Centuri Finless Finished!

I like the finished look, the aluminum trim helps set it off.

Lessons learned:
I didn't get as good a seam fill as I'd hoped. I tried a new smaller can of Dupli-Color Perfect Match Scratch Filler Primer after my larger can ran out. The larger can is a different formulation and filled the seams on other models much better. I'll be using the larger can style of primer next time.

Don't fly this model with a C6-5! The extra weight of the C engine will add too much weight to the rear and it will be unstable!
I have flown it with an A8-3 and B6-4 with good results.
From the Centuri plans, the recommended engines were the A8-3, A5-2 and B4-2.

Centuri Finless Build Part 17 Nose Weight

To balance with the trim colors, the nose cone was spray painted aluminum.

The instructions call for .40 oz. of clay weight to be added to the payload section.
Clay was weighed and pressed it into the upper payload section.
The upper section isn't used as a payload, so the nose cone was glued in place.

I won't get into any boring parachute making here. (You're welcome)
A 12" Semroc parachute was made and tied to a snap swivel.

Normally I'd attach the parachute 1/3rd the way down the elastic shock cord from the screw eye. Because the upper section is longer than normal, the snap swivel was joined at the screw eye.

Centuri Finless Build Part 16 More Trim

This is how the model looked using the Centuri instructional drawing.
I liked the looks of it, but it needs a third trim color.





Time to find the aluminum Monokote Trim sheet.
More detail was added by using my rotary punch again.

I punched some 3/16" diameter dots and placed them at random.




The dots are a little hard to get off the backing sheet.
Sometimes they stay on the backing, other times you have to pick them out of the cutter tube on the punch with the tip of your hobby knife.

Centuri Finless Build Part 15 Upper Black Trim

The upper trim roll patterns were strips of the black Contact vinyl.

These were rolled around the body tube using the Centuri instructional drawing as a rough guide.






Use a sharp knife and straightedge to cut the stripes and rolls.







Position the black vinyl using a clean knife blade, not your fingers.
You have less chance of transferring dirt and oils by using the blade.

Centuri Finless Build Part 14 Black Shroud Trim

The white areas were cut away from the template.

While cutting away the template, I tried to cut through just the top black layer of the Contact paper.
Be careful cutting the inside corners, you don't want the vinyl to tear when placing the trim.



The paper template was removed.

The open "white" areas were removed from the backing sheet.



The great thing about the Contact vinyl, it's respositionable.
Here's the trim in place after moving it a few times.

It's a little tricky to get it to line up, but worth the extra effort.
Try to keep dirt off the sticky underside. A speck of crud will show through the thin material.

Centuri Finless Build Part 13 Black Shourd Trim

I had printed an extra shroud on some 20 lb. paper to make a template for cutting the Contact paper trim.
The shroud sides were extended up to their center point. A compass was used to draw the horizontal lines.
The vertical lines were simply extended from the center point.




The sides of the cutting template were cut a little wide to allow for coverage and (hopefully) a matched edge.






An oversize piece of Contact paper was taped to my cutting board.

The cutting template was taped on top.

Centuri Finless Build Part 12 Shroud Paint and Trim

I applied some medium CA around the lip of the shroud to strengthen it a bit.





Here's the shroud after the white undercoats.

To do the stripes and roll patterns, there won't be any hard masking.
All the trim will be done with Contact Paper, almost the same stuff my Mother used to line her kitchen drawers with.
You can read about the Contact Paper trim material HERE

Centuri Finless Build Part 11 Fill and Finish Prep

Even after the first primer coats, I was still trying to fill the lug / shroud gap with white glue fillets.

It took a few applications to smooth it over.




The nose cone was brushed with thinned CWF and sanded smooth.
Some CWF was over the edge of the nose cone shoulder.

TIP: To get a better edge on a nose cone shoulder:
Pull the nosecone slightly out of the body tube.
Sharply fold a piece of 400 grit sandpaper.
Set the "V" of the folded sandpaper in the nose cone boldy tube gap.
Lightly sand, taking off the excess CWF.

