I don't mean to speak out of turn, respecting this obviously very high quality product from a respected firm, but my understanding is that an all-metal fin can, whether factory made or scratch built, violates the NAR High-Power Safety Code, and would negate the liability insurance that comes with NAR/Tripoli membership. I recently designed a high-performance L3 tubefin rocket for K-L-M power, using 4" diameter metal soup cans for the tubefin cores, and was told by one of our senior club RSOs that it could not be approved for flight, due to Safety Code violation. I subsequently redesigned the rocket with non-metal epoxy composite tube fins, and limited it to just K-L power. Comments?

Concerning metal in the airframe, I've copied this off of the Tripoli site: _______________________________________________________ There are two parts of NFPA 1127 (HPR Safey Code) that mentions metal in rockets; 1-3 Definitions, High Power Rocket, (e), and 2-6 Rocket Airframe Materials. The typical mistake made by most people is to "narrowly interpret" the Safety Code, focusing on a small part rather than the whole. For example, take both sections as quoted above (from the 1995 edition): 1-3 Definitions, High Power Rocket, (e) -- (High Power Rocket) That is made of paper, wood, fiberglass, or plastic with a minimum amount of metallic parts (Note: most people arguing for the non-use of metal in rocket construction stop right here.) necessary for airframe integrity dependent upon the installed total impulse, and whose primary use is for purposes of education, recreation, and sporting activities. and 2-6 Rocket Airframe Materials -- A high power rocket vehicle intended to be propelled by one or more high power rocket motors shall be constructed using lightweight materials such as paper, wood, rubber, plastic, fiberglass, or, when necessary, ductile metal so that the rocket conforms to the other requirements of this code. Now for the example of how people typically focus on a small part rather than the whole code when trying to promote a point of view. The very last sentence quoted says that the rocket must conform "...to the other requirements of this code." Look at the paragraph in the Safety Code immediately above the one just quoted (2-5): "A high power rocket shall be constructed in such a manner and with suitable materials to withstand the operating stresses and retain structural integrity under conditions expected or known to be encountered in flight." In reality, with the excepted mention of what the metallic material is to be (ductile material), we could end the chapter on rocketry construction with that statement. If we do not construct our rockets "in such a manner and with suitable materials to withstand the operating stresses and retain structural integrity under conditions expected or known to be encountered in flight" we are, at best, unsafe both to ourselves and to others. Here are most of the elements of everything referenced or quoted so far, including some obvious conclusions: -- HP rockets made of many materials, including metal. -- When made of metal, if complying with the Safety Code, HP rockets are made of ductile metal. -- When made of ductile metal, HP rockets are made with a minimum amount of metallic parts. -- When made of ductile metal, the HP rocket must have a minimum amount of metallic parts for the purpose of sustaining airframe integrity. Minimal may include whatsoever percentage of ductile metal NECESSARY TO ACCOMPLISH THAT REQUIREMENT. -- The HP rocket using ductile metal to sustain airframe integrity must do so to withstand conditions expected or known to be encountered in flight. This will require a sufficient amount of ductile metal to accomplish that requirement. -- The duration of the flight is from start to finish, which includes recovery. (Not quoted above, but a part of the safety Code. This means that the rocket must contain whatever materials are required, along with the recovery system, to withstand "the operating stresses and retain structural integrity under conditions expected or known to be encountered..." during recovery as well.) -- Factors to consider when using ductile metal in HP rocket construction, and what amount of ductile metal to use for "airframe integrity" are (a) installed total impulse, (b) rocket to conform to the other requirements of this code, (c) rocket to withstand the operating stresses, and (d) rocket to retain structural integrity under conditions "expected" or "known" to be encountered in flight." -- Since the rocket is to be recovered and reused as stated in this Safety Code, the rocketeer may determine that a particular rocket will be flown more than once, i.e. several times. A particular rocket may fly well one time using materials other than ductile metal, but continued use and the resultant stresses after the first flight may prove to render a non- metallic rocket unsafe. Such circumstances may include, but not be limited to, continual flights using M motors. Therefore, a particular rocket, in order to conform "to the other requirements of this code," shall be so constructed using any materials specified as being approved for use in a HP rocket, and in what ever amounts are required for compliance. Keep in mind that this interpretation takes into account the "installed total impulse (continual use of M motors), and whose primary use is for purposes of education, recreation, and sporting activities." -- Finally, note the use of the words "when necessary" [2-6]. This is subjective and really up to the interpretation of (a) the person building the rocket, weighing all the factors of the entire Safety Code, and (b) the RSO who will ultimately make the decision whether or not to allow the flight. _______________________________________________________ Whether or not a metal fincan violates the safety code appears a bit nebulous to me. I think it should be fine. Some narrow minded persons in position of authority might think otherwise. I think someone can make reinforced carbon fiber fins that are much stronger and sharper than aluminum and could slice the heck out of somebody probably more efficiently than metal. Nobody is outlawing composites yet are they?

