Cluster Ignition

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costnerk

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So I'm planning to do my first cluster rocket, and I want to run my plans by you people with more experience to make sure I'm thinking of everything.

I'm planning a cluster of 3 G motors. I'm planning to just use the First Fire Jr. igniters that come with the motors. I'm currently gathering equipment to build my own relay box which will hold a 12V SLA battery which should provide plenty of current to the igniters, cables and switches permitting.

I see the biggest problem coming from the igniters. I've come across several posts saying First Fire Jr's aren't the best for clusters, and that I'd be better off hand dipping my own igniters but I don't really want to do that.

Thoughts? I'd really appreciate your feedback on this.
 

dhbarr

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I've done 4x composite D's on FFJR's. All four igniters lit, but only 3/4 motors lit. If I do that flight again I'll likely:
- dope the igniter
- dope the topgrain
- use the recommended clustering schema from NARHAMS pub
- use an at-pad battery relay
 

tomsteve

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theres a few vendors that sell ignitors id suggest lookin into. Wildman sells small "lighters" that I found to be pretty good.

what propellant type are ya planning on?
 

jimzcatz

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You need a high current launch system for the First Fires. What motor exactly? Reload?
 

Dave A

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When I do clusters I like to have an extra 4-6' of wire such that the rocket can head up the rail and maybe all will light.
Single stranded leftover igniter wires are typically used. I would use some larger stranded speaker wire. All I know is it works.
 

Trident

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If you want to get into composite clustering, I'd suggest you rethink your position on making your own igniters. It's really pretty simple. Check out https://www.firefox-fx.com/ignition.htm

They sell dip kits that require no soldering, using a conductive pyrogen. Use a wire pair from inside tegular CAT5 LAN cable, strip the insulation, keep the two wires separated by a small gap, and dip. Easy, and cheap. For hotter igniters, you can add one of their secondary pyrogen dips.
 

costnerk

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Thank you all for your input! The motors I'm planning to use are single use Aerotech G80T-14A's. The relay box I'm planning to build should be able to deliver up to 20A before exceeding the ratings for most of my components. One thread I read said to plan for 3A per igniter, and anther said 5A. I went ahead and planned for 5A. I bought 10' of 10AWG wire, or maybe it was 12AWG... I don't remember, to run between the box and the rocket so the igniters have a little extra time to ignite.

That being said, am I still better off making my own igniters? I've looked into the quickburst igniter kit, I'm just not sure what that will gain me besides cost savings.
 

dhbarr

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A middle ground is just to upgrade pre-made igniters with a hotter-burning outer layer.
 

Dave A

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When I was staging to E and F motors I found faster burning motors lit faster and more consistantly. I had several F25s never light with a good igniter.
 

tomsteve

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Thank you all for your input! The motors I'm planning to use are single use Aerotech G80T-14A's. The relay box I'm planning to build should be able to deliver up to 20A before exceeding the ratings for most of my components. One thread I read said to plan for 3A per igniter, and anther said 5A. I went ahead and planned for 5A. I bought 10' of 10AWG wire, or maybe it was 12AWG... I don't remember, to run between the box and the rocket so the igniters have a little extra time to ignite.

That being said, am I still better off making my own igniters? I've looked into the quickburst igniter kit, I'm just not sure what that will gain me besides cost savings.

reads like ya have a good plan. them G80's, being the blue thunder propellent, should light easily. are ya better off making your own ignotors? welp, iffen youre going to be launching a lot and also doing clusters often, id say yes. if youre only going to do occasional clusters, welp, I don't know. I find the ignitors with AT motors are pretty reliable, but the green and red propellents- sometimes they don't light on first try and I need to put another ignitor in, so having extra ignitors is nice, whether self dipped or bought premade
 

new2hpr

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I've flown my Ultimate on a 7x G80 cluster with the FF igniters (10+ yr. old G80s, to boot) and all lit. You just need good connections and AMPS. A relay system with SLA battery should be fine. Long leads and hold that button until it's long gone.
-Ken
 

DavidMcCann

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G80's light if you sneeze hard at them. good call :)



other propellants.... I haven't had as much luck >< Number two lit here around 50 feet in the air.

 

costnerk

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Cool photos DavidMcCann! So it sounds like I have a pretty good chance of success with the G80 motors. So here's another question. Have any of you experienced a voltage sag when lighting your igniters? I was testing out my relay box the other day connected to a resistor bank load. The battery was fully charged and read about 13V before the test. During the test it sagged down to about 11V while I was drawing around 11A. Is this something I should address with a large capacitor, or should I be okay?
 

Handeman

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13 to 11 volts isn't much of a sag. If you had dropped to 6V or less, I would be concerned. How much resistance was in the load? With that value and the voltage you can calculate the current when it was at 11V. I'm sure it was still significant.
 

dhbarr

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That sounds like a picture perfect amount of juice, and with blue propellant it'll catch pretty much instantaneously. Pshoom, gone!

Be advised that 3x G80's puts you well outside Model Rocket territory; that's an H200+ish equivalent.
 
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Dave A

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Yes, I always try to use faster burning propellant when doing staging or clusters of primary engines. They light quickly and more consistently.
When I do clusters, i like to use some longer wire, lay it loosely on the ground, even drape over part of the rail. This allows the last motors to light a chance before they leave the rail.
 
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costnerk

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I used 3 3ohm power resisters in parallel and measured the current along one of the branches. The equivalent resistance of the circuit was measured to be 1.2ohms. The current I measured was a little more than 3.6A. Multiplying that by 3 gets me just shy of 11A. Though ohms law says I should be getting around 13A so there was probably more current down the other two branches of my resistor bank. Now I did test it with an electronic load tester, but I think it was broken. To draw 7A it would drop my voltage down to 2.5V. It's sad to say that i trust my crude resistor bank over a $5000 piece of equipment.
 
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