Casing Overheating and Rupture diagnosis 3

[sugarshot]

Some folks I'm working with on a hobby project could use some insights into addressing an experimental motor casing overheating problem.

It's a low energy solid fuel core-burning motor;

Say you have a long-ish motor casing with the usual bulkhead and nozzle. This casing has a midjoint, which is also a convergence-divergence zone. The fuel is located in the top half. The bottom half functions as a chamber.

The problem is midway up the lower half, there is a rapid heating effect (heat appears to accumulate more rapidly here, by measurement). On the test stand, the lower half splits open during the burn.

We don't want to approach the problem just by making the tube thicker or by using titanium.

So then, at considerable pressures and heating, how do we
reduce the heating effect and eliminate bursting? Between us, we have offered the following guesses;

1) This heating effect is mostly due to conductive thermal gains from the hot gases to the tube, and could be
addressed by adding a thin liner, and maybe use more resistive metal in the casing. The liner would be ceramic fiber with generous epoxy binder.

2) This heating effect is mostly due to thermal gains by thermal radiation from the gases to the tube, and could be addressed by changing the emissivity of the tube surface, and diffusivity of the tube metal. (my guess)

3) This heating effect is mostly due to the effect of
the convergence-divergence zone at the mid-joint. The shape of this 'shaped restrictor', results in gases interacting with the tube at steep angles, creating frictional overheating of the tube.

Again, the question is ultimately how to avoid casing rupture without increasing thickness. The next test casing will be with 4130 steel. The first two casings were standard electrical tubing. Someone suggested a fiberglass and thermoset polymer casing, which sounds unusual to me.

Ideas ?

 

[Compositepro]

Unless you have active guidance you do need spin to go straight. Otherwise any imperfection in symetry of thrust or fin angle will cause a constant drift in one direction and a circular flight path back to earth.

I find it surprising that you would consider EMT for a rocket casing. It is not designed for pressure or structural loading and therefore is very crudely made. With as much effort that goes into making a rocket why start with something so inappropriate? Wet-laid 7781 e-glass would be lighter and stronger.

 

[btrueblood]

I've launched a lot of hobby-level rockets, none of which were built with intentional spin, and none of which deviated by more than 10 degrees from a vertical launch. Thrust vector alignment, in an engine with a scale as large as yours, with reasonably precise machining (to profile and concentricity tolerances of, say 0.005 inches, which is very do-able with modern machine shop equipment) will be extremely accurate compared to a typical cast-clay nozzle of an Estes model rocket engine. The directional stability afforded by the aero forces acting on the fins will (or should) dominate the trajectory. So, no I don't think spin is a necessary component for what you're trying to do.

Yes, I think somebody there should have a simulation, or at least a hand-calc on the back of an envelope, for every event that occurs during the flight. From there, you should generate a what-if analysis tree. This would include the minor what-if affects of the worst-case fin and engine misalignments, and the major what-ifs of such things as an anomalous delay in the 2nd stage ignition.

 

[sugarshot]

To be clear EMT was only to make the first two 1/4 scale test motors. The point of the tests was to check heating effects and that the propellant and motor perform as expected.
"Epoxy and Fiberglass" is what we will possibly fly. It sounds sketchy to me, but it's not really epoxy, but a thermoset polymer .. if I recall correctly. Ameron.
Thanks for the views on spin, and the tip about fiberglass. I'm not sure if we have someone who knows aerodynamic and structural analysis in the truest sense, but it would not surprise me.
Active guidance is out of the question. There are many boundaries in amateur rocketry and that's one of them.
...........
I understand the case you make for no-spin, that the aerodynamics would keep the tail (and "center of pressure"-hobby term) behind the center of gravity. Seriously, I'm thinking of modeling the whole flight in MatLab, especially to simulate separation and drogue deployment. I'd have to learn a few new tricks to do this.
A simulation program, well written, could contribute to review process for getting permission. Simulating apogee and drogue deployment would make a lot of people happy, as I'd think others too feel edgy about deploying 'chutes far outside of any personal experience with rockets.

Regarding casings, any one of us could make a test casing for a simple motor and put these material recommendations to test.

thanks

 

[btrueblood]

" A simulation program, well written, could contribute to review process for getting permission. Simulating apogee and drogue deployment would make a lot of people happy, as I'd think others too feel edgy about deploying 'chutes far outside of any personal experience with rockets. "

Yes!

For that matter, even a poorly written simulation, backed up by sub-scale tests and re-written to cover the weak spots, should help the endeavor.