NAR L3 Requirement Argument

FredA said:

Assuming you are using [the wise choice of] a single four-channel altimeter.

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Had an incident on one flight at LDRS this past weekend that proves this false YET AGAIN.

H_Rocket said:

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.

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One 4PDT switch per altimeter. Absolutely meets the rule.... simultaneously removes power to the altimeter, isolates the wires from the altimeter to the e-match (satisfies the requirement), AND shunts the e-match leads together (not a requirement but added safety).

troj said:

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

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Safety glasses are always a great idea, but assumes the unintended event occurs when you are ready for it. Murphy calls the alternative. Static electricity, RF interference, any other unknown failure and Poof... a charge goes off. The expanding cloud of hot gas may not be the only hazard. It might be your aluminum-tipped fiberglass nose cone on a 40' shock cord, and everybody "down-range" from that projectile is not going to have eye protection in place. Obviously there are two schools of thought here, and one side isn't going to "win" the argument. Historically, we have had charges blow inadvertently. We have had motors light inadvertently. Small steps to mitigate low-probability, high-risk events are not unwarranted.

One 4PDT switch per altimeter.

An alternative way to wire for two altimeters.

You need to decide if they behave better if powered on with the pyro connected or without.
Some glitch on power on.
Some check their pyro continuity once.
Some check their pyro continuity late or multiple times.

I like to turn on my altimeters then arm the pyro....looking for altimeters that behave well that way.
One switch for power and one pyro's achieve this.
This way I only enable the pyro when the rocket is on the pad, upright and the altimeters are powered up and "in control" of their outputs....

If you have a "picky" unit, wiring the pyro and power together all on one switch avoids this, but exposes you to turn-on transients.

Learn what your altimeters does then be safe and use a disconnect and shunt.

FredA said:

You need to decide if they behave better if powered on with the pyro connected or without.
Some glitch on power on.
Some check their pyro continuity once.
Some check their pyro continuity late or multiple times.

I like to turn on my altimeters then arm the pyro....looking for altimeters that behave well that way.
One switch for power and one pyro's achieve this.
This way I only enable the pyro when the rocket is on the pad, upright and the altimeters are powered up and "in control" of their outputs....

If you have a "picky" unit, wiring the pyro and power together all on one switch avoids this, but exposes you to turn-on transients.

Learn what your altimeters does then be safe and use a disconnect and shunt.

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Good point, Fred. I am strictly a PerfectFlite guy (MAWD, SL100, mini timer 3 and 4). They check continuity repeatedly after power up, so it is possible to arm the altimeters and THEN the charges. On a side note, my Tiltometer requires 3 separate switches, thrown in order, to properly self-test and power up.

SMR said:

One 4PDT switch per altimeter. Absolutely meets the rule.... simultaneously removes power to the altimeter, isolates the wires from the altimeter to the e-match (satisfies the requirement), AND shunts the e-match leads together (not a requirement but added safety).

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Somehow I interpreted the rule to indicate you needed separate devices for each function. If that is not the case, then I really like your idea.

H_Rocket said:

Somehow I interpreted the rule to indicate you needed separate devices for each function. If that is not the case, then I really like your idea.

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Many things can be left to the discretion of the builder, as part of the certification process allows/encourages the candidate to demonstrate his/her experience, abilities, and understanding of electronics. The paragraph could be worded better, but I do not believe it requires separate switches, or even a switch at all, just that the charges can be safed by physically removing the power source from the energetic. I think it has the potential for people to read too much into it.

Looking at this from the other direction, the physical disconnect between the power and the deployment charges are to exclude the use of shunts for protection as the combination of low current ematches and batteries with high current output result in a situation where enough current would still flow through the ematches that they could fire with the shunt in place. Also some of the old altimeters had a bad habit of activating the firing channels during their power-up cycle. Then again there are some altimeters that are good to go without additional switches besides those designed into the product. With all of those possibilities it was simpler to state the minimum safety requirements, and let the certification team determine what is acceptable.

John

Johnly said:

Looking at this from the other direction, the physical disconnect between the power and the deployment charges are to exclude the use of shunts for protection as the combination of low current ematches and batteries with high current output result in a situation where enough current would still flow through the ematches that they could fire with the shunt in place. Also some of the old altimeters had a bad habit of activating the firing channels during their power-up cycle. Then again there are some altimeters that are good to go without additional switches besides those designed into the product.

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Yup, just adding a shunt alone does not break the circuit. The schematic in Post #32 breaks the circuit AND shorts the lead wires to the e-match, but would not stop an older altimeter from firing during the power up cycle. Fred's solution in Post #34 would.

Johnly said:

With all of those possibilities it was simpler to state the minimum safety requirements, and let the certification team determine what is acceptable.

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TheAviator said:

My interpretation would be that you could demonstrate that the system is safe when the power is removed. However, the onus is on you, not your L3CC to prove that point.

