Tricky, 54mm Motor Mount in 3" Rocket Questions...

[deandome]

[This post straddles 'Construction', 'Recovery' and 'High Power', so I figure this is the best place]

I'm starting up on THREE kits at the same time (Mach 1 'The Jericho', WM AGM-78 and a custom finned WM Jart); it sounds good in my head, will see how it goes. Anyhow, as you see in the pic, the MM in the Jericho is long (18") and they chose a weird, 2-'tab' TTW fin setup for the 4 centering-rings instead of just using 2 slots cut in fins for the center ones. With my old 4" Thor, I had no trouble with a similar quad-ring setup; I did them one at a time, snugging the rings against the fins as I worked aft-ward & keeping the MM clear of epoxy, doing internal fillet, etc...there was plenty of room to work this with a 4" BT. and attach a fat eyelet thru the top CR so you could link on as many cords as you could ever want, reaching your arm into the fat tube.

But with a 3" tube, it'll be a LOT trickier to pull this off! My 2 main issues/questions are:

ASSEMBLY ISSUES:

• This is one big fin, not two separate ones I could attach one row at a time, so I can't add rings to the mount one by one like I did on the Thor; the front 3 rings have to be pre-attached to MM
• I also think I'll need to attach all 4 fins at the same time, and without 'real' internal fillets, as epoxy will flow around the BT in areas I can't mask or clean before it sets...so fins 2-4 won't have a clean bonding surface.

ASSEMBLY THOUGHTS:

• Attach all foremost 3 CRs on MM at same time, wiping away any excess epoxy carefully, but making sure they're attached strongly. First part of install would use only a little epoxy spread on MM above the fin slot, so when it pushes in only the top of the top ring will have a proper amount of epoxy. THEN apply thin bead of higher-viscosity epoxy onto each fin root & attach fins to BT, being careful no glue drips where the other fins will go*. After all fins attached, do some decent internal fillets in the back 'fin cavity', as I'll have access to this part cuz I haven't attached rear ring. then attach it.
• Then I'd seal the fin slots & rear ring VERY well, and inject a decent amount of THIN epoxy into the top 'fin-cavity'...seal that hole...and then twist & spin rocket like an idiot for 20 min until epoxy sets up. this idea is based on how rotomolded coolers are made...epoxy will fill all the joints that were only tacked into place and create admittedly-tiny internal fillets, too. yeah, it adds weight, and a lot of the epoxy is wasted coating ALL the open surfaces of BT and MM with a thin layer, but it'll be strong. Then repeat for other 'fin-cavity' THEN I'd foam all of that to make up for the lack of internal fillets
• *Another idea I had would be to glue close-fitted strips of balsa vertically on the MM tube to act as 'epoxy dams', so I can use a decent amount of glue on each fin & not worry about epoxy dripping onto other fin-attachment areas

SHOCK CORD ATTACHMENT ISSUES:

• I'm not a fan of gluing kevlar cord to MM and hoping it never breaks...cuz my 3/16 or 1/4" TK DID fray & break on my old Wildman Jr. rocket. I'd prefer to have a metal attachment point, but one strong eyebolt won't fit in the small centering-ring 'gap'. Not just 'metal', I want to be able to easliy replace the short, internal shock-cord if it gets damaged...#3 below covers that

SHOCK CORD ATTACHMENT IDEAS

1. Use two...or three....smaller eyebolts. I got some SS 5mm ones on amazon, a little smaller than normal 1/4". I'd then loop kevlar thru the rings, figuring any all forces pulling on the cord will be evenly split between the 2 or 3 eyelets. And if Kevlar ever frays, I can get in their with some tools & sticks and make another loop. I'd laminate a plywood CR I have to the top glass one to make it strong enough not to have the eyelet break or pull-out the CR with the large forces of J and K ejection events
2. what do you think of drilling a hole in all 4 CRs, and then add in an 18" metal or plastic tube placed thru the rings just large enough to get a piece of 3/8 kevlar thru, and then you knot it at the bottom, outside the rear CR, next to the retainer. Again if it ever gets damaged, you can easily fish another cord thru the rod and create a new anchor
3. OR, I get some really heavy duty 7/16" or 1" kevlar, mount it to the motor tube the normal weigh, creating a stubby look that only sticks about 1" out from the top CR. Then I loop some 3/8"kevlar thru that loop to make a loop that extends outside the BT, where you then attach the 20" shock cord harness stuff.

