones for planets are also cool. Just harder when the planet is heavy like Earth and Venus.)
However, rramatically improving rockets might be like trying to improve the carriage to get a much faster personal transport, while keeping the horse.
It seems to be the case, that the cost of launching commercial payloads are still "low" enough compared to the cost of the payloads since there are few big budget ventures that tries to make lifting much cheaper through alternative methods.
I'm not saying that space elevators or other alternative lift devices are feasible, but we tend to get huge investments even into tech that seems absolutely implausible if there is a big market. The lack of these makes investments makes it plausible that the current launch costs are actually acceptable for all current, and expected near future commercial actors.
Space elevators, skyhooks, and launch loops are all big enough that it's an all-or-nothing deal. You can test individual components, but a whole end-to-end space elevator has to be exactly the distance between ground and geosync or else it just doesn't work. At best, you could build smaller versions of this stuff on the moon first and then scale up to Earth-size later, taking into account such factors as water, atmosphere, and the fact that the Earth is not tidally locked with the sun. But that requires hauling metric fucktons of parts and equipment to the moon, even if you're using lunar resources to build the space elevator itself, so there's no guarantee you end up ahead in the end.
The basic physics behind rockets is simple as all hell, and it still required von Braun to spend his entire adult life building progressively bigger and bigger rockets and refining the design details. The V2 only needed to be able to land nose-first 200 miles from where it was launched, but without that knowledge and experience, the Saturn V would have been impossible. That's the kind of iteration that's fundamentally impossible with these other launch systems.
Nuclear has amazing power ratios but detonating a nuclear warhead to get moving is somewhat of a problem when you're close to earth.
Fission Heat Engines are quite good but not good enough for takeoff. Ion engines suffer the same fate.
Almost no alternative propulsion method can replace takeoff boosters, thusly limiting the mass you can put into space. With limited mass you can only make engines that big. Or bigger if you assemble them in space which opens other cans of worms.
It's not really a 'near earth' issue. Fallout concerns can probably be mitigated (and Freeman Dyson came up with a few ideas in that direction). Research in this direction, however, got killed as part of the various test ban treaties.
From the perspective of trying to reduce any country's desire for having a nuclear bomb program, Project Orion, and other nuclear solutions, is a no go. So even if you were to promise "We'll only start the nuclear component when we hit the moon" the proliferation concerns would still kill NASA's interest in pursuing it.
So yeah; it's not really a technical issue at it's core (though you definitely have technical issues to solve), but rather a political one (which certainly doesn't invalidate it).
[1] https://en.wikipedia.org/wiki/Project_Timberwind
There’s a lot of reason why they haven’t been used yet, but I think most the compelling argument against them is that they won’t be used until fuel costs become the competitive factor in launch prices.
https://youtu.be/K4zFefh5T-8
https://space.stackexchange.com/questions/3004/why-arent-lin...
Nuclear rockets are quite the concept but have their obvious limitations. This video is worth checking out if you haven’t seen it yet.
https://youtu.be/eDNX65d-FBY
But maybe there won't be one dramatic change that makes rocket travel cheaper. Maybe we keep iterating on rocket and rocket engine designs and eventually looking back it seems revolutionary. As composites get better and better more and more of rockets will be made from composites instead of the current material of choice, lithium aluminum alloy. Plus the increased use of oxygen rich combustion[3] with it's improved efficiency. Also it looks like Ch4/LOX might be more common in the future[4][5][6], which is good because Ch4/LOX has a higher ISP then RP-1 and also is significantly more dense then H2 propellant. There's also so called "slush" or "super chilled" propellants[7], Spacex has even been using it for awhile[8]. So maybe the future will be extremely light, reusable, oxygen rich, staged combustion, methane burning rockets with super chilled propellants?
[1]: http://www.astronautix.com/g/gnom.html
[2]: http://www.astronautix.com/p/pr-90.html
[3]: http://www.americaspace.com/2015/05/29/new-oxygen-preburner-...
[4]: https://spaceflightnow.com/2017/10/20/worlds-largest-methane...
