Model Profile - MX-774 Kitbash

 

Semroc has just released a new MX-774 kit.
If you have a Estes Bullpup kit sitting around -

The plans for a sport scale MX-774 by A. Roger Wilfong are at:

http://www.spacemodeling.org/jimz/mx-774.htm

It's a kit-bash of the Estes Bull Pup 12D kit using the engine mount, nose cone and tail cone. You supply a longer BT-55 and some additional balsa.

TIP: The picture on the left shows how the electrical tape trim has shrunk up over time.
Always overlap vinyl trim tape wraps!

MX-774 Fix!

A while back, Bradycros on TRF referred me to some MX-774 scale plans.
Every picture I'd seen showed the small squares at the rear of the fins as black.
Of course they were black, every picture of the MX-774 is a black and white photograph!

As it turns out, the little squares are red.

I finally made the correction in time for the May TTRA launch.
Sure it's subtle, but it does make a difference!

MX-774 Kitbash Fix! Post 20

After reading Mr. Wilfong's Kitbash instructions, I realized I made a mistake!
Nothing major, I cut the upper band to 1/4" width. It should have been 3/8" wide.

Here's how I cut tape strips.

I lay the tape on the glass of my sliding glass patio door.

Set your straightedge on the tape and cut with a razor blade.

Don't worry, if the window is glass, the blade won't scratch it.

I find it easier to see the cut in daylight and if the window is clean, the underside of the tape will be clean too.

Here's the 3/8" wide strip in place. It's a subtle difference but closer to the truth.

MX-774 Kitbash Part 19 Finished!

Did you ever have one of those builds you just want to put a fork in?

I probably could have spent more time getting the masks cleaner or actually painting on the black band and small squares on the fins.

For a quick build and kitbash, I'm happy with the finished product.

While not true scale, it certainly captures the feel of the MX-774.

At the launch field, who'll know the difference?

Here's the rear view, you can see the black and white separations halfway over the trailing edges.

For the first flight, it's loaded up with a Quest B6-4. It should fly this next Saturday at the N.E.F.A.R. launch in Bunnell, FL.

MX-774 Kitbash Tape Trim Part 18

I didn't feel like masking the ring on the upper body tube so I did a 1/4" wide wrap of black electrical tape. Electrical tape is an easy trim material for bands and stripes. When applying it, try not to stretch it.

I used a Sharpie pen to mark around the top of the tube.

At the outside edge of the white side of the fins is a black rectangle. I used electrical tape again.

The small tape squares had to wrap halfway around the sides of the rounded fin edges. I made a small puddle of thick CA on my work table. A toothpick was dipped in the CA, then transferred the small drop to the edge of the fin. The tape side was rolled over and down into the CA.

MX-774 Kitbash Painting Part 17

Here's that mask, almost ready for paint.

I still have to add some cut up grocery bag pieces between the fins, but wanted to show it before all is covered up.

Scotch tape wouldn't stay down and roll over the fin edges. The black/white color separations run right down the center of the fin edges.

I had to use masking tape.

Jumping ahead, here's the paint after removing the tape.

There was quite a bit of cleaning up with an XActo knife tip. The masking tape over the fin edges didn't give a great line.

I'd scrape off a bit of too much black, then touch up the white side with a Sharpie pen.

Is it perfect? No.

Close enough for sport scale!

MX-774 Kitbash Masking Part 16

This is a tough mask. Roll patterns are never easy, this one had it's own challenges.

I wanted to be sure the horizontal mask lines were at 90 degrees to the line of the body tube.
I cut some cardstock at 90 degree angles and measured from the tip of the leading edge to the outside end of that edge. It was close to 1 1/2" long.

I taped it to the fin making sure the edge against the body tube was straight and flush. The side of the cardstock to the right in the picture would be the guideline for the tape mask. In the picture you can see the Scotch tape set next to the right side of the guide.

When rotating the body a quarter turn the next black mask is on the lower third of the fins.
I make another 90 degree cardstock piece for a tape guide.

To be sure it was set to 90 degrees, I used another cardstock square.
I layed it down the fin root edge against the rear guide. When it butted against the upper guide piece, the lower piece was taped from behind.
Scotch tape was set with the edge right right above to the lower square.
Both guideline cardstock squares were removed after the tape was in place.

MX-774 Kitbash White Spray On Part 15

After gray primer and sanding, I applied the white spray.

Overall, the finish is coming along well.

I'll have to do a little sanding to take off a few raised spatters before masking for the black roll pattern on the fins.

Looking closer at the fin servo pods, I noticed some air bubbles.

A little wood filler and sanding will close it up.

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.

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.

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.

MX-774 Kitbash Part 9 Filling Fins

I brushed Carpenter's Wood Filler onto the fins earlier today.

My filler is starting to dry out a bit and get thick.

You'll learn the proper consistency after a few tries. After applying, most of the brush lines will disappear when dries. You should let it dry overnight before sanding.

Here's a fin after sanding.

Ideally, with the CWF you want to leave a thin yellow coating overall.

In this picture you can see the pink balsa showing through in the center.

I'll have to go back, reapply and sand again.

In this last picture you can see two different layers of filler.

On the top is the dried filler before sanding. You can still see the brush strokes.

Below is the sanded surface. There is a thin overall coating of the sealer above the surface of the balsa.

Use a bright light when sanding to be sure you are removing all the brush lines and getting to the surface.

MX-774 Kitbash Part 9 Rounding Fins

I don't know if the Centuri MX-774 kit used 1/16" balsa, I would assume it did. Mr. Wilfong's kitbash instructions suggested using 1/16" balsa. I decided to go with 3/32" thick stock.

