The /dev/rocket Story/dev/rocket is a trademark of MontaVista Software, Inc. ...and that's how this all started. Starting with a logo for our /dev/rocket product, I decided to try to make a flying replica of it. This is that story. All photos are links to larger versions which will open in a separate browser.
When my company produced a version of the Eclipse development software and named it /dev/rocket (pronounced dev-rocket, the slashes are silent), Patrick in marketing came up with a design for a rocket logo that was somewhere between Jack Hagerty's Myst Rocket and the Wallace and Gromit rocket from A Grand Day Out. It was basically an orange football with fins. In fact, it was a football from the graphic library, with the end cut off and fins added. I seriously considered cutting the end off of my old Nerf football (which was already pretty close to the same orange color), but better judgement (or was it just a lack of time?) won out. But last year when the second version of the product was released we hired a firm to redo the logo and come up with a new design. This was when I thought I just might be able to make this fly. Literally. This is one of the evaluation shots that I used as a basis for the model.
01_splash_003a.jpg This is the prototype. This is a pair of Estes BT-50 sized nose cones with a 13mm central tube. This is the third flight, at the May 7, 2005 Snow Ranch launch (the previous two flights were the month before). Photo by David Flournoy.
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By measuring the drawings of the logo, it seemed that the nose/tail cones for this design needed to be a tangential ogive shape, with about a 3:1 length to width ratio. I didn't have any larger nose cones that shape, but I did have a pair of Apogee 20070 nose cones with approximately a 5:1 ratio. One had a cracked base (even better!) from the moderately disasterous flight of my Illumi-Roc in the fall of 2004. (Actually, the flight was spectacular, the landing was, well, even more spectacular - but luckily no one got hurt). To account for the thinner profile, I squeezed the shape of the fins and came up with this rocket, which I dubbed, Deviant Rocket (since it deviated from the /dev/rocket shape). Also, not being a corporate trademark, I took some liberties with the color scheme. It flew well with 1/2 oz of lead in the nose but I suspect a full ounce will work even better. This is the photo that is featured in May in the 2006 EMRR Calendar. Another excellent photo by David Flournoy.
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The starting point. These are the basic raw materials: two Public Missiles PNC-3.9 nose cones, a sheet of 3/16" aircraft plywood, and the logo file. The template has yet to be cut and assembled.
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Here we go. The fin templates have been cut out and traced onto the plywood and the bases of the nose cones have been cut. The band saw that will be used soon is in the background.
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Here's a close up of the tracing on the plywood. I just managed to get them to fit by shrinking the through tab a bit. Some measurements and a bit of geometry and I assured myself the tabs were still long enough (they were).
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All three fins, cut out and stacked up. I used 3M spray mount to hold them together as I shaped them. After my first attempt I felt it wasn't a strong enough bond so I redid it with a thicker coat on both surfaces. This would come back to haunt me later.
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Here's a close up of nose cone that was to be the bottom half of the rocket, the tailcone. You can see the ridge on the seam, as well as the ridge inside. A Dremel tool with a sanding drum made short work of the inside ridge. The seam turned out to be more of a challenge, but much hand sanding, filling and such eventually did well.
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Here are the fins and the initial markings on the tail cone. Also here's a look at the original prototype. This has flown successfully three times on 13mm motors.
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Here are the fins again, showing some detail on the shaping of the leading and trailing edges, sitting on the band saw. On top of the fins is the centering ring with a 5° bevel on the outside edge and hole for a 29mm motor tube. Next to the ring are the 6-32 thread T-nuts that will be the mounting point for the motor retention.
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Here's the tail cone, slotted and the end cut off. There were some thin spots inside which I tried to fill with some epoxy (thus the gray mess inside). After marking them, I free-handed the slots with the aforementioned Dremel tool and a small cutoff wheel. Worked better than I thought. I've since discovered that Public Missles will do this work. Next time!
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Here's the centering ring drilled and painted. This ring sits in the bottom of the tail cone, just a bit below the tabs of the fins. Besides centering the motor tube, I expect the ring to transfer the force of the ejection charge into the tail cone. The nose and tail cone are made of HDPE (high density polyethylene) which is amazingly difficult to get anything to stick to. I tried to rely on physical fit rather than epoxy adhesion to transfer forces. On at least one image of the logo, the motor nozzle is black, so I wanted the tail assembly to be black, but no paint on any surfaces to be glued.
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The centering ring, with (unpainted) T-nuts, epoxied to the motor mount tube.
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The inside of the tail cone, also painted black, but hopefully not past the point where the centering ring attaches.
