Aurora: A Scratch Built L3 Cert

[Cherrywaves89]

I’ve been lurking around the forum for some time now and have yet to contribute in a meaningful way for one reason or another. Maybe a little fear of my builds not being up to par with some of the incredible things I see on here. Putting that behind me because I know there is someone out there that stands to gain something from what little I have to offer just as I have. Here it is.

This is my first scratch built rocket, my first L3, and my first 98mm motor mount. Excited is an understatement. I’m grateful to have two incredible TAP’s that I consult with regularly and my club in Bayboro NC of course. (Best field on the East coast)

Without further ado, here is the first portion of the build.

Aurora will be a head end, dual deploy, 5” Wildman airframe with a 98mm motor mount and supersonic biased fin design. Here is the preliminary OR sim to gather basic Cp/Cg information.

 

[Cherrywaves89]

Fiberglass arrived, and fins were beveled using a 15° tungsten carbide burr bit on a table router I’ve only ever used to bevel fins. A wooden jig is required to make it work safely on the table. You can see we got a nice airfoil. May consider a layer of carbon reinforcement down the line.

 

[rfjustin]

Nice! A 98mm hole in a 5" airframe will be great, subscribed.

 

[Cherrywaves89]

First time using a mechanically attached retainer. Got her nice and secure to the centering ring, then bonded the assembly to the motor mount using west systems w/206 hardener and colloidal silica. I prefer to take deep breaths when opening the can as it allows the nano sized silica to enter the lungs much faster. Broke out the trusty painters tape and created a dam around the nuts and bonded the aft centering ring/retainer assembly to the motor mount with a little fillet and a lot more confidence while leaving 4 areas dry for fins of course. She’s not going anywhere now.

Anyone prefer to put the fins on the motor mount first, and slide the assembly into the airframe? If so, then how do you keep things straight. I typically inject at this point, but I’d like to try something new.

 

[Cherrywaves89]

About to start working on the avionics bay and I thought this would be a good time to run through the electronics before Fusion 360 consumes my foreseeable future. From the right, we have a Blue Raven as our redundant altimeter, a RunCam split 3 lite, an Eggtimer Quantum, and an Eggtimer WiFi switch to power on the notoriously hot RunCam. I’ve been able to partially crack the code on overheating by adding a heat sink to the chip with thermal epoxy and switching it on with a WiFi module as late as possible. On a hot day, you’re still looking at mere minutes before a crash. As always, Eggtimer is really fun to assemble, and adds to the thrill of the process.

Next update will likely be the bonding of motor mount/fins to the airframe. Does anyone else absolutely love this part?

 

[loopy]

Anyone prefer to put the fins on the motor mount first, and slide the assembly into the airframe? If so, then how do you keep things straight. I typically inject at this point, but I’d like to try something new.

After extending the fin slots out the back of the BT, you are able to slide the motor tube into the aft end and use the slots to hold the fins straight. Basically, I'd have the first 1 or two CRs inside the tube to hold it, get the base of the fin coated with epoxy, place it on the motor tube, then slide it in to let the slot hold it in place and use a little tape from the top of the fin to the body tube to brace it while the epoxy set.

After all the fins are in place, pull the motor mount out with the fins and do the internal fillets.

 

[Cherrywaves89]

After extending the fin slots out the back of the BT, you are able to slide the motor tube into the aft end and use the slots to hold the fins straight. Basically, I'd have the first 1 or two CRs inside the tube to hold it, get the base of the fin coated with epoxy, place it on the motor tube, then slide it in to let the slot hold it in place and use a little tape from the top of the fin to the body tube to brace it while the epoxy set.

After all the fins are in place, pull the motor mount out with the fins and do the internal fillets.

I love it for the first fin since I have the Macklin jig, but I am quite the psychopath about getting fins perfectly straight. Is there a way to do this without essentially eyeballing it or printing a jig?

 

[loopy]

I love it for the first fin since I have the Macklin jig, but I am quite the psychopath about getting fins perfectly straight. Is there a way to do this without essentially eyeballing it or printing a jig?

Printing a jig is really the only way to make sure they are perfectly straight. I don't really do much in the way of extreme speed or altitude, so my calibrated eyeball is good enough for my stuff, but if you truly want to make sure, print an alignment jig and use that to make sure they are perfectly straight. Removable fin can or not, that's the only way, I would say.

 

[David_Stack]

I can't recall who's thread/post it was, but someone showed how they 3D printed fixtures that were used in conjunction with traditional centering rings to capture the fins at the leading and trailing edges of the fin tab. They slipped onto the motor tube to index the fins, and were sandwiched with/by the centering rings.