Sometimes we sit around and wonder what it would look like to shoot… (fill in the blank) in slow motion. Well if you decided to place a bullet through a really nice telescope and the light was just right, you might see this:
Before you give up on star gazing and turn your telescope into a backyard bazooka just know that last statement was a little exaggerated. That is a schlieren video of a bullet in free flight.
Schlieren imaging was essentially discovered by a French physicist and telescope optics expert, Jean Bernard Léon Foucault in 1859. He developed the “knife-edge” test as a way to test the quality of optics. The “knife-edge” test was quickly refined and brought to the fore by August Toepler sometime between 1859 and 1864 envisioning its broader application. Toepler is credited with the German term schlieren meaning an internal glass streak and most credited for discovering various schlieren imaging techniques.
In brief, schlieren imaging uses telescope components to create parallel rays of light directing them to pass through air (or any transparent media). Any heat or pressure differences in the air throw the light off course. It’s almost the same as a highway mirage except for the addition of a knife-edge cut-off that acts as a gate to block stray rays of light. The knife-edge cut-off is one feature that separates schlieren imaging from its cousin shadowgraphy, the latter being less sensitive.
Ernest Mach enters the picture, about the same time as Toepler, eager to study acoustic waves with schlieren imaging techniques. He studied shock waves by creating fast duration sparks as the flash method to take pictures on photographic plates. Eventually he partnered with Peter Salcher of the Austrian Naval Academy to capture the first schlieren picture of a bullet in free flight.
This picture revolutionized the way we’ve approached ballistics ever since. Their spark technique has continued to be used at advanced ballistic “spark ranges” where data is collected at stations and used to form complex ballistic modeling programs. Now designs can be tested in the office before ever turning a part.
Photographs are great but they lack the dynamic aspect of video. Only recently has digital camera technology surpassed film camera speeds. Schlieren images of supersonic flows can now be captured as they develop. This can be helpful in understanding the little known field called transitional or intermediate ballistics. Transitional ballistics is the study related to the interaction of pressurized gases in the barrel with atmospheric conditions outside the barrel; how they relate to the projectile and how they relate to the gun.
The video below of a .410 shotshell cartridge makes it obvious that most projectiles have to traverse very turbulent air currents that escape around the projectile. In this case those currents are initially supersonic, the key give away are the Mach diamonds.
The first wave propagating as a bubble is supersonic and is the perceived gunshot sound. A donut-like turbulence then emerges called a poloidal vortex. The diamond patterns that form are the Mach diamonds in an underexpanded flow. (The end of the barrel is acting like an underexpanded rocket nozzle.) The projectile is expelled and the pressure in the barrel billows out in a similar sequence as before, the Mach diamonds becoming more pronounced then disappearing into subsonic currents, the flow slowing until a negative pressure is reached pulling some air back into the barrel and then puffing it back out.
Now that we’ve seen a typical transitional ballistic sequence we can more fully appreciate the design that goes into muzzle designs. Below is the video of an AR-15 with top and side ports. Rifles cartridges are manufactured with better tolerances so less gases escape around the projectile. Still one or two Mach diamonds form in front of the projectile. When the projectile exits higher pressure gases escape through the ports in a forward-diagonal direction. Those bright diagonal lines at the front of the top port are oblique shock waves.
Returning to projectiles, schlieren imaging has allowed aerodynamic studies to perfect “ideal” shapes. These can be seen in wing design as well as automobiles. Below is a video of a .223 projectile. Notice that the bow shock wave touches the nose and only faint oblique shock waves can be seen along the body compared to the projectile in the opening video of this article.
Generally we have design rules for spin-stabilized projectiles travelling at subsonic (up to Mach 0.90), transonic (between Mach 0.90 to 1.35), and supersonic (Mach 1.35 to 3.0) velocities. Usable range also plays a role because a bullet can decelerate through one or more of these divisions along its trajectory. Ogive length and radius, meplat diameter, boattail angle and length, and cannelure are some of the biggest factors in design. Handgun cartridges can violate these rules due to the expected short range and the priority of terminal effects. For subsonic projectiles the aft design can make the most drag reduction contribution but it makes less contribution the faster it flies.