Two Stage Redundancy

[roytyson]

Is redundancy in the second stage ignition a thing? I'm trying to imagine what that would look like: two ignitors? I know some put Pyrodex pelletin the motor for easier ignition. Or is this a one time shot, with recovery if it does not light? Thanks

 

[OverTheTop]

I ran two igniters in my head-end ignition setup last year. One was the massive igniter that came with the O3400 motor, and one was just an eMatch. Both in parallel in the igniter grain. That was nestled in most of an E motor grain.

Details here: https://forum.ausrocketry.com/viewtopic.php?f=8&t=6385

Put more effort into thinking how to keep the whole thing safe during preparation, that is where you need to be working everything through. Yes, a missed ignition should be recovered as normal, but make sure you have a plan on how to safe the motor when you recover. Always design for off-nominal events!

 

[Steve Shannon]

Is redundancy in the second stage ignition a thing? I'm trying to imagine what that would look like: two ignitors? I know some put Pyrodex pelletin the motor for easier ignition. Or is this a one time shot, with recovery if it does not light? Thanks

In my opinion, no. Full redundancy in staging increases your chance of staging unsafely (including on the pad or during preparation) and undermines safeguards that inhibit off normal staging.

 

[micro]

I agree I wouldn't use redundant electronics but it wouldn't hurt to have a second ematch if you can reliably light them together. I have had two instances where a sustainer igniter burned the bridge wire but didn't ignite the pyrogen. Sometimes there's just no mitigation- I had a Cesaroni motor in a sustainer burn the starter pellet but failed to ignite the propellant. The important thing is to have trusty electronic recovery.

 

[cerving]

If you have the room in your AV bay, you should have redundancy in the recovery side, but for the actual ignition event I'd go with what Steve Shannon says. If you have the room in your motor, a second ematch/igniter wouldn't be a bad idea, but almost all of the non-ignition events that I've personally had have been due to igniters shifting during launch and failing to light the motor, not due to the igniter not firing. With a 38mm+ motor you can usually stick some kind of stick up the nozzle to hold the igniter in place, but not so much with a 24mm or 29mm motor.

 

[roytyson]

Ok thanks. I do have redundancy set up for recovery. I will see if I have room in a 38mm for two ignitors.

 

[OverTheTop]

For the ignition side put your redundancy efforts into the lockout safety mechanisms for before and after flight. That is the main risk you need to mitigate in the sustainer.