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10 motor cluster question

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stevem

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I built my Delta III to be a 10 motor cluster. The largerst cluster I have flown so far is a 4 engine. I am currently using a 12v Gel Cell battery like the ones the R/C flyers use to run thier glo-engine starter motor. So far the battery has reliably lit the 4 motor clusters every time.

Now thinking about this 10 motor cluster - do you think this battery will have enough juice to light the 10? Or do I need to come up with an automotive battery?
 

Stymye

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assuming estes motors with estes igniters?
 

stevem

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yes - Estes motor and ignitors (haven't ventured into clustering AP motors yet)
 

rocketsonly

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I know D.M.B. Rocketry makes some large cluster cables. I positive they make a 9 motor cable, not sure about a 10 though. If they make a cable for that amount of motors, I'm assuming it would be possible.
 

stevem

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Thanks for the replies guys - I'll probably make up my own clip whip for this -
main question is ;

Will a hobby gel cell battery handle the current or do I need to get a larger automotive style battery to light 10 BP motors reliably?
 

edwardw

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What amp rating is the gel cell rated at? Is it sealed lead acid? I know I have 3 12V SLA cells that put out plenty of umph - I can cold weld 1/8 steel with them in series. Mine have a 4 Amp Hour rating on them. In cyclic use (on off, on off) they have a max 1 Amp pull capacity, but if you don't use them in a cycle they don't have a limit to the amount of current you can pull (well, it says that but it's really limited by the resistance in the circuit and internal resistance).

Edward
 

stevem

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the battery I have is a 12v 7amp sealed lead-calcium - I should have posted that right away - sorry!
 

bobkrech

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Stevem

The answer is yes provided the resistance of your launcher is low enough.

Estes ignitors have a resistance of 0.8 ohms and require 2 amps to fire, so using Ohm's law we can calculate minimum all fire voltage of only V=ri=0.8x2=1.6 Volts! Voltage is never the problem, it the current draw.

Using Ohm's law, we can figure out the current that can be supplied by the launcher into an ignitor. It will depend on the internal resistance of the batteries, the resistance of the wires, and the switches, and the ignitors.

Below is a typical 7 AH Gell Cell data sheet that you can purchase for $18-$25 at a good electronics supply store or from a number of on-lines sources.

http://www.panasonic.com/industrial..._LC-R127R2P.pdf

The internal resistance of the battery is listed as 0.040 ohms, so it can theoretically deliver I=V/R=12/0.040=300 amps on a dead short (for a short time anyway).

With the potential of delivering 300 amps, (3.6 KW) it's not a toy, so you won't find one in your local department store. So to fire 10 Estes ignitors you need at least 20 amps. Note that the gell cell actually has a curve at 23 amp current drain, and it will actually deliver that current for 8 minutes, however to get this current out, you need a low total load resistance.

The total circuit resistance that will draw 20 amps is R=V/I=12/20=0.8 ohms. The total resistance is the resistance of the battery, the resistance of the ignitors, and the resistance of the launcher.

The Resistance of 10 Estes ignitors in parallel is 0.08 ohms.

To calulate the maximum launcher resistance you can tolerate and still deliver 20 amps you need to solve the equation 20=i=V/r=12/(0.04+0.08+R(launcher))

0.040+0.080+R=12/20 and R = 0.6-0.12=0.48 ohms

It would better to have a launcher resistance that is half of this value or 0.25 ohms.

Read http://www.esteseducator.com/Pdf_files/1924_launch.pdf and you'll learn how to answer these questions.

You might want to go to the Estes Educator site, http://www.esteseducator.com/cfusion/publications.cfm to get some further info.

Bob Krech
 

vjp

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Originally posted by stevem
Thanks for the replies guys - I'll probably make up my own clip whip for this -
Steve- look into adding wire-wrap extensions to the igniter leads. I do this with 100% reliability on up to 5-motor clusters. They take a little time to pre-assemble (the night before the launch, in front of the TV) but make it a cinch to hook up on the launch pad, so you save time there. Also, you can insert the igniters and igniter plug one at a time, and check the resistance individually before twisting all the leads together.
 

stevem

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excellent info guys - thank you!