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Getting close now. Note the motor tube is quite long at this point. I had thought to have the central tube come all the way up into the top of the nose cone, but abandoned this as it seemed the tube would be a tangle hazzard for the recovery system and would take up needed room. The plan is to put some expanding foam into the tip of the nose to prevent the parachute from getting wedged into the tip of the nose. The cardboard ring (cut from the band saw blade package) will be used to center the motor tube during the glue up.
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Just about ready to paint the end of the motor tube.
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With the end of the motor tube painted, I wanted to be very sure no epoxy got into the threads of the T-nuts and so masked them off. The brass clips and screws for the motor rentention are visible just below the tail cone.
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Another view of the masking of the motor tube/centering ring. I made sure I could reach the tape and pull it off once inside the tail cone, which is why the tape comes up so far.
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Yet another view...
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Almost ready to glue it up. I've added a sleeve of newspaper to the motor tube (now cut to final length) and put the cardboard upper centering ring around the motor tube which will be used to hold the whole assembly above the table. A bit hard to see in this picture (but visible in the detailed version) is an extraction string on the cardboard ring.
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I mixed up some Devcon 30 minute epoxy and went for it. I had already roughed up the inside of the tail cone with some 120 grit sandpaper first.
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I was pretty happy with the way the assembly went. I got a nice, even fillet of epoxy around the outside of the centering ring. Then I got a bit impatient and tried to remove the tape from inside and pulled the whole thing apart. Luckily it went back together just fine and this time I kept my hands off of it for a day.
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The view from the other side. Another nice fillet of epoxy. If I seem overly proud of my technique, well, not everything turned out this neat.
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Another view from the rear. I thought for a while whether to glue up the fins at the same time, but felt that was asking for trouble - too many things to get right all at once.
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...but, it turned out the central tube was just a tiny bit off center. I used the clamp shown to pull it over just enough to get all three fins to sit against the central tube, and sit flush against the body on the outside. Finding the right combination of elastic to hold it together without the whole thing flying apart was fun. Dry runs (literally) were critical to getting a method that worked. The bottom two rings are fabric elastic and at the top is a rubber band. Some folds of masking tape that stood up from the edge of the fin gave the rubber band something to bite into. I used GC brand epoxy for both the body and central tube. I've had good luck with this, though I think it's a bit heavy. It's a pretty slow cure. This was not a job for a 5 minute epoxy.
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The view from above. Not nearly so neat a job on the epoxy here. But, I think it's going to hold just fine. This also features the first appearence of the contraption I hope to use to secure the nose weight and shock cord. At the right side of the picture are bits that were cut off of the tail cone and nested in each other. Later the tip was cut off and pushed through from the other side, and nested yet again. This worked well on the Deviant Rocket, but that nose cone was styrene and easily cemented. This is HDPE and well, we shall see...
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Here it is, assembled and standing on its own. What I haven't shown yet is the notches in the base of the top cone to allow for the fin tabs.
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A close up showing the launch lug. I had to take some care to make it vertical, since there is no vertical surface on the body. I also put two launch lugs (made from 1/4" body tube stock) on nearly opposite sides to try to balance the aerodynamic affect (at the cost of creating more drag).
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This is how it looked after the first coat of gray, sandable primer. I'm always amazed how much better a rocket looks once the bits of stray epoxy and the different shades of the plastic and wood are smoothed out to one shade. That is, until I look closer...
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and this close up shows some of the defects yet to be dealt with. They are a bit exaggerated by the primer, but there is still a lot of sanding in my future at this point.
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This shows, especially in the detailed version, the problem using too much 3M Spray Mount caused. When separating the fins after getting the edges all lined up, some of the wood grain pulled out. Oops. This is after a pretty thick coat of primer.
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After some sanding (at this point with 240 grit sandpaper), I used some balsa filler to try to fill in some of the worst of the remaining defects.
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I switched to white primer, partly because it seemed to me to be a better undercoat for the silver, partly because it gave me a contrast to the gray primer, but mostly because I had a big can of it about.
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Now sanding with 320 grit, I started to get a surface closer to what I had in mind. That seam is still pretty prominent.
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After another coat of white primer and a good part of a day sanding, now getting down to 600 grit, the seam and the surface in general is starting to look more like what I had in mind.
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This is a look at my painting setup. The two motorcycles on the left (an 82 Yamaha Vision and 81 Yamaha Seca 750) are probably sold. As I was struggling with the blue top coat, my neighbor stopped by and asked about buying both. See - rocketry can be profitable. (The '83 Seca 900, just visible on the right is not for sale. Not yet at least).
