Wednesday, October 31, 2018

Odd'l Rockets Cutaway Engine Build, Part 4, Gluing It Together

Test fit the end caps without glue to get an idea how they fit the curved ends.
Spread a thin coat of wood glue around the tube end and slide the end cap from the inside towards the outside edge.

Press the folded triangles into the glue with a tapered burnisher like this pen tip. The ink tip is retracted, I'm using just the plastic end.

There is a white glue coat on the end flap piece.

Press the ends of the long engine print into the end cap glue flap.

Turn over the half tube. The long side flaps should be opened out.
Apply a line of glue stick down the flap.
Burnish the long glued side flaps onto the tube edges.

Here's how the nozzle end will look.
If you don't get it right, chalk it up to practice. You can always print another and try again.

Odd'l Rockets Cutaway Engine Build, Part 3, Rounding Over End Folds

The end cap on the left has been folded.

Fold the triangles under with your fingers first.
The triangle corners can be rounded a bit more by rolling over the corners with the burnisher.

Round out the small bent back corners for a better fit in the end of the half body tube.

Fold the side flaps back on the large engine face print.

Tuesday, October 30, 2018

Odd'l Rockets Cutaway Engine Build, Part 2, Pre-Fold Embossing

Cut the 6 1/2" long BT-60 tube in half, lengthwise.

Smooth the cut edges with a sanding block with 220 grit.

The dashed lines are embossed with a dull butter knife and straight edge. Pick a butter knife without serrations on the tip.

Here the knife tip is drawn down the straightedge on the fold line.
TIP: Under the print is some corrugated cardboard. Be sure the corrugations are running the same direction as the embossed line you are making.

The small fold under triangles on the end caps are traced with the knife tip. These small folds are actually small arcs, trace in a smooth half circle.

Odd'l Rockets Cutaway Engine Build, Part 1, Parts

Here's what the PDF looks like. It prints out on 65 lb. card stock.
You'll also need a 6 1/2" long piece of BT-60.
Tools: Sharp knife, wood glue, glue stick, straight edge, butter knife with no serrated tip and a burnisher.

This is simply a flat face print with end caps glued to a half tube. It is not a 3D model like the FlisKits Cutaway Engine kit. I drew it up hoping it can be used in the classroom, science fair display or for simply explaining the internal components of a black powder model rocket engine.

If you would like the PDF to print and assemble your own engine model,
Email me at: and request the Cutaway Engine Model 

Monday, October 29, 2018

Cutaway Engine Background

Here's the Cutaway Engine I remember, it was there in the 1970 Estes catalog but I never ordered one. Here's a picture of that Cutaway Engine I found on Ebay. Pretty rough, it even has band saw marks!

Centuri had a Cutaway Engine model but it wasn't around long. It was only available in the 1971 catalog. To see it: CLICK HERE

Fliskits had a large cutaway engine kit available.
Jim Bassham did a build/review on CLICK HERE

I drew up a new Cutaway Engine PDF. It's smaller than the FlisKits's cutaway engine, and this isn't 3D. The flat print simply wraps and glues onto a BT-60 half.
I changed the nozzle shape so it looks more like a typical black powder engine. It still does the job showing the major sections of an engine.

Semroc Snake Jumper, Finished

I was never a big fan of the Centuri Sky Cycle and the whole Evel Knievel Snake River jump. But, I liked the idea of a larger and "Goonied" Sky Cycle kit.
When Semroc discontinued this Groonie kit , you could get the decals, fins and instructions cheap. I already had most of the parts from my Odd'l Pigasus and Little Green Man kits.

The Estes Goonys and this Centuri kit were launched with 13mm A mini engines. The Semroc Groonies used 18mm engines. You have more choices with A, B or C engines.

TRIVIA: Centuri introduced fiber fins in the "Bunches Of New Kits" page in the 1974-75 catalog.
The Hummingbird, Moonraker, Jayhawk, Nomad, Excalibur 2 and Arrow 300 kits had them - CLICK HERE

Here's those blue wheels with the 1/8" launch lug in the middle. Black would have been better, the blue looks cartoony.

Sunday, October 28, 2018

Semroc Snake Jumper Build, Part 8, Decals

The gold (not a real shiny gold) cockpit cover folds over the front of the long top fin. Looking back I probably should have rounded the leading edge.

I had to lightly pinch and pull back on the decal to get it to conform to the edge.

