Dumb questions regarding Aerotech RMS igniter positioning

[overklock]

I always try to follow Aerotech's instructions regarding assembly of their motors. Some motors assemble slightly different from others. If the directions say to push the ignitor against the delay, I do it.
If the directions say to push the ignitor against a piece of tape at the top of the fuel grain, I do it.
Maybe I've been fortunate, but have never had an AT motor failure.

The problem is, what the directions say, and what the directions illustrate, are two different things.

 

[gldknght]

The problem is, what the directions say, and what the directions illustrate, are two different things.

Do you have the directions from a specific reload that give an example of this?
I don't want two different sets of instructions from different reloads, one instruction from any reload that shows this?

 

[overklock]

Do you have the directions from a specific reload that give an example of this?
I don't want two different sets of instructions from different reloads, one instruction from any reload that shows this?

Sure. This is a picture from my G76-7g. It shows the igniter butting up against the delay grain which is FLUSH with the propellant grain. Meanwhile the g76 itself has a delay spacer which prevents it from being flush with the delay grain. Pushing it up into the delay grain past the delay element is not what is visually portrayed in the instructions.

Pushing the igniter up past the propellant grain, past the spacer and into the delay grain can (and has for me) caused early ejection issues.

My guess here, as I mentioned earlier, is that these instructions were written to be for both motors with and without a delay spacer, and the gap between the two was not taken into consideration.

 

[dhbarr]

I was talking about reload kits where the fuel grain isn't as long as the casing, for instance the various E and F reload kits for the 29/40-120 casing. There is a pretty big open space between the front of the fuel grain and the delay charge.
View attachment 545973

You're right, of course. I was thinking of two full grains and a short delay. Makes sense why you'd tape the stubbies

 

[gldknght]

Sure. This is a picture from my G76-7g. It shows the igniter butting up against the delay grain which is FLUSH with the propellant grain. Meanwhile the g76 itself has a delay spacer which prevents it from being flush with the delay grain. Pushing it up into the delay grain past the delay element is not what is visually portrayed in the instructions.

Pushing the igniter up past the propellant grain, past the spacer and into the delay grain can (and has for me) caused early ejection issues.

My guess here, as I mentioned earlier, is that these instructions were written to be for both motors with and without a delay spacer, and the gap between the two was not taken into consideration.

I'm not certain what you are getting at here. The instruction step 11 says to push the ignitor up against the delay element and figure 7 shows the same thing, the ignitor against the delay element.

Are you saying that the instructions don't match the actual parts in the reload kit?

 

[overklock]

I'm not certain what you are getting at here. The instruction step 11 says to push the ignitor up against the delay element and figure 7 shows the same thing, the ignitor against the delay element.

Are you saying that the instructions don't match the actual parts in the reload kit?

That's correct, the instructions do not match what is actually happening during assembly.

We can see that in the illustration, the delay element is extended and flush with the propellant grain.

In the actual kit, there is about a half inch gap between the delay element and the propellant grain caused by the delay spacer. This means that if you were to push the igniter in until it "stop against the delay element", you'd actually be pushing the igniter past the propellant grain, through that half inch gap until it's touching the delay grain.

I had a CATO on my last launch where the ejection charge went off far too early, because the delay grain lit long before the propellant did.

Here's a picture of just how far up the igniter would need to go past the propellant grain in order for the igniter to "stop against the delay element".

 

[gldknght]

..., because the delay grain lit long before the propellant did.

How do you know this?

 

[overklock]

How do you know this?

This is an educated guess.

Regardless, it doesn't negate my original concern, no?

 

[gldknght]

This is an educated guess.

Regardless, it doesn't negate my original concern, no?

Maybe, maybe not. It seems to me there is not enough evidence to conclusively state that pushing the ignitor against the delay element can cause the ejection charge to fire early.

It certainly DOES NOT change my recommendation to follow the AT instructions regarding ignitor placement.

 

[overklock]

Maybe, maybe not. It seems to me there is not enough evidence to categorically state that pushing the ignitor against the delay element can cause the ejection charge to fire early.

It certainly DOES NOT change my recommendation to follow the AT instructions regarding ignitor placement.

Generally, I would agree with you, if it wasn't for the fact that Aerotech literally replied earlier in this thread stating the opposite of what their direction say.

