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kavy8

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While I’m wrapping up my third year on my uni rocket team, I am aiming for my L1 in late April (we’re gonna pretend the launch isn’t 3 days before my last final exam, and the day after another final). I got everything in earlier this week and briefly started, but I wanted to make a build thread as well. 8443663D-52CE-49A2-9858-6CF3845752E0.jpeg

I ordered 2 kits and a bunch of other parts for others members on my uni rocket team, and this was everything out of the box! 3 of us are doing scratch built 4” diameter with 38mm motor mounts, hence the duplicated parts.

I got started by finalizing the physical dimensions of the body tube and MMT, and then modelled a fin slot cutout jig and a fin alignment jig, which were then 3D printed on Tuesday. I marked out the slots, removed the cutout jig and then cut out the slots. They weren’t the prettiest cuts cuz I didn’t have a fresh blade, but the fins ended up fitting nice and tight later on9F74D252-0D0E-436F-AAB5-120FE7D12B35.jpeg
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After cutting the slots, I got a 2’x2’ sheet of birch plywood from home depot for the fins and centering rings. I got them laser cut at the uni, and the second I got home yesterday I went to dry fit everything instead of working on my senior capstone project, because this is way more fun. I dry fit the fins with the motor tube and my 3D printed alignment jig, and everything seems to fit really nice!

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I took the aft centering ring out just for the last photo, but it also fit snugly. Will need a bit of light sanding on the OD to slide through the tube a bit easier, but turned out decently.

I’m hoping to make slow but steady progress on this, life is crazy busy rn with my last month of undergrad AND pushing for our hybrid to be ready to start doing cold flow tests and mock launches, while I’m also rebuilding a motorcycle, but I’m sure I’ll squeeze in time to make it for the April Launch
 

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Those fins are pretty small, have you sim'd this design in OpenRocket yet?
Yeah, I started in OpenRocket. I know they look small, I am planning on adding 150-250g of nose weight and I went with 4 fins as well. I plan on only flying H's and Baby I's. I did my best to position everything as realistic as I expect it to be in OR, with mass overridden values for everything, and on a baby I, I get a stability of around 1.5. With the H I plan to use for L1 I get about 2 cal stability
 
Yeah, I started in OpenRocket. I know they look small, I am planning on adding 150-250g of nose weight and I went with 4 fins as well. I plan on only flying H's and Baby I's. I did my best to position everything as realistic as I expect it to be in OR, with mass overridden values for everything, and on a baby I, I get a stability of around 1.5. With the H I plan to use for L1 I get about 2 cal stability
Fair enough. Any particular reason you went with the smaller fins + nose weight, instead of using larger fins? Aesthetics?
 
Fair enough. Any particular reason you went with the smaller fins + nose weight, instead of using larger fins? Aesthetics?
I wanted to sweep the trailing edge forward and minimize the height of the fins so that they’re less likely to be the first thing to impact the ground on landing, and lower the chance of them snapping off since that seemed to be the number 1 reason for failed certs at the launches I’ve been to. That logic may be totally flawed though, I wouldn’t be surprised haha. I know that adding weight to compensate for small fins brings it down faster under chute, which also increase the risk of snapping fins, but I’m hoping the pros and cons swing in my favour
 
Any updates?
Yep, I flew it back in May and got my CAR L1 (H motors) and L2 (I motors). My buddy built an identical one to me and he certified as well. We used roughly 250g of BB’s/epoxy in the nose cone. Mine cured on an angle unfortunately so I decided to retire the nose cone, but my buddies flew a fair bit nicer than mine. I decided it would be better to learn all the things I want to learn on mid power rockets, so I am slowly building a couple to fly on E’s-G’s with dual deploy. I started my masters degree, so I will be very tight on budget for the next couple years. Figured I can have just as much fun with mid power as high power for the time being.

Looking back, I 100% should have gone with bigger fins to save me the trouble of nose weight. Lesson learned!
 
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Yep, I flew it back in May and got my CAR L1 (H motors) and L2 (I motors). My buddy built an identical one to me and he certified as well. We used roughly 250g of epoxy in the nose cone. Mine cured on an angle unfortunately so I decided to retire the nose cone, but my buddies flew a fair bit nicer than mine. I decided it would be better to learn all the things I want to learn on mid power rockets, so I am slowly building a couple to fly on E’s-G’s with dual deploy. I started my masters degree, so I will be very tight on budget for the next couple years. Figured I can have just as much fun with mid power as high power for the time being.

Looking back, I 100% should have gone with bigger fins to save me the trouble of nose weight. Lesson learned!
Oh cool! I was thinking L2 I motors? I see your a little north! Yes you'll learn as you go, I've been building low/mid power for 33 years now. I'm ready to move forward, working on one right now for my L1 here in New York. Hopefully November 4th or 5th with URRG. Thats cool though, I'm actually going to experiment with DD and L1 stuff too but that will fly on F/G. Also I've been using high power build techniques even to low power. It's really made assembly of a HPR very easy. Knowing how all the glues or epoxies work and dry/cure. Making my own parts, cutting fin slots, using spill holes, etc.. And testing them out on the low/mid ones has been so much fun, especially the JLCR. I've have a few 29mm rockets I've been flying and its going good so far. I'm still pacing myself, taking it one day at a time with the L1 certification build.
 
So, I have had an idea and I would appreciate input from anyone who might see this. Originally, I decided to retire this rocket because of the nose weight epoxy+bb's which cured on an angle causing this to fly pretty poorly. (Still flew twice though, and the higher thrust motor it flew much straighter! I was surprised the RSO let me fly it again after the first flight, but it did prove to me that it was close to being good enough). First thing I will do to fix this rocket is mix another small batch of epoxy and bb's and pour them into the nose cone, anchored with a wooden dowel. But hopefully cured straight this time!

At about 5am this morning while struggling to get back to sleep, I wondered if it would be feasible to put "fin extensions" on this rocket to fix the original issue of fins that were too small. These extensions would be laser cut out of the exact same 1/4" birch plywood I originally used, and would essentially have a cutout missing from them that is the shape of the fins currently on the rocket.

To ensure they are as strong as they can be, I would drill small holes into the leading edge and tip of the old fins, as well as drill small holes into the corresponding edges of the fin extensions. I would use BSI slo-cure epoxy to bond the edges of the fins and extensions, paying special attention to get epoxy in the drilled holes. These drilled holes would act as epoxy nails. After that, I would do a tip-tip fiberglass layup. The fin extensions would also have a fin tab of their own which extends down to the MMT.

Photo of the holes drilled in the original fins which would act like epoxy nails
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Screenshot of the concept in CAD, existing fin highlighted in blue with 1 fin extension in position.
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Does anyone think this is feasible, or does it scream out this is a bad idea? And yes, since this rocket I have built a number of low power kits and mid power scratch builds, flying on estes and econojet motors. In all honesty, these are just as fun an so much cheaper, so I will probably fly 5 mid/low flights for every high power flight. The building is my favourite part anyway!
 
Should work. I would probably throw some fiberglass on the fins when done for good measure, but I tend to overkill my repairs.

@JimJarvis50 frequently lengthens fin tips and may have some pointers.
 
Should work. I would probably throw some fiberglass on the fins when done for good measure, but I tend to overkill my repairs.

@JimJarvis50 frequently lengthens fin tips and may have some pointers.
Cool, thanks for your input! I was planning on doing a tip-tip fin layup as well when I was done adding the extensions
 
You might run some simulations just adding to the existing fin tip rather than increasing the root and tip cords. You might find a lot of bang for the buck doing that.

Jim
 
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