Question about igniting backup B.P. charges

[Bogdanoffi]

Hey all, I had a quick question,

So I am intending to have a backup black powder charge for the drogue and main parachute deployments. I have two altimeters that I am going to use.

The first question is: I was going to have the main altimeter (an RRC3) fire the main charges. Then have the backup Stratologger wired to fire the backups seconds later if need be, but I don't know how to ensure that if the main charges DO fire the backups don't. Does it even matter if the backup charges fire after the mains successfully deploy the parachutes? Should I worry about this?

Alternatively I was thinking I could just hook both altimeters up to both the backup and main charges for drogue and main-parachute, but I need to test if the altimeters can light two ematches at once considering they'd have to be in parallel and the current would be split, and I would have to make sure that both charges dont total to be too much black powder and overdue the separation. I am brand new to rocketry, I have some experience making circuits with sensors, but I am hoping I can get some thoughts from someone with actual experience. Thank you in advance.

 

[Titan II]

Let the secondary (what I call backup) fire. No...do not worry about it.

 

[sriegel]

It's normal for the backup charge to fire into open air if the primary has already done its job. Don't try to have both go off together, staggered is the way to go.

Typical sequence might look like:
apogee
+1 second backup apogee
700' main
600' backup main

It's also common for the backup charges to be a smidge larger than the primary. Ex, if primary is 1.5 g, backup might be 1.75 g. That helps mitigate against a jammed tube, etc.

Always conduct ground tests of your charges, shear pins, recovery pack, etc. and then fly that configuration. "Test what you'll fly, fly what you tested."

 

[Voyager1]

Don’t disable backup charges - let them ignite at the appropriate time after the primary charges. Don’t deliberately allow the rocket to land with unburnt charges. If they were to accidentally ignite after landing you could start a fire, or someone could be injured.

The intention of your alternative method in the final paragraph is not very clear. However, for true redundancy, use separate drogue and main charges for each of your primary and secondary altimeters.

 

[OZRoc]

Don’t disable backup charges - let them ignite at the appropriate time after the primary charges. Don’t deliberately allow the rocket to land with unburnt charges. If they were to accidentally ignite after landing you could start a fire, or someone could be injured.

+ 1. Wot he said. Much Safer.
Cheers

 

[Bogdanoffi]

Okay cool, that actually makes the problem way simpler and safer too.

 

[Rocketclar]

It's normal for the backup charge to fire into open air if the primary has already done its job. Don't try to have both go off together, staggered is the way to go.

Typical sequence might look like:
apogee
+1 second backup apogee
700' main
600' backup main

It's also common for the backup charges to be a smidge larger than the primary. Ex, if primary is 1.5 g, backup might be 1.75 g. That helps mitigate against a jammed tube, etc.

Always conduct ground tests of your charges, shear pins, recovery pack, etc. and then fly that configuration. "Test what you'll fly, fly what you tested."

Agree. This is what I do. I've had experiences where the first charge didn't push all of the laundry out but the second did (as observed on the Stratologger CF's printout.)

 

[Handeman]

Agree. This is what I do. I've had experiences where the first charge didn't push all of the laundry out but the second did (as observed on the Stratologger CF's printout.)

I've had a main/primary charge fail and the 20% larger backup didn't work either. I've gone to 33% to 50% larger backup charges.

 

[David Schwantz]

All of the above, plus, when the backup charge fires into the open air it allows you to locate the rocket from the "pop".

 

[Abilene Sam]

All of the above, plus, when the backup charge fires into the open air it allows you to locate the rocket from the "pop".

Agreed. My backup charges are significantly hotter than the primary charges. The idea of one coming in ballistic gives me the willies so the bang and smoke of the "air-cannon" is fine by me. If the primary fails and the backup charge damages the airframe, it'll probably still be easier to repair/salvage than a large lawn dart at terminal velocity -- and the risk of injuring someone on the ground is significantly less.

 

[OverTheTop]

+1 to what Voyager said. Try to have all the charges fired before the rocket lands, for safety reasons.