Click here for Part 1.
The three-motor cluster mount of the FlisKits Tres is canted, which means that, rather than pointing straight up through the rocket's body tube, the engine tubes stick out at an angle. Because of this cant, the tubes have to protrude through the body tube a little bit - you can't just slide the mount in like you would with a standard model rocket motor tube and centering rings.
This means you have to do some cutting through the body tube, and here's the part that might make a newcomer to model rocketry a bit nervous. You have to make some cuts with a hobby knife, and you have to do it freehand, so there's a possibility of making a critical mistake.
But it's not to worry. Unless you're extremely good with a hobby knife, the cuts will not be perfect, and even if they are, you'll probably have to do some cleaning up and fitting anyway. Go slowly, and even if the cuts are somewhat messy, they can be fixed. That's not to say you couldn't mess up and cut too far if you're not careful. Again, go slowly. You can do this.
On the card stock template sheet included with the kit are two fin marking guides. The "Main Fin Marking Guide" includes marks not only for the fin placement, but also for the motor tube cutouts.
On this rocket, the fins are evenly spaced between the canted motor tubes. Cut out this guide as you would with any fin marking guide.
On the guide are areas that will need to be taped to the rocket to keep the guide from moving. You'll be cutting right through this guide, and it's important that it not slide around. The guide gets wrapped around the BT-60 body tube and placed right at the end of the tube. Then put tape in the spots indicated on the guide.
It's important to be careful when taping. You're not only taping the guide to the outside of the rocket; on the bottom the tape will wrap around to the inside of the tube. You don't want to rip the tube when you take the guide off. So, either use a low tack tape, or stick the tape on your forehead or pants leg before taping it to the rocket. That will make the tape less sticky and you can remove it without pulling off any layers of paper from the tube.
Make pencil point marks where the arrows indicate for your fin lines. Then, you begin cutting.
What you want to do here is go slowly. Trace the cutout with your hobby knife. The goal is to cut through the guide, and lightly score the tube - so you are in fact cutting the tube, but you don't want to try and cut all the way through at this point.
Trace that line slowly, especially at the top end where it curves sharply. Don't expect it to be perfect - it almost certainly will not be, and that's OK. Try to keep your knife on the line, or as close to the inside of the line as possible.
When you're done cutting, it will look like this:
Remove the guide from the tube, taking care not to tear anything as you lift the tape.
Before continuing to cut, use an aluminum angle or door jamb to extend your fin marks all the way up the tube. It's best to do it now, because once the cutouts are removed, the tube end won't be as strong, and you may have a hard time drawing a straight line on the tube without the tube distorting.
Then, carefully go over the score marks to cut out the engine cutout area completely. Go slowly, don't press too hard, and just do your best to retrace the score marks you've already made. If you press too hard, you may find your knife suddenly cuts through and you end up with a slash mark all the way up the tube!
Once you've cut everything out, it will probably look pretty jagged.
Again, this is fine. You're going to have to clean it up anyway.
At this point, take your finished motor mount and test fit it into the body tube with the motor tubes in the cutouts. When you install the motor mount, the custom centering ring - that black disc with cutouts - should fit just inside the body tube and be flush with the end.
But, most likely, at this point, you won't even be able to fit the mount all the way into the tube. Your cutouts will be too narrow, and the motor tubes won't pass far enough into the tube. I suspect that even if you cut exactly on the lines of the cutting guide, the slots may still be too narrow.
Here's where you do your cleanup. You'll need a bit of sandpaper (probably about 220 grit will be just right) and a used rocket motor - one of the standard, 18mm A/B/C ones. Wrap the sandpaper around the motor casing, and use that as a tool to sand where you need. If you find it too awkward to hold, you can glue the sandpaper on.
I use self-adhesive sandpaper, so I just stuck that onto my used motor casing.
Test fit the mount into the body tube, and start by sanding away any edges which are too narrow and block the engine tubes from passing far enough into the rocket. You should be able to slide the motor mount in closer and closer to flush with each test fit.
When I first posted Part 1 of this build, there was something I forgot to mention. I've since gone back and edited it in. But I didn't think to do it when I was building, so that's why I forgot about it. Check to see that the centering ring actually fits into the body tube before building the motor mount!
Now, if, like me, you didn't do that, you may find, like I did, that the fit is too tight. In my case, the centering ring was just a little bigger than the inside diameter of the tube. You see this all the time in kits, and if your centering ring doesn't fit inside the body tube, just sand the edges lightly until it does.
If you've already built the motor mount, and the centering ring won't fit, you can still sand the edges with an emery board or sanding stick. It's not as easy as sanding with a bit of sandpaper before gluing everything together, but it's not too tedious.
When the motor mount fits and the shape of the cutouts is right, and the edges are relatively clean, you're supposed to sand a bevel on the inside forward edge of the cutouts with your motor casing, to give a better surface for gluing.
It should look something like this:
When everything looks like it dry fits correctly, you're ready to glue the motor mount into the rocket.
Put a thin bead of glue around the inside edge of the body tube where the centering ring will fit flush in the end, and a thin bead of glue on those beveled edges inside the tube where the motor tubes will butt up against them. Install the motor mount, and carefully tape all the edges of the body tube to the bottom of the centering ring.
This will ensure that the edges of the tube adhere to the centering ring, and that the motor mount doesn't push its way back out before the glue dries (a tight fit can make the assembly act a little springy).
Carefully remove the tape when the glue is dry. The mount should be solidly in place. You can pat yourself on the back. The hardest part of this build is complete.
You may notice there are gaps between the motor tubes and the BT-60 body tube. Not to worry - we're going to take care of that next.
Click here for Part 3.
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