How many switches on a dual-alt recovery system?

In my L3 cert build, I am running dual altimeters.

On each altimeter, I have always used three switches:

- Power for altimeter
- Shunt for apogee output
- Shunt for main output

<a href="https://s124.photobucket.com/albums/p10/GatorFanFL/L3%20build%20-%20Competitor%206/?action=view&current=competitor6avionicsschematic.jpg" target="_blank"><img src="https://i124.photobucket.com/albums/p10/GatorFanFL/L3%20build%20-%20Competitor%206/competitor6avionicsschematic.jpg" border="0" alt="Photobucket"></a>

I use the shunt switches as a safety measure, to help eliminate any chance of a charge going off while the rocket is being loaded on to the pad.

I am wondering if KISS (Keep It Simple, Stipid) applies here, and perhaps just turning off power to the alt is enough. So my question is...

Better to have the extra measure of shunt switches, or just use a power switch and keep it simple?

Cris

I would just go with the power switch and keep it simple..Tho I have been told that NAR L3 reqiure the shunt switch, but this is for your TRA L3 right?
KISS by all means..If the power is off to the Altimeter, what are the chances of the charges going off? Pretty small to none I would think...

If you cert with TRA you can use one switch. Just make sure with your alt manufacturer that the ematch leads are latched low on power up. (This is the case with most alts and I believe so with the MWAD).

Just go with 1 switch on each alt. If by slim chance it accidently goes off on the pad no big harm. Reset everything and do it again. I've seen those switches go bad. Keep it simple and easy this isn't NASA.

As a person that has had altimeter charges fire on power up twice, and witnessed it several other times, I think that additonal safety provissions are warrented.

A single DPDT switch will safe both channels of a dual deployent altimeter. Ask your local L3CC member for a schematic.

John

I've always used just a power switch. I've never had a charge fire on power up ad never seen it personally. Sure, the risk is there, BUT... I've seen numerous switches fail under high G's. Are you better off with the risk of a charge firing at power up, or one of 6 switches failing in flight...?

Ron

Johnly said:

As a person that has had altimeter charges fire on power up twice, and witnessed it several other times, I think that additonal safety provissions are warrented.

Click to expand...

I'd really appreciate hearing details on those, John! Were you able to nail down the cause(s)?
Thanks in advance!
Will

One switch per alt., the less to fail the better off you are. Every case of charge firing on the pad I have seen was traced to user error. Polarity reversed, alt turned on-off-on to quickly etc.
This is just my experiance.

If you want to shunt your ematches, all you need to to due is use 4 switches, as the power switch can shunt at least 1 ematch when in the "off" position.

One switch per altimeter, keep it simple.

Edit: to keep it safe I put the switch between the battery and the altimeter, and jumper the altimeters switch connectors. This keeps screw drivers from shorting out the bottom of the altimeter boards etc.

6 or 12 switches are a failure waiting to happen.

BTW, you do not need to shunt charges, even with a NAR L3

Edit Again: I'm not sure I would use RC servo connectors between my charges and the altimeter. I'm sure in all respects they are OK, but again; it is another place for something to go wrong.

I wire my ematches direct to the altimeter's terminals with nothing in between.


Art Upton
NAR #26255 L3CC
TRA #492 Prefect #076

Pantherjon said:

Tho I have been told that NAR L3 reqiure the shunt switch

Click to expand...

You have been given incorrect information. NAR does not require a shunt.

SynergyAero said:

I use the shunt switches as a safety measure, to help eliminate any chance of a charge going off while the rocket is being loaded on to the pad.

Click to expand...

Your design has a potential flaw. Without the switch being in series with the battery the potential exists for current to flow through the circuit board even though the switch is open.

snowman2 said:

You have been given incorrect information. NAR does not require a shunt.

Click to expand...

Maybe not a shunt, but at least a disconnect that is separate from the power switch on the altimeter(s) per NAR L3 High Power Certification Requirements, Section 2.4 which says:

In either case NAR is asking you to add switches that could be a source of failure during the flight.

dixontj93060 said:

Maybe not a shunt, but at least a disconnect that is separate from the power switch on the altimeter(s) per NAR L3 High Power Certification Requirements, Section 2.4 which says:

The capability must exist to externally disarm all pyrotechnic devices in the rocket. In this context, &#8220;disarm&#8221; means the ability to physically break the connection between pyrotechnic device and the power source to its igniter. Simply turning off the deice controlling the pyrotechnic(s) is not sufficient.​

In either case NAR is asking you to add switches that could be a source of failure during the flight.

Click to expand...

Putting the switch between the battery and the altimeter removes the current from pryotechnic device and its power source.

We have hashed this out on the L3CC email list for quite some time.

