Shunt for 2 stage?

Is a shunt necessary for the sustainer igniter in a 2 stage J-I powered rocket?


I have been getting mixed opinions.

subscribed for ideas


I have my eggtimer wired so battery to Drogue, Airstart , and Main is switched , I haven't presented this to a RSO yet so not sure if will be approved.

It is a good idea (don't ask me how I know... :blush.

You didn't trust me?? Unless you want a Motor firing right in front of you or even worse when your holding the rocket, use a shunt it will break the circuit until your ready and set up when u can turn the switch or whatever you use closing the circuit.

conman13 said:

Unless you want a Motor firing right in front of you or even worse when your holding the rocket...

Click to expand...

Ahem... I resemble that remark...

dixontj93060 said:

It is a good idea (don't ask me how I know... :blush.

Click to expand...

May I ask, did this "unplanned event" you allude to occur right as the electronic system controlling a 2nd-stage/airstart was armed, or was it at some point after the system was armed, while the rocket was sitting on the pad and you were working on some other portion of the rocket?

I just wonder if the majority of accidentally ignited stages/airstarts occur because of a fault/short that manifests itself immediately as the altimeter is powered on (or later when you close the circuit to the motor), or because of a false launch detect or similar anomaly minutes after the electronics are armed as the rocket sits on the pad. If the former is true, I don't really see how a shunt changes anything. To me, the only way to prevent that situation from occurring would be to arm the electronics and ignitor while the ignitor was still outside of the engine, so that an electronics malfunction causing the ignitor to fire would not ignite the motor.

IMHO yes, and if it's not required, it should be... (assume you're talking about a "disconnect" device between the ignitor and the upper stage ignition electronics). IMHO it should be required between ejection charges and the DD altimeter until it's on the pad and ready to fly as well IMHO. Seen altimeters get a gust of breeze across the ports, drop the pressure, and fire off an ejection charge on the ground, nearly impaling one guy with with the nosecone who was sitting at the next EZ-up over (which I was standing in front of the table he was assembling an RMS motor at) and the upper section of the rocket kicked back from the ejection charge and nearly impaled the guy working on it... he got the wind knocked out of him and a near-perfect "ghostbusters" type ring-with-a-slash-through-it mark on his abdomen where the electronics board hit him in the gut, snapped off the rails it was mounted on, and then the rest of the tube hit him in the gut hard enough to leave that red welt ring around it... (which I'm sure turned into a bruise).

That incident where someone got burned at a HPR desert launch a few years back when the upper stage lit off when the rail was lowered to work on the rocket (what the idiots were doing on a four wheeler in front of the rail is another question, albeit a valid one) would have been 100% prevented by a properly used upper-stage ignition shunt to either short the two ignitor leads to each other (to prevent firing) or opening the circuit either when inserted (normally closed circuit) or removed (normally open circuit). Last thing out on the pad should be the safety shunts... and if the rocket requires service on the pad before launching (altimeters re-armed, batteries, new ignitor on the booster, whatever) the FIRST thing that is done when approaching the pad is to REINSTALL all the safety shunts to prevent accidents...

"Remove before flight" pins/tags are just common sense insurance and basic safety equipment IMHO... Like I said, if they're not required, THEY SHOULD BE!

Later! OL JR

I was at an adjacent pad when the sustainer of a two-stage rocket decided to make a hasty vertical retreat. In this case, it was due to not ground testing the electronics. The auxiliary port used goes high for a brief moment during power-up. We had a second or two to move back from the rocket as it came to pressure. That incident reaffirmed my belief in ground testing AND shunting igniters. Also, plan for any failure mode; if you forget to disable the shunt, make sure the electronics can handle a "single-stage" failure mode.

Doug

The best way would be to both shunt (dead-short) the upper stage igniter AND to disconnect the power. The bigger problem is that once you've armed the electronics and removed the safeties, you have to trust that the electronics isn't going to fire it until it's appropriate to do so. Most of the stories that I've heard about this happening (I've been fortunate enough not to have seen this first-hand) have had something to do with inadequate testing or relying on a single arming mechanism for the sustainer. There needs to be multiple arming mechanisms... baro altitude, minimum sustained velocity, accelerometer time after launch, timer-activated time after launch, physical breakwire, gyro attitude, etc. Most single-factor arming mechanisms have some way of accidentally defeating them, but having two makes it very difficult and unlikely for that to happen.

The message from my mentor....

I assume you mean for the motor igniter? Shunts are never necessary (unless a crazy RSO tells you otherwise). Some people think they can save you from an early firing of the igniter, but unless you test it you don’t know if it will really work.

Best to have the electronics that fire the motor off until the rocket is in the upright position ready for launch.

I turn on the electronics with the igniter attached but not in the motor to make sure the transistor/relay is not stuck on. Then turn it off again. (A common failure mode for a transistor or relay is to be stuck on or off.) Usually do that on my prep table before I take it out to the pad.