The igniter should be positioned at or near the top/forward end of the propellant grain.

 

[gldknght]

Generally, I would agree with you, if it wasn't for the fact that Aerotech literally replied earlier in this thread stating the opposite of what their direction say.

Yes, at or NEAR the top of the propellant grain. That doesn't contradict anything I've seen in the AT motor assembly instructions.

In the photo you posted of your partially assembled motor, that ignitor IS at or NEAR the top of the grain.

 

[overklock]

Yes, at or NEAR the top of the propellant grain. That doesn't contradict anything I've seen in the AT motor assembly instructions.

In the photo you posted of your partially assembled motor, that ignitor IS at or NEAR the top of the grain.

I mean, that’s semantics at this point. Id argue that’s not near the top of the propellant grain, that’s past the propellant grain.

Truth is, until @AeroTech can give us an absolute answer, there are too many variables at play here. Many in this thread stating the exact opposite of what you are, and a vague official response leaves too much up in the air.

 

[n3tjm]

I have seen several times where the delay grain ignites but the propellant never ignites.

 

[gldknght]

I mean, that’s semantics at this point. Id argue that’s not near the top of the propellant grain, that’s past the propellant grain.

Truth is, until @AeroTech can give us an absolute answer, there are too many variables at play here. Many in this thread stating the exact opposite of what you are, and a vague official response leaves too much up in the air.

Let's try this from a different point of view. The AT 29 / 40 - 120 hobbyline case is a "one size fits all" reload case. I forget how many different reloads will fit in this case, but there are several. I have an E23-8 and a G64-7 that both go, but have very different propellant grain lengths. The top of the G64 grain is very close to the delay grain, whereas the E23 grain only fills about half of the case. So if we shove the ignitor for the E23 all the way up against the delay grain, it probably won't ignite the propellant. Agreed? So AT says put a piece of tape across the top of the propellant grain, push the ignitor against the tape and "go flight."

For the G64 reload, the top of the propellant grain is "close enough" to the delay grain, AT says to push the ignitor against the delay grain and it will do what it's supposed to.

Neither of these instructions contradict what the AT rep said on this thread. Actually I think what he said was not vague at all, but quite specific.

And It's NOT semantics. It's ok for the ignitor to be a little bit past the top of the propellant grain. What we never want, is to have the ignitor BELOW the top of the propellant grain.

 

[OzHybrid]

I always try to follow Aerotech's instructions regarding assembly of their motors. Some motors assemble slightly different from others. If the directions say to push the ignitor against the delay, I do it.
If the directions say to push the ignitor against a piece of tape at the top of the fuel grain, I do it.
Maybe I've been fortunate, but have never had an AT motor failure.

I follow the guidelines of the manufacturer but, the instructions supplied are unfortunately NOT based on reality for a lot of people. Lighting a 20 year old reload is my reality. 5 year old reloads would be a reality for most. The instructions will work for brand new stock as was supplied to TRA/NAR for testing. Let's not forget copperheads and the change to firstfire igniters.
The advantage of compressed BP slice is that it ignites readily from the firstfire and is unlikely to cause overpressurisation at ignition.

Whatever YOUR reality is, pick a method that works for you. Your reload type, stock age and igniter type/augmentation.
Norm

 

[overklock]

Let's try this from a different point of view. The AT 29 / 40 - 120 hobbyline case is a "one size fits all" reload case. I forget how many different reloads will fit in this case, but there are several. I have an E23-8 and a G64-7 that both go, but have very different propellant grain lengths. The top of the G64 grain is very close to the delay grain, whereas the E23 grain only fills about half of the case. So if we shove the ignitor for the E23 all the way up against the delay grain, it probably won't ignite the propellant. Agreed? So AT says put a piece of tape across the top of the propellant grain, push the ignitor against the tape and "go flight."

For the G64 reload, the top of the propellant grain is "close enough" to the delay grain, AT says to push the ignitor against the delay grain and it will do what it's supposed to.

Neither of these instructions contradict what the AT rep said on this thread. Actually I think what he said was not vague at all, but quite specific.

And It's NOT semantics. It's ok for the ignitor to be a little bit past the top of the propellant grain. What we never want, is to have the ignitor BELOW the top of the propellant grain.

