Newbie question - rocket casings

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Hello,

We are slowly working on "DIY" rocket motors. Yes, we've read many pages of safety advice, and are taking a number or safety precautions. At this point, the build is roughly as follows:
3/4" tube, packed with
3/4" clay plugs at each end
3" sugar fuel (white mix)
7/32" nozzle punched through one of the clay plugs.

There is no recovery charge yet, and nothing else fancy. We're just trying for a build that works.

For the casing, we have been following some popular advice of schedule 40 PCV pipe. However, as a part of that safety research, I have learned that although it is popularity recommended, PVC has the tendency to shatter if it fails. Schedule 40 PVC has a 480 PSI capacity rating. For better or worse, we have used this PVC pipe in successful tests, and the PVC has not failed so far. We haven't dared to reuse the PVC, but if that is the only disadvantage, that's fine with us.

Our fuel is the basic [ed. - formula removed by mods] blend, used in powder form (and packed firmly)

My questions are:
1. Does anyone have some idea what kind of PSI this build contains, or suggestions on how to measure it?
2. What kind of casings are viable and fairly safe? Suggestions?
3. Any other thoughts, suggestions, or dire warnings of catastrophic explosions?


We have made a build using layered paperboard for a casing, but we have not been able to set up a safe launch environment to test it yet. My concerns with that are that
1. The casing might burn out too quickly, and fail catastrophically in flight (we considered treating the casing with baking soda, be decided to try it plain)
2. The casing might stretch enough to eject the clay plugs (including the nozzle)

Any thoughts on this are also welcomed.

Thank you!
 
We don't talk about homemade motors here as it is against forum rules. Expect to see this thread locked or deleted very soon.
 
Even though you aren't discussing formulas, this thread may get locked. But before it does, I want to discourage you from using PVC. I have built similar kinds of motors, but I have stayed away from PVC. I use 3/4" polyethlene tubing. You can buy it at Home Depot in the irrigation section. It is lighter than PVC, and it doesn't shatter. Going forward, I'd like to try cardboard tubing, like Estes BP motors. You can buy stuff like that at fireworks supply websites.
 
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Took just one eye-opening incident to convince me that PVC is not, and never will be, suited for rocket casings.

"Oh, I've never had one blow up" isn't far from "Hold my beer and watch this!" Ya really need to remember: it only takes one. Please. People learn from experiences; wise people learn from others' experiences. Had mine. Don't want nummore. Learn.

Best --Terry
 
Took just one eye-opening incident to convince me that PVC is not, and never will be, suited for rocket casings.

"Oh, I've never had one blow up" isn't far from "Hold my beer and watch this!" Ya really need to remember: it only takes one. Please. People learn from experiences; wise people learn from others' experiences. Had mine. Don't want nummore. Learn.

Best --Terry
BTW, prfesser is the guy who's book is/almost is required reading for entry level composite motor fuel making...
 
Since the OP has no real chance of following a discussion if I moved the thread to the Research forum, I just removed the formula in his post. The rest of his questions are valid and you are giving him good answers and directions. For sugar rockets I have often heard that Richard Nakka's website is the "go to" standard. I also want to discourage the use of PVC and the best reason, besides being prone to shattering, is that PVC is transparent (or nearly so) on x-rays so when it lodges inside of you, the ER docs are going to have a really hard time finding it. That's not a good thing.
 
Curious Amateur
Get your level 2 cert and join the Research area, that is what we have done. There is many formulas for igniters, fuel. Discussion on about anything.
Get Prof. Terry McCreary's and John H Wickman's books. They are my go to reference
Bill
 
samb: I have been there a few times, thanks! In many cases it is a bit advanced for us at this point, but it definitely looks like a great resource! We've already learned a few techniques from him.

BDB: Thanks for the specific suggestion! We will look for that tubing. Cardboard does seem to be the material of choice, and from a safety standpoint, I can see why. I wish we could get it any way other than mail order.

prfesser: That's what we've been hearing--thanks for the confirmation.

rharshberger: We haven't tied composite fuels yet, just the previously mentioned (apparently secret) sugar formula. Still, I've made note of this. Thanks!

