Centuri Skylab Build, #KB-4, Part 10, Panel Supports

This is one of the few kits where pre-cut (die-cut) balsa needs to be recut. It makes me wonder if the die-cut razor blocks was made too long. Or, maybe the extra length was needed for strength when doing that center punch?

From the instructions:

You are told to cut off 1/8" off each side of the die-cut panel supports. Then bevel the ends to a 45 degree angle.

 

Later on, there has to be a 1/8" space between the top of the panel support and the bottom edge of the wrap.

On the left is an early dry fit of the wrap and top of the panel support.

In the inset picture is the beveled top.

Centuri Skylab Build, #KB-4, Part 7, Fitting The "Super" Shroud

Mentioned in the previous post - 

Here's the smaller end showing the inside stepped shrouds.

This gives a better fit and adhesion area onto the smaller tube.

One of the previously made centering rings is glued into the doubled shroud. 

After the glue dries, the edge should be stiff enough to sand off, even with the face of the centering ring. 

The fit is good -

Even if the low end were a hair wider than the larger tube, the 3nd printed instrument wrap would cover any shroud lip.

In the next post:

Now that all the parts are almost ready, the actual build can start. 

Centuri Skylab Build, #KB-4, Part 6, Making A "Super" Shroud

Cardstock shrouds have always felt like the weakest part of a LPR build. You can make a stronger two layer "nested" or super shroud.

For a complete explanation of the technique, I wrote an article for the Apogee Peak Of Flight: CLICK HERE
 

Go to payloadbay.com and click on "Tools", then to "Transition Tool".

Enter your tube diameters and length of the transition.

I printed the transition on 110 lb. cardstock.

The unused lower half of the page is cut off and taped behind the printed transition. Two layers are cut out at the same time. 

Note the glue tab in the upper right, added it with a pencil line.

I don't pull the shroud under a straightedge or pull it over a table edge.

Form the shroud in the heel of your hand with a smooth dowel. Press the dowel in an arc matching the curve of the shroud. 

Your shroud should be formed until it can stand on its own.

I don't put glue on the outside glue tab,

the glue stick goes on the inside where the tab will touch.

This makes the glue area easier to handle and prevents any glue getting on the outside of the shroud.

Burnish the glue tab area over a dowel set on the inside.

Repeat with the second shroud.

Pick the best shroud to go on the outside.

 Apply glue to the outside of the central shroud and press the two together placing the seam joints on opposite sides. 

After drying you'll notice how much stronger a nested shroud is. It almost feels like sheet plastic.

Inside the smaller end  the tow shrouds are "stepped" making a better joint over the upper tube. 

Look at the bottom and you can see the inner shroud overhanging the outside shroud. This will be sanded off in the next post.

Centuri Skylab Build, #KB-4, Part 5, Making Centering Rings

The outside circle is cut with scissors.

The traced pencil line will be a bit large.

Stack the rings on the smaller tube and reduce the diameter with 220 grit on a block.

Again, sand and check the fit.

Here's a good fit in the 2" lower tube.

I used my rotary punch to make an engine  hook notch in the nozzle bell centering ring.

The inside wall of the 3D printed nozzle was thicker than the original kit vacu-formed part. The ring sat high, almost over the top.

The diameter was cut down with scissors.

After rounding the edge - 

The final fit of the nozzle ring is recessed.

Centuri Skylab Build, #KB-4, Part 4, D.I.Y. Centering Rings

TIP: One of the best centering ring materials is the left over mat board found at a frame shop. Ask the manager if they have any scraps, leftover pieces too small to be used in a frame.

If you're lucky they might give them to you for free! If not free, cheap anyway.

Trace around the diameter of the tubes. Trace the larger tube first, it's easier to center the smaller tube afterwards. 

It might take a few tries to get the smaller pencil ring centered.

With a new blade, cut the center out first. 

Cutting the center hole first is easier when you have more of the cardstock to hold onto.

That small circle in the center was on the mat board from an earlier build. I should have erased it for the picture.

Some 320 grit was rolled around my finger to enlarge the diameter..

Sand a bit, check the fit - repeat until you get a good friction fit.

When the hole is to size,

you might find a raised lip around the edge.

Leave the lip. It gives you a tighter fit and a wider gluing edge later on.

Centuri Skylab Build, #KB-4, Part 3, Making A Centering Tube

From the original instructions:

There is a side Kraft centering tube set over the engine tube and engine hook.

I don't have a 3/4" long centering tube, so we'll "make" one.

Three 20/50 centering rings will be used, stacked and glued end to end.

For tighter glue joints, sand the ends flat.

Slide the rings over a BT-20 tube and glue just the rings together, don't glue them to the tube yet. 

Sliding the rings over the tube is for a true, straight alignment. 

The first picture shows how little glue was used.

After the glue dries, slide the three glued rings off the tube.  

The diameter of the centering rings will be too small for a good fit in the BT-52 tube.

Using a glue stick, I wrapped some cardstock around the rings to build up the outside diameter. The ends were then sanded flat.

Here's a good fit in the BT-52 tube.

Don't glue the engine mount in place yet -

Centuri Skylab Build, #KB-4, Part 2, Parts Fitting

The 3" long BT-20 tube is notched for a standard engine hook.

The inside of the nozzle doesn't have a relief slot for the engine hook.

A square diamond file cut a notch for the hook.

Don't go too deep, the top of the nozzle is thin.

I marked the inside notch with pencil so you can see the fit through the plastic.

I can now slide the nozzle over the engine mount tube without distorting the tube.

No glue yet, there are more pieces to dry fit.

Centuri Skylab Build, #KB-4, Part 1, Parts Fitting

I have to wonder if the Thingverse 3D designer made the pieces fit to Estes tubes. The two pod nose cones and nozzles fit a BT-20, but the nose cone and main nozzle fit a tube slightly larger than a BT-50.

The large nozzle bell didn't fit a BT-50, too small at .976" diameter. The older Centuri ST-10 tube was 1.04" diameter. 

I didn't have any ST-10 tubing. Before ordering parts, I always check the spare parts drawer.

I found some BT-52 tubes left over from an old Thor Agena B build. One piece was 7 1/8", the other at 11 5/8" long. I checked the nose cone and the shoulder fit was good in the BT-52.

Strange thing about those two BT-52 pieces of different lengths. The kit used two 9.4" lengths for a total of 18.8". These two pieces when held end to end were exactly 18.8"! 

The smaller rear engine pod nose cones and nozzles fit a BT-20 perfectly. Two pod BT-20s are needed at 2.95" and a engine mount tube at 3" long. 

The BT-52 tubes will have to be joined together with a coupler. I don't think one is available to order.

Here's the fit of a BT-50 coupler showing a loose fit in the BT-52 tube end.

The coupler diameter was built up with some cardstock wraps and a glue stick.

After the coupler was sized, I tried a standard BT-50 and it would work as a coupler! Oh well . . .