A shunt alone isn't safe. In combination with a switch that opens the connection to power it adds safety.

Two-stagers and airstarts are inherently more hazardous than single-stage flights. Taking care of the low-hanging fruit might help and is just part of the process of making these flights as safe as possible.

With respect to a shunt, the best way is to use a shunt (short) and also open the circuit to the match ahead of the shunt. If you choose a shunt only, then it has to be designed to do its job. If the wires that constitute the shunt are too thin or too long, there can be enough resistance through the shunt to allow significant current through the ignitor (with a shunt, there is always current through the ignitor, it's just a matter of how much). The specifics depend on the electronics, the ignitor and the circuit you use. I did some testing a while back, and with a 4-foot wire to the shunt switch, the ematch ignitor would fire. It's a good idea to test your shunt in its exact configuration.

Although you didn't ask, here is my list of other safety practices that can make these flights safer:

- Always do an all-up test of the electronics just prior to flight. This means turning everything on, recovery electronics included, but with the ignitor our of the motor.
- Use an altimeter with an altitude check feature. There are many of them available now.
- When it comes time to arm the electronics, get people away from the pad, or wait until everyone has left. The attached pic is what it looks like when I arm a rocket.

Jim

My biggest suggestion is that you learn by committee, then apply individually.

Understand your circumstances and apply your learnings to your very specific problem. Don't just slap a shunt on there without fully understanding what you're doing. A two stage rocket is quite complex and to quote Jim "inherently more hazardous."

Ground test, ground test, ground test.

blackbrandt said:

Shunts are never necessary (unless a crazy RSO tells you otherwise).

Click to expand...

That would a "crazy RSO" following the explicit Tripoli guidelines from https://www.tripoli.org/Launches/Safety/RSOGuidelines/tabid/184/Default.aspx

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.

Click to expand...

Bold mine.

First off, thank you for all of the input. I will be redoing my multistage launch prep after reading this. Thank you mikec for pointing out that in the TSLP.

I figured that I would restate how I am arming the upper stage starter.

Before I even bring the rocket out to the pad, I hook the igniter up to the altimeter and turn on the altimeter. I then check the terminals with a voltometer. This allows me to check if the igniter will fire on power up of the altimeter.


The altimeter for the ignition of the sustainer is in the sustainer.

The booster is placed on the pad, and is left unarmed.

The sustainer (which has no rail buttons) is now lifted up on top of the booster with a few inches in between them. The upper stage altimeter is now powered on, and I confirm that the igniter did not fire. Then, the igniter is inserted into the motor, and the sustainer is placed on the booster.

I have a higher than normal arming altitude for this, to reduce the chance of the moving of the rocket to cause arming of the altimeter.


Again, I am not saying that this is the right method, but this is the method I was taught. I would prefer not to say the mentor's name who taught me this.

Thanks again for all of your help!

Cris Erving and John Dermiggio,,
Good men,, you both said the same thing in a different way..

I'd like to ask a question..

An altimeter's pyro output that is configured to do an airstart,, whether it be the Eggtimer's "B" channel or any one of the Marsa's channels....

First-- Will it check for continuity on that channel as part of it's normal "prepare for flight" routine just as it would if that channel was configured for a deployment charge???
And if it doesn't "see" continuity will it fail it's preflight checks just as it would if it were configured for a deployment charge and it didn't see continuity on that channel ???

Second-- If a shunt / dead short is put in place on the airstart motors ignitor leads for safety reasons and the altimeter goes through it's preflight checks looking for continuity
on this channel,, can the short damage the output of the altimeter because there's so little resistance on the short,, where there should be an ignitor that does offer some resistance ????

Teddy

First-- Will it check for continuity on that channel as part of it's normal "prepare for flight" routine just as it would if that channel was configured for a deployment charge???
And if it doesn't "see" continuity will it fail it's preflight checks just as it would if it were configured for a deployment charge and it didn't see continuity on that channel ???

Click to expand...

I can answer this from experience with the Eggtimer.... Yes the Eggtimer does check for continuity on power up and will give you an error code if it's programmed for a certain channel and there is not continuity. This was the problem of why my Calvin&Hobbes didn't ignite the second stage or have deployment on the sustainer......

onebadhawk said:

Cris Erving and John Dermiggio,,
Good men,, you both said the same thing in a different way..

I'd like to ask a question..

An altimeter's pyro output that is configured to do an airstart,, whether it be the Eggtimer's "B" channel or any one of the Marsa's channels....

First-- Will it check for continuity on that channel as part of it's normal "prepare for flight" routine just as it would if that channel was configured for a deployment charge???
And if it doesn't "see" continuity will it fail it's preflight checks just as it would if it were configured for a deployment charge and it didn't see continuity on that channel ???

