3-in 38mm Doorknob Rocket Builders Kit

Update:  5/11/2024:

I have upgraded the nosecone for this kit, found here.  If you have a printer, that is capable of printing up to 290mm, then you may wish to give that one a try.  There is nothing wrong with the nosecone within the kit.  It was designed to print on my Ender 3 with a z-axis height of 250mm max.    Also, my modeling skills have grown to the point that I don't need to build off the work of others.  Please continue to support the other designers, and so them appreciation by printing and adding Makes to their posts.  For those starting out, please use my models as a reference and build from them and make them better.  That makes this community stronger.  Also, please always give attribution to the designer so they can get the credit due them.  Thank you.

This is a builders kit for a scratch clone of the venerable Estes Doorknob.  Only the fins will require support for the root cord between the tabs.  It includes the following:

• Nose Cone
• Shoulder
• Fin
• Forward Centering Ring
• Mid and/or Aft Centering Ring
• Avionics Sled
• Motor Retaining Ring
• Motor Retaining Cap
• OpenRocket Design - In a zip file

Notes:

• Print the Nose Cone with the highest resolution for your printer.  I did not use support for mine
   
• The Shoulder incorporates the base from @Boyson-IPL's 4-in ebay.  It was scaled to fit in the shoulder.  The shoulder also includes two ribs for extending the size to accommodate the tube.  Adjust to your needs
   
• The Retaining Ring and Cap were adapted from the 29mm model found on Thingiverse.  I have had great results with this since I beefed up the aft end of the retainer with ~ 1.5mm more meat.  YMMV.  You can opt to use the Launch Labs 38mm retainer or an aluminum Aero-pak retainer
   
• You will need to print 4 fins.  Build supports for the root cord just forward and aft of the fin tab.  You may need to sand/file this area after printing
   
• You should print two of the mid/aft centering rings If you want to launch larger motors (see recommended motor mount tube size below)
   
• The Forward Centering Ring has a hole that will accommodate a 3/16-inch eye bolt.  You can drill that out to accommodate a ¼-inch
   
• The Avionics Sled has not been modified other than rescaling.  It incorporates a slot in the middle of the threads that will accept a nut for use with an eyebolt at the bottom, but it is too small for a 3/16-inch nut (rescaling issue).  I opted for a 3/16 eyebolt without the nut, using epoxy and treading the bolt in.  There is no way that is coming out without significant damage.  It should be oriented with the threads down as seen in the cover image
   
• I did not modify the sled since most modelers have their own ways of setting up their ebays.  I will be mounting an Eggtimer in mine with nylon standoffs and hardware with the battery at the bottom/aft of the sled.
   
• Print (or at least measure) each part separately.  This way you can get an accurate mass calculation for each part.  Then compare this mass to the OpenRocket part to get a more accurate simulation.
   
• The OpenRocket simulator is awesome because it's very versatile, configurable, modifiable, and FREE!  However, it is an approximation and accuracy is important to get your simulation to align with real-world results.  The sim very accurately approximates your Center of Pressure (CP) and is helpful for verifying your stability in the field.  The more accurate you are with measurements and mass calculations, the more accurate your simulator will be.  Use this simulator to your advantage to get the best possible performance.  If you cannot, at least do a swing test.  If you are unfamiliar with what that is, head over to The Rocketry Forum to learn more.  

For building this rocket, it's up to you but I have provided some measurements for mine (also included in the OpenRocket Simulator)

Motor Mount Tube (MMT): 38mm diam MMT  at 9 inches or 228mm in length

Body Tube: 3-inch dia. at 18 inches or 457mm

Important Note:  Always double-check your dimensions and do test prints.  The nose cone was designed for the Estes Pro Series II body tube.

Tubes and accessories can be ordered from Loc/Precision Rockets or Estes Rockets

Subscribe to my YouTube Channel to see this rocket in action when the video posts

Combined several models from other creators (with some modifications) to make this kit.