I would recommend using aluminum for the retainer. Steel will rust very quickly if it isn't a good stainless.
L2 Certification Build -- Five Finned Folly
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And, steel might be a violation of the safety codes, whereas brass, copper, or aluminum would not.
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PBic
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Hasn't Estes been using spring steel motor retainers for over 50 years?
Yes, in minimal amounts and where needed its specifically allowed by the safety codes. Same for eye screws. Its needed in the case of spring steel retainers because steel can be spring tempered.
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jlabrasca
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It will be a big washer, not too different from the retainers Madcow sells.
https://www.madcowrocketry.com/motor-retainer-38mm/
If I make it from 28 gauge steel, it will mass something less than 10 grams. No part of it will extend outside of the tailcone -- so it will be, arguably, inside the rocket.
Its light enough that it might be allowable, but why not make it from aluminum? For this application steel is not really required.
At the same time I realize its not much different than the nozzle washer for a snap ring case, which is heavier and steel.
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RocketFeller
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I was thinking the same thing. Aluminum is so much easier to cut that it seems like a better choice. It also shines up nice with a bit of fine sandpaper.
I made this thrust plate and retainer ring on my drill press for my Bigger Daddy.
jlabrasca
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Well, the steel isn't necessary, so I will use something else.
Dan's motor mount/retainer is beautiful. The Moonbat's will be hidden by the tailcone so I will not try to make it conspicuously pretty.
I sat for, and passed, the L2 written test yesterday. I also received my certification-allowance motor -- so I'll be able to get a real launch weight and COM when it is finished.
Not worth a picture, but I worked a little on the nosecone. Its slow going. I don't want to screw it up more than I've already screwed it up.
Started building the tailcone.
I cut three conical transitions, each smaller that the last by the thickness of the card stock (a chipboard box), and stacked them like matryoshka dolls. Lots of fitting, and trimming, and refitting. I mis-measured the diameter of the blue tube by 0.4 mm, which meant all of the transitions were about 1.5 mm too long.
I had to do a little more fine-tuning on the fin slots to get the fins in tight to the tail-cone. I am not going put a tube inside the tail cone. After I get the final wall thickness I will cut plywood formers -- narrow rings -- for the forward and aft openings.
Dan's motor mount/retainer is beautiful. The Moonbat's will be hidden by the tailcone so I will not try to make it conspicuously pretty.
I sat for, and passed, the L2 written test yesterday. I also received my certification-allowance motor -- so I'll be able to get a real launch weight and COM when it is finished.
Not worth a picture, but I worked a little on the nosecone. Its slow going. I don't want to screw it up more than I've already screwed it up.
Started building the tailcone.
I cut three conical transitions, each smaller that the last by the thickness of the card stock (a chipboard box), and stacked them like matryoshka dolls. Lots of fitting, and trimming, and refitting. I mis-measured the diameter of the blue tube by 0.4 mm, which meant all of the transitions were about 1.5 mm too long.
I had to do a little more fine-tuning on the fin slots to get the fins in tight to the tail-cone. I am not going put a tube inside the tail cone. After I get the final wall thickness I will cut plywood formers -- narrow rings -- for the forward and aft openings.
Beautiful work!
jlabrasca
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<blush> thank you.
While stacking the cones you see up in #39, I noticed that they they were tending towards a circular profile without a former or any real coaxing from me. I printed up another set of patterns with a tighter pitch between layers, and cut them from a thinner and more flexible cardstock (the white-on-one-side stuff commonly referred to as "shirt cardboard"). I guessed wrong about how tightly I could get the cones to stack and ended up leaving off the outermost cone (which is why it says "middle" on what is clearly not the middle layer). I used Beacon Frabri-Tac glue between the layers because it is flexible and has a long open time. Pressing the stack between pieces of plywood scrap, the tailcone came out nearly perfectly circular. I'll seal the edges with CA, and the whole assemble will get a coat of finish epoxy before I paint it. I will have to see how long/tall a cone I can fabricate in this way.
I cut a retainer from 3mm aluminum/plastic composite panel. Light weight, high tensile strength and fire-proof. It bends a lot more easily than steel or solid aluminum, but not too worried about that with five points of attachment around the circumference. I dropped in a 38 mm DMS, and everything fits together.
