Project Pluto: Going Nuclear

[Neil1993]

I have been seeing this crop up in a few of the books that I have been reading. While Project Pluto was one of the more cruel and ruthless weapons designed by the United States during the cold war, it still remains an interesting technical concept. I have summarized the history of the project, while mainly focusing on the technical aspects, in the following paragraphs.

Conception and Requirements

It was the mid '50s when the need for a weapons project such as Project Pluto was first rationalized. The United States wanted something that would be invisible to enemy radar and unattainable by conventional anti-aircraft weapons. The proposed solution was a supersonic, low-altitude missile (SLAM). The idea is that the missile would fly so close to the ground that radar couldn't detect it, and it would go so fast that it could not be shot down. However, sustaining speeds that great (approximately mach 3) at such a low altitude required ridiculous amounts of power. The answer? A nuclear ramjet, of course!

Design: The Vehicle

The vehicle itself had obviously been designed with supersonic speeds in mind. It had a pointed nose and very swept wings. The vehicle, in fact, was practically a rocket, as the three control surfaces that it had were small compared to the rest of the vehicle and evenly spaced at 120 degrees. Underneath it was the large ramjet intake which earned it the nickname "The flying crowbar." Since the vehicle was to be very large and complicated, the designers figured that it would be a waste of resources to use it to deliver a single warhead. So, it was designed to carry more than a dozen thermonuclear warheads which could be used on multiple targets.

The challenges of such a craft were, understandably, enormous. For instance, the body had to endure a dynamic pressure greater than that felt by the X-15. As well, the ramjet had to handle heat loads in excess of 800 degrees Celsius and most of the components within the rocket had to be able to operate in an irradiated environment. The vehicle was supposed to be launched with three strap-on boosters. Once reaching sufficient speed, the reactor was engaged and the nuclear ramjet kicked in.

Design: The Powerplant

By far the most difficult part of the vehicle to design was the nuclear motor. This was something that had never been previously attempted. It had to produce more than 500 megawatts of power while still being small enough to fit in a plane. Obviously, it had to be unshielded. However, this meant that the conditions within the reactor were more extreme than anything had ever been designed for. the temperatures in the center of the reactor exceeded 1370 degrees Celsius, which is well above the operating temperatures of most alloys. In the end, the core was made of ceramics, molybdenum, advanced steel and Hastelloy R-235. Even then, it was designed at a low factor of safety, as the materials were operating very close to their thermal maximum while still resisting the incoming air, which created up to 2400 Kilo-Pascals of pressure on the core.

Testing the Nuclear Ramjet

Despite the almost impossible technical requirements of the motor, two powerplants were actually built and tested. The first one tested was the Tory II-A. It was a scaled down version which only generated 155 megawatts of power, processed 320 Kilograms of air every second and burned at 1230 degrees Celsius. A large facility had to be built to compress and preheat the air required the run the engine. The first test lasted only a few seconds before all the pre-heated and compressed air ran out, but it was a complete success.

Next, a full scale test engine, called the Tory II-C, was built. As well, the facilities required the store the compressed air were upgraded so that they could fuel the bigger engine for a longer period of time. After a "small" test with reduced thrust, the scientists put the pedal to the metal and tested the engine at it's maximum operational limits. It produced 156,000 newtons of thrust and 513 megawatts of power during its five minute burn and consumed almost a ton of air every second.

The End of the Program

Thankfully, this missile was never deployed as an actual weapon. In fact, a full prototype was never even built and tested. There are a myriad of reasons proposed as to why it was shut down. It could have been that this weapon was just too deadly and provocative. Another plausible reason is that the hundreds of ICBMs and submarine based missiles rendered it redundant. Finally, it could be that it was almost impossible to test the missile without irradiating its test range and rendering the area uninhabitable. (Edit: Read rdfox's reply for a more in depth explanation of the program's cancellation)

Despite the project's sinister purpose, many of the technologies developed went on to be used in commercial and civilian applications, meaning that the money dished out by the government did not, in fact, go to waste.

Sources

Spaceship Handbook, by Jack Hagerty and Jon C. Rogers

http://en.wikipedia.org/wiki/Project_Pluto

Note: This post tries to focus on, and create discussion over, the technical challenges of such a device.

