PVC Hatred

[Jeffmhopkins]

Hello all,

 

My name is Jeff Hopkins and I build sugar motors out of PVC with great success.

 

There seems to be a common hatred of PVC due to the risks involved if a CATO occurs.

 

I posit that this is not a real issue, but one perpetuated by individuals without significant understanding of the materials science behind the claims. Further, as with all experimental amateur rocketry, personnel should be completely out of the hazard zone when testing or launching, so even if there was a CATO, injury is highly improbable.

 

Full understanding of solid rocket propellant characteristics and critical materials properties with appropriate safety factors can make PVC as safe as any other case material.

 

From John Wickman who actually teaches rocketry and has a book "How to Make Amateur Rockets":

 

"We have been testing PVC pipe motors for almost 20 years and have seen them fail on the test stand. What happens is the end cap or nozzle lets go while the PVC pipe portion remains intact. We have deliberately put in pressure spikes to chambers and can only fragment them with explosive over-pressurization. We have had motors go from 0 to 650 psi in 200 milliseconds. Way beyond the failure pressure of the motor and guess what happens? No fragments, just the end caps or nozzles blow off. The failure point is at the glue bond line.

We can fragment PVC motors when we over pressurize them to levels of 1,500 psi in about 50 milliseconds. But, if your motor is overpressurizing at these levels, there is a lot more wrong with your motors than using PVC pipe and fittings. You have made a serious error in the design of your motor or have a propellant that detonates."

 

Basically, if you don't have significant understanding of what you are doing you can explode a case. But this will happen with paper, pvc, aluminum, or steel. PVC shouldn't be the risk, it's the uneducated people using then without significant research.

 

John Wickham answers this question as well: Wouldn't reusable aluminum cases be better and safer than PVC pipe and fittings?

 

"Aluminum permits higher pressures which means higher specific impulse or better propellant efficiency. That's certainly better. Being able to reuse the chambers, bulkhead and nozzle would save money, too. If you want to go this route, we recommend buying the reloadable motor hardware set. You can make your own reloads for it using the methods in the bookset.

From a safety standpoint, aluminum is not better than PVC. If the propellant detonates or overpressurizes at almost detonation rates, aluminum will fracture just like PVC. It is a myth that aluminum only splits and does not fragment. We have seen aluminum chambers fragment.

If a case is going to fragment, the PVC plastic is actually better since the plastic fragments will not go as far or penetrate skin as well as metal fragments. To see for yourself, take a small piece of plastic and throw it. Now, take a small piece of metal the same size and throw it. The difference in speed and range is dramatic. The metal fragment is much more dangerous."

 

I hope this will open a reasonable discussion on whether PVC hatred is reasonable, or if the hatred should be placed on inexperienced individuals making rocket motors.

 

Edit//

 

Often I see posts of "my motor CATOd" and they give specs, but often lack things like KN ratios or specifics on the fuel they are using, are using poor or improper fuel preparation and casting methods. This is where as a community we should place our concern.

 

And particularly motors lit by fuse by hand, and are recorded without proper safety conditions.

 

Cheers,

 

Jeff

Edited November 9, 2015 by Jeffmhopkins

[Maserface]

What casing would you choose, if you didn't understand Kn, ISP, burn rate exponents etc. and were hand lighting ?

[Jeffmhopkins]

I wouldn't recommend anyone create rocket motors without significant research and understanding of the topics and understanding the safety provisions that are required.

 

If someone is insistent upon going down the path, I try to get them to be more informed by suggesting websites and literature, and I've offered to mentor a few individuals on the phone/in person to the point where they are no longer at risk of injuring themselves or others by reckless behaviour.

 

But too often I see individuals make hasty comments online about how dangerous PVC is, recommending steel or aluminum enclosures instead, or even loki reload kits etc. I think it's MORE dangerous to recommend metal enclosures to the neophyte over PVC without the proper knowledge.

 

For small motors, those making bottle rockets etc, paper is fine to experiment with, but once getting to HPR range where death is a possibility in a CATO, electric ignition and adequate static testing IS required.

 

If someone is illiterate in the engineering/math/science and wants to use ANY kind of case we should recommend they do more research. If someone is still learning the basics, but is making motors with PVC, we should recommend proper safety requirements and recommend reading/mentor, not recommend metal casings because PVC fragments.

Edited November 9, 2015 by Jeffmhopkins

[Maserface]

Is this a problem you have observed on this site?

[Arthur]

The problem with PVC tubes probably comes from it's use in mortar tubes for shells, which are likely to be hand lit. and if they fracture they break into small shards that may not show on X-ray.

 

The other problem is that several people don't have a 1000m safety radius and don't have the high power firing system to fire at a safe range, there are still questions about how to put visco in a rocket motor.

