Curious Ultra-High RPM Rotary Tether Finding
Weight -to-power efficiency of rotating tether
mechanical energy transmission, such as
Italy have demoed, can be greatly increased by a higher rotation rate.
If the tether can be rotated at ultra-high speed close to its aero-thermal
limit (>100 k rpm) spectacular performance seems possible.
Of course an aero-turbine of itself cannot turn so fast by wind, so some
sort of step-up gearing is needed. This is a weird mechanism with a
critical dependence on maintaining a narrow range of tether tension
to operate properly, but should be possible with the right controls. I
DaveL's tether software can model this exotic mode. There are
probably some very odd dynamics to discover; and failure-modes are likely
~Dave Santos May
Comment and development of this topic will be occurring here.
All, send notes, drawings, and photographs!
Terms and aspects:
- UHrpmRT Ultra-high rpm Rotary-Tether
- aero-thermal limit, aerothermal, aerothermodynamics,
- step-up gearing
- range of tether tension
Commentary is welcome:
- Beware of using gearboxes where high amounts of power (over 1 MW) is
concerned . . . . gearboxes involve weight and expense. ~
- Consider the very shroud of a rotary cable or tube as a stator with
the cable as rotor to have the tether itself as an electric generator.
- Harry is right about conventional gearboxes scaling poorly by
weight & my use of "gearbox" is in the broadest sense; several methods
of mechanical advantage do avoid gears. Also this particular idea is so
far-out that its not (yet) serious. Joe's idea seems to be constrained
by the low Re of the characteristic dimension of the tether
cross-section fundamentally limiting the top "tip" speed and resulting
I neglected to mention that this idea's best advantage is that one
can greatly reduce the required tether tension for operation. High
power-to-weight efficiency at low RPM is possible, but by a tether so
tight that the amount of kite lift required is prohibitive.
- The suggested consideration has the shroud of a tether be kept
rotationally static by anchor and aloft hold while the interior shrouded
part of the tether complex be rotated by wind-driven kite-lifted lofted
elements; then the tether become an electric generator.
In your comment, DaveS, is that what you got from my first concept
presentation, or did you envision something else for your comment?
- Yes a driveshaft carries more power at less torque at higher RPM,
and a series
of small rotors naturally spin the tether fast without a gearbox. The
small blades are the aerodynamic equivalent of many gear teeth. We've
simplified it by making the turbine itself act as its own gearbox. That
whole SuperturbineŽ has only a single moving part. Steady-state rotation
who knew? :)
I'd share some characteristics of these long, fast driveshafts, but it
take all day of explaining. DougS