In the 1950s, the first autonomous nuclear monster almost came into existence. Autonomous, because no one was controlling it – once sent to crisscross the world, it would destroy everything it flew over. Nuclear, because its engine was a ramjet powered by an unshielded 600MW fission reactor. Monster, because it was supposed to carry 26 hydrogen bombs and kill just by flying over. If it were ever released, there would probably be no tomorrow for mankind.
Fortunately for us, future generations, this missile was never completed. On the other hand, it was not far off, and the only reason it was never completed was the practical introduction of ICBMs at around the same time.
From 1946, nuclear engine research had been carried out in the USA as part of the NEPA (Nuclear Energy for Propulsion of Aircraft) project – an atomic-powered bomber. Since the mid-fifties, the development and research of cruise missiles also began.
The Cold War was in full swing, so in 1956 the U.S. Air Force applied for an atomic-powered, winged missile. It was intended to be capable of delivering atomic bombs over enemy territory, which at the time was otherwise only possible using conventional bombers.
This is how the project SLAM (Supersonic Low Altitude Missile), which was dubbed Flying Crowbar, came into existence.
It was a nearly 27 meters long and 1.5 meters wide, aerodynamically rounded missile, capable of carrying up to 26 one-megaton hydrogen bombs, or a smaller number of bombs with greater effect. The drive was to be provided by an unshielded nuclear ramjet engine, which had several advantages and several disadvantages.
Among the advantages of this type of nuclear propulsion is the relatively low weight of the missile - just 27,549 kilograms, which is much less than if a shielded ramjet and reactor had been used. Another advantage was the missile's enormous range, which could theoretically reach up to 182,000 kilometers, enough for more than four and a half orbits of the earth. All this at speeds between mach 3.5 and 4.2, depending on the altitude.
Thanks to its high velocity, unshielded reactor and ramjet engine, where air flows directly through the reactor, the missile was supposed to kill and destroy just by flying over. The intensity of neutron radiation should have been sufficient to kill and cause radiation sickness in the areas over which it would have moved. At a speed of Mach 3.5 at 300 meters altitude, the missile would have produced a noise of 162 dB, which supposedly alone would have been enough to instantly destroy buildings and kill people within hundreds of meters.
Once launched, the SLAM would rise to an altitude of about 9 kilometers, where it would remain until it reached enemy territory. There, it would drop to 300 meters to avoid detection by radars. Thanks to its speed, there would be virtually no defense against it – hitting something approaching you at almost a kilometer per second and zigzagging at three hundred meters of altitude would be a big problem even today, even though we live in the age of computers with frequencies in the order of gigahertz.
Once over the target area, the missile would drop one of the 26 hydrogen bombs and continue to the next target. It was even suggested that after dropping all the bombs, it would fly across the Soviet Union, irradiating as many people and as much enemy territory as possible. At the end of its flight life, which was counted in days and weeks, the missile would self-destruct by hitting a priority target, which it would destroy both by kinetic energy and by contaminating it with a destroyed reactor.
The project was discontinued after seven years, on July 1, 1964, when the Americans began to wake up from their nuclear infatuation (have you seen an atomic bazooka?). The SLAM began to seem like a very harsh weapon – the idea of a missile the size of a locomotive circling the air for days and weeks, searching for its enemy as a horseman of the apocalypse, was ultimately too frightening even for the dismal times of the Cold War era. The fear was that the Russians would be forced by the mere existence of the missile to create a similar concept of unstoppable weapon, and that wouldn’t be good for anyone. At the same time, the introduction of ballistic missiles as a deterrent began. Eventually, the project was cancelled, despite the huge amount of money that had been put into it at the time – 260 million pre-inflationary dollars.
There were 177 full-time engineers and scientists at SLAM just before its discontinuation, and over 350 researchers at the time of its greatest glory.
Developers of the missile had to contend with big problems, which led to the development of several technologies that are still in use today. The missile was built on the very edge of the technical possibilities of that time and had practically unlimited budget, which was something unprecedented until then. Remember, that the development took its place during the fifties and the early sixties, before the Apollo project.
Metallurgy, aerodynamics, navigation and shielding of electronic components against radioactivity made great progress thanks to the missile.
The body of the missile was made of materials previously unknown, capable of withstanding high temperatures over a long period of time. These were created both by friction with air, by the speed at which the missile moved, and as a product of the fission reaction inside the nuclear reactor.