[5]: https://en.wikipedia.org/wiki/Raptor_(rocket_engine_family)
[6]: http://www.russianspaceweb.com/soyuz5-lv-ptk.html
[7]: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/197900...
[8]: http://www.businessinsider.com/spacex-deep-cryogenic-cryo-li...
Does Skylon fit the bill?
https://en.wikipedia.org/wiki/Skylon_(spacecraft)
It pushes the limits of materials and design. But I think a reusable 2-stage rocket—as boring as it sounds—will end up working better in the near term. It can be done with safe margins.
Chemical rockets are also much more efficient devices than is given credit for. A good, very optimized two stage rocket like SpaceX BFR (and the ITS concept before it) can get payloads to orbit using just ~300 Megajoules per kg (250MJ/kg in the more efficient tanker configuration). The specific kinetic energy of orbit is about 25MJ/kg (40MJ/kg if we include gravity and aero losses and changes in altitude), so rockets are total 10% efficient at converting chemical energy directly into orbital energy, and with an optimized, high pressure and variable mixture ratio hydrogen rocket engine architecture with some improvements in materials science, we could improve that to 20% or more. That beats the chemical to mechanical efficiency of the typical car.
That translates to between $1 and $10 per kg to orbit. That's 3 orders of magnitude less cost than today, if we can master reuse. So arguably, chemical rockets are perfectly fine for achieving orbit cheaply.
built on the success of a rocket-based program! no way would a country fund a vehicle that spews out a constant stream of nuclear bombs as their species' first foray into orbit!
That's not how the NERVA was supposed to work.
https://en.wikipedia.org/wiki/NERVA
[0] https://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propu...
I like space, lots of neat things to learn, but the discussion always seems to get a little religious and doomsday-ish, and I find it less than genuine. Like have people thought about where the energy to get massive numbers of people off the planet will come from, and how that will leave the planet in even worse shape for those that can't leave?
It is folly to think technology can fix everything, and I sometimes wonder if it has actually "fixed" anything, aside from helping to enable overpopulation.
A billion people is a PWh. That's 0.5% of the earth's annual energy use. It's a blip on the radar, especially since it will subsequently reduce annual consumption in the 10% range.
plus where are you going to send %10 of the population where they will be immune to human nature, whatever that is.
I've always felt uneasy about this, as it seems to imply that the only way to put things in orbit is via rockets.
For example, there is the idea of a space crane (assuming it can be built) that is simply a very, very tall building, assembled one chunk at a time, and that once built can simply lift things in orbit.
Would a space crane be slave to the same equations?
But again, this is still a materials science problem.
We don't really know how to reliably manufacture materials that can sustain the loads required for something like this.
. . .
"doesn't the hypothetical space elevator break the tyranny of the rocket equation?" (which you did answer).
No it would not, it has its own cross to bear, tensile strength.
If you start from the ground, then as the crane gets taller, the forces on its outer edges are the torque applied when a lateral force pushes against it (think wind). As with a stubborn nut, the longer the wrench handle the more force you can apply to the nut. If you think of the crane as your wrench handle, the taller it gets, the more force it will exert on its mounts when air moves past it. (this it true of sky scrapers as well).
Eventually you move past the point where there is anything that can stay vertical (steels rip and bend, carbon fiber breaks, Etc.) There is a working solution, a pyramid, but to build a pyramid that reached low earth orbit requires a base that is larger than the state of Kansas and will sink down to bedrock. There is an entertaining discussion of this from 2011 here: https://www.wired.com/2011/07/does-the-slope-of-a-pyramid-re... We know from the Himalayas you can stack up a lot of rock into a really really tall mountain, but its really really a lot. This might occur naturally on some planet, Olympus Mons on Mars is 16 miles high (about 85,000 ft or almost 26 km) that is 25% of the way up to space (assuming the 100km is your line for 'space'). Eventually you hit the limit which is that gravity prevents you from having a feature stick out too far from a sphere.
Another way to do this is to put a weight in geosynchronous orbit so it stays over the same spot all the time. If you start building a cable downward and adding mass further out you can keep the "effective" mass at the geosyncronous point. Eventually, (materials science not withstanding) the part you are lowering down to the planet reaches the surface and voila you have your jack and the beanstalk type beanstalk.
