Rocket 2: L2 High-Power Certification 

PROJECT SUMMARY

As part of the Tufts SEDS Rocketry team, I built a high-powered rocket for my NAR L2 certification. My objectives were to create a sturdy fin assembly for the J-class motor, optimize stability via careful positioning of center of mass and center of pressure, and intentionally limit the apogee to 3,000 feet AGL to increase launch flexibility. The rocket sports a sturdy laser-cut birch fin assembly, a 3D printed nosecone, and a large Blue Tube airframe, all designed in Onshape and simulated in OpenRocket. The rocket successfully flew to 3000 feet AGL on a J285 motor in February 2023, earning me L2 certification!
MAX THRUST (LBS)
MAX SPEED (MPH)
MAX ALTITUDE REACHED (FT)

DESIGN

The fin assembly's jigsaw fin design accounts for vertical loading from engine thrust and parachute deploy, as well as torsional loading and drag from the fins. Notably, the larger thrust plate translates thrust directly into the walls of the body tube, reducing stresses on the fins and risks of body tube "zippering." The rings and fins both have notches that slot together for optimal structural integrity. The fin assembly is intentionally heavy for improved stability; the fins and fins are ¼" thick, and without airfoils, to increase drag and keep the apogee below 3,000 feet.

The nosecone is a scaled-up version of the V1 nosecone from my first rocket, with an ogive shape and an integrated ring for connection with the shock cord. Although a rounded nosecone shape is better for subsonic flight, the pointy ogive profile looks cooler and reduces the apogee.
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Version 1 two-part nosecone design (transparent view)
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The final assembled nosecone
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Final fin assembly. Note the red tape on the top ring

CONSTRUCTION

I laser-cut the fins, thrust plate, and top ring from ¼" birch plywood, and cut the thinner centering rings from ⅛ " birch plywood. I fit these parts together, then epoxied them in place; I also added several layers of thin tape to improve friction fit with the body tube.

Due to size restrictions on the available 3D printers, I made the nosecone in two parts, then epoxied them together. Because the friction fit with the body tube was a bit loose, I added a few layers of tape to the base of the nosecone.

I then carefully cut the fin slots manually (because I did not have a large enough jig to use the table saw), painted the body tube, added rail buttons, and fit-tested everything. With all tests complete, I attached the shock cord to the eye bolt on the fin assembly and epoxied the fin assembly directly to the body tube, in order to avoid it popping out during parachute deploy the way it did on my first rocket.
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The fully assembled rocket before painting

TESTING

Following fit tests, I connected the fin assembly to a shock cord and strongly yanked it to simulate a parachute deployment at high speeds. I then fit the fin assembly in the body tube and applied a 54kg load to the bottom of the thrust plate to simulate the motor's maximum thrust. The nosecone attachment point also underwent yank testing, and I carefully adjusted the fit with the top of the body tube.

FLIGHT

Immediately prior to flight, I passed the mandatory L2 certification exam with a modest score of 100%. I then performed final checkouts, installed the motor, set the parachute release altitude, and loaded the rocket on the pad. After final pictures, the rocket powered up to 3000 feet, deployed the nosecone at apogee, and fell earthwards.

I had set the release altitude to 300 feet AGL but watched with a sinking feeling as the rocket continued to fall closer and closer to the ground, with no parachute visible. Then, to my immense relief, the parachute opened at about 90 feet and slowed the rocket down just enough for landing, about a quarter mile from the launchpad.

The late chute release, combined with the frozen-solid ground, posed a significant risk to the landing rocket. However, the beefy fillets, resistant design, and careful construction meant that the airframe survived landing with only two minor cracks in the fins. The late parachute release was likely due to the Jolly Logic chute release catching slightly in the parachute fabric, and only releasing on the second failsafe release attempt. 
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The rocket leaving the pad at full thrust

FUTURE PLANS

Following inspection, minor repairs to the fin fillets, and some wet wipes to remove the mud, the rocket will be ready to fly again. One potential plan is to retrofit the rocket with cameras and sensors, and relaunch it on a more powerful motor to a higher altitude. I could also use it as a test bed for using a dual-parachute system with only Jolly Logic chute releases.

However, my main goal is to obtain L3 certification next year. This will require significantly more experience with complicated deployment and recovery systems, as well as much more powerful motors and heftier airframes. As such, I want to ensure I learn as much as I can by designing, building, and flying progressively more complicated rockets. If I am pressed for time, I will prioritize these projects over re-flying my older, simpler models.

Regardless, this rocket was very fun to build and exciting to fly!