Securing the second stage igniter

SDramstad

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When I launch a single stage rocket with a larger 54 or 75 mm motor I use a wood dowel to hold the igniter in place at the top of the motor so it doesnt slide down and light the motor from the bottom. How do you do this for the second stage? With the G forces involved with the first stage boost how do you keep it in place?
 

JimJarvis50

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This is a key issue for two-stagers. The problem is not so much what happens on the boost, but rather, what happens when the igniter fires. If firing the igniter causes the igniter to be spit from the motor, it often won't light. Many times, I see people using the nozzle cap to hold in the igniter (with or without a stick). The cap doesn't prevent the stick from getting blown out though.

What I do is glue a couple of strings, like kite strings for example, on the top of the nozzle or the bottom of the bottom grain. Then, when I insert the igniter, I wrap the string around the bottom of the stick and super glue the string to the stick. That keeps the stick in just long enough to light the motor.

Jim

IMG_2463.JPG
 

cls

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Depending on the core diameter, I have good results by physically jamming the ematch head in between several pyrodex pellets at the top of the forward grain. I bought boxes of each caliber, usually some combination will fit.
 

JimJarvis50

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I use Pyrodex, a dowel, and the red cap. I need to think about Jim’s suggestion to use string. Few people have his experience with staging.
The problem with the red cap is that it gets blown off right away and doesn't hold in the stick. I use string, but masking tape or the like would be fine as long as it doesn't completely block the nozzle. You just need to hold in the igniter for a very short time.

I have had two instances where sustainer motors didn't light over the past few years. In both cases, I relied on the red cap. The attached video shows a comparison of identical flights, one with strings and the other with the red cap. The red cap motor lit, but it took 5 seconds to do so, which was a problem. The one with strings was instantaneous. I now have a third, nearly identical flight (same motor, same pyrogen) with strings where the motor lit after about 1 second.

I assisted with 5 Spaceport two-stage flights a couple of months ago. We had the sticks either stringed or taped. All of the ones that should have lit did. Briefly retaining the stick is the key.

It's the same on the ground. You use a stick that is taped to the pad in some manner (most people do that). If you don't, you can spit the igniter. Same issue.

Jim

 

waltr

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Very interesting video comparing the two flight.
Without sustainer ignition, chute deploys and is considered correct as it recovers on a chute. BUT, sustainer motor then comes up to pressure.
Big problem- Without chute it would be pointed not up and would go far away from launch site.
With chute out guessing there was lots of recovery system damage and not sure what else.

I've been using the red cap to hold the igniter in. I will now add some tape first to ensure igniter does stay in.
I've done 3 2-stage flights and on the second one the sustainer took about 3 seconds to light and I did not have a reason. Sustainer motor is a RMS24/20 D15T. Possibly the igniter got blown out. On that flight the sustainer had arced over and was pointed downward a bit. It then powered through a tree and chute ejected within the tree. Not good but at least it was pointed away from the flight line.

Thanks for posting video on another failure mode.
 

Jmhepworth

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The problem with the red cap is that it gets blown off right away and doesn't hold in the stick. I use string, but masking tape or the like would be fine as long as it doesn't completely block the nozzle. You just need to hold in the igniter for a very short time.

I have had two instances where sustainer motors didn't light over the past few years. In both cases, I relied on the red cap. The attached video shows a comparison of identical flights, one with strings and the other with the red cap. The red cap motor lit, but it took 5 seconds to do so, which was a problem. The one with strings was instantaneous. I now have a third, nearly identical flight (same motor, same pyrogen) with strings where the motor lit after about 1 second.

I assisted with 5 Spaceport two-stage flights a couple of months ago. We had the sticks either stringed or taped. All of the ones that should have lit did. Briefly retaining the stick is the key.

It's the same on the ground. You use a stick that is taped to the pad in some manner (most people do that). If you don't, you can spit the igniter. Same issue.

Jim


Jim, what glue do you use to secure the string to the motor?
 

JimJarvis50

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Jim, what glue do you use to secure the string to the motor?
If it's a CTI phenolic nozzle, I just superglue to the nozzle above the throat. If it's a graphite nozzle, I glue to the bottom of the grain with a little epoxy. It's possible I superglued to the top of a graphite nozzle (can't rule out having done that).

Jim
 

cls

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I'm confused about "use the red cap". I always cut a wedge out of it to vent the pressure so it doesn't pull the igniter out. Are folks not cutting any vents in the cap?

Also, depending on how high up the second stage is lighting... A burst disk might be appropriate. Keeps the core pressurized until it lights completely.
 

Jmhepworth

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I'm confused about "use the red cap". I always cut a wedge out of it to vent the pressure so it doesn't pull the igniter out. Are folks not cutting any vents in the cap?