Second-- If a shunt / dead short is put in place on the airstart motors ignitor leads for safety reasons and the altimeter goes through it's preflight checks looking for continuity
on this channel,, can the short damage the output of the altimeter because there's so little resistance on the short,, where there should be an ignitor that does offer some resistance ????

Teddy

Click to expand...

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.

Please define "SHUNT"

Is it a "continuity" switch in the staging igniter circuit that's simply "open" during setup and "closed" for flight? OR

Is it a "dead" short (conductor) across the igniter/ematch augmented igniter circuit in parallel to "shunt" the electrons "away" from the igniter/ematch? If so, I doubt its effectiveness with an ematch augmented (low current) igniter to prevent it's firing if the
circuit was accidentally activated. Most altimeter/electronics will say in the instructions to avoid testing into a dead short so's not to damage the FET's. If you can't safely prove a "shunt" will prevent the firing of the ematch augmented igniter, it's a fallacy
to believe in its safety. It's a false premise.

I have never seen a sure-fire (sic) safety circuit I could trust for a staging ignition channel short of remote arming. Short of that, I believe a flier would have to trust that their device setup is as safe as it can be for its planned purpose. Kurt

onebadhawk said:

Cris Erving and John Dermiggio,,
Good men,, you both said the same thing in a different way..

I'd like to ask a question..

An altimeter's pyro output that is configured to do an airstart,, whether it be the Eggtimer's "B" channel or any one of the Marsa's channels....

First-- Will it check for continuity on that channel as part of it's normal "prepare for flight" routine just as it would if that channel was configured for a deployment charge???
And if it doesn't "see" continuity will it fail it's preflight checks just as it would if it were configured for a deployment charge and it didn't see continuity on that channel ???

Second-- If a shunt / dead short is put in place on the airstart motors ignitor leads for safety reasons and the altimeter goes through it's preflight checks looking for continuity
on this channel,, can the short damage the output of the altimeter because there's so little resistance on the short,, where there should be an ignitor that does offer some resistance ????

Teddy

Click to expand...

The Marsa preflight test routines looks for a normal load on the channel not just continuity. If the resistance is too low or too high it will fail the test and give you the "not good" beeps. So a shunt will normally fail the output test.

However if you designate the airstart channel as a "Mtr-Timer" event (as you should with airstart motors) the logic will ignore the shunt and open failure and give you a "good" beep. This is because it is common to close switches or remove shunts to motors as the LAST thing you do at the pad. The consequence of a failed airstart is much lower than a failed deployment event.

Thank you very much gentlemen,,
Kurt I believe we are all on the same page with a shunt defined as a dead short across the air-start ignitor leads
so if current was mistakenly put to those leads the ignitor would not fire....

John,,
I had forgotten that the Marsa gives you resistance values as opposed to just continuity confirmation,,
so by that it would fail for either high OR low resistance values on an "enabled" channel......

Cris,,
As always,, multiple fail safes,, multiple parameters must be met to verify everything is correct for the air-start
and not wind in an open vent in the av bay......

But I was under the impression that an output would be damaged by a continuity test if you shorted that output ???
That's what actually generated this question,, under that circumstance,, how to do this...
I don't recall why I thought that a short on the output would damage it....

Thank you both very much,,,,
Always informative and helpful....

Teddy

onebadhawk said:

Thank you very much gentlemen,,
Kurt I believe we are all on the same page with a shunt defined as a dead short across the air-start ignitor leads
so if current was mistakenly put to those leads the ignitor would not fire....

Click to expand...

Right. But........If one is using a low current ematch augmented igniter, short of a dead short test with the altimeter in question (which BTW could dork the altimeter) one doesn't know if the shunt is effective. I suspect a fair number of units (like a Marsa) could pump enough electrons through a parallel shunt and pop/set off an ematch augmented igniter. So you people running around thinking that shunts are "safely" protecting everyone from a premature staging accident are fooling yourselves.

Kurt

The short across the deployment channels will only fry the transistors if it fires with a high-current power source like a LiPo. The deployment continuity test current is limited to 1/2 mA, so no worries there. Now if you forget to remove the shunt... that's another story.

I think it's worth pointing out that the check an altimeter does is probably only looking for an open-circuit, not checking for shorts. The problem is that the ignitors are all very low-resistance anyhow, so the difference between a true short and a properly-functioning ignitor is negligible, and likely well within the tolerances of the circuit used to test the ignitor continuity, especially given that it has to use a very low current to avoid prematurely setting off the ignitor/ematch. The ignitors are typically around 1Ω, and with ½ mA like Cris mentioned above that's 0.5mV, which would be next to impossible to read using a 3.3V micro's ADC. Heck, even reading this resistance with a typical multimeter is difficult, as the test lead resistance generally isn't compensated for (at least on DMMs with detachable leads, amazingly enough the $10 pocket DMM I ordered online appears to have compensated for it's attached leads when taking resistance measurements), so just measuring resistance between the two leads shorted together often reports something around an ohm, so whenever I check an ignitor with a multimeter I take two measurements, one with the leads shorted together and one with them empty, and take the delta of the two readings to get the ignitor resistance.