Peartree: Thanks for keeping this accessible to me! I haven't heard the x-ray visibility aspect of PVC, but it is a good insight. Unfortunately, unless we use metal, this will be true with most casings. That brings us back to the shattering issue, I suppose.

r66astro: It seems I need to do more research on this forum, to learn what this level 2 certification is. I may do this, thanks! We do have a number of fuel formulas, but we're staying with the simple sugar composition for now, since it was our assumption that less energetic materials are safer when doing experiments.
 
heada: it seems I missed you post initially. Thanks for the suggestion--this is pretty much what we did with paperboard, and have not yet tried. We will try paper too. It doesn't burn through too quickly to work? No fire retardants required?

BDB/others: I thought poly tubing sounded familiar, but I couldn't remember why. I looked back in my notes and found out. We did consider it a while back, but here's the thing: Sched. 40 PVC is rated to 480 PSI, and should not be used due to risk of failure. The only poly we could find was rated to 100 PSI, which is much weaker than the PVC. Is 100 PSI rated poly really good enough, or do we need higher quality stuff from somewhere other than Home Depot? The 100 PSI stuff is also quite thin, and often comes on a roll. My concerns there are that it might explode (I still have no idea how much PSI is in there), or it might melt through. Are these fears unfounded? Sure, it's safer if it fails, but it sounds much more likely to fail. Just holding the stuff, I really don't feel safe packing pyrotechnics into it. If you've had this type work then we'll test it, but I just wanted to make sure that this was the stuff you are talking about first.
 
Since you're using a sugar based propellant, read and understand Richard Nakka's site and data. He covers most of what you're doing. I would highly recommend using the parallel wound paper casings for a few reasons. 1) as paper, if for some reason you do have a case rupture, there will be no shrapnel of any kind. 2) since the nozzle is compressed clay, if there is an over-pressure, the nozzle should blow out before the casing ruptures. 3) paper is a very good insulator and could be re-used if you wanted but I wouldn't recommend it for anything important, mostly just testing modifications to propellant/process/etc. 4) they're really cheap compared to aluminum cases 5) fireworks industry (and Estes!) have been using paper cases for many decades and they have a very long safety record

The second link I included was a direct link to a vendor that sells the paper casings in 30inch lengths. From the 10 tubes, you cut them down and can get 40 cases for the 1 purchase of $37, so less than $1 each casing

https://www.skylighter.com/mall/product-details.asp?id=203

That company is also a good place to get other things needed like your oxidizer of choice.

The recommendation to join one of the national organizations and get your level 2 certification is that it will make many more resources available to you including peers and mentors so that you don't have to invent the wheel and you skip the dangerous steps that have been done hundreds of times before you.

Good luck and be safe
 
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Also find a mentor. There is a lot to learn from "hands on" and you get your questions answered in real time. Of course, unless you are TRA L2, this may be a mute point.....
 
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I agree that paper would be better, though I don't know it’s pressure rating, so I can't compare it to PVC on that basis. As for the pressure rating for the polyethylene tubing, it is considerably less than PVC, but it’s not brittle like PVC, so explosive ruptures are not as much of a threat. As others have pointed out, any failures I have seen resulted in the nozzle being ejected.
 
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heada: I'll do more reading on his site, then. Thanks for the suggestion of that and this store! I thought at first when you mentioned the cert, you meant something on this site only, but I found the page discussing what it actually means. I do not have this certification yet, but I will consider it.
pondman: I would love to find one, but I'm not sure how/where. Without the cert, and hundreds of miles from the nearest rocketry club, our options are limited.
BDB: We are using clay plugs in our current builds for this purpose. It was our understanding that 3/4" of clay would fail before the 480 PSI PVC. However, general consensus seems to be that PVC is just a bad idea anyway. I just don't see how the Poly will have a chance in, well, this pyrotechnics simulated hell, of surviving the heat and pressure if PVC with much higher ratings tends to fail. Still, we'll give it a try, since you say it will. Perhaps we will need a poly/paper hybrid casing....
 
