Overview
Motor cases come in a variety of designs and styles. Broadly, you can classify a motor case as as either COTS or SRAD. COTS cases come from manufactuers such as Loki Research, CTI, and AeroTech. SRAD cases are designed by the team and either manufactured in house, or by an external machinist, such as Central Machine Shop or Loki Research.
The cases themselves can follow several design styles.
Snap Ring
Snap ring closures use a ring of spring steel that slots into a groove to hold a closure in the motor. Snap ring closures work best on rocket motors up to 98mm in diameter, due to the force required to compress the snap ring to fit it in place. Snap rings enjoy an exemption from Tripoli Research Rules forbidding the use of steel components in a rocket motor. These cases are relatively cheap and easy to manufacture. Due to the case thickness required to achieve sufficient snap-ring groove depth these cases can weigh slightly more than an equivalent threaded case at smaller diameters. These cases are frequently used for research motors due to their low replacement cost in the event of an anomaly.
Threaded Closures
Threaded closures use threads formed into the case. These are more challenging to manufacture, but can allow for thinner cases. They also are easier to integrate on the field, requiring fewer tools than either a snap ring case or a bolted case. These case designs usually use compression seals on their o-rings, due to the higher compression force that can be achieved with this design. The thread depth also allows for some tolerance in the length of the sealing surface. Cases of this style are available COTS from CTI or Aerotech. The team used this style of case for Xaphan Blue. We retain 2 1 grain and 1 4 grain case of this style from this project.
-Case design article under construction
Radial Bolted Closure
Radially bolted cases use through wall bolts to retain closures. This case design scales well and is relatively easy to manufacture, though the actually manufacturing time may be longer. This design is challenging to impliment at smaller diameters due to the countersinking requirements on the bolt heads required to use this design with a conventional motor mount. This was the case design chosen for the Ocean Water propellant.
Hoop Stress Calculations
MATLAB: case calcs.m
Python: case_calcs.py