The nose cone decals never want to fit the contour shape. Wrinkles will show up.
Roll a damp Q-tip over the surface smoothing out the wrinkles. Check it as it dries, sometimes the wrinkles come back.

Align the SNAKE JUMPER side decals by looking down the body from the front. Check the spacing against the root edge of the wing.

The X2 decals go on last, set along the same base line of the SNAKE JUMPER decals.

There were two small intake decals that go between the Snake Jumper and X-2 name decals. I must have thrown them away. I might draw and print them later on.

I would have thought the wheel decals should have been printed in black, not blue. There is other black ink on the decal sheet. Checking the online Centuri decal scan, they were printed blue.
To see the original kit decals: CLICK HERE

Saturday, October 27, 2018

Semroc Snake Jumper Build, Part 7, Paint and Decals

The Centuri instructions say you really don't have to paint the model, in that kit, the parts were already white. They did say the model would look better with an overall sprayed gloss white.
Mine has a white tube (with sanded primer on it) and the nose cone was balsa so I had to paint. No masking, just white.

As it usually goes, there was a blob of paint on the model.

TIPS: Sometimes you can carefully shave off the paint blob with a new single edge razor blade. This beats sanding and shooting with more paint.
This area will be covered by a decal. That paint blob would have raised the decal surface.

Many of the decals had to be cut right on the color line so there wouldn't be any clear borders.

Dry fit before soaking to see what needs to be trimmed.

Joining these two decals at the root edge of the long top fin was tricky but it can be done.

This is one of those models where the decals make the kit.

Friday, October 26, 2018

Semroc Snake Jumper Build, Part 6, Fin Gluing and Gussets

The side rudders sere glued onto the wing halves using my 2" machine square. This insures a perfect right angle. Notice the wing is set into the top of the gusset.

Here's the model up to this point. It's an interesting sport "profile" fin design.

On the left is the Semroc instructions. The suggested gusset position didn't seem like it gave the best support for the wings.
On the right is the old Centuri instructions. The flat side of the gussets are glued onto the wing with the narrow root edge against the body tube.

Here's how I glued on the wing gussets, Centuri style. To me it seemed like a stronger joint.

A dowel and engine casing has been slipped into the engine mount. A paper towel is rolled and pushed into the engine mount to keep the spray paint out.

Thursday, October 25, 2018

Semroc Snake Jumper Build, Part 5, Fin Gluing

Card stock fins are thinner than balsa and have a thinner root edge for gluing.
Just like the old Centuri kit, "gusset" reinforcements are used. The gluing position is lightly burnt into the side fins.
You can go ahead and fillet the gussets on the bottom, rounded side.

The long top fin and cockpit piece is easy to glue on crooked. Be sure it is straight before gluing. I used my aluminum angle to double check it down it's length.

The launch lug is sandwiched and hidden between the "wheels". The wheel halves glue on either side.

Before applying glue, do a dry fit and mark where the glue contact points are. Use less glue and there will be less to clean up.

Looking down from the front, double check to be sure the wheel halves are straight and parallel.

Wednesday, October 24, 2018

Semroc Snake Jumper Build, Part 4, Fin Prep

The nose cone was a bit rough, some "chatter" marks. Nothing some sandpaper can't take care of.

The picture on the right jumps ahead a bit. I had bad luck with this nose cone. As careful as I was, dents showed when the white was sprayed.

Here's both sides of the fins. All are laser cut in thick centering ring style stock, .050" thick.
As usual, one side of the laser cut is wider.

The burnt edges were cleaned up, sanded with 400 grit. You'd have a hard time covering up the black edges with white spray paint.

Sometime you'll find a de-lamination. Don't use CA (Super Glue) to close these up. For some reason it doesn't work as well in this application. Use wood glue. Sneak the glue in deep enough on the sides of a knife blade.

After all the sides are sanded clean, some medium CA glue can be brushed on using a Q-Tip. Sand the edges smooth with 400 grit after it dries.

First Man - Go!

I went to a matinee to see first man. That first attempt at the newest theater in the area was charging $15.00, I declined.
The theater I saw it in wasn't new, but still clean. It was two miles farther away but the ticket price was an agreeable $7.45.

No comments except to say it is a worthwhile watch for any space race fan. My big compliant were the ads before the start of the movie. The movie start was advertised at 4:45, the commercials ran from 4:30 until 5:00 p.m.