I’ve proabably built and fired 100 of these motors with no failures of the polyethylene tubing, so I can attest that it works. That said, I want to move to cardboard for safety and environmental reasons. I don’t really want to intentionally litter our launch site (public property) with plastic casings.
 
I’ve proabably built and fired 100 of these motors with no failures of the polyethylene tubing, so I can attest that it works. That said, I want to move to cardboard for safety and environmental reasons. I don’t really want to intentionally litter our launch site (public property) with plastic casings.

Thanks, we got some Poly to try, but haven't built anything yet. On a side note, we finally got a good day to try the paperboard casing. It didn't rupture (probably thanks to copious amounts of poorly balanced duct tape...) but the nozel failed so it never experienced the full stress.


This is the third time we've had a nozzle failure. It simply blows out at about 15', throwing the rocket body onto an unpredictable course. It quickly discharges all of its fuel (on the bright side, this looks something like cheap fireworks) and dies. The rocket reaches about 30' before it veers off and crashes. This probably is a typical nozzle failure, but we're not sure how to fix it. We are roughing up the interior of the casing (though with the paperboard, this is difficult), but that is not holding the powder clay plug. We could try a "band-aid" solution like actually using glue or heat-resistant tape, but I'd be surprised if this even worked reliably. I'm tempted to just make it out of wood, but I want to keep the safety factor of clay.


Our nozzles are currently 3/4" thick. Perhaps this is an issue better posted in a new thread, though.

Thanks again for your guidance!
 
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How are you ramming your nozzles? (over a mandrel, flat disc that is then drilled and if drilled, are you creating a divergent/convergent configuration or truly a flat disc) Any additive to the nozzle mix like grog (ceramic additive, not the drink)? If you're following normal process and still blowing the nozzle, then you more than likely have too much chamber pressure and you're failing in the safest way possible. Look at ways to lower your chamber pressure. I don't want to go into details as that will more than likely get this thread closed. Spend a few days and read all the development process on Richard Nakka's site https://www.nakka-rocketry.net/ He walks through the process you're following and its better to learn from someone else than to make all your own mistakes.

I'd also build a small test stand using a load cell, amp and arduino (all available on Amazon for less than $100) so that you can test your motors without launching a rocket and you actually know the performance of the motor afterwards.
 
How are you ramming your nozzles? (over a mandrel, flat disc that is then drilled and if drilled, are you creating a divergent/convergent configuration or truly a flat disc) Any additive to the nozzle mix like grog (ceramic additive, not the drink)? If you're following normal process and still blowing the nozzle, then you more than likely have too much chamber pressure and you're failing in the safest way possible. Look at ways to lower your chamber pressure. I don't want to go into details as that will more than likely get this thread closed. Spend a few days and read all the development process on Richard Nakka's site https://www.nakka-rocketry.net/ He walks through the process you're following and its better to learn from someone else than to make all your own mistakes.

I'd also build a small test stand using a load cell, amp and arduino (all available on Amazon for less than $100) so that you can test your motors without launching a rocket and you actually know the performance of the motor afterwards.

It's just a flat disk (well, more like a 3/4 inch tall cylinder). We'd considered enlarging the nozzle aperture, but have not done so since we know someone who uses this method successfully.

I have visited that website in the past, and found some great information there. I will visit again.

This test stand sounds like exactly what we need. At present, all we can do is measure relative force, and of course, temperature of the outer shell.
 
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For those looking for paper tubes:

https://www.hobbyhorse.com/pyro_tubes.shtml

They have 0.51" id x 0.70" od (same as Estes 18 mm), 5/8" id x 7/8" od (tape or make adapter for 24 mm), 3/4" id x 1 1/8" od (29 mm), plus half a dozen other sizes.

Best -- Terry

Thanks! I was confused for a moment by the mortar casings, but found the tubes just below. If all else fails (or we get tired of wrapping, gluing, and taping) we will give these a try.
 
Just discussing in vauge terms about my design for variable thrust hybrid landed me in the research section. Now I understand why and so should this thread. If you don't qualify to join that group find a mentor then work your way to Lev 2 as mentioned above. You'll gain tremendous knowledge and have a better respect for the reasearch hobby.
 
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