Goblin Extreme Model Rocket - BT60 with a 29mm Motor

Update: The maiden flight was conducted on August 17th, 2024, at the C.R.A.S.H. launch event.  It was flown with an Aerotech E30-7, achieving an apogee of 1,003 ft -- seen here.

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The Extreme Series of Builders Kits were designed to be easy to print, assemble, and fly.  These rockets can be flown on lower-powered motors in a park, or with higher-powered 24/29mm motors.  Typically, an Estes E15 can loft these rockets to nearly 1000 ft, thus the name Extreme.  

The construction is simple.  The fin can is solid plastic with cut-outs for the fins to be joined using epoxy.  I have included a one-piece fincan with the fins attached.  The inside diameter of the fincan was designed to be lined with a 29mm motor tube.  Between this tube and the fin can is a small channel for a piece of kevlar kite string (~1mm thick).  This should also be epoxied in.  

The motor retention ring will slip onto the end of the motor tube and be glued with epoxy.  The nose cone can be flown in two configurations, with the electronics sled or with the plug.  Both are identical except the plug does not incorporate the sled.  They will accept a ¼-20 eye-bolt.  Both also include two holes for venting for altimeters and using e-matches for electronic deployment.

Also included is a detailed OpenRocket simulation file with potential motors that should work.  You can configure this model to fly with electronic deployment using the nosecone ebay found in the Extreme Accessories kit.  I have included motors in the simulation file to accommodate this, you will simply need to switch the parachute deployment setting from “First ejection charge at this stage” to “Apogee” in the simulation file.  All the motors listed in the simulation file should work fine for electronic deployment but the ones NOT marked “Electronic Deployment” will fail miserably without a deployment mechanism.  Be Vigilant with your motor usage.

If you print and enjoy this model, please consider clicking the LIKE (Heart) button and posting a Make.  It really helps me and I would love to see other models made from this.

NOTE:  I have not flown this model yet. It is very similar to the other Extreme kits, and I will post more pictures and the results post-flight, but please keep in mind that this is an untested model.

Models Included:

• Nosecone - Requires support for the shoulder
• One-piece Fincan - Requires support

Additional Files:

• OpenRocket Simulation File - This may not be entirely accurate, but good enough for CP/CG and motor selection.  You will need to download OpenRocket.  If you update the file, please share it with me and I will update the model for others and give you credit!

Materials Needed:

• Extreme Series Accessories Kit - These parts are interchangeable with other Extreme Kits
• BT60 Estes Body Tubing
• Approximately 6 ft or 1800mm of Kevlar shock cord – approx 1mm diameter.  I use Emma Kite String made of Kevlar from Amazon
• 29mm Motor Mount tubing (MMT) -  I use LOC tubes but Estes or Apogee will work too
• 15 (or more)-minute epoxy
• ¼-20 eye-bolt (or two) for the sled/plug
• 18in parachute and wadding or fire blankie

Printing Guide:

These parts work well with PETG, though PLA+ will work too.   I printed mine with 5 walls at 0.2 resolution @ 100% infill for durability.  This will make it heavy, but strong.  You could get away with 2-4 walls and 15-30% infill, but you probably want to use a stronger infill like Cubic or 3D Honeycomb.  Your field conditions will need to be a consideration.  You can print the fincan lighter and the nosecone heavier to help with stability.  Let me know how you configured yours.

Building Guide:

Starting with the fin can, your motor tube (MMT) should be as long as the fin can plus the retaining ring.  I suggest using a longer tube, and after the MMT and ring are glued, trim the excess off at the ring.  Dry fit your MMT into the fin can.  If you encounter moderate resistance, either sand the inside of the fin can or sand/peel the top layer of the motor tube until you can slide the tube in without kinking it.  If you kink the tube, the motor will not insert easily and can become jammed after flight.  

Once the dry fit is complete, run one end of your shock cord down from the forward end until about 25mm (1 in) is sticking out the back.  Apply a thin coat of epoxy to the aft end of the MMT and slide it in from the forward end until about 15mm sticks out the aft section.  You must align the shock cord in the internal channel that was modeled to accommodate it.  Take your retention ring and slide it onto the end of the tube and push it up until the ring is seated on the aft end of the fin can, pinching the shock cord.  Make sure everything is aligned and seated then set aside to dry.  Clean off any excess epoxy from the fin can, retention ring,  or inside of the MMT with Rubbing Alcohol.

Next, you will need ~ 9-in (228mm) of Estes BT60 body tubing.  Use the body tube cutter in the Extreme Accessories kit to trim it to the desired length.  You can cut the tube back to the desired length after assembly (mine was about 6 inches).  Apply a film of epoxy to the inside aft end of the tube and slip it into the fin can.  Give the tube a little twist to make sure there's good coverage.  Make sure to feed the shock cord through.  Apply a little epoxy to the aft end of the motor tube and slide the retaining ring on until it bottoms out.  Your fincan is now complete.

Safety Police Disclaimer:  

Rocketry is a safe and rewarding hobby.  With knowledge and experience, you can experience the excitement of yeeting a rocket thousands of feet into the sky.  There is nothing else like it.  

Having said that, you are responsible for the rockets you fly.  If you build and fly one of these rockets, you should understand the NAR Safety Code and the potential risks involved with launching any rocket, especially these.  They were not designed to be toys.  They are real rockets that can potentially go very high and carry significant energy.  This rocket is intended to be built and flown by experienced rocketeers who understand the risks and can mitigate potential issues through experience.  By printing and flying this model, you do so at your own risk.  You are solely liable for any unexpected results and you agree to hold me and any manufacturer harmless of any liability.  I recommend you join NAR or Tripoli, join a club, and ask lots of questions.  Launching a rocket as a NAR/TRA member at a sanctioned event provides you with insurance you don't get at the park.  </end of Safety Rant>