NAR L3 Requirement Argument

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Walldiver7

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I am just starting down the road to my L3 cert. Below is something that I want to make sure that I am interpreting correctly.

2.4 The capability must exist to externally disarm all pyrotechnic devices on-board the rocket.
In this context, ‘disarm’ means the ability to physically break the connection between a
pyrotechnic system and its power source. Simply turning off the device controlling the
pyrotechnic(s) may not be sufficient.


My interpretation of this means that my L3 project will have SIX switches aboard. That would be two, to switch power to the flight computers, two to arm the drogue charges, and two more to arm the main charges. Have I read this correctly? Note the last sentence of 2.4 where it states "... may not be sufficient." Does this mean that this is up for discussion?

With the requirement of SIX switches, would I not be introducing more failure points?? When I sever the power to the flight computers (switches in the power line leading to the computers), there is no way for any of the charges to be energized..... So why would anyone purposely put in more switches that could fail?

(Moderator, Have I posted this in the correct area?) Yes, this is the correct area. Bob
 
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This is exactly why I moved my membership from NAR to Tripoli after I decided to pursue my L3. Being a Quality/Reliability Engineer for a time early in my career, I could not resolve added-value or any real-life benefit from this requirement, and in fact, was instead adding complexity and potential fault states. A long time tenet of mine... "Never let anyone impress upon you their insanity." Easiest course was just to transfer my membership.
 
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DixonTJ,

Unfortunately, This may be my course of action too. I am a diver (scuba) that dives solo and beyond recreational limits. I have redundant systems in my setup. I can assure you that I don't add anything into the path of gas delivery that might fail and serves no purpose. The extra switches appear to be nothing more than a test of a rocketeer's ability to solder.

This is exactly why I moved my membership from NAR to Tripoli after I decided to pursue my L3. Being a Quality/Reliability Engineer for a time early in my career, I could not resolve added-value or any real-life benefit from this requirement, and in fact, was adding complexity and potential fault conditions. A long time tenet of mine... "Never let anyone impress upon you their insanity." Easiest course was just to transfer my membership.
 
When I sever the power to the flight computers (switches in the power line leading to the computers), there is no way for any of the charges to be energized.....

I would suggest, then, that you would only need those two switches since opening then would break the connection between the charges and the power supply.

That might not be true, though, if the electronics have capacitors or whatever onboard that might stay energized.

-- Roger
 
I am just starting down the road to my L3 cert. Below is something that I want to make sure that I am interpreting correctly.

2.4 The capability must exist to externally disarm all pyrotechnic devices on-board the rocket.
In this context, ‘disarm’ means the ability to physically break the connection between a
pyrotechnic system and its power source. Simply turning off the device controlling the
pyrotechnic(s) may not be sufficient.


My interpretation of this means that my L3 project will have SIX switches aboard. That would be two, to switch power to the flight computers, two to arm the drogue charges, and two more to arm the main charges. Have I read this correctly? Note the last sentence of 2.4 where it states "... may not be sufficient." Does this mean that this is up for discussion?

With the requirement of SIX switches, would I not be introducing more failure points?? When I sever the power to the flight computers (switches in the power line leading to the computers), there is no way for any of the charges to be energized..... So why would anyone purposely put in more switches that could fail?

It reads more ominous than it really is. It is just a requirement to safe the charges in your rocket by isolating from them any source of electricity that might inadvertently set them off. An example is your rocket is on the pad, you arm your recovery electronics, and then realize you need to pull the rocket back down off the pad to (fill in the blank). Twist-N-tuck no help here. So, a switch that takes the battery off the altimeter MAY work, if the battery is the source of power to the charges. The "may not be sufficient" comment is to cover the possibility that your particular brand of altimeter uses a capacitor to fire the e-match. In this case, the capacitor would remain charged, so yes, you must break the connection between the altimeter and the e-match. That is a specific requirement to specific altimeters, check with your manufacturer. It is really not much different than there desire we all have to ensure the igniter leads are unpowered before connecting them to your igniter, with your face near the business end of your motor.

