20th January 2017, 03:07 AM
I found a set of Starett 6" digital calipers with 1" digital micrometer on ebay many years ago. They've been working well. There are so many on the market these days.. I'm not sure what would be the smart choice. I'd go for stainless steel with a known brand name over the cheap plastic ones. Digital over Dial over Vernier.
Originally Posted by rocketsam2016
20th January 2017, 03:48 PM
With the MMT rings sanded so that the assembly fits, next up was cleaning/sanding where the fin roots will touch and placing foam board dams as discussed earlier in the thread. I had planned to set them 1/4” outside the fin root, but that looked big so I made it 3/16”.
Here’s how the epoxy will be placed:
As you can see I placed the dams radially instead of parallel with the fin, which I’m regretting now since the extra width near the outer body tube isn’t all that useful for strength. I’m seriously debating not filling the chambers up all the way since it’s going to make a mess when I’m trying to fine tune straightening the fin. The good news is that even if I fill them all the way, it’s only 6oz total of epoxy to do all the fins which isn’t as bad as I was worried it would be when I realized that I shouldn’t have placed them radially.
Here’s my math for how much epoxy it will be in case it is useful for anyone. There was no need to do these calculations so precisely but I like math. Sorry for the mixed units, but google doesn’t care :-)
The dams are placed 3/16” + 3/16” (fin) + 3/16” = 9/16” apart on the MMT. This represents 5.7% of the MMT exterior diameter:
(9/16”) / (pi * 79mm) = 0.057
Thus, the block of epoxy will be 5.7% of the area between the MMT (79mm exterior diameter) and the outer body tube (99mm interior diameter), minus the area of the fin root (which is 3/16” wide and 10mm tall)
((99mm/2)^2 - (79mm/2)^2)*pi * 0.057 - 3/16in * 10mm = 111mm^2
The combined root chord length of the fins is 13.5”, so for each pair of fins I’ll be using 38mL of epoxy:
111mm^2 * 13.5” = 38mL
We have 3 fins and rocketpoxy weighs 1.5g/mL, so the total weight of epoxy will be:
38mL * 3 * 1.5g/mL = 171g = 6oz
Tonight I’m planning installing the MMT assembly and possibly doing my first fin. Woot.
20th January 2017, 04:51 PM
It's looking great, Sam. I'm going to copy this method on my next FG build.
NAR #100940, RIMRA & CMASS
L1 - 4/17/16, Tyrannosaur (by Binder Design), Loki H144
Dual Deploy - 11/12/16, Tyrannosaur + Eggtimer TRS, Loki I405
20th January 2017, 05:22 PM
Thanks! Ask me again once I'm done though :-) Negatives that I can see so far
Originally Posted by BDB
- It took more time than I expected to cut out the little foam board dams
- A lot of the added weight isn't doing much good. The most egregious part is the middle (halfway between the MMT and the body). The only benefit I can see that region adding is that we've formed a solid epoxy block around the fin base that adds a compressive/mechanical force against the fin pivoting or being ripped out. It does nothing much though to help adhere the fin to the MMT.
- If I don't manage to completely fill the chamber with epoxy there will be a gap at the top and it'll have been no better than just filling it up a third of the way.
- I anticipate a giant mess of squirting epoxy when I insert the fin if I try and fill it all the way, especially when I try and adjust the fin angle. I'm worried this will be even more of a mess with the rig I'm using to ensure the fin is straight
- There is going to be quite substantial force resisting me adjusting the angle of the fin since the fin is flush with the centering rings and the MMT and is therefore a well-sealed piston. I may actually chamfer the front and back of the fin root tab so that epoxy has at least some path to take instead of just squirting out the body tube
On the plus side:
- Using a dowel to add internal fillets to such a small space going nearly 15" into the rocket would be brutal and hard to do well!
- No drilling holes in the frame for injection
- No trying to inject rocketpoxy (I'd rather not invest in another epoxy, I like this stuff)
- Compared to injection, it's OK and even preferable to have some gaps between the fin root and MMT and centering rings
Overall, I'd be thinking harder about whether to use this method if it was on a leaner and meaner rocket or a rocket that had marginal stability. The Frenzy is a heavy, draggy and very stable beast though so the extra weight isn't the end of the world.
Holy cow the foam board added a ton of friction. I got the mmt assembly about 75% of the way into the body tube before realizing I wasn't going to make it.Fortunately I hadn't put any epoxy in yet. My full body weight applied through a pole wasn't enough to get it to even budge to get out, nor was trying to jump to apply my weight. I had to put a bulkhead down agiant it and use a mini sledge hammer with force transfered through a metal pole to get it moving.
I cut down the foam board so that the top edges were angled and this time got it in. I put a solid epoxy bead at the top centering ring. The foam board dams might be a wee bit off center but they are close enough. Off to ice my bruise hands from trying to push it out... Good news though is my mmt is installed! And friction alone apparently would be enough to keep it in place for a lot of motors...
Picture of the rear CR. Only showing since it's kind of cool how you can see the dams and centering rings from the inside of the MMT.
I also went ahead and attached the aeropack retainer. Smart move would've been to wait in case I screwed the fins up badly and/or needed access through the rear centering ring for some reason but I was feeling enthusiastic :-)
Time to attach fins! Cleaned and sanded (still dubious about whether I'm passing the water break test, but I cleaned and sanded the snot out of this thing with 100 grit)
I estimated 14mL (21 grams) of rocketpoxy would be needed for a single rear fin and ended up using around 24 grams, not bad! I originally was trying to get the rocketpoxy into a syringe to get it cleanly into the slot but that was a pain and it turns out it wasn't so bad to just pour it in from the cup. Next time I think I'll put saran wrap on my jig since the most stressful part of this was worrying about accidentally gluing my jig to the fin or body.
As you can see my jig is inspired by other folks on the forum. Two boards of particle board with a beveled angle cut perfectly straight on each of them. I lined them up then drilled 5/8" (I think) holes for dowels to keep the boards perfectly lined up no matter the spacer size. I use 3 #8 screws to hold it tight around a spacer (one of the forward fins), and then keep it tight to the rocket with rubber bands. First time using it and it seems to work great! I wish I hadn't tried to size it to work with everything from 29mm to 98mm tubes. If I'd just sized it for 3" plus (aka used a steeper angle and a thicker or doubled board) then I could've had more clearance between the jig and the rocket, minimizing the risk of getting it glued on as well as making it more powerfully hold the fin straight. Oh well.
Instead of saran wrap use clear or brown packing tape the it can be attached to the jig and epoxy wont stick.
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