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Absolutely.

NAR L3 here. I used 1 featherweight screw switch between the battery and the altimeter... Adept 22 - switch - battery; Stratologger SL-100 - switch - battery.

No problems on the L3CC front.

First, I would say notice the word "may." I have seen altimeters that allow a separate power source for firing the charges from the power for the altimeter. If the altimeter had an off button, that might not suffice as well. And if your altimeter has a capacitor, you may need w way to ensure that it won't set off your charge.

I would follow the advice earlier about contacting your L3CC team and not assume that you need four switches. Beyond that, keep in mind that the common dpdt witch can switch two wires and even short wires in the off position. Or you could make a hatch and not have a battery in place until after in the upright position.

On the EasyMini, this switch physically breaks the positive line from the lipo. There is no need to do what you have described. Did you check your schematic; Maybe you didn't need to do this?

COrocket said:

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.

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Makes perfect sense to me! I fly redundant EasyMinis in the Av-bay controlled by two screw switches. In the NC I have the TeleGps with a Featherweight magnetic switch. Since the TeleGps nor the magnetic switch emits any type of sound upon activation, I carry my ht (radio) with me to the pad to assure that the TeleGps is transmitting.

mrwalsh85 said:

NAR L3 here. I used 1 featherweight screw switch between the battery and the altimeter... Adept 22 - switch - battery; Stratologger SL-100 - switch - battery.

No problems on the L3CC front.

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SMR said:

One 4PDT switch per altimeter. Absolutely meets the rule.... simultaneously removes power to the altimeter, isolates the wires from the altimeter to the e-match (satisfies the requirement), AND shunts the e-match leads together (not a requirement but added safety).

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Nice bit of kit there. Let me make sure I understand: This setup fires both apogee charges simultaneously? And it fires both main charges simultaneously? Do I have that correct?

Best,
-Carl

WOW, what happened to the good 'ol days? I have 5 rockets and 7 altimeters, and zero switches. Never felt the need for a switch, not gonna change now. But like I always say, what ever works for you, go with it!!

Mick2b said:

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.


Sent from my iPhone using Rocketry Forum

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I thought that was the case with NAR. I griped about this on a list-serv some years ago. Was an absolutely stupid rule for deployment charges.
I was told like you said Mickey this was dropped. Extra switches are a point of failure that are totally unnecessary. Kurt

jimzcatz said:

WOW, what happened to the good 'ol days? I have 5 rockets and 7 altimeters, and zero switches. Never felt the need for a switch, not gonna change now. But like I always say, what ever works for you, go with it!!

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They let you L3 with wire twisting Jim? :wink: I thought that was a big no, no for a cert flight? OR did you cert in the bygone days before the ivory towers started thinkin' about that stuff?:grin: Kurt

cvanc said:

Nice bit of kit there. Let me make sure I understand: This setup fires both apogee charges simultaneously? And it fires both main charges simultaneously? Do I have that correct?

Best,
-Carl

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It could, depending on how the individual altimeters are set up. I put an apogee delay (1 sec) in one, and set the main deployment altitude lower (i.e. 500' vs 700'), so it functions as a backup. The second (Hail Mary) charge to fire usually has a little bit extra BP.

SMR said:

It could, depending on how the individual altimeters are set up. I put an apogee delay (1 sec) in one, and set the main deployment altitude lower (i.e. 500' vs 700'), so it functions as a backup. The second (Hail Mary) charge to fire usually has a little bit extra BP.

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Ah, got it. The altimeters are programmed differently to give a staggered triggering of the charges. Thanks!

cvanc said:

Ah, got it. The altimeters are programmed differently to give a staggered triggering of the charges. Thanks!

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A lot of altimeters such as the old Perfect Flight MAWD allowed a 1 second delay of the apogee charge to be set for a second altimeter to act
as backup. Many now outside of the $29.99 specials allow time delays to actually be programmed into the channels.
I'm not slamming the $29.99 specials as I own a few and they've performed well for single unit dual deploy sport rockets.
Prices are more economical now than in 2006. Kurt

SMR said:

One 4PDT switch per altimeter. Absolutely meets the rule.... simultaneously removes power to the altimeter, isolates the wires from the altimeter to the e-match (satisfies the requirement), AND shunts the e-match leads together (not a requirement but added safety).

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I want to make sure I'm not missing something here. Wouldn't a 3PDT switch be adequate for each altimeter?

Its amazing to me how a simple and common sense rule can cause so much confusion.

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.

How to do this shouldn't be a matter of L3CC analysis. You ask your altimeter manafacturer "Does disconnecting power to the altimeter break the power to the pyrotechnic devices?". Two possible answers:
Answer 1. Yes it does. -> Then show this to your L3CC and use a single altimeter switch per altimeter if you desire.
Answer 2. No it doesn't because the altimeter has persistent energy storage on-board. -> Then you need a switch to break the pyro circuit. Even if you belong to TRA.

jderimig said:

Its amazing to me how a simple and common sense rule can cause so much confusion.