The same could also be said in reverse. The E23 has absolutely no issue igniting the delay element, despite the fact that it is quite a distance away from the propellant grain. YES, it is at the TOP of the propellent grain. Just as in my scenario, the igniter would be at the TOP of the g76's propellant grain. In both scenarios, the internal pressure of the propellant grain burning ignites the delay element.

However, in a worst-case-scenario with the igniter pushed beyond the the propellant and into the delay grain, this could create an environment in which the delay grain begins burning before the propellant, resulting in early ejection or CATO.

The igniter being "as close to the top of the propellant grain as possible" means that the igniter is not inserted too far down, but it could also be interpreted as not being too far up either.

Let's be real, when inserting the igniter, it has about a 50/50 chance of getting stuck behind the forward insulator anyway, meaning it's not going to be going past that spacer and into the delay element.

 

[gldknght]

I follow the guidelines of the manufacturer but, the instructions supplied are unfortunately NOT based on reality for a lot of people. Lighting a 20 year old reload is my reality. 5 year old reloads would be a reality for most. The instructions will work for brand new stock as was supplied to TRA/NAR for testing. Let's not forget copperheads and the change to firstfire igniters.
The advantage of compressed BP slice is that it ignites readily from the firstfire and is unlikely to cause overpressurisation at ignition.

Whatever YOUR reality is, pick a method that works for you. Your reload type, stock age and igniter type/augmentation.
Norm

I don't disagree with any of this, for certain.

There are AT reloads that are notoriously hard to light, even when they are new, regardless of where the ignitor is put (the F40W comes to mind). I've always had to augment the ignitors for these.

 

[gldknght]

The same could also be said in reverse. The E23 has absolutely no issue igniting the delay element, despite the fact that it is quite a distance away from the propellant grain. YES, it is at the TOP of the propellent grain. Just as in my scenario, the igniter would be at the TOP of the g76's propellant grain. In both scenarios, the internal pressure of the propellant grain burning ignites the delay element.

However, in a worst-case-scenario with the igniter pushed beyond the the propellant and into the delay grain, this could create an environment in which the delay grain begins burning before the propellant, resulting in early ejection or CATO.

The igniter being "as close to the top of the propellant grain as possible" means that the igniter is not inserted too far down, but it could also be interpreted as not being too far up either.

Let's be real, when inserting the igniter, it has about a 50/50 chance of getting stuck behind the forward insulator anyway, meaning it's not going to be going past that spacer and into the delay element.

It seems to me that you really just want to argue.

Nothing you've said changes my opinion that following the AT instructions is always the best policy.

If you want to continue beating this horse, might I suggest you get a hold of AT and convince THEM to change their instructions?

 

[overklock]

It seems to me that you really just want to argue.

Nothing you've said changes my opinion that following the AT instructions is always the best policy.

If you want to continue beating this horse, might I suggest you get a hold of AT and convince THEM to change their instructions?

Honestly I'm not trying to argue, I'm trying to look at this through every point of view to avoid any issues in the future. I greatly appreciate your input, as well as everyone else's. There are many different opinions here, including those that directly contradict your own, and I'm simply trying to find the correct one. AeroTech's current instructions have not worked for me.

I've contacted Aerotech via email, although I have no heard back.

 

[smstachwick]

We might be overthinking this.

My guess is that the instruction to put the igniter in contact with the delay element is specific to the full-length reload and the full delay grain. For shorter propellant and delay grains, i.e. the ones that use a spacer and/or tape, the safest placement is at the top of the propellant grain, which is the same positioning relative to the grain as the full loads.

If you’re using a shorter-delay G load and both components light immediately as designed, there’s no difference. But if you only get the delay grain or if the propellant takes its time, you might be in trouble. You wouldn’t be able to light the delay early in an Estes end-burner, so don’t put yourself in a position for that to happen in a top-ignited C-grain motor.

I agree that AeroTech’s instructions are not clear and could use some work, but I think the answer reveals itself with an understanding of how these motors work (and how they sometimes fail).

 

[overklock]

We might be overthinking this.

My guess is that the instruction to put the igniter in contact with the delay element is specific to the full-length reload and the full delay grain. For shorter propellant and delay grains, i.e. the ones that use a spacer and/or tape, the safest placement is at the top of the propellant grain, which is the same positioning relative to the grain as the full loads.

