Monthly Archives: December 2012

Three hole bulkhead: revision 2 of a stressed skin approach

Rev 2 of the stressed skin approach to the bulkhead. Holes are added to the PMMA disk, and covers are added over the holes. Each cover is 1.495″ in diameter, to fit through the 1.6″ diameter throat.

Material is changed to 7075-T6 for the aluminum parts to increase the FOS. Contours are simplified to make machining simpler. Top and bottom of sandwich are now identical parts.

three hole laminate-Study 1-Results-Factor of Safety1.analysis

three hole laminate-Study 1-Results-Displacement1.analysis

Cross section view in midbulkhead:
three hole laminate in bulkhead

The holes through the PMMA are 1.3″ in diameter. This give a full area just under 4 square inches, compared to 2 square inches for the nozzle throat. This should ensure that there is no sonic lock in the flow as it goes through the delay disk. Having sonic flow into the aft chamber would result in a large increase in heat flux into the walls of the chamber, and should be avoided if at all possible.

Sandwich construction melting bulkhead

PMMA was shown to be the fastest to erode in engine tests, and is one of the weaker materials tested. My original mass-effective solutions to the problem were discarded when the midbulkhead was machined, so I started over with a stressed skin approach, similar to the concept of honecomb or foamcore composites, where faces of a part take tensile and compressive load while a weaker interior takes shear.

Here the faces are made of 2024-T3, though it could be a steel of similar strength. The upside of the aluminum is that it’s much lighter, the downside is that the firing will likely anneal it and require new parts.

The aluminum faces are bonded to a simple disk of PMMA. Ideally with a perfect layer of cyanoacrylate, as it bonds extremely well to PMMA, but a high strength epoxy would probably suffice as well and give more working time.

The parts assembled in the midbulkhead, the retainer and screws are omitted:
bulkehad in midbulkhead

The analysis of the sandwich with 1000psi on the bottom surface; displacement:
laminate sandwich-Study 1-Results-Displacement1.analysis

laminate sandwich-Study 1-Results-Factor of Safety1.analysis

It could be bumped up to over 2 for the entire part by increasing the 2024 thickness beyond the current quarter inch. The coaxial groove on the top surface isn’t needed for a boilerplate motor, it is a weight optimization. The lower surface does need some thickness removed from near the edge so as to fit in the space provided in the as-machined part.

The port through the center is not shown for simplicity in simulation. A plurality of ports would reduce the odds of the destruction of the lower casing.

This approach is generally inelegant, and would be best replaced with a flapper or retained pop-off valve mechanism, which would eliminate the supersonic jet of gas into the lower casing that this concept is bound to create.