Howard, List,
One of the things that has disturbed me has been seeing the main tether snap
and leave the canopy fully deployed but just staying there. In that case
the size of the chute and vehicle speed determine the pull on the tether.
If the canopy can come out and stay deployed it can handle the pressure,
just makes me wonder what type of tensile strength is needed to hold it to
the car. Having done some fiber tensile strength testing at much lower
pressures I know that there are some fibers and yarns that are very much
stronger than others. In this case the flexibility is a factor as is the
way the fibers are twisted into a yarn/thread. When the tethers snap as
cleanly as they did on Burklands's car or on the Hoffman/Markley car a few
years ago there has to be some way to strengthen the fiber in the tether and
prevent it. The fibers were only shredded over a 3/4" length, a pretty
clean snap. Obviously they were the same strength across the width of the
tether. If there were some other fibers such as a carbon fiber that were
core spun with the nylon in the tether, that would result in some stretch
and a tear at different places along the line, not all in the same exact
area. This might help to save the line and keep the chute on the car.
Nylon is stronger on that kind of a tug than a steel wire of the same size
and I'm wondering if carbon fiber could be strong and flexible enough to do
the job.
I'm guessing Jim Diest will suggest two things about the canopy, either a
way to allow less air to be captured at the speed in question, a smaller
canopy probably, or a ribbon canopy that will have slots to allow the air
through. The ribbon will be more easily damaged and the smaller canopy
won't slow the car as well. Dragging a chute over the salt can't be good
for the fabric either way. The "slow" speed chute on Burklands car was in
ribbons, obviously the tensile strength of the fabric was not up to the
task. The military contractors developed ripstop nylon for that originally
but the weight of the canopy as used in military applications is a much
higher tensile strength and a heavier fabric. A jeep also has a different
problem with canopy depoyment coming out of an airplane and only freefalls
at around 147 mph anyway. Even the space shuttle isn't as hot as these
liners. Burkland is using compressed air to kick the chute out of the tube
so I'd guess deployment isn't the problem. He is using the clamshell air
brake then three chutes in sequence, Perhaps the triggers are set at too
high a speed for the canopy size.
Maybe if Rick Vesco is right about the salt, we'll be back on the long
course and stopping won't be such a major problem next year. A three mile
shut down is much better than what we have had the last few years.
Wes
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