As fantastical as that sounds, it should actually work, if, and it is a big if, you can make a material that can withstand the tensile force of being both pulled up from the counter weight, and down by gravity. Once built you could attach a vehicle that would ride up and down the cable moving things from ground level into geosynchronous orbit, with only the energy needed to climb up the cable. These "space elevators" are not precluded by physics so they show up in science fiction stories.
But once you get away from building structures you are left with either chemical rockets or some how pushing against the Earth. Creative ideas there are balloons that take you part of the way up, and electrostatic systems that use an electric field to push against the charge on the Earth. These ideas start off far fetched and move right into crazysauce fairly quickly.
The tallest structure ever built by humans is a little more than half a mile tall.
A space elevator to geostationary orbit (required for a circular orbit with no rockets) would have to be 22,000 miles tall.
Low earth orbit is a lot closer, what is considered LEO is pretty wide, but the ISS is at about 250 miles.
If you get an crane ride up 250 miles, you aren't orbiting. You haven't even done half of the work, to be in orbit at that altitude means you have to accelerate to several miles per second.
How orbital mechanics works is not very intuitive.
Something more feasible would be a Skyhook[1] which only provides part of the energy needed to get to space but which due to the exponential nature of the rocket equation could make a huge difference while being conceivable to build.
[1]https://en.wikipedia.org/wiki/Skyhook_(structure)
Also, the economic implications of the equation don't matter so much when you have a reusable rocket, and the BFR is supposed to be able to reuse both stages.
NASA source is from 2012, before BFR was fleshed out.
This is selling rocket science a little short, I think. The availabilty of gravity assists and aerobraking make it possible to achieve large variations in delta V necessary for a trip, as long as you can make the right tradeoffs elsewhere (travel time, etc).
See: https://en.wikipedia.org/wiki/Interplanetary_Transport_Netwo...
The challenge is to get from the Earth's surface, to orbit. Yes, there's oxygen along the way, but the speeds you have to go will wreck you if you stay in the atmosphere at all. So you get into vacuum ASAP.
Once you're in orbit, you have a lot more options. For example, you don't need a high-thrust engine—instead, high-efficiency is fine, even if it's low thrust. Etc.
Beam microwave or laser energy to ship
Launch loop
Sky hook
Launch fountain
Some company is even working on a launch slingshot.
I’d provide Wikipedia links but I’m on my palm pilot. Look them up though. Interesting stuff.
The problem is the same for all of the ideas like space-elevators, or asteroid mining, or spacetravel as a normal thing - given our current politics, there's no way you'd avoid occasional rock-droppage with catastrophic consequences for people still stuck on earth. It would just be too easy. If you start thinking along the lines towards nuclear-fueled rockets, the problem just gets worse.
Non-weapon rockets are high-energy, fiddly devices - and their dumb purely-destructive siblings are far simpler.
If launching rockets became cheap enough to put people on Mars, that would also mean they'd be cheap enough so even small states could afford ICBMs. Because rockets are ICBMs.
Lots of modern technologies do that. Like a child growing up—at what age do you use sharp knives? a lawnmower? a car?—humanity has matured ethically alongside our technology.
I agree we have to be careful not to Great Filter ourselves.
That's exactly the problem. I don't think we have at all. We do no serious study of ethics in our schooling, and frankly don't devote serious thought to the subject. It's very uncommon to meet somebody who has read even a single book about ethics. We're a society of meat-eaters who consider people who harm animals to be scum, a society that simultaneously considers the protection of children fundamental, and where the most likely age to die on a world scale is in childhood. These are reflections of a deep lack of ethical maturity.
Regardless of all of the inconsistency, I think it's hard to look at the times in which the world came to the brink of nuclear catastrophe in the cold war, and state confidently that this is a society with the maturity to hold nuclear weapons.
Even more intuitively: it's about distance. Flying from Los Angeles to Salt Lake City requires less fuel than flying from Los Angeles to New York City. Flying from Los Angeles all the way around the world and landing again in Los Angeles takes even more fuel.
https://what-if.xkcd.com/58/
https://what-if.xkcd.com/126/