Can't be stealthy, the skin temperatures at that speed will be up to around 900K = 600+ deg C, or a nice bright orange colour, you just can't cool all that surface area, so it will stick out in IR like the proverbial canine's gonads. You can also forget stealthy radar absorbant coatings.
Temperature also makes it extremely difficult to do things like communications, radar, landing gear, and especially optics at speed without heavy cooling (fuel cooling is the only real option, and there isn't that much fuel available).
You also have a ridiculously bad lift to drag ratio - about 5 if you are lucky, (concord was 8 and 747 about 18), which of course means that you have a lousy range of maybe 2-3000km.
Noise is also going to be huge, with a power output of more than twice what the blckbird was, even if you can't see it, you will hear it once it is 100km past you.
I think the difficulty of hitting this baby is also overestimated. You don't need a Mach 6 missile, you only need an accurately guided Mach 2-3 missile with a range of 50-100km lying in the path of the SR72 and an integrated aerial defense network that is smart enough to fire the missile to intercept. Think something a little like the Patriot which is 30 year old tech. SR71A might have survived in an era of primitive missile defence systems, but no longer.
Weapons deployment is also likely to be hugely difficult at such speed, given how finicky hypersonic aerodynamics are, and the added kinetic energy of 1800m/s is still only about half the energy content of TNT per kg, so it's of doubtful benefit.
Development cost would be huge, and utility against other than low tech enemies (like maybe middle east) is probably not great.
For the same money you would be better off creating a reusable sub-orbital rocket powered ballistic craft that could be both faster and stealthier (aerodynamics less important) as it does its fly over. It could be built to have long range efficient subsonic self-ferrying and it would be a much much more difficult target to hit. It could also deploy weapons, bigger better optics and other sensors without having trouble with hypersonic vibration and heat. And as a final added bonus, might help develop tech for the naescent space tourism/RLV industry.
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Can't be stealthy, the skin temperatures at that speed will be up to around 900K = 600+ deg C, or a nice bright orange colour, you just can't cool all that surface area, so it will stick out in IR like the proverbial canine's gonads. You can also forget stealthy radar absorbant coatings. Temperature also makes it extremely difficult to do things like communications, radar, landing gear, and especially optics at speed without heavy cooling (fuel cooling is the only real option, and there isn't that much fuel available). You also have a ridiculously bad lift to drag ratio - about 5 if you are lucky, (concord was 8 and 747 about 18), which of course means that you have a lousy range of maybe 2-3000km. Noise is also going to be huge, with a power output of more than twice what the blckbird was, even if you can't see it, you will hear it once it is 100km past you. I think the difficulty of hitting this baby is also overestimated. You don't need a Mach 6 missile, you only need an accurately guided Mach 2-3 missile with a range of 50-100km lying in the path of the SR72 and an integrated aerial defense network that is smart enough to fire the missile to intercept. Think something a little like the Patriot which is 30 year old tech. SR71A might have survived in an era of primitive missile defence systems, but no longer. Weapons deployment is also likely to be hugely difficult at such speed, given how finicky hypersonic aerodynamics are, and the added kinetic energy of 1800m/s is still only about half the energy content of TNT per kg, so it's of doubtful benefit. Development cost would be huge, and utility against other than low tech enemies (like maybe middle east) is probably not great. For the same money you would be better off creating a reusable sub-orbital rocket powered ballistic craft that could be both faster and stealthier (aerodynamics less important) as it does its fly over. It could be built to have long range efficient subsonic self-ferrying and it would be a much much more difficult target to hit. It could also deploy weapons, bigger better optics and other sensors without having trouble with hypersonic vibration and heat. And as a final added bonus, might help develop tech for the naescent space tourism/RLV industry.