Ken Burnside, about your space elevator.
- Thread starter Blue Ghost
- Start date
Hi,
I guess there may be a difference between what Traveller thinks might be feasible and what may actually be feasible in real life.
Vladika
SOC-14 1K
Okay, my non-engineering little mind is confused. You're orbiting in space. You drop one end of a line down and the other end up; they balance each other. Eventually one reaches the ground and the other is out in the opposite direction. Now you've got the Hindu rope trick, a rope that stands straight up from the ground by itself, courtesy of the pull imparted by the section of rope that's out beyond the orbiting midpoint of the rope - except you're saying that when the snakecharmer tries to climb the rope, applying a downward pull at the ground end, they're not in balance anymore: he's pulling the rope down instead of pulling himself up.
But add something: anchor your end in bedrock, attach a mass to the other end. The mass on the other end is like a rock in a slingshot, pulling hard on the rope. Let the anchor give, and the mass and rope go flying off. While the anchor holds, you can climb: you're subtracting tension, but there's already more tension in the line than it needs to stay upright, and your downward pulling mass isn't enough to overcome that upward pulling mass. You're like an ant climbing up a rope held by a great big giant hammer-tosser.
Am I looking at that right, or have I missed something in the physics?
Orbital mechanics.
the cable is hung, not "held up" - the orbital anchor station is (1) massive and (2) in orbit- it's NOT coming down unless it's hit.
The cables hang; put a pair of balls on a chain, and in orbit, they naturally orient so the long axis is pointing towards the barycenter. Put a bar in orbit, same thing. FIrst little bit of spin, and tidal forces begin to go to work.
Skyhooks are doable with current tech, because the material strengths are low enough (it's only 20-50 miles of cable, not 22,000), and the use of atmosphere to spin it is actually desired. The problem is putting them into service in the first place.
Vladika
SOC-14 1K
OK Tie a rope to a thick branch and climb. No problem. The tree is acting in compression and the branch is transferring both a moment and a force resisted by shear to the truck. Try the same with the twig at the end of the branch. Isn't going to work.
There is NOTHING up there to hold the rope up. Nothing at all. Similar to the twig. Yes, he's pulling the rope down, just like you are pulling the twig down.
The thing is, it isn't acting like a slingshot. The mass (of whatever) in space is in a geosynchronous orbit. In that case the object is in fact falling toward the Earth at all times. That is it's vertical vector. It also has a horizontal vector, tangent to the orbital circumference. The vector sum of the two components results in a "fall" that is just exactly keeps the object the the exact distance.
If you add ANY weight at all, no matter how slight, the vertical vector increases and the whole thing becomes an unstable orbit. You must then compensate by changing altitude and or speed.
That mass you put up there stays because nothing changes, neither it's velocity, nor weight. The moment added weight starts up the line, the mass starts to come down.
Satellites eventually fall back to Earth as they slow. They are often boosted into a higher orbit to keep them up longer (another topic).
IF you increase the speed of the orbiting mass you can in fact lift a load. Problems are exactly balancing that thrust required with the fuel reduction of the expended fuel and overcoming a lot of inertia acting over a huge distance. Your mass needs a "motor" and that "motor" needs fuel. You need a heck of a lot of energy and some serious calculus with way to many outside variables. (Density of Earth atmosphere, wind force at all stages, wind direction, humidity, etc. Pretty much what a tank computer does in a firing solution, on a much larger scale.)
There's nothing wrong with your mind, just not enough data or knowledge to fully grasp the situation. You are doing a good job of critical thinking and asking good, thought-out questions.
If this was even a remote possibility I'd think NASA would have been working on the idea for some time. They, after all, are the space experts.
Vladika
SOC-14 1K
"Hung" "Held up" You pull on a rope and something either moves or the rope snaps.
The barycenter of anything you could possibly launch into space would lie beneath the earth's surface. This does make a huge dent in your argument. In any case the barycenter of mutually revolving space objects is traditionally a gravitational phenomenon.
Your argument, with a "chain shot" analogy holds some water. The problem is that the masses are way to disproportional. Swing a bucket of water around in a vertical circle. Everything is just fine. Now pull it toward you. It moves toward you, You don't move toward it, providing you are in fact standing on Earth.
