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Payloadbay.com has many free tools for building rockets. Fin alignment guides, transition templates, fin slotting guides, centering ring templates, etc. Tip: When printing guides that wrap around the body tube, add about .009″ to the diameter to account for the paper thickness.
ThrustCurve.org is a database of nearly all commercial motors. There is also a very good motor guide tool that works great for quick altitude and delay simulations.
Open Rocket is a free and very popular rocket simulation program. Can open most RockSim files.
RockSim is a rocket simulator that has a few more features than Open Rocket, but it isn’t free.
BurnSim is a simulation tool used for research motors.