Chronically confused

We’ve seen a dramatic increase in the biodiversity in the neighborhood since. We have birds that we haven’t seen since we moved into the city from living rurally and those birds are starting to nest in the trees and gardens in spring. The swallows and bats are back at night. The single woodpecker we would only occasionally see visits daily and even has a friend that has joined it. We have more squirrels and rabbits and shockingly fewer moles and voles. We heard an owl outside last fall.

Outraged by the Jan. 6 Capitol riot, a wilderness survival trainer spent years undercover climbing the ranks of right-wing militias. He didn’t tell police or the FBI. He didn’t tell family or friends. The one person he told was a ProPublica reporter.

King of the table

good morning

okay hi anon idk what your base of knowledge is with any fluid mechanics or rocketry stuff so I'm writing this for a very general audience with no experience. hopefully i won't xkcd average familiarity myself.

Part 1 LAMINAR VS TURBULENT FLOW AND WHAT THE HELL IS A BOUNDARY LAYER

Laminar flow is a flow condition that describes how a fluid is moving in relation to a surface. It means all the fluid is moving more or less with the bulk flow, and there's no rapid changes in velocity anywhere. The flow speed changes gradually from zero at whatever surface the fluid is moving over¹, to the free-stream velocity at some distance away from the surface.

Through a pipe, it looks roughly like this. Parallel streamlines all moving in one direction all niceys. The area between the surface and the free-stream region is the boundary layer, and it's the primary source of energy loss in fluid systems. Laminar flow occurs at relatively low flow velocities² along very smooth surfaces. If you try to push the fluid too fast, you start getting turbulence.

Turbulent flow is when a bunch of tiny vortices start forming along the molecular-scale imperfections on a surface. You get a region of flow where the fluid is swirling and disorganized, full of tiny eddies which change very dynamically from instant to instant.

It's usually sketched out looking something like this, but in actuality is it's fairly fractal; you can observe vortices at basically every scale in a turbulent flow. From how disordered and messy the flow is, you might think that it's less "efficient". However, this isn't actually the case! With about a heap of "it depends", turbulent flow tends to have less energy loss than laminar flow!

As it turns out, those tiny little vortices are really efficient at transferring velocity between regions of the fluid. You still have to have a boundary layer between the surface and the free-stream velocity, but the velocity can change much more easily without fluid getting dragged by viscous forces. This means the boundary layers in a turbulent flow are typically much thinner, and this kind of flow is fairly desirable in a lot of engineering applications.

Laminar flow is definitely used in a few applications. For example, wind tunnels can't have any turbulence over the test section because it'll mess up your experiment. However, the main reason laminar flow has such a grasp on the engineeringtuber zeitgeist is because it looks pretty. And it does look pretty! You can make a hose that spits out a glassy, static-looking stream off water that's perfectly clear and looks cool as hell in a youtube thumbnail.

¹ Zero velocity on a surface is called a "no slip condition", and it's a pretty important thing in fluid mechanics. Every surface will have some amount of adhesion (stickiness) with the fluid, so it can't have any velocity directly at the interface.

² Actually the quantity we're concerned with here is called the Reynolds number, not velocity. The Reynolds number adjusts the flow speed for the viscosity of the fluid, its density, and the size of the object its interacting with. This lets us compare flows in very different substances, like water and air.

Part 2 HOW DOES A ROCKET ENGINE WORK ACTUALLY

A rocket engine is, fundamentally, a device for turning very angry chemicals into a high speed stream of gas.

An injector injects a liquid fuel and oxidizer³ into the combustion chamber, which burn violently producing a bunch of hot, high pressure gas. The propellants have to be separate until they enter the engine (otherwise what you have is a bomb)⁴, so the purpose of the injector is to mix the propellants together while they're inside the combustion chamber. There's a number of different ways injector designs accomplish this, but most of them involve smashing together two streams going in opposite directions. This creates a very fine mist of propellant which burns very quickly. If a rocket engine has poor mixing, the propellants will burn quite slowly, and much of the combustion will occur outside of the engine. In the best case, this results in a loss of efficiency, in the worst case, it'll result in the combustion instability and the rocket flaming out altogether.

