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Picture this, an elastic tube in an elliptical orbit around the Earth, the apogee is in space the perigee is in the atmosphere. This tube takes up the entire orbit around the Earth and every part of it is in orbit, the part that is closest to the Earth, within the atmosphere is moving the fastest, the part that is at the maximum altitude is moving the slowest in its orbit as per Kepler's laws, to accomplish this the tube stretches when it is nearest the Earth so it can move faster than its slower portion while the part that is furthest away bunches up so it can move more slowly and every part of it is moving at orbital velocity for its given altitude. Since its a continuous loop it displaces no air as it moves through the atmosphere, but it moves through the atmosphere at orbital velocity. Embedded within this tube is a maglev track, the tube orbits through a ground station which precesses the tube so that the orbit stays fixed relative to the Earth's surface, and maglev cars move at orbital velocity relative to the tube at the ground station so that relative to the Earth's surface it starts out stationary. Passengers board the car at the ground station as the elastic maglev tube moves under the car at orbital velocity. As soon as all the passengers are aboard and strapped into their seats, the doors close and the car magnetically brakes slowing down relative to the elastic launch loop but accelerating relative to the Earth's surface, the car eventually matches the velocity of the track while in space and then continues to accelerate till it reaches a velocity at the top of the loop where it is at the orbital velocity of a circular orbit or greater all the way out to escape velocity. What do you think of this launch method?
Last edited by Tom Kalbfus (2013-11-18 01:50:54)
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(1) I cannot understand how any structure moving through the air does not displace air, causing drag, and my education and experience is very heavy in aerodynamics engineering. You are talking about super-hypersonic orbital speeds, too. Drag will be enormous, as will the aero-heating problem. The space elevator centered at the geosynch point, and non-rotating in any way, does not move through the air. What you described doesn't sound as if it is centered on any geosych point in any way at all. Therefore it has to move through the air. If it does, there will be drag.
(2) Structures like this loop of yours have the same problem space elevators have: they require some sort of "unobtainium" material, even supposing you are right about no drag. Carbon nanotubes offer a possibility of making progress toward materials like that, but we cannot even successfully spin a long thread with them yet. They may or may not ever be strong enough for applications like that.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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What about an orbital ring 1000km up, using maglev to anchor elevators? When you get up to it, you transfer across to another part of the ring, and decelerate to orbital velocity...
Use what is abundant and build to last
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(1) I cannot understand how any structure moving through the air does not displace air, causing drag, and my education and experience is very heavy in aerodynamics engineering. You are talking about super-hypersonic orbital speeds, too. Drag will be enormous, as will the aero-heating problem. The space elevator centered at the geosynch point, and non-rotating in any way, does not move through the air. What you described doesn't sound as if it is centered on any geosych point in any way at all. Therefore it has to move through the air. If it does, there will be drag.
(2) Structures like this loop of yours have the same problem space elevators have: they require some sort of "unobtainium" material, even supposing you are right about no drag. Carbon nanotubes offer a possibility of making progress toward materials like that, but we cannot even successfully spin a long thread with them yet. They may or may not ever be strong enough for applications like that.
GW
Maybe I didn't describe it well enough, let me try again.
First example: Lets say you have a car going 60 miles per hour on a highway, as the car travels down the highway at 60 miles per hour it pushes air aside as it passes through the space occupied by the air, since the air has mass, it resists being pushed aside by the car and this creates drag which slows the car down if the engine isn't adding power to maintain the car's speed. There is definitely drag here
Second example: Lets say there is a sphere made of stainless steel, and a smooth nearly friction less surface, it is in a room full of air the sphere is spinning in place on this frictionless surface, the tangential velocity at the equator of the sphere is 60 miles per hour.
Now if you are standing on the side of a highway as a car just misses you by 1 foot as it passes by at 60 miles per hour, do you feel air displacement? I think the answer would be yes, you would feel a gust of air pushed aside by the car as it passes you.
