length diameter ratio?

[Peter Bech Svalgaard]

Were building a rocket for sac. The current aspect ratio is 24.5. What would you guys recommend as the maximum aspect ratio?

 

[Ben Martin]

Were building a rocket for sac. The current aspect ratio is 24.5. What would you guys recommend as the maximum aspect ratio?

My recent scratch build is 62in long with a 3in diameter. This means that it has an aspect ratio of just over 20. It flies perfectly with 2-3cal stability due to longer rockets needing greater stability margins. I suspect that as long as your frame is solid with adequate reinforcement, the 24.5 aspect ratio should be just fine.

Do you mind posting a picture of your design for us to look at? It would make the discussion much easier.

 

[Cameron Anderson]

I built a 3" diameter two-stage that had 94" of airframe and flew fine. Just finished a 54mm diameter three-stage that has 89" of airframe that is very sturdy.

I would say the limiting factor in aspect ratio is construction material.

 

[dr wogz]

Longer rockets can be "overstable", and do suffer structural integrity as they get longer..

That's what I've been lead to believe.. no other suggestions on my part..

 

[Andrew_ASC]

The construction material determines modulus of elasticity. The modulus of elasticity relates with math to how much deflection a simply supported beam (the rocket body tube) will have at its center. You may want a stiff material such as fiberglass or carbon fiber for Mach plus environments with longer body tube lengths to help reduce deflection. On paper you can adjust the fin span to reduce overstable behavior and design stability as actual stable nominal for a high tube length if say you wanted to design around a hybrid motor for example. Practically there will be a length limit which when exceeded the tube buckles from flight loads and real life harmonic vibrations which influences how far the airframe tube deflects. A thicker wall tube also has a different moment of inertia then a small diameter thin wall tubes which again affects tube deflection. Tube deflection is not the only failure mode. Shearing is possible from too high acceleration and forces when tube wall is too narrow or material selection is not adaquate. The tube should be in compression from thrust and drag force in theory. I’ve computed simply supported beams, rotating shafts, and such for undergrad mech. I’ve also been involved in several university rocket multistage designs one had 1500N Of thrust with a CFD analysis of 40N of drag at m1.7. We asked the university for a FEA to help answer structural questions similar to yours but were declined. But with a rocket other than drag force,thrust, and weight you are dealing with aero induced external forces on the surface of the rocket that are pressure related for supersonic designs such as oblique shocks from compressible gas dynamics theory which are not trivial and may require a CFD and FEA to compute accurately. You also have to include thermal expansion loading on the tube for extreme temperature environments. I hate to rant but with my limited mechanical engineering knowledge this is a complex situation not easy to answer. I may have missed some other important factors which are outside of my knowledge and best left to an Aero or Civil background.

If you want to look into the beam deflections with math try finding mechanics materials books. Also machine design books cover some basic dynamic loading. The problem with rockets is they become dynamically loaded and aren't as simple as straight beams to compute. It’s bad when your in undergrad for engineering then have to ask some graduate students and professors to pinpoint an exact numerical answer with a computer then they tell you they don’t have the research time after a windtunnel analysis is done, that’s kinda my impression on theoretical analysis of rockets outside of just openrocket it becomes hard for exact.

 

[Bat-mite]

Depends on the size and number of fins.