Doorknob Scratch Build

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Feb 10, 2023
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Centennial, Colorado
Doorknob Scratch Build

Since I gave my Estes Doorknob kit with Vanderburn upgrades to my son, I have regretted that move. I really wanted to build this kit. His build was fine and he needs the practice with bigger models, but it doesn't help me with my need to build.

I have been tinkering with different designs for the past couple of weeks and then set out to scratch-build one with 3D Printed parts.

I chose to stay with the 3-inch BT platform to start. It's within the capabilities of my printer without having to get squirrely and I still have 3-inch tubing left over from my Hobgoblin and SpiderMax builds. While I launch most of my rockets on 29/240 RMS, I still like the versatility the 38mm MMT can offer. This is the foundation of my build. I also built this kit entirely with Duramic PLA+ filament. It's been fine for all my builds and it's what I know. I am looking at doing other types of filament later but ABS/PETG seems like a lot of maintenance for the payoff.

I started off modeling the fins and centering rings in OnShape, based on the ID/OD of the MMT and BT provided in OpenRocket. While OpenRocket's material database is probably pretty accurate, it seems that I am not, despite my best efforts but we'll see how that plays out later.

Here are some pics of the Fin and rings:
Doorknob Fin Drawing (1).pngDoorknob Fin Drawing Forward Centering Ring.png
Parts Designed in OnShape

I mentioned that the dimensions used in OR were not accurate. I certainly don't mean to blame OR. I have tried using calipers with limited success but I have had better luck using tube dimensions right from OpenRocket. I used the dimensions for the LOC 38mm MMT ID/OD and thought I was using the Estes 3-in body tube. I have something else which is close but not the same. This resulted in the ID of the centering rings being a little snug, which of course means some sanding but not a reprint, most likely due to the material shrinking because the rings are loose in the body tube. Also, I noticed that the tab on the fins is about 1mm too short. This is not ideal but I think I might try to make it work since I have about 25 hrs of printing time on the line.

Next, I scaled up my 3D Printed Retaining Rings in TinkerCAD. I have noticed on HPR launches, that the opening around the nozzle gets melted away, as expected. I print a couple of these for each launch and replace them as needed. For this build, I increased the thickness of the aft part of the cap, to make it beafier. I will probably be launching mostly 29mm on this, so the nozzle will be narrower. This worked fine on the Hobgoblin during my L1 Cert flight, with minimal soot and no deformation. A 38mm motor will probably cook this. When I do get a 38mm in this or the Hobgoblin, I will test the durability of this retainer. Should be fine for a few flights.

Finally, I set out to design a nose cone. I'm 'ok' with nose cones but not a wizard. While I was mulling over some different ways to do shock cord retention and ebay housing, I came across a model by Argon67. I have built my Hobgoblin and SpiderMax nose cones from his design with some modifications. This specific model was a near-perfect replica of a Doorknob/Big Daddy nose cone. It lacks an integrated shoulder or shock cord retention but includes the base from his previous models. The base/shoulder would work fine if you glued it in but I want to utilize the nose cone for an ebay. I adopted the method of stuffing a coupler into the nose cone as seen on Wildman nose cones by designing a modified coupler from a previous design. It worked great. I came up with a way to attach an eyebolt to a piece that would be epoxied to the interior tip of the nose cone and that worked but the heat from the Expoxy kicking off warps my print.

It's not the end of the world thanks to Bondo Spot Filler but I didn't like that approach. Last night, Inverted Pursuits Labs uploaded a model that converts your Loc 4-in nose cone into an ebay. I thought, "Why not try that?" It worked out brilliantly and I have incorporated this approach into this build. I scaled it down for 3-in format, fused the ebay base into my coupler/shoulder, and printed it. It's perfect. I can either modify the sled after printing to match my electronics or redesign it with built-in standoffs.

Nose Cone Assembly Parts

Ebay with Nosecone shoulder coupler

Nose Cone Assembly

Testing the fit on my BRM

That's where I am at the moment. I have epoxied the forward and mid CRs to the MMT, along with the MMT shock cord mount, cut the slots for the fins, and test-fitted everything. Time to fire up some PPE and start gluing things to other things.

Dry-fit to get aft CR spacing

Doorknob Mockup

I'll try to capture the events as they transpire, but I'm really bad about target fixation, so pictures end up getting taken at the end.