Modern quality switches have a very low failure rate, which is further mitigated by the requirement to have complete redundancy in the initiation of recovery events. Care still must be used in the design of the system... i.e. I would discourage using multiple switches on the same altimeter, which could allow the altimeters to be powered and the charges to be unpowered. (This would hopefully be caught by the lack of continuity beeps.) I personally use 4PDT POPO switches on my sleds, which powers the altimeters when turned on and shunt (short) the charges when turned off. Never had a switch-related failure of any kind, (though plenty of "other" failure modes.)

We have a pretty good safety record, considering we deal with things that conflagrate with great enthusiasm. And we still have to deal with un-informed public perceptions and any appearance of being reckless.
 
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Talk it over with your L3CC member. As far as I heard is that statement was known to be a little bit confusing.

As far as I know the intent of that requirement was basically you have to physically disconnect the battery from your altimeters with a switch, meaning you cant only use the launch disable pin on your altimeter.

So meaning you only need two switches (one for redundancy) to disconnect the batteries from your altimeters, and then can use a terminal block to terminate your pyro leads. At least that was how my certification electronics was set up.
 
jadebox/SMR,

Point taken regarding caps in the pyro circuitry. I fly Altus Metrum's EasyMinis. Here's the link to the schematic:

https://altusmetrum.org/EasyMini/v1.0/easymini-sch.pdf


Hopefully Bdale or Keith will come up on the thread and make a comment as to whether or not it would be possible for the EasyMini to fire the BP charges with the power disconnected.





It reads more ominous than it really is. It is just a requirement to safe the charges in your rocket by isolating from them any source of electricity that might inadvertently set them off. An example is your rocket is on the pad, you arm your recovery electronics, and then realize you need to pull the rocket back down off the pad to (fill in the blank). Twist-N-tuck no help here. So, a switch that takes the battery off the altimeter MAY work, if the battery is the source of power to the charges. The "may not be sufficient" comment is to cover the possibility that your particular brand of altimeter uses a capacitor to fire the e-match. In this case, the capacitor would remain charged, so yes, you must break the connection between the altimeter and the e-match. That is a specific requirement to specific altimeters, check with your manufacturer. It is really not much different than there desire we all have to ensure the igniter leads are unpowered before connecting them to your igniter, with your face near the business end of your motor.

Modern quality switches have a very low failure rate, which is further mitigated by the requirement to have complete redundancy in the initiation of recovery events. Care still must be used in the design of the system... i.e. I would discourage using multiple switches on the same altimeter, which could allow the altimeters to be powered and the charges to be unpowered. (This would hopefully be caught by the lack of continuity beeps.) I personally use 4PDT POPO switches on my sleds, which powers the altimeters when turned on and shunt (short) the charges when turned off. Never had a switch-related failure of any kind, (though plenty of "other" failure modes.)

We have a pretty good safety record, considering we deal with things that conflagrate with great enthusiasm. And we still have to deal with un-informed public perceptions and any appearance of being reckless. Refer to this quote from the lawyer representing the plaintiff in a recent lawsuit.

"It is appalling to us that you have a group of individuals who literally are playing with dynamite, with no safety precautions, and obviously not playing with a full deck," he said. "Who in their right mind would lower a rocket, point it in the direction of a young woman, knowing that it could explode and ignite?"
 
My L3CC has interpreted it to mean SIX switches.

You flew with only one switch per flight computer, or two per computer?

Talk it over with your L3CC member. As far as I heard is that statement was known to be a little bit confusing.

As far as I know the intent of that requirement was basically you have to physically disconnect the battery from your altimeters with a switch, meaning you cant only use the launch disable pin on your altimeter.

So meaning you only need two switches (one for redundancy) to disconnect the batteries from your altimeters, and then can use a terminal block to terminate your pyro leads. At least that was how my certification electronics was set up.
 
