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Old 01-03-2021, 04:39 PM
MarkB.'s Avatar
MarkB. MarkB. is offline
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Default Frangible Arcas

Comrades:

So the September/October Sport Rocketry had Chris Timm's article on the Frangible Arcas. It had color pictures and cool MacDraw drawings. A Frangible Arcas was an attempt by the Atlantic Research Corp. to develop an Arcas rocket that would blow itself to smithereens on the way down so as to decrease the chance of hurting somebody or something. Basically, a wrap of explosive fabric was added to the motor section, a section was added under the parachute canister with detonator stuff and a new fin unit that I'm willing to bet came from or lead to the HV Arcas fin can was slipped on the nozzle end. Four were launched in the mid '60's as proof of concept, one apparently remains.

I thought it would be fun to build a model in the same scale as the Alway/BMS kit (1/4.61 scale) as it meant I had an available nose cone from BMS (BNC-50AC). So an afternoon with the calculator revealed that the main body tube would be ST-10. I wanted to make a slip-on fin can using the BT-50+ that eRockets sells. A sheet of heavy cardstock was used through out, as you'll see below.

Picture 1 shows the initial assembly. I made the little fin can first out of 2.154" of BT-50+ and 1/16 basswood fins. I slid this on to about a 4" piece of BT-50, marked it and then added a single wrap of cardstock on the remaining length of BT-50. This would be sufficient for a snug fit into an ST-10. I did the same to create the nose cone. I added about a 5/8" wrap to a piece of BT-50 for the shoulder section. Inside the BT-50, I added about a 5/8" length of a balsa BT-50 coupler with a piece of hardwood dowel inserted to hold the screw eye. The nose cone shown is not the one I eventually used. I used more cardstock wraps to build up the small transition section from BT-50 to ST-10 which I shaped, sanded and sealed with superglue. It ended up slightly oversized at 5/16" long as opposed to 3/16" but good for a first try. The main tube also got a section of cardstock wrap at the front end to replicate a bulged section there. I had hoped to use a 13mm motor mount like the Alway version but I'm glad I waited to the end to make the decision, as it came in heavy due to the thick-wall ST-10 and I went with 18mm.

Picture 2 shows the painted subassemblies. I decided to use the remaining artifact scheme. Note the difference in nose cone shapes. Paint on the fins is Rustoleum Flat White with one quadrant of Flat Black. The body tube is Krylon Fluorescent Red-Orange with a clear flat acrylic overspray. The nose is Rustolem Flat White with a Rustoleum Shell White parachute canister.

Picture 3 shows it assembled in this morning's bright Texas sunshine

Picture 4 shows it next to my son's Alway Arcas. It's slighty taller because of the extended nose section and the longer fin can.

Very Cool. Thanks for the plans, Chris
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Last edited by MarkB. : 01-03-2021 at 09:51 PM.
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Old 01-04-2021, 10:57 AM
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Chris_Timm Chris_Timm is offline
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Default Detail pics

I tried in vain to duplicate the "bulge" portion.
Even going so far as to use the RIGID contour tool.
It is a lumpy, resin-like coating, not unlike fiberglassed cloth.
The coloring is similar to an aged amber. (burnt orange as I noted it)
Reminds me of varnished yellow pine after a few decades of aging.

The 2 screws and jack port are the only sizable details.
There are other smaller holes here and there, but those could be modeled with a fine point Sharpie.

The fin detail shows how small the 1/20" wrap thickness looks in real-life.
Notice the almost imperceptible wedge cross-section on the fin.

Cool model!

I am pleased someone actually uses the scale data to build models with.

There is a Frangible-Arcas model produced by Rocketarium of the test round flown at Pt. Mugu.

Kudos Mark!
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Last edited by Chris_Timm : 01-04-2021 at 11:23 AM.
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