But it also means that since the delta between a proper ignitor and a dead short is so small, you're not going to do any damage by having the altimeter do its continuity check into the shunt, it won't know the difference. I should note based on what Cris just posted that I'm specifically talking about the continuity check. Firing the deployment charge into the shunt is not good, you might find the weak link in your circuit and fuse it.

ksaves2 said:

Please define "SHUNT"

Is it a "continuity" switch in the staging igniter circuit that's simply "open" during setup and "closed" for flight? OR

Is it a "dead" short (conductor) across the igniter/ematch augmented igniter circuit in parallel to "shunt" the electrons "away" from the igniter/ematch? If so, I doubt its effectiveness with an ematch augmented (low current) igniter to prevent it's firing if the
circuit was accidentally activated. Most altimeter/electronics will say in the instructions to avoid testing into a dead short so's not to damage the FET's. If you can't safely prove a "shunt" will prevent the firing of the ematch augmented igniter, it's a fallacy
to believe in its safety. It's a false premise.

I have never seen a sure-fire (sic) safety circuit I could trust for a staging ignition channel short of remote arming. Short of that, I believe a flier would have to trust that their device setup is as safe as it can be for its planned purpose. Kurt

Click to expand...

My use of shunt is a dead short. Closed during prep and opened for flight.

They are effective for low current ematches as long as the resistance of the shunt is low (short wires, good switch, etc.). This can all be measured and tested. Some altimeters have a short enough latch time that testing will not damage the altimeter - others do not, and it may also depend on the battery you use. Everything in rocketry is equipment specific. If you don't understand how your electronics work, my advice is don't do staging.

Jim

woferry said:

I think it's worth pointing out that the check an altimeter does is probably only looking for an open-circuit, not checking for shorts. The problem is that the ignitors are all very low-resistance anyhow, so the difference between a true short and a properly-functioning ignitor is negligible, and likely well within the tolerances of the circuit used to test the ignitor continuity,

Click to expand...

The Marsa54/54L does indeed check for a short and can detect abnormally low (relative to ematch) resistances quite reliability. The difference between a short and a 0.8 to 1.2ohm resistance is not negligible at all. The Marsa54L is also "shunt safe" in the case of accidentally firing into a shunted or accidentally shunted load.

Ideally, you would have a switch to tie the igniter leads (no chance of firing) together and have a resistor/LED circuit (view status of circuit) active across the outputs during powerup. This starts getting fairly non-trivial quickly. I plan to have a switch to disconnect the igniter during setup, check Tele-(metrum/mega) status via telemetry, then reconnect igniter via the switch and finally, double-check status via telemetry.

2013 might have been the year of the two stage, but I plan on getting around to flying one this year...

Doug

cerving said:

The short across the deployment channels will only fry the transistors if it fires with a high-current power source like a LiPo. The deployment continuity test current is limited to 1/2 mA, so no worries there. Now if you forget to remove the shunt... that's another story.

Click to expand...

Well, if one is so sure that it's just a low continuity test current, why worry/shunt at all? The deal is if "somehow" a high current source
leaks "into" the channel that is used for staging/ignition with a low current ematch augmented igniter. One can test the channel just before a launch with a bare ematch to be certain the channel functions properly with the continuity test.
If one forgets to remove the shunt with a lipo, I bet enough current will get across to blow the low current match/igniter if that's what's being used. The flight will be saved but the electronics might be gorked that channel thereafter.
A shunt is an unproven false sense of security unless tested with the equipment it is meant to be used with period. Kurt

mikec said:

That would a "crazy RSO" following the explicit Tripoli guidelines from https://www.tripoli.org/Launches/Safety/RSOGuidelines/tabid/184/Default.aspx

Bold mine.

Click to expand...

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

RocketHunter said:

May I ask, did this "unplanned event" you allude to occur right as the electronic system controlling a 2nd-stage/airstart was armed, or was it at some point after the system was armed, while the rocket was sitting on the pad and you were working on some other portion of the rocket?

I just wonder if the majority of accidentally ignited stages/airstarts occur because of a fault/short that manifests itself immediately as the altimeter is powered on (or later when you close the circuit to the motor), or because of a false launch detect or similar anomaly minutes after the electronics are armed as the rocket sits on the pad. If the former is true, I don't really see how a shunt changes anything. To me, the only way to prevent that situation from occurring would be to arm the electronics and ignitor while the ignitor was still outside of the engine, so that an electronics malfunction causing the ignitor to fire would not ignite the motor.

Click to expand...

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.