Igniting 2nd stage Aerotech motors?

[Sparrowhawk]

Hello! I'm currently developing a small flight computer that captures telemetry on flights, and I want to build in the ability to fly 2-stage High-Power rockets. However, as far as I can tell, Aerotech motor igniters need ~12v 3A, or 36W, to fire. At present I'm struggling to find a MOSFET that can dissipate so much power in an SMD package. Is there a lower-wattage solution to igniting Aerotech motors mid-air?

The board is powered by a 3S lipo with a voltage stepdown to 3.3v for the IC's

 

[prfesser]

The AT motor starter isn't often used for upper stages. I think most people use a low-current starter, such as an e-match, boosted with a strip of an easily-ignited composite propellant (almost any blue propellant).

 

[Sparrowhawk]

How would I get my hands on said composite propellant? Are there any resources on how to "boost" an e-match with propellant?

 

[jsdemar]

You can light a second stage composite motor with an e-match dipped in ProCast pyrogen from Quickburst.

The MFG FireWire initiators/e-match are shippable by mail.

As to your original problem, there are many MOSFETs that will handle 10+ amps for a momentary ignition. Just do some more searching on Digikey, etc.

 

[Sparrowhawk]

As to your original problem, there are many MOSFETs that will handle 10+ amps for a momentary ignition. Just do some more searching on Digikey, etc.

I've seen several with pulsed current over 10 amps that fit my needs, but those are for micro or millisecond pulses. How long would an ignition pulse need to be?

 

[cerving]

Most ematches will fire with a 300 ms pulse, so 1 second should do it. And you definitely DO NOT want to use the Aerotech First-Fire igniters... I've tried them, and they don't work very well, even with an altimeter rated to 10A and with a 3-second pulse with a 2S Lipo. Use a dipped ematch instead.

 

[Sparrowhawk]

Great, thank you so much!

 

[roytyson]

Use a piece of pyrodex in the top grain and an ematch. Works great.

 

[Steve Shannon]

I took the black powder pellet out of a CTI Pro38 once and wedged it into the top of the core of a 29mm AT motor for staging. Then I used the ematch that came with the CTI motor to light that motor and I used the AT igniter to ignite the Pro38 which was in the booster. This was before CTI made the Pro29 line.
It worked very well.

A pyrodex pellet in the top grain, like Roy suggested above, is what I would do now.

 

[SDramstad]

What sized pellets do you use? I found 50 and 45 caliber pellets in a quick search but that seems big for say a 38mm motor. Can you safely cut them up?

 

[Jmhepworth]

Use a piece of pyrodex in the top grain and an ematch. Works great.

That’s what I do. Works every time. I also use a 1/16” dowel to make sure the ematch stays put against the pyrodex pellet.

 

[Banzai88]

How would I get my hands on said composite propellant?

Most of us who go this route simply purchase a cheap reload and it becomes the donor. One or two simple 29mm reloads is probably a lifetime supply.

 

[Jmhepworth]

What sized pellets do you use? I found 50 and 45 caliber pellets in a quick search but that seems big for say a 38mm motor. Can you safely cut them up?

I have some of each. The 50 fits very snuggly in the top grain of many 38mm reloads. The 45 is usually rather loose, but when the 50 doesn’t fit, I just attach a 45 to the top grain with a couple of drops of CA. So if you only get one, I would get the 45. And you can break them up. It takes very little effort to break them.

Pyrodex works really well as an initiator. Bob Brown showed me how he uses three of the pellets with an ematch down the center as his standard initiator for big (M and N) motors. It gives off a nice puff of white smoke when the ematch goes off, so you know it lit. Then a second or so later the motor is up to pressure and away it goes.

 

[Banzai88]

http://www.pro38.com/news.php
Look up the Pro38 1G pellet modification instructions from April 2019 to get an overview of how simple it is to split a pyrodex pellet.

 

[waltr]

I make igniters to first replace bad AT ignitiers and for Air-starts.

These are twin-lead 'shooter's wire (or ematch wire) with nichrome elements. For launch I use 34ga and 40ga for air-starting. Then dipped in Quick burst ProCast. These always fire from an Eggtimer Quantum or Proton.

http://quickburst.net/qb/making-igniters/http://quickburst.net/qb/procast-order-page/
Article of making these:
https://www.jacobsrocketry.com/aer/homemade_wire-wound_igniters.htmI use the 'crimp' method and nichrome wire. then ProCast.
Also, do 3-4 turns of the 40ga.

Another way is the MJG low current igniters dipped in ProCast.

