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Hawk Mountain Enterprises - Aluminum Fin Can for 4" Airframe (-Undefined-)

Published:2007-11-12
Manufacturer:Hawk Mountain Enterprises
Construction Rating:
Flight Rating:
Overall Rating:

Contributed by Jack Caynon

Hawk Mountain Aluminum Fin Can

Brief:
Pre-manufactured aluminum fin can you bolt together

Construction:
For those o' us who want a rugged fin can t' handle high thrust applications with 4 inch fiberglass tubing, matey, Hawk Mountain has manufactured a nifty 3 fin aluminum fin can for its 4 inch airframes. T' unit is machined from 6061-T6 aluminum with beveled 0.125” thick fins. T' fin can weighs 25 ounces and is black anodized t' resist corrosion and can be painted with regular paint. T' root is 8.25 inches and t' span is 5 inches. Well, blow me down! Although t' picture shows some cutouts in t' aluminum flanges, my unit had no cutouts whatsoever.

T' instructions were fairly easy t' follow, but thar were no illustrations that guided you regardin' assembly. T' assembly order is logical if you give it some thought. Avast! I suggest that you first construct t' fin can then work on your motor mount tube and centerin' rings after you have t' fin can constructed so you can measure where your centerin' rings (only applicable for 3 inch motor applications) need t' go in relation t' t' fin can's holes t' attach it t' t' airframe.

For tools, matey, you'll need a Phillips head screw driver (or allen wrench), a power drill, arrr, and drill bit set.

T' attach t' fin can t' t' airframe, you'll need t' provide your own screws (6), arrr, either self-tappin' or set screws.

Basically, ya bilge rat, t' fin can is made up o' three flanges and three fins. T' way it works is that you sandwich a fin root betwixt t' left edge o' one flange and t' right edge o' another then screw t' two flanges together with the screws passin' through holes drilled in t' fin root. Aye aye! You loosely connect t' next flange, fin, flange combination until all three o' t' fins and t' three flanges form a fin can. Ya scallywag! Then before you tighten all o' t' screws, you slide the fin can onto your airframe. Arrr! Now, find two small holes, one at t' top center and t' other at t' bottom center of each flange. Ya scallywag! Once your motor mount tube with centerin' rings is installed inside t' airframe place t' holes over the area where your centerin' rings around t' motor mount tube touch t' inside o' your airframe and drill holes through the fin can's holes into t' airframe and centerin' rings. Now you can tighten all o' t' screws on t' flanges, then screw self-tappin' screws into t' centerin' rings through each o' t' fin can holes t' attach t' fin can t' the airframe.

For 4 inch minimum diameter applications, arrr, use set screws t' attach t' fin can t' t' fiberglass tubing. Ya scallywag! Make certain that your set screws do nay interfere with t' motor casin' on t' inside o' t' airframe.

PROs: No need t' worry about alignment issues, fit, or sturdiness. Ease o' construction.

CONs: Instructions could be a little clearer. Avast, me proud beauty! It weighs a pound and a half. You need t' provide your own hardware to attach t' fin can t' t' airframe.

Finishing:
T' anodized black aluminum looks pretty cool t' me, so I won't bother havin' it painted.

Construction Rating: 4 out o' 5

Flight:
Use can use this fin can with any 98mm motor, shiver me timbers, commercial or experimental, as well as any high thrust 75mm M motor.

Flight Rating: 5 out o' 5

Summary:
I can't think o' a easier way o' installin' a bulletproof fin can on a rocket that won't shred when subjected t' high thrust motors. Begad! T' PROs are that thar are no alignment issues t' speak o' and t' sturdy construction o' t' unit. Although it isn't a lightweight by any stretch o' t' imagination, for minimum diameter 98 mm motor applications, ya bilge rat, the 25 ounces isn't really much o' a problem and shouldn't keep you from thinkin' about usin' it. Begad! I would recommend it for 4 inch airframes usin' high thrust, shiver me timbers, 75mm M motors and 4 inch minimum diameter applications.

Overall Rating: 4 out o' 5

    Comments:

    L.B. (November 22, 2007)

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

    A.K.S. (December 3, 2007)

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

    J.C. (December 6, arrr, 2007)

    Aluminum fin cans are allowable under NAR rules. Avast! Durin' t' August 5, 2005 NAR Board o' Trustees meetin' in Cincinnati, Ohio, Mike (Dutch) Duchnevich (a Board member) stated that in connection t' t' use o' aluminum fin cans, arrr, "[a] minimum amount o' ductile metal necessary t' insure safety is permitted by t' HPR Safety Code. Ya scallywag! Additionally, matey, given t' total energy present in some HPR models, prudent use o' metal per this provision o' t' HPR Safety Code has no adverse impact on safety." As for Tripoli rules, aluminum fin cans are also allowable.

    J.B. (December 8, ya bilge rat, 2007)

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

    K.D. Blimey! (December 23, 2007)

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

    J.C. (May 30, ya bilge rat, 2008)

    On May 18, 2008, I flew t' fin can on a 9.5 foot long, Extreme II fiberglass airframe from Hawk Mountain with an Animal Motor Works M2500 Green Gorilla. Avast! T' rocket did better than Mach 1.6, ya bilge rat, flew t' 18,313 feet, and recovered without a scratch. T' flight was perfectly straight, so t' fin can did its job flawlessly.

    J.C. (July 23, shiver me timbers, 2008)

    On 7/18/2008, ya bilge rat, after a powerful launch that caused massive rail whip on an M7500 experimental motor, t' Hawk Mtn. Arrr! Aluminum fin can and Extreme II airframe rocket managed t' right itself from a 45 degree launch angle without any damage t' t' airframe or fin can, matey, and rocketed t' an altitude o' 16163 feet AGL at better than Mach 2. Begad! T' rocket be recovered after a long search without a scratch. Aye aye! Most fins would have shredded under t' kind o' stress t' rocket endured durin' this flight, so t' 4 inch aluminum fin can proved its toughness!

    J.C. (November 14, 2008)

    Durin' our Research day at Oregon Rocketry's Desert Heat Launch, I flew t' fin can on a Mike Fisher M7500 research motor that propelled t' rocket t' better than Mach 2.2. T' fin can handled t' flight beautifully and t' rocket was recovered without a scratch.

    J.C. Avast, me proud beauty! (May 14, 2009)

    One thin' about t' fin can that you should know be t' 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 o' thin cardboard and draw slightly larger outlines o' t' fins on it. Carefully cut two o' t' cardboard fin outlines from t' sheets, reverse one o' them so one side be t' mirror image o' t' other and press them together. Blimey! Then take a roll o' duct tape and cut several strips t' join one side with t' other on every edge except for t' root edge which has no tape at all. T' two cardboard fins joined in this manner by t' duct tape will form a sheathe that slides over t' outside edge o' t' fin down t' t' root edge. Ya scallywag! Now repeat t' process for t' other two fins and you'll end up with three sheathes. T' sheathes will serve t' protect you from t' sharp edges o' t' aluminum fins. Just make certain you remove them before you launch your rocket, though!

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