[The Following is an excerpt from a letter to Allen Meece]
[Updated 4 September 2002]
Though some active control of bouyancy is
necessary, I believe we may be asking too much of an active control system
to have it compensate for every jerk and strain of the tether. I think we
should focus on passive means of stabilizing the tether. A constant higher
line tension with positive bouyancy would go a long way toward this.
It would inherently provide the additional force we'd need to stabilize the VBP
without the added response time that an active control system would
require.
It's important to note that I'm mainly concerned with the more
extreme short term behaviors of the VBP system. I fear that an active
bouyancy and/or tension control system could not make a half million cubic meter
balloon on a 25km rope respond quickly enough to damp them out. The
capacity to deal with these sudden changes in load has to be inherent in the
design.
Superpressure lift cells might open up an option for lightening
the platform design. The zero pressure lift cells require heavier
struts to hold them in position -- a keel -- and can support almost no
weight on the tops of the cells. The only place they can bear the load of
the platform is at their base. Superpressure cells can
bear almost as much weight on their topsides as they can
below. This means that a Spectra mesh draped over
the cells can be used to carry the entire load of the platform without any
aluminum struts. A lighter platform is a smaller platform, and smaller
platforms have less drag.
Another aspect of bouyancy control is related to tether
tension: what to do if the tether breaks. Breaking the tether will require
20 to 50 tons of force, depending on the rope thickness we ultimately decide
on. That's several times what the rope itself weighs and will produce
quite a recoil. I seriously believe that we should
design at least one or two of the lift cells to fail short
of accelerations that might overwhelm the tether (also easier to do with
superpressure balloons.) ) It should be possible to keep enough cells
intact that we would stabilize at a lower altitude. If the lift cells pop,
the worst case scenario is that we would only have seven minutes to review
our skydiving lessons before we came up on 5000 ft. If the tether snaps,
it's likely to come up and explain to us why we shouldn't use ten tons of rope
as a bullwhip. I gave it some thought, and I know which of those two
scenarios I would prefer.
There is still considerable safety margin. Conditions
will be rough, but I believe we can come up with a design that can just tough it
out.
It may just be a balloon on a string, but I think it's going
to be a tough balloon on a string.
:)
