The Cold War had plenty of disadvantages for the world as a whole, true, but there was never a better time to be a mad scientist with crazy doomsday devices. No longer limited to freelance work delivering threats to the UN, the Cold War meant governments would actually hire you to make things. Things like Project Pluto.
Now, I'm not saying the scientists who came up with Project Pluto were mad scientists, but there was a pretty high degree of mad science going on in this thing. In some ways, it was the perfect embodiment of Cold War era thinking taken to its utmost extreme. Project Pluto, also known as "the Flying Crowbar," would have been an incredibly potent weapon. Potent, and also cruel, terrifying, and ultimately uncontrollable.
At its core, Project Pluto was just a cruise missile. Well, not "just" a cruise missile. It was a cruise missile designed around a nuclear ramjet engine, and the acronym for it should give another big clue about the pants-filling nature of it: SLAM, for Supersonic Low Altitude Missile. That means that this giant, nuclear-powered beast would be screaming around at oh, treetop level or so.
The nuclear ramjet engine at the heart of Project Pluto is key to what made this weapon so horrific. A ramjet is a very simple engine design, with essentially no moving parts. Once the missile was launched with conventional rocket boosters, the air velocity going into the ramjet's intake would be fast enough to let the engine function, and an essentially unshielded nuclear reactor would heat the air as it entered, where it would expand and be expelled out of the engine's nozzle, providing both lots of thrust and plenty of radioactive material.
Thanks to the nuclear reactor, the missile could stay aloft almost indefinitely. That means after flying across the Earth to its targets in the Soviet Union, where it would dump its payload of 16+ hydrogen bombs, the missile itself was still good to keep flying. Which allowed for all kinds of extra terror-fun:
...a locomotive-size missile that would travel at near-treetop level at three times the speed of sound, tossing out hydrogen bombs as it roared overhead. Pluto's designers calculated that its shock wave alone might kill people on the ground. Then there was the problem of fallout. In addition to gamma and neutron radiation from the unshielded reactor, Pluto's nuclear ramjet would spew fission fragments out in its exhaust as it flew by. (One enterprising weaponeer had a plan to turn an obvious peace-time liability into a wartime asset: he suggested flying the radioactive rocket back and forth over the Soviet Union after it had dropped its bombs.)
This crazy bastard had so many ways to kill you, it was like a death buffet: should I die in the nuclear blasts of the bombs themselves, or just let the shockwave of the overpassing missile kill me? Maybe I'll just wait for the radiation sickness as this thing circles endlessly overhead, like a colossal demonic robot vulture. It's so hard to choose!
From an engineering standpoint, Project Pluto was certainly impressive, and pushed the absolute limits of the technology of the time. The reactor that powered the missile was one of the smallest, lightest ever built — partially achieved by eliminating almost anything that had to do with such candy-assed ideas as "safety." The reactor's operating temperatures were so high (2500° F) that most alloys would melt, forcing the use of components like fuel rods to be made of ceramic, developed by a little porcelain company named Coors. Coors's ceramic-lined brewing vats eventually spawned a profitable sideline you may have heard of.
Lots of exotic materials were needed for Pluto's demanding parameters, and some came from unexpected routes. To coat and protect electric motor armatures, exhaust manifold paint was used, which the developers ordered from an ad in Hot Rod magazine. Which is really kind of perfect.
As advanced as it was, Pluto was riddled with bigger issues. Even setting aside the moral issues of using such a weapon, the logistics proved tricky, too. In order to keep it from being detected by Soviet radar, Pluto would need to fly extremely low. That was expected. What wasn't so well considered was the fact that to get to the USSR at those low altitudes, Pluto would have to fly over the US and/or much of our allies in Western Europe. Which means we would have been terrorizing and killing our own people and friends.
Oh, and deafening them as well — Pluto flying overhead would have been about 150 decibels. To compare, a Saturn V moon rocket at full thrust is 200 decibels. So there's that.
Of course, testing an unmanned, unsheilded nuclear reactor was tricky as well, since any tests originating in Nevada could have easily put Vegas or Los Angeles in harm's way, and most proposals (like a really huge tether) to keep things under control seemed pretty inadequate.
And, it's probably worth mentioning the thing couldn't be turned off, as such. If this was actually used, that's less of an issue, since if you're using it you're pretty much writing off habitability for wherever you've sent it to, so you may as well circle it around until it either crashes or everyone's dead. For testing purposes, the best option they came up with was ditching it deep into the ocean.
Eventually, a combination of cheap and effective ICBMs and a few moments of terrified lucidity put Project Pluto to rest. As engaging as some of the underlying technology was (airplanes/rockets that can fly without refuling) the Flying Crowbar was simply too mean, even for a weapon.
As an aside, it's interesting to read comments like this one on pages that reprint an old 1990 Air and Space article about Project Pluto:
This is thinly disguised environmentalist whacko propaganda, designed to make nuclear power seem dangerous in the sheeple's mind. If we had gone ahead with this weapon, no telling what kind of technology we would have today.
Yes, a building-sized unshielded nuclear reactor flying at rooftop height does "seem dangerous." I'm all for nuclear propulsion, but I think this may be the wrong poster child to pick. Just a hunch.