The Maple Seed - Part 2: The balanced seed fins

The Thrifty Rocketeer continues...

We continue our build thread for the Maple Seed by Semroc, marketed by eRockets.biz

We've already covered the motor mount, body tube and the nosecone last installment.

Houston, we have a problem.  Most all the photos from the critical build of the seeds are gone. My computer crashed.

However, I can describe a lot of the steps through the existing photos and point out the critical error I made when it comes up.

The Instructions are clear with photos to help orient your seed wing/fin in a particular direction. Unfortunately, I can't orient my above photo right now, as my operating system has changed and flipped several photos top for bottom. (But I'll try to correct that.)

You start with one of the large pre-cut sheets in the shape of the seed wing. (I'm going to use the terms seed, fin and wing pretty interchangeably here.)

You'll note that the head of the seed is to the right, and the sail of the rest of the fin is to the left.

It's hard to make a mistake at first, because the next several pieces are small and shaped just like the seed pod.  There is a square hole in each that must either be punched out, or will arrive already empty. The base of the seed pod has a flat, square notch about an inch & a half long, that overhangs the head.  There are six of these seed pod pieces that are identical.  One gets glued on each side of the head end.  I used wood glue here as it gives you a little wiggle time to adjust. CA would grab too quickly but is light.

The seed wing not lay flat when working on it, if you chose to work both sides of one seed fin at a time. In other words, they no longer will lay flat once you've added something to the underside.

Remembering to lay the seed wing fin properly, the top edge of the wing has a nice graceful curve that is perfectly mimicked by the first turbulator. (I don't know what a turbulator is, but assume that it either affects the aerodynamics of the wing, or throws the wing off balance to make it spin.)  Of the two sets of turbulators (3 pieces each), one set has more square to the head end than the other, which is more of a Y. (The second TOP turbulator is fractionally shorter than the first, bottom one.)  BOTH sets resemble a sperm cell, which is in effect what we are building here.*

The instructions suggest using thin CA to attach these to the spine of the fin. I used wood glue the first time I did this, and thin CA the second time. A slight re-positioning is possible with both, but that thin CA grabs quick, so you need to dry fit first, and know where you are going with this. 

With wood glue, I found it was immediately necessary to place the fin under a heavy book to prevent warping as it dried.  In fact, I did this with most of each piece of the assembly of the fin, just to make sure...checking that I didn't glue the fin to the book, table or anything else. 

The next step takes some of the clay provided, and fills the square hole. If you use a razor blade, you can almost shave off the right size and quantity like slicing bread off a loaf. My clay was tan, but it makes no difference. With just a little practice, you can lay your slice down into the square and almost fill the hole right from the start. A little bit of kneading of the clay fills the square, and may leave slight bit of excess. Scrape off that excess flat by dragging the razor blade across the flat of the seed head. With a little practice, you'll squeegee the excess off without pulling the underlying clay to one side, creating a gap that must be filled again.  Practice makes perfect...  It can be done.

Upon filling the square hole with clay (for weight), we switch to the hook assembly. Now last installment, we assembled half the hook to the tube hook mount. Those were glued in line with the body tube line, with the saw tooth pointed up. 

This will engage the hook when the fins are completed. This orientation is pretty standard when launching a parasitic glider or similar booster rocket kit. Assuming you have oriented the hook properly, and that glue is drying, the only remaining half of the hook is available to glue to the bottom of the seed pod in the square notch. It is critical that this be positioned so that it is centered along the seed pod, and also with the teeth pointed toward the tail of the seed wing. This means the seed will eventually hang from the hook on the body tube.

I found I had to sand some of the sides of this basswood hook after the glue had dried, so that it was smooth and the right width to allow an outer pod cover to be added flat and flush.

After the first bottom turbulator has been glued onto the spine of the seed wing, with the head end up tight to the seed head, a cover plate is ready to be applied over the head. This will trap the clay in place like a sandwich, but also forms two side rails on either side of the basswood hook. Imaging the saw tooth steps of the seed hook like an escalator. The side walls of the escalator helps to keep the passengers in place. That's where this step is going.

