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.
Design:
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:
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.
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.
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.
Design:
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:
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.
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.
