Rocketry Warehouse Reaper 3 Kevlar strap question

I’m working on assembling the motor mount for the Reaper 3. It has a 54mm motor mount and 6 fins. With the 6 fins, there’s not a lot of room to epoxy the Kevlar to the sides of the MMT without it getting in the way of the fin roots.

It’s a 3” kit, so the centering rings are too thin to drill and install an eyebolt or U-bolt.

I can see two ways to attach the Kevlar strap to the booster section of the rocket:
1. There’s some exposed length of the MMT forward of the fins (a couple of inches). It’s enough to epoxy some strap to. I could wrap the Kevlar around the forward end and epoxy, instead of running through the centering ring and down the sides. Only problem I see here is the strap will be coming off the MMT at an angle, creating a stress point.
2. I could epoxy the strap to the inside of the body tube above the motor mount and below the coupler. This is the system in the Madcow instructions for their 2.6” FG Nike Smoke.

Does anyone have any advice on either of the above? Or another idea that would work?

I have seen build threads where someone runs a piece of plywood across the top CR and mounts a U-bolt thru the wood into the CR.

I'd rather not run anything across the top of the motor tube in case I want to use a 6XL casing in it.

What I've done for multiple builds is to sand the top corner of the fins on an angle, then use a lariat approach with the shock cord. Hole through the top CR, loop around the MMT, tie, then glue/epoxy into the corner between the MMT and CR. Knocking down the corner means that the fins don't interfere with the shock cord. Just make sure you get all the sanded corners going the right way!

Done this with several of the Estes PSII Builder's kits rather than their tea-bag approach. Couple of feet of 1,000# kevlar, then tie on the supplied shock cord. I know their approach works - I just like this one better.

Exactimator said:

I'd rather not run anything across the top of the motor tube in case I want to use a 6XL casing in it.

Click to expand...

I would guess the same thing would work with a small eyebolt to one side.

75Grandville said:

What I've done for multiple builds is to sand the top corner of the fins on an angle, then use a lariat approach with the shock cord. Hole through the top CR, loop around the MMT, tie, then glue/epoxy into the corner between the MMT and CR. Knocking down the corner means that the fins don't interfere with the shock cord. Just make sure you get all the sanded corners going the right way!

Done this with several of the Estes PSII Builder's kits rather than their tea-bag approach. Couple of feet of 1,000# kevlar, then tie on the supplied shock cord. I know their approach works - I just like this one better.

Click to expand...

Have been mulling this over, but those smaller upper fins should be fine mounting in what I call a TAB method.

Visualize a MD rocket with slots cut for fin..like if you would have made the X-celator a MD rocket . In that case you would have taken a Pro75 6xL case and single wrap of wax paper or mylar and slid in the tube as a back stop ..cut most of the TTW tab off the fin and epoxied into place with adhesive to sides instead of bottom...and of course carefully remove motor case to not glue to rocket.

Technically you don't need them flush just straight and I bet you could secure them into proper position with Estes Plastic door frame for support while they are curing. The rest of the MMT is standard including the traditional encapsulated bridal on the tube. Heck you could even glue the fin to the encapsulated tether..sand encapsulated tether mostly flat ..remove TTW tab as needed til fin will seat properly then double butter mount.

Kenny

Hmmmmmm, Haven't looked at mine closely yet. Getting the Go-Devil done first. Those parasite fins are pretty small. How about applying a bridle strap a' la Wildman flush with the 54mm motor tube on both sides and coming thru two notches on
the forward bulkhead. Those parasite fins aren't very big so if you have epoxy one on top of the kevlar bridle strap, so be it. Adjust the size of the parasite fin(s) so it still is symmetric to the others and epoxy it right on top of the strap. You'll still
have exterior fillets so that would likely be enough.

Again, I don't remember how long the motor tube is. Would want to be able to leave room so the "main" fins have complete contact with the motor tube and not epoxied on top of the strap. If the tube is short, might be hard to do the above technique
and might warrant ordering a longer MT. Kurt

Weelllllll....... I stand corrected. The MMT is a bit short but 1" strapping would fit between the fin slots.

I'm considering doing this for a couple kits....

https://shop.macperformancerocketry...kits/products/copy-of-centering-ring-kit-3-00

I was faced with same problem on another rocket.

Just drilled 2 holes in the top CR, inserted Kevlar & tied knots to hold.

Use some masking tape and make it come to a "point" on end of kevlar to get it through the hole.

Or file a half hole on edge of ring and hold Kevlar in place with knots. It will work fine.

Another method I use:
Small section of coupler 2in. [pic shows my first before I cut the size down]

Another section....cut slice out of it, so will fit inside coupler.
Glue shock cord between the 2.
Slide down the fin can & glue onto of CR.

This is a great way to replace a burned or broke cord also.....

Thanks to everyone for the pointers. Here's as photo of what I'm working with. I've got an extra centering ring and 2-1/2" of MMT forward of the fins to work with.

You could wrap the shock cord around the motor tube behind the aft centering ring.

I think I'll do a combo of knotting the strap under the forward centering ring and epoxying a portion of it to the 2-1/2 inches of available MMT. It's the easiest for me to do with the tools and skill I have. Should be enough. The bottom half isn't going to be THAT heavy.

I'll post a photo when it's done and follow-up with a report if it tears loose at a launch.

Does anyone know where the CP is located on this thing?

I think 1" kevlar strapping would be acceptable and fit between the fins. Form a bridle loop and grind a small slot on both sides of the upper rings to allow the strapping to pass-through. Epoxy both sides to motor tube. It will allow a long motor casing to be used. I will do mine in that fashion and will apply tape to protect the strap as it exits the upper ring so it doesn't get gobbed with epoxy and loose strength. I tie a loop in the bridle so it falls just aft of the forward end of the sustainer tube so it doesn't suffer wear and tear rubbing on the tube edge after deployments. Kurt

I just carefully marked where the 6 fins would touch the mm and epoxied my 3/4 " strap between fins.

Reaper flew super straight on a J357 Skidmark to 3500 & change.

I changed my recovery plan at the last minute and ruined my L2 attempt. I stuffed too much 'chute into the body for the motor deploy while leaving the payload bay empty. Motor did not eject the chute and may not have separated the booster and av-bay. The back-up altimeter charge may be what separated the halves, but only succeded in tearing the chute protector* instead of getting the chute out. The halves were still joined by about 4 feet of kevlar strap. Fell in a flat spin. Had a very short walk to review the crash scene - not even a scratch. Of course the chute slid out when I gave it a gentle tug. (I'm not sure Rocket Warehouse wants this kind of "review" of the Reaper.)

I will fly it again with a small drogue at FITS.

* Chute protector was cotton welding sleeve, not Kevlar.