I understand the SR-71 had to deal with a lot of issues in order to keep its speed (special fuel which leaked on the runway, titanium fuselage and probably other stuff). But wouldn’t the same type of engine be able to power a relatively slower fighter jet capable of easily cruise at match 2-2.5, so it doesn’t have to deal with so much friction as the SR-71 at match 3.
But while the engines exist since the 1960s, relatively few fighters go faster than match 2 and it took all the way to the 21st century to have fighters capable of supercruise (and still below match 2). So I guess there has to be a reason for that.
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Because Ramjets get more efficient at higher speeds and is actually a terrible engine at mach 2 and only on par with turbojets at 2.5 (while turbojets have an amazingly better performance at speeds lower than mach 2).
Since a ramjet engine has no active compressor (unlike a turbojet) and instead passively gathers it through the jet inlet/shockcone and pushing it through the engine it becomes more and more efficient as it approaches mach 3. After mach 3-ish it starts to lose efficiency until it’s not really capable of pushing a missile much faster than mach 6.
So overall. Ramjets make absolutely no sense for aircraft/missiles that don’t go really really fast. And even if it’s going really fast but not faster than mach 2.5-ish (Mach 3 preferably) it only makes sense because it lacks moving parts (so it makes sense for a mach 2 missile which is a one use weapon with a relatively short lifespan, but not for an aircraft given the reliability and power of turbojets).
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