[roc-chat] Re: Tether setups

On Jun 17, 2012, at 11:05 AM, Jeff Gortatowsky wrote:

> (let me know if not appropriate for ROC-CHAT)
> 
> Greg thanks that is helpful, all my rockets have been single deploy and I 
> simply test for continuity during prep (no  charge, just the match), then cut 
> one lead, strip it, tape over one end of the bare wire, then put the charge 
> in. At the pad I take off the tape, twist the two bare ends together, and 
> place the tape back over it. Now with a larger dual deploy, and to MAYBE use 
> it as an L3 project, I need to change that I guess. So forgive another dumb 
> question...
> 
> What is the difference between shunting at the pad, and safeing should it 
> come down in an indeterminate state? 
> Seems like
> 1) Shunts are a possible failure point under boost, opening or closing under 
> G load and vibration (depend on what you used of course - maybe that is NOT 
> so?)
> 2) The screw switches would provide both functions anyway(???)
> 
> No?



Well, some of what I'm going to say might be more in the realm of opinion than 
fact, but I'll wade into it...

A relatively likely (that is, I've seen it happen) possible failure is that 
charges fire when power is applied to the electronics.  To guard against this, 
one would either want to completely break or completely short out the match 
connections.  The former makes me a little nervous because if it can break on 
the ground, it can break in the air; also it screws up your continuity check.  
The latter also makes me nervous, because there is no such thing as a shunt 
with *zero* resistance (unless it's a superconductor, but most of my rockets 
are not equipped with liquid helium dewars).  So there will still be *a little* 
current passing through the match and you must take it on faith that your 
calculations line up with reality and don't exceed the minuscule firing current 
for a match.  (I never wanted to test this scenario because I was afraid of 
damaging my electronics by dumping the output into a dead short intentionally 
-- perhaps too much worrying on my part?)

OK, so then the actual history of what I chose and why...  I agree that screw 
switches could do it all (except for the continuity check issue), but in my 
original design I chose the shunt method.  My original L3CC signed off on this, 
but was not in attendance on the day I tried my first launch attempt.  Andy 
Woerner was also an L3CC and stepped in, but his interpretation of the rules 
was different.  In order to sign off on the flight, he required that I have a 
method to break the match connections from the outside of the rocket.  So 
already trembling with launch jitters, I proceeded to take a cordless drill to 
my beautiful rocket and make holes to pull a loop of the match leads to the 
outside.  That way the matches could be disabled with the snip of a wire cutter.

As it turned out, the flight didn't happen that day due to a hybrid ignition 
problem.  Later on when I was making other modifications in preparation for the 
ultimately successful flight, I replaced the inelegant wire loops with nicer 
looking screw switches.  At that point, the pre-existing shunts became pretty 
redundant.  However, in practice I never actually opened the screw switches, on 
the theory (mine) that the more times they are cycled the more likely they are 
to wear out and fail.  So to this day the screw switches remain closed for use 
in emergency only, and I use the redundant shunts for my pad safety function.  
But as you can see, this is more of a preference/superstition thing than any 
hard and fast logic.  (although I still don't know how to use switches only and 
have the electronics properly sense continuity...)

                          - Greg


-------------------------------------
Gregory A. Lyzenga  <lyzenga@xxxxxxx>           ***     (909) 621-8378
Dept. of Physics, Harvey Mudd College           *** cell(626) 808-5314
Claremont, CA 91711-5990                        *** fax (909) 621-8887
<http://tinyurl.com/LyzengaPhysics>




Other related posts: