[The following is an excerpt from a letter to Allen Meece]
[Updated 6 September 2002]
I'm starting to think that the VBP platform keel should be
somewhat narrower than some of our earlier suggestions. First, judging
from manufacturer's information on stand-alone radio towers, a keel of
radio tower struts just over 1.1m across should be adequate to distribute the
VBP's mass if it is reinforced at selected points (such as where the hab is
connected) with additional aluminum and supported with a network of spectra
cable. Bolt-together towers of this size are comparable in strength to
welded ones, and it is possible to make spar sections of this size very
lightweight. The disassembled parts for a 40ft section are less
than 150kg and could be brought up on the elevator.
Making the keel thinner means that it can be used for fewer
things, but also frees up mass for additional uses. For example, rather
than running a rocket launch rail on the top of the keel, a rail of the same
length or longer can be slung underneath the keel, saving all the space on the
top deck that would otherwise be used.
A detached launch rail could be suspended by cables under the
platform, allowing it to be winched right up the deck for loading and then
reeled out to deploy for launching. A solid hinge is not necessary if the
rail won't be reeled out far enough to twist in the wind. This could be
done with a rail of any size, and would allow the rocket to be angled in
any direction, regardless of the direction of the wind. The angle of a
fixed rail, on the other hand, is determined by the wind direction, since
the VBP platform must ultimately always face into the wind.
Here are some sketches of a short rail, in its loading
position (reeled right up to the hab) and deployed like a kite behind the
platform.
A short rail becomes less stable the further it is reeled out,
but a launch rail of any size can be deployed in this fashion.
