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Space Train?

  1. Sep 16, 2011 #1
    Everyone has heard of the space elevator using a cable(s) that attaches to a geosynchronus satellite. Now trying to lift things is always a very energy expensive process, since just making a payload weightless cost energy and you get zero velocity and travel. Also how fast could an elevator travel using friction to move up the cable? An elevator moving 200 mph would take five days or more to get to orbit!

    What if the cable positioned wasn't striaght up? What if the cable actually ran accross the earth to the horizon striaght to the satellite? When the cable is actually horizontal then you're only having to push mass. Obviously as the cable rises because of the earth's curvature the angle with respect to the ground increases and there is some gravity that must be overcome but its still a lot less effort when compared to moving striaght up.

    If the cable could be magnitized then the space train could use a solinoid principle for propulsion and actually move at supersonic speeds since there is no contact with the cable the travel is fictionless across the cable!

    My first question is would the cable droop?

    My impression is perhaps not since the tension from the attached satellite should hold the cable taught.
     
  2. jcsd
  3. Sep 16, 2011 #2

    Drakkith

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    What do you mean by "using friction to move up the cable"?

    You would see no gain, as you would still have to make it out of the Earths gravity well just as far. Consider a handicap ramp at a building. It takes a much longer route to reach the same height, but is still equal work to someone using the steps.

    Definately. The enormous weight of the cable itself makes me doubt whether this kind of space elevator is feasible.

    I am not very familair with this kind of space elevator. How would the satellite keep the cable taught? Other than by continually using fuel I mean. Perhaps I am not understanding the principles behind this.
     
    Last edited: Sep 16, 2011
  4. Sep 16, 2011 #3

    DaveC426913

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    No it isn't. It takes at the same amount of work to move an object up a distance y regardless of the distance x. And that doesn't include extra the work involved to move it distance x, so your sloped cable will use more energy.
     
  5. Sep 16, 2011 #4
    Going straight up is fine. The reason an airplane doesn't go straight up is because of the flow of air is important for the motion of the airplane. With an elevator powered by electric cables, it can go straight up if it is positioned vertically (and has sufficient electrical power for movement of course).

    But how would you hold the cables in place? They would need to be very strong, very thick, and we would need a lot of it.

    Also, would we would need something in space stopping it falling back to Earth, or could it be a bit like satellites? I mean you know how if a ring spins round it doesn't look like it is moving (but it is)... if the cables were in (near-)frictionless tubes, and some were in space, and the cable kept moving round like a conveyor belt... the part of the cable furthest out in space would "pull" the cable into space and the part of the cable on Earth would "pull" the cable back down to Earth, and there is a chance for equilibrium. Is this possible or not (ignoring - for the moment - the effects of EM force when an electric current flows through the cable).
     
  6. Sep 16, 2011 #5

    DaveC426913

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    This would still need power. Otherwise you're trying to describe a perpetual motion device.

    Otherwise, yes, it is the basis for quite a few designs. Look up space fountain for a start.
     
  7. Sep 16, 2011 #6

    Drakkith

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    What is pulling the cable? Centripetal force? Or something like a satellite actually pulling it?
     
  8. Sep 16, 2011 #7

    DaveC426913

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    In the case of a space fountain? There are many designs. Some designs don't use a cable; they use discrete objects like slugs or beads.

    Regardless, generally, magnetism is the propulsion method. The force imparted on the cables/slugs/beads will both support the structure itself, keeping it vertical and rigid and it will serve as the backbone of the payload propulsion system.
     
  9. Sep 16, 2011 #8
    Getting to height x is always the same work, you're right, but the velocity when you reach height x is going to be higher because your not having to work against g to get the final velocity.

    The profile for jet fighters to intercept is a thin S or wave profile where the jet levels off or will actually lower its angle so it can get some gravity assit, but in either case its to gain velocity. That S profile is a faster route than moving in straight line and burns less fuel to reach velocity x and distance x, but the fuel needed to reach height x is always the same.

    Also birds with small wings like sparrows use that wave technique to travel long distances. In particlular you'll notice sparrows traveling like curise missles, with their wings folded their trojectories dip and then you see the little bird make a few flaps and then fold in its wings and then dip again, they continually repeat the cycle. Its a pretty cool trick to gain speed and enturance.
     