[DavidF]

Jeff, could it be that the 'haters' you describe are people involved with pyrotechnic rockets, and not involved with HPR? Those of us that make pyrotechnic rockets are regular folks that make rockets that sometimes explode on the launch pad. Often we put headers on them that are designed to explode. In some cases the header, which may be MUCH more powerful than the rocket when it explodes, deploys early (blowthrough or CATO).

 

From what I can see, pyrotechnic rocket enthusiasts are completely different than HPR enthusiasts. I have read many times that the HPR folks look down upon us 'simple' pyro rocket guys.

 

Pyro rocket guys don't need engineering degrees to make good rockets, and have no need to know what kn even means. When we make rockets with paper tubes, it's a pretty straight forward process. Lots of people that make good (and bad) pyro rockets are teenagers or young adults. And they hand light. Why introduce yet another hazard like something that can produce flying projectiles, whether they be whole chunks or slivers?

 

The paper tubes we use are sometimes not recovered, but that's OK because they are biodegradable. Good pyros are also dismissive of plastic shell casings, partially for environmental reasons. As far as I know, HPR folks are dismissive of pyrotechnics on rockets.

 

So, this being a pyrotechnic site, we are generally happy to make simple rockets. We don't go to school for this. We learn from each other on forums like this one. For our purposes, paper works well.

 

Myself, I would never consider the use of PVC or metal for the casing of any rocket I make. Why fix what ain't broke?

[Nessalco]

Nicely said, Dave. Except that the HPR folks don't allow PVC either.

 

I am involved in both HPR and pyrotechnics, and you've summed up the situation pretty well. I'd comment that I don't think it's snobbishness that keep the HPR folks distant, but the regulatory environment and the need (in their eyes) for HPR to disassociate themselves from things that explode. They make things that fly....that sometimes rapidly disassemble themselves Their national clubs provide liability insurance along with membership, and they protect themselves by insisting that certain rules be followed. One is that no PVC cases (or steel, for that matter) may be used, and that failure modes be predetermined. Those are the rules you play by.

 

The danger with PVC is simple - if it ruptures, it creates knife-edge shards that don't show up on x-rays. Absolutely not safe for mortars. An unnecessary risk for a rocket motor, and inefficient to boot.

 

Kevin

Edited November 9, 2015 by Nessalco

[Jeffmhopkins]

Thank you guys for the replies, I do appreciate the comments.

 

I can see a little of the divide that I couldn't before.

 

As Nessalco said even the big HPR clubs don't allow PVC. I think it stems from the plethora of people watching YouTube videos and CATOing at close range to a camera that gives PVC a bad image.

 

I feel that PVC is actually safer than aluminum. Sure, it fragments, but so does aluminum at detonation level pressure increases. And the xray comments..., you shouldn't be close enough for that to be an issue. Plenty of distance at away cells for it to be a non issue.

 

I feel that people who have invested time into metal casings has generally spent more time on their research than someone who can just run down to Home Depot and have less chance blowing it up.

 

But rocketry clubs have away pads for a reason, HPR level 3 need a lot of room in case of a CATO, yet a 1" PVC sugar motor is looked at like an explosive.

 

I think I understand the reason why pyro stays away from it now, but I feel the HPR crowd stay away from it because of a risk that isn't really there.

[Nessalco]

Jeff, the national HPR clubs stay away from PVC because NFPA 1122 specifies acceptable motor construction - and PVC is specifically prohibited as a case material. Their insurance depends on compliance with the regs.

 

You are right about distance - but this is pyro, not HPR. We are often in close proximity to the devices we create.

 

Kevin

[Nessalco]

 

...... a 1" PVC sugar motor is looked at like an explosive.

 

 

 

As far as BATFE is concerned, sugar propellant is an explosive, and requires a magazine for storage and a permit if you want to transport it away from your property.

 

Kevin

Edited November 9, 2015 by Nessalco

[Jeffmhopkins]

NFPA 1122 is for model rocketry, prohibits custom motors at all. NFPA 1127 covers high power rocketry, but the interesting thing is that neither cover experimental rocketry. They have to be commercially available and certified motors. Though they do list paper and metallic cases specifically.

 

And BATFE classifies potassium nitrate explosive mixtures as prohibited. One could argue due to the low deflaguration rate (under sonic) that it is not explosive at all. But I wouldn't want to try that without a team of lawyers like the APCP case

[mikeee]

The other issue with pvc motor casings are the ones not recovered and left in the field.

Plastics have become a major waste stream product and take decades to break down in the environment.

Cardboard motor casings will break down in several years when exposed to the outdoor environment.

