Longburners

Hi,

I heard on some thread that longburners with offset cores produce a lot of heat on one side, and heat up the case so that damage might be done to the side of the rocket. Is this true?

I'm a big fan of longburning motors (flying mostly 29-38mm, but also 24mm and 54mm). My own experience is that the liners definitely get more scorched and/or burned through, and occasionally I have some discoloration on the inside of my hardware casing, but I've never had any actual damage done to casings or rockets.

ssix

As I recall, Loki uses a casting liner that is thicker on the core side to reduce the effect.


[emoji1010] Steve Shannon, L3CC [emoji1010]

Certified motors aren't permitted to raise the temperature of the case above 220C. https://www.nar.org/pdf/NFPA 1125 Proposals & Disposition - Unofficial (Feb 2010).pdf

The results of this effort has indicated that exposure of a reloadable motor casing to temperatures of 225ºC +/- 5ºC for less than one hour will not
adversely affect the long-term structural proprieties of the motor casing.

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I've never had any damage from longburn motors up through 75mm.

mikec said:

Certified motors aren't permitted to raise the temperature of the case above 220C. https://www.nar.org/pdf/NFPA 1125 Proposals & Disposition - Unofficial (Feb 2010).pdf



I've never had any damage from longburn motors up through 75mm.

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The danger comes if one "rolls their own" (spell that research motors). I'm not surprised with Steve's comment above. I'd expect as long as one uses a commercial load as it's designed, they would be fine.
As an aside, that's why there isn't high L/D APCP end burning motors. Yes there are motors that may have a solid grain in the stack but not its entirety. Kurt

mikec said:

Certified motors aren't permitted to raise the temperature of the case above 220C. https://www.nar.org/pdf/NFPA 1125 Proposals & Disposition - Unofficial (Feb 2010).pdf



I've never had any damage from longburn motors up through 75mm.

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That brings up a question, for which I have no answer. If a reloadable motor casing is blistered or bulged by a reload, should we file a MESS report? I'll contact a friend on NAR's S&T and see what he says.

Steve Shannon said:

That brings up a question, for which I have no answer. If a reloadable motor casing is blistered or bulged by a reload, should we file a MESS report? I'll contact a friend on NAR's S&T and see what he says.

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Like the 29/40-120 G138T's??? Sure, it a real short burn, but from the threads I've seen here, it's long enough to blister the case...

Adrian

Steve Shannon said:

That brings up a question, for which I have no answer. If a reloadable motor casing is blistered or bulged by a reload, should we file a MESS report? I'll contact a friend on NAR's S&T and see what he says.

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Steve, I absolutely would file a mess report, and probably email the manufacturer.

jd2cylman said:

Like the 29/40-120 G138T's??? Sure, it a real short burn, but from the threads I've seen here, it's long enough to blister the case...

Adrian

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I would say file the report- that should not happen. If it is then the motor's certification should be examined.

My experience with CTI 29mm 3-grain longburns is that the black plastic liner tends to glue itself to the case on the core side, but not other damage.

Do you mean one like this?
It's called a Moon burner.
The core is right next to the edge of the casting tube.
Giving it a thicker web thickness leading to a longer burn time that tails off more.
The thrust spike is in the first 1/2 sec or so.




JD

That's because of inadequate thermal insulation.
Deleting the delay might help.
I also wrap the outside of the grain with at least 1 wrap of tape.

I've flown 3 or 4 of them with no blistering.
Discolored but, not blistered.

JD

jd2cylman said:

Like the 29/40-120 G138T's??? Sure, it a real short burn, but from the threads I've seen here, it's long enough to blister the case...

Adrian

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I have noticed on CTI the plastic casing tends to melt more and the casing needs extra cleaning on long burners.

John Beans said:

I have noticed on CTI the plastic casing tends to melt more and the casing needs extra cleaning on long burners.

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For sure, I got a 29-3G with a melted liner, keeping me from getting the forward closure out of it. I may just end up buying a new 3G case... I'd be worried about damaging the case if I tried to pry it out or heat it up a little.

cerving said:

For sure, I got a 29-3G with a melted liner, keeping me from getting the forward closure out of it. I may just end up buying a new 3G case... I'd be worried about damaging the case if I tried to pry it out or heat it up a little.

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I don't think I've ever had one melt so bad it wouldn't come out pretty easily, or in one piece. Though I vaguely remember knocking one out with a thin piece of wood that just fit into the narrow end, and hammering/tapping a bit.

John Beans said:

I don't think I've ever had one melt so bad it wouldn't come out pretty easily, or in one piece. Though I vaguely remember knocking one out with a thin piece of wood that just fit into the narrow end, and hammering/tapping a bit.

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Tried that, it ain't budging. What ticks me off is that I greased the liner first... never had an issue until now.

You gonna make it to LDRS?

cerving said:

Tried that, it ain't budging. What ticks me off is that I greased the liner first... never had an issue until now.
Snip..,

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I don't know if this will work, but you might try putting it it your freezer overnight. Maybe the difference in contraction will loosen the plastic liner.

