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Interpretation of NAR safety code

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cornyl

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Hi All, Spring can't get here soon enough!

I am working on a scratch rocket that is some what like a combination of a saucer and a "porta john" or "phone booth" by design.

I have used some short pieces of light weight aluminum tubing for strength to externally duct the ejection charge to the nose cone.

I have not used the aluminum for the nose cone, fins, or body as stated by the NAR code.

Just wanted to get some feedback on this use of aluminum.
Thanks,
Cornyl
 

Pantherjon

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How light weight is it? Can it easily be crushed with your hand like a soda can? If so, I don't see anything wrong with it..I would first fly it in a desolated location first before flying it in front of crowd however..Just for the 'uh-oh' factor..
 

Peartree

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I would think that for ducting ejection gasses to the nosecone (essentially not different than the Fliskits USS Grissom or Borealis) paper tubing would still be adequate. What is the need for aluminum?
 

shreadvector

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Aluminum can burn through pretty fast when exposed to flaing bits of ejection charge and especially the blowtorch like flame of the delay afterburn. See sticky thread at top for link to "The Hibachi Effect".
 

cornyl

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I would think that for ducting ejection gasses to the nosecone (essentially not different than the Fliskits USS Grissom or Borealis) paper tubing would still be adequate. What is the need for aluminum?
Potentially this will show up in as a scratch build but I need the aluminum to support the outrigged motor pods and then duct the gas to the nose cone area.
It's kinda hard to explain.
If the build goes well i will put it up on the scratch build thread.
Cornyl
 

adrian

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If the outrigger pods are far enough away that aluminium is required, they're also probably far enough away that the rocket will be in trouble if the pod motors don't ignite exactly simultaneously or don't produce exactly identical thrust.

On the other hand, a couple of light aluminium tubes probably don't pose any greater hazard than a pointed plastic nose cone stuffed with epoxy and lead weight, and nobody seems to complain about those. ;)
 

Peartree

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As I recall, Micromeister has used rectangular brass tubing to duct the ejection gas from the pods of the Estes SR-71 to the main BT since nothing round would fit.

[disclaimer on] As far as I know [disclaimer off\]

using soft metals for short runs of ducting *should* be okay. It would be a shame to need an L1 to fly a LPR rocket.
 

shreadvector

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Potentially this will show up in as a scratch build but I need the aluminum to support the outrigged motor pods and then duct the gas to the nose cone area.
It's kinda hard to explain.
If the build goes well i will put it up on the scratch build thread.
Cornyl
This sounds like the metal will be a primary load bearing structural component, not "secondary structure" (i.e. a screw eye, launch lug, motor hook, mesh ejection baffle, etc.). Metal cannot be used as a primary structural material for Model Rockets.
 

powderburner

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Maybe you could use booster motors and then something like http://www.picoalt.com/ for deployment?
I'm not sure that this suggestion is a step in the right direction for Cornyl. (And I'm not picking on Will either; there are many people who make these sorts of alternate suggestions.)

Yeah, I know lots of you guys use these electronics all the time in bigger rockets and it's no big deal, but you have to remember that there are also many of us who specifically do NOT want to use electronics:

-- A low-power rocket often simply does not have extra internal volume to install a timer, a battery, switches, and wires, or a good place for a usable-sized access door without completely ruining the rest of the rocket
-- A low-power rocket often cannot stand the added weight of an electronics bay without seriously impacting the flight performance (in a bad way)
-- It is definitely more expensive, often many times the cost of the rocket kit, if you have to go buy the electronics just for this one job
-- It is definitely more complex, not that it is "impossible" to manage, just that it makes simple low-power rocketry into something different
-- Electronic timing goes with external (non-motor-based) ejection systems, which generally require electrical igniters and black powder, both of which usually require LEUPs and all sorts of other government nonsense, which completely changes low-power rocketry into a whole different animal
-- Sometimes you just like to try to master the design/engineering/craftsmanship aspects of a more complex configuration, and to try new techniques to complete a new rocket concept
-- It doesn't "feel" like model rocketry
-- Keep it simple (and something else?)

I have also asked questions about the acceptability of unconventional materials and design approaches. These questions do not seem to receive much serious evaluation, instead generally triggering only knee-jerk reactions like "you CAN'T do that!!!" or "use electronics." I would instead encourage everyone to look over the NAR rules, to consider common sense, and to give the question more than two seconds of thought. If you have a safety concern, by all means DO voice it, but we should avoid blanket condemnations when we don't even know the details of the rocket design at hand.

