After the structural failure at STF 4, the structures team analyzed the failure and the events that led to it. One of the main causes of the failure was an epoxy joint that was only analyzed for the previous regulated system. In the Spring 2012 semester, the system was changed from a regulated to a blowdown system. The regulated system was run at around 500 psi, while the blowdown system was eventually increased to 2000 psi. With this new, higher pressure in the system the epoxy joint could not hold and failed.
This semester (and the very end of last semester) there has been a lot of work put into the redesign of this structure. First, we are adding threaded rods to our system so that the rods themselves will take the full force produced from the pressure inside the chamber rather than the epoxy joint. Below is the redesign.
Before the rods, the two aluminum end pieces (injection system on top and the nozzle on the bottom) on the combustion chamber were just epoxied to the phenolic. Now, the rods will be preloaded so that it will be pulling everything together, even under the pressures seen in the chamber.
In addition, the rings through which the threaded rods are attached have also been modified. The rings have been strengthened and optimized for our current system. The longerons and u-channels will also be strengthened to be able to handle the loads through the structure during fire. Finally, we are implementing another system similar to the structural threaded rod system so that the structure itself is more easily adjustable. This system will not bear any of the loads seen, it is just for ease of assembly.
The full structure (in blue) is shown below. We are very confident in this new design and look forward to testing it at STF 5.