Hang a seed wing onto the body tube hook mount.
Dry fit a seed pod cover on each side of the seed head, aligning all sides with the shape of the seed pod. Note that the only flat edge of the seed plate overlaps the saw teeth of the hook.  It is critical that you get the placement of these two cover plates right.  A little wiggle and adjustment will not only keep the hook teeth engaged, but also align the flat of the seed pod cover plate against the body tube.  This makes more sense as you do it.

Holding the dry cover plates in place with one hand, uncouple the fin while pinching the plates tightly in place. Thin CA is spread around the edges of the seed plate to soak in and seal the sandwich together. Be certain not to place glue on the saw teeth of the hook AND DON'T GLUE YOUR FINGERS TO THE SEED PLATE.  A few seconds to air dry and you can set this seed aside to rest your fingers.   If you've been clever, you've done this for all three fins simultaneously.  Conversely, you could assemble one seed fin at a time, as intended.

BUT HERE IS THE CRITICAL POINT WHERE I WENT WRONG:  The instructions are clear. The pics warn you to keep the orientation of the seed fin constant through all of these steps. The second turbulator lays atop the first and builds one turbulator on only one side of the wing.

The side of the seed wing that has the first turbulator glued along the spine is the TOP SIDE of the seed wing.  The backside (WITHOUT a turbulator) is the bottom face and remains against your table or work pad. IT IS CRITICAL THAT THE SECOND (shorter) TURBULATOR BE GLUED DIRECTLY ON TOP OF THE FIRST ONE for each fin.

There is NO TURBULATOR on the back side.  It remains blank.  This is where I went wrong, rushing ahead to glue the second turbulator to the wrong side on all three fins.  I didn't catch my error until all were dry. I had used thin CA for the top turbulator, and there was NO chance to pull it off and correctly position it.  I think this is a most critical error that kit builders fall for. I had to use a wood chisel to pry each one off, ruining each one.

By correctly laying the second turbulator atop the first turbulator, you'll note the Y-head nestles the curved head of the seed cover plate perfectly, and allows the second turbulator to match the curves of the first turbulator perfectly. They wind up being exactly the same length and build together to make a single unit on ONE SIDE of the wing.

After checking the dry fit of the second turbulator, apply thin CA and position over-top the first turbulator. I found by laying the piece head first, and tail last, I could just lay it down into place without any gap at the head.  A few seconds holding will allow the CA to grab.  Be careful not to glue your fingers to the balsa.

At this point, the fins are completed, with clay sandwiched inside the seed head, two cover plates holding that clay in place and shielding the saw teeth of the seed hook.  Once dry, you can hang the seed fins back on the body tube. They should grip the body tube from one direction as the rocket lifts off, but release at the slightest ejection charge in the other direction.

OK, now comes the really challenging part:  Each of the three fins must be tuned or balanced from the remainder of the clay.  

A single card  template (CG Marking Tool) is included. It fits each fin only one way, and is applied to the BOTTOM surface. It will not fit on the turbulator side.  Lay the template against the fin up snug with the seed pod head, and make a light but legible dot through the hole. This marks a critical balance point for each fin.

Remove the template.

Also mark in pencil a section of the turbulator 2.5 to 3.5 inches down the spine from the seed head.  This is the region where you apply clay.  The second region is the extreme tip of the turbulator tail.

Using a small gauge screw bolt (NOT SUPPLIED) and anchored in a mound of clay (or  held in a vice, or through an eraser), balance the first seed fin on the dot that you have marked. 

It can't be done...  The seed will not balance until you add small amounts of clay between the 2.5-3.5" range, AND at the end of the turbulator tail.  Some advice suggested adding the clay to the tail of the seed wing first, to balance head to tail before trying to balance left to right (or Turbulator to opposite edge).  Both balances are apparently critical to making the seed fin spin gently on the way back down.

