24mm vs 29mm motor choice

[SailBag]

Hi everyone, we are new students that are new to rocketry but attempting the American Rocketry Challenge. We've designed our rocket but can't decide on the size on our engine mount. We've simulated the rocket design but everyone says you can't trust it until you fly it and adjust the drag in the software, so we don't know if we should use it as is to pick a motor. But we do need to pick a 24mm or a 29mm mount. What are the advantage/disadvantages of the two sizes? Is there a way to design a rocket that can use both (without being over engineered/complicated)? Just looking for general advice about the two sizes and what we should consider (we need to design the rocket ourselves according to the rules).

 

[Titan II]

https://www.apogeerockets.com/Build...pters/Estes-24-29mm-Plastic-Motor-Adapter-2pk

 

[DigBaddy]

It's easy to adapt down, but impossible to adapt up.

The Estes 24-29mm adapters work great, are cheap and are very light.

 

[kramer714]

Use the 29mm, per the comments you can use an adapter to use the 24 in the 29mm mount.

I have designs for 3d printed disposable adapters. Msg me and I can send you a STL file

 

[dhbarr]

It's very handy to be able to fit and simulate a broad variety of approved motors into your project. In this way, an oversight in one phase can potentially be corrected by substitutions in later phases.

 

[Back_at_it]

When in doubt go bigger and adapt down as necessary.

 

[kramer714]

One other thing you may not have thought about yet is the overall weight and the CG.

Using the 29mm mount gives you a few more 'possible adjustments'

• 24 mm motor and lightweight adapter - low weight cg moves forward vs 29mm
• 29 mm motor - low weight - moves cg aft vs 24mm motor
• 24 mm motor and 'heavy adapter' - increases overall weight, moves cg aft
• 24 mm motor and 'heavy adapter' + nose weight - increases overall weight, allows tailoring of cg fore or aft
• 29 mm motor mount and a pizza - allows for lunch, for safety use a "remove before lunch' flag on the rocket

 

[spaceship<=>operator]

• 29 mm motor mount and a pizza - allows for lunch, for safety use a "remove before lunch' flag on the rocket

Mmmmm pizza.

I really like the Aeropack 24mm adapter for the 29mm motor retainer. Good way to go.

https://www.apogeerockets.com/Build...Motor_Adapters/AeroPack_24_29mm_Motor_Adapter

 

[kramer714]

Disposable Motor Adapters

https://www.rocketryforum.com/threads/disposable-motor-adapter-dma.179262/

have fun

 

[boatgeek]

24mm motors will get you to ARC target altitudes every time unless you're building something giant (like 3" diameter) or really heavy. That said, in general the tradeoffs are as follows:
29mm can get to the higher end of the F impulse range than 24mm
24mm motors tend to burn a little faster for the same impulse, meaning that you'll get more acceleration off the pad. This is dependent on propellant and manufacturer, so it's not an ironclad rule.
24mm motors tend to be a little lighter for the same impulse, with the same disclaimer.
24mm motors tend to be a little cheaper, with the same disclaimer.