Estes Mini Bertha Clone, #TK-3, Part 12, Finished

This was a fun little build. My original 1970s fleet had quite a few Mini-Brutes and this was a favorite.

The double layer decals got me close to the full black color density on the nose cone.

I tried to clean up the "bug" decal. In the original art the lettering was thicker and a bit hard to read.

This one represents the 1970s Estes décor. Many catalog  rockets then were only two colors, one fin and the nosecone the same color.

Estes Mini Bertha Clone, #TK-3, Part 11, Two-Layer Decals

This was one of those home print decals that didn't give a good opaque coverage. With the nose cone in place you can really tell the difference between the two blacks.

On the left is a single layer decal, on the right is a two layer. There is a big difference. Sometimes home print decals don't cover over white paint.

Lay down the first decal and let fully dry overnight.

To apply the second decal - 

Soak the decal, wet the first decal already set on the tube. The surface is wetted down so the second overlay decal can easily slide into position.

As you are positioning the second overlay decal, simply line up the ink lines on the decal underneath. It's easier than it sounds.

I only had to double the two wrap around decals. The two Mini-Brute bugs looked fine.

Here's the shock cord ties.

The nose cone screw eye got the Duncan Uni-Knot.

The Kevlar / Elastic shock cord got interlocking overhand knots.

There was some clear decal overhanging the top of the body tube. When the decal is completely dry, you can shave off the edge with a razor blade.

Use a "sawing" back and forth motion when cutting off the clear overhang.

Apply some clear acrylic Future on the exposed edge so it won't peel back. I don't overall clear coat models, just decal edges like this. 

The tube edge got some Sharpie to blacken up the white paint.

 

Estes Mini Bertha Clone, #TK-3, Part 10, Black Fin & Nose Cone

The single black fin was spray painted black.

The nose cone was painted black separately, off the model.

Note the masking tape doesn't completely cover the shoulder. The edge of the tape is a little below the shoulder lip. The gives you a little paint below the joint. You won't see balsa peeking out if the nose cone isn't completely seated into the body tube.

I drew up some decals based on the old catalog page graphics.

These were home printed on clear decal paper.

PATREON - The Mini Bertha decal PDF is only available to Patreon supporters. Email me at oddlrockets@bellsouth.net and ask for the Mini Bertha PDF.

The fit was good, the decal start and stop overlap was set in line with the launch lug.

The Mini-Brute "bugs" were set on the body tube and both sides of one white fin.

Estes Mini Bertha Clone, #TK-3, Part 7, Gluing On The Fins With No Wobble!

Well, I did it again! This is a missing, out of order post, 

It's an important one, showing how to get rid of standing wobble of four finned models. 

Here's post #7. I'll put them into correct order in a few days.

 

Extend the fin position lines under and onto the centering ring.

I also mark the intersection of the root edge and trailing edge on the fin itself. It gives me another alignment point.

I've scraped the glue lines for the four fins and the launch lug.

After scraping the pencil alignment lines are redrawn.

TIP: Four fin rockets can wobble when standing up on the fin tips. To fix this - 

Glue on three fins. The model might stand up by itself now, but can tip over.

Apply glue onto the root edge of the fourth fin.

Set the root edge on the pencil line.

While the fourth fin glue is still wet - You can slide the fourth fin up or down. 

Set the fin and check by standing the model on a tabletop. 

Pay attention to the last glued, fourth fin position. By making slight up and down movement on the fin down the alignment line you should find the spot where the model will stand on all four fins without wobbling.

Estes Mini Bertha Clone, #TK-3, Part 9, White Paint & Masking

Here's the model after the first white coat.

This is an easy mask, the body is white, the nose cone and one fin are black.

There are always glue fillet ridges.

Here's the same fillet area after sanding.

The single fin is painted black.

This shows the Scotch tape masks.

These are followed with brown masking tape covering the larger areas. A plastic bag covered the upper tube.

Before spraying the black, the edges are soft burnished with a Q-tip.

Estes Mini Bertha Clone, #TK-3, Part 8, Prep For Paint

Here's an old school idea:

TIP: On a four fin model, look across the trailing edges to check the fin alignment. 

While they didn't wobble,

The trailing edge tips weren't perfectly flat. It took very little 400 grit sanding to square them up.

If I had sanded through into the raw balsa, sometimes you can rub some wood glue into the small trailing edge to reseal it. Keep the glue only on the trailing edge.

You can only use this glue seal on small areas like the tip of this fin. Follow with some light sanding.

 

The model is ready for white paint.

Look close to see the masking tape pieces out the back of the engine mount tube, sticky side out. This keeps the paint out. A dowel was slipped in for a painting wand.

To get full paint coverage, the nose cone is pulled out about 1/16". This puts paint over the shoulder lip so you won't see a white line if the nose cone isn't fully seated in the tube.

Estes Mini Bertha Clone, #TK-3, Part 6, Cutting Out The Fins

I had to search to find some 1/16" thick balsa. 1/16" thick balsa is rarely used in rocket kits anymore. You can still see the SIG logo in purple ink on one fin.

Cutting them out by hand makes some small inconsistencies. On the left the four fins are squared up. 

On the right side, the fins have been stacked and the sides sanded even. 

Current laser cut fins are uniform, you don't need to gang sand them. But, you should still sand off the black ash on the fin edges.

Here's another view next to the fin pattern from Jim Z's.

The fins got one coat of CWF and sanding.

Gray filler/primer followed.

On the right side of the picture the fin was sanded with 400 grit. I tend to sand most of the filler/primer off leaving some in any remaining balsa grain.