Centuri Finless Build Part 10 Shroud Gluing

The original Centuri instruction plan has you glue the ring into the shroud then install it on the body tube.

Because of the retaining ring, you'd be better off gluing the ring on first. Then, slide the shroud down the tube from the top and onto the ring.

If you were to glue the ring and shroud together first, it'd be hard to get the shroud over the retainer ring if slid on from the bottom up.




To get the shroud tight enough around the tube, I applied a small amount of white glue under the sides of the lug slot. The sides were pressed down onto the tube using a clean, rounded dowel.

Centuri Finless Build Part 9 Lug Slot

This is the first try at the lug hole in the shroud.

The shroud sides are still raised too high from the pressure of the lug underneath. I'll make the slot longer by punching another hole a little farther back.
In the end the punched hole was 11/16" back from the smaller front of the shroud.

Making a accurate lug hole and slot in a shroud will be different with every shroud size you make. It varys because of the shroud length and how the lug sits under the shroud angle.


TIP: Sand a slight angle on the edge of the centering ring. Angle your sanding block to the same angle as the shroud side.

By doing this, you'll get a better fit of the ring against the inside of the shroud. You'll also see less of the ring "imprint" on the outside of the shroud after gluing the two together.

Centuri Finless Build Part 8 Lug Slot

The shroud was dry fitted down the tube and over the centering ring.
A (longer than needed) 5" launch lug was pushed in from the rear lug hole until it hit the inside of the shroud seam.



A pencil mark was made where the lug raised the shroud from the inside. This is where the lug slot will be punched and cut.

I should say, this is where the first punch will be made. You usually have to go back and make additional adjustment holes with your rotary punch.



Use the largest tube on your rotary punch which should be 3/16" diameter.
Punch a hole right behind that pencil mark made earlier.

Centuri Finless Build Part 7 Shroud Cement

Pre-roll the shroud before cementing together.

I learned a valuable tip from the Apogee website about using rubber cement instead of white glue when gluing a shroud tab.

Lay a coat of rubber cement on 1/2 of the length of the tab. Stick the tab under the long edge of the shroud.
Apply more rubber cement on the exposed tab and stick the other side of the shroud on the glue tab.
After the cement dries, you can cleanly remove any excess by rubbing over it with your finger.

TIPS on rubber cement:
Using the cement on one side gives you a joint you can place and reposition if needed.
Using rubber cement on both sides gets you a permanent joint you can't reposition.

Centuri Finless Build Part 6 Shroud Prep

The shroud was printed up using the Transition Tool from the Payload Bay website HERE:




Print up the shroud cone on 110 lb. cardstock.
Tube 1 size is an ST-7 at .759"
Tube 2 size to fit the centering ring is 2.08"
Length is 3"
(The original adapter fits a ST-20 tube which was 2.04" diameter. I made mine slightly bigger for a longer lip on the bottom.)

You can see I've drawn the gluing tab in pencil.
On this shroud, the tab is cut off and glued inside (or under) the two butted shroud sides.

Cut the straight sides and glue tab with a straight edge and single edge razor blade.
Cut the curves with scissors.


To be sure the top and bottom curves are round, run a sanding block and 400 grit sandpaper around the sides to remove any high spots.






Set a small piece of 400 grit around a finger to round up the upper inside curve.

Centuri Finless Build Part 5 Hook Retainer

Here's how the split 1/4" long piece of BT-20 will hold down the engine clip.
Before gluing the split ring, a piece of masking tape was set on the lock after making sure it was in line, straight down the tube.

Enlarge the picture and you'll see the tape has fine blue fibers stuck to it. I reduced the stickiness of the tape by placing it on my jeans pant leg and removing it a few times.