Steve Shannon said:

I don't know if this will work, but you might try putting it it your freezer overnight. Maybe the difference in contraction will loosen the plastic liner.

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Thanks, I'll give it a try.

Talk to your dealer about it if that doesn't work. They may be able to warranty the thing or suggest a solvent that you can safely soak the case in (I am thinking acetone, which is normally safe with anodized aluminum) which would dissolve the plastic.

cerving said:

Tried that, it ain't budging. What ticks me off is that I greased the liner first... never had an issue until now.

You gonna make it to LDRS?

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Dunno yet. Try.

JDcluster said:

Do you mean one like this?
It's called a Moon burner.
The core is right next to the edge of the casting tube.
Giving it a thicker web thickness leading to a longer burn time that tails off more.
The thrust spike is in the first 1/2 sec or so.


View attachment 289419

JD

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Hi JD, Is that a thick garolite liner there? Is that a "Wimpy Red-ish" grain? :smile: Kurt

cerving said:

Tried that, it ain't budging. What ticks me off is that I greased the liner first... never had an issue until now.

You gonna make it to LDRS?

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You may have converted the grease to glue.

Try using a penetrating oil to work down between the liner and the casing. Let it sit for a day or two and then carefully use a wooden dowel that fits inside the BP well and inserted from the forward end of the motor to push the liner out. A steady load is probably better than a hammer load. A press or push clamp would be preferred.

Bob

I'll mention this method I saw used to get a stuck liner out of a casing. It involved using a forward closure with an eyebolt on it meant for the case and inverting it so it's inside the case. A piece of kevlar harness material is threaded through the case and tied to the eyebolt on the closure. The other end thrown over the rafter of the garage and hoisted the case up with the end securely tied.

Next, one guy dons super thick leather welding gloves. My gosh, I've never seen gloves this thick before. A second fellow lights a large propane torch and goes up and down quickly to heat the case up fast. I could just see smoke coming out from the inside from warming oil/residue. Second guy with torch tells first guy to get ready and when he steps away with the torch, first guy (with gloves) gives a yank on the casing and it comes right off the liner the first try. He sets casing outside to cool and the liner is allowed to cool suspended for later disposal.

I inquired about heat damage to the casing but was told it's not an issue as indeed the process of heating was done pretty quickly and the number one yanked on it quickly.

If you have the facility, might be a last resort to try unless the casing maker offers to take your casing back and send you a new one. Kurt

I would never take that approach with aluminum. If you are seeing smoke, it is already too hot. It won't be T6 when it is done. What it will be, you won't know. Get it a little too warm for a little too long and you can lose up to 80% of the strength. Some of that will recover over the next week as the metal "ages". But the case afterwards is an invitation for a bubble or CATO.



Gerald

I'm freezing it, I'll let you know how it comes out. I know about 6061 Al tempers, so I would highly reticent to heat up the case at all... it would end up as T0 instead of T6, most likely, assuming it didn't pick up some oxidation in the process too.

Yeah,
I would presume that crazy heating would affect the Al but in reality it was a few quick swipes with the torch and a yank. Freezing might cause contraction to break the hold and allow it to be moved when warmed to room temperature.
In the end, I'd email the motor maker and see what they say if the freezing thing doesn't yield results. If you're SOL, as a last resort a quick trial at heating might be worth a try if the case is going to be trash anyways. Of course as long
as you have two people and the appropriate glove protection. Kurt

Ahem. Try the heating method without the heat. Penetrating oil, then pull.

Get a jaw of a needle nose pliers between the liner and case. Grab. Twist. The "glue" joint gets encouraged to break when the load applied is not shear.

G_T said:

I would never take that approach with aluminum. If you are seeing smoke, it is already too hot. It won't be T6 when it is done. What it will be, you won't know. Get it a little too warm for a little too long and you can lose up to 80% of the strength. Some of that will recover over the next week as the metal "ages". But the case afterwards is an invitation for a bubble or CATO.

...

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Unless I have been mislead about metallurgy, the chart you show is the strength at that temperature, not the resulting strength after being at that temperature and cooling down. Here's from an Alcoa reference:


So it seems unlikely a quick pass with a torch would do much harm. Think about how hot the cases are after a flight - most motor mounts don't allow quick heat dissipation. Even doing a static burn in open air produces a case that's way too hot to handle for several minutes.

But as always, YMMV.


Tony

The requirement is < 200C for any part of the case as part of a motor burn, during the burn and the post burn heat soak.

I posted the curves because of the mention of "smoking" of the liner. Heated from the outside to achieve a temperature capable of producing smoking on the inside is going to put one around or over the upper end of the temperature range you mentioned.

Gerald

PS - You are correct about that chart.

cerving said:

For sure, I got a 29-3G with a melted liner, keeping me from getting the forward closure out of it. I may just end up buying a new 3G case... I'd be worried about damaging the case if I tried to pry it out or heat it up a little.

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My approach is to get the spent motor out of the casing as soon as possible, preferably while everything is still warm. It seems to leave a bit of melted plastic on the side of the casing that needs some elbow grease, again as soon as possible.