Recently, I was hired to design a low-velocity shotgun slug fired from a non-rifled, smooth-bore barrel.
The bullet would be manufactured entirely from injection molded elastomer, and would be used as a nonlethal bullet at very short range and very low, subsonic velocity. As a plastic solid, it would be inexpensive to manufacture.
Fired from a non-rifled barrel, this bullet will have to be stabilized by use of aerodynamic fins instead of the usual spin-stabilization used in cannons, rifles, pistols, and in some types of slug-firing shotguns.
Design work began with hand sketches followed by 3D CAD model generation, and a computer-generated simulation using a CFD code – Ansys CFX. The fundamentals of the design are that the aerodynamic center of pressure needs to be behind the center of gravity in order to be stable in flight, resulting in good accuracy.
Once the CAD model was generated, a computerized air mass to serve as the wind tunnel was constructed around it with appropriate boundary conditions such as fluid properties and the bullet design velocities for air, and inlet and outlet surfaces at the ends of the air mass represented in the computer program.
After some research and a few iterations a stable design was achieved, which was then manufactured by 3D printing. These test bullets were then loaded into 12-gauge shotgun shells along with a small amount of propellant and fired.
To my pleasant surprise, all of the test bullets clustered into a two-inch group from 50 feet.
I cannot minimize the advantage I had by having managed over 20 wind tunnel tests over the years, to say nothing of reviewing well over 200 wind tunnel reports over the decades, the tests having been run dating from the early 1930’s to the present day.
One of the details in wind tunnel testing is the great costs involved. Just to operate the tunnels, great amounts of electricity are needed. So much so that the runs are typically scheduled for evening and early morning hours due to not only the load on the local power plants but to the lower costs for off-peak power.
Wind tunnel model construction is very costly. In the days before 3D printing, the models were made by machining and milling the parts from blocks of aluminum or steel. Within the model could be pressure ports (sometimes close to 100!) that record various pressures throughout the model. It does not end there.
An electronic force balance is attached to the model to record forces and torques generated by the model These balances are all hand made.
Wind tunnels are never located where you can easily drive to them, so the test crew has to pack up all the stuff and fly there, rent cars and hotel rooms, and work the hours from about 7:00 P.M. to 9:00 A.M. 2 crews are needed of at least 2 people each, and usually more.
If I did not have access to the CFD code, a time-consuming, manual pre-test analysis would have to be conducted along with several versions of test bullets manufactured and fired.
Chalk up one for the computer! The analyst did a fair job as well!
Norman T. Neher, P.E.
Analytical Engineering Services, Inc.
Elko New Market, MN
www.aesmn.org