If you’re using a shorter-delay G load and both components light immediately as designed, there’s no difference. But if you only get the delay grain or if the propellant takes its time, you might be in trouble. You wouldn’t be able to light the delay early in an Estes end-burner, so don’t put yourself in a position for that to happen in a top-ignited C-grain motor.

I agree that AeroTech’s instructions are not clear and could use some work, but I think the answer reveals itself with an understanding of how these motors work (and how they sometimes fail).

All of my thoughts summed up eloquently.

 

[gldknght]

My final thought on this: I've been flying AT reloads for almost 25 years, following the instructions as written, every time. I've never had an early ejection charge fire, even in a reload that was left loaded in a rocket in my living room where temperatures and humidity fluctuate greatly, for over two years.
You can disagree with the AT instructions all you want, in my EXTENSIVE experience, following those directions has never ever caused me a problem. That's my experience, not affected by anyone else's OPINION.

 

[overklock]

My final thought on this: I've been flying AT reloads for almost 25 years, following the instructions as written, every time. I've never had an early ejection charge fire, even in a reload that was left loaded in a rocket in my living room where temperatures and humidity fluctuate greatly, for over two years.
You can disagree with the AT instructions all you want, in my EXTENSIVE experience, following those directions has never ever caused me a problem. That's my experience, not affected by anyone else's OPINION.

Just wanna say I really appreciate your input. As I’m trying to collect all the data I can, do you mind answering just a few more questions that are relevant to your experience?

I know you said you always follow the directions to a tee. But when you’re building the G reloads, do you always make sure that the igniter extends past the forward insulator and past the delay spacer? If so, how are you sure it’s it’s made it that far? Or do you simply insert the igniter until you feel it stop?

 

[heada]

Just put the igniter at the top of the propellant grain. If the propellant ignites the delay grain will also ignite and nearly instantly.

You're way over thinking it. It really isn't rocket science.

 

[Steve Shannon]

My final thought on this: I've been flying AT reloads for almost 25 years, following the instructions as written, every time. I've never had an early ejection charge fire, even in a reload that was left loaded in a rocket in my living room where temperatures and humidity fluctuate greatly, for over two years.
You can disagree with the AT instructions all you want, in my EXTENSIVE experience, following those directions has never ever caused me a problem. That's my experience, not affected by anyone else's OPINION.

This!
This is how the motors are tested and certified, including the timing of the delay.

 

[overklock]

Just put the igniter at the top of the propellant grain. If the propellant ignites the delay grain will also ignite and nearly instantly.

You're way over thinking it. It really isn't rocket science.

Hey, told you it was a dumb question. Thanks again

 

[loopy]

This is not true regarding some reload kits for the 29/40-120.

And the reloads where that is true instruct the user to put masking tape over the top of the c-slot in order keep the igniter from extending past the top of the fuel grain. This was discussed earlier in the thread, so I assumed we were speaking with the understanding that comments were in regard to motors where the fuel grain is a full length fuel grain since the instructions clearly point out specifically where to position the igniter head in those other motors...

 

[loopy]

It seems as though the continued best option is to push the igniter up until it touches the delay grain, and then pull it back a quarter inch, yes?

Yup.

 

[Antares JS]

Maybe, maybe not. It seems to me there is not enough evidence to conclusively state that pushing the ignitor against the delay element can cause the ejection charge to fire early.

It's happened to me three times. Twice with Aerotech motors, and once with a Loki motor. (Though Loki actually does say to back the igniter off a little bit, so that one was definitely my own screwup.)

The igniter lit the delay, and the motor sat and smoked for several seconds before the propellant lit. All three of these incidents resulted in premature ejection.

Edit: I do want to emphasize again though, that I would never say you are definitely screwing up by pushing the igniter up against the delay. You are only increasing the risk that the delay will ignite without the propellant.

 

[overklock]

It's happened to me three times. Twice with Aerotech motors, and once with a Loki motor. (Though Loki actually does say to back the igniter off a little bit, so that one was definitely my own screwup.)

The igniter lit the delay, and the motor sat and smoked for several seconds before the propellant lit. All three of these incidents resulted in premature ejection.

Edit: I do want to emphasize again though, that I would never say you are definitely screwing up by pushing the igniter up against the delay. You are only increasing the risk that the delay will ignite without the propellant.

Officially got word back from Aerotech email support. Looks like they're on the same page (info redacted for privacy)