Cornyl;

There are many examples of "accepted" design practices using these sorts of materials in the immediate neighborhood of motors. The one that stands out to me is the use of a (low-power) steel motor retaining clip, positioned literally right on top of the motor. Or, steel retaining hardware immediately next to the nozzle end, from which a clip or bolt or nut could be thrown. Then you have the widespread use of styrene plastic (relatively brittle) fin cans that surround the motor mount, and could easily generate sharp shrapnel in the event of a major CATO. And steel eye-bolts in motor mounts, and steel all-thread in motor mounts, and.......

I am reading into your question that you have a proper awareness of the relative safety aspects of using MOTC (materials other than cardboard). I suspect that you have already puzzled through some alternate design approaches and have settled (painted yourself into this corner?) on using aluminum tubing as the "best" way to go. The fact that you are even asking this question shows that you are sensitive to the problem, and probably not trying to create some terrorist disaster. Unless your design is clearly and obviously beyond the NAR safety rules, or goes against common sense and wisdom, at some point you just have to go try it.

Before you give up completely on ducting ejection gas with cardboard BT, remember that there are things you can do to reinforce it. You can coat the inside with CA or glue, or glue a metal tape liner inside (wait a minute....). You can wrap the outside with another layer of cardboard BT, or with fiberglass. You can add braces in structurally strategic places, and they can be visually unobtrusive if you use thin bamboo sticks (skewers) or carbon fiber rods, or if you paint them black. You can use clear plastic sheet products to fashion a reinforcing web that is still structurally useful (but be sure to use polycarbonates, not acrylics, or you will trigger the wrath of Micromeister).

Make a few ground "flights" to test the durability of your materials in the severe thermal environment of ejection, and see whether your design requires repairs between each launch. (Ground testing has challenges too, to figure out how to safely immobilize your rocket and still allow all the moving parts to be free.) Make some test flights by yourself, without risking bystanders. Find another experienced hobbyist and talk it over with them. Contact the NAR safety guys (there's some experience!) and talk it over with them.

And when you get it working, and master the new materials and techniques, and get published in a magazine or post your build-thread online, resist the temptation to tell us "I TOLD you so!"
 

Micromeister

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As Shread sort of explained Thin walled alum tubing is a poor material for ducting ejection gases. If its in any way being used as a structural part of the model that violates the MR safety code, thus shouldn't be done.

I'd have to see what your doing, but I wouldn't worry a bit about using cardboard tubing for your outboard pod structures and ducting. these can easily be reinforced in several ways without adding to much extra mass or complexity.

What Peartree was referreing to on the SR-71's are embedded 1/8 x 3/16" rect. brass tube ducts installed as part of the balsa connection inner wing spacers not a structural part of the model. If I could have found or made a phenolic tube ducting system small enough I'd have used it;)
have you thought about CA soaking standard tubing? these become a form of phenolic tubing making them very strong and hold up well against ejection gas and after flame.

PS: Apparently Powder and I were typing at the same time..... "the Wrath of Micromiester" I mean Really???... Yea! stay away from acrylic plastics in model rocket use, it's simply to brittle to have any real purpose on a model, Polycarbonates, styrenes, PVC's, even Butyrates are a much better choice;)
Hope this helps a little.
 
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Porthos II

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This sounds like the metal will be a primary load bearing structural component, not "secondary structure" (i.e. a screw eye, launch lug, motor hook, mesh ejection baffle, etc.). Metal cannot be used as a primary structural material for Model Rockets.
The NAR safety Code:1 Materials.* I will use only lightweight, non-metal parts for the nose, body, and fins of my rocket.

It doesn't sound like he is using the metal tube for a nose cone, fins, or body tube...


Erik
 

Micromeister

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The NAR safety Code:1 Materials.* I will use only lightweight, non-metal parts for the nose, body, and fins of my rocket.

It doesn't sound like he is using the metal tube for a nose cone, fins, or body tube...


Erik
Sounded like structural part(s) of the body to the way I read his post Erik.
Word-play isn't the important thing here; the INTENT of the code is NOT to use metals as structural parts or members on our models. Really; it's pretty cut and dry....no need for pink book lawers "depends on what the daffynition of "is" is... Sheech!
 
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Peartree

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Sounded like structural part(s) of the body to the way I read his post Erik.
Word-play isn't the important thing here; the INTENT of the code is NOT to use metals as structural parts or members on our models. Really; it's pretty cut and dry....no need for pink book lawers "depends on what the daffynition "is" is... Sheech!
Everybody's had their say on this definition and it has potential to get heated. Arguing intent is not helpful.

Let's not take this any further.
 

dr wogz

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Just to throw another wrench in the fire..

Could we also assume the NAR codes intent refers to: No metal parts for more than __% of the total rocket? Like, say, 5% or less of the rocket 'can be' a metal part?

