Skylights

Ryan

--- In spacesettlers@y..., Ryan Healey wrote:
> Is http://www.islandone.org/LEOBiblio/SPBI131.HTM the idea you were
refering
> to?

I don't think so. The above concept seems to be one of using eddy
currents to accelerate an object in a tube, but it seems the object
is launched above the atmosphere first. I really don't understand
much of what the endless tether is actually used for, in that concept.

I'm thinking of a circular ring, at as low an altitude as possible,
from which tethers can be lowered to the earth's surface. A really
thick ring wouldn't even have to have any velocity relative to the
earth, but this puts an incredibly long structure under compressive
loads, which is unstable (it would probably buckle). So instead, the
ring rotates so that centrifugal forces render it neutral, or under a
bit of tension. The ring's rotation is rather irrelevant to the
tether, which is connected to the ring via a car using a mag-lev
system. The ring's rotation is just there to provide structural
stability. Once the car is up to the ring, the car can take
advantage of the rings velocity, in order to accelerate the car up to
speed. However, this still means that the ring's velocity must be
replaced, but this can be done using high specific impulse engines,
running continuously at low thrust levels.

The hard part is figuring out how such a ring would react to the
point loads from tethers. Viewing the ring as an assemblage of
particles, its behavior gets more comples. The weight of the tether
and payload adds an impulse to the section of the ring that's passing
by, so this part will be kicked downward (action/reaction) and will
want move to a more eccentric orbit, with a periapsis 90 degrees past
the tether point. Then the ring section we're discussing climbs out,
and at 180 degrees opposite reaches the same altitude that it was at
when it passed under the tether car. At this point, it hits another
tether car (on the other side of the earth), and this impulse re-
circularizes the orbit.

Another view would see the ring as a fixed solid, and having opposed
tethers just guarantees that the net external force is zero. This
ensures that the ring is not accelerated relative to the earth's
center, so it doesn't drift around.

A more realistic view would study how the waves generated by the
tethers would propogate through the ring. After a lift, would there
be standing waves running around? Is so, how would they be damped?

I don't know how important it is to use a fiber which has a higher
speed of sound propogation than the tether car relative speed.
Carbon fiber, kevlar, berylium, etc. would be able to transmit the
force of the car along the fibers, faster than the cars velocity.
With steel, the force from the car is travelling faster than the
speed of sound in steel, which may affect how the structure behaves.
I don't know enough mechanics to figure this out.

BTW: You can treat the centripetal acceleration times the mass of the
cable as an internal pressure in a cylinder, and then calculate the
velocity, in excess of orbital velocity, that will stress the cable
to the ultimate strength of the material. It turns out that this is
a fairly small value, less than 100 m/sec as I recall.

Best Regards,

George Turner