Half a croissant, on a plate, with a sign in front of it saying '50c'
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self-supporting railgun

launch spaceships through a vacuum tube supported by kites and bouyancy of the vacuum
  [vote for,

This is a way to launch into solar orbit with no fuel. The first step is to make a sphere with vacuum inside that is lighter than air, where the supports needed to keep the sphere from collapsing from air pressure are lighter than the air they are keeping out. This hasn't been done, but I suspect it is doable. Next step is to make it a hollow sphere, have the supports leave an open center. Next step is to make it a hollow tube instead of a hollow sphere.

Have an evacuated tube 2000km long, with the eastern 500km freefloating and supported/steered by kites and its own buoyancy, rising as high as possible, tens of km up. Stratospheric winds above 30 degrees latitude consistently blow from west to east. Have an opening at the eastern end of the tube, and actively evacuate the tube as the air rushes in. Accelerate spaceship to beyond escape velocity via railgun through the tube. The kites need to hold the tube steady and straight enough that the spaceship do not hit the sides, and so that they go out the small opening at the eastern end. There would actually be two such tubes side by side, with connecting tubes to dynamically switch from one to the other in case of maintenance or accident.

Unlike space elevators, this can be built with existing technology. It's just big and expensive. The ships it would launch would be big, around the size of jumbo jets. They would accelerate at 5G, so people could ride them. You could launch the whole human race into space with a few of these over the course of a few years if you could build ships fast enough.

rjenkins, Jul 05 2019

Obligatory link Cat_20food_20can_20shotgun
[normzone, Jul 05 2019]

NASA: recent NIAC proposal for vacuum balloons for Mars https://www.nasa.go..._for_Mars_Missions/
Mentioned in my anno [notexactly, Jul 22 2019]

LONG Gauss gun
[xaviergisz, Aug 27 2019]


       Err...how about point the 2,000 km tube vertically, then people would be able to slot satellites in, without a ladder?
not_morrison_rm, Jul 05 2019

       Well thinking of those "flailing man" tubes that are outside garages and car salesrooms, where the escaping air causes the tube to flail about wildly. Wouldn't the escaping vacuum cause similar flailing here? If you timed it right you could flip your sattelite out the end when the tube was flailing in the appropriate direction, aided by a puff of vacuum
pocmloc, Jul 05 2019

       Is it possible to have puff of vacuum?
not_morrison_rm, Jul 05 2019

       Yes. Justin Bieber.   

       // You could launch the whole human race into space //   

       Possible, but not necessary; the list of those who need to be relocated is actually quite short.   

       // two such tubes side by side //   

       If you used it to launch politicians, would it be a double-barreled shitgun ?
8th of 7, Jul 05 2019

       Obligatory link to cat food can shotgun ...
normzone, Jul 05 2019

       //Surface area grows with the square of radius while volume grows with the cube, so this seems doable if it is big enough.//   

       I think your thinking is wrong. Suppose you have a material which is **just** strong enough to let you make a 1m- diameter vacuum sphere with 1mm thick walls.   

       Now you want to make a 2m-diameter sphere. The compressive force will be increased 2^2-fold, i.e. it will be 4x greater. But the material supporting that load is only 2x greater. (If you have problems imagining it, think about a thin band of material running around the sphere's equator; it has to support the compressive force from the two half- spheres.) So, the sphere will fail. To make it work, you need to make the material twice as thick.   

       So now your 2m sphere has 4x the surface area, and 2x the wall thickness of the 1m sphere. Which means it needs 8x as much wall material. And it also displaces 8x the volume. So, you haven't gained anything.   

       Therefore, it follows (ipso calendro, ipsiur thesaurus) that if you *can't* find a material strong enough to make a small vacuum balloon, you *won't* be able to make it work just by building it bigger.   

       This is all based on the assumption that the structure will fail by crushing. You can also model it for failure by oilcanning (which is more likely for an imperfect sphere made of metal), but then it actually comes out worse as the balloon gets bigger.
MaxwellBuchanan, Jul 05 2019

       MaxwellBuchanan, I agree, the surface area vs internal volume reasoning was wrong, and you are right that scale doesn't matter. I was thinking an internal scaffolding, sort of like a tree, but your reasoning still holds: if a 1x1x1 cube can support n kg, then eight copies of that 1x1x1 cube (arranged in a 2x2x2 cube) can support 4n kg.   

