Back in December 2013 I posted on YORF about the repair of my Big Daddy 4x18. I was reviewing the site today and realized I never posted the information here... so this post will repeat and expand what I posted on YORF.
A couple of years back, my Big Daddy 4x18 had a partial failure of the Kevlar harness to which the shock cord was attached. (It was during this launch.) The harness consisted of a piece of 100# twisted Kevlar attached at each end to opposite sides of the engine mount; on that particular flight, one side broke free. Fortunately, the rocket recovered without damage.
It sat around, unrepaired, until November 2013. I couldn't figure out a good way to attach a replacement harness to the engine mount... it's like building a ship in a bottle in the dark.
I was reading about various replaceable Kevlar shock cord mount ideas when I remembered my Big Daddy, and I started thinking, how can I do replaceable Kevlar in that rocket?
Then I remembered it has only one centering ring, on the bottom. The centering ring is made of photographic mat board, pretty flimsy but plenty good enough for the necessary gas seal. (The TTW fin tabs support the engine tubes.) That's when I thought of running the Kevlar out the engine hook holes.
But that flimsy centering ring would never hold the screws, and the Kevlar might just cut right through it. I needed to beef it up without adding too much weight.
So I took a piece of 1/16" birch plywood to some friends of mine who have a laser cutter, and came home with a reinforcement ring. I attached it with yellow glue, then applied a thin fillet of epoxy to add a bit of extra strength.
Here's the result:
On September 20th I was able to launch the refitted Big Daddy. The parachute was damaged during the flight, but the harnesses held up fine. I used a long piece of mason's line as the shock cord, tying each end to one of the two harnesses; then I pulled up the midpoint of the cord, slipped it through the nose cone loop and then over the cone, thus anchoring the cord. I realize now that I should have done that part differently, as a break at either end of the cord could result in the cord slipping through the nose cone loop, but that didn't prevent the rocket from performing almost as well as I intended.
Click here for the fleet page entry for this rocket.
I got it put together last night; the assembly went almost as I had described it. Before inserting the engine mount, I attached both ends of a Kevlar cord to opposite engine hooks, adding a couple of layers of paper to secure them to the tubes, as a shock cord harness; after assembly, I tied a length of tent cord (the lightweight kind I get cheap at Wal-Mart) to the harness and tied the other end to the mondo huge nose cone loop.
And actually, the assembly was really, really easy. I sanded and tried and sanded again on the fins to make them fit correctly, then put glue into the valleys between the engine tubes and put it all together. Without glue on the ring, I was able to turn the engine mount until the fins were straight; then, I pressed the fins in a bit more firmly and put the whole thing upside down to dry.
I applied glue to the unglued joints after it dried; it all feels rather solid now, and seems straight.
Regarding noseweight: OpenRocket said I'd need 24 grams of weight at the nose to get the 0.38 calibers of stability the stock model has; that comes out to just over 31 computer screws at the 0.77 gram average weight I measured long ago. I put 33 screws in, and just enough polyurethane glue to cover them (which of course foamed up nicely).
I decided that, instead of making the engine tubes all the same length as the stock BT-50 engine tube, I'm going to make them the "normal" length, cut just one centering ring to fit them, and then use the fin tabs to stabilize the whole thing. In other words, no upper centering ring. Shouldn't be needed with the fin tabs to hold it all at center.
I will put glue in the troughs between the engine tubes (where the fin tabs will go) and radially out from there to the edge of the ring, then insert the engine mount into the tube without any glue to hold it in; then, I'll fit the four fins in place (pushing them into the aforementioned glue) and line them up so they are straight out from the body tube. Since there is as yet no glue between the engine mount assembly and the sustainer, I'll be able to turn the assembly to get things lined up right.
When all is straight, I'll let it dry. THEN, I'll put more glue around the centering ring (at the aft of the rocket), and when that is dry, I'll put glue fillets on both the outside and the inside of the fin "roots" (using dowels or long swabs to do the inside fillets). The end result should be plenty strong. Omitting the upper ring and excess engine tube length will lighten the rocket some, and being able to fillet both inside and out should help make up for any strength lost from the upper ring's absence. I simmed it without engine hooks, but I'm darn well having them on the final rocket; hopefully the things omitted will make up for the extra weight of three more hooks.
And I'll put in 24 grams of computer screws in the nose cone (just over 31 screws on the average), and then add some polyurethane glue to hold them in. I've had good luck with that sort of glue for this purpose, and big as that cone is, unless I just go nuts with it, it shouldn't overflow.
REALLY looking forward to this one. I'm thinking I'll paint it orange (not fluorescent) with one white fin, much the same as I painted my Baby Berthas (the standard BB is blue with a white fin, the 2x18 is red with a white fin). Definitely not going with package-front dress.
The fin tabs have been sanded to fit between the tubes, and I'm just waiting for the small dribble of polyurethane glue I put in the center (held in at one end with a small square of balsa and some yellow glue) to finish expanding and actually set up. Probably finish it this weekend.
I got a Big Daddy kit on clearance last year, and I'm considering building it as a 4x18 cluster. A quick check with my centering ring generator shows that the fin tabs should fit almost perfectly in the "notches" between the engine tubes, so only the rings need modification for this conversion.
When I said on Ye Olde Rocket Forum that I needed to figure out the stability of this rocket, Greg Gleason pointed me toward OpenRocket. So I downloaded OpenRocket and spent a little time with it. It's not Rocksim, but it's pretty good... and it's free, which is pretty cool.
According to OpenRocket, with 4x C6-7's mounted, my Big Daddy would have -0.06 calibers of stability... oops. It takes 32 grams to get to 0.5 calibers, or 69 grams to get to 1.0 calibers. Ouch, man, that's a lot of screws and epoxy. HOWEVER... Also according to OpenRocket, a "stock" Big Daddy with a D12-5 has 0.38 calibers of stability. The 4x18 version needs just 24 grams to reach 0.38 calibers... not a problem at all. Given that the rocket is stable at 0.38 calibers in stock trim, that's what I'm going with.
Also according to OpenRocket, the 4x18 with 24 grams noseweight loaded with 4x C6-7's deploys with the rocket descending at a bit more than 40 feet per second; with 4x C6-5's, just about right at apogee (the rocket has just turned over). So that's my planned flight configuration.
911 feet altitude in that config. I like it.