I was concerned about the director, he also did Whiplash. I hated that movie. His other big picture was La-La Land, I haven't seen it yet.
One interesting thing for me was the problems they had with the Gemini 8 docking mission. I don't remember Walter Cronkite talking about that -

Tuesday, October 23, 2018

Semroc Snake Jumper Build, Part 3, Replaceable Kevlar Ties

Some Kevlar is stiff, other Kevlar is soft and can start to unravel.
TIPS: I'll usually tie off the end with an overhand knot to prevent unraveling.
Run a bead of wood glue (never use CA glue on Kevlar or any string) and rub it into the Kevlar. After it dries you can cut off the knot in the Kevlar (wire cutters are easiest for this) and the line won't fray.

Here's how the replaceable Kevlar is tied on.
On the left is the low end of the engine mount. A tight loop is tied around the back end, over the hook on the other side. Feed the Kevlar line through the tube and out the top. Don't glue the Kevlar loop to the centering ring!

On the right is the upper side of the engine mount. A loop is tied in the end of the Kevlar for attachment of the shock cord.

After a few flights you can slip the loop off the back of the engine tube, slide it through the tube and see if it needs replacement.

A forward fillet was applied using a Q-tip taped to a dowel. Put a drop of glue on the Q-tip and spread it around the ring / tube joint. It'll take a few applications of glue drops to get all the way around the outside of the ring edge.

A Run-in at the N.E.F.A.R. Monthly

On October 13 I had an interesting interaction with a another flyer.
While waiting for the next rack, a flyer talked up his 3D printed model. He was pleased with his work. Larger than a BT-60 diameter, it looked thick and heavy. I asked which engine he was going to use. He said: "A D12-7.
I asked to see it to feel the weight. It felt like it weighed a pound.

I suggested: "That seems like a long delay. You might want to go with a D12-3. With a seven second delay it could be on the ground before the ejection charge goes off."
He followed: "I'm not worried about that, I'm just checking stability." Laughing he added: " My last model came in and stuck itself into the ground."
I saw that previous flight after hearing a "Heads Up!" It came in nose first and hit hard near the parked cars.
I said: "I don't want to argue, just check with the RSO.
He raised his voice and said angrily: "I don't want to argue with you either!" He stormed off.
I never raised my voice or insulted him. I just didn't want anybody (or a car) to be hit by a heavy rocket from an inexperienced flyer.

(The rocket pictured above was not the rocket described. It's just a picture I found online.)

Monday, October 22, 2018

Semroc Snake Jumper Build, Part 2, Engine Mount Upgrades

This build will incorporate the new Odd'l Rockets heavy wall BT-20H, an extended length engine hook and replaceable Kevlar.

The BT-20H diameter is a bit wider than a standard BT-20. To make it fit the centering rings sand the inside hole edge with 220 grit sandpaper. Roll the sandpaper around your index finger and rotate around the inside hole. Sand a bit, check the fit and repeat until the tube slides in. It's easier than you think.

To prep the rings for the replaceable Kevlar mount -
Punch a small hole directly opposite the engine hook slot on both rings. A plastic tube from Dollar Store Q-tips will slide through both hole
To use the new eXtended Length Engine Hook (3" long) the BT-20H tube is cut to 3" long.

A notch is cut for the upper bend of the hook. Make the notch 1/4" from the top of the tube.

On the far right is a dry fit of the hook, tube and centering rings. Black electrical tape will wrap around the tube, over the hook and tube, centered between the centering rings.

Slide the engine block in from the bottom, no glue yet.
Use an standard engine casing to push the block forward until it ends up under the top bend of the hook.

A glue fillet is applied around the top of the engine block. You can leave the engine casing in place until the glue dries.

Using the XLEH (eXtended Length Engine Hook) places the engine block under the top bend of the hook preventing movement and tearing of the engine mount tube. The fit of the engine top is also better, not resting at a slight angle in the tube.

Sunday, October 21, 2018

Semroc Snake Jumper Build, Part 1, Parts

The instructions, decals and fin stock were picked up when Semroc discontinued the kit. The rest of the parts are from my Odd'l Little Green Man kit. It uses the same nose cone, body tube and engine mount.

The face card says "Laser Cut Balsa Fins". I don't know if the first prototypes had balsa fins, the other Semroc Groonie kits had balsa fins.
The fin material may have been changed to card stock like the Centuri kit.

I will be using the Odd'l Rockets thick wall BT-20H and XLEH Extended Length Engine Hook for the engine mount. Replaceable Kevlar will be added.