Scratch Craft Orion

[billmi]

Getting back into building after a multi-decade hiatus, I decided to kick things off by cloning a favorite rocket, Estes' Attack Craft Orion.

In high school, I rolled BT-20 tubes out of office paper with Super 77 spray adhesive using taped together spent engines as a mandrel, and cutting the engine casings for thrust rings and the bases of rolled paper nosecones. They were lumpy and sloppy, but they flew.

Today, new materials and tools.

By 3D printing stackable 5cm long mandrel sections I can quickly get a mandrel in the size I want.



I've started parallel rolling construction paper tubes with watered down wood glue. First challenge has been shrinkage, tweaking the mandrel diameter to compensate. I've gotten to workable with a little variance and a tiny sandable wrinkle here or there. Good enough for now, but I will probably next try white glue, letting it dry on the paper then hot-rolling with a covering iron as described in one of Apogee's newsletters.

 

[billmi]

A few years ago my wife built us a laser engraver. This was my first go at using it to cut instead of engrave. Lesson learned: LaserGRBL imports vector art in SVG format. Instead of engineering units, SVG uses pixels as its dimensions, with no scaling info. Illustrator exports SVG files at 72 dpi, but Laser GRBL assumes 96dpi. That's easy to resolve once I figured it out, but I made the mistake of not measuring my first test cut before cutting fins. Now I have a 75% scale set of fins to spare. Maybe a BT-20 sized Orion clone will be a followup.

 

[billmi]

I decided early that I'd be 3d printing the engine tube spacers and a thrust ring.

I came up empty when hunting for 1/8-inch steel strip to make an engine hook from. Back in the day I used steel street sweeper bristles, easily found by watching the gutters while walking home from school, but these days I live in a commumity with no street sweepers and a lot of gutterless streets.

Then I ran across pictures of screw in engine retainers in this forum and that seemed like a good upgrade. So I modeled and printed working screw threads for the first time. That took a couple of versions to tweak from too-tight to smooth but snug.

 

[billmi]

Estes' kit had a paper to cut out a wraparound fin guide as well as a jig to set the canted fin angle. I expanded on that and made a fin alignment cradle. In the process I discovered our blue laser cut the paper of foamcore board fine, but disburses in the translucent foam. It worked well to transfer the design to some scrap foamcore, with an X-Acto and some hot glue finishing the job. This will probably become a storage cradle, too.

 

[billmi]

Some DAP Plastic Wood and Duco Cement later and it holds an engine and has fins on. Draft 1 of the nosecone came in at 33g. Draft 2 is next with a thinner shell and internal supports to be Dremeled out.

Also I discovered that the cloned Orion I'd used as reference (from posts here) had an Asteroid hunter nosecone which had a longer neck, so version 2 will get the short neck.

 

[billmi]

When I bought the balsa at Hobby Lobby, they included a parachute canopy free of charge, so I trimmed away its excess and added shrouds.


I always thought the launch lug on the original Orion looked out of place and asymmetrical, so I decided to make a pair of under-wing pods one for 1/8-inch and the other for a 3/16-inch launch rod. I planned on two small prints back to back with a tapered back, nestled into the 12 degree joint.



But... I didn't pay attention to the shafts being slightly off-center relative to the fin joint so they didn't quit line up back to back.

Version 2 of the nosecone (now with gunports in the top and bottom halves and a shorter neck) printed cleanly with some adjusted print settings coming in at 17g.

While thinking about fixing and reprinting the launch lugs, I decided I liked the look of them square-tailed and separated so that's how they went on with Duco Cement and everything got white glue fillets.

 

[BABAR]

When I bought the balsa at Hobby Lobby, they included a parachute canopy free of charge, so I trimmed away its excess and added shrouds.
View attachment 611088

I always thought the launch lug on the original Orion looked out of place and asymmetrical, so I decided to make a pair of under-wing pods one for 1/8-inch and the other for a 3/16-inch launch rod. I planned on two small prints back to back with a tapered back, nestled into the 12 degree joint.

