Wiring Ejection Charges

I read somewhere that people use a switch to short out an e-match and just before launch they open the switch so the ejection charge can be activitated by the altimeter. But doesn't this make the altimeter continuity check useless? It will show continuity with the switch in either position so it would be possible to launch the rocket with the ejection charges still shorted. Wiring the switch in series with the e-match and altimeter would eliminate this problem. Then the altimeter would only detect continuity if the switch was closed and so there could be no false continuity signal. I realize this would not prevent the e-match from being set off by static electricity, etc., but I've never heard that static is a real problem for HPR fliers. Any thoughts on this? Larry Lobdell Jr.

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An SPDT switch could both disconnect the ematch AND shunt it to protect against stray RF.

Bob Kaplow wrote:
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You mean DPDT?

Steve Humphrey wrote:
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Single pole would be sufficient.

A waste of time, and an additional potential point of failure.
Never used a shut, and have also never heard of an ematch going off due to RF. Most electronics actually provide an "internal shunt" thru circuitry anyway, so you're "shunted" if it's simply attached to the altimeter.
The "shut" requirement is only needed for a NAR L3 cert, and was added by a type AAAAAAAAAAA LC33, when the head of the LC33 was still a L2 flier.
Do you worry about an ematch "suddenly going off" when it's sitting in your range box?
(Kaplow has one less 'A' in his personal traits than the guy that purposed the shunt for L3 certs. The guy probably has double locks on his bathroom, and an auto unlock if there's a power failure!)
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AZWoody wrote:
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A shunt is a low resistance path and no altimeter that I have heard of provides this internally. There are high resistance paths but these are only good against static charges.
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Shunts are not required for NAR L3 certification.
"2.4 The capability must exist to externally disarm all pyrotechnic devices in the rocket. In this context, ‘disarm’ means the ability to physically break the connection between a pyrotechnic device and the power source to its igniter. Simply turning off the device controlling the pyrotechnic(s) is not sufficient."
The built-in safe/arm screw in the AltAcc2 (although I actually used an external switch) and the pyro power switching on the RDAS were sufficient for my L3 flight.
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No BP to make things interesting but I keep them shunted anyway.
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Try dealing with "real" range safety people at one of the national ranges sometime.
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Inline...
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to
circuitry
Isn't that what RF noise is considered? Seems clear to me!
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by a
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Uh, have you checked the NAR L3 requirements - and all the friggin forms - in the last 5 years? This IS the major difference between NAR and TRA L3 certs ( unless NAR got wise and dropped the requirement and I've not noticed the change)
(something deleted from the original post, as I'm not sure where it came from, but seems to be part of some document).
I did my L3 with TRA and the main reason being, is the "shunt rule" made zero sense. I'd be stuck with Steve if doing NAR L3CC, and at that time, he still didn't have his L3!
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purposed
bathroom,
You have absolutuly no idea of my background. Tell me, of ONE case where an ematch went off unexpectedly! You can't..
Ever heard of NSL? I was involved i the hosting of the event (including RSO). Ever heard of GHS, again involved in hosting and RSO, and also a NAR section advisor. Ever heard of Springfest in NV? (a Prefect and NAR section advisor at the time) Ever heard of the Gila Monster? (up to 7 M's) built many a motor for that. I've RSO'd the Gate Bros flights.
Ever heard of AHPRA? Or their "Balls " launch? (an international event in NV). The TAPS for my L3 were AHPRA members.
Don't give me this crap about "real" range saftey. If the "real" range saftey people are saying this, it's only beause thay don't have a clue.
When you connect an ematch to an altimeter (without a shunt) it is safer than it was in your range box.

AZWoody wrote:
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No.
Yes it is. The NAR L3 certification requirements document.
http://nar.org/pdf/L3certreq.pdf
Shunting ejection charges has _never_ been a NAR L3 requirement.
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LDRS a few years ago. Documented on the Discovery Channels three hours of TV. Altimeter connected to battery at pad with guys at side of large rocket when it happened.
That's one. There are many more but that one is publicly documented.
I have no idea of your background but my point was that if think that the NAR's requirements are excessive, you should try dealing with one of the ranges.
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Range safety requirements are the result of fifty odd years of experience. But you obviously are better qualified.
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I can, but not because of RF.
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David, that incident happened because the altimeter's design allowed current to be fed to the ematches if battery polarity was reversed, which is a bad design. It was not caused by RF. If the ematches had been shunted they would have immediately fired when the shunt was removed and the result would have been the same because the person on the tower would still have been there to remove the shunt. A shunt in that situation would have been worthless.
I do not know of any cases of RF causing an ematch in a rocket to fire unexpectedly. I've flown rockets that contained radio transmitters and receivers and ematches. I've flown rockets with wireless launch systems. I've stood next to hundreds of ematches with cell phones and two-way radios. None of the ematches accidently fired. However, that does not mean that it is impossible under the right circumstances, but those circumstances are very rare and finding those circumstances on a rocket range is even rarer. In my opinion a shunt is more likely to cause problems with a rocket than solve them.
I am not in any way saying that I have no concern about ematches accidently firing. A large motor, an ematch and thermite for ignition is a recipie for disaster if the correct procedures are not used.
Dean