Have you thought about dual pole switches? One switch that controls the power between Battery 1 and Altimeter 1 on pole 1, and between battery 2 and Altimeter 2 on pole 2? Then repeat for the other 4 circuits. You will make your L3CC happy and cut the number of switches down to 3. Or maybe find a 4 or 6 pole switch? And cut down to 2 or one switch?
 
This surfaces every now and again and for the same reason Tim mentions, I chose to go TRA for my L3. The ridiculous number of switches that the NAR rule calls out may be safe on the ground, but left me uncomfortable with the multiple potential failure points in flight. My concern was always that the switches we use are not rated for the environment (acceleration/vibration) the air.

SMR has an interesting solution that I am not sure actually meets the letter of the rule.
 
My L3CC has interpreted it to mean SIX switches.
I flew with two switches (between the batteries and altimeters), and my L3CC was fine with it. It says "may not be", though the statement is confusing.
 
The CAR go even more further, they will like us to shunt and disconnect the igniters, the reasons been that some static electricity can build in the rocket and fire the charge as the igniter wires can act as antenna and capture the static electricity, I don't study the circuitry deeply, but are not those FET or other output transistor serve as shunt by themselves if they have no power apply to them ? If so a simple switch between the battery and the altimeter will do the job.
 
Whether or not it would be possible for the EasyMini to fire the BP charges with the power disconnected.

Unless the the device is damaged, there is no path for current to get to the igniters when the power switch is off. The ballast cap we use powers the computer while the igniters are being fired, and there is a blocking diode between that capacitor and the ignitors. Also, the ballast capacitor only has enough capacity to run the flight computer for perhaps 100ms.
 
I used no switches on my L3 Talon 6. I used 2 MW RRC2 old style boards next to each other. I used one twist and tuck wire for each unit. Each came out the body tube, twisted and put back in the body in a second hole just below the first. Total of 4 holes. Each wire was taped to the tube. If there is a problem you just pull the tape and clip the twist. The Talon has flown a total of 8 flights without a problem. As a matter of fact I have never used anything but a twist and tuck method on any rocket I have ever flown, from G to N. I have never had a failure in over 10 years. There is nothing in the NAR book saying you must use a switch. KISS man KISS. Tim Thomas L3
 
I flew NAR L3 and did two switches - the rule just states that you need to prove that you won't fire charges if the switch is off...I think that this is aimed more at the old style altimeters then the modern ones.
 
I flew NAR L3 and did two switches - the rule just states that you need to prove that you won't fire charges if the switch is off...I think that this is aimed more at the old style altimeters then the modern ones.

If that is the case it makes more sense. Saying that, if the above is true, it seems that NAR should update their requirements to reflect improvements in altimeter design and only ask that the flyer provide proof that they pre-tested their electronics configuration to sufficiently rule out any faulty logic, electronic transients or intermittent/incorrect connections.
 
Thanks Keith. Ok, you heard it from the Man (designer). There is no reason that would require me to use more than two switches to disarm ALL charges.
Unless the the device is damaged, there is no path for current to get to the igniters when the power switch is off. The ballast cap we use powers the computer while the igniters are being fired, and there is a blocking diode between that capacitor and the ignitors. Also, the ballast capacitor only has enough capacity to run the flight computer for perhaps 100ms.
 
thumbs up that's the way I do it. any more splices or switches are another point for failure
I used no switches on my L3 Talon 6. I used 2 MW RRC2 old style boards next to each other. I used one twist and tuck wire for each unit. Each came out the body tube, twisted and put back in the body in a second hole just below the first. Total of 4 holes. Each wire was taped to the tube. If there is a problem you just pull the tape and clip the twist. The Talon has flown a total of 8 flights without a problem. As a matter of fact I have never used anything but a twist and tuck method on any rocket I have ever flown, from G to N. I have never had a failure in over 10 years. There is nothing in the NAR book saying you must use a switch. KISS man KISS. Tim Thomas L3
 
Not even switches, just twist and tape all the wires to the outside of the rocket! Physically shunting I would interpret to be breaking the connection between the match and the board, not necessarily the battery. In that sense a two circuit switch (On turns both on/off) would be fine for closing the on/off circuit and shunting/arming the e-match.