One L3CC member even had to quit rockety&#8482; for awhile during the debates, he has since come back

That is why I say jumper the switch contacts on the altimeter board, so no stray shorts can do anything, and put the switch between the battery and the altimeter.

You do not need to add extra switches that can lead to failures irate:

Work with your L3CC member or your TAP member instead of the forum rumors

Art Upton
NAR #26255 L3CC
TRA #492 Prefect #076

Art Upton said:

Work with your L3CC member or your TAP member instead of the forum rumors

Art Upton
NAR #26255 L3CC
TRA #492 Prefect #076

Click to expand...

Art,

Respect the statement, and frankly agree with your position. But I don't spew "forum rumors." What is posted is from NAR documentation posted on their website.

-Tim

I vote for ZERO switches. I twist the alt power wires together. I used to use one switch to power on at the pad. But, after I had a switch failure during flight (resulting in a lawn dart from 5,000 ft) as well as witnessing one other, I don't trust switches anymore. Twisting the wires together is a 100&#37; sure connection.

WillMarchant said:

I'd really appreciate hearing details on those, John! Were you able to nail down the cause(s)?
Thanks in advance!
Will

Click to expand...

One was caused by cycling the battery power switch on-off-on quickly.

The other is still a mystery, but was with an old timer (Alts 941) that is rarely seen today.

There are several altimeters on the market today that will fire the charges if the battery polarity is reversed. In this cases, the visual and audible altimeter power up indicators will not occur, would be a big clue not to close the swiches to arm the charges.

John

I actually have seen this type of connection fail.. The wires when twisted broke where it came out of the insulation. Didnt catch it immediatly but the altimeters kept cycling on and off while the pad prep went on. I have had failures in the power circuit but was due to either a battery pack breaking free on a high g load flight or a cold solder joint on the switch.

Dennis

MarkM said:

I vote for ZERO switches. I twist the alt power wires together. I used to use one switch to power on at the pad. But, after I had a switch failure during flight (resulting in a lawn dart from 5,000 ft) as well as witnessing one other, I don't trust switches anymore. Twisting the wires together is a 100% sure connection.

Click to expand...

dixontj93060 said:

Art,

Respect the statement, and frankly agree with your position. But I don't spew "forum rumors." What is posted is from NAR documentation posted on their website.

-Tim

Click to expand...

Hi Tim, I wasn't try to say you were spouting rumors, just not to trust them.

That link is correct, and the L3CC and safety board has said that depending on altimeter and charge design, putting a switch in-between the battery and the altimeter "CAN" do 2.4 in that document.

It was then placed to the L3CC member theirself to determine the issue with 2.4 with each L3 Rocket they were accepting.

I can't point to a website for that, as it was discussed on the L3CC email list. I have copies of those emails stored in my yahoo mail abys, but they are protected by privacy issues so I can't just post them.

Art Upton said:

That link is correct, and the L3CC and safety board has said that depending on altimeter and charge design, putting a switch in-between the battery and the altimeter "CAN" do 2.4 in that document.

It was then placed to the L3CC member theirself to determine the issue with 2.4 with each L3 Rocket they were accepting.

Click to expand...

Hi Art:
Can you give us some hints as to what is and is not an acceptable altimeter design? Is an on-board capacitor the deciding issue?
Thanks,
Will

WillMarchant said:

Hi Art:
Can you give us some hints as to what is and is not an acceptable altimeter design? Is an on-board capacitor the deciding issue?
Thanks,
Will

Click to expand...

Hi Will,

{weezle mode}
I will defer you to your local L3CC member for that question

One, but 4PDT - switches power on two legs (redundantly) and disconnects & shunts two pyros all in one housing.

I use one and have never had an issue. I think more switches increases the probability of failure, so keep it simple.

Also, the altimeter should be the last thing you do before you install the igniter, which means the rocket should already be standing up and the only person near it should be the owner of the rocket.

MarkM said:

I vote for ZERO switches. I twist the alt power wires together. I used to use one switch to power on at the pad. But, after I had a switch failure during flight (resulting in a lawn dart from 5,000 ft) as well as witnessing one other, I don't trust switches anymore. Twisting the wires together is a 100% sure connection.

Click to expand...

I know others have replied to this as well. It depends on the altimeter, I remember a discussion that began after the Rocket Challenge show when an ejection charge on a full scale Nike Smoke went off while they were filming. The gist of the discussion was about the arming procedures we use on our rockets. I believe that some electronics are prone to firing when the power switch is "bumped". Which does happen when you just twist the wires together. I beleive the Adept (amoung others) were really succeptable. I have seen this twice. You can now purchase rotary switches that are cheap, robust and bumpless, just use them. On the other hand, 6 switches is a bit much. You can use 3 of the same switches to accomplish exactly the same circuit.