As the high temperature, high pressure combustion products travel down the rocket engine, the combustion chamber starts to constrict. The constriction causes the flow speed to increase, like putting your finger on the nozzle of a garden hose. The flow speed keeps increasing until it reaches the speed of sound, and at that point it can't be accelerated any faster. This condition is called choked flow,⁵ and it occurs in the narrowest part of the nozzle, called the throat.

In order to accelerate the exhaust stream to supersonic speeds, the flow is now expanded instead of constricted, in a part of the rocket engine called the nozzle. In supersonic flow regimes, the Bernoulli's principle you're familiar with (or maybe not) flips, and flow gets faster when its expanded.⁶ By the time it reaches the end of the nozzle, it's going many times the speed of sound. The average velocity of the exhaust is what determines the efficiency of your rocket engine, so this is optimized to be as fast as possible.

³ Not every rocket engine uses a liquid fuel and a liquid oxidizer, but that's not important to understanding here. Anything you can stick in a pressure vessel and burn can be a rocket fuel.

⁴ Monopropellants are a single liquid sitting in the tank, but they usually require some catalyst to burn so they don't explode. And they still do sometimes.

⁵ You can think of the speed of sound in a gas as the speed at which "information" can travel through a gas, since pressure waves like sound are the only way one region of gas can be effected by neighboring regions. As the flow travels through the constriction, the gas has to be pushed along by the pressure from the gas behind it. When you reach the speed of sound, that pressure wave can't move any faster than the gas, so the upstream gas can't accelerate the downstream gas.

⁶ The math here is actually quite helpful! Without going into too much detail, the energy of a gas flow is proportional 1+ΓM², where Γ is a constant related to the molecular properties of the gas, and M is the speed of the fluid divided by the speed of sound, aka the Mach number. This term shows up in a lot of gas dynamics equations, and since it's a quadratic term with two roots, basically every equation has two solutions: a supersonic one and a subsonic one. The two solutions tend to have flipped behavior, so a lot of stuff in supersonic flows is backwards from how it works in subsonic flows.

Part 3 THE PUNCHLINE

Remember when I said laminar flow requires smooth, symmetrical flow? As you might of noticed, at no point is that the flow condition in a rocket engine! In fact, every one of those components I discussed benefits from turbulent flow!

The injector is, perhaps, the most obvious. Chaotic, swirly flows are ideal for mixing propellants as fast as possible. You need that turbulent small scale motion to drive that mixing, otherwise you're relying on diffusion⁷ which is very slow. This benefits the combustion chamber as well, as the turbulence more evenly distributes the temperature and prevents hot spots. Uneven combustion can greatly stress the structure of the engine, and cause really undesirable combustion instability.

As the flow travels through the choke and the nozzle, it's moving very very fast. Laminar boundary layers in supersonic flows aren't exactly feasible since you're just moving so damn fast, but as I mentioned earlier, they're not even more efficient. The huge boundary layers you get in laminar flow slow down the fluid a lot, which is really not something you want in a rocket engine. Any energy loss is a loss in efficiency, so thin, turbulent boundary layers are fairly desirable.

A "laminar flow rocket engine" is an idea that just sounds so absurd to anyone who has spent actual time doing aerospace engineering. And if you look at it, it's not even really shaped like a rocket engine? There's no expansion nozzle! The flow is limited to Mach 1 at the most, and there's no way its even getting that fast if you're going for laminar flow. And what the fuck is that injector???? There's so much surface area! That'll result in huge pressure losses and it'll melt if you try to operate at any type of substantial temperature. It's just. Bizarre all around.

⁷ If you've ever watched tea or dye or something slowly spread itself through a container of water without mixing it, that's diffusion. It does not happen on rocket engine timescales.