It is not clear to me how this clay stays against the turbulator, except by natural stickiness of the clay, molded against the ridge of the turbulator spine.  I'm STILL working on balancing my wings.

Setting them aside for now, let us return to the body tube. By careful masking of the raw basswood  hook mounts, you can prime and paint the body tube as you like. Generally, lighter paint coats means a lighter load on lift-off and higher flights, but suit yourself.  Second, I sealed my nosecone and painted it a bright red (just cause I usually do that) separate from the body tube. Eventually, you tied the Kevlar string to the elastic shock cord and eventually to the nose cone screw-eye, attaching the parachute to the screw-eye first.

Once you're finished with the body tube and nosecone finish (use future floor coat or clear coat if you like), your fins are going to look pretty naked by comparison. I think that's intentional, because any layer of paint or sealant or coating is going to add weight and impact their performance.

Now for the adult part of the build. Kids, look away for this part.

*It occurred to me that we were building maple seed wings from the start, that had a peculiar shape to them. The smoothness of the seed pod head and the sperm-like tail of the turbulator was not lost on me.  This appearance helped me to select my color scheme for the body tube, nose cone and fins. 

I know, I just said NOT to paint the fins.  As I mentioned, I screwed up the second turbulator and had to scrape them off with a wood chisel when I discovered the error. And a scar was left where the thin CA had adhered.  As a result, I had to cut a replacement second top turbulator three times out of scrap balsa that was thin enough, and shaped enough to complete my first set of fins.  I had no confidence that I had done it right, nor that it would fly.  So I secured a second set of fins through a friend, and reassembled a virgin set of three more fins the right way, with both turbulators on the correct side.  The original scared set were available for painting.  So, I decided to play with colors and patterns, and well, I have an adult version that may never fly in public.  I have a perfect second set of naked seed fins that WILL debut in public, if this self-quarantine ever lifts.

I have been struggling with tuning both sets of these wings a lot. While a larger gauge screw bolt will give me an easier balance point to work with, I am told that is not necessary. A single pivot point supposedly is all that's necessary.... like the tip of a wood screw. I question that, as the slightest mis-balance makes my fins fall off the pivot.  So, after much practice and trial and error, I am still not sure I've got them "tuned" right.  And I want to see how critical this is in my maiden launch.

Second, I have been told that the original design called for a C11-3 motor first flight.  I can't find them commonly available.  And I will NOT pay hazmat charges for a single pack of these motors at this time.  The second motor suggested is a D12-3, which IS available but not during this quarantine.  

Here's the thing:   Someone else in the production chain added the D12-3.  It may be that a D12-3 is too much motor, but I won't know until I try one. Again, if the painted first set of fins go down in flames or bust up under too much force, I'll know.  But again, I have an extra set to practice upon.   

I want to encourage you to try this Maple Seed rocket. The designers were foresighted enough to see that replacement seed fins were going to be needed. They've produced a kit to build an additional fin: It ain't cheep, but it may be necessary for a number of reasons.  Let us hope they have included some additional clay to balance the fin.  My replacement 3-fins did not, but I had saved and salvaged enough from the original kit, that I literally had just enough for a second set of fins and some tuning of the wings (though I am certain that was never the intent.)

That's as far as I have gotten on this project by April first.  I want to assure you this is no April Fools Joke, but I think you've already figured that out. I'll add  photos as available, but my computer operating system has changed and all my other photos are not available to me right now, so this is an embarrassing end to the build thread.

PS: Remember the first and second turbulators stack on the SAME SIDE to build one complete turbulator. (I don't know if it matters which side they build upon, but they MUST be on the same side.)

Thanks for staying with me this long, and remember to let me know how your kit goes for you. Remember not to glue your fingers to the balsa, cap your glue, and always save your spare balsa.... cause I really needed it for this one!