BTW I second, third (depending how many times Art has said it already) talk to your TAP.

Todd Knight said:

I believe that some electronics are prone to firing when the power switch is "bumped". Which does happen when you just twist the wires together. I beleive the Adept (amoung others) were really succeptable. I have seen this twice.

Click to expand...

I and 2 other fliers I know fly almost exclusively Adept Alts25's. Out of 100+ flights between us (although I just began twisting) I have yet to see charges go off on the pad due to twisting or even traditional switch arming.

You can now purchase rotary switches that are cheap, robust and bumpless, just use them.

Click to expand...

Which are these?? The missile works are cheap, IMO and in switch failures I've seen (including my own), the missile works were at fault. I won't use them, again. I'm also not a big fan of simple "screw-in" type switches. The construction on the ones I've seen are not very robust either.

I can easily adapt to a new switch. Frankly, I do like the simplicity of using a single switch to arm rather than twisting, but I haven't found a switch I'd be willing to use that gives me the security and peace of mind that it will not fail.

I could have been wrong on the manufacturer. But... this was verified by the manufacturer in the thread that it could happen and probably would. He took an incredible amount of heat from the twisty croud.

Still....those cheap rotary switches have personally been flown in ultra extreme conditions and regimes with zero problems. I was also holding the booster section of a loc syonic when charges went off while the flyer was twisting switch wires together. He had done this several times before on the same rocket, that time the odds caught up to him and me.

Look through the archives of Rocketry Online for the thread. I promise I am not making this stuff up.


BTW I've forgotten o-rings, failed to replace parts that were marginal and many more things that in hindsight not very bright but in the end the equipment was not at fault.

MarkM said:

I vote for ZERO switches. I twist the alt power wires together. I used to use one switch to power on at the pad. But, after I had a switch failure during flight (resulting in a lawn dart from 5,000 ft) as well as witnessing one other, I don't trust switches anymore. Twisting the wires together is a 100% sure connection.

Click to expand...

I agree with Mark on this one. It isn't worth the risk. Switches just equal another point of failure.

Ask your TAPs or L3CC.

Rocket Guy 1317 said:

I agree with Mark on this one. It isn't worth the risk. Switches just equal another point of failure.

Click to expand...

Switches also equal more reliability than twisted wire. I'm in the one switch per alt camp, and I have used two switches in parallel, both wired to the power contacts before (such that if either is touching, the full circuit is good).

I use 4PDT slide switches on all my rockets from 38mm to 6" P's.
I've never seen a failure.

The switches I use are identical to this Digikey part (p. 1990)
4PDT Standard Size Switch PCB Mount DigiKey #:SW106-ND* $1.62

I use one or two per rocket depending on the altimeter choice.
If there are only two pyro channels, then one switch does it all.
If more, then I'll use two switches.

If one switch, then I'll wire 2 poles to switch power and 2 poles to switch the pyros. Pyro's are always wired in "disconnect and shunt" mode using both throws. Power is wired as a redundant set of poles if a single battery or, if twin batteries, then each on its own pole....more on this later.

If two switches are needed because of pyro channel count, then one switch will dedicate all four poles to pyro and the other switch will handle power. Since you all know I don't fly multiple altimeters, my power switch in this case will be wired so that the two batteries will each get a redundant pair of poles. Yes, using double-throw switches for power is unnecessary, but this way I use the same switch everywhere.

When wiring power, I'll use as many contacts as available as a bit of insurance against contact opens. I'll also space these away from each other on the switch with the thought that any virbration problem would be more likely out of phase if distant. However I don't really worry about contact opens and here's why.....

Most of these switches have "reasonable" vibration resistance. I've disected the ones I use and they have a nice V-groove contact that slide over and captures the other contact in a positive mate. Their "Opens under vibration" specs are talking about millisecond wide possible opens. I've looked at thier behavior with a storage scope and I'd have to say they suffer more from microphonics than real opens....good enough for our use....

Milliseconds are nothing to pyro firings that last 1/2 second or longer.
Milliseconds of no power to a altimeter CPU could be bad if unfiltered, but if there is on-board bulk by-pass, a ms-wide event won't get to the CPU. If you use an altimeter without such bulk capacitance, just add a cap on the far side of the switch.

I bought a bag of these switches bulk at a surplus store for dirt cheap.
I've been handed them out to my flying pals and we all use them.
Combined, we have had 100's of flights using these without incident.

Using a switch is a lot more elegant than twist and tuck plus provides an easy way to disarm the unit. You might think twist & tuck is cool till you need to dis-assemble and disarm a rocket on the pad after the wires are gone inside. I've seen more than "pro" change their mind (and shorts) after a few hariy encounters trying to get a hot rocket off the pad.

YMMV,
FredA