Edited December 18, 2013 by Neil1993

[Themohawkninja]

Engine: High-pressure radioactive flamethrower

Warhead: Nuclear

End-result: Your target, and the 100km path up to the target are incinerated and uninhabitable.

In case you're interested, both the U.S., and the Soviet Union tried a nuclear jet engine. Only the Soviet Union ever got one to work with any sort of thrust-to-weight ratio, as they disregarded the safety of the crew by ditching the heavy lead/concrete radiation shield to reduce weight. Only two of the crew members ever survived the radiation.

[rdfox]

In case you're interested, both the U.S., and the Soviet Union tried a nuclear jet engine. Only the Soviet Union ever got one to work with any sort of thrust-to-weight ratio, as they disregarded the safety of the crew by ditching the heavy lead/concrete radiation shield to reduce weight. Only two of the crew members ever survived the radiation.

[citation needed], to quote Wikipedia.

As for SLAM, there's no mystery as to why it was cancelled--it was a whole combination of things. First off, our allies--over whom the missile would have to fly on the way into its target--were less than amused by the thought of a Mach 3 missile roaring past at 500 feet, flattening buildings with its shockwave and literally cooking chickens in the barnyard as it flew overhead with the radiation from its reactor. (Ironically, this last issue was proposed as being an asset in wartime, with the missile being programmed to pretty much perpetually fly back and forth over the Soviet Union after it had deployed its last warhead, to essentially render the place uninhabitable.) Secondly, there was the little issue of where you test-fly an unshielded nuclear reactor--and even if you find a safe place to do so, how do you guarantee that the guidance system won't go nuts and make a low-level run through Las Vegas, or even Los Angeles? There was a proposal to test-fly them at the Nevada Test Site's nuclear testing ranges, on an extremely long tether so that they couldn't run away (and that would have been a HELL of a tether!), but a more realistic one had them test-flying them off Johnston Island, then having the test missile end the flight with a deliberate dive to an ocean burial in the Marianas Trench. As Air and Space Smithsonian commented, "even in an era when the Atomic Energy Commission was trying to get people to think of radiation in terms of 'sunshine units,' the thought of dumping dozens of unshielded nuclear reactors into the ocean was enough to give people pause."

However, the biggest reason it was cancelled was much simpler than any technical or political issues--those could have been resolved with enough work. By 1960, the Atlas ICBM was starting to show signs that it would be a reliable weapon, at a time when SLAM had yet to fly, or even test a flight-rated engine. What's more, whereas SLAM would have taken roughly four hours to reach its first target in Soviet territory, and saw genuine risk of being intercepted and shot down, either by fighters or ground-based air defenses, the Atlas could put a warhead on the same target a mere thirty minutes after launch, and was invulnerable to all then-existing defensive measures--AND it looked like it would have a significantly lower unit cost than SLAM, on top of that.

So now, not only do we have a project where the final weapon would have a number of fundamental problems, but there's an alternative program that's at a much more advanced state of development--despite being started later--which avoids most of those problems, can do the job just as well, and will cost less. There wasn't any real question which one was going to be cancelled and which would be developed into an operational weapons system, even BEFORE the Atlas people started claiming that "SLAM" actually stood for "slow, low, and messy"...

[Nuke]

this was supposed to be a doomsday weapon, so who cares about the path of death left behind it. you could probibly do a design with a heat exchanger to better isolate the core from the atmosphere, but the risks of using a flying nuclear reactor on earth are still to great (look at how frequent plane crashes are). but that doesnt mean you cant fly around on saturn with one.

[jwenting]

They cared not so much for the path in operational use, but the path during test and training missions.

[Nuke]

it was canceled not because of safety but because icbms pretty much made them obsolete. this was the '50s, everybody wanted nuclear and we didnt have hippies to protest it yet.

[K^2]

In the 50's, such a system might have been an ultimate weapon, but now a modern AAA takes something like that out with no trouble.

[NGTOne]

In the 50's, such a system might have been an ultimate weapon, but now a modern AAA takes something like that out with no trouble.