The printed fin marking guide didn't quite match up after rolling around the tube.

I went to payloadbay.com and printed out one of their fin marking guides: CLICK HERE

Estes Mini Bertha Clone, #TK-3, Part 5, Odds & Ends

Before marking and gluing on the fins - 

I fill the gap between the outside of the centering ring and the edge of the tube.

Apply some CWF around the outside of the joint. Let dry and sand. You can sand and work around the engine hook with a sanding block.

The upper inside edge gets a wipe of CA glue. 

After drying, the glue gets a light sanding with 400 grit wrapped around a finger or dowel.

The nose cone weight is attached to the nose cone.

NEWBIES: Don't just blindly start the hole with the screw eye. If you are off center, that washer might come in contact with the inside walls of the body tube.

Mark the center with a pencil point. Screw the screw eye in first, going in about 3/4 of the threads. Remove the screw eye and squirt some glue in the hole. You know the drill.

Place the washer weight on the screw eye and reinsert the screw eye, this time screwed all the way in. 

Estes Mini Bertha Clone, #TK-3, Part 4, Slightly Longer Engine Mount?

An engine block wasn't included in the original kit.

I added one, glued in so it's flush with the upper centering ring and body tube edge.

Here's why that engine hook was designed to hang 1/2" beyond the bottom of the mount!

There isn't much clearance and flex between the BT-5 and BT-20 tubes. The low bend of the engine hook scrapes along the outside of of an engine casing when slid into the mount.

It's not impossible to get the engine in the mount, you'll just have some friction when being slid into place.

I pressed the mount into place with the flat face of my sanding block. The Kevlar was fed out the back and layed to the side while pressing the mount into the tube.

The mount is certainly useable with the shorter overhang hook. 

TIP: This is a BMS 1 3/4" engine hook. The cut off ends were a little rough. I smoothed out the edges with a few passes of a small, flat diamond file. The engine slides in easier now.

With the Bertha's fin trailing edges in the way it takes a moment to work an mini engine into the mount.

I'm sticking with the modification I made.

Estes Mini Bertha Clone, #TK-3, Part 3, Slightly Longer Engine Mount?

Here's a second dry fit engine mount using a 1/4" longer BT-5 engine mount tube.

With the BT-5 tube now at 1 3/4" long you end up with the engine hook extension only 1/4" out the back.

NEWBIES: I don't recommend using tri-fold shock cord mounts in smaller BT-5 and BT-20 diameter tubes. The bulky triple fold paper and double fold of rubber can block a chute or streamer ejection.

I substituted a Kevlar line.

The Kevlar line knot loop goes under the upper bend of the engine hook, Semroc style.

Like in previous builds,

The flat face of the exposed lower centering ring will be seen, rough layer wraps and all.

Rub in some white or wood glue and let dry. Sand with 400 grit. On the finished model that flat ring face will be sealed, smooth and shiny.

To hold the split ring closed while the glue dries - 

Slide the lower ring dry into the open end of the BT-20. 

Here you also see the cardstock hook retaining wrap between the two centering rings.
 

Estes Mini Bertha Clone, #TK-3, Part 2, Short Engine Mount?

Here's the engine mount drawing and some of the parts list.

The kit engine mount tube was a BT-ST, at only 1 1/2" long. With the engine hook slit 1/4" from the top of the tube, the engine hook and engine will extend 1/2" out the rear. This seems long to me. I found out later why it is longer.

The upcoming snap-together Min Bertha kit has a new screw on engine retainer.

A dry fit of the 5/20 centering rings found they were too tight. This required some inside peels for a good friction fit.

Here's a dry fit of the engine mount using the 1 1/2" BT-5 tube. It's short! I didn't like how much of the engine hook hangs out the low end.

More (and a fix) in the next post.

Estes Mini Bertha Clone, #TK-3, Part 1, Parts

All the parts came from the spare parts drawer. I was lucky this time and didn't have to order a balsa nose cone.

That nose cone is a BNC-20L, 1.4" exposed.

The parachute will be an Odd'l chute, trimmed way down to 8" diameter.

The BT-20B main airframe tube is 8.65" long.

The motor mount tube is a BT-5T at 1 1/2" long. More on that later - 

The LL-2A launch lug is 1.25" long.

I'll be adding a Kevlar shock cord line, a BT-20 tube is too tight using a tri fold mount.

This is one of the few kits that uses 1/16" thick balsa.

I found some nose cone weight washers on a trip to Home Depot. I don't know if these are the same weight as the kit part. I just needed them to fit inside the BT-20 tube diameter.

Estes Mini Bertha Clone, #TK-3, Background

In the new Estes 2024 catalog, there are some new Mini models. 

The Mini Alpha, Mini Arcas, Mini Bertha and aq reissue of the Mini Max kit from 2010.

These were referred to as Mini-Brutes in the Narcon manufacturers forum video.

In 1973, the original Mini-Brutes were designed for the new 13mm mini engines. To see them: CLICK HERE and page through the catalog. 

The upcoming Mini Bertha and Mini Alpha are snap together. The Mini Max and Mini Arcas are traditional kits you build.

This blog build will be a clone of the Mini Bertha from 1973. I had a Mini Bertha then, it was a favorite.

The original Mini Brute kits had Kraft tubes with balsa fins and nose cone. Assembly is much like an 18mm motor kit, just smaller.

The descriptive copy on the Mini Bertha says a 8" chute is included, but a 10" parachute is in the instructions parts list.

The build starts tomorrow!