Agent_47
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It's not really "obscure", but most people - even airplane geeks - don't really appreciate how ingenious the Blackbird's engine is.
In most ways, the J58 engine is like a normal afterburning turbojet. It's got an intake/diffuser (with a neat spike that helps convert airspeed into pressure, but that's not substantially different from similar spikes in other airplanes like the MiG-21 and F-104, or from the sharp intake edges in airplanes like the XB-70 and almost every fighter jet out there since the F-4), then a compressor, then a combustor, then a turbine, then an afterburner, then a nozzle. All that is just like any fighter jet engine, nothing unusual yet.
But it ALSO has a bunch of big bypass pipes that go from the intake to the afterburner. And it is these unique pipes that allow the Blackbird to exceed MACH 3 with late-1950s engine technology. It's pure genius. The reason is as follows.
What the front half of a jet engine (the intake/diffuser, and the compressor blades, i.e. all the stuff that happens before fuel is burned) does is; it heats up the air until it's hot enough for fuel to ignite. First, the intake/diffuser slows the air down, which raises its pressure, which raises its temperature. The faster you're going, the more of a temperature rise you get just from slowing the air down as it enters the big hole in the front of the engine (because the air has more speed, and more energy, to convert to heat). Then the air goes through those spinning blades that compress it (and thus heat it) further, until it's hot enough to burn fuel. THEN, you burn fuel, which gets the air (now with fuel and exhaust mixed in) to the hottest temperature anywhere in the airplane. Now we enter the rear half of the engine. The air - at its peak temperature - goes through the turbine, which is a bunch of blades that act like a windmill and absorb some of that energy in order to spin the compressor. The amount of fuel you can burn in the combustor (or, rather, the rate at which you burn fuel) is therefore limited by your turbine's materials: The more fuel you burn (per unit time), the hotter the turbine air becomes, so you can only burn fuel so fast before you melt your turbine. Once your turbine temperature is as hot as it can sustain, the only way you could burn more fuel (to get more thrust) is if you had a bigger engine. (Or by burning fuel in the tailpipe - the afterburner - which does not have meltable spinny parts in the middle). So you make your turbine out of the highest-melting-point materials known to man, and that determines how much fuel you can burn and how much thrust you can get. (All this is true about every jet engine since the Nazis and Whittle started experimenting with jet engines in the 1930s). But remember that the faster you go, the hotter the air becomes just by going into the big hole at the front. That means that, if your turbine is at the hottest temperature it can sustain, then you're at your top speed: if you go any faster, you won't be able to burn as much fuel in the combustor before you exceed your turbine's critical temperature. By the time you get to MACH 3, the air in the engine intake is already hot enough to start melting turbomachinery. You can't burn almost ANY fuel in the combustor, or your engine melts. This is why a conventional jet engine (read: any jet in the world other than the Blackbird or ramjets like the D-21, X-43, X-51, etc) just can't get you past MACH 3.
What the J58 engine does is, it routes more and more air from the intake (the space just behind the spike, in front of the spinning compressor blades) into big bypass pipes that go around the engine's spinning parts and take the air straight to the afterburner. The air in the intake is already more than hot enough to burn fuel, so you don't need spinny parts at all at that speed. Even at subsonic speeds, a good intake can heat the air up enough to burn fuel without a compressor's help. That's called a ramjet. The problem is, a ramjet doesn't work at all until you get to that speed; until you're going fast enough that just slowing the air down in the intake gets the air hot enough to burn fuel. So you need a spinning compressor - and a turbine to power it - in order to take off and get up to speed.
So the Blackbird takes off with its engine working just like any fighter jet engine. But as it goes faster and faster, and the air in the intake gets hotter and hotter, so less and less of that air goes through the spinning "core" of the engine (and less fuel is burned in the combustor) and more and more of that air goes around the core and into the afterburner (where more and more fuel is burned). By the time you get to MACH 3, the spinning core of the engine is basically just sitting there and windmilling around, burning little or no fuel... and the afterburner is getting almost all the air and generating almost all the thrust.
Each J58 engine in a Blackbird are like a turbojet sitting inside a ramjet. At first, the turbojet does almost all the work. As the Blackbird accelerates, the turbojet is fed less and less fuel, until it's basically just dead weight, a useless lump of spinning metal sitting in the middle of a ramjet. You needed the turbomachinery in order to get to supersonic speeds, but once you're at MACH 3, you route your air around it.