The trailing edges are behind the body and would be prone to breaking on landing. I felt 1/16" balsa would be too thin.

Mr. Wilfong also suggests airfoiling the fins. Again for strength, I simply rounded the edges.

Here I'm removing most of the edge by rolling the edge over a block with 220 grit sandpaper.

That block is a Great Planes Easy Touch Hand Sander, a great tool. You buy the sandpaper in a roll, the paper has a sticky back to hold it on the flat side. Their 220 grit paper isn't a real 220. It's more like 320 so it's perfect for this kind of work. That roll of sandpaper is a little pricey.

I use a block for the initial rounding of the edges. It helps keep an even roundness across the entire length of the edge. Most younger modeler's tend to take off some of the corners when using too rough a grit held in their fingers.

After using the block I go back and knock off any small edges with a piece of 400 grit held over my index finger.

MX-774 Kitbash Part 8 Sanding Tube Seams

Here I'm sanding down the filler already dried in the body tube seams.

Sand down the length of the tube, up and down, against the seam direction.

Don't sand in a circle or along with the seam, you could knock the filler right out of the tube.

The sanding block is holding 400 grit sandpaper. Carpenter's Wood Filler will not load up sandpaper the way sanding sealer used to.

Look close at the next picture to the left. You can see the filler has been knocked out of the seam. This will require me to go back and fill again.

While some hate the white tubes, it is easier to see the tan filler in the white tube seams.

MX-774 Kitbash Part 7 Fin Trailing Edges

The MX-774 has four flat bottomed fins that stand flat and square.

Sometimes you won't know if the trailing fin sides are square until the model is finished. You stand your finished rocket on a table and the bottom edges aren't flat!

Here's one way to check them before gluing to the body.

Stand your straight edge up on a flat surface. You could use anything long enough that has a true square bottom. Set the fin's root edge against the vertcal side, keeping the bottom flat against the surface. Your fin is now standing straight. (I used a wooden ruler just for the example. It didn't reflect light in the picture.)

Check out the light coming through the right side of the trailing edge!

I'll have to sand the left side of the trailing edge to square it up.

Better to fix it now than see the rocket wobble when standing up after it's finished.

MX-774 Kitbash Part 6 Seam Filling

Some builder is going to read this and say: "He's doing what?"

I've gone back and forth over the years on the best way to fill body tube seams.
Thirty years ago I used white wall spackle, back when it was real wet and gritty. It was too wet and gritty for the kraft paper tubing.

More recently I've been using Elmer's Carpenter's Wood Filler mixed at the same consistency you'd use on to fill balsa.

I first used small brushes, painting over the seams. Then I'd lightly "squeegee" off the excess with a razor blade, leaving as much as I could in the seam.

For now I use a old, dull X-Acto blade to apply it. I dip just the tip of the blade into the filler, leaving a drop on the blade tip. Wipe off the back side of the blade on the filler jar lip. You won't need all of it.
The bead of filler is set into the seam. I can drag and spread the drop of filler down the seam. The Blade edge easily stays in the seam trench as I push the filler forward and back, in a sawing motion.
You are doing about an inch at a time, then going back for another drop of filler.

Because the filler will shrink when it dries, I don't "Squeegee" off the excess. I simply sand the raised ridge down to the surface. Tube seam widths will differ between tubes, a smaller width seam will hold the filler better than a wider width seam. Let the filler dry thoroughly before sanding.
I use 400 grit on a block to sand off the dried filler. Sand lightly, you don't want to knock the filler out of the seam. Using Carpenter's Wood Filler will usually fill 3/4 of the seam depth. The last 1/4 will fill with spray primer.

MX-774 Kitbash Part 5 Engine Mount Install

After a couple of dry fits I found the upper centering ring would be about 2 1/4" from the rear of the main body tube.

To insure the ring would have glue around it, a dowel was marked at 2", 1/4" below it's final resting point. You can see the pencil mark in the picture at the right.

A bead of glue was applied to the dowel top. It was set inside the tube up to the 2" mark on the dowel. The glue bead was spread evenly around the inside of the tube, using the mark to keep it around the 2" distance from the end.

The tail cone was used to push in the mount against the lower centering ring. When the tailcone was about 1/4" from the end of the body tube I removed it and twisted the engine mount to spread the glue evenly around the top centering ring. The tailcone was replaced and pushed in the final 1/4" against the body tube. With the tailcone butted up against the body tube, the mount is in the correct position.

The picture above is taken from the top looking in. You can see how the upper glue bead is even from twisting the mount and evening out the glue bead.

With every engine mount installation, you are working against the clock. This install had three distinct movements. Partial install slide in, twist of the mount to spread the glue evenly and final push into position.

I know some are thinking: "Why bother? Nobody will see the inside anyway." I try to get both - good, clean internal and external construction.

Here's the dry fit of the tail cone, both engine mount tube and the end of the tailcone are even.

MX-774 Kitbash Part 4 Fin Profiling

Here's technique loosely borrowed from guitar repairmen when they level frets on a fingerboard.

They'll mark over the tops of all the frets with a Sharpie pen before leveling them flat. When sanding over the fret tops, the remaining dark marker will show them the low spots on the fret. The low areas remain black.

Here I've marked the master (already fitted fin) with the Sharpie around the edges. Go light with the marker, you might need that fin later.

Four more fins were cut slightly oversize for this demo. All were pinned together making sure all the edges were overhanging the master, blackened fin.

Here's two edges sanded down to match the master template fin. You can see where the sanding was just starting to go into the blackened edges of the master.

I worked off a master fin this time because I didn't want to have to sand and match up three other fins to the angles of the body tube and tail cone.

That first master fin took too much time to get the correct fit.