Snap Dragon
Scratch-Built Model Rocket

Part 4:   Fins


A strip of modeling hardwood is selected.  It is 1.5 inches wide by 3/32 inch thick.  The best wood is maple.  It is hard and strong.  Next best might be basswood.  Then perhaps poplar.  If you can't get any of these, balsa will do.  It is easy to work, but not nearly as strong.  

One foot length is needed to make four fins.  You don't have to use four fins, three are enough for good stability.  If you are using balsa, a spare fin is a good thing to have.  And if you make three rockets, you have enough leftover fins for a fourth!  No extra charge.


Mark the strip at the halfway point.  It will be cut into into two 6-inch sections shortly.

Mark a point one inch from each end, and one inch from the centerline on the opposite side as shown:

Connect the one-inch points with diagonal lines.  These are the cut lines.  

They are also the "root edge" which will be glued to the rocket body tube.


  One good thing about balsa - it can be cut with a sharp knife.  

So can this basswood, but it takes several heavy strokes.  

Hey!  Watch that knife!


Isn't there a better term for what I am calling the "Adjacent Edge?"  Please inform!

Fin Conformity

As you can see, the fins are not cut uniformly.  And the root edge is not flat.  That must be fixed, as the edge must mate well with the body tube to glue well, and it must be squared with the flat so the fin will stand up straight..

The fix is a bit labor-intensive without power tools, but doable.  I am the proud owner of a cheap little belt sander which does this in a jiffy.  But it can be done quite well with just a sheet or two of sandpaper, a strong grip, and a few minutes of reciprocal motion.

First, align the top edges and the 90-degree adjacent sides as closely as possible.  This will minimize the amount of sanding you have to do.  Pinch them together and hold them tightly so that they do not shift in relation to each other.  If you have some small clamps, this might be a good time to use them.

 

Lay a piece of fine sandpaper sandpaper on a flat surface, and rub the root edge of the fin group on it until they are all flat and even.  Balsa won't take long.  Hardwood might take a few minutes.

I'm using 150-grit here, but other sizes will work too.

If the fins slip out of line, take a moment to get the top edges lined up again before sanding any more.

Here they are all sanded pretty much flush on the root edge.  Might want to check and make sure they haven't slipped, though....

Do the same thing for the trailing edges, and the adjacent edges connecting the trailing edges and the root edges.  

Having been sanded as a unit, they should all now be pretty much the same size and shape.

Now work on one at a time.  Use the sandpaper to round all the edges except the root edge of each fin.  

Here I simply rub the fin against the sandpaper laying on the counter, and change the angle to make a rounded edge.

Be sure to leave the root edge flat.  Do not round it.  
The root edge needs to stay flat to maximize contact surface with the body tube when it is glued.  

If you wish, the trailing edge may be tapered to a thin edge.  This will make for slightly better aerodynamics.  

Draw it at an angle on the sandpaper held on flat surface.

Attaching the Fins

  I found this steel angle iron at Home Depot, cost was only a dollar or so for 3 feet.  I can't resist any thing that is both cheap and sturdy, so grabbed one.  Bad idea.  It had a garbled bar-code tag.  Homer didn't wait around long enough for me to tell him where it came from, and apparently had to search the whole store.  I waited about 20 minutes while he tried to find it.  

But it was worth it.  This bar is just about the right size for spacing four fins around this body tube.  So I cut it into 1-foot sections.  Rubber-banded to the body tube, it makes for accurate fin alignment, much better than eyeballing it, or using a drawn line.  

Now that the aligner is in place, I can glue a fin.  A thin bead of Elmer's is applied to the fin root.  Actually, it was a thick bead, but a finger removes most of it and spreads the rest evenly.  

The iron channel is used both for spacing and for fin alignment.  Thus the only thing I need to worry about is for/aft fin placement, and a pencil mark where the tip of the fin should go makes that pretty easy.

Sometimes I apply candle wax to the iron to keep glue from sticking to it.  It helps.

Hey, look!  I have two of these channels!  Isn't that cool.  

As it turns out, the iron channel is much better for alignment than for placement.  Since the thickness of the fins varies somewhat and so does the diameter of the body tube, a distance that is right for one airframe will be wrong for another.  

In this case, it's OK, it will fly stably.  I could even call it an intentional "X-Fin" design.  But it isn't.  

This is a very simple fin design.  It is elegant in that it is very easy to cut and shape, does not waste material, and creates a very stable airframe. It is also well adapted to the "paint job cheat" technique I will describe shortly.

But other designs are certainly possible with this method.   Please let me know if you come up with any really good ones!


Launch Lug.  Also made from scratch.
Scratch paper, that is.  I cut a piece of plain copy paper 2 inches wide, 3 inches long, and pre-roll it on a short section of 1/8 inch brass rod.  

A dollop of Elmer's is spread by finger....

...and the paper rolled around the rod, loosely.  


No doubt there is a precise way to do it, but this happens to work just fine.

Once it has dried, a long, sharp knife is rolled over the tube which is supported by the brass rod, allowing a nice, clean cut to be made.

Two sections of lug tube are cut, each about 3/4 inch long.

The first is glued adjacent to a fin.  This gives it good reinforcement, as well as alignment.  
After the first lug has set... 30 minutes or so... the second one can be glued on using a bamboo skewer to make sure they are aligned.  Sometimes I use a rubber-band to hold the skewer down while the glue sets.  In this case I just held it for a minute then gently withdrew the skewer and didn't touch it for awhile.  Elmer's sets fast on porous stuff, as long as you don't use too much.  

Thanks to Flint Hazpirat for suggesting the use of drinking straws for the launch lug.  That would certainly save a little time.  Here, such straws are usually made out of plastic, so Elmer's is not an appropriate glue.  So far the hot-melt glue from my glue-gun has worked the best, as Flint also suggested..


Next:  Nose Cone!  No lathe required.

Jimmy Yawn

Recrystallized Rocketry

rev. 12/18/05