Purpose
Over the summer of 2023, I designed an SRAD solid rocket motor with my team to compete in the Friends of Amateur Rocketry 51025 competition during the following summer. We designed our motor to be able to carry up to two payloads to an altitude of 10,000 ft. using four 98mm tubular White Lightning COTS grains simulated to produce a total impulse of 10,250 Ns.
The purpose of this project was to bring rocketry back into our club and spark more interest for new and existing members to get more involved in hands-on projects for years to come. Competing in the FAR 51025 competition with an SRAD solid motor seemed like a very doable option for us, while still drawing interest from other students.
The purpose of this project was to bring rocketry back into our club and spark more interest for new and existing members to get more involved in hands-on projects for years to come. Competing in the FAR 51025 competition with an SRAD solid motor seemed like a very doable option for us, while still drawing interest from other students.
Design
Nozzle:
During the design process, I used the isentropic flow equation for ideal expansion ratio. Given our propellant is White Lighting, and using the data sheet to get chamber pressure, I was able to calculate the ideal expansion ratio for the nozzle with the equations from our Critical Design Review (CDR) document on the right. This number was verified through a graph from "Rocket Propulsion Elements, Ninth Edition" that associated expansion ratio with pressure ratio, and specific heat ratio. The ideal expansion ratio, taking into account the specific heat ratio as well as chamber and ambient pressures, is 8.1672. Through iterative calculations using Microsoft Excel and data from OpenMotor, I calculated a throat diameter of 3/4" and exit diameter of 2.14". Initially, we were going to machine our own nozzle out of graphite, however, there were restrictions on use of graphite in our university's machine shop. We then looked into sourcing the machining of the nozzle, but the cost of machining the nozzle was exponentially above what we expected and were willing to pay. We even looked at other materials, but there were also restrictions on using stainless steel nozzles at the competition. So, instead we purchased a COTS nozzle from Rocket Motor Components (RCS) made of phenolic glass which was actually very comparable to our designed nozzle. |
Motor Casing:
To be able to hold the propellant and contain the phenolic liner, the inner diameter (ID) of the motor casing is 3.75". With our Max Expected Operating Pressure (MEOP) 864.22 psi and our Factor of Safety (FoS) of 2.5; our motor casing should be designed to hold in 2160 psi. Using the hoop stress equation on the left I calculated the required thickness of our motor casing to be 0.1". Our universities machine shop didn't have 1/10" thick 3.75" ID aluminum 6061-T6 tubing, so we went with with 1/8" thick tubing at the expense of some added weight. Bolt-shear and bolt tear-out calculations were conducted to make sure the bolts don't shear off during the combustion process, that we have enough bolts, and the minimum distance the line of bolts are to the edges of the motor casing. Using the equations on the left I was able to calculate that we would need 12 1/4-20 bolts on each side of the motor casing 0.375" away from each edge. |
Forward and Aft Enclosures:
My contribution for the forward enclosure was sizing the O-Rings needed to seal each end of the motor casing. I sized the O-Rings by using the "Parker O-Ring Handbook", where 2 different sizes were needed. One O-Ring is used to seal the forward enclosure and the phenolic liner, and the other O-Rings are used to seal the forward/aft enclosures to the motor casing. Taking into account the piston outer diameter, bore diameter, I could properly size each O-Ring. I also had to take into account for higher temperatures from the combustion of our propellant. So I opted for Viton O-Rings as opposed to Buna-N, which is a more standard and cheaper option of O-Ring. |
Manufacturing & Assembly
Our team was able to source all material and machine all of our parts in our university's machine shop. I put together engineering drawings for each of our parts including the motor casing, forward enclosure, aft enclosure, and nozzle plug for conducting a hydrostatic test.
Once all of the components were either finished machining, or were delivered, I began the assembly process to make sure the forward and aft enclosures fit in the aluminum tube. I also made sure the O-rings fit and had to make adjustments due to some variability in the diameter listed in the Parker O-ring handbook and what Amazon provided. Everything ended up fitting nicely and is ready to interface into our rocket to compete for the FAR 51025 competition! |
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