Dumb questions regarding Aerotech RMS igniter positioning

[overklock]

Hey folks, I just have a couple questions I was hoping I could get help with.

First of which is regarding igniter positioning within an RMS. In the instructions for the G76 motor, it specifies that the igniter be pushed all the way up and into the delay element. SO, what would happen if the igniter instead got stuck and stopped behind the behind the forward insulator? See example below:


How it "should be", igniter coming up and going into delay grain:



Mistake that could easily happen; igniter stuck behind forward insulator:




My assumption here is that if the second option happened, it may prevent the delay element from burning?




Second stupid question:

I'm looking for a complete guide as to how an RMS motor works. For example, the purpose of the forward insulator, what the rubber gaskets do, etc.

Thank you for any help!

 

[Steve Shannon]

I agree with your assumption. Either the igniter might not ignite the delay grain or the timing of the delay would be affected, probably considerably.

I’ll take a stab at explaining functionality, but the best explanation would probably be contained in the patent text for the RMS system (I assume.)
The forward insulator and the delay grain insulator protect the metal surfaces of the forward closure from hot combustion gases.
The rubber o-rings keep the hot gases inside the case liner. The liner is the actual pressure vessel and insulates the aluminum case from the heat of the combustion. The aluminum casing supports the liner so the liner doesn’t rupture.
Occasionally you’ll see a motor where the liner fails and the metal case is bubbled or even burned through.

 

[overklock]

I agree with your assumption. Either the igniter might not ignite the delay grain or the timing of the delay would be affected, probably considerably.

I’ll take a stab at explaining functionality, but the best explanation would probably be contained in the patent text for the RMS system (I assume.)
The forward insulator and the delay grain insulator protect the metal surfaces of the forward closure from hot combustion gases.
The rubber o-rings keep the hot gases inside the case liner. The liner is the actual pressure vessel and insulates the aluminum case from the heat of the combustion. The aluminum casing supports the liner so the liner doesn’t rupture.
Occasionally you’ll see a motor where the liner fails and the metal case is bubbled or even burned through.

Thank you for the in-depth explanation.

It seems like such an easy mistake to make, especially since once the igniter goes in, there's no real way to tell if it's touching the delay element or not. Even more interesting since a reload E motor doesn't touch the delay element at all, it actually stops where it's taped up.

I just wanted to make sure I'm not making a mistake by actually following the directions when ensuring that the igniter is touching the delay element.

Again, thanks for the explanation, makes total sense about the O rings now.

 

[Steve Shannon]

Thank you for the in-depth explanation.

It seems like such an easy mistake to make, especially since once the igniter goes in, there's no real way to tell if it's touching the delay element or not. Even more interesting since a reload E motor doesn't touch the delay element at all, it actually stops where it's taped up.

I just wanted to make sure I'm not making a mistake by actually following the directions when ensuring that the igniter is touching the delay element.

Again, thanks for the explanation, makes total sense about the O rings now.

I’ll be honest that I don’t know how the delay grain burns accurately in the E motor when the instructions call for tape over the grain to prevent the igniter from being inserted too far. Also Gary has told me that the best position for some igniters is to insert it all the way in and then pull it back a short distance. So, based on that your and my assumption is probably wrong.

 

[heada]

Speculation: once the propellant igniges and the motor comes up to pressure, the delay grain will start. If you put the igniter against the delay grain and the gap prevents the propellant from igniting then bad things will happen.

If you design the delay grain timing such that it onky starts burning after the motor comes to pressure, then it can work. It all depends on how the motor is designed.

 

[overklock]

I’ll be honest that I don’t know how the delay grain burns accurately in the E motor when the instructions call for tape over the grain to prevent the igniter from being inserted too far. Also Gary has told me that the best position for some igniters is to insert it all the way in and then pull it back a short distance. So, based on that your and my assumption is probably wrong.

I'll chalk it up to "magic" on the E motor. Honestly it doesn't make a whole lot of sense why the G has to be against the delay element, while the E motor is miles away. Perhaps it's just factored into the actual delay time.

