Thanks for the additional information. As I said to start with, I
didn't find your objection unreasonable, but as it was phrased it was
hard to see how firm the conclusion was.
[SAL:]
>> Megawatts are a measure of power. "Load" on a cable is a measure
>> of force. The two are completely different; the phrase "... a load
>> of 10 MW" is meaningless.
[ ... ]
>> Power generated by a laddermill would depend on cable tension and
>> on how fast the mill turned, and without knowing (or guessing at)
>> the latter you can't say anything about requirements on the former.
[Jed:]
> The mill would have to have gears.
>
> It is easy to guess at how quickly the cable would move. For one
> thing, it would have to move a huge unwieldy string of kites which
> can only rise and fall at a certain speed. Second, the cables would
> not move much faster than the fastest cables used in excavation
> equipment, elevators, cable cars, ski lifts and the like. People
> have been using cables for a long time. If they could make them move
> much faster, I expect they would.
>
> There are enormous cables on excavation equipment that are actuated
> with megawatt motors, but these cables are extremely heavy and I do
> not think any excavator motor is as large as 10 MW. I think the
> largest in history was "Big Muskie" which had a 2000 hp dragline
> motor (1.5 MW).
[ ... ]
> I believe elevators are the fastest moving cable driven
> equipment. Hitachi has developed monster high-speed elevators that
> will travel at 480 m per minute (28 kph). They are driven by 240 kW
> motors.
Ah -- now we have something we can apply.
28 kph, with 10 MW of power being transfered, implies a major pull on
the cable. (I'm going to wimp out and skip the calculation, in which
I'd undoubtedly mess up the units...) This is going to be in the
vicinity of what several of those monster 5 MW locomotives you
mentioned, pulling in series, would be able to do to the freight train
hooked on behind them. I don't know what kind of power they can
actually apply to the wheels but I'd guess that, at intermediate
speeds, it's a good fraction of the nameplate power.
This is one seriously heavy duty piece of kite string they're talking
about here! Railroad car couplers can take that kind of stress, of
course, but as you point out it's hard to see how a cable that strong
could be made light enough.
[Jed:]
>>> I recall reading that some of them could hold back something like
100 railway locomotives.
[SAL:]
>> You have carried the simile too far and it fell over a cliff.
>>
>> You're no longer talking about power from the engines, you're now
>> talking about power applied to the wheels. Power applied to the
>> wheels is the product of torque and rotational velocity. When the
>> locomotives are stationary, they are applying ZERO power to the
>> wheels.
[Jed:]
> Obviously I meant that 100 locomotives from a standing start could
> not pull hard enough to break the cable. If it moves fast enough
> even 1 locomotive can break any cable. For example, if you drop the
> locomotive from the Oort cloud to Earth.
Yes, of course. But charming as the analogy is, the question would
really be, how strong must the cable be to _brake_ the locomotive,
given that it's traveling as fast as it would be if it fell from the
Oort cloud? And the answer to that is that most likely just about any
cable would do, because the engine's torque curve will long since have
crossed zero and gone negative at that speed ;-)
