Space Elevator ribbon harmonics or “Don’t go all wobbly”…

In the most recent issue of Acta Astronautica, Lubos Perek (of the Astronomical Institute, Czech Academy of Sciences) writes of his concerns with the stability of a Space Elevator.  From the abstract:

“The size of the Space Elevator and its lack of resistance against buckling or bending require a detailed study of its stability, both in its initial phase as a geostationary (GEO) satellite as well as in its operational phase as a “sling”. Lunisolar perturbations and other minor forces may affect the stability in the initial phase and will cause oscillations in the operational phase. Station-keeping thrusters will have to be mounted at selected points along the cable in order to maintain stability. In addition, the thrusters will perform local maneuvers for avoiding collisions with passing space debris. The control system of thrusters has to be adaptive, reacting fast to actual situation and rectifying the attitude of the Elevator whenever necessary. A further advantage of the thrusters is a possibility to locate the Elevator at any longitude, possibly looking for a region with minimum traffic at GEO distance. Extensive numerical simulations will have to be performed in order to determine elements of the thrusters and their control system.”

Columnist David Shiga writes about this study in the latest issue of NewScientistSpace (available here).  Rather than just sensationalize this study, David checks with others who may have an opposing point of view.  Both Anders Jorgenson and Dr. Brad Edwards comment in Shiga’s article saying that a) Perek’s study does not definitively quantify whether or not thrusters would be needed on the ribbon to stabilize it and b) these thrusters would introduce their own problems.

In addition Blaise Gassend (via Andy Price) comments on the study as follows:

A rather disappointing paper as it makes plenty of claims but substantiates none. As far as I can tell, the author considers the elevator to be identical to a geosat and applies results from geosats to the SE (though he doesn’t actually detail where his numbers are from). However, the elevator’s North-South mode is a bit less than 24 hours, which limits how much the resonance with the moon and sun can build up. I had run a back of the envelope a couple of years ago and the steady state oscillation was far from alarming (though I don’t remember precisely what it was anymore).

Of course I’m sure that neither Jorgenson nor Gassend nor Edwards think that there’s absolutely nothing to worry about, but rather it’s just one of those engineering issues which will have to be examined and, perhaps, dealt with.  The Tacoma Narrows tragedy is certainly a cautionary tale to engineers who neglect seemingly insignificant factors:

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And, perhaps, such a “wobbly” problem might look like this in a ‘real’ space elevator…

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(A ‘thank you’ to reader rb for ensuring that I knew about this story)

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