Same theory with this magical "space elevator".
Well, I guess NASA just doesn't have a clue and is pursuing outdated theories and technology. They aren't considering this line of thought, much less research, for a reason. The reason is that it's bogus.
Why? Do you want whatever is at the free end to spin around rapidly or the cable to snap from torsion? The connection to Earth would have to spin too.
aramis, your an intelligent guy, but you don't have a degree in either physics, or engineering. Reading a technical treatise just isn't a substitute. In the same vein, I wouldn't begin to read an article on computer anything and argue it with you. I simply would not know what I didn't know.
You haven't been listening. Once the cable is anchored, you let it out just a bit. ANd the counterweight cable gets adjusted for upcoming loads. It's really quite simple, but you're ignoring that we've all said there is a counterweight cable. A counterweight cable which is out past the anchor, and going faster than it's distribution cares for, and thus is trying to raise the system of anchor and cables, because the center of mass is no longer at the anchor, but beyond. Likewise, when you put the load on the planet side, it's climbing up, but the cable is being let out on the far side to counter it's pull and lack of centrifugal acceleration outward. (Which said, it's gaining orbital speed as it climbs the tether, because the orbit period is still 24 hours, but the circle inscribed by it's would be orbit is larger and larger.)
It's not "free", but it is orbital dynamics, and the best minds in the physics world all say the only problem is the materials tech. And carbon nanotube cabling might make that a non-issue, too. They've already shown that ribbon climbing bots can work.
It's not "free", but it is orbital dynamics, and the best minds in the physics world all say the only problem is the materials tech. And carbon nanotube cabling might make that a non-issue, too. They've already shown that ribbon climbing bots can work.
Vladika
SOC-14 1K
In that I have agreed. I stated clearly that the orbit, and or speed, would have to increase.
No, it proves you are clueless about it. If 10 people who've done the readings all say X, and you say !X, odds are, you're the one in the wrong. And some of the people saying what you claim is wrong are major physics professors, others major engineering professors.
Objects in orbit have a tendency to rotate on their center of mass, and orient their long axis towards the barycenter. Which, ideally, is in the center of the planet or star, but in real, practical terms, for earth, is off by several hundred miles, due to the moon. This is due to tidal forces - that is, if gravity at X is Y, gravity at (X-A) is ((X/(X-A))^2)Y, and at (X+A) is ((X/(X+A))^2)Y. So, if a long object isn't PERFECTLY perpendicular, the near end experiences slightly more gravity, and the far slightly less, and thus it rotates to the point where it can't rotate further.
Skyhooks should be a set of radial arms, held under their own centrifugal force into near rigidity, and spun by being dipped into the upper atmosphere. Think a ship's paddle wheel, but without the rim. Unlike a beanstalk, the skyhook elevator is passive and is not connected to the planet. It uses it's orbital velocity to stay in orbit, and spin its wheel portion. (A hub station with the solar gear for electric orbital maneuver is probable.) You don't climb the arms; you get caught and lifted; the station uses electric field acceleration to accomplish energy management as it throws you into orbit at slightly faster than itself (Vo +Vr), and above its own orbit, where the Vo needed is lower, and so it literally throws you into a higher orbit, due to the principle of orbital equilibrium.
Vladika
SOC-14 1K
They did. So have a lot of people.
"have build a tower about 50 km high, with a cable to be connected at the top" is where the argument by arimas fails. You DON'T want atmospheric effects.
Now, forgetting the cable for a moment, who claims to be able to build this 50Km Tower? Exactly nobody.
Rigel Stardin
SOC-13
(Was going to post this before atpollard made his post)
Centrifugal force keeps the tether tight. Even though it's in geocentric orbit the counterweight exerts enough force to keep the tether tight. The mass of the elevator climbing up the tether does not exceed the centrifugal force exerted on the tether, therefore maintaining the counterweight in space. Particle engines could be used to counteract the elevator movement up and down the tether. They can also be used to keep the counterweight stationary in orbit.
One way to compensate for the elevator climbing the tether is to use a maglev system. The force exerted by such a system would be much less than actual contact of the climber to the tether. This system would add weight to the tether and increase the size of the counterweight.
