Shunt for 2 stage?

It's relatively easy to determine if your shunt is sufficient. A simple example.....

I use a 9-volt alkaline battery for my altimeters (I specifically do not use LiPo's in this service). The maximum current a 9-volt battery can produce is around 5 amps. The statistics for the JTEK ematches I use are:

- Maximum no fire - 300 ma
- Minimum all fire - 750 ma
- Maximum recommended test current - 40 ma
- Recommended firing current - 1000 ma

Let's say that the resistance for the ematch part of the circuit is 2 ohms (the match and some wire). If my shunt wiring is 1 foot long and I use 24 gauge wire, the resistance of the shunt could be as low as 0.026 ohms, depending on the switch used. If the altimeter fires and shorts the battery to the ematch, the maximum possible current through the ematch would be 0.026 / 2.026 x 5 = 64 ma. This is above the maximum recommended test current but well below the maximum no fire current. And, since the circuit has some actual resistance, the battery won't actually produce 5 amps.

In the above scenario, the shunt would prevent the ematch from firing if the altimeter fires. However, if you used a battery or cap that could source more current, or a longer piece of wire for the shunt, or finer wire, you could approach the maximum no fire current. I ran this test case with 4 feet of wire (0.104 ohms) and a shorter ematch (1.4 ohms perhaps), which might have given 0.104 / 1.504 x 5 = 346 ma. That fired the match.

Although my example case would be safe, I have gone to adding a resistor to the ematch pathway for sustainer ignitors. For a 9 volt battery, and with a recommended firing current of 1 amp, the ematch circuit could have a resistance of as much as 9 ohms. I use a 3 ohm resistor (plus or minus), giving 5 ohms total, to drop the maximum current to below the recommended maximum test current.

Jim

luke strawwalker said:

Exactly...

Sounds like a "mentor" that is passing on bad habits and bad information... and has a bad attitude when it comes to safety.

Exactly the reason I won't allow HPR on our farms... One bad apple spoils the whole barrel...

Later! OL JR

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Mikec quotes the RSO quidelines incorrectly:

At all times prior to a safe firing position on the rod, rail, tower, or other suitable ground support facility, the igniter shall not be inside the motor, and all ejection charge related electronics must be off!
Exception: Igniters used in the initiation of upper stages and those of complex clusters may be inserted early but must be shunted to avoid accidental ignition

Shunts are only required if the igniters are inserted prior to the launch position. And I would argue "shunt" should be replaced with "switch disconnecting the battery from the igniter". As an RSO I would not allow electronics to be turned on with installed igniters until the rocket is vertical.

I'm the mentor who matt quoted. By "crazy RSO" I meant one who follows guidelines/rules he does not understand. And I tried to express to Matt that if he runs into one, he has to do what the RSO (god) says.

The only failure I've had in 14 years of successful airstarts is a timer module that had a transistor that failed "closed". If I had had a shunt in place I would have had a false sense of security. Immediately upon removing the shunt, the igniter should have fired. Matt understands to test the electronics with the igniter not in the motor to insure the transistor is not stuck on.

Dave Morey

dixontj93060 said:

It occurred as I turned on the electronics. Saying that it was totally my fault as I was trying to comply to the club's "smart 2nd-stage monitoring/ignition" so I subbed out a "modern" altimeter for a dead-nuts "old" reliable timer that I had used for 4 or 5 previous flights. I inadvertently programmed the "smart altimeter" incorrectly and ignited the second stage while on the pad. With a shunt (or open) I would have experienced the pre-ignition beeping sequence as "off kilter" and never would have transitioned to connected igniters for the second stage.

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My question here is "Do you know for a fact that a shunt would have prevented the mishap that you experienced?" Unless one has actually tested the failure mode, one can't be certain. If you tested it out after the fact, then o.k. I can accept a shunt would have prevented your accident. Now if the altimeter can discern between a dead short that a shunt presents as opposed to a plain jane low current igniter/ematch, O.K. that might work as long as no high current was presented to the circuit during the powerup.

If you mis-programmed the device, I don't know if an emplaced shunt would have prevented the firing "unless" you tested and proved it. Some devices can put enough current through the parallel circuit to pop the ematch/igniter even with a shunt in place.

Again, unless one actually tests the failure mode putting shunts in willy-nilly is no guarantee that an accident won't occur. If one tests the setup in question and the shunt performs by not allowing the igniter to fire in a failure mode then they're fine.
The problem here is can that test be safely done without dorking the electronics? Gee, "It works" but now you gotta buy a new altimeter is not very cost effective. Kurt

ksaves2 said:

My question here is "Do you know for a fact that a shunt would have prevented the mishap that you experienced?" Unless one has actually tested the failure mode, one can't be certain. If you tested it out after the fact, then o.k. I can accept a shunt would have prevented your accident. Now if the altimeter can discern between a dead short that a shunt presents as opposed to a plain jane low current igniter/ematch, O.K. that might work as long as no high current was presented to the circuit during the powerup.

If you mis-programmed the device, I don't know if an emplaced shunt would have prevented the firing "unless" you tested and proved it. Some devices can put enough current through the parallel circuit to pop the ematch/igniter even with a shunt in place.

Again, unless one actually tests the failure mode putting shunts in willy-nilly is no guarantee that an accident won't occur. If one tests the setup in question and the shunt performs by not allowing the igniter to fire in a failure mode then they're fine.
The problem here is can that test be safely done without dorking the electronics? Gee, "It works" but now you gotta buy a new altimeter is not very cost effective. Kurt

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Sorry, for not covering enough detail for you. No I didn't test it after the fact, but don't need to, as I have used the circuit in other builds (air starts). BTW, not a shunt. I took the original question in the broadest sense as a safety disconnect from the source and a shunt isolation of the second stage ignitor. After turning on appropriate electronics and validating beeps and states, then, and only then, do you turn the switch to engage the ignitor(s).

cerving said:

An Eggtimer puts about 1/2 mA of current through the igniter to test for continuity. No chance of anything getting damaged with a dead-short. However, a shunt will of course show up as continuity... you need to have good deployment channel power AND the shunt removed for the test to be meaningful. It has several mechanisms for arming sustainer igniters: minimum altitude, minimum velocity, physical breakwire, and time-after-launch detect. It also disables the airstart channel after the rocket slows down prior to apogee... this is to prevent misconfigured timer parameters from firing it in a non-vertical position. Short of having a gyro/magnetic attitude sensor, that's about the best you're going to be able to do with a baro-only flight computer.

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I have been meaning to ask you if you can disable the break wire option?

TA

thobin said:

I have been meaning to ask you if you can disable the break wire option?

TA

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Yes you can disable it if you don't want to use it.

All this talk about shunts has me wondering- I get the idea behind it (and why some say they're not as safe as you may think), but I'm not sure how one would be applied? Obviously the easiest for me to visualize is two additional wires coming out of the airstart channel in parallel with the ignitor leads. Prior to flight the ends of the auxiliary wires are twisted together. On the pad, the leads are clipped or otherwise permanently separated- kind of the opposite of twist-n-tape. Is this correct in theory?
I see a more elegant solution is to wire a 1/8" N.O. audio female jack in parallel with the ignitor harness, and short the mechanism inside the male jack such that when installed, it shorts across the leads. I do this on my Eggtimer for reset, and it's a convenient solution, but is not being used for safety. How do others accomplish this?