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This is a better look after the first coat of silver. My best guess to match the color and look of the graphics was transparent blue over silver. This worked reasonably well on the prototype. This is Rust-oleum Specialty Metallic. It's what I had when I made the prototype and seemed to work well, though I had some problems with the blue top coat.
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A look at the silver coat.
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A close up of the nose after going over it with some 600 grit. Gonna need another coat...
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A bit better - but the camera seems to have trouble focusing on the surface - that's a good sign, right? At this point I'm getting pretty excited. The rocket seems sound, the RockSim simulations are confirming my expectations for stability, the major flaws are sanded out and the silver looks pretty good. Then there was the transparent blue.
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In the imortal works of The Three Stooges (Larry Fine, actually), "Something went wrong!". I was probably trying to get too much done at one time, probably shouldn't have been painting during the rainstorms around the first of the year, 2006 but, clearly this is not what I had in mind. The paint ran with with little provocation. If I put the paint on thin enough that it wouldn't run, I got a miserable looking speckle pattern. Luckily, when having this problem with the prototype, I discovered that Testor's Brush Cleaner dissolves the 1257 Transparent Blue, but leaves the silver undercoat apparently untouched. I removed this coat, and gave it a second try. It turned out a bit better, but still pretty bad.
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The second try was better - but a look at the detail will show that it's just not right.
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Part way through stripping it back down to the silver base coat. At $2 for a small bottle of brush cleaner (and the fear that I might run out even though I'd picked up three bottles - very handy having Sheldon's Hobbies within walking distance!), I decided to try acetone, which I had a lot more of. This worked. A bit too well, it also started taking the silver off. I did manage to get all the transparent blue paint off, and still have a bottle of brush cleaner left.
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I decided I had to have some sort of a paint incompatibility (heaven forbid I had simply used Testor's all the way through - do need to test that). Anyway, the thought that a clearcoat layer might make for a better looking transparent coat seemed reasonable. This is the test I did. A bit of a newspaper glossy insert, rolled into a tube and painted with the Rust-oleum silver. I then masked off half of the tube and sprayed on Rust-oleum Painter's Touch clear and let it dry a bit. Then I unmasked it and hit the whole thing with the #1257 Testor's Transparent Blue. The effect is more obvious in the full sized picture, but you can see, especially at the ends, how the paint was being repelled by the silver but it not only stuck to the clearcoat, but actually seemed to spread out over time.
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Here we are, almost all of the blue stripped off the rocket. getting ready for another try - this time with clearcoat.
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I pulled it into the shed a bit to cut down on swirling (don't worry, in spite of the angle of the picture, I sprayed out the door). At the tip, and especially in the large version, you can see the gloss finish. I applied this over time, letting it get tacky. I wanted the clearcoat to be pretty thick, and hopefully even, before applying the blue.
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This picture does't really show it, but the clearcoat worked like a charm. Part of the reason it looks so splotchy, is that it's wet and is very reflective. Not only did the paint go on better, but by the next morning it looked better yet. More pictures later.
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With the paint coming along, it was time to finish up. That left securing the recovery attachment points and filling with foam. These next few pictures didn't turn out too well, I too often put the white surface in the focal area and the camera had nothing to focus on. These are the screws and clips that make up the motor retention along with a couple of spacers and some heat shrink tubing. The plan is to create something that will keep the foam and the screws apart.
PICT0663.jpg I put the spacers into the ends of the tubing to keep them from collapsing around the screw and causing problems if I ever had to remove the screw. I inserted the screws and used the heat gun to shrink the tubing around most of the length of the screw. After the tubing cooled, I moved the screw back and forth to stretch the tubing a bit; I didn't want a tight fit, just one snug enough to keep the tubing on the screws when filling with foam. I then used a small blob of hot glue to close off the end of the tube and flattened the end with a pair of pliers while the glue was still hot.
PICT0665.jpg Here's another blurry close up showing me using a tool I knew would come in handy, to put the cover over the screw.
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Here's a look into the body of the rocket showing one of the covers on its retaining screw.
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Here's another view from a different angle.