View attachment 611091

But... I didn't pay attention to the shafts being slightly off-center relative to the fin joint so they didn't quit line up back to back.

Version 2 of the nosecone (now with gunports in the top and bottom halves and a shorter neck) printed cleanly with some adjusted print settings coming in at 17g.

While thinking about fixing and reprinting the launch lugs, I decided I liked the look of them square-tailed and separated so that's how they went on with Duco Cement and everything got white glue fillets.

View attachment 611093

Very cool.

I also like the Hobby Lobby bags for parachutes. They seem a bit tougher than the plastic grocery bags, but not as thick as trash can liners.

I like your asymmetry solution. Not having a 3D printer, I probably would have put on bilateral launch lugs (I use paper straws) and called them Pew Pee guns.

 

[jqavins]

A few years ago my wife built us a laser engraver. This was my first go at using it to cut instead of engrave. Lesson learned: LaserGRBL imports vector art in SVG format. Instead of engineering units, SVG uses pixels as its dimensions, with no scaling info. Illustrator exports SVG files at 72 dpi, but Laser GRBL assumes 96dpi. That's easy to resolve once I figured it out, but I made the mistake of not measuring my first test cut before cutting fins. Now I have a 75% scale set of fins to spare. Maybe a BT-20 sized Orion clone will be a followup.

I'm confused by this. SVG is a vector format, so it shouldn't be pixelated at all.

 

[billmi]

I'm confused by this. SVG is a vector format, so it shouldn't be pixelated at all.

There's no bitmap involved, so nothing got pixelated. The cut lines were nice and straight vectors, they were just the wrong length and position.

Svg is an xml based description of all the elements used to draw the image. Pixels are the units Illustrator and Inkscape use when writing svg files. For a bit of text, a line or an arc, among other attributes, the svg describes its location using x and y coordinates relative to the upper left corner of the virtual canvas on which it is drawn. The xml below describes various attributes of a block of text, including its x and y coordinates.



My problem was that Illustrator used a scale of 72 dpi when converting from inches that I drew in to pixels for the svg, but when LaserGRBL opened the file it assumed a pixel size of 96dpi as it converted from pixels to mm.

Since LaserGRBL assumed each pixel was 75% the size of what Illustrator used, the lines it had the laser cut were only 75% as long as they should have been. The lines were still straight diagonal paths, its just that they weren't the size and position they were supposed to be, and I hadn't bothered to check that - or even check the on screen dimensions in mm in LaserGRBL before committing the first piece of balsa to the laser.

Now I have an extra set of fins at 75% scale. I could toss them as scrap, or take the environmentally responsible approach and use them to build a 75% Orion.

 

[billmi]

My first foray into making waterslide decals yielded good and bad results.

Using Koala no-spray clear waterslide laser print paper there was poor adhesion of the toner to much of the left side of the letter sized page. Fortunately, the decals only took half the page, and I'd doubled them up for print, so I got a good set out of it.

 

[billmi]

After testing paints and decals on some scrap balsa, a single coat of Krylon primer, sanding and couple of coats of Krylon gloss white yielded a better finish than the Estes kit I built in the 80s, but a a finish that is quite a ways from perfect - including some minor wrinkles that I sanded down because I was a bit impatient with that second coat.

I decided to paint the windows rather than decal them, as with the kit. Painting in the gunports with clean edges was as much of a pain, with a similar number of touch-ups as I remember doing with the kit.

 

[NTP2]

or take the environmentally responsible approach and use them to build a 75% Orion.

Or plant a tree… but that’s no fun.

 

[billmi]

The decals help distract from the imperfections.

I'm pretty stoked about the decals overall. Until starting down this road, I didn't even realize consumer printable decals existed.

 

[billmi]

Few people are familiar with the X-75, a 75% subscale technology demonstrator program that played a key role in the development of the Attack Craft Orion.