Now lets say you walk up to this spinning stainless steel sphere, the sphere is spinning at 60 miles per hour, yet it sits on its axis of rotation as it spins, and you are standing 1 foot away from its spinning surface. Do you feel any displaced air? I think not, or at least not very much depending on the smoothness of the surface of the sphere, but I would imagine a sphere spinning as 60 miles per hour would displace less air than a car speeding by at 60 miles per hour, even if both the sphere and the car had the same volume and mass, the car would displace more air because it would move into an area where it previously was not, while the sphere just spins, but no part of the sphere moves into an area where it was not previously, so a perfectly smooth sphere would displace no air as it spun and feel no friction as it slid past the air if its surface was perfectly smooth.
Now lets consider an endless loop moving through our atmosphere at orbital velocity, no part of it is moving into an area filled with air that it wasn't in previously, so what's going to experience more friction an endless loop moving through the atmosphere or a meteor just entering out atmosphere at orbital speed?
Now lets suppose this loop has an internal maglev track, and you have a maglev car just above its rapidly moving surface but not touching. The car levitates off of the magnetic field and moves relatively slowly through the air, it ascends up the loop without touching it rising higher in the atmosphere, as the atmosphere thins it moves faster until it reaches space, and its speed catches up with the loop and then exceeds it reaching the loop's farthest point and it then releases its payload into space, the car then descends on the loop slowing down as it approaches the atmosphere and then descended slowly through it without experiencing much friction and hardly any heating at all, and then it parks in the ground station in which it originated ready for the next load to be taken into space.
Does this description help?
Last edited by Tom Kalbfus (2013-11-18 15:06:31)
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What about an orbital ring 1000km up, using maglev to anchor elevators? When you get up to it, you transfer across to another part of the ring, and decelerate to orbital velocity...
The more elliptical the orbit the more the belt or tube has to stretch, it elongates at its closest point because that is where its moving the fastest and shortens at its furthest point because that is where its moving the slowest.
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In that scenario it would still be necessary to have compressive structures to get to 1000 km, of course (although 200 km, maybe even 150 km would probably be low enough that atmospheric drag would effectively be nil). This is not a trivial challenge.
What you might get away with would be to spin the ring at slightly faster than orbital velocity and then use tensile members that hang down to the ground to climb up. You'd be able to do it with kevlar given a mild taper. This would of course induce stresses in the ring and make it more complicated to operate and construct (I don't want to be the one designing the 8 km/s bearings, that's for sure!). This does, of course, have the benefit of being physically possible. Probably even using known materials.
The downside, of course, is the 40,000 km long structure that you need to build. Good luck!
-Josh
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Thanks, the spinning sphere concept helped me very much, to understand your loop launcher concept better. There is a problem, the air in the room around your spinning sphere does not remain still, even though no air is displaced by translation-type motion.
There is still a "skin friction" effect, parallel to the moving surface. Using your example of the spinning sphere in the room, yes you will feel airflow, moving circumferentially in the room, same direction as the sphere spins. The friction of the air, no matter how smooth the sphere, will cause the spinning sphere to drag the air adjacent to it into spinning with it.
At some radius not far from that of the sphere, the air will be moving almost like a spinning solid cylinder, spinning at the sphere speed. Further out radially, the velocity distribution is inversely proportional to the radius, with fluid shear between every "layer" as you move outward. Basic vortex flow. There is also a speed decrease axially, but I don't know the variation formula off the top of my head.
Move this into the realm of supersonic speeds, and you complicate it greatly with shock wave formation and what is called viscous dissipation, which is an incredible "black hole" for converting useful energy into waste heat. Plus, everything starts to get really, really hot. The air, solid surfaces, everything.
Move this from 1 or 2 Mach numbers to 25-ish Mach numbers for orbital speeds, and you magnify all this friction and energy dissipation exponentially, in part because the hot air isn't air anymore. It has ionized into individual nitrogen and oxygen nuclei immersed in an electron sea. That's another immense black hole sopping up all the energy being used to drive the sphere.
