RE: starship-design: Tugs

["L. Parker" <lparker@cacaphony.net>]

Jim,


Uh, just a note folks, really quick (I'm working on my rebuttal for my
long range tanker idea).  Stretching the sail will not stop catastrophic
vibrations.  Even systems in pure tension suffer from vibrations.  And
one of the biggest fears of any engineer worth his salt is something
called the RESONANT FREQUENCY (go talk to your engineering professors,
ask them about the Tacoma Narrows Bridge).  With many tension lines, a
HUGE sail area, and a microwave wind that we must admit won't be
absolutely homogeneous (nothing in life is, sigh), there will be some
vibration in the sail (ever see a taut line vibrate in the wind?)  If the
frequency of this vibration approaches something called the resonant
frequency, and this frequency may well be rather small in such a huge
structure, the amplitudes of the vibrations will compound with increasing
force until the structure fails.  ANY structure, device or whatever, if
operated upon continuously by a forcing function at or near its resonant
frequency, WILL tear itself apart, even if the forcing function is
relatively small.

[L. Parker]  I remember the Tacoma Narrows Bridge, pretty impressive. But this is merely an engineering problem. The resonant frequency problem in bridges has long since been taken into account in bridge design. Most current concepts for sails are not solid sheets but more of a screen or net where the whole diameter is less than the wavelength of the radiation it is designed to reflect. This design has some inherent give or stretch that should serve to dampen any oscillations, resonant or otherwise. 

I don't remember ever seeing any of the authors of sail ideas ever covering this topic though. I would like to restate the objection to tugs though, this fabric is not going to be strong enough to be pulled on by a tug, it will simply rip. Any steering or other "pulling" would have to be done buy the tether lines and even that would have to be gentle.

The thrust figures I've seen are for 0.05 G which is not very much. We might be able to get it up to 0.15 G with advanced materials, but you would still have to place these "tugs" at some sort of reinforced point on the sail.

Lee

                                                          (o o)
------------------------------------------------------oOO--(_)--OOo---------
Among the stars lies the proper study of mankind; Pope's aphorism gave only
part of the truth, for the proper study of mankind is not merely Man, but
Intelligence.

Arthur C. Clarke, The Exploration of Space, 1951