Or go TRA, if you're paying for L3 you should be able to swing for both memberships :)
 
When I did my L3 I put a jumper over the "switch" input and put the physical switch in line between the battery and the altimeter. That way turning off the switch both turns off the altimeter and creates a physical disconnect between the battery and the charges. This is a trick I learned from my L3CC and I really like the extra degree of safety it provides. I can post pics if anyone is interested.
 
At the end of the day, the people whose opinion matters are those that you're asking to sign the certification.

If one or more of them is demanding six switches, personally I'd find a different L3CC.

At an LDRS several years ago, I once saw this taken to an absolutely ludicrous extreme by a NAR L3 candidate which required flipping switches, moving wires between switches, etc. All at the pad. At the end of the day, the opportunity for recovery failure was so very much higher because of all the additional wire stress and failure points introduced.

If you can't find an L3CC whose requirements are palatable to you, check with TAPs to see if you can find the requisite pair whose viewpoint agrees with yours.

-Kevin
 
Add a few switches that should eliminate the possibility of a charge going off in your face [or other bad times] with the trade off a slightly higher chance of a lawn dart....

Hmm...save my eyes or my rocket....

NO BRAINER!
 
Add a few switches that should eliminate the possibility of a charge going off in your face [or other bad times] with the trade off a slightly higher chance of a lawn dart....

Hmm...save my eyes or my rocket....

NO BRAINER!

Your playing the worst case of an on-pad failure against the best case of an in-flight failure. What if we reverse it and go with best case of on-pad failure and worst case of in-flight failure. Charge goes off but nobody near it, or lawn dart into people.

Hmm.... Save my rocket or save my life....

NO BRAINER! (after the rocket goes through yours or someone else's)
 
Add a few switches that should eliminate the possibility of a charge going off in your face [or other bad times] with the trade off a slightly higher chance of a lawn dart....

Hmm...save my eyes or my rocket....

NO BRAINER!

A pair of safety glasses resolves the eye safety issue, without introducing other potential problems.

Not to mention that I don't tend to have my face right in the line of fire when I arm things, anyway.

-Kevin
 
Just reading about altimeters popping on the pad due to flakey mag switches.
Just read about a guy getting his facial hair fried...lucky he did wear glasses....
Just reading about people rewiring on the pad.
All ugly!


All my rockets have a 4PDT switch between the altimeter and the pyros.
Never had a switch problem.
Do what you want -- I think it very prudent to keep my charges disconnected and shunted until the last minute.
 
I've had a mag switch set off a charge in a rocket about a foot from my face. Luckily it was cold gas. I've also seen a bp charge go off during prep.

I've always kept a physical barrier when connecting wires, usually the rocket itself, between my eyes and the charge.
 
I had a charge go off on the pad due to a faulty NPN diode that failed between my pre-test at the bench and the journey to the pad. Weird stuff really does happen occasionally, the best set-up, if one has the room that I could see is four switches. One for the Main charges, one for the Apogee charges, and one for each altimeter battery, separate ones so you can hear the beeps of each altimeter. But some of my rockets simply do not have room for two extra switches, I am looking at you Wildchild with a Raven on-board.
 
Buy Digikey Part Number SC1821-ND which is the Switchcraft 50212LX.
Use one for the power and one for the four pyros.
This gives you 4 sets of contacts for altimeter power (for you paranoid) and one per pyro.
Assuming you are using [the wise choice of] a single four-channel altimeter.
If you must use two altimeters, use two contacts for each.

You MUST epoxy over the four tabs on the sides -- plus I epoxy over the terminals once all soldered up.

Now -- these are NOT the exact units I use -- I get mine surplus in a slightly different configuration, but the switching body is the same.
I just looked these up to use as an example.
These also say "locking" - not sure what that means but it sounds like a bonus. Mine don't lock - never had it be an issue.
 
My L3 CC told me the shunting rule had been removed. You just need to prove you can shut the power off and it disarms the charges.


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