One problem I see with a space elevator is harmonic vibration caused by the motion of the vehicle climbing the tether and atmospheric forces on the part of the tether exposed to Earth's atmosphere. There would have to be some way of dampening the force from these vibrations.
Centrifugal force keeps the tether tight. Even though it's in geocentric orbit the counterweight exerts enough force to keep the tether tight. The mass of the elevator climbing up the tether does not exceed the centrifugal force exerted on the tether, therefore maintaining the counterweight in space. Particle engines could be used to counteract the elevator movement up and down the tether. They can also be used to keep the counterweight stationary in orbit.
One way to compensate for the elevator climbing the tether is to use a maglev system. The force exerted by such a system would be much less than actual contact of the climber to the tether. This system would add weight to the tether and increase the size of the counterweight.
One problem I see with a space elevator is harmonic vibration caused by the motion of the vehicle climbing the tether and atmospheric forces on the part of the tether exposed to Earth's atmosphere. There would have to be some way of dampening the force from these vibrations.
Vladika
SOC-14 1K
In all fairness, we have digressed from an elevator to "Climbing a pole" two very different things.
And your
is exactly why NASA would require the 50Km tower.
"harmonic vibration" is a real nightmare for any thin structure subjected to fluid forces.
Do you have any links to any technical discussion of a space elevator as a box lifted by a cable rather than as a car climbing a pole?
I don't ask to be flippant, but because I have never read anywhere about a space elevator other than a moving car climbing a fixed cable.
[with the one caveat that I remember reading about a plate of solid fuel and a laser to burn the plate to create lift ... But that never felt like an elevator to me.]
Vladika
SOC-14 1K
Hey, I started responding to the OP and the definition of elevator he was refuting. I agreed with him. As for a climbing pole? It could theoretically work. An elevator? No.
Here is two good links on the "climbing a pole" by the elevator name.
http://lupuvictor.blogspot.com/2012/09/space-elevator-to-moon-over-8-years.html
http://www.cosmosmagazine.com/features/orbital-express/
There aren't going anywhere with either.
BTW, I've never though you were being flippant. We're just friends here having an interesting discussion.
Hi,
I agree about it being a fan recommendation but I mentioned Traveller because he referenced Traveller armor levels (which I have grown to become a bit disillusioned by, because they kind of seem to me to be not particularly realistic)
There are TWO KINDS OF SPACE ELEVATOR.
Beanstalks, where what you say is marginally correct (in that atmospheric effects have to be accounted for)...
and Skyhooks, where you use them to spin the skyhook's wheel.
Both are Space Elevators... and both have similar base issues - tensile strength, atmospheric interaction, and orbital mechanics.
Skyhooks need less tensile strength, and use the atmosphere to ensure spin... just as waterwheel spins; the top resistance is sufficiently less that the bottom's drag induced aftward motion is converted nicely into spin, as the whole thing orbits forward. Moreover, Cable based (rather than fully rigid) can extend/retract to decrease/increase spin rate.
Skyhooks are one of the NASA proposals that was shot down for being "doable but pointless"... getting up to 50 miles for a 150 mile orbit is impractical as a low-volume system. We couldn't, in the early 80's, conceive of needing them in the near term, and didn't have craft capable of making them feasible due to reentry issues.
Beanstalks are likely impractical at all non-gravitic tech levels. At Traveller TL 11+, you can take 90% of the weight off the cable, and drive the endpoint, all using nothing but electricity to power it. But, by the same toke, when they fail, they fail BIG.
Hi,
One thing very unclear to me has always been if you attach the tether to the ground at the bottom end then you are introducing additional forces into the cable, whereas if you leave it free (like in a Skyhook system) then what keeps the end from flailing about due to the various forces that it will be experiencing. As such, the tether should in all likelihood be subjected to far more forces and bending, etc than just the tension due to supporting its own weight.
One thing very unclear to me has always been if you attach the tether to the ground at the bottom end then you are introducing additional forces into the cable, whereas if you leave it free (like in a Skyhook system) then what keeps the end from flailing about due to the various forces that it will be experiencing. As such, the tether should in all likelihood be subjected to far more forces and bending, etc than just the tension due to supporting its own weight.
Similar threads
