Enerjet by Aerotech Strong Arm Build, Part 18, Waiting For Paint To Dry and Planning Ahead

You may have noticed a slowing of posts - 

That's because I'm building three Strong Arm kits at the same time.

One is a flight version - no seam fills and less attention to details. These are sometimes referred to as: "They'll look good on the pad."

Duplicate builds slow things down, I can't repeat the same build posts three times.

Planning ahead,

You might assume this roll pattern sticker would just roll around the tube -

These will have to be cut apart and placed individually.

But, there is 17/32" space between each vertical bar. 

To make placement easier, I'll print a spacing guide. It'll be a fin marking guide from payloadbay.com

To get to the marking guide tools: 

CLICK HERE

Hit TOOLS and then FIN WRAP TOOL.

There are eight bars going around a 2.638" diameter tube.

Enerjet by Aerotech Strong Arm Build, Part 18, Overall Paint Coverage

The larger the rocket, the more difficult it is to get a good gloss coat of spray paint. The Strong Arm is big enough to be a painting challenge.

To get an idea how I spray models horizontally, 

I'll refer you to a previous post: CLICK HERE 

I'd recommend reading the post to understand the technique:

Enerjet by Aerotech Strong Arm Build, Part 17, Adapting A Painting Wand

I use painting wands to hold my rockets horizontally when spray painting.

The wand is a 1/2" dowel shoved into a used 18mm engine casing. 

This is too small a diameter to fit this larger rocket engine mount.

I wrapped some corrugated cardboard around the casing and closed it up with tape. To fit the 29mm diameter mount, the cardboard is just over a full wrap around the casing. This wider diameter gives a good friction fit. 

With the dowel wand slid into the mount,

Some masking tape is placed sticky side out inside the main body tube.

A rolled up paper towel is press into the area around the mount and up against the inside wall of the main tube.

Any open areas are covered with additional pieces of masking tape.

Now the rocket can be spray painted without getting any paint inside the low end.

Enerjet by Aerotech Strong Arm Build, Part 16, Setting The Lowest Centering Ring

I'm back from vacation, hoping to paint the lower body assemblies - but, it's been raining! So, onto other steps.

 

Check both sides of the centering ring, pick the smoothest side to be exposed out the bottom.

On one side, there was a chip. I marked this side with "X's". This side will be facing up, not seen. 

The low edge of the body tube is slightly "squared", out of a perfect round. The four fins are locked into the rings and pull in the body tube. 

It would be very difficult to "slide" the ring in with the tube slightly out of round. 

On the left, I sanded an angle into the edge to make the insertion easier.

Notice the laser charred edge has been sanded.

The center of the ring also got a taper to slide over the engine mount tube.

The ring is slid in and rests against the root edge tabs of the fins.

Here I've applied a wood glue fillet around the ring/tube joint.

Enerjet by Aerotech Strong Arm Build, Part 10, You're Not Alone!

I see paint problems on the Facebook rocketry pages. It happens to all of us!

    

I usually have better luck at spray painting once the weather cools and the humidity drops.

No fancy masks on this one, the nose cone is sprayed a medium gray.

I did mask off the shoulder.

The initial two coats are sprayed on light, the third coat is sprayed on a bit wetter for a good, smooth gloss.

I checked them an hour later. Most of the nose cone was fine, the tip was not. The paint didn't craze or wrinkle, but there were bubbles!

I'll wait a day and sand smooth. After 48 hours I'll try again!

Enerjet by Aerotech Strong Arm Build, Part 9, Filling The Tube Joint

Here's the tubes together, the coupler inside glued together using 15 minute epoxy.

The pencil lines are lined up before the glue sets up.

 

Most of the joint gap was first filled with CWF, the excess smoothed out using a single edge razor blade.

The picture shows the joint after the CWF was sanded to surface.

The high spots end up fuzzy after sanding.

I wipe on some medium CA Super Glue using a Q-tip applicator.

After it dries, sand using 400 grit on a block.

A final fill is a shot of filler/primer and sanding with 400 grit.

Taking these extra steps, you shouldn't see the tube joint seam on the finished model.

Enerjet by Aerotech Strong Arm Build, Part 8, Gluing In The Engine Mount

This may seem like a lot of pre-glue fitting, but everything must be in the correct position so I double check the fit of all parts. 

The engine mount is glued in.

I applied a ring of 15 minute epoxy from the low end, just above the top of the fin slots.

Why use epoxy? You'll have more working time. You can slide and turn the engine mount into the correct position without it locking up in the wrong place.

As you slide in the mount - 

Keep your eyes on the fin slots making sure the centering rings remain visible, just above and below the top and bottom of the slots.

As the mount is slid into position, TURN the mount to even out the epoxy making a more even internal fillet.

With the centering rings just above and just below the ends of the fins slots -

Slide in the fin tabs without clicking them into the lock rings.

Epoxy is slippery, the TTW fin tabs will keep the mount in the correct position as the glue dries. 

TIP: If you haven't used epoxy before, wear gloves. You might be allergic or could develop a epoxy allergy over time.

Enerjet by Aerotech Strong Arm Build, Part 7, Engine Mount Fit & Coupler Gluing

Before setting the engine mount, 

the TTW fin slots are removed. 

