:eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek: <–passengers looking out the window of a jetliner using afterburners
Concorde used afterburners but I guess flying on that thing would’ve prepared the passengers for something out of the ordinary anyway.
This context makes me think you may have meant low-bypass. An important part of the appeal of high-bypass engines is their efficiency (c.f. the final paragraph of this link).
The Concorde engines exhaust was not visible from the passenger windows.
Nothing worse than having to explain a joke.
Okay, so the message I’m getting here from you and from **1920’s Style Death Ray ** (post #13 below) is that a turbofan cannot operate if the air coming into the engine is itself travelling at supersonic speeds. Why is that? The Wiki link didn’t provide any illumination.
Can you expand on this a little? Why does this happen? In baby language. OK, toddler language.
Mostly it’s an issue of cavitation. Basically, a turbofan engine is a turbine engine with a ducted fan. The thrust does not come from the exhaust of the turbine, it comes from a ducted fan. The ducted fan is a bunch of propellor blades that just barely fit inside of a tube and is powered by the shaft of the turbine.
A propellor blade works by creating a low pressure area infront of the blade as it spins. The air is then “sucked” into this low pressure area and is moved past the blade.
If these blades get spinning too fast, the pressure in front of the blade gets to be too low. This creates a low pressure bubble, you can think of it as a vacuum, but it isn’t quite. When that bubble gets back into higher pressure, it explodes and can pit metal and destroy turbine blades.
You can look up cavitation if you want a better explanation.
Cavitation doesn’t occur in air; the pressure differences aren’t great enough, and air is compressible enough (even at supersonic impingement speeds) that this would never be the case. However, at supersonic speeds air behaves differently; it can no longer be treated as inviscid (without internal “friction” or viscosity) and forms shock waves which create higher drag and push back on incoming air. In essence, it becomes dispropotionately harder to compress the air which reduces flow, and no matter how hard you drive the fan you get progressively less suction until the drag actually sucks air back out. There are a few tricks that engine designers use to moderate or slow the flow of air while maintaining thrust. but the only way to really get around this at higher supersonic cruising speeds is to effectively get rid of the fan entirely (or at least minimize its size) and carefully shape the interior of the engine chamber to compress and stagnate the air prior to injecting fuel and igniting it. This turns it into a ramjet, which gains compression not from a spinning fan but by the maintanence of air pressure from the motion of the engine itself (and the plane it is attached to).
The problem is that you have to get it up to considerable speeds before the ramjet effect is efficient enough to propel itself. Very high bypass turbofans have been designed to transition between one regime and the other, but they’re expensive and not terribly efficient at lower speeds (due to minimizing the size of the fan to allow bypass). Modern airliners use subsonic, high bypass fans which are efficient at the low transonic cruising speeds they spend most of their time at, while modern fighter jets and high speed, high altitude reconnaissance craft compromise between decent efficiency and performance in the supersonic regime. Scramjet (supersonic ramjet) engines don’t even try to slow the air down, instead focusing on minimizing stagnation and allowing high throughput of air at supersonic speeds; the problem is that at these speeds, the air doesn’t remain in a normal sized engine long enough to combust and provide thrust, so either you have to have prohibitively long engines (dozens of feet long) or you have to play games with the pressure waves to induce rapid combustion or detonation (see Pulse Detonation Engine for an example).
Stranger