       Atmosphere at sea level is 0.1MPa and weighs 0.001g/cm^3. Glass is about 3g/cm^3, so the scaffolding has to be less than 1/3000th of the volume in order to float. So it has to resist bending and not be crushed at 3000 atmsopheres (300Mpa). Glass has a compressive strength of 1000Mpa, so it's 3x stronger than needed to keep from crushing. Don't know about bending. Plain glass breaks easily, too, which is not a useful property here.
rjenkins, Jul 06 2019

       Pocmloc, no, the air rushing into the end will eventually hit the insides of the tube (adding momentum), but it's cancelled by an equal momentum should have hit the outside of the tube but didn't due to the hole at the end. And it's pretty high up where air pressure isn't much.   

       A worse problem is sonic booms (and air spontaneously catching fire) due to the escape-velocity spaceship inside the end of the tunnel going through the not-yet- evacuated air. The fix for all that is a big bulb at the end, to keep the walls further from the spaceship and to spread out the air.
rjenkins, Jul 06 2019

       //Atmosphere at sea level is 0.1MPa ...// On the other hand, at high altitudes the pressure is much much less, so a vacuum balloon becomes more feasible (but then again, the bouyancy is much less, so the balloon structure has to be much much lighter).   

       //it's 3x stronger than needed to keep from crushing. Don't know about bending.// Agreed. If you could make a perfect sphere, glass would work. But any deviation from sphericity, or any knock at one point, will cause a deformation and then you get a sort of positive-feedback collapse. It's a bit like trying to built a very tall, thin column with a weight on top: as long as it's perfectly vertical and not knocked, it will be OK; but if it bends even slightly to one side, it'll buckle.
MaxwellBuchanan, Jul 06 2019

       MaxwellBuchanan, lengthwise the tube is actually under tension not compression, due to outside wind drag. I expect the empty center of the tube to be about half the diameter of the outside, with the remainder filled with a lattice of supports. Buckling, the weight a column fixed at both ends can support without buckling is proportional to 1/length^2. And it takes close to 0 force in the middle to keep it from buckling. So, a light latticework of small triangles can stabilize large supports so they crush before they buckle.
rjenkins, Jul 06 2019

       Do levitating magnets have that same crush weight, if stacked on top of each other? Can they be stacked?. If not, might make a good powered exoskeleton.
wjt, Jul 07 2019

       wjt, elaborate? I don't follow.
rjenkins, Jul 07 2019

       Make it a cylinder. Spin it to use centrifugal force to help maintain the shape against air pressure. The thicker your walls the more force you have.
Voice, Jul 07 2019

       But a spinning cylinder, if there's any incident wind, will act as a Flettner rotor and experience large forces normal to its primary axis ...   

       As it's extremely long, the leverage at the lower end will be enormous.   

       It will probably start to "snake", become unstable, and disintegrate.
8th of 7, Jul 07 2019

       Super conducting magnets will lock at a levitating position. Can they be stacked (Stargate like)?and , a lot of if's , if they can be made room temperature and toroidal, wouldn't that help form a rigid tube structure against gravity.
wjt, Jul 07 2019

       // Can they be stacked (Stargate like)? //   

       Yes, of course they can, but if you have the technology to do that, why not just use your network of Stargates ?
8th of 7, Jul 07 2019

       //network of Stargates// You don't get a discount for repeat uses. The fees mount up quickly.
pocmloc, Jul 07 2019

       It might be more practical to simply pump the Earth's atmosphere into a large tank, thereby removing the air drag out of the equation.   

       Over millennia, humans will evolve to hold their breath come launch time.
not_morrison_rm, Jul 07 2019

       On holding one's breath at lunch time, Is it in fact possible to breathe and swallow at the same time?
pocmloc, Jul 07 2019

       Anatomically speaking, no.
MaxwellBuchanan, Jul 07 2019

       [8th] When extraterrestrial tech gets in human hands, whats the bet it won't be used how it was intended. Point in case, a magnetic structured ground escape tube.
wjt, Jul 08 2019

       By the way, the reason not to launch into earth orbit with no fuel is you'll collide with the earth again within an orbit. Solar would miss if the orbit isn't exactly a year. Also a year is more time to adjust orbits by slow means.
rjenkins, Jul 08 2019

