Overview

The design for this fin can uses 4 fins, with a phenolic tube overwrapped with carbon fiber as the base and ⅛” G10 fin cores with 12 layers of tip-to-tip carbon fiber on each fin.

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Below are pictures of the Hermes flight fin can. 

Manufacturing

Starting with the base, also referred to as the fin collar.  We used a 22in long phenolic airframe tube with a 6.007” ID, the next size up is 7”.  While both you and me look at this and think, “Oh that will fit over the 6” OD motor case perfectly!”, think again.  The motor case was conveniently 6.4ish” OD and while looks were deceiving, it was well out of the sand-to-fit range. So to create a fin collar which fits the motor case the phenolic was cut, slid over the motor case, using it as a mandrel and gluing it back together with 5-minute epoxy. Tips from experience: don’t use slow cure epoxy, don’t tighten the tube around the mandrel too much and use a somewhat stiff release film.  At this point you may be asking yourself why we even bothered jumping through all of these hoops some silly phenolic tube. Fear not, I asked myself this same question while spending hours using percussive maintenance to unsuccessfully remove a ratchet-tightened tube from the mandrel. However, since the motor case is structural and the fin can which normally melts at 300F sits directly on the aft end of that case, we need some thermal protection and so we used phenolic, an excellent thermal insulator.  So once we had the phenolic tube at the appropriate size, it was ready for overwrap, which was far less dramatic as most overwrap issues were related to the phenolic cutting and re-gluing nonsense. Three wraps of 2x2 5.7oz twill weave carbon fiber was used (plain weave works just as well). A fair amount of extra carbon was used for this because an extra inch or two in the front of the 16” long fins is needed as well as extra length to make clean cuts on the front and back of the fin can, ie. if the CF fabric ends at the edge of the fin can, you will have a messy edge.  We heat shrinked this part when we let it cure because being smooth was the priority, wrinkles would be sad. Also peel ply is one of your best friends for all the layups in this fin can.

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