Newbie here: How can I figure out the point at which body tubes buckle?

[OverTheTop]

I tested BT55 to destruction once. Doesn't account for off axis loads - so fly straight!

This ^. Off-axis loads cause the tube to banana. A tube nicely accelerating straight might fail if a non-zero angle of attack happens. This can be caused my many reasons, but the usual is different wind velocities/directions as the rocket ascends. Non-zero AoA puts a bending moment on the airframe and the axial load does the rest. The further the rocket is from zero AoA the worse the problem is.

The stiffness of the tube can help/hinder this. An aeroelastic resonance will likely cause it to fold. Stiffer tubes (phenoicl, bluetube, f/g, cf) drive that deflection down and resonant frequency up. That makes it harder for it to fold during flight.

As per the Euler formulae if you looked it up, the longer the rocket then the more likely it is to buckle under load.

 

[DavidMcCann]

I believe this was quantum tube pushed to failure. I can't find the photo, but the next frame, 1/8 of a second later, this rocket was confetti. I got about 2 frames of the nose slowly pushing over, then this one, then POP. I'd imagine a failure like this being very difficult to calculate.

 

[Nytrunner]

I love that photo, never fails to crack me up when I see it.

It looks like it folded instead of fracturing or tearing (initially) which is expected for a material like that . QT seems very tough, but not as strong as other materials (for the non-mechanically inclined, look up the difference between strength and toughness)

Any idea how fast that was going? PML suggests not to exceed M.85, so that's probably where their safety factor is. Like any safety factor you can exceed it a bit *cough, Steve Shannon, cough*
but you have to be a lot more careful. That one looks like a noseblow rocket so it probably had a long unreinforced section of empty tube more susceptible to buckling as Over-the-top reminded us.

My bet: Too fast for the material, angle>0, nose gets twisted sideways in the already stressed material, and that extra stress got amplified by any buckling or bending concentrations. Longer nose shoulder could have helped spread the twist load, or a stuffer tube with centering ring further up to reduce the unsupported length.

And if anyone has ASME material properties for Quantum Tube, those would make the calculations a lot simpler lol

 

[DizWolf]

I forget the motors, and who’s rocket it was... but I know it was a large cluster, and it was when the outboards kicked in that the trouble began.

 

[Larry Curcio]

Chris Flannigan did a NARAM R&D project on the subject some years back. He did it in an LP context, but he has a nice program.

Here is a summary of his stuff in an Apogee newsletter

https://www.apogeerockets.com/education/downloads/Newsletter353.pdf