3-in 38mm Builders Kit.png
Builder's Kit on Printables

I have also put the 3D parts up as a builder's kit on Printables. Feel free to download it and give it a try. I will be racing to get this built for Labor Day weekend's launches, and I will have some videos to share as well.
Last night, epoxied the MMT, mounted the fins, and epoxied in the aft centering ring. I used a generous amount of epoxy on the interior (between the mid and aft CRs both at the MMT and where the body tube and fin meet). This turned out great with the fins straight and extremely strong. I floated a small bead of 5-minute epoxy right down the gap between the fin tab and the MMT. Used a wooden chopstick to keep it from running while the epoxy kicked and that worked well enough, but it was fiddly. These turned out surprisingly strong. I am very happy with the result and that the aft CR prevents anyone from seeing my shame. 🤣

I went to mount the retaining ring, but it's too long. I cut it down to 15mm and reprinted it. Fits perfectly but I might have to make some mods to the cap now. I'll see once I get the ring mounted on the rocket permanently.

Also tonight, I will get the shoulder for the nose cone epoxied to the NC itself and start working on the external fillets. I still need to mount electronics, print and mount the camera shroud, and get this gal painted. I might be able to fly this at CRASH for our launch this Sunday!

The plan for flight will be to maiden with an F67, then a G74 (if all goes well). I will probably save the H250 because... SCARY. I am planning to attend the Chili Blaster at SCORE next month. This will be a fun rocket to launch there on an H-motor.
I got side-tracked by nose cone ebay modifications. I posted a bunch of makes and remixes on Printables. I now have a customized ebay sled that houses my Eggtimer Quantum and a couple of 2S batteries. This is gonna make life a lot easier since I don't have to dismantle the quantum install to fly multiple rockets at the same launch. Yes, I could buy more Quantums (and I really should) but I'm saving my nickels for a tracking system b/c I live in Colorado and I wanna get high... I mean launch rockets really high. Yeah, that's what I meant.

I need to epoxy the shoulder to the nose cone, attach the retaining ring to the MMT, and do my fillets, sand, prime, fill, etc... I'm not getting color on this before CRASH this weekend. I might slap a sticker on one of the fins so it's not completely naked and send it. All the rigging for the recovery is done and it's ready to go!


  • Ebay Sled for Eggtimer Quantum.png
    Ebay Sled for Eggtimer Quantum.png
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Tonight, I epoxied:
  • The nose cone to the coupler
  • The motor retention ring on the MMT
  • The aft centering ring external
I am currently prepping to do fillets. I think I will wait until tomorrow night. I want to be rested.

Oh, and I have been playing around with names and I think I'll call this one, "PoleCat". I'll start playing around with some designs for the decals this weekend.
I ended up calling this rocket, "Cookie Monster" by request of my youngest. We set out this weekend to launch it at CRASH in Ft. Lupton, CO on Sunday, 9/3/2023. The weather and wind were great but it was a little humid. I tested the electronics and the nose cone and everything looked good but the nose cone was giving a little resistance, so I took off 1/2 wrap of tape. I maiden launched it on an F67, but in hindsight, it wasn't a great choice as the launch was weak off the rail. It left the rail, with a little wiggle and then arched over very early, probably 200ft. It did fly pretty good... all the way to the crash site! I never heard the deployment charge and it lawn-darted. When I found it, the top 2" of the nose cone were buried in the dirt with the rest laying off about 3 feet away. The nose cone (or what was left of it) was fused to the crinkled body tube. After we surgically removed the NC, we discovered that the Eggtimer Quantum Altimeter was missing along with the wiring harness. I spent the next 40 minutes searching the area but unfortunately, I was unable to recover the altimeter. My camera survived but it stopped recording before the launch, so there isn't any on-board footage. #TripleShotToThePills

I did remember to arm the Chute Release 😜

Lessons learned:
  • Don't undersize your motor selection (aka double-check the thrust-to-weight ratio)
  • Double-check the nose cone fit (I seem to need to relearn this lesson)
  • Double-check the CP/CG
  • Secure the electronics - Don't assume the ebay will protect everything
  • Take a GPS reading of the landing area
  • Check the Electronics (Camera) -- AGAIN!
The good news is, that the fincan, Ebay sled, and the batteries survived unscathed. I will need to reprint the nosecone/coupler after I redesign it to be a little more robust. I have plenty of 3" tubing left and will need to graft on a new donor BT. Cookie monster will fly again!

20230903_094116.jpgCookieMonster Launch2.png