For more:
airshowfan comments on This SR71 holds the flight airspeed record at 2193mph (3530kmh) and I got to see it!!
In most ways, the J58 engine is like a normal afterburning turbojet. It's got an intake/diffuser (with a neat spike that helps convert airspeed into pressure, but that's not substantially different from similar spikes in other airplanes like the MiG-21 and F-104, or from the sharp intake edges in airplanes like the XB-70 and almost every fighter jet out there since the F-4), then a compressor, then a combustor, then a turbine, then an afterburner, then a nozzle. All that is just like any fighter jet engine, nothing unusual yet.
But it ALSO has a bunch of big bypass pipes that go from the intake to the afterburner. And it is these unique pipes that allow the Blackbird to exceed MACH 3 with late-1950s engine technology. It's pure genius. The reason is as follows.
What the front half of a jet engine (the intake/diffuser, and the compressor blades, i.e. all the stuff that happens before fuel is burned) does is; it heats up the air until it's hot enough for fuel to ignite. First, the intake/diffuser slows the air down, which raises its pressure, which raises its temperature. The faster you're going, the more of a temperature rise you get just from slowing the air down as it enters the big hole in the front of the engine (because the air has more speed, and more energy, to convert to heat). Then the air goes through those spinning blades that compress it (and thus heat it) further, until it's hot enough to burn fuel. THEN, you burn fuel, which gets the air (now with fuel and exhaust mixed in) to the hottest temperature anywhere in the airplane. Now we enter the rear half of the engine. The air - at its peak temperature - goes through the turbine, which is a bunch of blades that act like a windmill and absorb some of that energy in order to spin the compressor. The amount of fuel you can burn in the combustor (or, rather, the rate at which you burn fuel) is therefore limited by your turbine's materials: The more fuel you burn (per unit time), the hotter the turbine air becomes, so you can only burn fuel so fast before you melt your turbine. Once your turbine temperature is as hot as it can sustain, the only way you could burn more fuel (to get more thrust) is if you had a bigger engine. (Or by burning fuel in the tailpipe - the afterburner - which does not have meltable spinny parts in the middle). So you make your turbine out of the highest-melting-point materials known to man, and that determines how much fuel you can burn and how much thrust you can get. (All this is true about every jet engine since the Nazis and Whittle started experimenting with jet engines in the 1930s). But remember that the faster you go, the hotter the air becomes just by going into the big hole at the front. That means that, if your turbine is at the hottest temperature it can sustain, then you're at your top speed: if you go any faster, you won't be able to burn as much fuel in the combustor before you exceed your turbine's critical temperature. By the time you get to MACH 3, the air in the engine intake is already hot enough to start melting turbomachinery. You can't burn almost ANY fuel in the combustor, or your engine melts. This is why a conventional jet engine (read: any jet in the world other than the Blackbird or ramjets like the D-21, X-43, X-51, etc) just can't get you past MACH 3.
What the J58 engine does is, it routes more and more air from the intake (the space just behind the spike, in front of the spinning compressor blades) into big bypass pipes that go around the engine's spinning parts and take the air straight to the afterburner. The air in the intake is already more than hot enough to burn fuel, so you don't need spinny parts at all at that speed. Even at subsonic speeds, a good intake can heat the air up enough to burn fuel without a compressor's help. That's called a ramjet. The problem is, a ramjet doesn't work at all until you get to that speed; until you're going fast enough that just slowing the air down in the intake gets the air hot enough to burn fuel. So you need a spinning compressor - and a turbine to power it - in order to take off and get up to speed.
So the Blackbird takes off with its engine working just like any fighter jet engine. But as it goes faster and faster, and the air in the intake gets hotter and hotter, so less and less of that air goes through the spinning "core" of the engine (and less fuel is burned in the combustor) and more and more of that air goes around the core and into the afterburner (where more and more fuel is burned). By the time you get to MACH 3, the spinning core of the engine is basically just sitting there and windmilling around, burning little or no fuel... and the afterburner is getting almost all the air and generating almost all the thrust.
Each J58 engine in a Blackbird are like a turbojet sitting inside a ramjet. At first, the turbojet does almost all the work. As the Blackbird accelerates, the turbojet is fed less and less fuel, until it's basically just dead weight, a useless lump of spinning metal sitting in the middle of a ramjet. You needed the turbomachinery in order to get to supersonic speeds, but once you're at MACH 3, you route your air around it.
For more:
airshowfan comments on This SR71 holds the flight airspeed record at 2193mph (3530kmh) and I got to see it!!