YOUR THOUGHTS, other than this guy overthinks stuff & talks too much ? Any options I' not thinking of? Thanks!
Dean

 

[boatgeek]

I'd probably do the MMT assembly as follows:
1. Glue all four centering rings to the MMT. Forward one with a good fillet on the fwd side, #2 on the aft side, #3 on the forward side, #4 on the aft side so there's no fillets to mess with the fins. I'd probably add a small fillet between fins on the "clean" side of the CRs, but that's probably not necessary with a good one-side fillet.
2. Add epoxy dams (aka popsicle sticks or other light material 1/4"-3/8" from where the fins will land on each side of each fin) in the gaps where the fins will land.
3. Put a good ring of glue on the inside of the body tube just aft of where the forward CR will land. Slide in the MMT/CR assembly and let that cure in the vertical position so any epoxy flow seals up the forward CR. Make sure the epoxy dams are aligned with the fin slots! Once that sets, put a good fillet on the aft CR and cure vertical pointing down.
4. Try to get some epoxy in through the fin slots to glue the middle two CRs to the body tube. It probably won't do much, but I'd feel better having tried.
5. Working one fin at a time, fill the space between the epoxy dams with epoxy, then drop in the fin and make sure it's aligned properly while the epoxy sets. If you have nice thick epoxy, you could probably do all of them at once.
6. External fillets however you normally do them.

For the retention, is the MMT longer than most or all motors you'll fly the rocket on? If so, you could tweak a long 2" wide U-bolt slightly so that the legs can go into holes drilled into the forward centering ring. You'll probably need to grind down the nuts a little bit to make them fit between the MMT and the body tube. Generous amounts of epoxy are a good idea here. Make sure they don't align with the fins! If you're planning on flying longer motors, you could do basically the same thing with another CR mounted higher in the body tube. with no inner tube.

 

[David Schwantz]

Glue the 2 center, centering rings in place. Leave the forward and rear CR loose. Use just for alignment, glue MMT into tube, on just the 2 center CR's. Allow to dry. Now you can either grind small hole at the fin slots and inject epoxy doing one fillet side at a time. I use the black dye for this as it will allow you to see the glue run down the joint. When done, allow joint to sit at a 45 angle till dry. Repeat as needed for other fillets. Myself, I would run my fillets from the rear after removing the CR that you did not glue in place, then glue in CR. For the front I would make my fillets using a long dowel to apply the glue. then drop in forward CR with eye bolt already attached, and run fillet same as the fins.

 

[cerving]

I'd probably do the MMT assembly as follows:
1. Glue all four centering rings to the MMT. Forward one with a good fillet on the fwd side, #2 on the aft side, #3 on the forward side, #4 on the aft side so there's no fillets to mess with the fins. I'd probably add a small fillet between fins on the "clean" side of the CRs, but that's probably not necessary with a good one-side fillet.
2. Add epoxy dams (aka popsicle sticks or other light material 1/4"-3/8" from where the fins will land on each side of each fin) in the gaps where the fins will land.
3. Put a good ring of glue on the inside of the body tube just aft of where the forward CR will land. Slide in the MMT/CR assembly and let that cure in the vertical position so any epoxy flow seals up the forward CR. Make sure the epoxy dams are aligned with the fin slots! Once that sets, put a good fillet on the aft CR and cure vertical pointing down.
4. Try to get some epoxy in through the fin slots to glue the middle two CRs to the body tube. It probably won't do much, but I'd feel better having tried.
5. Working one fin at a time, fill the space between the epoxy dams with epoxy, then drop in the fin and make sure it's aligned properly while the epoxy sets. If you have nice thick epoxy, you could probably do all of them at once.
6. External fillets however you normally do them.

For the retention, is the MMT longer than most or all motors you'll fly the rocket on? If so, you could tweak a long 2" wide U-bolt slightly so that the legs can go into holes drilled into the forward centering ring. You'll probably need to grind down the nuts a little bit to make them fit between the MMT and the body tube. Generous amounts of epoxy are a good idea here. Make sure they don't align with the fins! If you're planning on flying longer motors, you could do basically the same thing with another CR mounted higher in the body tube. with no inner tube.

Fin dams/pockets work really well, and you don't necessarily need fillets either. The bad thing is that it adds some weight to the back side... depending on the rocket, you may need to add some nose weight to offset it. (GPS trackers make great nose weights, BTW...)

 

[Troy3003]

I'd glue on the centering rings like Boatgeek said, then drill holes & inject the internal fillets like on wildmans directions. Dont really need dams, Easy peasy.