After some playing around with the motor, I do notice that the first fire Jr's that come with the G have quite a long patch of pyrotechnics on it, meaning that when it burns, it's more than likely igniting both the grains and the delay element simultaneously (vs the delay element lighting first and then igniting the grains). That is assuming it's been inserted correctly and is actually touching the delay element at time of ignition.

Speculation: once the propellant igniges and the motor comes up to pressure, the delay grain will start. If you put the igniter against the delay grain and the gap prevents the propellant from igniting then bad things will happen.

If you design the delay grain timing such that it onky starts burning after the motor comes to pressure, then it can work. It all depends on how the motor is designed.

I'm a bit confused on your wording here. The directions say specifically to put the igniter against the delay grain, no? I do agree that the propellant igniting late or not at all would result in bad things happening. This happened to me recently.

 

[heada]

On the 29/40-120 E reloads, they have you put tape to block the igniter from going all the way forward to the delay grain. That is what I was referring to.

 

[Steve Shannon]

If you put the igniter against the delay grain and the gap prevents the propellant from igniting then bad things will happen.

I don’t see how that could happen. A delay grain is just a smaller propellant grain and it gives off lots of fire. The only place that can go is through the c-slot on its way to the nozzle. I don’t get it.

 

[overklock]

On the 29/40-120 E reloads, they have you put tape to block the igniter from going all the way forward to the delay grain. That is what I was referring to.

Got it! Wasn’t aware you were referring to the E motors. Thanks!

 

[heada]

I don’t see how that could happen. A delay grain is just a smaller propellant grain and it gives off lots of fire. The only place that can go is through the c-slot on its way to the nozzle. I don’t get it.

Ideally both delay and propellant ignite ar the same time. If the propellant ignites first, it'll produce enough flame and gas to bring the motor up to pressure and ignite the delay. If the delsy ignites first, it'll produce flame and gas but not as much as the propellant so it can burn for a period of time before the propellant ignites. That cuts into the delay time and is "a bad thing"

 

[overklock]

Ideally both delay and propellant ignite ar the same time. If the propellant ignites first, it'll produce enough flame and gas to bring the motor up to pressure and ignite the delay. If the delsy ignites first, it'll produce flame and gas but not as much as the propellant so it can burn for a period of time before the propellant ignites. That cuts into the delay time and is "a bad thing"

This could possibly be why as Steve mentioned, some people say to pull the igniter back slightly. Puts more of the pyrotechnic against the grain.

 

[smstachwick]

once the igniter goes in, there's no real way to tell if it's touching the delay element or not.

You can measure and mark the igniter prior to assembly to indicate when it’s in the required spot. It’s a bit like the “marked stick” trick to get rings of glue in a tube.

 

[overklock]

You can measure and mark the igniter prior to assembly to indicate when it’s in the required spot. It’s a bit like the “marked stick” trick to get rings of glue in a tube.

This makes sense.

I know this goes without saying, but the igniter does indeed need to be touching the delay element, yes? This is how everyone else does it?

 

[smstachwick]

This makes sense.

I know this goes without saying, but the igniter does indeed need to be touching the delay element, yes? This is how everyone else does it?

Haven’t flown a G in that casing, sorry. The E loads I’ve flown use shorter grains, they tell you to put a bit of masking tape on the top end of the grain so that you can feel the motor bump up against it and know that you positioned the igniter correctly. Then you close up the motor, the nozzle cap holds the igniter in place.

I’d assume it’s not super critical that the igniter touch the G delay element for this reason, but if at first you don’t succeed, try following the instructions.

 

[Antares JS]

This makes sense.

I know this goes without saying, but the igniter does indeed need to be touching the delay element, yes? This is how everyone else does it?

No igniter EVER needs to touch the delay grain. Aerotech most likely used this language to ensure people fully inserted the igniter.

@heada has the right of it. If the propellant ignites properly, the delay grain will certainly ignite at almost the same time. If the delay grain ignites, it may or may not ignite the propellant, and your delay grain burning by itself before burnout is never desirable.