You'll want to see the centering rings position through the slots.

Remove the launch lug slots, too.

Slide in the engine mount. 

Slide in the fins, don't fully snap them in place yet. You are just checking that the centering rings are above and below the top and bottom of the fin slots.

The end of the engine mount tube should be even with the end of the main airframe tube.

I set my sanding block onto the end to make sure both tubes were even.

The mount will be locked in place once the TTW tin tabs are snapped into the locking rings inside.

On the left, 

The upper tube is flipped over for the coupler to be glued up to the 3" halfway pencil mark.

The shock cord is draped over the end.

TIP: To keep any epoxy glue off the shock cord, I applied some tape and slid it down the cord to where it might contact any squeezed out epoxy.
 

Enerjet by Aerotech Strong Arm Build, Part 6, Fit & Joining Two Tubes

On the left are the two tubes slid together using the coupler inside. No glue yet.

On the right is after sanding the tube ends square, the sanding shown below in the left side picture. Again, no glue yet.

Square up both tubes with a sanding block with 220 grit.

Slide the tubes together using no glue over the inside coupler.

Rotate the upper tube until you get the best, tightest join of the two tube ends.

Mark that best joint with a pencil line. When glue is applied, line up the pencil lines.

Having a tighter joint of the two tube ends will make it easier to fill the seam. 

The seam fill is coming in a few posts.

Enerjet by Aerotech Strong Arm Build, Part 5, Tube Seams

   

Back to the Enerjet by Aerotech Strong Arm build. 

To see the first four posts: CLICK HERE

I had to go back and do a little more CWF fill where it fell out of the seam. This happens on occasion. I mark them as I find them using pencil.

After sanding the CWF to surface,

The tubes got a shot of filler/primer.

Notice the masking tape set inside the tube end to keep the interior surfaces clean.

On the right is the same tube after sanding. Filler/Primer fills what seam lines were left after the first CWF pass. Only this two-step fill is necessary.

The edges of the tubes got a wipe of CA glue using a Q-tip.

Use your fingers as a depth gauge keeping the CA line even.

The centering ring glue fillets are up inside the tube, not near the CA'd edge. Wood glue or epoxy won't soak into the tube if the CA glue has sealed it off.

The tube ends are sanded after the CA glue dries. 

This gives you a hard, stronger edge tube.

Enerjet by Aerotech Strong Arm Build, Part 4, Strakes

On the left is the front of the strake, on the right is the rear.

The final tip cuts are marked with pencil.

Cut them a little wide outside the pencil lines. Then sand to shape with a block.

TIP: It's easier to visualize the shape by turning over the strake and checking the bottom.

I drew a centerline to better see the wedge shape.

Enerjet by Aerotech Strong Arm Build, Part 3, Strakes

Here I'm cutting the back of the strake.

Notice I'm cutting a little outside the pencil line.

The top picture shows the rough cut using a knife.

Here's the same cut after sanding with 220 grit on a block, followed by 400 grit on a sanding block.

Another before and after picture - 

220 to take down the rough cuts then 400 grit to smooth.

This inside angle is tough to smooth out.

Mark the side with a pencil and use a finger to block the block edge from cutting in too deep.

Enerjet by Aerotech Strong Arm Build, Part 2, Strakes

The strake material is white plastic, 12" long.

Be sure to check the lengths of the four strakes.

Set back to back, one strake was about 1/16" longer than the other three.

Even them up before trimming the strake shape.

Use a pencil to mark the side of the strake cuts.

The top picture shows the rear of the strake.

The bottom picture shows the front half of the strake.

At first I tried a razor saw but the raised base made it difficult.

It's also hard to get a straightedge onto the surface to use as a cutting guide.

I ended up simply cutting the lines freehand. I cut a little wide knowing I would do the final shaping and cleaning up with a sanding block.

In the two pictures on the right side - 

This obtuse (inside) angle cut is the most difficult.

Start and stop your cuts right at the apex of the inside angle.

The upper picture shows one cut.

The strake is flipped over in the second picture showing the start of the cut from the center to the outside edge.

Enerjet by Aerotech Strong Arm Build, Part 1, Parts

Out of the box, all the parts are of high quality.

There is a second body tube, same length as the tube on the left, without fin and launch lug slots.

The tube in the upper center is a coupler to joint the two 2.6" diameter tubes.

The instructions mention "Aero-Fibre" centering rings. The kit now includes plywood rings.

The black ring in the lower left is an screw on aluminum motor retainer.

 

The strakes are molded in white plastic.

These are trimmed to shape. The inset picture shows the profile.

The conformal rail guides are 3D printed.

The black launch lugs fit a 1/4" diameter launch rod.

The decals are peel and stick.

Enerjet by Aerotech Strong Arm Build, Background

 

Here's my latest build for Aerotech.

The Aerotech mid-power kits now carry the Enerjet name.

It's a big model for me at 44" tall.

It resembles the General Dynamics AGM-78 Standard ARM. 

All the Specs:

This design brings up memories of the Centuri Nomad, the Estes and Fliskits Tartar designs.

The build starts tomorrow!