Defender Post Flight 3X C6-7s

After a launch, I'll inspect, repair and clean up any soot.
The three friction fit C6-7 engines were pulled from the Defender right after the flight.
TIP: Hard to believe, but it's easiest to pull a friction fit engine from the engine mount tube while the engine is still warm, immediately after a flight!

For the picture, the three engines were rotated out, away from their center positions, to show the charring on the casings.
This end burning was twice what the first B6-6 engines showed. There must be a lot of central "flash" during the thrust stage of the burn.



Something else I found interesting:
The paint on the black fin trailing edges was charred.
The paint on the white fins is still like new!
(But, the white fins are a little farther away from the engines.)




The exposed decal lip, below the largest transition, is peeling back!
I'd mentioned how fast these clusters are off the pad.
While the other decals are fine, this roll decal has the upper edge exposed to the windstream right below the adapter.

Semroc Defender Finished!

COOL retro model with a three engine cluster!

Lessons learned:
Look and read ahead in the instructions. The decal hitting the upper launch lug could have been avoided by gluing the lug higher up.
If I checked the finishing instructions the payload section would have been painted aluminum before the adapters were glued together.
Double check the wrap on the decals before soaking them in water.
Two of the wraps didn't quite make it around the tubes.
The larger white fins didn't match up to the long white strips as well as I would have liked.
But, I couldn't see any other way to pre-paint and glue it together afterwards.
The old Centuri catalogs say it'll launch up to 3 oz. of payload. Considering there's less than 1/2 oz. of clay, I might fly this with B6-6 and C6-7 engines instead of the recommended B6-4 and C6-5s.

Semroc Defender Build Part 23 Decals

The checkerboard roll pattern decal intersected the upper launch lug.
There wasn't enough decal to go around the lug so it was trimmed down on one side.





Whenever I print up custom decals on my ink jet printer, I always print some extra thin trim lines. These were placed where the illustration showed.

The upper checkerboard roll pattern wasn't wide enough to meet around the payload section.





I cut a small rectangle off an old decal sheet and filled in the gap.
I figured one wide square was less noticeable than a gap between the squares.

Semroc Defender Build Part 22 Ends and Edges

Here's how the back end lined up and glue was applied at all motor mount and ST-16 contact points.





The front page of the instructions shows a color rendering of the Defender. It looked black and white to me.
But in step 28, you are told the lower payload section is painted silver.
I would have preferred to paint the tube off the model without masking. I felt it needed a third accent color.




I went with aluminum instead of the suggested silver.
Here's how it looked after the mask was removed.




The Kevlar extended 5" over the top of the main tube. I'll add a few wraps of masking tape to prevent any zippering.

24" of elastic is provided in the kit. But, with three engines and three ejection charges, I'll feel better if the shock cord was longer.
I found a 12" piece of elastic and tied it to the end of the 24" long piece.

The snap swivel hooks were pushed through the knot. This put the two 12" parachutes about 1/3 the way down from the payload section.

Semroc Defender Build Part 21 Assembly

The long strips were slipped underneath the upper body tube section and between the black tubes.

The upper end of the strip goes 1/8" under the upper tube.
Look underneath the lug and you can see the pencil mark at 1/8".


Glue was applied under the lower end of the strip.

Before it set up, the fin can was slipped over just the end of the three tubes.
Here I could check the alignment of the strip end and white fin.
Any adjustments were made before the glue could set up.

A piece of masking tape held the strip in line with the fin.

Semroc Defender Build Part 20 Assembly

Because I didn't follow the instructions I had to make some special considerations before gluing everything together.
For one, alignment of the launch lugs.
The motor mount tubes got a roll of masking tape 1 1/8" from the top. This is how far down the upper body tube will be glued.




After I made sure the lugs were in line, alignment marks were made on the tape.
Another piece of tape was added to the upper body tube for alignment.





When it was slid together and all the tick marks were in place, a glue fillet was added inside. The fillet was placed drop by drop using the rounded back end of a paint brush.

The centering ring isn't really that far down inside, it's just a camera angle illusion.

Semroc Defender Build Part 19 Fin Can Mask Part 4

Here's one of the rounded leading edges that didn't turn out as well as the others. It seems there is always one!
This is a simple fix.

TIP: You can take your knife and carefully push the black paint back onto itself!
That is - if the paint hasn't set up. Don't try this tomorrow when the paint isn't malleable.
You aren't cutting or scraping the paint off, just push the overhanging paint line back.




I did have some tearing of the paint line on one fin.
I have been using Rustoleum 2X Painter's Touch brand paint.
It takes touch-ups very well.





TIP: For years I'd spray a bit in the plastic cap and dip the brush in that. But that paint would dry in the cap and make a mess if I had to spray in the cap a second time.

Now I turn over the top of a snack size baggie and press it into a small mixing cup. Spray into the bag and paint!
When you are done, turn the edges up, zip the bag closed and throw it away.
You can use the cup again and again.

Semroc Defender Build Part 18 Fin Can Mask Part 3

Here's the complete mask on the fin can, ready for the black paint.






Before spraying, press down the edge for a better seal.

I've been using a sharpened dowel with good results.
The dowel has a blunt point. I don't want to take a chance and scar the balsa.




TIP: When removing tape, try to pull the tape down and away from the mask line.
If you were to pull straight up, you could get chips and tears.

