Reinforcing spiral wound motor tubes

[JB3DG]

So before you ask why I even mention spiral wound tubes and motors in the same sentence, I made the mistake of buying 600 tubes in 19mm, 25mm, and 29mm outer diameters from a company that only produces spiral wound tubes. After a couple CATOs I realised my mistake. But now I wish to dispose of the mass of tubes I have left over and get my moneys worth at the same time.

 

So I had an idea of using a PVC tube sleeve around the motor tube, which I would tap on each end, and then screw another short length of PVC tubing into the threaded sections to hold the motor firmly in place so the tube cannot expand in any direction.

 

The problem? I can't find PVC tubes with inner diameter that matches my motor tube outer diameters, let alone tubes of the right diameter to thread and screw into the main tube casing.

 

Anyone got any suggestions? I am reluctant to resort to aluminium as this is meant to be disposable and I don't want metal missiles raining down in free fall from 2000ft or higher.

[Col]

I would try to figure the cause of the cato first. Toning down the fuel, using less pressure on the comp and using a tube support can make a difference. Encasing the tubes in pvc isnt a good idea, in the event of a cato it can create razor sharp shrapnel. Pvc isnt biodegradable like paper, so its not environmentally friendly. If the tubes turn out to be no good for rocket motors. you could always use them for other things like cakes, gerbs and wheel drivers.

[JB3DG]

It is definitely the tube. The fuel is an ordinary sugar fuel which I have had no problems using with convolute home rolled tubes. I have a slow motion video of the CATO and I can clearly see the spiral bind unwinding under the pressure. I have also found several tubes in the mix with obvious defects like voids between the layers.

[MadMat]

PVC is not a good idea. I know there are people that use it but, if your rocket does rupture, PVC will shatter into lots of flying shrapnel. To make matters worse PVC is invisible on x-ray and MRI, so doctors would have no real way to find any pieces inside of you, if you get hit with it. That being said, I tried to reinforce tubes myself some time back and found that there is no easy fix.I first tried impregnating the tubes with various resins and glues and, at best, got limited success. I then tried rolling more paper over the O.D. of the tubes. That worked fairly well, but you end up with much fatter tubes, not to mention, it is very labor intensive.

[JB3DG]

PVC is not a good idea. I know there are people that use it but, if your rocket does rupture, PVC will shatter into lots of flying shrapnel. To make matters worse PVC is invisible on x-ray and MRI, so doctors would have no real way to find any pieces inside of you, if you get hit with it. That being said, I tried to reinforce tubes myself some time back and found that there is no easy fix.I first tried impregnating the tubes with various resins and glues and, at best, got limited success. I then tried rolling more paper over the O.D. of the tubes. That worked fairly well, but you end up with much fatter tubes, not to mention, it is very labor intensive.

 

I'm quite certain my motors are low enough power not to make PVC rupture. I'm not interested in going high power enough to get to that risk .

[justvisiting]

You might try waxing the interior with paraffin. It won't make the tubes any stronger, but it will block up the fire pathways in the inner spirals. There are spiral-wound tubes that are plenty strong enough for rocket motors (Phil's).

[BetICouldMake1]

Right. This issue is likely the spiral pattern letting fire pass by the outside of the fuel grain, which isn't going to be fixed using outer reinforcement. That said, aside from waxing the tubes, I would try treating the tubes with wood hardener or water glass before using PVC. You could also add a few wraps of paper. If none of that works save the tubes for headers and/or inserts and stick with the convolute tubes for motors. It's not that hard to blow up a PVC tube and aside from that they soften when heated. Bad news all around as far as I'm concerned.

Right. This issue is likely the spiral pattern letting fire pass by the outside of the fuel grain, which isn't going to be fixed using outer reinforcement. That said, aside from waxing the tubes, I would try treating the tubes with wood hardener or water glass before using PVC. You could also add a few wraps of paper. If none of that works save the tubes for headers and/or inserts and stick with the convolute tubes for motors. It's not that hard to blow up a PVC tube and aside from that they soften when heated. Bad news all around as far as I'm concerned.

[NeighborJ]

I'm willing to bet those tubes would work fine with nozzleless r-candy motors. The hard fuel lends added strength to the casing when it is first lit and pressure is the highest, as the core opens the pressure drops quickly just when the motor will start relying on tube strength alone.

[JB3DG]

Right. This issue is likely the spiral pattern letting fire pass by the outside of the fuel grain, which isn't going to be fixed using outer reinforcement. That said, aside from waxing the tubes, I would try treating the tubes with wood hardener or water glass before using PVC. You could also add a few wraps of paper. If none of that works save the tubes for headers and/or inserts and stick with the convolute tubes for motors. It's not that hard to blow up a PVC tube and aside from that they soften when heated. Bad news all around as far as I'm concerned.

