Design Comment / Build Thread Tuna Rocket

[boatgeek]

TRF newbie here asking for design comments on a new scratch rocket. Here's the scoop:

Just after WWII, my great-grandfather was working as a commercial photographer in the LA area. He was hired by the Starkist company to take pictures of their tuna and make a promotional movie. As you can see from the picture below, these were the days when fishermen's lives were pretty cheap and the sea was a limitless resource. That really is four guys with bamboo poles standing on a grating outside the boat yanking one several-hundred-pound tuna over the rail.



I fell in love with that fin shape for a rocket, especially the lower pectoral fin in the picture. On the other hand, I knew that an 8:1 aspect ratio would make it awfully hard to keep the fin attached to the body tube. So after some tracing, a little judicious scaling in AutoCAD and a trip through OpenRocket, we have this design:



The motor mount is 29mm since I have a 3-grain Cesaroni kit with spacers and a couple of single-use motors hanging about. The bulkhead/centering ring near the front of the motor mount will likely become a bulkhead with a hole to stop forward movement of the larger casings. I've had trouble with recovery harness attachment on MD rockets, so the body tube is 38mm, with TTW fins laser cut from 1/8" plywood. It's good to have a friend with a laser. Stability is kinda high for the 1-grain/disposable motors at over 2.5 calibers, but I want to be able to launch it on low-thrust 3-grain motors, ideally some of the longburns like the Mellow or G/H54 Red Lightning. I also expect the CG to move aft a bit to account for epoxy and fillets around the fin can and motor mount. The overall rocket is about 30" long, although it's possible I'll go with a 34" piece of LOC tubing for the body tube. Less cutting that way. The launch lug shown is for drag purposes. I expect to launch on 1010 rail buttons at club launches in WA.

On to the questions:
Do you see any fatal (or non-fatal) flaws in the design I should address?
Would it be a good idea to put in a coupler a little above the fin can so that I could add a dual deploy and/or access the recovery harness root for maintenance/repair?
Is it going to be impossible to work between the 29mm and 38mm tubes?
Does anyone do non-BP dual deploy systems (springs, co2, etc.)?

I'll be back with pictures as I build. Thank you!

Note: These pictures came in really tiny. Is there an easy way to adjust that?

 

[Rex R]

the pictures are pretty good size, once you click on the thumbnail.
Rex

 

[Englertracing]

im curious
did you test the rocket with standard delta fins just for comparisons sake?

if you did what was the difference in velocity and apogee?

 

[boatgeek]

I hadn't run a comparison to standard fins, but it's an interesting exercise. I hacked together some swept trapezoidal fins with roughly the same stability as the curved fins here. I stopped at a couple of iterations, although I could probably cut weight down even more with some more optimization of the fin shape. The two versions are pretty evenly matched, with altitudes and max velocities within a few percent of each other. I'd guess that this is within the margin of error of the program. The trapezoidal fins are slightly heavier, but appear to be slightly less drag.

 

[boatgeek]

OK, here's the first pictures of the build. Fins and centering rings are cut (it's nice to have a friend with a laser!), fin slots are cut, and everything is dry fit together. The 38mm rocket in front is mine, the 54mm behind is my daughter's. Next stop is to sand the fins and clean up all of the char from the laser. After that, it's epoxy time.



I also picked up some parachute shroud line and tubular nylon for recovery. I'll build a chute for this one, but not sure if it will be a boring flat one or a more complex shape. I'm also going to experiment with running a piece of light bungee up inside the tubular nylon to make a shock-absorbing harness. This will give it a 1.5:1 or 2:1 stretch, sort of like the shock absorbing fall harnesses you see on work sites. It should ease up the post-event loads on the recovery harness significantly. We'll see how it works in practice...

 

[crazyed]

Apogee has a CO2 deployment set up ,not sure the manufacturer. What speeds and altitudes are you expecting from the two.?

 

[boatgeek]

Altitude is ballpark 1,000-4,000 feet from F to baby H motors (basically anything you can get in 3-grain 29mm Cesaroni). I don't remember speeds, other than that they're comfortably below transonic, somewhere in the 0.5-0.7 range.

 

[boatgeek]

Progress is being made, even if updates are slow. First of all, fin cans and recovery harness anchors. I saw this thread (https://www.rocketryforum.com/showt...ood-centering-ring&highlight=shock+cord+mount) about mounting shock cords, and I remembered a spare piece of 3/32" 7x7 stainless cable I had laying around. It had been used in tying the canoe to the van, but is no longer needed. It had loops swaged in both ends, one with a thimble and one without. It also happened to be long enough to make 2 shock cord connections when cut in the middle. One is for me, one is for my daughter's rocket.



