Shockwave Shadows in Ultra Slow Motion (Bullet Schlieren) - Smarter Every Day 203

• Hey, it's me Destin. Welcome back to "Smarter Every Day." As long as I've understood the physics, I've wanted to visualize the shock wave on the front of a supersonic bullet. But the problem with doing this is you have to have access to some pretty expensive optical hardware. Which is why I'm pretty excited about this giant 16" parabolic mirror. Which is going to let us play around with the speed of sound. Let's go get "Smarter Every Day."

Okay, we're going to use a Schlieren setup to get these bullet images. Here's how it works. So we have the camera right here, right. So this is a Phantom v2511. We have a mirror way over there and the way we're doing that is that we have a point source here. So we're shining that light and it's spreading out; and as it spreads out, it hits this mirror, and it bounces back and as it bounces back, it comes down to a point. One point right there and we're cutting off half of that light. Long story short, it comes right here, flips back over, goes into the lens. If the air is less dense, then light travels through it faster. If air is more dense, light travels through it slower. That causes the light to move or bend as it goes through different types of air; and then we can detect those shadows using this Schlieren setup.

This is my buddy Coop. He's gonna shoot through his own shop. We're not gonna do this, you know (cow mooing) - I'm not shooting at his shop. I think we're ready. - On target. - Three, two, one. Fire. (gunshot) Whoo huhu (gun clicks) - Weapon clear. - That's a big boom. So let's see if we got it. (laughing) - [Male] Come here Coop - [Destin] Oh Coop, come look at this. (sound wave) - [Male] Oh cool - [Destin] That's unbelievable. Okay, so I'm gonna save that. I like to guess where the shock waves come from. The initial one looks like it's coming off the bullet itself. But then later there seems to be this weak one that I'm guessing that reflected from the plywood table that the mirror is sitting on. Not really sure about that. But then there's this stronger one that comes after that and my guess for that one is that it's coming off the concrete floor, because it's more of a pure reflection. Then later if you fast forward the video, you can see there's a very weak shock wave coming across, and my guess is that's coming from the muzzle itself. I'm not sure on all these. I canna have to extrapolate, but that's kinda part of the fun.

Three, two, one. Fire. (gunshot) (laughing) (sound waves) That's legit man. Okay, so we've dropped it to 28 thousand frames per second. We're trying to get a really good single image. Fire. (gunshot) (coughing) I actually put air in my lungs. (sound waves) This footage looks amazing. But to understand exactly what we're looking at, why don't we build a little bitty model with a bullet and a stick and talk about it here.

So let's imagine that we've got this bullet sitting here and we've got circles emitting from the front. As we move this thing, watch what happens. Those circles will stack up in the front and they kind of relax in the back and they expand, right. So that's a normal flight. What happens if the velocity at which we move things outruns those circles moving away? Check this out. As we start to outrun it, you'll notice that we have this expansion in the back here and it tapers down and we get this angle. We get this, I don't know what you'd want to call it, right. We've got a cone here.

What happens if we move forward even faster? (whooshu) That angle that we made there is even shallower or pointier. That's called the Mach Angle and people use that to measure the exact velocity of a bullet traveling through air. There's a simple equation. The Mach number is equal to one over the sine of that angle and you can figure out how fast it's moving. I use the circle illustration because that's the way I was taught in my aerodynamic book. But if you look at the high-speed imagery, you can actually see circles being formed at regular intervals. I don't understand what they are, but they're clearly there.

What we're about to do is what I've wanted to do from the day I understood what Schlieren imagery was. We're going to shoot a subsonic and a supersonic round, and we're going to compare the difference in the shock wave. Coop, whatta we have? - [Coop] So this is a 300 blackout. - [Destin] If I understand correctly, the whole point of this particular round is to keep it subsonic. - Correct. [Destin] This is a heavier bullet. - This is by 100 grains. Almost twice the weight of this bullet here, and so this bullet right here you'll get somewhere around 1080 feet per second at the muzzle. Whereas this bullet right here will give you somewhere in the neighborhood of 1500. [Destin] Do you know what the sound of speed is? - Yeah, it's like 12 something. [Destin] I don't know. We'll put it on the screen, right here. That's the speed of sound. So this one is above the sound, that one's below. All this is on purpose because that weapon and that cartridge design was made specifically because of these physics.