I am still going to cut a steel one. I'll keep this on hand in case the RSO says no.
I filled in the the seam on the one bad slot and I shot some more paint tests on earlier versions of the fins (when it was going to be a a Four-toed Moonbat). So far, I like the Duplicolor Metalcast yellow the best. The Spaz Stix color change paint is very cool, but it takes a lot of coats to get the effect.
Final assembly this weekend. Because I am a grown-up with responsibilities I won't get out to Brothers to launch next weekend -- which is breaking my heart. If I get it finished in time, I might try to drop in on the June Tri-Cities launch. Otherwise it is looking like NXRS or Summer Skies. I've got too many rockets that have never flown.
jlabrasca
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Started painting.
Pushed the fins into the airframe, to see how the colors clashed.
I really like the Metalcast yellow. In the bright sun its actually a painful to look directly at the fins. Which is good, because I did a crap job painting them.
The Rustoleum automotive metallics continue to baffle and vex me. That purple was a LOT darker the last two times I used it. I really don't want to inhale any more solvent, so I might be inclined live with the lurid Mardi Gras colors and only take the rocket out on overcast days, except that the dog saw somebody in the street and knocked over the painting stand in her rush to get the fence -- so I've got some dings and scrapes to repair.
Looks like we will have decent weather for painting tomorrow, so I will give the airframe a few coats of something less super-villainy.
Pushed the fins into the airframe, to see how the colors clashed.
I really like the Metalcast yellow. In the bright sun its actually a painful to look directly at the fins. Which is good, because I did a crap job painting them.
The Rustoleum automotive metallics continue to baffle and vex me. That purple was a LOT darker the last two times I used it. I really don't want to inhale any more solvent, so I might be inclined live with the lurid Mardi Gras colors and only take the rocket out on overcast days, except that the dog saw somebody in the street and knocked over the painting stand in her rush to get the fence -- so I've got some dings and scrapes to repair.
Looks like we will have decent weather for painting tomorrow, so I will give the airframe a few coats of something less super-villainy.
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jlabrasca
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Made the holes for the rail buttons. Something I absolutely should have done before I started painting. I was dithering, trying to decide between screw-on buttons and glue-on guides. After I repainted the airframe, I decided that black delrin buttons would be less obtrusive than whatever that white plastic is that is used for the 3D printed guides.
I glued the rest of the obsolete aft centering ring to the back of the forward centering ring, then I printed a template (a large format inkjet printer is at least as useful as a 3D printer for rocket building). It took me a minute to figure out how to mount the round-stock holders the Stanley plastic saw horses.
I didn't want to drill, because the blue tube is so plastic, I figured I'd end up sanding away a burr on the outside and then have to repaint. So I hung the tube on a piece of 2" round stock, strapped everything down with blue-tape, and hit the tube with a 6mm arch punch. The holes are clean. Only lost a little paint at the edges -- a defect completely covered by the button.
Last dry fit, with a piece of 1" X 1" standing in for a 1010 rail.
The tail cone gave me a lot of grief. After the finish epoxy cured, I sprayed it with coat of Dulpicolor Metal Cast ground coat. Instant orange peel. I sanded, primed it, sanded it again, but it looks like the outer layer of the paper is wrinkled. I put two coats of Rustoleum High Heat silver on it and am moving on.
Prepping the balsa cone for paint -- two coats of thin CA, and lots of sanding.
ProTip <grin>: The little paint brush you can see in the background above is one of THESE -- they are less than useless for paint, but they are great for CA. They don't smoke like cotton swabs, the CA in the bristles stays liquid for a long time, and the (eventually) hardened CA comes right off the bristles.
Also, I put two light coats of Rustoleum Automotive Metallic blue over the Metallic purple. It makes a two-tone effect. A dark blueish purple, with bright blue highlights.
I really expected this build to be done a month ago.
I glued the rest of the obsolete aft centering ring to the back of the forward centering ring, then I printed a template (a large format inkjet printer is at least as useful as a 3D printer for rocket building). It took me a minute to figure out how to mount the round-stock holders the Stanley plastic saw horses.
I didn't want to drill, because the blue tube is so plastic, I figured I'd end up sanding away a burr on the outside and then have to repaint. So I hung the tube on a piece of 2" round stock, strapped everything down with blue-tape, and hit the tube with a 6mm arch punch. The holes are clean. Only lost a little paint at the edges -- a defect completely covered by the button.