Meaning: The screw eye, swivel, motor hook would equal less than 1% of the total model composition. that's acceptable (for LPR)

The motor hook, screw eye, swivel, and AL (Steel, titanium, etc..) body / stuffer tube would constitute 51% of the model composition: Not acceptable

(Also owing to the metal body tube.. But for example's sake!)
 

mperdue

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The NAR Safety Code is based on NFPA 1122 which says:
4.2.1 A model rocket’s structural parts, including the body, nose
cone, and fins, shall be made of paper, wood, or plastic and shall
contain no metal parts.
As you can clearly read, structural parts include the items listed in the safety code but are not specifically limited to them.

Mario
 

Micromeister

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Our Model Rocket Saftey Code was "simplifed" recently, by the NAR so that children could understand and abide by it.

Don't see anything, anywhere in it about percentages or formulas. It's as simple as is written. We don't use metal parts to build more models.
 

Pippen

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Everybody's had their say on this definition and it has potential to get heated. Arguing intent is not helpful.

Let's not take this any further.
John's already asked that this not be taken any further and I've just pulled a post for language and disrespect. Again, let's not go any further with this.
 

cornyl

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WOW,
I was not my intention to create a such a firestorm on the aluminum.
I think everybody had great input and made very legitimate points. I think for now I will scrap the aluminum and double up some bt-5 or use bt-5 coupler and go the CA route. Again thank you everybody for your input!
CornyL
 

Pat_B

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I've never understood how HPR flyers can have 16-lb bowling ball nosecones and we're not suppose to have any metal in our small nose cones, or a small piece of metal tubing like the OP's example. Same with glassed parts- a nice glassed nose cone or fins can be just as lethal as any metal parts, yet they aren't prohibited.

I support the safety code, but think they need to find a way to be more consistent with what's going on with the larger rockets that can do a lot more damage than the LP versions that weigh a matter of ounces.
 

MarkII

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I've never understood how HPR flyers can have 16-lb bowling ball nosecones and we're not suppose to have any metal in our small nose cones, or a small piece of metal tubing like the OP's example. Same with glassed parts- a nice glassed nose cone or fins can be just as lethal as any metal parts, yet they aren't prohibited.

I support the safety code, but think they need to find a way to be more consistent with what's going on with the larger rockets that can do a lot more damage than the LP versions that weigh a matter of ounces.
Well, for one thing, high-power rockets are not simply model rockets with large diameters. And they do have their own safety code, which differs in some respects from the code for model rockets. And bowling ball lofts are conducted in specific places under specific conditions where they will present little to no hazard to the public. And anyone seeking to launch a high power rocket needs to obtain an FAA waiver and meet a number of FAA regulations, which are exempted for anyone seeking to launch a low power model rocket. And in the case of high power rockets, a prohibition of the use of metal or hard composites in their construction would actually make them more hazardous by limiting the ability of such rockets to maintain their structural integrity under the thrust loads of high powered engines. High power rocketry and model rocketry share the same culture and ethic of safety, but neither discipline's safety code can be directly transferred to the other discipline without some modification.

It's a moot issue now, since cornyl has decided to use a different material in place of the aluminum, but I don't think that anyone could provide a definitive answer to his question without seeing the actual design. It is indeed debatable whether or not his original plan would have violated the MR safety code, based on the information that he provided. And, for very normal and understandable reasons, he probably did not want to "unveil" his design to the entire forum until he had completed his build, so any advice provided in the thread would by necessity have been limited and tentative.

Model rocketry is not in the same stage of development that it was in 1958, when the use of any metal in a model rocket's construction was to be avoided at all costs. Fifty years of experience has demonstrated that the LIMITED use of metal in small non-structural components can be done without compromising the safety of model rockets built out of high quality modern components and flown with modern high quality and highly reliable commercially produced rocket engines and reloadable motor systems. The use of such small components as eye lag screws that have been embedded into balsa nose cones, small, thin flexible engine retaining clips that have been glued or taped to the side of paper engine tubes, or small screws, nuts and washers embedded in paper or light wood centering rings for use in positive motor retention have not been shown to cause any increased incidence of injury. Metal composition structural components, such as metal airframes, nose cones and fins, continue to be prohibited now, just as they have always been. Fiberglass composites are very rarely used in model rocket construction; they are used mainly to construct competition models for FAI events. Even then, such composites are quite thin and frangible. The use of dense, rock-hard composite components is very seldom seen in model rocket construction simply because it is not necessary, because it is not practical and because it has a disastrous effect on the performance of models propelled by low power rocket engines.

MarkII
 
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