• So let's do supersonic first. - [Coop] Sure. - Okay. Three, two, one. Fire. (gunshot) (sound waves) This shock wave looks very similar to the 50 cal with one major difference. The angle is more obtuse, which indicates that the bullet is traveling much slower. So he's chambering a subsonic round now and so on the Schlieren we should see a drastically different image. Two, one. (gunshot) (sound waves) It looks like, it looks like (laughing) - [Coop] (laughing) I can make it, I can make it. (sound waves) - [Destin] Alright, so this is a subsonic bullet. So why are we seeing these little flickery things on the side of the bullet? That doesn't make a lot of sense to me. So this what I always do in a situation like this. I went looking for an expert, which led me to Dr. Kanistras. A well-respected aerodynamics professor at the University of Alabama in Huntsville.

One thing we saw on the subsonic bullet is we saw, we didn't see that shock wave but we saw these weird lines out to the side of the bullet, you know. - [Dr. Kanistras] What is the speed, when you say subsonic? - [Destin] It's about 10 percent less than the speed of sound. - You have supersonic flow there. It's a normal shock. What you see there. - [Destin] On a subsonic bullet? - Uh-huh, what is happening on a commercial aircraft? It goes between point eight to one point two. Even if you're flying subsonic, you have acceleration of the flow. So you're reaching one point two. - [Destin] What? - Then you're going back subsonic. - [Destin] So what you're saying is even though I have, - [Dr. Kanistras] It's a normal shock. - [Destin] I have a subsonic bullet but the speed of the air is supersonic right there, halfway up the bullet. So on this subsonic bullet, those things I'm seeing, you're saying that's supersonic flow. - That's right. - [Destin] And it's because the air has to move out of the way around the bullet. So it has to accelerate. - That's right, yeah - [Destin] Really. - So the flow accelerates, and creates a normal shock there. - [Destin] It's because this particular bullet is so close to the speed of sound. - It reaches a transonic speed, which is point eight to one point two. So even if it's not supersonic at this point, at some point on the surface it will become supersonic. - [Destin] That's cool. Thank you. (laughs) That's really cool. Thank you for explaining that.

After this, we decided to move closer to the mirror, so we could see what the muzzle blast (gunshot) from the weapon itself looked like. (sound waves) After that, we moved the muzzle back about a foot so we could see that transition period (gunshot) where that bullet pierces through the blast itself. What's even more impressive is each one of those tiny grains of propellant had that visible shock wave you could see associated with it. (sound waves)

One more thing real quick, we took video of a revolver. It literally leaks shock waves out of the side. But before I show you that, I want to say thank you to the sponsor, which is Audible. Audible has been a partner for "Smarter Every Day" forever, and I'm very grateful because I enjoy getting smarter by listening to books as I drive. This is like the perfect brand match for me. It helps me get smarter every day. You can get a free book by going to audible.com/smarter or text in the word "smarter" to 500 500.

I've a special book to recommend today. It's called "THEM, Why We Hate Each Other and How to Heal," and it's by Senator Ben Sasse. I steer really far away from politics at all times, but this is not a political book. It's about the things that are eroding away at our culture. Things like loneliness and the importance of your family support structure, and how we need a scapegoat; and sometimes we get angry at other people when the problem is things that we are doing in our own life. It's a really good book. So I highly recommend it. I'm not quite done with it, but up to this point, it seems to be apolitical. So check that out. "THEM, Why We Hate Each Other and How to Heal." You can get that by going to audible.com/smarter or texting the word "smarter" to 500 500. It's about how we can improve ourselves by working on our communities around us. I really like it, and now definitive scientific proof of why your hands should never leave the grip of a revolver.

One. (gunshot) (sound waves) One. (gunshot) - Weapons clear. - [Destin] That is the best video I've probably ever participated in. (sound waves) Look at that, oh my goodness. Okay, well I've a feeling we're going to be doing something like this again. Can you see us stopping at this? - No. - Okay. - No way. - Yeah, so there you go. Feel free to subscribe to "Smarter Every Day" if you're into this kind of thing. If you're not, I don't know what's wrong with you (laughs). That's amazing. Alright, cool, thanks.