Do ensure that the Flight computer does all the Deployments even if the sustainer motor does not fire.
Lots of good threads about this in the "Cluster and Airstart" forum topic.

 

[Sparrowhawk]

So it sounds like Pyrodex is a great option, but I'm not sure if I can store that in a college dorm room
Are dipped e-matches equally as effective as a Pyrodex pellet?

 

[gtg738w]

I think you just have your terms crossed. The power dissipated by the mosfet is the heat wasted when it is on or switching, not the load power. It's the maximum heat load that can be conducted out of the package to the pcb and ambient before the junction over temps. If you're running at low frequency like just an on/off switch you just need to worry about the current and Rdson. If you pull 3A and the on resistance is 10 mOhm, I^2*R is only 0.09W dissipated. You should have no issue firing an AT igniter with a 3s lipo. The problem of actually starting the motor in an airstart arrangement is a little harder as others above have noted.

 

[Sparrowhawk]

I think you just have your terms crossed. The power dissipated by the mosfet is the heat wasted when it is on or switching, not the load power. It's the maximum heat load that can be conducted out of the package to the pcb and ambient before the junction over temps. If you're running at low frequency like just an on/off switch you just need to worry about the current and Rdson. If you pull 3A and the on resistance is 10 mOhm, I^2*R is only 0.09W dissipated. You should have no issue firing an AT igniter with a 3s lipo. The problem of actually starting the motor in an airstart arrangement is a little harder as others above have noted.

Ah, I see. I was using P=VI rather than P=I^2R, I never know which one to use.

 

[seth_cooper]

So it sounds like Pyrodex is a great option, but I'm not sure if I can store that in a college dorm room
Are dipped e-matches equally as effective as a Pyrodex pellet?

Do not keep black powder, Pyrodex or e-matches in your dorm room.

 

[Sparrowhawk]

Do not keep black powder, Pyrodex or e-matches in your dorm room.

Duly noted o7

For MJG initiators, I would still need to dip them in ProCast the same as if they were just chipboards without pyrogen right?

 

[LithosphereRocketry]

Ah, I see. I was using P=VI rather than P=I^2R, I never know which one to use.

P = VI is always correct, but you have to use the voltage across the component that's dissipating the power. In this case, with a reasonably low RDS(on), very little voltage will be across the MOSFET, so the dissipation will be pretty low. P = I^2*R is just derived from V = IR and P = VI for things that are more or less resistors.

 

[Jmhepworth]

So it sounds like Pyrodex is a great option, but I'm not sure if I can store that in a college dorm room
Are dipped e-matches equally as effective as a Pyrodex pellet?

You will likely violate your lease if you have any of that stuff in your dorm room. Probably also with a rocket motor. You need to figure out how to get what you need at the field.

 

[jimzcatz]

Use a piece of pyrodex in the top grain and an ematch. Works great.

EXACTLY, why would anyone use a high current igniter to stage? Ematches and pellets are the way to go.

 

[jimzcatz]

You will likely violate your lease if you have any of that stuff in your dorm room. Probably also with a rocket motor. You need to figure out how to get what you need at the field.

My opinion its safer with the added OOMPH of a pellet instead of relying on a dipped match.

 

[Sparrowhawk]

So how much of a Pyrodex 45 pellet would I want to put on the end of an MJG initiator? I'm sure its not the whole thing, right?

 

[wsume99]

So how much of a Pyrodex 45 pellet would I want to put on the end of an MJG initiator? I'm sure its not the whole thing, right?

You should probably specify what kinds of motors you plan on using. Lighting a 29mm probably has different amounts compared to a 54mm.

 

[Sparrowhawk]

You should probably specify what kinds of motors you plan on using. Lighting a 29mm probably has different amounts compared to a 54mm.

True. For now, any 2-stagers I attempt will be either 29 or 38, most likely 29 for the sustainer. Would 45 even fit in there?

 

[Jmhepworth]

So how much of a Pyrodex 45 pellet would I want to put on the end of an MJG initiator? I'm sure its not the whole thing, right?

I’ve not tried staging to a 29, but I use the whole pellet in the top grain of a 38.

 

[Sparrowhawk]

I’ve not tried staging to a 29, but I use the whole pellet in the top grain of a 38.

Does embedding the pellet in the top grain count as modifying a motor, thereby making it an experimental motor?

 

[wsume99]

Does embedding the pellet in the top grain count as modifying a motor, thereby making it an experimental motor?

Yes, I was just going to caution you about that. Unless the motor was shipped like that such as some CTI motors are.