Jadon Walters
TRA 18935 L1 Madcow Aerobee-Hi 4/20/19
Looking for some guidance on this problem. Would like to know what physics term this is that i'm looking for.
Im trying to understand what forces would be required for the interstage coupling area to fail and how to calculate these forces. As in the forces required to rip a sustainer away from a booster.Could you be more specific about what you're trying to understand?
I guess that you are looking for structural design loads. the most important of which would be the bending moment.Just browsing around, Is what i'm looking for the Bending Moment?
flew this earlier in the year and there was less than 1/8" slop at the top of the stack when mated.
Im trying to understand what forces would be required for the interstage coupling area to fail and how to calculate these forces. As in the forces required to rip a sustainer away from a booster.
But I guess I should mention that my last two-stage experience was my BALLS project, 3-inch MD first stage to 54mm MD second stage, that broke in half at the interstage coupler 3 seconds into the boost on a CTI M1590.
For rockets small enough, the interstage coupler need not be that complicated, but for a long, heavy, somewhat flexible supersonic rocket, it's a much bigger deal that it might seem.
The ISC was a machined piece of Delrin that held the bottom of the second stage airframe and the CTI tapered closure of the second stage motor and formed the top of a coupler/avbay in the first stage. (I'll post some pictures when I get a chance.) The Delrin broke from a side load -- I haven't decided if it simply wasn't up to the task (could have gotten some wind shear) or if the second stage wasn't properly seated in the ISC at the pad so the stack was a little off-axis at launch. I suspect the latter, though I am unlikely to use Delrin again in this fashion.Did your coupler fail? Or the airframe below the coupler? Construction? Would love to hear more details.
The ISC was a machined piece of Delrin that held the bottom of the second stage airframe and the CTI tapered closure of the second stage motor and formed the top of a coupler/avbay in the first stage. (I'll post some pictures when I get a chance.) The Delrin broke from a side load -- I haven't decided if it simply wasn't up to the task (could have gotten some wind shear) or if the second stage wasn't properly seated in the ISC at the pad so the stack was a little off-axis at launch. I suspect the latter, though I am unlikely to use Delrin again in this fashion.
Not following. In which direction is the slop
I believe this is what I'm looking for, thank you for the guidance. Just starting to dive into the more math heavy bits of rocketry.Bending moments are what causes them to fold in half. Thrust pushing up, air drag pushing down, and if your rocket is not stiff enough it bends. The largest bending moment is in the middle typically, so for a homogeneous tube (say a column, or a single rocket) that is where the maximum stress is:
https://en.wikipedia.org/wiki/Euler's_critical_load
Staging complicates things as it can lead to a weakness at the interstage coupler, and any slop in the joint shows up as more bending, just making matters go south really quickly. If done correctly there is almost no slop in the coupling and everything stays axial. If you are clever with your design the fit will exhibit minimal movement.
I flew this earlier in the year and there was less than 1/8" slop at the top of the stack when mated.
View attachment 395383
The sustainer motor protruded about 8" past the back end of the rocket, into the interstage coupler, to provide the stability of the stack.
Discussion on aeroelasticity will probably be interesting for you also.
Here's a picture. The white thing is the Delrin ISC. You could say that I didn't have enough depth of engagement into the ISC and you might be right, but I flew a test flight on a K1000 before the BALLS flight and it went fine. So I still suspect that the second stage wasn't seated correctly. If I do it again, I will probably make the ISC out of aluminum and increase its length all the way up to the second stage fins.The ISC was a machined piece of Delrin that held the bottom of the second stage airframe and the CTI tapered closure of the second stage motor and formed the top of a coupler/avbay in the first stage.
Here's a picture. The white thing is the Delrin ISC. You could say that I didn't have enough depth of engagement into the ISC and you might be right, but I flew a test flight on a K1000 before the BALLS flight and it went fine. So I still suspect that the second stage wasn't seated correctly. If I do it again, I will probably make the ISC out of aluminum and increase its length all the way up to the second stage fins.
View attachment 395449
I will probably make the ISC out of aluminum and increase its length all the way up to the second stage fins.
i setup my ISC to engage 1.5x body tube width in both directions
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