Hinged or Mechanical Fins.

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Well-Known Member
Jul 25, 2009
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I have been thinking alot on this concept since I read this thread.


Have any of you built a rocket with hinged or mechanical fins and what
building techniques and materials did you use?

My thoughts are 2 fold on this...

First, I like what Jim did to hide his second stage fins prior to staging
separation. That would be fun to do in either a multistage scale or scratch

Second, would be to do that with a Harpoon and launch it out of a Canister
(tube) launcher similar to what we had on my Spruance class destroyer back
in the day. All 3 fins folded down for insertion into the launcher and would
snap out on leaving the tube. Cool to see in slow motion video.

Not to mention I have a TLP Harpoon on the bench just begging to be
Well, for your Harpoon, you could do it the ugly, simple way, with elastic on the outside (kinda like that Saturn example referenced in the other post)

Or you could be clever (and complicated) and try to rig a coil-spring-driven deployment, maybe even hidden inside a fin bump, just like on the full-scale version

Either way, for model rocket applications, a hinge is going to leave your rocket with a weak point (or three...or four). These fins are prone to breakage and misalignment, and can be a lot of "headache" to maintain and keep in good operating trim.

The fin deployment force is going to need to be something substantial enough to hold the fin firmly in the extended position, so it doesn't fold back down if the rocket tries to fly sideways. That means the folded fin needs to have some sort of "downlock" to hold it folded, so that it doesn't press against the inside of any possible launch tube, and drag fin tip against tube, and slow the rocket or jamb it inside the tube. Then the downlock would have to be released as the rocket clears the launch tube (lanyard? pull-pin?) If you use a "scale" launch tube, will the rocket be moving fast enough when it clears the tube to be up to "safe" flying speed? (fast enough for fins to be aerodynamically effective?) Last but not least, is the RSO going to let you launch if you don't have a back-up plan to keep your model rocket stable if the fins don't deploy?

Ya know, it might not be "perfect scale" but you could launch your model rocket with fixed fins from inside a slightly larger diameter tube...
Several of the international FAI flyers have used custom formed spring activated fins with a very thin mylar or film PETG as the actual Hinge. Hiding the attachment points inside the previous stage. Not sure what that would look like on a Harpoon.