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This view shows the attachment of the recovery harness to the bottom half of the rocket. There was enough room under the through tabs of the fins to get a washer tied to a piece of thread to go under the fin and through the space. I could then use the thread to pull the flat kevlar cord through. (Tricky, huh? I think I know what you're thinking - and I did think this out before I glued it all up!) I'd gotten several sizes and types of kevlar from Commonwealth Displays and I decided to use this flat cord. With a harness around two fins, I can use a knot to adjust the nominal descent attitude and if either attachment should fail, I've got another. (Heh, I said "nominal descent attitude".) With foam going in soon, I wanted a backup in case of a failure; I was pretty sure "going in" was not something I wanted to do once the foam was in place. I was inclined to use an overhand knot on the loops, but a bit of research showed that a figure-8 loop puts less strain on the cord. This is tied by first putting a figure-8 knot into the cord, feeding the free end around the fin tab, then feeding the free end back through the knot. I got my knot information from the Ropers Knot Page. I considered a bowline, but I don't like the fact that the free end points back towards the loop. This shows the state of things with two pieces of heat shrink tubing (I do love the stuff) covering one of the knots (one piece covering the knot, a slightly smaller piece covering the end of the larger piece). The tubing for the exposed knot is on the loop, just off the picture. Rumors to the effect that I actually got everything to this point, including adjusting the knot and taking all extra twists out and then discovered I'd left off the heatshrink tubing, are no doubt exaggerated.
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This is a picture of an initial test of the two part foam using the "plug" shown before, made from the bits of the tail cone.
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Here I am, ready to go a foaming. I've got carefully measured amounts of each part. I mixed them in a third cup to be reasonably sure I kept the proportions equal. With these volumes, the amount left in the cup is enough to change the proportion. The scotch and soda is there to help with the cure, but not necessarily of the foam.
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For some reason, I stopped taking pictures after I poured the foam. I guessed at the volume of the space and per the instructions on the PML two part foam, I mixed up about 1/25 that volume. It turned out to be about half the total volume after it finished expanding. I added about that much more and that took it over the top. I had dreams of these nice, rounded domes of foam at just the right level. Reality crept in and I used a steak knife to cut the foam to the height I wanted. This went well, but I managed to nick a bit of the paint off the edge. That's what's shown here.
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My experiments with a couple of scrap pieces made from the base of one of the nose cones confirmed that HDPE is very difficult to bond to. It seemed to me that the foam did stick reasonably well to the surface, but trying to use it to bond two pieces didn't seem to work well at all. The pieces would come apart easily, but the foam was sticking to the pieces, it was simply sheering at the joint. I decided to use contact cement, which had worked better than epoxy or even two part plastic weld, to hold the plug in place and make the surface bond and then let the foam fill the space in front and behind it to help lock it into place. Here's the inside of the nose cone after using a stick (chop, type) to apply a decent coat of contact cement to the inside.
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I applied a similar amount of contact cement to the plug and let both get tacky. I wrapped some newspaper around a wooden spoon to make a tool that would let me keep the plug centered as I inserted it. It worked pretty well.
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Here's a close up, showing the nose cone after I'd poured in 3 oz of lead shot. You can see a couple of balls of shot that got caught in the ridge of the cap. Unfortunately, again, not shown is the foam. After pouring the lead shot into the hole, I then poured in foam. It came out of the confined area of the nose tip pretty quickly (i.e., it was cool!). It then spread out, but because of the way the foam expanded, the sort of mushroom cap got pushed up and away from the sides. This actually worked out well as I then poured foam into the gap between the foam cap and the inside of the nose cone. This had a tendency to push the second batch of foam into the sides and it seemed to adhere surprisingly well. I'm hopeful that this arrangement will keep the weight in the nose and the shock cord attached to the top half of the rocket that I spent so much time filling, sanding and painting.
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Here's a descent shot of the beast in it's finished (so far at least) splendor. One fear working on this, and basically making it up as I went, was that I'd let up at the end and something would go wrong. I had decided to use a urethane top coat to give the rocket a nice shine and to protected the paint from chipping. PML's web site recommends urethane. I did a test of the urethane over the silver/clearcoat/transparent blue combination, but because I wanted to paint the next morning I put the test in the oven at about 200°F to speed things up. The surface bubbled up a bit, but I was convinced there was no horrible incompatibility. I was right, but the urethane still left a rather mottled appearance compared to the clearcoat. It doesn't really show up here. In spite of that it looks pretty cool.
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It's now January 17th and /dev/rocket is ready to fly. I'm still considering taking some 600 grit to the urethane and (after some careful tests) putting a layer of clear coat over that. Though, since I plan to fly this, it might be better to try building a non-flying version and put the finish effort into that. I plan to get a few more pictures out here, including inside shots of the foam work at both ends and my somewhat unusual (so far as I know) shock cord arrangement. Still need to weigh it and consider if I want to put it up with an F-50, or go first with the G-80. I'll let ya know. Of course, I'll get some flight pictures out here as soon as they're available. Could be as soon as the February Snow Ranch launch. |