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6.13 Rocketry: How can I prevent balsa fins from breaking off on landing (especially for models with swept fins)?




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This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.

6.13 Rocketry: How can I prevent balsa fins from breaking off on landing (especially for models with swept fins)?

From: craddock1@aol.com (Bob Craddock)
Take your fin pattern, reduce it by ~90% on a xerox machine, and make
as many copies as you need to glue one pattern on both sides of each
fin. Put about two coats of sanding sealer on the new paper surface,
sand, and then paint the fins all over again. A friend of mine was
having the exact same trouble on his Super Big Bertha, and the paper
reinforcement was his solution. It worked great, but next time I
say use bass wood on everything.

From: silent1@ix.netcom.com (The Silent Observer)
There's a variation on this technique, that needs to be applied during
building, that can make balsa fins stronger than bass (and still
lighter). What you need to do is simply to cover the fins before painting.
I used silk tissue (like model airplane tissue made from silk fibers)
on my Big Bertha, and in a dozen flights (before it lodged high in a
tree) never had so much as a crack, even when flown on a D21 (and
including one "plastic wad" recovery when the rocket hit the ground
fairly hard). You could use ordinary Japanese tissue, or Silkspan (R),
or you could even use something like nylon cloth or very light
fiberglass (attached with epoxy or CA in this case). With tissue, you
need to cover the entire surface -- I simply wrapped it over the rounded
leading edge, and trimmed it off at the tapered trailing edge, leaving
the square "bottom" edge and the root uncovered. You can attach
Silkspan with almost any glue, but silk tissue (as I found) "fuzzes"
if you get it damp and handle it, so something like Testor's model
airplane glue or thick, clear nitrate dope might be a better choice; it
won't soften the binder that holds the fibers in the tissue together.
Any of these, done after sanding (and filling, in the tissue cases)
will add significantly to the strength of the fin, while adding very
little weight. Making fins out of basswood or ply is probably okay with
a Bertha derivative -- they tend to be overstable in any case -- but may
lead to an unstable model if you have a design with less margin.

From: dbucher@mcn.org (David Bucher)
There are two things you can do, both of which lower the rocket
in a "fin up" attitude. The first works by making a "yoke" or
harness for lowering the rocket body horzontally ( if you choose).
Install an anchor (screw eye, inch worm shaped brass wire clip, etc.)
through the body tube wall between the fins at the rear end of the body.
Attach a squid line or kevlar thread to the anchor and run it up the
outside of the body (tightly) and attach to the nose cone or 'chute.
Configure it to lower rocket as above.

The other (and better!) way for the rocket you describe is to use rear
ejection. This will not help you with the present rocket, but any other
rocket with sufficient body width will work just fine. When making the
motor mount assembly. substitute a longer motor tube (29mm LOC
tube for instance) and make up some ply or G10 centering rings including
two with a fair spread between where you can wrap the 'chute around
the motor tube. Install a solid bulkhead with cable lanyard to serve
as a thrust ring and pressure block. Make sure the motor mount unit
slides well in the body and attach elastic to the cable lanyard and now
you've got a rocket that ejects to the rear. Just cut a small notch
in the farthest foward centering rings to allow the shock cord to pass.
This method works great and if you're confused by what I just wrote (a
not unheard of possibility!) just think of the internal "power pod" in
some BGs. It works the same way except you must make provision to conn-
ect ALL parts together.

 

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previous page: 6.12 Rocketry: I've built several BT-80 based models. How can I strengthen future models to take larger motors, such as Aerotech 24mm or 29mm reloads?
  
page up: Model Rockets FAQ
  
next page: 6.14 Rocketry: I just lost my favorite rocket and the kit is discontinued. How can I make another one just like it?