Convert the sphere shape to your loop shape, and all the same phenomena still apply. It might work, but it will take an enormous amount of energy to keep it in place against all the viscous shear effects, and it will get very hellishly hot.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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However most of the loop is above the atmosphere, effectively the loop is in low Earth orbit almost the same as the International Space Station except part of that orbit is within the atmosphere, actually the troposphere. An example of such an orbit. The Earth is 6,400 km in radius, so you have an orbit that has a perigee of say 6,402 km and an apogee of 6,700 km, this is still a very circular orbit, it would make a complete circuit of this orbit every 88 minutes, everytime a part of this ring dipped into the atmosphere the skin would get hot, then once it left the thick part of the atmosphere it would radiate this heat into space or the heat would be conducted away by the material of the ring. The amount of heat would be proportional to the ring's surface area that is within the atmosphere. The thicker the ring the better because with thickness you have more material that can absorb heating of the skin due to friction, and probably some central portion of the ring would stay cool if it was large enough. As for what's keeping the ring at orbital velocity, I would say the ground station, it is pushing the ring forward and bracing against the ground as it does this.
As for where the ring would be constructed, I would say it most likely would be constructed in Low Earth Orbit out of an asteroid. First the asteroid is brought into low Earth Orbit, then construction robots would build the ring in orbit until it completely fills the orbit's circumference, then retrorocket fire causing the ring to slow down and part of it to dip into the atmosphere, a waiting ground station then receives the ring and maintains the velocity it has. The ring would probably be moving slightly faster than orbital velocity for its particular orbit as it will have to support the weight of various maglev cars so they can travel into space.
At the apogee of this ring is another ring in a more circular orbit that stays above the atmosphere, this orbits every 90 minutes, and cars along it can acceletate to super-orbital speed with this track holding it into a circular path until the payload is released into a higher elliptical orbit, the apogee of which is a ring even further out, and the process is repeated until the cargo/passengers are in the desired orbit. Some orbits are at the escape velocity of the Earth. An interplanetary ring, just outside the Earth's orbit, at for example 1.1 AU orbits the sun, a payload could then be accelerated along this ring and then released so it flings outward in an elliptical orbit around the Sun. Another ring around Mars intercepts this vehicle and slows it down dropping it toward Mars where another ring similar to the one around Earth has an orbit with an perigee of 3,400 km at a Martian ground station and a apogee of 3,700 km, and other planets have similar arrangements, one for Venus, one for the Moon, one even for Saturn perhaps. The interplanetary railroad.
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Do you have any calculations wrt what kind of elasticity would be necessary to make this workable? How does this compare to existing materials? What would the magnitude of the drag be? My expectation is that it would be pretty high, and therefore your ground station will need to exert a lot of force, and what's more exert it on a continuous basis.
How do you plan to exert this force on your loop if it to deform elastically in response to deformations? What is your plan to deal with the increasing brittleness of a material as you cycle it?
-Josh
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I think first thing to do is examine the difference in velocity from the top of the orbit to the bottom, and then imagine the ring as a stream of particles, that accelerate as the approach perigee and decelerate as they approach apogee. We know according to Kepler's laws of orbital motion than an object in orbit will map out equal areas of its orbit in equal time periods.
The transverse orbital speed is inversely proportional to the distance to the central body because of the law of conservation of angular momentum, or equivalently, Kepler's second law. This states that as a body moves around its orbit during a fixed amount of time, the line from the barycenter to the body sweeps a constant area of the orbital plane, regardless of which part of its orbit the body traces during that period of time. This law is usually stated as "equal areas in equal time."[citation needed]
This law implies that the body moves faster near its periapsis than near its apoapsis, because at the smaller distance it needs to trace a greater arc to cover the same area.