This is where a little knowledge can get in the way of doing things right. I know from my job on the marine side that when you cast a socket on to the end of a tugboat's tow wire, you broom out the wire, separating the strands. That gives you a much stronger bond, since the socketing compound (high grade epoxy or molten metal) can get in around the wire better. So I thought that would be a good idea here, too. The only problem was that I pulled the masking tape that was holding the wire flush to the motor mount too early, and the wire strands lifted off the mount. It was a little bit of a struggle getting it into the body tube, but it all worked out in the end. I slathered some more structural smoodge over the wire, stuffed it into place, and put the aft centering ring in place until the epoxy cured.

The wire is bent back into the front of the motor mount in this picture because I pulled it there when I glued the mount in so I wouldn't glue the wire to the body tube. After the epoxy cured, I used a piece of string helpfully pre-attached to the loop of wire to pull the wire out into the body tube. I'll use the string to pull a loop of recovery harness down through the loop of wire and back up again. That way I'll be able to rotate the recovery harness and replace it as needed.

I use Fiberlay epoxy, which is really made for boat building, rather than rockets. It flows nicely, sets up when it says it will, and comes in a helpful 2:1 ratio. They also sell various fillers, cloth, etc. for any of your needs. Plus, they're local to me, only a few miles from my office. Years ago, I saw an article that called any kind of filled epoxy glue or fairing compound "smoodge." I like the word, so I kept using it. For structural smoodge, I usually mix the epoxy with wood flour, which is basically fine uniform sawdust. It's pretty strong, stays where it's put if it's mixed right, and does seem as nasty as the microfibers you see elsewhere in structural work. For fairing smoodge, I use microballons. I think the ones I have now are phenolic. They also make a nice mixture that stays put and is pretty easy to sand. Microballoons do insulate, so they make the epoxy in your cup kick off faster than it would otherwise.

Enough about epoxy, let's get back to rockets. The next step was to glue in fins and the aft centering ring. I had about 3-4mm between the outside of the motor mount and the inside of the body tube. Since that wasn't enough room to work in, I forced a bunch of structural smoodge in to the fin slots, filling them up down to the motor mount. I then pushed the fins in, which ended up making fillets to the motor mount and inside of the body tube. I was ready to use a straw to push the smoodge into place, but didn't end up needing to. Final step was gluing the aft centering ring into place. Slather in some smoodge, push it into place, and let it set. Here's what it looked like then:



Over the weekend, I added fillets to the outside with fairing smoodge and sanded them down. My technique for not getting smoodge all over everything needs work, but with the application of a couple of files and some sandpaper, it all looks pretty good.



I also cleaned up the mold marks on the nose cone. Next step is to add the launch lug platforms, prime, and sand. And prime, and sand. I have some thoughts on a nose cone AV bay, but need to percolate more on that before I commit to anything. I like the notion of cable cutter dual deploy, but need to work out details in my head and watch a bunch of YouTube how-tos to learn from others.

 

[boatgeek]

I have been thoroughly delinquent in updating this thread. Much has happened.

Here are the pads that the rail buttons sit on:


Here's the first coat of primer, on a broomstick that fits a 29mm motor mount perfectly. I need to paint more rockets flat black.


I scratch built the chute, although it's nothing fancy. 18" octagon.


And here's final assembly. Paint is silver with a touch of blue on the fins.


The AV bay is under construction. I cut off the end of the nose cone and fit a piece of plywood in the end that can be a mount for a sled, but there's nothing smart in there right now. Maybe for the taller flights Memorial Day weekend, but almost certainly not any kind of dual deploy.

I did get first flight in at a Washington Aerospace Club launch in the Seattle area a couple of weeks ago. That was on an Aerotech single use F42 that was leftover from last year's flights. It was a nice straight flight with recovery close to the pad. Simulations said 1500', but I think that was optimistic. No damage, although the pointy fin went about an inch into the soft dirt. We'll see how those fins hold up in the harder soil of Eastern Washington. Next flights on CTI motors, a G54 redline and a G33 Mellow. Those flights will be up around 3-4000 feet.

 

[Bat-mite]

Definitely unique! Did you think about adding a dorsal fin farther up the booster?

 

[boatgeek]

Did you think about adding a dorsal fin farther up the booster?

I didn't, but mainly because I am a little gun-shy about stability. My younger daughter's first rocket was a MD 29mm jobbie that went unstable off the pad. It did some lovely loops before spitting the nosecone at the crowd at about eye level. [edit] The recovery harness held, so it was merely a crowd gasping moment, not a risk of major injury moment. [/edit] OpenRocket said it was stable, but we did a bunch of small mods at the end that I think pushed it over the edge. I want to be a little more cautious for a while so I don't get the RSO sidling away from the table as I walk up.

 

[gerbs4me]

Very cool looking design!
Great job

 

[455 Buick]

I'm diggin' those fins. Might be cool to make another with fins mounted further forward and a long boat-tail. But it's already cool!

 

[Nick@JET]

Very cool, I like the rail buttons, they go with the theme.

 

[boatgeek]

Thank you all for the kind words!