Last dry fit, with a piece of 1" X 1" standing in for a 1010 rail.
The tail cone gave me a lot of grief. After the finish epoxy cured, I sprayed it with coat of Dulpicolor Metal Cast ground coat. Instant orange peel. I sanded, primed it, sanded it again, but it looks like the outer layer of the paper is wrinkled. I put two coats of Rustoleum High Heat silver on it and am moving on.
Prepping the balsa cone for paint -- two coats of thin CA, and lots of sanding.
ProTip <grin>: The little paint brush you can see in the background above is one of THESE -- they are less than useless for paint, but they are great for CA. They don't smoke like cotton swabs, the CA in the bristles stays liquid for a long time, and the (eventually) hardened CA comes right off the bristles.
Also, I put two light coats of Rustoleum Automotive Metallic blue over the Metallic purple. It makes a two-tone effect. A dark blueish purple, with bright blue highlights.
I really expected this build to be done a month ago.
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After drilling leaves a burr, I just chamfer it with a countersink bit or even a pocket knife.
Thats looking really good. Ive seen L3 rockets with less attention to detail.
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rharshberger
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You are welcome to drop in on any Tri-Cities Rocketeers launch you chose. June 2018 is a 3 day event from the Fri-Sat-Sun Fathers Day weekend.
jlabrasca
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Thx. It will be awesomely phenomenal when its done, thats for sure. I really wish somebody sold some kind of "kit rocket", where you could buy a bag of parts and assemble them. <sigh>
Tried out the the new action camera for size. Its going to be a right carbuncle, no matter how I mount it.
Got some primer on the balsa cone.
Frustratingly, the plastic cone-now-transition is just slightly out of round. I suspect that it happened after I cut it down -- something relaxed after I separated the tip, or when I sanded it out to try to even up the thickness of the rim. It makes a little lip under the balsa cone on one side or the other, unless I wrap 10 turns of tape around the shoulder of the balsa cone and jam it in. This pushes the transition back into shape. I was planning for the balsa cone to be removable. That would make it easier to add ballast to the very top of the rocket, where it would have the greatest affect on moment of inertia. Now I am thinking I will just epoxy it in place and sand the seam flush.
Got the motor mount epoxied into the airframe, and the first fin is epoxied into the motor mount. There isn't a whole lot of room for epoxy in that trough, but it really shouldn't take a large mass of adhesive to secure the fin -- as long as all five glue surfaces of each each fin tab are well adhered -- at least that is what I am telling myself.
In the first image, you can see the bad slot. With all the fittings and measuring and refitting, repeatedly jamming fins into that slot, I managed to scrape off all the body filler that I put on to fix the ragged seam. So. Yay! Another problem to solve!
Igotnothing
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"This is what it looks like when a 20th century brain gets tired of pushing pixels around and reverts to old habits."
Amen!
Amen!
jlabrasca
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Had to break out the rifflers to clean up some paint from the tabs on the fins.
Fins are and tail cone are attached. There is a little epoxy squeeze-out adhering it to the fins, but tail cone mostly just trapped by the fins.
The tail cone was just slightly oblate, so I attacthed fins in positions 1 and 5 (adjacent to eachother) and put the cone in so that its long axis was aligned more-or-less half way between them -- lined up with the slot in position 3, where I attached the next fin. The last fin to go on wouldn't quite seat, because the tail cone was pushing it out. I put a drop of CA at each end of the fin, where it made contact with the airframe, and held it down until the glue set. Should probably have made a jig to do that -- my hands are still cramped and sore 5 hours later.
Assembled and installed the bulkhead in the forward section of the airframe.
glued-up the nosecone and and started paint
jlabrasca
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Its not finished, but it is rocket-shaped.
Rail buttons are installed -- and straighter than I'd've predicted, given the sloppy and misshapen threaded inserts. I stripped the cap on the aft insert before I got it quite flush with the surface of the tube. I had grab it with a pair of reverse pliers to finish driving it in.
