What minimum electrical circuitry do I need for a composite igniter?

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Tinker

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Hello all,

I would like to wire up a composite motor to ignite as an air start any time after liftoff. I was going to use some sort of device (wooden pin?) to break a circuit that connects straight to a battery while on the pad and then as the other motors lift the rocket, the circuit is completed as the wood pin is removed and fires the composite igniter. Can I just wire a composite (Aerotech) igniter to a 400 or 900 LiPo battery and have it fire? Do I need a little circuitry to boost the amps from the LiPo? I know just enough electronics to be dangerous to myself and solder my fingers together......

Tinker
 

Tinker

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Mostly because why pay $45 when I am good with it being a simple circuit. This is really an attempt to cluster BP and a composite motor. I read that composites often don't light as the BP motors go so much faster that the composite igniter is ripped out before ignition. If I need the battery in any case but I can use a simple circuit to lite the composite after liftoff, why add the extra circuit board? *IF* on the other hand, I need to boost the amperage and put in voltage regulators, then you may be on to something. I was trying to determine the need first.

Thank you for the feedback though, and the link.

Tinker
 

MikeyDSlagle

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You shouldn't need extra circuitry to boost the amperage. Alot of the computers used for airstarts uses a safety feature called a breakwire. When the rocket lifts off the breakwire trips and either opens or closes a circuit telling the airstart controller the rocket has lifted off and starts a timer or lights the motor then, depending on the flight profile. The controllers have a lot more than that going on of course.

I see what you are trying to do and seems to me it would work, but you have to make darn sure you have enough speed before the composite lights. You wouldn't want to clear the rail while the composite is still pressuring up and the black powders don't have enough left to stabilize the rocket. That could be very bad.

I am not the one to be saying whether or not it is a good idea, I guess the folks you launch with will make that call. I'm guessing you are talking about smallish motors.

Mikey D
 

Tinker

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I'm guessing you are talking about smallish motors.

Mikey D
Yea, like two d-12's and maybe a f32 as far as engines, but I think the design must assume the composite dose not lite until a 100 feet or so. The problem is if only 1 of the BP engines lights. The only thing I can think of is some kinda of magnetic pull/shear bolt on the rocket to keep it on the pad if it only has the thrust of 1 BP or to change the design such that it has three boosters and only needs 2 for safe flight or something. But I get off topic. Thank you for the feedback on the battery simple circuit.

Tinker
 

Handeman

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Why not lite the composite on the pad and air start the BP motors? All the BP need is a little spark from an ematch. Those tend to be very low current and easy to light.

You could light the composite on the pad and use long wires that are fired when the rocket lifts to fire the BPs before they pull out.
 

MikeyDSlagle

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Have you ran a sim to make sure 2 D12s is enough to safely lift the F32? You are basically doing an airstart.

But you are really wanting them all to light at once correct? I don't know how stubborn the F32 is to light, or how long it takes to get to pressure. Maybe use two circuits at the pad. Light the composite first then quickly light the BPs. You would have to be quick, I don't think I could do it. Don't know. Surely someone on here has done this and will chime in.

Even with expensive flight controllers there is still a chance that one of the BP motors won't light. The controller has nothing to do with what happens before liftoff anyway. Cant your motors slightly to help the rocket keep going up in the event only one lights. I don't know how helpful that is. It is in the Tripoli safety guidelines (?) I think.
 

rharshberger

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Odds are if you are flying with a club they will want more than a simple circuit to fire the airstarts, a Perfectflite MT4, Missleworks PET2, Marsa 54, Eggfinder Quantum (airstart mode), MissleWorks RRC3, Raven, or someother electronics with safeties built in would be required.
 

Steve Shannon

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You don't want something that can be fooled into ignition by range personnel pulling it off the pad, or a wind gust blowing the pad over or anything like that. Mercury switches and roller switches are specifically prohibited by the Tripoli Safety Codes, from use in directly closing a simple circuit to ignite a motor for that very reason. That's a poorly worded sentence but I think you get the idea.


Steve Shannon
 

Tinker

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I think we are getting a little off track. While I think cluster starts are sketchy at best, I don't see this as any more inherently dangerous than a BP cluster in terms of power/weight calculation and number of motors ignited. The simple circuit can be made relatively safe by adding a second 'manual' safety that is armed when the rocket is ready on the pad with the first dowel inserted. Adding an air start timer just really complicates things, besides the money. Mounting, protection from ejection gases while mounted in the rear somewhere..........

I model (40k) with rare earth magnets, and I have been using them in rockets also. I was thinking about having about 40N of magnetic pull on the rocket while on the pad such that without both d-12's lighting, it won't break free of the pad, preventing an unstable flight (and by extension the composite ignition) .

Tinker
 

Tinker

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As this being said, I think I may have thought of a cool place to put that mini timer........ :madnote:

Tinker
 

new2hpr

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Why not lite the composite on the pad and air start the BP motors? All the BP need is a little spark from an ematch. Those tend to be very low current and easy to light.

You could light the composite on the pad and use long wires that are fired when the rocket lifts to fire the BPs before they pull out.
What he said!

Also, size your composite such that the rocket can fly safely with it alone. Once that's set, airstarting the additional motors (BP or AP) is relatively straightforward. A pull pin or break wire is the simplest, but then you're not getting much more interesting than just lighting the cluster all at the same time using "normal" methods. Going with a timer or altimeter with additional channels lets you play around with delays and real airstarts.
-Ken
 

Tinker

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Why not lite the composite on the pad and air start the BP motors? All the BP need is a little spark from an ematch. Those tend to be very low current and easy to light.

You could light the composite on the pad and use long wires that are fired when the rocket lifts to fire the BPs before they pull out.
The long wire idea is really cool, and I may give it some real thought. Airstarting the BP boosters would be problematic in the envisioned rocket design with a single timer. Maybe I could rig two simple BP air starters but then I am doubling the weight.....

Would there be any additional concerns about having long leads for the cluster (BP) setup? I figure you could be ranging from 8"-12" to up to 3 feet on those BP leads. Does impedance start to be an issue in that danger close timing? Wouldn't I still have stability issues if the composite and one BP motor lit?

Tinker
 
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