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.

How to do this shouldn't be a matter of L3CC analysis. You ask your altimeter manafacturer "Does disconnecting power to the altimeter break the power to the pyrotechnic devices?". Two possible answers:
Answer 1. Yes it does. -> Then show this to your L3CC and use a single altimeter switch per altimeter if you desire.
Answer 2. No it doesn't because the altimeter has persistent energy storage on-board. -> Then you need a switch to break the pyro circuit. Even if you belong to TRA.

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What is persistent energy storage onboard? Magic? A battery? A capacitor? I will concede that a device with two batteries, one to power the altimeter and one for the pyro circuit could still have power going through the ematch circuit for continuity detection. Two come to mind immediately. The out of production Parrot altimeter that had two screw switches, one for the onboard battery to power the unit, one for a 9V pyro battery. I believe the pyro battery was in the circuit for continuity purposes even if the altimeter was off. Was also a PITA to screw both switches in on such a small sized altimeter. The ARTS 2 has two battery capability but only one switch. I don't know if both batteries are off with the one switch connection.

The EggTimer altimeter in one of it's forms has separate batteries. One for the electronics, one or two additional for the pyro. Now current is going through the pyro circuit with the altimeter off. How do I know? I had LEDs attached for testing and there is a faint glow with the pyro battery attached and the altimeter shut off. It is the limited current going through for continuity even with the altimeter turned off. Is this a safety hazard? NOOOOO!!!! Is it a nuisance?
Can be. It's a nuisance because one CAN'T preload a rocket ahead of time without the pyro battery being drained (without a switch) If I was going to do that
with an EggTimer, I'd use use a DPST switch to turn the pyro batt and the altimeter on/off at the same time.
If one connects the pyro battery directly at the launchsite then goes and flies, no problem. Letting the rocket sit for weeks could lead to a drained pyro battery. The ET would beep a fault on the pad and in that case if the flier ignored it, it would be a ballistic flight. (Safety hazard if one ignores a pad altimeter fault)

Oh, I did put live matches on the ET circuit with the pyro battery and guess what? Nothing happened.

John, that logic is impaired and has nothing to do with safety. An ET with a "live" pyro battery is no more dangerous than one that has no pyro battery.
It is not going to "blow" when one connects up the ematches. I simply recommend a DPST switch to make sure maximal juice is there when ready to fly.

I know the old NAR rule of switches on every charge is gone now and that's fine. Was just a multi-point failure risk and further reason to consider TRA for L3 flight (that isn't there anymore of course). If one doesn't want to worry about it, use MAWD's, Stratologgers, Ravens or any of the single battery altimeters out there. Kurt

ksaves2 said:

What is persistent energy storage onboard? Magic? A battery? A capacitor?

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Magic no, Battery yes, Capacitor without a charge drain yes.

ksaves2 said:

I will concede that a device with two batteries, one to power the altimeter and one for the pyro circuit could still have power going through the ematch circuit for continuity detection.

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In that case to comply you at least need the ability to break the power to the pyro circuit. Your altimeter manufacturer will have to advise you on whether you need another switch for the altimeter power to prevent the possibility of any potential to develop across your ematches.

jderimig said:

Magic no, Battery yes, Capacitor without a charge drain yes.



In that case to comply you at least need the ability to break the power to the pyro circuit. Your altimeter manufacturer will have to advise you on whether you need another switch for the altimeter power to prevent the possibility of any potential to develop across your ematches.

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I wouldn't need to contact the maker in that regard as I've already seen for myself. I simply would use a DPST switch to activate both the pyro and the electronics at the same time. The ET has a pretty long startup sequence and should be perfectly fine. I've allowed the altimeter to startup and give the pyro battery fault mode and turning on the pyro battery doesn't change the fault mode. The pyro battery would either have to be turned on first or at the same time as the power to the altimeter electrics. Kurt

When I was planning my L3, NAR was just getting into L3 certs and the L3CC had just been formed. At that time, it was 6 separate switches, with each ematch being shunted. The nearest L3CC was just planning his own L3 and had this "switch plan" he'd hand out to other folks looking for a NAR L3. It made things very complex, as it was not just the number of switches, but there was a specific order they had to be turned on. I recall a big issue at that time was the shunting, because a few of the altimeter vendors came out and stated that their devices were "self shunting", in that the design was such that the ematchs were internally shunted until they were fired (The shunting was so that an ematch wouldn't fire due to strong radio waves in the area). Then the debate moved to shunting the ematch on all flights that used altimeters, and not just L3..... (not sure about now, but back then, when you bought ematches, the leads were twisted together so that strong radio waves wouldn't set them off in shipping)

While all this was going on, I just got my L3 with TRA. (and one switch per altimeter)