http://en.wikipedia.org/wiki/Orbital_speed
I'll have to do some research and get back to you. I think for an orbit of this sort the eccentricity is not that great so neigther would the ellasticity required. There are a couple of ways to achieve this elasticity, one way is with a rubbery material that stretches, another is with altermatinc cylinders, one with a slightly smaller diameter than the one next to it or a slightly larger diameter so the smaller cylinder can slide inside the larger cylinder as one would collapse a telescope. Electric actuators would determined by how much the cylinders extend or contract so as to achieve approximate zero gravity pull at the center of each cylinder as it moves in its orbit. Perhaps to protect this arrangement from air friction the portion within the atmosphere could move within a vacuum sleeve that surrounds the portion that moves within the atmosphere, it would be magnetically levitated against the track and its purpose would be to keep most of the gases from making contact with the track. This would be a version of a launch loop. From the point of view of the ground station the sleeve would seem to extend in both directions toward the horizon in a slight curve that is less than the curvature of the Earth, so the ground station would have to be at a high enough elevation so the rest of the track can clear all other topological features in its path. Perhaps the best place to have this station would be an island in the Pacific, as the track would intrude upon a minimum of other country's air spaces before reaching space. I think the best way to build it would be from space, as you have the fewest obstacles in its path as you build it rather than building a structure on the surface of the Earth and trying to accelerate it to orbital velocity while on the ground.
That is it starts out in an orbit over the equator that stays in space, for instance at an altitude of 300 km above the surface of the Earth. When the ring is complete, you fire retro rocket at a certain point, so that the opposite end of the ring dips into the atmosphere. (Before this happens you build a sleeve around a portion of the ring and you use the maglev track to decelerate the sleeve so that it is stationary in relation to the Earth.) The sleeve maintains a vacuum within so there is no atmospheric friction. The part that dips into the atmosphere is lowered until it is mated to a cradle in the ground station. The ground station the hold onto the sleeve and precesses the orbit to match the Earth's rotation. That is the apogee moves around the Earth at the same rate as the Earth rotates.
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Here are my results:
I find that an orbit with a maximum altitude of 700 km works best with a perigee equal to the radius of the Earth 6378.1 km (0 km altitude) which will give us a apogee of 7078.1 km and a mean radius of 6728.1 km.
Long : Altitude : Velocity : Expansion/contraction
0 : 350.0 km : 7.70 km/sec : 1.000
30 : 525.0 km : 7.50 km/sec : 0.974
45 : 597.5 km : 7.42 km/sec : 0.963
60 : 653.1 km : 7.36 km/sec : 0.956
90 : 700.0 km : 7.31 km/sec : 0.949
120 : 653.1 km : 7.36 km/sec : 0.956
135 : 597.5 km : 7.42 km/sec : 0.963
150 : 525.0 km : 7.50 km/sec : 0.974
180 : 350.0 km : 7.70 km/sec : 1.000
210 : 175.0 km : 7.90 km/sec : 1.026
225 : 102.5 km : 7.99 km/sec : 1.037
240 : 46.9 km : 8.05 km/sec : 1.046
270 : 0.0 km : 8.11 km/sec : 1.053 (Ground Station)
300 : 46.9 km : 8.05 km/sec : 1.046
315 : 102.5 km : 7.99 km/sec : 1.037
330 : 175.0 km : 7.90 km/sec : 1.026
Looks like the maximum expansion/contraction is +/- 5%, I think there are materials that can expand and contract by this amount.
Last edited by Tom Kalbfus (2013-11-21 00:22:59)
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I'd say so. My next question would be what kinds of stresses you would expect to have in the loop. This should take into account the fact that this stretching will generate a force, for most materials proportional to the magnitude of the expansion and contraction, as well as atmospheric drag forces and the impeller used to cancel them out. Keep in mind that the forces that result from a local expansion or contraction of the loop will modify the orbit. You may find that you'd rather have the point of zero stress be at the highest altitude rather than the middle.
To minimize this, use a material with a low Young's modulus. The downside there is that these materials tend to be low strength.
Have you considered that it might be more accurate to model the loop as a circle that is subject to shear stresses? If the loop is only slightly off center the shear stresses might not be too high.
-Josh
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I think high strength is not required in this case, unlike say a space elevator hanging down from geosynchronous orbit, in this case the entire loop is in orbit.
