Interesting high lights of the NAR BoD Meeting and ...

Kevin Trojanowski wrote:


Yes it was. And it re-enforces what I've heard about ejection charge mishaps being the #1 cause of injury in HPR.
When I was preparing for my NAR L3 I questioned the NAR requirement to physically disconnect the ejection charge from the electronics until after the rocket is on the pad and ready to fly. I now understand it and agree with it. I don't know the full details of what happened on Master Blasters, but it seems to me that a proper safety disconnect (with the charges shunted when disconnected) would have prevented the initial problem.
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What's wrong with just having the battery disconnected from the electronics until everything is set up and ready to go?
Chris
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Chris Eilbeck
MARS Flight Crew http://www.mars.org.uk /
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On Tue, 09 Aug 2005 17:35:14 +0100, Chris Eilbeck

Nothing. Things were being set up and being made ready to go when it happpened. I don't have the specifics of what happened. A shunt is about all you can do to prevent that from happening. It can greatly reduce the possibilty but not totally 100% eliminate it.
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Shunting has its own safety issues. Your electronics can't tell if the shunt has been removed (because a shunt looks like an igniter), and if the shunt is not removed then the charge is not going to fire and the rocket is going to crash.
The odds of some bizzarre power-up surge is a lot less likely than forgetting to un-shunt or a failure in the un-shunting, IMHO. Most (but not all) ejection charge problems are a result of user error. I know that some Missileworks altimeters will fire the charges immediately if the battery polarity is reversed, and to make matters worse they reversed the location of the + and - terminals from one model to the next. That's just downright poor design.
-- David
writes:

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And that's a good reason to design your airframe so you still have motor backup, at least on drogue.

I agree.
Chris
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Chris Eilbeck
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says...

That is not an option with bigger motors.
--
Tweak

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On Tue, 09 Aug 2005 17:35:14 +0100, Chris Eilbeck

There's always the possibility of "startup transients". During the fraction of a second between connecting the battery and the electronic components all seeing steady, full-on power, there is the possibility that digital outputs will be in an "undefined" state for a moment. In other words, the output MOSFETs may see a "fire" signal for a moment, even though they will be set to "off" as soon as the electronics finish coming up to full power.
NO electronic circuit is capable of switching from zero volts to some positive voltage instantaneously. Due to the fact that all circuits have some resistance, and there is always some capacitance between conductors (not to mention the effects of power "filtering" capacitors), there is always a "ramp-up" of power over some time interval. It is not always possible to guarantee (especially in digital circuits) that the outputs of circuits will be predictable and stable during that ramp-up time.
Disconnecting the charges entirely, and/or shunting them, is cheap insurance against this sort of momentary "glitch" during power-up.
In theory, there's no difference between theory and practice.
- Rick "In practice, there is." Dickinson
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National Association of Rocketry (NAR) # 73975 Level 2
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It's pretty damned easy to add a time constant to the fet gate control and guarantee it'll be less than the switching voltage by the time the processor is in control.

Has this effect ever actually been seen? I'd be staggered if the amount of energy transferred into an ematch in this case was even measurable without seriously good test gear nevermind anywhere near the guaranteed no-fire spec for commercial igniters.
Chris
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Chris Eilbeck wrote:

On the Discovery Channel special there was a G-Wiz that blew in the user's face because he hooked up the battery backward. A physical disconnect would have prevented this, wouldn't it have? As Rick said, its cheap insurance.
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says...

Just out of curiosity, what happens when you pull the shunt?
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Tweak wrote:

That, I don't know. In the case of the G-Wiz I don't know if the immediate fire current is sustained or if it burns out the FET, or what. I would HOPE that the controller signals an error condition about the polarity.
In the case of an accidental arming of a timer or a power transient at power-on, the shunt/disconnect will minimize the odds of being connected at the same instant as the pulse.
Regardless, hopefully safety procedures will be followed that you assume the charges will blow when you pull the shunts and you do it in such a way as to minimize the risk of injury (i.e. not sitting on a ladder with your face up against the access port).
In the case of the G-wiz wouldn't it be wise to add a diode to the pyro channel to prevent this? Or a reverse polarity alarm, perhaps part of the safe switch (i.e. when in safe mode, anything over the all-fire voltage/current sounds a loud buzzer). The diode would be trivial to retrofit, wouldn't it?
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On Tue, 09 Aug 2005 14:01:48 -0500, Alex Mericas
There is one. It sounds just like an ejection charge. 8-)
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Designing the thing properly in the first place would have prevented it!
When you remove the disconnect it would have fired in any case. I fail to see why that is any safer than firing when you turn on the altimeter.
Chris
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Following proper procedures will prevent almost all accidents. They are accidents because they're not done on purpose. Somebody screwed up. Safety systems should provide a margin of error to allow a system to gracefully fail (by burning out the altimeter, as and example or better yet not powering on at all) instead of blowing up in your face. Literally.
Would the charge have blown when the shunt was removed? Don't know. I do know the G-Wiz only pulses the pyro channel (doesn't leave it hot for long) so maybe it would reset. But it wouldn't have blown when it did. Would the rocket have survived if flown? Not on THAT pyro channel, but again it wouldn't have blown up in your face. All the more reason for redundant deployment devices.
Again, cheap insurance. And other than forgetting to remove the shunt, no downsides.
Chris Eilbeck wrote:

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Where's Ray?
I'm sure he'll tell us about the time at Plaster Blaster where a rocket and it's pad flew over his head and hit the truck behind him.
Had one guy at a local launch with a week old truck that got damage from a free-fall nose cone it weighed something like 7 lbs) Someone got a magnetic sign that looked like a bullseye that would show up at launches on his repaired truck.
These were NAR launches.
As certs get easier, and the crowd grow at launches grow, the chance of an ingury increases.
That's one of the reasons I don't fly much anymore. I've seen birds with 4 K's turn into cruse missles because the bird was built poorly. People who barely passed there L1 get L2 and build some really wackey stuff that didn't turn out well. In this case the flier would cause not a "heads up flight", but a "hard hat" flight. (I still have an actual hardhat with the fligher's name on it, as this flier was known for bad flights) (this was a NAR range)
Also seen folks that ignored fire restrictions and launched sparkey motors in a dry desert (causing a fire)
I've also been awakened by a group throwing full 38mm grains into a smokey joe (webber) grill less than 10' from my nylon tent (another NAR launch). Once I was awake, I did get this shut down and told the guys to sleep it off...
Ending comment - if NAR/TRA expects the government to cut hobby rocketry some slack, it won't come from court actions. The first thing to do is to do self regulation - at the local level.
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Woody,
The Plaster Blaster Launches are a hosted by a club that is affiliated with both NAR and TRA. Here are the links and information.
http://nar.org/NARseclist.asp Diego Area Rocket Team (DART) #317 Andy Woener 9547 Abbeywood Road Santee CA 92071
http://www.tripoli.org/prefecture/prefusca.shtml Tripoli San Diego(5) ANDY WOERNER 9547 ABBEYWOOD RD SANTEE CA 92071-2668
I'm not really sure that it's all that productive to get into the finger pointing game, as both organizations have suffered their share of close calls. When the first life threatening accident occurs in rocketry, both rocketry groups will be adversely effected. I think you'll find the leadership of both organizations in agreement with this position.
I totally agree with your statement that self-regulation at the local level will be the most effective tool in preventing accidents. If someone is doing something stupid at a launch, we can't expect Ken or Mark to hop on to a plane and visit the flying field to deal with the issue. I'm sure that you've offered "guidance" to many a flier over the years; it isn't something that one wants to do, but unfortunately in needs to be done from time to time.
See you out on the playa,
John
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snipped-for-privacy@tek.com says...

In what way?
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Tweak

Camptown ladies never sang all the doo dah day
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Pretty simple. The insurer of one organization talks to the insurer of the other following an accident and up go the rates.
John
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snipped-for-privacy@tek.com says...

That's a reasonable assumption. Thanks.
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Tweak

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On Tue, 09 Aug 2005 18:50:49 +0100, Chris Eilbeck

I don't remember which ones they are but some people have reported that some altimiters have a tendency to fire charges when power is applied. Maybe it was a malfunctioning altimiter but either way shunting should help. ALso some altimiters will blow the charge verytime when the power is connected backwards. Sorry for being somewhat vague. This is from memory. A lot of the information I received was on the TRA list when doing research for an article I wrote on shunting. It was written right after the LDRS accident that someone mentioned.
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