Mach 3 at 150 meters? As if. AA guns of ANY type are out of the question - NO weapon mount EVER developed tracks that fast. As for missiles, if they managed to get a lock ("flying below the radar" is still a thing), Mach 3 is roughly the speed at which the SR-71 Blackbird flew. You know what the standard evasive procedure for it was? "Hit the gas". It out-RAN missiles. An aircraft like this, though preposterous in every POSSIBLE respect, would actually be nigh-impossible to intercept using even modern technology. Best interception profile? Rig up large explosive landmines along the flight path, triggered by the shockwave - bury the detonator a distance from the actual mine, and pray to whatever deity you believe in that the plane will fly low over it. With a little luck, you'll either a) starve it of air, or fill it with so many holes that it won't matter, and in both cases, pray even harder that it doesn't land on something important.

[Neil1993]

As for missiles, if they managed to get a lock ("flying below the radar" is still a thing), Mach 3 is roughly the speed at which the SR-71 Blackbird flew. You know what the standard evasive procedure for it was? "Hit the gas". It out-RAN missiles.

The Russian's planned Hypersonic missile should be fast enough to fit the bill, that is, if it is ever finished.

[K^2]

If you spotted it at 150m? No, it's gone before the round can reach it. But you don't. Typical acquisition distance to a low flying plane is about 10km. These systems are usually placed pretty strategically. That gives you over 20 seconds at Mach 3. If you've ever seen AAA guns in action, you know that it's more than they need. The actual reaction time is measured in seconds. So a couple of kilometers lead is sufficient. Even in tricky terrain, you aren't going to fly by even a mid-cold war AAA system, like Shilka.

Modern systems? Forget about it. Not only are they equipped with passive radar, meaning holes in your fuselage are your first warning, but they have much better reaction time. Some of the systems, such as these that are set up on US carriers, are designed with maneuvering ICBM warheads in mind. If it has a chance to shoot down a re-entering warhead, do you really think it would have any trouble with Mach 3?

The only possibility of sneaking by one is if it's not armed. There, you rely on human reaction time, and that you can beat with a low flying plane easy enough. Might be good for a sneak attack, but experience shows that major wars rarely start completely out of the blue.

[DerekL1963]

However, the biggest reason it was cancelled was much simpler than any technical or political issues--those could have been resolved with enough work. By 1960, the Atlas ICBM was starting to show signs that it would be a reliable weapon, at a time when SLAM had yet to fly, or even test a flight-rated engine

Not just Atlas, by 1960 Titan I was entering service, Minuteman I was well along in development, IRBM's (Thor, Jupiter) were being widely deployed, SLBM's (Polaris A-1) were showing great promise, stand-off weapons (Hound Dog) were being deployed... The need for SLAM to penetrate the thicket of Soviet air defenses was rapidly evaporating.

[Nuke]

If you spotted it at 150m? No, it's gone before the round can reach it. But you don't. Typical acquisition distance to a low flying plane is about 10km. These systems are usually placed pretty strategically. That gives you over 20 seconds at Mach 3. If you've ever seen AAA guns in action, you know that it's more than they need. The actual reaction time is measured in seconds. So a couple of kilometers lead is sufficient. Even in tricky terrain, you aren't going to fly by even a mid-cold war AAA system, like Shilka.

Modern systems? Forget about it. Not only are they equipped with passive radar, meaning holes in your fuselage are your first warning, but they have much better reaction time. Some of the systems, such as these that are set up on US carriers, are designed with maneuvering ICBM warheads in mind. If it has a chance to shoot down a re-entering warhead, do you really think it would have any trouble with Mach 3?

The only possibility of sneaking by one is if it's not armed. There, you rely on human reaction time, and that you can beat with a low flying plane easy enough. Might be good for a sneak attack, but experience shows that major wars rarely start completely out of the blue.

also pretty sure a low flying aircraft at mach 3 would make a flir scanner light up like a christmas tree, even at several kilometers distance. control systems could handle that, pepper its flight corridor with lead, you just need to hit it once, the plane's own velocity will destroy it. if thats not good enough, ground based laser weapons aren't far off.

[Kerbin Dallas Multipass]

Might be good for a sneak attack, but experience shows that major wars rarely start completely out of the blue.

Nuclear first strike weapons systems like this one were quite popular during the cold war and are pretty much designed to hit the enemy completely out of the blue

[kiwi1960]

also pretty sure a low flying aircraft at mach 3 would make a flir scanner light up like a christmas tree, even at several kilometers distance. control systems could handle that, pepper its flight corridor with lead, you just need to hit it once, the plane's own velocity will destroy it. if thats not good enough, ground based laser weapons aren't far off.