First step is to build the loop in orbit, this orbit will start out circular.
Then you build two sleeves in space, each sleeve is a giant maglev car that completely surrounds the loop. One covers an area equal to 90 degrees of a circle, the other a much smaller portion of it. Both parts start off at orbital velocity, then the sleeves accelerates in a direction opposite to the loop's orbit, after a while both synchronizes their motions with the Earth's surface. The small sleeve has a rocket attached, it fires its rocket to push itself in the opposite direction to the loops orbit while breaking magnetically against the track so that it maintains its position above Earth while slowing down the loop at that point, this changes the shape of the loop's orbit so that it is more elliptical, as it does so the opposite side of the loop with 90 degrees of sleeve lowers itself into Earth's atmosphere and dock with the ground station.
Last edited by Tom Kalbfus (2013-11-21 12:17:39)
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My gut feel is that the atmospheric drag forces and friction heating will be catastrophic, no matter whether the loop moves streamline to the air or not. Any off-angle just makes things worse.
Plus, Josh is right about vibrations induced by vehicles using this thing. There's as yet no "unbelievium", "unobtainium", or "manurium" from which to make this thing. Same basic problem as the space elevator, which at least hangs static in the air.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Vibrations can be countered and compensated for. I explained how to deal with atmospheric drag. Space elevators require tensile strength because they must lift their own weight as well as the weight of the payload, the loop of course is in orbit so weighs nothing. I think with a small amount of tensile strength is can orbit slightly faster than orbital velocity and keep its shape, the forces pulling the loop apart are much less than those weighing on a space elevator, you could call this a space train if you like. All that's really required is the ability to build on a really large scale in space, much the same is required in building solar powered space lasers to push interstellar light sails to Alpha Centauri, the laser is 1000 km wide and so is the sail it is shining on, if you can build that, you can also build this loop. The tech parts are dealt with engineering, not unobtainium. Unobtanium is for people with little imagination on how to solve problems, such as the scrith that Larry Niven uses to build his imaginary Ringworld out of.
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Ignoring the drag/heating problem (a mistake, I might add), look at the velocity distribution of the "connected-particles" around your orbiting loop. If the orbit is circular, they all have the same speed, and thus want to stay the same distances from each other.
If your orbit is elliptical, and it has to be to function as a launch device, then at perigee the velocities are large while at apogee they are lower. The particles will want to be a lot farther apart at perigee, and a whole closer together at apogee.
Since the velocities and their differences are measured in km/s, I suggest that we are looking at compressions and extensions measured in km, relative to gage lengths (typical construction dimensions) measured in perhaps 100's of m. I don't know how to figure the numbers for stress-strain in this orbit situation, but this m vs km disparity suggests we would have to have a material that is elastic over strains measured in the hundreds to thousands of percent. That's not a material available to humankind at the present time in history.
I've worked with polyether polyurethane (PEPU) which was 800% elongation at failure, but only the first dozen-or-so percent was at all elastic. That's the biggest stretchiness I have ever heard of. I used it as a one-shot shock cord in a parachute rig. Its normal use is absorbing the shock of fins opening on bombs. This is all one-shot stuff, the elongation is almost entirely inelastic. And the stuff was a molten mess at only 300 F. Metals, rock fiber, carbon fiber, all have single digit elastic elongation numbers, and small digits at that.