Dropped the baffle in. It will be weird, after all these weeks, not to have it in my way every time I go to put something down on the bench. Its been kind of mascot for me. Sliding it in from the forward end, there was no obvious way to get glue between the top plate of the baffle and the wall of the airframe -- so I just tacked it in place with some gap-filling CA. The coupler will do most of the work of keeping it from being blown out/yanked out at deployment.
I don't know if blue-tube is subject to coupler grabbing with yellow glue -- decided not to risk it. Used a LOT of 15 minute epoxy.
I am moderately pleased with the nosecone. The balsa sanded up much prettier than did the plastic.
Its too tall to get a good shot of the completed rocket in the clear -- at least not while avoiding the robins nesting on our front porch and the sparrows nesting in our pergola.
Nothing left but to build he recovery harness (took delivery of the parts for that last week) and getting the final weights and measures with the motor, chute, and harness installed.
Edit: Shot of the competed (almost) Moonbat. I decided to go a little longer than the original design (couldn't think of anything to do with an 8 in off-cut of 76 mm blue tube) so the final length is about 154 cm.
Rail buttons are installed -- and straighter than I'd've predicted, given the sloppy and misshapen threaded inserts. I stripped the cap on the aft insert before I got it quite flush with the surface of the tube. I had grab it with a pair of reverse pliers to finish driving it in.
Dropped the baffle in. It will be weird, after all these weeks, not to have it in my way every time I go to put something down on the bench. Its been kind of mascot for me. Sliding it in from the forward end, there was no obvious way to get glue between the top plate of the baffle and the wall of the airframe -- so I just tacked it in place with some gap-filling CA. The coupler will do most of the work of keeping it from being blown out/yanked out at deployment.
I don't know if blue-tube is subject to coupler grabbing with yellow glue -- decided not to risk it. Used a LOT of 15 minute epoxy.
I am moderately pleased with the nosecone. The balsa sanded up much prettier than did the plastic.
Its too tall to get a good shot of the completed rocket in the clear -- at least not while avoiding the robins nesting on our front porch and the sparrows nesting in our pergola.
Nothing left but to build he recovery harness (took delivery of the parts for that last week) and getting the final weights and measures with the motor, chute, and harness installed.
Edit: Shot of the competed (almost) Moonbat. I decided to go a little longer than the original design (couldn't think of anything to do with an 8 in off-cut of 76 mm blue tube) so the final length is about 154 cm.
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jlabrasca
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Snuck down to the shop for an hour and assembled half of the recovery harness.
I've left myself about 1470 cm3 (about 90 in3) for the recovery gear. This is NOT how I will pack the parachute for a launch, I just wanted make sure that I didn't do something really stupid when I glued things together. I thought it was going to be much tighter with 15 feet of 1" tubular nylon in there. It actually goes in really easily -- with more than a little room to spare.
Also got some better shots of the rocket assembled.
I've left myself about 1470 cm3 (about 90 in3) for the recovery gear. This is NOT how I will pack the parachute for a launch, I just wanted make sure that I didn't do something really stupid when I glued things together. I thought it was going to be much tighter with 15 feet of 1" tubular nylon in there. It actually goes in really easily -- with more than a little room to spare.
Also got some better shots of the rocket assembled.
jlabrasca
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Thank you.
Just need to figure out how to get it to a launch.
Finished the recovery gear. The forward section of the airframe is nose-heavy and too aerodynamically stable by itself. On deployment, it is liable to be blown away from the aft end like a dart. I can't think of a way to make it tumble, so I am including a long section of elastic shock cord attached in parallel to a strip of velcro -- to act as a shock absorber. The idea is that the velcro will peel apart like a loomed-fiber fall-arrester -- dissipating the energy of the ejection. The worry, of course, is that something will go wrong that the tension exerted by the shock-cord will act parallel to the velcro -- and it won't peel. At which point, the integrity of the rocket will depend upon my joinery and the ultimate strength of the well-perforated piece of 3/16" aircraft plywood at the the top of the baffle.
I decided to test the "fire-resistance" pf the aluminum composite retainer. I've used a torch to soften this stuff up, so it could be bent. It occurred to me, however, that I'd never exposed a cut edge to flame. The nozzle of the 38mm DMS motor is recessed, so the plastic is going to be in the exhaust. "Fire resistant" is not an idle boast. 3 minutes of direct exposure to a propane torch, and it did not burn -- but it smoked and melted.
