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.
Reply to
Larry Lobdell Jr.
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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!)
Reply to
Nope. SPDT can break the firing circuit AND shunt the ematch. Note NO center off position for this application.
. ematch . altimeter .
Reply to
Bob Kaplow
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.
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.
No BP to make things interesting but I keep them shunted anyway.
Try dealing with "real" range safety people at one of the national ranges sometime.
Reply to
David Schultz
I agree with AZWoody. Adding a switch or shunt to the ematch adds another failure point to the system. If you want a switch, add it where it will do the most good - to power your alt off and on. A voltage selector switch from a computer power supply is my favorite switch to use for that purpose.
Reply to
J.A. Michel
Yes , the proper way to do this is to use a keyswitch (use a Jameco mini key switch) and place it on the negative/black wire line from the 9V battery. You throw the switch to test continuity when packing the rocket, the light goes on (I use GWiz) and the system is good to go. Switch it off, then carry it to the pad. Switch on just before leaving the pad and it's blastoff time.
Reply to
I just butted in on the conversation. This all sounds quite interesting. However, I don't know a thing about electronics. I would like to learn so I can start putting them in my rockets. Is there a book I can buy that is a good teacher for a beginer.
Thanks, KT
Reply to
Isn't that what RF noise is considered? Seems clear to me!
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!
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.
Reply to
ROL "infocentral" is a good place to start
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,but the site just changed (I won't say upgraded) a few days back, and I've yet to find infocentral!
The basics are (were) there
Reply to
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Another good source, go to the Perfict Flite site and download the manual for the MAWD:
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AZWoody wrote:
Reply to
Yes it is. The NAR L3 certification requirements document.
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Shunting ejection charges has _never_ been a NAR L3 requirement.
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.
Range safety requirements are the result of fifty odd years of experience. But you obviously are better qualified.
Reply to
David Schultz
Have you checked out Mark Canepa's Modern High Power Rocketry 2? I'll bring a copy to the launch this AM and you can check it out.
Reply to
James L. Marino
I can, but not because of RF.
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.
Reply to
Did I say it was caused by RF? Did AZWoody ask for instances where RF caused it?
The challenge was to cite just one case where an e-match went off unexpectedly. I did that.
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.
Reply to
David Schultz
The altimeter beeps will indicate if one of them opens up. When you unshunt, listen for any changes in the tones coming from the altimeter.
Reply to
Phil Stein

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