Back in the 50's, flir was still only science fiction. Like most weapons of the 50's era, they would be obsolete these days because modern technology renders them useless. Case in point, the naval Battleship. The army and its trench warfare, the air force and any propeller powered bomber...

Most of the proposed weapons was just to get the other side to spend more to develop its own version or the defences for it... as soon as the B1 was flown, it was promptly cancelled because the USSR had developed effective countermeasures for it... oh, and of course, the cost for each copy was too much (over a billion US dollars per plane) .... having said that, President Reagan vetoed that cancellation and a few were built but only because the USSR was expanding its military... he also reactivated a few WW2 era battleships.

ICBM's negated the need for any silly nuke powered missile... and lets face it... it was a silly idea.

The Pentagon also wanted to nuke the Moon back in the 50's... why? Because it could and to put the fear of God into those commies.... typical Hawk thinking.

[Kryten]

You know what the standard evasive procedure for it was? "Hit the gas". It out-RAN missiles.

No, the standard evasive procedure was to not overfly significant amounts of territory with modern missile systems (i.e. USSR or PRC); they'd learned their lesson after declaring the U-2 'invincible' due to altitude and then having seven shot down. Why did you think they came up with the M-21/D-21 concept? They knew they needed something even faster and higher to survive under those conditions, and that it had a good chance of not doing so regardless.

[jwenting]

The Russian's planned Hypersonic missile should be fast enough to fit the bill, that is, if it is ever finished.

And even if not, you could detect it coming from hundreds of miles away and put up a barrage of steel and lead in its path that would knock it out of the sky. No accuracy needed, just volume.

[Themohawkninja]

The Pentagon also wanted to nuke the Moon back in the 50's... why? Because it could and to put the fear of God into those commies.... typical Hawk thinking.

How would that put the fear of God into the Soviets? Nuking the moon wouldn't really do anything. I mean, it might look cool when viewed through a telescope (if you remember not to let yourself get blinded), but it wouldn't really do anything...

[Themohawkninja]

And even if not, you could detect it coming from hundreds of miles away and put up a barrage of steel and lead in its path that would knock it out of the sky. No accuracy needed, just volume.

That's what we did in WWII, and yet planes were still able to physically crash into the deck of the warships (Kamikaze). Those we flying at mere fractions of mach 3.

[Neil1993]

That's what we did in WWII, and yet planes were still able to physically crash into the deck of the warships (Kamikaze). Those we flying at mere fractions of mach 3.

the point he (jwenting) was trying to make is that the shrapnel is considerably more deadly to a vehicle possessing a higher velocity. Say a piece of flak moving at 50 m/s and weighing 0.25 Kg impacts a plane moving in the opposite direction at 150 m/s. In this case, the flak's speed relative to the plane will be 200 m/s and its kinetic energy relative to the plane will be 5 KJ. However, if the same piece of flak hits a SLAM moving at 1020 m/s (~ mach 3), its relative kinetic energy will be more than 143 KJ. This would be able to cause instant and catastrophic damage to the vehicle.

However, if it were shot down over your country, the fallout from the crash (or the plane being shredded into a million pieces in midair) would still cause significant damage.

Edited December 18, 2013 by Neil1993

[KASASpace]

In the 50's, such a system might have been an ultimate weapon, but now a modern AAA takes something like that out with no trouble.

Not of course considering max altitude which just might be cruising altitude. Plus, if shot down, the core could go into meltdown and then the whole thing would go up in flames, along with all the stuff within a 15 km radius.

[Neil1993]

Not of course considering max altitude which just might be cruising altitude. Plus, if shot down, the core could go into meltdown and then the whole thing would go up in flames, along with all the stuff within a 15 km radius.

The cruising altitude of this missile while it was engaged in its supersonic bombing run was no more that 150 m from the terrain. Also, if shot down, the core would probably spread itself over a very large area. It's unlikely that the core inside such a vehicle would remain in one piece long enough to go into meltdown during a crash.

[Minecrafter1]

Well to add insult to injury, you might as well arm the warheads when you pass a waypoint on the flight into enemy territory.

So, even if you do shoot it down, the warheads might "fail deadly" and the entire plane would go up in a multi-megaton flash...that would spread the pieces of that hot reactor as fallout.

[Nuke]

it would be about as dangerous as a dirty bomb. not super dangerous but enough to cause some deaths.