I stopped at the scrap-metal recycler on the way home from work, and picked through the bin of aluminum sheet. One of the technicians saw me looking at a big piece of 15 gauge. When I told him what I needed it for, he squared up a piece about 30 cm by 20 cm. No charge. Which almost makes up for the fact that I forgot to check the tension on my z-axis before I started to cut it. That endmill had been through a lot in its short life in my shop, as I fumbled my way through various parts of the CNC learning curve. I think I will try to use the shank as a shaft for a drag-knife.
Tightened up the belts, wrote a CAM program that took a shallower first bite, put in a heavier end mill, and ran it again
I will tighten up all the belts before I cut aluminum again, I left a little foil at the bottom of the ends of the long cuts, which suggests that the mill is climbing.
Last detail, tried on the "please return-to" decal for size. This looked a little large to my eye, so I will reprint it at 90%. It'll be attached with 777 adhesive, before the whole rocket gets light coat of automotive gloss clear.
Got the as-built dimensions measured and into OR this morning. It is about 200 grams heavier than I'd originally guessed, but the balanced-on-a-straigh-edge COM is pretty much exactly where the simulation put it. With no motor, the rocket masses 1590 grams. With the the J270 installed it'll have a pad-weight of about 22 newtons; better than 10:1 thrust-to-weight. OR has it coming off a 1.3 meter rod at about 17 m/s (about 55 ft/s). Seems little slow to me, but the rocket is over-stable and the fins don't present much of a cross-section to the wind -- so maybe not too big a worry?
Still have to figure out how/if I will attach a camera but I am done with the build. Hopefully my next post will be the flight report. Thanks for reading, and for the advice and encouragement.
snrkl
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I am interested in how this velcro shock absorber works - I think I understand roughly how you are using it (the velcro peeling absorbing ejection energy part) but I can't visualise how it is actually rigged (relation of elastic, velcro and shock cord). Is this something you can elaborate on a little?
jlabrasca
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A quick sketch
Do a search for "loomed fall arrester" and you might find some videos or better graphics illustrating the idea I am stealing for this application.
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I keep waiting for one of the vendors to discover what a hook pile tape lowering line is for and why I would want to buy a harness with that type of rigging baked in.
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I had to Google that phrase.....
I think a Velcro energy dissipation system is a great idea. I considered it once but was worried that the hook side might snag the nylon parachute. I think I was overly cautious and that youre onto something smart.
Im not so sure about the riveted grommets in the ends of the strap (if Im understanding correctly). Ive not seen that for load bearing applications. I usually see loops sewn or tied into the ends of nylon straps. I would be concerned that the grommets would pull out under load.
Youre doing a lot of really clever things with your design; Im happy to see that.
Im not so sure about the riveted grommets in the ends of the strap (if Im understanding correctly). Ive not seen that for load bearing applications. I usually see loops sewn or tied into the ends of nylon straps. I would be concerned that the grommets would pull out under load.
Youre doing a lot of really clever things with your design; Im happy to see that.
jlabrasca
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Me too. I have to say, apart from learning (I think) that it is something to do lowering equipment as you descend by parachute, I didn't find much detail on HOW you use hook pile tape lowering line. I'd like to know more about this, if dhbarr can point me at a resource?
That is a real concern. The canopy doesn't catch on the hooks, but I am using a tangle-free chute with nylon netting in place of shroud lines and the hooks do snag that. The chute will be packed at the far end of the harness from the velcro, with the rolled up shock cord between. The manufacturer includes a deployment bag (the parachute is bagged in the images above) which doesn't stick to the hooks. I am not sure I will use the bag, though. I'll be flying a JLCR, so it ought to be enough to fold the canopy over the mesh. By the time the release opens, the velcro should be peeled and flapping harmlessly 4 meters away at the other end of the line <fingers crossed>.
Yeah, I'm not sure about those either <grin>. I can't sew. Two grommets at each end of the nylon strap, with the end folded over to make a loop, with the quick link through both grommets. I don't suppose it will last for as many launches as would a sewn loop, but I think it will support the weight of the rocket for few descents. How it will perform if it has to take the full shock at deployment -- I can't even guess. It might be that one or the other grommeted ends would fail before the anchor in baffle, but I kind of think its a coin toss.
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