Aluminum fin cans are allowable under NAR rules. During the August 5, 2005 NAR Board of Trustees meeting in Cincinnati, Ohio, Mike (Dutch) Duchnevich (a Board member) stated that in connection to the use of aluminum fin cans, "[a] minimum amount of ductile metal necessary to insure safety is permitted by the HPR Safety Code. Additionally, given the total energy present in some HPR models, prudent use of metal per this provision of the HPR Safety Code has no adverse impact on safety." As for Tripoli rules, aluminum fin cans are also allowable.

Respectfully submitted: This is what the NAR safety code says: Materials. I will use only lightweight materials such as paper, wood, rubber, plastic, fiberglass, or when necessary ductile metal, for the construction of my rocket. I suppose this would be up to the interpretation of individual RSO's, but I don't see that this wording excludes the use of metal fincans. Having been RSO for all of our club high power launches, I have allowed the Hawk Mountian fincan to be flown numerous times, and would have allowed your tube fin fincan also. This is assuming the rest of the rocket was flight worthy. My interpretation is that fins on high power rockets, whether made of metal, or composite, or basic plywood are dangerous during recovery.....hence the rule that in no circumstance should an individual attempt to catch a recovering rocket. This also the responsility of the RSO.....to control the range. Just my opinion.

Weighing in quickly on the metal vs non-metal debate: You can apply the NAR, TRA or CAR safety codes by either the literal text, or by the intent. There are certain applications where using metal fins or a metal fin can are appropriate. There are other cases where other materials would work just as well. My opinion only: if the flight profile requires metal fins, go for it. Have the backup to show the RSO. If you're using metal "just because you can"... don't expect it to fly. Why is that? Bear in mind that metal is *perceived* as being harder and thus more dangerous, in the minds of the general public. As the recent incident with the Estes X-15 shows, typical model rocket plastics can be every bit as dangerous.

On May 18, 2008, I flew the fin can on a 9.5 foot long, Extreme II fiberglass airframe from Hawk Mountain with an Animal Motor Works M2500 Green Gorilla. The rocket did better than Mach 1.6, flew to 18,313 feet, and recovered without a scratch. The flight was perfectly straight, so the fin can did its job flawlessly.

On 7/18/2008, after a powerful launch that caused massive rail whip on an M7500 experimental motor, the Hawk Mtn. Aluminum fin can and Extreme II airframe rocket managed to right itself from a 45 degree launch angle without any damage to the airframe or fin can, and rocketed to an altitude of 16163 feet AGL at better than Mach 2. The rocket was recovered after a long search without a scratch. Most fins would have shredded under the kind of stress the rocket endured during this flight, so the 4 inch aluminum fin can proved its toughness!

During our Research day at Oregon Rocketry's Desert Heat Launch, I flew the fin can on a Mike Fisher M7500 research motor that propelled the rocket to better than Mach 2.2. The fin can handled the flight beautifully and the rocket was recovered without a scratch.

One thing about the fin can that you should know is the aluminum fins are thin metal that can scrape skin or cut a person if one isn't careful around them. I devised a simple fin sheathe that you can easily make yourself. Just find some sheets of thin cardboard and draw slightly larger outlines of the fins on it. Carefully cut two of the cardboard fin outlines from the sheets, reverse one of them so one side is the mirror image of the other and press them together. Then take a roll of duct tape and cut several strips to join one side with the other on every edge except for the root edge which has no tape at all. The two cardboard fins joined in this manner by the duct tape will form a sheathe that slides over the outside edge of the fin down to the root edge. Now repeat the process for the other two fins and you'll end up with three sheathes. The sheathes will serve to protect you from the sharp edges of the aluminum fins. Just make certain you remove them before you launch your rocket, though!