[gregkdc1]

Yes there is an undue hatred toward PVC in HPR. I have felt the same way and I have even fragmented PVC motors when I first started out. Like previously mentioned the PVC rapidly looses velocity due to it low density and in my opinion is no threat at stand normal off distances. It is also true that it has a lower working pressure so a lower performance than aluminum motor case but how much performance do you need? I find it funny that the guys at the research launch optimize everything in their motor only to place it in a fat over weight rocket they wont loose. Last I wouldn't want to use sugar motors if I was concerned about performance, some of us just like to fly rockets as many times as possible. I don't have anyway of proving it but I always got the impression that the clubs put the needs of the motor makers over the members. I say this because obviously this was true with Estes from the beginning,but the APCP lawsuit shouldn't have been about just one type of propellant as rockets motors are PADS by definition. They could have cleared the way for all of rocketry but they didn't, instead the focused on one propellant that had the biggest benefit to the manufacturers. I also think this is true of the cases, most people don't have the ability to machine a case so they have to buy one instead of running down to the hardware store to buy PVC pipe and fittings. I have reluctantly switched over to APCP and aluminum cases due to the regulations but it is way more money, complexity and time to make a motor. At one time I had a reloadable F sized (60 gram) PVC motor with a ceramic nozzle that I flew over 2 dozens times. It cost just a few dollars to make and around .50 cents/ flight. It was and still is my favorite motor because I could fly it all day long.

[dagabu]

While this is all interesting and fun, lets focus on the reality that is present in this world and not focus on items subject to personal like/dislike.

 

#1, PVC does indeed show up well on X-ray.

http://www.amateurpyro.com/forums/topic/7287-what-size-pipe-and-what-material/?p=95620

 

#2, PVC has its place in PYRO as long as distances and recovery are taken into consideration. (Post #8 & #12)

 

#3, PVC does deform at low temperatures allowing for expansion at firing times above 2 seconds.

http://www.nakka-rocketry.net/pvcmot9.html

 

#4, Material selection: Even LDPE will tear off pieces or fragments when exposed to gross overloads. My club found fragments of LDPE and HDPE a hundred feet away from the firing line, many of these fragments were sub-ounce in weight, therefore the velocity must have been exceptional.

 

All-in-all, the selection of materials for rocket casings is s personal decision and all implications of such should be fully understood before attempting to use one over the other.

 

On a personal note: The Rocket Rodeo at PGI is a great demonstration as to why my personal decision to only use paper tubes for rocket motor casings is truly important. Since 2009, not a single injury to eye or other extremity has been reported at even close range (CATO) from the rocket line. Paper only for all hobbyist motors is mostly responsible for this phenomenon IMHO.

[Arthur]

Like many things pyro it's partly about safety distance. If you want to use a two inch piece of visco then you dig a bunker and don't use suspect materials. If you have a 100m command wire then the risk an hazard of motor failure is dramatically reduced.

 

If you haven't the safety distance you shouldn't be making rockets.

[dagabu]

Like many things pyro it's partly about safety distance. If you want to use a two inch piece of visco then you dig a bunker and don't use suspect materials. If you have a 100m command wire then the risk an hazard of motor failure is dramatically reduced.

 

If you haven't the safety distance you shouldn't be making rockets.

 

In the real world, 90% of pyro rockets are ignited within 30' of spectators. 100m is a silly notion at best.

[OldMarine]

I can guaran-damn-tee you no one will be thirty feet from a rocket I make, including me! I'll have to fire a few dozen before I do any hand lighting. I'll burn a whole case of Chinese ematches!

[Col]

You dont need 100m for the rocket if its reliable, being that far away from the launch its more likely to hit you on the way down

[Arthur]

You dont need 100m for the rocket if its reliable, being that far away from the launch its more likely to hit you on the way down

 

 

Agreed!

 

But there are vids of folk doing their first tests of big rockets from the decking by their house with the neighbour's decking in clear view. When you do tests distance is your friend. when you show off a tested design UCP is good.

[Col]

Its best to test big motors upside down a few times before adding a stick. You have the option of bunkering it with sandbags or burying it in the ground to reduce the distance.

[dagabu]

[OldMarine]

https://www.youtube.com/watch?v=K9WwcUSOmfo

Possibly the most spectacular failure I've seen. Already feeling sorry for the poor sap who built the Dresden Special.

 

 

No way in Hell that was planned. Was it?

Edited November 20, 2015 by OldMarine

[OldMarine]

Upon further review I'm probably wrong. Was there powder spread out to catch fire?

[Col]

lol, one or two cato`s there, that blue would`be been a nice one. Around 26-27 seconds looked like something, possibly with fins on it headed for the cameraman, definitely wasnt a fish

strike that ..it was a bird hehe

Edited November 20, 2015 by Col

[dagabu]

Possibly the most spectacular failure I've seen. Already feeling sorry for the poor sap who built the Dresden Special.

 

 

No way in Hell that was planned. Was it?

 

That's just the rocket rodeo. It is all planned, the three driver girendola, the flights and even the cake in the background. The only thing NOT planned are the CATO's but we need to get rid of ALL the stuff we made so some of the interesting stuff makes it in.

Edited November 20, 2015 by dagabu