Also, depending on how high up the second stage is lighting... A burst disk might be appropriate. Keeps the core pressurized until it lights completely.
I also cut a wedge to vent things until it comes up to pressure. I do that in both the booster and the sustainer. I’ve not had a problem getting sustainers to light (except when I had a break in the wire to the sustainer), so I have not experienced the red cap pulling out the starter. But Jim has so much more experience than I have with staging (by orders of magnitude) that it would be pretty stupid not to listen to him.
 

jFlds

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The problem with the red cap is that it gets blown off right away and doesn't hold in the stick. I use string, but masking tape or the like would be fine as long as it doesn't completely block the nozzle. You just need to hold in the igniter for a very short time.

I have had two instances where sustainer motors didn't light over the past few years. In both cases, I relied on the red cap. The attached video shows a comparison of identical flights, one with strings and the other with the red cap. The red cap motor lit, but it took 5 seconds to do so, which was a problem. The one with strings was instantaneous. I now have a third, nearly identical flight (same motor, same pyrogen) with strings where the motor lit after about 1 second.

I assisted with 5 Spaceport two-stage flights a couple of months ago. We had the sticks either stringed or taped. All of the ones that should have lit did. Briefly retaining the stick is the key.

It's the same on the ground. You use a stick that is taped to the pad in some manner (most people do that). If you don't, you can spit the igniter. Same issue.

Jim


great insight Jim. I'm taking all this into account when I start finishing my first stage build. Thank you
 

waltr

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I do cut a wedge in the red cap to vent as per AT's instructions.

On one 2-stage flight the Red cap embedded itself on the threaded end of the ISC eyebolt. Unknown if this was when the igniter fired or the motor lit.
 

JimJarvis50

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I do cut a wedge in the red cap to vent as per AT's instructions.

On one 2-stage flight the Red cap embedded itself on the threaded end of the ISC eyebolt. Unknown if this was when the igniter fired or the motor lit.
I was not aware of the AT instructions on this. If the result is that the cap can vent but not get blown off, and if that keeps the stick in, then it sounds fine.

Jim
 

waltr

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Yep, cutting a vent in the red cap is in the AT RMS reload instructions.
However, is the vent large enough to prevent the red cap from blowing off when the igniter fires? what about igniter and some propellant?
What about if the igniter is enhanced (I do add ProCast to AT igniters and DIY 2-stage ignites with 40ga nichrome and ProCast), is the cap vent still large enough?
Ideally, the red cap stays on until motor comes up to operating pressure. But a chuff will blow the cap off.
 

SDramstad

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This is a key issue for two-stagers. The problem is not so much what happens on the boost, but rather, what happens when the igniter fires. If firing the igniter causes the igniter to be spit from the motor, it often won't light. Many times, I see people using the nozzle cap to hold in the igniter (with or without a stick). The cap doesn't prevent the stick from getting blown out though.

What I do is glue a couple of strings, like kite strings for example, on the top of the nozzle or the bottom of the bottom grain. Then, when I insert the igniter, I wrap the string around the bottom of the stick and super glue the string to the stick. That keeps the stick in just long enough to light the motor.

Jim

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Thanks Jim! Great info.
 

SDramstad

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I havent done any 2 stage rockets using composite propellants yet. Lots of questions..... Any way, I will be at Airfest this year so will probably go bug some of the guys about staging and see what I can pick up there.
Steve
 

Jmhepworth

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I havent done any 2 stage rockets using composite propellants yet. Lots of questions..... Any way, I will be at Airfest this year so will probably go bug some of the guys about staging and see what I can pick up there.
Steve
Steve, find me at Airfest. All I’m bringing are two stage composites, a Wildman Junior, a Darkstar Junior, and a 3” Darkstar.
 

JimJarvis50

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I expect to be there, too. Oddly, with a two-stager (and hoping for no skywriting this year). Not sure which days, but probably just for a day or two. I should be somewhere near the Texas compound.

Jim
 
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The problem with the red cap is that it gets blown off right away and doesn't hold in the stick. I use string, but masking tape or the like would be fine as long as it doesn't completely block the nozzle. You just need to hold in the igniter for a very short time.

I have had two instances where sustainer motors didn't light over the past few years. In both cases, I relied on the red cap. The attached video shows a comparison of identical flights, one with strings and the other with the red cap. The red cap motor lit, but it took 5 seconds to do so, which was a problem. The one with strings was instantaneous. I now have a third, nearly identical flight (same motor, same pyrogen) with strings where the motor lit after about 1 second.

I assisted with 5 Spaceport two-stage flights a couple of months ago. We had the sticks either stringed or taped. All of the ones that should have lit did. Briefly retaining the stick is the key.

It's the same on the ground. You use a stick that is taped to the pad in some manner (most people do that). If you don't, you can spit the igniter. Same issue.

Jim


Will someone please explain for the still unexpirienced why the fins spunin the video? It looked super dope (!) but is there a practical purpose?
 

Titan II

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We have a great search function on the forum. It is described in great detail in this thread...........


and....

 
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JimJarvis50

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Will someone please explain for the still unexpirienced why the fins spunin the video? It looked super dope (!) but is there a practical purpose?
I fly a vertical orientation system with canards at the top of the rocket. The downwash from the canards can interact with the fins, and in some cases, the rocket can turn in the opposite of the intended direction. The spin can minimizes this effect.

Jim
 
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