Loki motor reload instructions actually say it and it made so much sense I now do it with every composite motor: Fully insert the igniter, then pull it back 1/4-1/2" so that the igniter is up against the PROPELLANT, not the delay.

If that "error" occurs, where the igniter is stuck against the forward insulator and not the delay, it is not in the slightest an issue. It may actually be desirable. To reiterate: Your igniter should be igniting the PROPELLANT, NOT the delay.

 

[overklock]

No igniter EVER needs to touch the delay grain. Aerotech most likely used this language to ensure people fully inserted the igniter.

@heada has the right of it. If the propellant ignites properly, the delay grain will certainly ignite at almost the same time. If the delay grain ignites, it may or may not ignite the propellant, and your delay grain burning by itself before burnout is never desirable.

Loki motor reload instructions actually say it and it made so much sense I now do it with every composite motor: Fully insert the igniter, then pull it back 1/4-1/2" so that the igniter is up against the PROPELLANT, not the delay.

If that "error" occurs, where the igniter is stuck against the forward insulator and not the delay, it is not in the slightest an issue. It may actually be desirable. To reiterate: Your igniter should be igniting the PROPELLANT, NOT the delay.

Well now this is conflicting information...

It's pretty specific, see step 7 here:



Also, Apogee in their video also states that composite motors should have their igniter place up against the delay element

Specifically at 1:54



So now I'm really confused. As mentioned I DID have a CATO where my ejection charge went off too early mid flight. Could this be the reason?

 

[overklock]

@AeroTech Could you chime in here?

 

[Antares JS]

As mentioned I DID have a CATO where my ejection charge went off too early mid flight. Could this be the reason?

Did your rocket sit on the pad with the motor emitting smoke for several seconds before taking off? If so, then 100% yes. Propellant igniting without the delay fixes itself in far less than a second, while delay igniting without the propellant can be very bad, and avoiding touching the delay with the igniter helps to avoid this issue.

 

[overklock]

Did your rocket sit on the pad with the motor emitting smoke for several seconds before taking off? If so, then 100% yes. Propellant igniting without the delay fixes itself in far less than a second, while delay igniting without the propellant can be very bad, and avoiding touching the delay with the igniter helps to avoid this issue.

Negative. Took off pretty quick before ejecting early. You can see the video here if you want https://www.rocketryforum.com/threa...t-could-have-caused-this.172452/#post-2273090

 

[Antares JS]

Negative. Took off pretty quick before ejecting early. You can see the video here if you want https://www.rocketryforum.com/threa...t-could-have-caused-this.172452/#post-2273090

Never mind then, that looks like something leaked and the ejection charge got set off too early.

 

[overklock]

Never mind then, that looks like something leaked and the ejection charge got set off too early.

Cheers. Also, here's another manual from Aerotech clearly showing that they want the igniter pressed up against the delay grain. Honestly I'm very confused now. I want to avoid an additional CATO.

 

[Antares JS]

Consider this: if the igniter tip specifically being against the delay element really mattered, would they tell you to put a piece of tape on top of the shorter grain and only insert the igniter that far when using the shorter grain?

The delay element is just being used as a stop to tell you the igniter is fully inserted to the top of the motor. Like I said, the propellant igniting without the delay fixes itself in a split second. It may be decades of rocketry experience talking in my case, but to me there is really nothing confusing about this. You just have to consider how the motor actually works.

 

[overklock]

Consider this: if the igniter tip specifically being against the delay element really mattered, would they tell you to put a piece of tape on top of the shorter grain and only insert the igniter that far when using the shorter grain?

The delay element is just being used as a stop to tell you the igniter is fully inserted to the top of the motor. Like I said, the propellant igniting without the delay fixes itself in a split second. It may be decades of rocketry experience talking in my case, but to me there is really nothing confusing about this. You just have to consider how the motor actually works.

What you're saying makes sense. One would assume that if an igniter could light the delay element, then so could the massive internal heat and pressure of the grain burning.

I think on my next launch, I will push it into the delay grain, but then back it out a quarter inch as others have mentioned so that it's rested on the top of the grain rather than the delay element. This makes the most sense to me.

