Nozzle blowout due to waxed tubes?

[Mumbles]

I think there is a lot that goes into this in my own personal opinion. It'd be hard to nail down specifics without a lot of careful investigation. I'll share a few antidotes that maybe will shed light or generate some food for thought for others.

 

Potassium nitrate I don't usually worry about absorbing water. It may clump up, but I don't believe it holds a lot of water. From personal experience under probably the most in undesirable conditions possible, I had potassium nitrate gain about 3-4 wt% water. This is with lower quality prilled nitrate, stored in a temperature uncontrolled environment in what at the time was probably approaching 100% humidity for about 5 years. The prills had become visibly translucent. Baking it out returned it to a perfectly acceptable state. Even still, this didn't appear to cause any problems when used as is, unless milled. It would cake a bit while milling on its own, but nothing a good rap on the side of the mill wouldn't dislodge or at the very least a light pass with a butter knife or ruler to get everything from the corners. I was having some clumping issues while milling at the time, but even baking it out first didn't solve that issue.

 

Charcoal was the more likely culprit. In my admittedly somewhat limited experience, it had more to do with the variety of charcoal than anything. Commercial airfloat didn't cause any clumping. Home cooked charcoal didn't exhibit any clumping either. It was only some willow I had purchased from Custom Charcoal. One thing to note about this material is that it was undercooked. There were some very hard pieces and even some uncharred wood in some places. There are reports of this causing hygroscopic issues. If you look in Davis where he describes cocoa powder, made from undercharred rye I believe, he notes it is more hygroscopic than typical BP. I think there is some merit to the thought that undercooking or perhaps a lower pyrolysis temperature may lead to a greater hygroscopic affinity of charcoal.

 

The other thing to note, as some others have mentioned, is that cardboard tubes can actually hold onto a lot of moisture. I think many would be surprised at the amount of water you can pull out of them. I used to know actual values off the top of my head, but I bet you can probably pull 10wt% out of them. Ordenblitz would know for sure, as he's the one who first brought this to my attention a number of years ago.

 

It'd be pretty hard to check accurately, but I wouldn't be the least bit surprised if pressing slightly damp propellant had the water pulled from the grain into the tube, at least to some degree. This would effectively "dry" the propellant, without causing an apparent decrease in mass of the overall rocket.

[Bourbon]

WTF? I wasn't even referring to you. Hell, you posted after I loaded this page and started my post. I was referring to the OP.

My deepest apology. The time line was spot on, and of coarse I posted after. I had no idea when you loaded this page. I must have mistaken it from your other remarks. Looked legit. I'm sure you can see that.

[Bourbon]

Mumbles.. Thank you for sharing your experience in the matter. Gives me a good idea of what I was hearing.

[shockie]

mabuse00, most of my black powder tests have been done with rockets or with 1/2" diameter X 2" thick pucks. The pucks were pressed to a density of 1.7 to 1.9, in general. The pucks were dried, and the weights tracked as they dried. After they dried, they were stored for probably a year before I pulled them back out again. When I did, just for a lark, I re-checked all the weights and all the dimensions. Part of the reason for this was because another (highly experienced) pyro suggested that the dimensions of the pucks might change as they dried out. He was right, kind of. The diameters all stayed the same. The thickness of all of them increased, by an average of .005". Strange! I've said it a hundred times that charcoal has elasticity, and it's been proven to me in my own life, as well as being documented in official studies on black powder.

 

With this in the back of my mind, my earlier concerns about shrinkage of a damp propellant grain evaporated. Even if I'm wrong, and they didn't 'grow'- they didn't shrink. What happens is that the structure of the black powder grain becomes more open instead. This is also documented information from published, available studies. I can provide references if anybody needs to see proof of these 'claims'.

 

When I damp-pressed my rockets, I fully expected them to dry right out. I expected it to happen slowly. But, with a core up the middle, and no nozzle, the weight didn't go down in 3 weeks. The surface of the propellant grain became dull rather than shiny, after a couple of hours. But no weight loss. I'm sure the water did move- it just didn't move out- in 3 weeks. I included this observation in my reporting, because my report would be incomplete without it. IF I had declined to include that observation, it would have been irresponsible. So I included it.

 

Dave Sleeter has written two books about black powder-type rockets. He's been hand-ramming damp propellant since the early 80s. For some reason, Sleeter's work has never really been accepted by the pyro community.

 

I think the chunky nature of the 'pre-densified' BP helped pull on the tube. It happened on 1lb motors. 1lb NEP tubes have the lowest amount of tube per unit of area of all the common sizes. Also, recently, NEP has taken away 10% of the paper from their 1lb tubes. The ones I have now fall through a rigid tube support like it's not even there.

Elasticity is a known but not well understood phenomenon with BP propellant motors.

 

The paper tube and its adhesive and the black powder grain itself have differing thermal coefficients of expansion.

 

For example it has been known for 30 years that if you increase the temperature of a stored BP cored rocket motor, the BP slug will expand against the paper casing.

Once the temperature comes back to normal ambient,the grain contracts and there leaves a gap between the BP slug and the paper casing. The flame can then race up the side because of more burning area,and kaboom.

 

This is classic thermal cycling.

 

These are the results of elasticity in the differing materials with elevated temperatures.

 

The best thing to do per storage is to keep them in a cool,dry place. Low humidity is good. But not so low it dries them out.

 

There's an excellent paper about why waxing tubes is a good thing at pyrobin. I highly recommend it to be read.

 

http://www.pyrobin.com/files/Reducing%20Catos%20in%20Pyrotechnic%20rockets-V2.pdf

 

Lloud S. has some asute insights ..

Edited January 5, 2021 by shockie

[mabuse00]

PS:

Coming back to my original question:

I made several endburners with very tight nozzles and fully waxed tubes. No problems, no blowouts.

So in future I will not bother to leave the nozzle area free of wax...

[WRAITH]

I use superglue on the ends of tubes, let it soak in then when dry, it should be as hard or harder than wood.

[WRAITH]

Why don't you wet the BP with a little alcohol as it dries faster & use a light dusting of boric acid on the cardboard walls as it's a very good dry lubricant. Amazing stuff really.

[royster]

I make these short 3inch 1lb end burners for small ball shells, they go up to fun height and quickly hand rammed. I used the classic bentonite clay with %10 parafin wax and waxed tubes. I never had nozzle blowout but after retrieving a few used rockets and looking at the nozzle i wasnt satisfied with the performance of the nozzle, a lot of it was burnt away.

 

So i was looking online how to improve it and i was reading somewhere about adding fire clay to the mix, couldnt find any so i just took a piece of pottery clay, smashed it with a hammer and blended the small piece to very small pieces, i probably added around %20 of it to the mix. The blender cup was heavily scratched, but after inspecting post flight rockets that mix was holding up much better.

 

I tried to peal off one rocket i knew was going to fail and i quickly gave up as it seemed impossible, the small sharp pieces of pottery clay just dig into the tube.