MX-774 Kitbash Part 14 Fins On

Here's the fins, in place viewed from the rear.

I used the fin alignment guide I made in an earlier post.

And the view from the side:

Here's the side tunnels. One goes on each side, at the top where the leading edge meets the body tube.

Embarrasing Picture - OOPS! - not Out Of Production

Maybe I should be more careful of how I hold my rockets!

Photo credit goes to Roger Smith for capturing the moment.

MX-774 Kitbash Part 13 Fin Position Marking

I cut the fin postitioning guide from the Estes Bull Pup kit instructions.

Look how poorly it fit. There is almost a 1/16" gap where the guide ends should match up.

I decided to make my own guide.

I wrapped a piece of 20 lb. copy paper around the tube and taped it closed where it overlapped.

The overlap point was marked with a pencil. That will be the first fin line.

I slipped the new guide off and carefully folded it in two.

At the creases I marked the folds with a pencil.

Using those two marks it was folded again into fourths.

Those new creases were marked with a pencil line.

The guide was slipped over the body tube and marked for the fins.

MX-774 Kitbash Part 12 Servo Pods

All the Servo Pods had to have "wedge cut" ends.

I found it easier to make the wedge on the open side first.

Look closely at the picture, you can see the side opposite the blade already had the angle cut made. It's easier to hold sand the first angle when it's still on the long strip.

It's cut to length, then the second side has the angle sanded. You could cut this wedge with a knife, I found it easier to sand the angle with a block.


Here's the servo pods in place on the fins.

MX-774 Kitbash Part 11 Fin Patterns, Tunnels and Servo Pods

To answer some detail questions, I used a combination of Mr. Wilfong's Kitbash and the old Centuri instructions.

In upscaling this model, I had to multiply everything by a factor of 1.456 to get the correct larger sizes.

At the top is my drawing of the Centuri kit size and Mr. Wilfong's kitbash fins, upscaled for the BT-55. I scaled up the Servo Pods - the small rectangles on the lower right end of the fin drawing.

Below are the Centuri instructions, showing a taper on the ends of the servo pods and the body tunnel strips.

Patreon members: If you'd like a printable PDF of the fin drawings,
Email me at: oddlrockets@bellsouth.net. Ask for the V2 Fin PDF.

Centuri had you cut 1/16" basswood strips to 3 1/4" long for the body tunnels. Multiply that by 1.456 and the new upscale tunnel length is 4 73".

I ended up cutting my servo pods to .6" long. The correct .75" looked way to big when compared to the Centuri drawings. I can play a little, This is sport scale model, not true scale.

I cut the strips from a popsicle stick, I didn't have any 1/16" basswood handy.

All the strips were sanded square and sealed with two coats of varnish, sanding between coats.

Simple Shock Cord Install Tool

This tool is so simple it almost seems ridiculous to post it.

Over the past few months I've been burning through a surplus of Micro Maxx engines. Some MMX rockets have minimum diameter tubing, too small to get my fingers in when trying to pack the Kevlar shock cords.

I've used pencils before, trying to get the floppy Kevlar line back into the tube.

That little notch at the top of the pictured dowel makes all the difference. It gives you just enough grip to position the Kevlar over the top.

1. Loosely roll the Kevlar over a finger.

2. Remove the loops, holding them together in a coil.

3. Lay the loops over the top of the body tube.

4. Set the notched dowel over the Kevlar. The notch keeps the loops in position.

5. Press the loops into the body tube. Press in until the nose cone is drawn up to the top of the tube.

MX-774 Kitbash Post 10 Tying Shock Cord

Many already know this technique, but for some who may not know it:

Since Kevlar was used in rocket building, zippers have occurred. If the rocket is traveling too fast at ejection (too short or too long a delay) the Kevlar cord can can cut through the body tube. Kraft tubing is no match for the 5X stronger than steel Kevlar.

One solution is to tie the elastic shock cord to the Kevlar BELOW the top of the body tube.

Picture 1: Feed the Kevlar out the top of the tube opposite the engine mount. Just below the lip of the tube, mark the Kevlar dark enough so you can find the mark later.

Picture 2: Feed the marked Kevlar back through to the rear of the rocket body, all the way through the engine mount. Tie a tight loop knot with the mark at the center of the loop. You can see the mark in the center loop bottom.

Picture 3: Tie the Elastic Shock Cord to this loop knot in the Kevlar. Be sure both knots are tight and apply a small drop of glue to the knots to set them. Feed Both tied cords back through the mount and out the top of the body tube. Remember the post about the long tweezers?

Picture 4: Here's the "Zipper Proof" shock cord out the front of the body tube. Both the Kevlar and elastic shock cord knots are below the top of the main body tube.