You can work around elongation to provide stretchier spring constants in non-trivial shapes, like a coil spring. Geometries like this would have even more drag and heating as they dipped through the air at orbital speeds. There's a very good reason we have to use sacrificial ablatives at the stagnation zones of entry heat shields.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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The loop is 42,400 km in circumference
Long : Altitude : Velocity : Expansion/contraction: Unstretched Segment length : Stretched length
0 : 350.0 km : 7.70 km/sec : 1.000 : 3,533 km : 3,533 km
30 : 525.0 km : 7.50 km/sec : 0.974 : 3,533 km : 3,441 km
45 : 597.5 km : 7.42 km/sec : 0.963 : 1,766 km : 1,701 km
60 : 653.1 km : 7.36 km/sec : 0.956 : 1,766 km : 1,688 km
90 : 700.0 km : 7.31 km/sec : 0.949 : 3,533 km : 3,353 km
120 : 653.1 km : 7.36 km/sec : 0.956 : 3,533 km : 3,378 km
135 : 597.5 km : 7.42 km/sec : 0.963 : 1,766 km : 1,701 km
150 : 525.0 km : 7.50 km/sec : 0.974 : 1,766 km : 1,720 km
180 : 350.0 km : 7.70 km/sec : 1.000 : 3,533 km : 3,533 km
210 : 175.0 km : 7.90 km/sec : 1.026 : 3,533 km : 3,625 km
225 : 102.5 km : 7.99 km/sec : 1.037 : 1,766 km : 1,831 km
240 : 46.9 km : 8.05 km/sec : 1.046 : 1,766 km : 1,847 km
270 : 0.0 km : 8.11 km/sec : 1.053 : 3,533 km : 3,729 km (Ground Station)
300 : 46.9 km : 8.05 km/sec : 1.046 : 3,533 km : 3,696 km
315 : 102.5 km : 7.99 km/sec : 1.037 : 1,766 km : 1,831 km
330 : 175.0 km : 7.90 km/sec : 1.026 : 1,766 km : 1,812 km
That's all I have time for now.
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I'd add that the nonzero elastic modulus (Young's modulus) of literally any material means that your loop will always fall somewhat short of truly being in orbit. This is where the stresses start to come from.
-Josh
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Not quite in orbit is still alot better than hanging down from geosynchronius orbit, as the loop doesn't have to lift most of its own weight, as it is mostly in orbit. You could have a rail road to Mars for example, A 100 car train accelerates up the loop to the top, detaches, rocket thrusters fire circularizing the trains orbit and it docks to a second circular track and accelerates along that one, building up 1 g of centrifugal force at 700 km above the Earth's surface and then releases, every one in the train is weightless once more, rocket thrusters fire to correct the train's trajectory to put it on an intercept course for Mars, then the train curls up in space and its front attaches to its rear, and the rockets fire once more causing the train to spin for gravity, the passengers can now get up of their seats and walk around inside the cars.
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Why npt just use a launch loop to get into orbit? Or a skyhook?
Use what is abundant and build to last
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The question is which is easier to build, which would be less expensive, what are the plusses and minuses for each system? A skyhook is a rotating tether in some orbit below geosynchronous orbit, one end of the tether touches down at certain points on the Earth's surface, and the rotation of the tether runs counter to its orbital motion so that the tether ends match the velocity of the Earth's surface when they touch down. the Tether still has to deal with centrifugal forces much as the space elevator does, the tether needs to be tapered and needs to be made of some strong stuff to hold itself together, one advantage is the payload does not need to climb the skyhook as it would a space elevator. With the ELL instead of an elevator car, you have a maglev train and it moves mostly horizontally along it with a slight ascent, unlike the space elevator which is a vertical journey.
Last edited by Tom Kalbfus (2013-11-24 09:09:22)
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You pretty much get that with a launch loop, which would be much easier to build...
I wasn't thinking of the skyhook that touches down, I was thinking of the one that you rendezvous with in a suborbital craft.
Use what is abundant and build to last
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The disadvantage is that then you need a suborbital craft. Would SpaceShipTwo, do or would you need some other kind of suborbital craft? I'm thinking that if you can build a space elevator, you also could build a skyhook that touches down, and if your going to do that, I'd say build a fairly robust one. Build a modular Mars colony on Earth, move it to the touch down site, when the skyhook touches down, hook the module with colonists inside and fling it into space, then some other tether higher in orbit will grab onto it and fling it onto a trajectory to Mars, where another skyhook awaits to catch the module and lower it onto the surface of Mars, the module is then towed to its final location in the Mars colony to make room for other modules touching down on the Mars surface at that same site. I think SpaceShipTwo is not the proper vehicle to send towards Mars, you need something bigger.
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