Bob, I appreciate your taking time to type that all out - great stuff and just what i needed!

vjp - can you elaborate on that more? - do you use a wire-wrap tool like I have seen at Radio Shack?
 

daveyfire

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thank u, next
Originally posted by stevem
do you use a wire-wrap tool like I have seen at Radio Shack?
That's how I did it when I wired up my 7-motor 13mm cluster I flew last year. Lotsa fun, I hadn't built a "model"-sized rocket in a long while, and this was one heck of a challenge. 7x13mm motors in a BT-60 to prove it could be done. I had a 1/4A3-3T in the center, two 1/2A3-4Ts in the outboards, surrounded by four A10-3Ts. What a rush!

To hook up the igniters, I stripped about 1/2" off the end of the wire-wrap wire. Then I wire-wrapped the wire around the end of the igniter lead, as if it were a post on a PCB. Took me about 30 seconds per igniter, I did it while watching The Simpsons. I then installed them all and checked for continuity. I then twisted all the leads from one side of the igniters into a bundle, and repeated it for the other. When I got to the pad, it was a matter of hooking up the two clips to the bundles, making sure there were no shorts in the igniters, and pushing the button. All 7 motors lit!

Here's a pic of the bottom of the model prepped on the pad.
 

vjp

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Steve-

What Dave said! Except I wrap the igniter leads right up against the tape separator, then bend the igniter leads over, up and over the wrapped area (one on each side of the tape). I think this helps lock the wrap in place and reduces the chance of a short when all those leads are in close proximity to each other.

I always prep each igniter with one blue and one red wrap extension, so there's no confusion when twisting the ends of the extensions together.

Also, I scrape the BP at the bottom of each nozzle, to make sure there's no clay where BP should be (I keep a small drill bit with a plastic handle from an inkjet reload kit, which works well for this).

Also, sometimes the igniter leads get too close to each other near the filament/pyrogen head. If they look like they might touch each other when inserted into the nozzle, grab the pair just behind the filament with a pair of thin needle-nosed pliers (to take up the strain against the filament/pyrogen), and tug them apart a little bit.

And finally, ALWAYS use the right igniter plug! I jammed an oversized plug into a C11 a few months ago, and as a result the nozzle overpressured and blew the whole nozzle out! Or, you can stuff wadding into the nozzle between the leads instead of using the plugs, and tape the leads over the end of the motor with masking tape. I've used this method as well as regular igniter plugs and both have worked well.

Oh and BTW, Wal-Mart sells a 12V lawn & tractor battery for about $20, which works great! Puts out lots of amps, and can be kept charged with a regular car battery charger. Or go "Tim Allen" and use a $45 deep-cycle marine battery. Urh Urh URH!

Please take lots of pics and video if you can and post back with your results!
 

stevem

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great info guys -

maybe this is obvious but I often need to have the obvious pointed out to me (at least that's what my wife says)

to do the wire wrap I'll need a wire wrap tool - right?

what gauge wire wrap wire do you use?
 

vjp

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Originally posted by stevem
to do the wire wrap I'll need a wire wrap tool - right?

what gauge wire wrap wire do you use?
Sorry - Yep! The tool is about $7 at Radio Shack.

The wire wrap I use is 30 gauge, and is a little more than $3 a roll.
 

stevem

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thanks vjp - time to hit the local Radio Shack!
 

Rocketboy25

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Why don´t you use a flashpan? Or masking tape cover with BP?Before you use thismethod press a little amount of BP into the nozzels to ensure ignition! Thats the simple way!And you will get alot of extra smoke!

Jochen
 

ronhill

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Regarding the use of extension wires, here's the simplest way I've seen: use nickel sleeves you find in the fishing supply section to crimp the leads to the igniter. Takes just a couple of minutes per igniter.

Ron
 

stevem

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Rocketboy25 - thanks for the reply - I am aware of this method of lighting clusters but from what I have read it is likely to scorch the botton of the rocket. I may try that another time, another rocket.

ronhill - thank, that sounds like another idea I may try.

I appreciate your help guys!
 

ronhill

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Daveyfire's idea of checking continuity is exactly right. Just as an obsessive step, I check the resistance of each igniter before attaching the extension, after putting on the extensions, and then after placing the igniter in the engine with the plug in place. For that last step, be sure to check the resistance of the individual igniter and not the bundled set.

The resistance of the Estes ignitors almost always is 7 or 8 ohms if you haven't cracked them or shorted them.

This resistance checking ensures you haven't messed up an igniter in the building/loading process.

Ron
 

ronhill

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A little correction on my resistance measurements for Estes igniters. That should have been 0.7 to 0.8 ohms.

Just an order or magnitude off.
 
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