       "self-supporting" led me into thinking "self- launching" which in turn led me into thinking about recursive railguns. Since they're just a bunch-o- magnets, could you construct a series of matroishka rail-guns such that each launched the remaining onion-like package inside of itself, up and up, smaller and smaller, until you were out in space?
zen_tom, Jul 08 2019

       Great idea [zent]. If they were infinitessimally small, each would require zero energy and therefore would be very cheap to run.
pocmloc, Jul 08 2019

       [zent], that does reduce the relative speed of the projectile vs the railgun.
rjenkins, Jul 09 2019

       //matrioshka rail-guns//   

       That would be possible, and it would be equivalent to a multi-stage rocket, sorta. For maximum efficiency, you'd want to control the mass and launch speed of each stage such that the recoil left the previous stage momentarily motionless. (Because, otherwise, whatever velocity your previous stage has is wasted kinetic energy.) But I suspect it would be, overall, no more efficient than a gun launching a gun which launches a gun...
MaxwellBuchanan, Jul 09 2019

       ^ Due to conservation of momentum and Galilean relativity, that doesn't make any sense.   

       Re the idea, vacuum balloons are well considered infeasible on Earth, but they may be non-infeasible on Mars. (Chrome was complaining about my use of "feasible" there, so I fixed it.) But we don't yet know about the winds at altitude or latitude on Mars (:-#). Anyway, if you somehow manage to do this on Earth (maybe with centrifugally supported vacuum tubes with sheaths to prevent them acting as Flettner rotors), at a latitude of 30°, then the minimum orbital inclination that you can launch to is also 30°. Therefore, it would probably be a good idea to launch to a parking orbit, reduce inclination, and then burn to escape, rather than launching directly to escape, which would result in a minimum inclination to the ecliptic of 6.5° (I think). That is, of course, unless you want a high inclination to the ecliptic. Then launching directly to escape is fine (orbital-mechanically at least, maybe not for other reasons), and you can also get a higher inclination if you can aim the exit end of the tube equatorward.
notexactly, Jul 22 2019

       // they may be non-infeasible on Mars //   

       Sp. "non-unfeasible" ?   

       Why not just say "feasible" rather than employing a double negative ?   

       Anyway ..   

       Buoyant flight using a vacuum lifting body is progressively less practical as the external pressure increases. On Mars, you have a thin atmosphere - lower density, lower pressure - but concomitantly less lift, which is directly proportional to the difference in mass between the envelope and the surrounding gas.   

       Gas filled buoyant balloons are typically at the same pressure at their lower end as the surrounding gas, indeed hot air balloons have a very large hole which has no effect on their lifting power. At the top, the force exerted is the "lift" of the envelope.   

       A balloon with 1kg of disposable lift in the Martian atmosphere will have to be proportionately bigger than an equivalent on your planet, and because of square/cube law the forces a vacuum balloon would be subjected to, given its much greater size, would be increased.   

       The best place to deploy a vacuum balloon would be on one of the gas giants - but it wouldn't be a "balloon", but a float.   

       Think of it this way; a thin steel sphere containing a vacuum would be buoyant in liquid water and could sustain an appreciable crush depth; but it ain't gonna fly ...
8th of 7, Jul 22 2019

       // Why not just say "feasible" rather than employing a double negative ? //   

       Because Chrome wanted me to change it to "possible", and I didn't feel like it.   

       See [link] re feasibility of vacuum balloons on Mars.
notexactly, Jul 22 2019

       There have been similar proposals. "StarTram" supported itself with magnetic repulsion between a superconducting cable above the evacuated tube with railgun and superconducting cables on the ground. They had a "plasma window" to keep air from rushing in at the end. Also a "space fountain", which had a similar evacuated tube with things rushing in it, but it slowed things down instead of speeding them up and elevated itself off the stolen momentum. They're all about 2000km long because that's what it takes to accelerate people to escape velocity safely.
rjenkins, Aug 27 2019

       // safely //   

       There's that strange word again ...
8th of 7, Aug 27 2019

       In this context "safely" means "without killing them by design".
rjenkins, Sep 07 2019

       Just as long as killing them by accident, carelessness, incompetence or indifference is still OK.
8th of 7, Sep 07 2019


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