 

[Antares JS]

What you're saying makes sense. One would assume that if an igniter could light the delay element, then so could the massive internal heat and pressure of the grain burning.

I think on my next launch, I will push it into the delay grain, but then back it out a quarter inch as others have mentioned so that it's rested on the top of the grain rather than the delay element. This makes the most sense to me.

You won't regret it. I would never say you are definitely going to have a problem by fully inserting the igniter up to the delay grain, but it does increase the risk of the delay igniting while the propellant does not, which leads to premature ejection as has been said. Backing the igniter off just a little bit, 1/4" is just fine, helps reduce this risk.

 

[loopy]

What you're saying makes sense. One would assume that if an igniter could light the delay element, then so could the massive internal heat and pressure of the grain burning.

I think on my next launch, I will push it into the delay grain, but then back it out a quarter inch as others have mentioned so that it's rested on the top of the grain rather than the delay element. This makes the most sense to me.

The reason they say to seat the igniter against the delay element in the F/G loads is to avoid the issue of people putting the igniter halfway up the delay grain and calling it "good enough". Basically, it's "keep pushing the igniter in until it comes to a hard stop". Doesn't matter if it's the insulator, the delay grain, or a piece of masking tape at the top of the c-slot. The goal is to avoid igniting the propellant grain halfway up.

 

[heada]

Cheers. Also, here's another manual from Aerotech clearly showing that they want the igniter pressed up against the delay grain. Honestly I'm very confused now. I want to avoid an additional CATO.

View attachment 545661

We all can give sage advice, "I woulddda...." etc. but all our words don't add up to a hill of beans compared to the instructions. Read and follow the instructions fully. They were written by the experts, not us. Also, if even after you followed the instructions perfectly your motor fails, then you can get it replaced under warranty. If on the other hand you ignored the instructions and followed advice given here that is different from the instructions, the motor manufacturer could deny warranty replacement.

That said, when propellant goes all the way up to the delay grain, I put the igniter at the top of the propellant so that it is touching both. If there is a gap between propellant and delay grain, I put it at the top of the propellant. That is how I do it, but you should follow the instructions.

 

[overklock]

The reason they say to seat the igniter against the delay element in the F/G loads is to avoid the issue of people putting the igniter halfway up the delay grain and calling it "good enough". Basically, it's "keep pushing the igniter in until it comes to a hard stop". Doesn't matter if it's the insulator, the delay grain, or a piece of masking tape at the top of the c-slot. The goal is to avoid igniting the propellant grain halfway up.

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?

 

[Steve Shannon]

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?

No, although that is best for some, it’s not for all.
I think the continued best practice is to follow the instructions. That’s how they are tested and certified.
Some motors, such as that E c-slot motor, should not have the igniter up to the delay and then pulled back.

 

[waltr]

I am not to sure the instructions are always correct.
There are a few reloads for the 29/40-120 case that are shorter and use a spacer but instruction do NOT say to tape across the end of the grain.
Since another instruction sheet (different reload for same case) does say to tape across the end of the grain I have been doing this on all where the grain is not full length.
Then push the igniter in until it hits the tape for a short grain or to the delay for a full length grain.

If the igniter is good motor always lights. Bad AT igniters is another subject.

Another Issue I have run across on some 29/40-120 reloads is the grain liner does NOT Fit into the case.
It is only through forum posts that I have read that one then needs to peel a layer off the outside of the liner.
Never saw this in any instruction sheet nor does the instruction say that the liner MUST move easily inside the case.
Part of my assembly procedure now is first check that the liner fits the case. If too tight then peel off a layer.

 

[smstachwick]

I am not to sure the instructions are always correct.
There are a few reloads for the 29/40-120 case that are shorter and use a spacer but instruction do NOT say to tape across the end of the grain.
Since another instruction sheet (different reload for same case) does say to tape across the end of the grain I have been doing this on all where the grain is not full length.
Then push the igniter in until it hits the tape for a short grain or to the delay for a full length grain.

If the igniter is good motor always lights. Bad AT igniters is another subject.

My E16W reloads needed scraping to light. The igniter went but the propellant surface had oxidized.