Nobody wrote:
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Did I say it was caused by RF? Did AZWoody ask for instances where RF caused it?
No.
The challenge was to cite just one case where an e-match went off unexpectedly. I did that.
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This is why at a "real" range, which AZ Woody disparages, a test is done on cables prior to connecting them to EED's (Electro Explosive Devices) to verify the firing circuit is shorted and there is no voltage present. Only then is the shorting plug removed and the cable attached. This is for devices that are required to withstand 1 Watt of input power (1 Amp into the typical 1 Ohm bridgewire) for several minutes without functioning.

The discussion was about RF causing an ematch to "go off unexpectedly". Yes, I've seen ematches go off unexpectedly, but ALL were caused by flier errors. Your LDRS example just proves the case. A shunt or open would not have changed what happened at LDRS, as the rocket was being armed at the time
There was a bozo in my club that wanted to check out his electronics, and he took his rocket apart to ask questions and then put it back together. "Proof", or more correctly "Boom", as he didn't use a power switch, and the change in pressure triggered the ejection charge, withing a few feet of other folks.
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David Schultz wrote:
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Why were all of the years odd??? <G>
David Erbas-White

David Erbas-White wrote:
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Yeah, if all the years were odd, wouldn't that mean only 25 years of experience?
:)

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Wasn't something changed here this spring for ALL electronic deployment flights in the HPR safety code revision as a result of the Safety Committee.
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NSL Muncie, a BP charge went off in someones prep area.
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What increased the danger of an ematch is not its connection to an altimeter, but it's proximity to a BP charge. Once in a container with several grams of BP, you have to treat them VERY differently.

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Sorry, but "never" seems to be a strong term.
At the time I was looking into my L3, it was a requirement. And to me, one that was foolish. I did my l3 with TRA. This was back when Lubliner (the head of the L3CC) was still trying to get his own L3 cert.
I was there and know the history.

AZWoody wrote:
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It is the appropriate one.
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The L3 program was developed by local NAR members so I remember it. If your memory needs jogging, just try the Internet archive. The oldest version of the L3 requirements archived there is from 1999 and the safe/arm requirements are identical to those of today.
http://web.archive.org/web/19991103084949/http://www.nar.org/hpcert/l3certreq.html
A shunting requirement might have been inappropriately imposed by your local L3CC person but with persistence you could have overcome that.

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Does one that happend on one of the LDRS shows that aired on the Discovery CHannel a few years count? THere are plenty of others.

Phil Stein wrote:
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RF in and of it's self can not set off an ematch... What can possibly happen: Extreme RF can cause transitory voltage to develop in some electronic circuits and the transitory voltage can then cause triggering of a firing circuit that would then in turn, fire the ematch. Although that is in theory, I have never known a case of this happening, with any of the devices we use. On the other hand, there is several altimeters that have or, allegedly have, a history of firing because of "switch bounce". Because of that problem I don't skimp on switches. I use either a parallel wired DPDT slide switch or a screw type arming switch to power up or down, all altimeters, Missleworks, Transolve, PerfectFlite, and yes even Adept. I sometimes use shunts with bigger charges because it makes me feel better, no other logical reason. I also cut the shunts before powering-up the altimeter unit as I don't like the idea of the unit circuit first reading 0 resistance, (dead short), and then + resistance from the ematch if I cut the shunt after power-up. Once again, no real reason, other than this "does seem logical".(:-)
Fred

On Sun, 04 Jun 2006 14:34:43 -0400, "W. E. Fred Wallace"
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TO clarify, I was not saying that incident was caused by RF. Neither do I believe that Woody was asking about accidental discharge being caused by RF.
Phil

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I was asking about RF and ematches.. Without a doubt, as that's what "shunting" was to prevent.
There's nothing that will prevent a flier from screwing up his/her wiring and having an ematch trigger at the wrong time, including shunts/opens.