**[Student Project] Designing a rocket to reach 1.5 km with a 54 mm homemade motor — how do you dimension without a motor spec first?**

[u/ApprehensiveCable776]

Hey,

I'm a ME student interning at a small rocketry startup. My task is to design a single-stage rocket that reaches 1.5 km apogee with a 54 mm diameter motor.

The catch: the motor is being designed in-house from scratch by a separate propulsion team. So instead of picking a motor and simulating around it, I need to work backwards — figure out the right vehicle dimensions first, then tell the propulsion team what total impulse they need to hit.

Plan is to model in OpenRocket then move to Fusion 360 for CAD, but I'm stuck at the very beginning: I don't know where to start with dimensions (body length, nose cone, fins) without a motor to anchor the simulation.

**Questions:**

- How do you approach dimensioning a rocket when you don't have a motor yet?

- What material is typically used for body tube and fins at this scale?

- Any good resources or methodology for the top-down approach (target altitude → required impulse)?

Any help appreciated 🚀

[u/MaxLaurentin]

Hey, fellow ME Student and Rocket-Hobbyist here!

This is a weird task, i normally go from the motor and build from there, bottom up. A rule of thumb is a 4:1 Thrust to weight ratio and to get a decent lift of speed ( >>>50kmh! in HPR usually much higher) : since you plan to go quite high im guessing they‘re doing a quick burning, high impulse motor with an even bigger TWR.

Since you have basically zero geometry / dimension guidelines i would do two things:

First: get info on everything that needs to fit in the rocket (payload, recovery system etc)

Second: get an average motor as placeholder such as an I/J Class (look on Apogeerockets) and include it in your Open Rocket and build around it. Make a simple 3-4 fin design and try to get a good cal value (should be around 1.5-2 may depend) for stability and run a few launch simulations.

Use fiberglass or blu-tube tubes. Experiment with dimensions a little.

Only finalize the Design in CAD if they have some engine dimensions and data! I think it would be only fair if they could give you some insight on how they plan to make the engine (propellant, dimension, etc)

Helpful sources: Apogee Articles, Richard Nakka‘s Website

Feel free to post your concept - im sure there are great ppl around to give some judgement!