The tape will pull up some denim fibers and that should cut back on some of the tackiness of the tape. I don't want to run the risk of tearing up the motor mount tube when removing the tape after the retaining ring is glued in place.

The split side is directly opposite the center of the hook.

A small piece of tape is holding down the split while gluing.
Be careful not to get any glue on the hold down tape.



Here's a dry fit of the centering ring with the launch lug in it's hole.

The launch lug sits right in the split in the engine retaining ring.

Centuri Finless Build Part 4 Engine Block and Kevlar

The engine block (or thrust ring) was notched for the Kevlar.
I've been using a small diamond file with good results.




After tying on the Kevlar, the block ring was glued into the rear of the ST-7 tube. An engine casing was marked 1/8" from the end.

The Kevlar was fed through the casing so it'd be out of the way when gluing the block in place.
The engine block was glued inside the ST-7 tube by sliding the casing up to the 1/8" mark.

Centuri Finless Build Part 3 Centering Ring Holes

The engine hook notch and launch lug hole will be on opposite sides of the body tube.
Draw a pencil center line across the centering ring.
Using a rotary punch, set it on the largest hole or 3/16" diameter.
TIP: A 3/16" hole will fit a 1/8" launch lug perfectly.




I also used the rotary punch to make a round engine hook notch on the other side.

It took two punches, one on top of the other, to make a complete notch.

Centuri Finless Part 2 Parts Prep

The ST-718 tube was cut 4 1/2" from the top for a payload section using my Odd'l Rockets CUTTER tool.

This upper section will also hold the clay nose weight.


A nose block was made for the payload section from a 7 tube coupler and the shoulder from a old, unusable nose cone.

I just kept sanding away at the outside of the scrap balsa shoulder until it fit into the coupler.
It was glued inside overhanging the edge. After the glue dried, it was sanded flat.

To the left is 1/4" long BT-20 scrap that was split to make a engine hook retainer.

Centuri Finless Build Part 1 Parts

The plans are from the Centuri American Rocketeer magazine, Volume 4, Number 1.
The plans are on page 15.
Go to: oldrocketplans.com HERE

Parts are available from Semroc:

1 ST-718 18" Long ST-7 Body Tube
1 TR-7 Engine Block
1 BTC-7 Balsa Nose Block or Coupler
1 BC-736 Nose Cone Conical, 3.6" long
1 CR-720 Centering Ring
2 LL-130 Launch Lugs 3" long
1 EC-136 1/8" X 36" long Shock Cord
1 SCK-24 Kevlar about 14" needed
1 CP-12BW 12" diameter Parachute
1 EL-28 Engine Hook
1 WC-5 Clay Weight .40 oz. needed
1 SE-2 Screw Eye
1 1/4" long scrap of BT-20 or ST-7 for retainer lock ring

Instead of slitting the tube for a shock cord, Kevlar will be tied to the TR-7 engine block.

Mylar lock rings aren't available in the ST-7 size, so I'll use a split piece of body tube. The split of the retaining ring will sit on either side of the lower launch lug location.
For the paint pattern, I'll be using the Contact paper self-adhesive blackboard paper. Black electrical tape could also be used.

I only used one of the 3" launch lugs, under the rear shroud. The Centuri instructions showed both being used, glued in line with each other.

You can print up the tailcone adapter on 110 lb. cardstock from the Payload Bay website HERE
Tube 1 size is an ST-7 at .759"
Tube 2 size to fit the centering ring is 2.08"
Length is 3"
(The original adapter fits a ST-20 tube which was 2.04" diameter. I made mine slightly bigger for a longer lip on the bottom.)

Centuri Finless Rocket Plans from Jim Z's

Here's a fun one:
http://www.spacemodeling.org/JimZ/finless.htm

I've had great luck with the 13mm BT-5 based Hot Rod Rockets Bell Bottom, I wanted to make one for 18mm engines.

I remember seeing this one in the mid 1970s and decided to give it a try with a few changes and updates.
I'll start the new build blog tomorrow.