Overview
Responsible Engineer: Sam A
The motor retention and payload bulkheads play a crucial role in the rocket’s integrity by ensuring the motor and recovery lines are fastened to the airframe during flight, as well as providing a compartment for to payload to reside in. The design of the vehicle is such that the motor acts as a coupler between the fin can and the main airframe sections. In turn, there needs to be a mechanism to retain the motor in order to hold everything together. This is accomplished by the motor retention bulkhead, which is a disk bolted radially to the airframe forward of the motor and secured to the motor with a threaded rod and nut.
Revision 1
The bulkhead design went through several iterations, each successively lighter while retaining the required structural properties. The first bulkhead design was a solid ½” thick aluminum disk that fit inside the airframe and was secured by radial grub screws. The center hole is the mounting location for the threaded rod from the motor.
This design also had off-center holes to accommodate the recovery system attachment U-bolt. The rocket design placed the CO2 deployment mechanism close enough to the bulkhead that it interfered with the U-bolt if it were placed at the center.
Revision 2
While this design worked well in the first test flight, it was quite massive (274g) and needed to be redesigned to meet the high level of performance and efficiency required to allow Virgo to fly to accomplish its mission. A new bulkhead was designed with recesses cut into the disk to remove mass. The machining was performed on a 3-axis CNC milling machine, and the results were satisfactory. The resulting bulkhead was 138g – 50% lighter than the previous iteration.
After completing the second iteration, team members used built in FEA (Finite Element Analysis) features of the Solidworks CAD program to analyze the structural properties of the new bulkheads and their performance under loading conditions. In the simulations, the bukheads were fixed at the radial bolt holes and applied a force of 2,000N to the center face. This was the worst-case scenario in which machining defects left a gap between the fin can and airframe, causing the motor to push upward and transmit the full thrust force to the bulkhead. While this is not expected in flight, testing the part with these loads allows us to be confident in our design.
Revision 3
The results from the FEA analysis on Revision 2 shows that the part is still well within the structural limits of the material, so a new bulkhead was designed to remove even more mass. The design requirements were analyzed and it was realized that this bulkhead did not need to hold pressure, allowing for the removal of even more material. The design was optimized for stress reduction with the addition of filleted corners and mass reduction with the cut pattern in the center. FEA analysis reveals that while the main stress concentrators are at the corners of the radial braces, while rest of the part remains well within the structural limits of the material. This bulkhead design weighs 43g – 84% lighter than the original design.
As was mentioned earlier, there will be two bulkheads – one for motor retention and one for payload and recovery retention. Little work was done on the payload/recovery bulkhead until revision 3, at which the following design was created:
The absence of cut-out features is indicative of the fact that the bulkhead must hold the pressure created by the CO2 deployment system. The bossed holes will provide mounting for the recovery U-bolt. As the threads on the U-bolt start ½” from the bottom of the “U”, it was important to stand the bolt off so that the nut on the other side could tighten completely flush with the surface of the bulkhead.