  10. Sep 16, 2011 #9

    DaveC426913

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    If you use a rigid structure to get to space, then you save that work cost, generally independent of its shape. The most efficient and shortest, therefore least-fuel-consumptive route is straight up. Sloping the cable gains you nothing.

    How is any of this relevant?
    1] we're talking about hanging from a structure, these are free, powered flight alternating with ballistic flight
    2] we're talking about mostly above any useful amount of atmosphere, these are entirely aerodynamic
    3] we're talking about climbing and keeping that height. The bird and jet plane are trading off potential energy (height) for kinetic energy (speed) - a zero sum game. To gain altitude and keep it requires the further expenditure of energy. There ain't no such thing as a free lunch.

    I'm not saying you have nothing here, I'm saying your logic is murky and you haven't thought through the mechanics and energy expenditures.
     
    Last edited: Sep 16, 2011
  11. Sep 16, 2011 #10

    Drakkith

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    What final velocity are you referring to?
     
  12. Sep 16, 2011 #11
    Why I said the work to reach height X is always the same, my objective is to get there in as little time and engery as possible. Since I know height is always going to cost me x I can strategize how to get velocity to save energy. That's not a free lunch it just a less expensive lunch than going straight up and having x velocity.

    If we look at like this: I push my plane to the height of 20 miles at a slow and steady pace of 600 mph, using a turbo jet, I now fire a rocket engine where that pushes my plane to 19500 mph. I then change my angle to gain altitude, my plane without rocket power is loosing 32ft/sec^2, after say 60 seconds my plane gains an altitude of 325 miles and I lost about 1400 mph during those 60 seconds! My final velocity however is 18,100 mph, I'm in a stable orbit. I save much more energy by changing the profile from straight up to a flight path that allowed the plane to be pushed in extremely thin atmosphere because I'm not continually working against gravity. In my final approach I do have to pay the piper two machs and it was well worth it!
     
  13. Sep 16, 2011 #12

    Drakkith

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    If you are using a space elevator, what does velocity have to do with anything? If you climbed a cable steadily up to geo-sync then you would be in orbit and only have to expend energy to reach that altitude. Since the elevator itself is being held up, it would supply the energy to increase your angular velocity to match itself.
     
  14. Sep 16, 2011 #13

    DaveC426913

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    OK, yes. You are reinventing the two-stage to orbit system - airplane to high altitude then rocket to gain velocity. (A pretty far cry from the slanty cable in your OP, but who am I to quibble...)

    There are problems with the two-stage system. Notably, it's more complex, having to be both plane and rocket and, being neither fish nor fowl, does neither as efficiently, eating into your fuel savings. eg: you're carrying useless aerodynamic mass all the way up to orbit.
     
  15. Sep 16, 2011 #14
    Not in the least in comparison to say the rocket engines of the space shuttle that weigh 7000 lbs each and are carried into orbit. Turbo jet engines are much lighter by at least 4 thousand pounds. Comine this with lower fuel demands, especially the O2, also using jet fuel rather than H2 removing the need for one of the heavier tanks and using lighter materials. The approach then is more competative than mutli-stage rockets that continually loose parts and require expensive launch facilities. Wings can double as fuel storage and actually integrating the rocket and jet engine is possible with approaches that I'm not at liberty to discuss, but the proposed unit utilizes much of the same hardware for both modes of operation.

    What most think of when trying to achieve orbit is altitude, but the altitude is trivial, the final velocity is the real objective.
     
  16. Sep 16, 2011 #15

    DaveC426913

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    I'm not sure what "most" you're refering to but we here know that. There have been many discussions here about the fluffiness of the X-prize for acheiving 100mile altitude, which does not consider the effort to achieve orbital velocity)

    Thing is, you've got to acquire the velocity outside the atmosphere. When trying to reach 18,000mph, the atmosphere is a serious liability, not an aide. Which is one more rather compelling reason to go straight up and gain altitude as soon as possible.

    The reason they use flight on some designs is because, if you do it in two-stages, you can get the altitude without the cost of hauling all that aeroplane and its fuel all the way to orbit - it just goes back down to land.
     
    Last edited: Sep 17, 2011
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