[roc-chat] Re: Tether setups - Shunts and Switches

  • From: Jeff Gortatowsky <indanapt@xxxxxxxxx>
  • To: "roc-chat@xxxxxxxxxxxxx" <roc-chat@xxxxxxxxxxxxx>
  • Date: Sun, 17 Jun 2012 13:10:49 -0700 (PDT)

Thanks for the info Greg and Dave. 

David I am surprised. Twist and tuck seem to be a tried and true almost 
(almost!) fool proof method. 

Greg that makes good sense! 

As for the charge going off on the pad. I've seen this as well. However the 
altimeters I have been using will power up, initialize, and go into 'Ready to 
Launch' mode without any continuity. Yet, once the charge is enabled (by 
twisting it together) they start to beep out continuity on the channels. So I 
'feel' safer (even if it is a placebo). IOW I am not so worried about it 
powering up in a weird state because it's already powered up (on the pad of 
course) and beeping it's 'ready' and sensing no charges, then sensing one (or 
two) charges as the screw switches would close. IOW you don't have to have 
continuity at power up on the charges for the device to go into Apogee or Dual 
Deploy mode.

Also, with accelerometer based electronics, like my old AltAcc, it's think it 
is important to not arm it (well IMO) while the rail is down. The rocket should 
be pointed straight up before arming so once armed, it knows which direction 1G 

I bought a few plugs years ago that could be used as shunts but was told by 
others, the contacts are too unreliable to know under G forces they'll stay 

I know this has been discussed a lot on chat over the years. I guess I should 
go dig into the archives and The Rocketry Forum build threads. 

Jeff Gortatowsky, Redondo Beach, CA | Twitter: JeffGortatowsky | Yahoo: 
"(Scientific) Skepticism is not a set of beliefs, it is a set of methods for 
asking questions about reality." -- Doctor Steven Novella

 From: Gregory Lyzenga <lyzenga@xxxxxxxxx>
To: roc-chat@xxxxxxxxxxxxx 
Sent: Sunday, June 17, 2012 12:22 PM
Subject: [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 
>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(???)

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

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

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  • » [roc-chat] Re: Tether setups - Shunts and Switches - Jeff Gortatowsky