heavy-duty 1/4 inch launch lugs part 1

[powderburner]

This tidbit is for you mid- and high-power guys who are using 1/4-inch launch rails/lugs:

Some heavy duty launch lug material can probably be had for free if you will go to your local archery store or shooting range. Arrow shaft materials have expanded beyond wood over past decades, and include high-quality tubular aluminum and carbon-fiber-composite. (You do not want fiberglass arrow shafts for rocketry applications: those are bow-fishing shafts and are solid.)

You can probably get some of this tubular aluminum for free because when someone purchases a set of arrows (generally a half or full dozen shafts at a time), there is some excess length trimmed off and thrown away. Arrow shaft raw stock arrives in lengths like 31 inches, but most archers need only 27, 28 or 29 inches, thus there are often pieces of high-quality aluminum shaft thrown away at the archery shops. If you ask nicely, I’ll bet they’ll be happy to hand you a bunch.

At a shooting range, you may find mostly-complete shafts that have been discarded due to minor damage. All it takes is a little nick or dent in the shaft and it is unsafe to shoot; most archers just dump ‘em in the trash. And even if the shaft is totally crunched in the middle, there is still lots of useable material at both ends.

Or, odds-and-ends show up occasionally on ebay at decent prices. Archers like to shoot a batch of the same-size shaft, for ballistic consistency. They tend to discard the “leftovers” when they get down to three or four remaining arrows, and they go buy a new dozen.

As a last resort, you can buy brand new shafts and cut them into pieces for launch lugs, but don’t tell me about it or I’ll cry. A good source of inexpensive stuff is FS Discount Archery, catalogs available free at 800-824-8261, also have a website. There are many others, look around.

The aluminum used in these shafts is often 7000-series, aircraft-grade stuff (like in Easton brand shafts). It is very high strength, extremely straight, stiff, very smoothly finished on the inside, and usually anodized on the outside in some form of camouflage pattern. The anodizing is an ideal surface finish for getting good epoxy adherence, as the microscopic surface roughness offers plenty of grip. Manufacturing quality is very high on arrow shaft material, and I think you will be impressed with the smoothness, straightness, and material quality.

Aluminum arrow shaft sizes (diameters) are described with a four-digit number. The first two digits are shaft outside diameter, in sixty-fourths of an inch. A 2117 shaft is 21/64 inch in diameter, or 0.32815 inches. The second two digits are the wall thickness, in thousandths of an inch. A 2117 shaft has walls 0.0170 inches thick. Simple math tells you the inside diameter; a 2117 shaft is 0.2941 inches in inside diameter, plenty to fit over a 1/4 inch launch rod.

 

[powderburner]

Sizes (diameters) will vary with what you find. I described a 2117 shaft in the example above because that is a common “utility” shaft size, and you will probably find lots of it. Easton is one of the big names in this business, but this info also applies to other aluminum shafts. Additional data is listed below (no, I am not gonna try to HTML it all) for the shaft size designation, O.D., and I.D., for a range of Easton shafts. (And I sure hope I did my math correctly so I don’t embarrass myself.)

1716 0.2656 0.2336
1813 0.2813 0.2553
1816 0.2813 0.2493
1913 0.2969 0.2709
1916 0.2969 0.2649
2013 0.3125 0.2865
2016 0.3125 0.2805
2018 0.3125 0.2765
2020 0.3125 0.2725
2113 0.3281 0.3021
2114 0.3281 0.3001
2115 0.3281 0.2981
2117 0.3281 0.2941
2212 0.3438 0.3198
2213 0.3438 0.3178
2215 0.3438 0.3138
2216 0.3438 0.3118
2219 0.3438 0.3058
2312 0.3594 0.3354
2314 0.3594 0.3314
2315 0.3594 0.3294
2317 0.3594 0.3254
2413 0.3750 0.3490
2419 0.3750 0.3370
2512 0.3906 0.3666
2514 0.3906 0.3626
2613 0.4063 0.3803

To cut aluminum shafts, the archery shops use these cute little high-speed saws that make a really clean cut. A Dremel with cutting disk would be a close equivalent. At home, I use a small roller-type pipe cutter (like for small copper tubing) from the hardware store. I make the basic cut with the pipe cutter, flare the cut edges back out to full diameter by rolling around a heavy metal rod (like a launch rod), and clean up the edges with sandpaper.

I’m not sure that I recommend aluminum arrow shafts for use as a body tube with MMX motors, because after all, this IS metal. And we are all good little rocketeers that follow the safety code at all times, aren’t we?

I am also not going to recommend using carbon-fiber shafts. In fact, I recommend against it. Carbon fiber is nasty stuff if you get any of the broken fibers under your skin or inhale it. Loose carbon fibers (like from cutting and snapping a tube?) floating in the air are a disaster for ANY electronics you have in the house (this was one of the weapons dropped on Baghdad in the Gulf War I). Besides, carbon shafts usually come in shorter lengths and smaller diameters (if you really want an explanation, email me). Some of it comes in “barreled” form, with smaller diameters at the ends and larger diameter in the middle. And carbon shafts are powerful expensive if you buy them new. Still, I suppose some of you high-power guys may be able to bond full-length carbon shafts to your rocket airframes for local structural reinforcement?