Semroc Defender Build Part 17 Fin Can Mask Part 2

Instead of completely covering the white fins and body tube with masking tape, I use plastic grocery bags.
Here I've cut a square piece large enough to cover the fin. It's a little smaller than needed to cover the body tube. This last uncovered edge is for the masking tape.






A strip of masking tape is set on one edge of the plastic square -
half on, half off.








The sticky exposed tape is set over the Scotch tape already on the body tube.

Semroc Defender Build Part 16 Fin Can Mask Part 1

Clear Scotch tape was set on my glass patio door.
Since the fin can is already painted white, I wanted the edges of my clear tape easy to see.
A permanent marker was used to draw a line down the middle of the tape strips. Cutting down the middle also gives you two masking pieces and a clean edge.

A sharp knife and straight edge cut through the middle of the thick, marked line.


I didn't want a square top around the rounded leading edge.
Here's one way to get the rounded mask.

Small pieces of tape are set around the leading edge tip. There is four small pieces around each side and a final piece over the tip.




The longer tape strips go down the fin sides.

I had used the Titebond M & TG for fillets. I wanted the color separation right down the middle of the rounded fillet.
With the tape edge black, it's easier to see the edge and judge how well it's centered.

Semroc Defender Build Part 15 Upper Mask

Here's the entire upper section of the rocket in gray primer.
This half is an easy mask, most of the color is in the roll pattern decals.





The uppermost section is painted black, halfway between the 3" long ST-5 tube to the top of the nose cone.

This is the mask, Scotch tape at the color separation line, masking tape and white paper behind that.




With that mask complete, you start to see how the upper section will look finished.

Semroc Defender Build Part 14 Nose Weight

I looked up the Centuri Defender instructions at:
http://www.oldrocketplans.com/

Here's step 11 from the instructions.
With three C6-5 engines, the recommended nose weight is .27 oz.





Here's the clay weight from the Semroc kit.
The clay square weighs in at .55 oz.





In order to fit the clay into the ST-5 tube at the top, you have to roll it into a round cylinder.

Take into account the shoulder length from the lower adapter and the nose cone shoulder. All of the supplied clay won't fit into the small ST-5 top section. I cut off the excess.
The cut clay weighed .40 oz. That's more than the Centuri instructions call for, but with three engines under it, the extra weight shouldn't matter.

Semroc Defender Build Part 13 Painting The Fin Can

I had started spraying primer and noticed there was no launch lug!

It was glued next to a larger fin and filleted with the Titebond M & TG.





This fin can assembly will be painted off the model, slid on and glued afterwards.

I didn't have anything of a larger diameter to make a painting wand, so masking tape was wrapped around a casing, sticky side out
This was pressed inside the fin can wall.




Masking tape was set over the body tube edges and up under the larger fin root edge extensions.

It'll be important to keep the paint off all gluing surfaces.

Semroc Defender Build Part 12 Large Fin Gluing

As the root edges were individually fitted, the body tube and fins were numbered in pencil.

Note the smaller three fins (that don't overhang the front) were glued on earlier.




Here's a good fit of a fin on the inside strips.

The strips are not glued in place yet, they are friction fit held in place by the lower ring.
Look from the rear to be sure they are sticking straight out away from the engine mount tubes before gluing the fins on the lower tube.



With the larger fins glued to the lower tube, you start to get a good idea of how the finished model will look.

The root edge of the larger fin overhang is not yet glued to the inside strip. It's only glued to the lower tube along the lower root edge.

Semroc Defender Build Part 11 Fitting Fin Strips

When setting in the strips, be sure they stick out straight away from the center line of the model.






This is how the larger fin will sit against the inside strip.

There's a small step in the root edge that overhangs the front edge of the tube and contacts the long strip.
I'll have to sand the two root edge contact points for the best fit.

Semroc Defender Build Part 10 Fitting Fin Strips

The three smaller fins were glued into position on the shorter ST-16.
This fin can assembly was dry slid (no glue yet) over the three engine mount tubes. In the first picture you can see the fins strips are taller than needed.



The bottom edge of the strips was sanded down using 220 grit sandpaper on a block. They were slid in place under the top of the ST-16 fin can assembly from the front, not the back.


The strip in front shows the height after sanding down the root edge for a better fit.
Behind it is a strip at the original height. You can see how much was taken off the fit inside the larger fin can tube.

Semroc Defender Build Part 9 Fin Strips

The fin strips are white and go between the three internal engine mount tubes. I would guess the Centuri designers were trying for a Saturn 1B look.

These fin strips will be painted white before assembly and slid between the shorter ST-16 tube at the bottom and the longer ST-16 tube at the top.

The picture at the right shows the position of the strips without the ST-16 segments over them. I set them down this way to mark for the mask.
The strips fit about 1/8" under the upper, longer ST-16 tube. the top and bottom of the strips are masked off for a better gluing surface.

Here's the masked strips set on end for the white spray.
I haven't sanded the strips height to fit under the ST-16 tubes yet.
When they are ready to slide into place, I'll just sand down the bottoms (the root edge that faces in) until I get a good fit.

Semroc Defender Build Part 8 Centering Ring and Kevlar

I found it easier to center and line up the CT-KV-60 centering ring with pencil tick marks on the ring and body tubes below it.

It's very easy to glue this ring to one side so use some reference marks to keep it centered.



White glue was used to fill in the gaps at the top of the ring and body tube joints.

The Kevlar is knotted and fed through the gaps between the tubes. After gluing the gaps closed, I had to open a hole with an awl.






Just to be sure, I set the knotted Kevlar in some epoxy down the joint between the two tubes.