[The following is an excerpt from a letter to Allen Meece]
[Updated 4 September 2002]
Although the peak stable tensions at the 100mb
level (average 16.5km) and 70mb level (19.1km) are more variable than that
at the 50mb level, on average they are actually less than the
stable tension at the 50mb level. This is because the drag
forces on the balloon are not significantly greater and there is less weight of
rope hanging under the platform.
Thus the platform could likely descend to the 100mb level
without any significant increase in line tension. A drop to 150mb is
problematic, however, because wind drag begins to increase markedly below this
level as the air density increases.
Thus, the altitude range in which we can safely deploy the VBP
is likely anywhere over 16km. Deploying to a lower target altitude would
allow us to lift more payload per unit of lift cell volume. It also
leaves a considerable margin beneath a platform deployed to
20km.
Again, note that the low line tensions at these high altitudes
do not necessarily reflect any decrease in wind speed nor any unusual stability
in the wind at these altitudes. The winds at all altitudes vary over the
course of a month in roughly the same pattern. Only the degree of change
varies with altitude. The low tensions are due to the fact that air
density is so low at these altitudes.