Thanks. I find it rather strange that it would be fire passing by the outside of the grain given that it is a core burner motor created using compression of dry powder, and the core doesn't run all the way through since I have a solid section for a slower delay burn. I am pretty sure it is just outer reinforcement. The hard fuel is not going to add all that much strength to the casing (then again I am using compressed clay nozzles. Not a fan of nozzleless). Once the tube unwinds (the pressure is not purely outward but also up and down as well which is a further strain on the spiral winding), the fuel grain doesn't have anything to contain that pressure and starts to crack, which of course increases burn rate and it goes boom. We are talking milliseconds after ignition while the rocket is halfway up the launch rod, so the flame front hasn't even had the chance to get to the outside of the fuel grain.

What about carbon fibre and fibreglass tubes? Will they withstand the heat and pressure?

[BetICouldMake1]

The fuel doesn't burn only inside the core since the bottom of the grain lights and if there are cracks or small voids in the wall this allows fire to pass up the outside of the fuel grain from the bottom. This happens almost instantly after lighting.

 

Carbon fiber or fiberglass would likely work but that's gonna be a pretty expensive option.

[justvisiting]

The wax blocks the firepaths in the tube, but also inerts the outside of the fuel grain so it doesn't pass fire easily. Lubricating a spiral wound tube causes it to compress less during filling as well. Were it me, I'd try the cheapest and easiest workaround first, and IF it didn't work, I'd re-think the issue.

 

One time I pressed an end burner in a Phil's tube, and got a powerful chuffer, which showed huge pressure spikes on the test stand. After waxing the tube, I got a smooth burning end burner.

[dagabu]

A few items to mention:

 

PVC is very visible on X-Ray in an MRI scan. I know firsthand, you can also find many pictures online as both MRI and X-Ray look for density differences and PVC has clay added to it, causing it to show quite readily.

 

All spiral wound tubes are inferior to convolute wound tubes, they split along the joint, fire travels down the paths at the joints, they are not made to resist pressure from the inside.

 

Waxing tubes both fills the intricacies and stops fire propagation. It helps keep the tube from being compressed but also prohibits a good bond of the nozzle and fuel to the tube walls if not done right.

 

Aluminum tubes over the spiral would tube is actually lighter in thin walled stock but really messes with farm equipment, I also know this first hand.

 

Bottom line: Wax the inside and wrap virgin kraft over the outside MAY be your best choice. Personally, I use all my spiral wound tubing for salute headers.

[MadMat]

 

 

 

"PVC is very visible on X-Ray in an MRI scan. I know firsthand, you can also find many pictures online as both MRI and X-Ray look for density differences and PVC has clay added to it, causing it to show quite readily."

 

Um, are you talking about X-rays or MRI? I ask, because they are two completely different things. I worked at a company that built MRI machines and they almost exclusively used PVC for parts. They did this because PVC did not interfere with the operation of the machine.

I'm not saying you are wrong, because the density difference thing makes sense, it's just kind of curious....

Edited December 29, 2018 by MadMat

[dagabu]

I think you may be confusing magnetic interference with density. Lead does not interfere nor does Titanium with the magnetic resonance, the clay in PVC makes it denser allowing for good shadowing. HDPE is visible under the skin while LDPE is not.

[dave321]

"......One time I pressed an end burner in a Phil's tube, and got a powerful chuffer, which showed huge pressure spikes on the test stand. After waxing the tube, I got a smooth burning end burner."

 

hmm, interesting.

 

I have a particular low toxicity white smoke which uses cinnamic acid, and is very prone to so called "chuffing".

am I correct in thinking this is due to pressure differences inside the core when burning?

 

ive never really managed to remove the chuffing completely.

any ideas ?

 

sorry its a bit off topic

[dagabu]

Not at all off topic, you used one of Phil's tubes...

 

Chuffing is the same principle as strobing, it's a smolder/pop burn going on. This is found in a lot in comps that still have some solvent in it. Often, solvents left inside create a non-uniform flame front, the vapors have to burn off then the grain can burn again. This happens very fast and gives you a chuffing sound. Wax, mineral oil and water can give similar results but usually just give very poor performance and no/little power to the fuel.

 

One of the solutions to make the chuffing diminish is to add a little more oxidizer and see if that helps.

[Mumbles]

This is just a gut feeling, but I've always thought cinnamon acid smokes chuffed due to sublimation and slight clogging of the nozzle. You can see this in some colored smokes too that aren't perfectly optimised.

[dagabu]

"cinnamon acid"

 

Never heard of that?

[Mumbles]

My computer died and I've been using a tablet. It thinks it's smarter than me. Cinnamic acid. It is one of the components of cinnamon though,as you may infer from the name.

 

https://en.m.wikipedia.org/wiki/Cinnamic_acid

[dagabu]

Is it used in chuffers? Honestly, never heard of it in pyro.