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Rocket Staging Circuit


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Hello all! I am a hobby rocket-eer. I am building a two-stage model using composite motors. For those who might not be familiar with the motors, the standard "black powder" rocket motors can be staged by "butting" the motors together. When the bottom motor burns through, it ignites the second motor. Composite motors cannot be staged this way. therefore, the upper stage must carry it's own electronic charge. A booklet from the mid to late 80's says you can store a charge using a capacitor (it calls for using a mercury switch to complete the circuit, but I won't be using one, due to the toxicity issue. I will instead use a "G-switch" from www.aeroconsystems.com).
The ignitor requires 12-volts at 2-to-3 amps!  Is it possible to store that much current in a capacitor? (I may not be using the correct terminology, as I am NOT an electronics guy.) There are commercially made barometric altimeters that require one or two 9-volt batteries. This is not an option for me due to the excessive weight.  Any help would be appreciated!

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Thanks Ante!  Now, what size capacitor? I have experimentet with a 1000-micro farad 18V capacitor but it wasn't enought to light the igniter.  Would two work? If so, do you wire capacitors in a series (+ to - to + to -) or do you wire them joining the +'s and -'s together? I have read online that they can explode if wired improperly. I certainly want to avoid that :-)
Thanks again!

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You'll need to wire those capacitors in parallel. Two 1000uF caps in parallel have a combined capacitance of 2000uF.

A 1000uF cap's voltage will drop 3V (from 12V to 9V, or 15 to 12, for example) in 1 millisecond, if it is supplying 3A. Is this long enough to ignite the rocket?

A 2000uF cap (or two 1000uF caps in parallel) supplying 3A will drop by 3V after 2 milliseconds (twice as long), and so on.

Use 'low ESR' caps to maximise the current available in these short bursts.

Capacitors discharge on their own, even with no load, so you'll need to charge them immediately prior to launch - otherwise by the time the second stage is ready to fire, the caps will already have lost a lot of charge.

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Cabwood.. Thanks for the info!  Usually the igniter will light within one second with 12V at 2-or-3 amps.. Usually the rocket will sit on the pad for 1-or-2 minutes minimum.. If you were building the circuit, what components would you use? Would you stick with the 1000-micro farad capacitor or would you use a different size?

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Or going the other way, you could use a couple of 1F 'supercaps', designed for memory retention.
Let's assume you have three supercaps in series (assume each has a voltage rating of 5V), to give .333F. Charged to 12 volts, that's 4C of charge.

Now take your igniter, which has a resistance of around 4ohms. Attaching the charged supercaps will produce a current of 3A initially, dropping exponentially to 1.5A after 1s.

That would probably do it, but I'm not sure that the supercaps will like a 3A current draw!

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hey ray a3 glad to be of help to youif you use solid feul like gun powder to  then you can use a nichrome wire with some initiator compound on the top to ignite the gunpowder, use sparkler compound scraped from a sparkler , those ones you lite up and they spray sparks out of.  at the tip of the nichrome wire file it thin then apply the sparkler compound to it with some contact cement or other  and use the foam wire cutter circuit i posted to heat that nichrome wire hot so that the thinnest bit with the initiater compound will ignite first

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hey ray a3 glad to be of help to youif you use solid feul like gun powder to  then you can use a nichrome wire with some initiator compound on the top to ignite the gunpowder, use sparkler compound scraped from a sparkler , those ones you lite up and they spray sparks out of.  at the tip of the nichrome wire file it thin then apply the sparkler compound to it with some contact cement or other  and use the foam wire cutter circuit i posted to heat that nichrome wire hot so that the thinnest bit with the initiater compound will ignite first


Not to start an argument, but I wouldn't attempt making my own igniters unless I knew exactly what I was doing. Anything you tried to bodge together by scraping, filing and gluing is liekly to be a)more dangerous and b)less reliable than anything you'll buy.

Try getting some igniters that use less current. I'm guessing you're using Estes igniters? Switch to electric matches. At least one website I found claims to have some which will ignite consistently at 1.5v with 0.5A:

http://www.firefox-fx.com/kits.htm
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;) its ok i seen a rocket site for model rockets on the nets agers ago and they showed the details of one of those launchers and the nichrome wire with its initiator was in the stand and the initiator was at the tip of the nichrome wire top of where it bends  so that when it glows at the thinnest piont in the bend it ignites the initiator and it initiates the ignition of the solid feul

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Yep, that'll be your basic Estes igniter. I haven't done model rocketry for a number of years, but I recall that Estes igniters needed a lot more power to ignite compared to electric matches.

That was probably intentional, as Estes stuff is used by people new to the hobby. If they're harder to ignite, less chance someone will launch a rocket up their nostrils (or something equally painful).

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