Some parts of the thruster reached the very limits of physics. For example, the area around the jet’s outlet would be coated with gold to improve heat radiation because the cooling was not sufficient despite the heavy draft. Even so, the missile was expected to fly with some parts red-hot. This was possible only because it contained virtually no moving parts.
The principle of ramjet is relatively simple:
It is an open chamber, narrowed on both sides. Air is pumped into the air inlet due to the movement of the aircraft, and then slowed down by an obstruction just behind the intake pipe, resulting in an increase in pressure. Fuel is then injected into the air stream, it burns and heats up the air, resulting in its rapid expansion and even greater increase in pressure. This makes the air rush out with high velocities. It can’t go through the inlet which is blocked by the air that is being sucked from the front, so it escapes through the narrow outlet, giving the aircraft the energy necessary for movement and further suction of air.
Ramjets can move at a high speed (~Mach 4), while maintaining a small mass and size. On the other hand, because it is a thruster engine, you first need to accelerate the aircraft or missile to a speed that produces sufficient air pressure at the inlet. This requires additional engines that are switched on during take-off.
Nuclear ramjet engine
The nuclear ramjet works in a similar way, except instead of fuel, only heat from the reactor is used. This heats the air, which expands and is forced out of the chamber.
It is practically possible to construct two types of nuclear ramjets:
- Indirect, which is wired in a similar way to a conventional nuclear power plant. The reactor heats the primary circuit, which heats the secondary circuit, which heats the air. This kind of reactor was created and considered for piloted atomic bombers, but the weight of shielding was eventually too great, so the Americans gave up. Funny thing is that the Russians had built a nuclear bomber, with relatively little shielding. This was not thanks to the revolutionary invention of better shielding, but thanks to "volunteers".
- Direct, where the air is pumped directly into the reactor, where it also serves as cooling. This approach is somewhat brutal because although it achieves higher power and the design is relatively light, simple and more/less trouble-free, the air passing through the reactor is heavily polluted by the radioactive elements produced in the reactor.
For SLAM, a direct method was chosen, as it was considered an advantage in a sense – the territory over which the missile would pass would be contaminated. On the other hand, it had the disadvantage that the missile could not be launched from allied territory. SLAM was therefore intended to be launched either at sea or in the desert, using additional rockets that would accelerate the missile to the necessary speed.
The designers had to deal with a wide variety of problems, such as unfriendly weather, snow, water and the corrosion caused by the salt water on the ramjet's hot body. Much of the progress was made possible by the simplicity of the engine, which had no moving parts. They succeeded, and the engine was tested and declared ready for aerial demonstration. The engine's thrust during the tests was a respectable 170 kN.
A small, compact ceramic reactor was specifically developed for SLAM. The code name was Project Pluto, the specific implementation was called Tori. The reactor was tested for several years on a ground station specially built for this purpose.
The temperature inside the reactor was up to 1400 °C and the radiation intensity was 4 x 1011 MEV. The reactor core was made up of 465,000 elements of fuel, with 27,000 holes in between, through which air flowed. The total weight of uranium in the reactor was 59.9 kg.
The reactor had no shielding - no lead and concrete, as is customary with conventional, atomic reactors, just a cylinder of fuel inserted into a channel through which air flowed.
A special facility in the desert was built for testing the reactor. Two miles of rails ran from the test site and were operated by a fully automatic system (in 1960!) that automatically took the reactor to the dismantling hall after the tests because the reactor was highly radioactive. More than 25 miles of compressed air piping was used for the tests to generate enough pressure on the engine in the ramjet simulation.
TERCOM navigation system
Thanks to the long range, it was clear that SLAM would need more than just inertial navigation system (hereinafter referred to as INS), which was used for missile navigation at that time.
The INS operate on the principle of recording changes in angle of motion and acceleration, which can be used to navigate in space if you know the position of the start. Inertial navigation at the time was not very accurate because it mainly used gyroscopes, which are mechanical instruments and as such can only be produced with limited accuracy. As a result, the system could only be used for relatively short distances (tens/hundreds of kilometers), as there was no mechanism to correct spatial location information.
Because of the shortcomings in the INS, a system called TERCOM was created for SLAM – TERrain COntour Matching. The radar antenna scans an "image" of the surrounding terrain – mountains, hills, and valleys. The image is then converted to digital information (at the time it must have been something relatively primitive because computers had minimal RAM) and compared to the stored patterns. Thus, the missile knows where it is at any moment and does not need active navigation systems such as GPS, which did not exist at the time (and can be jammed, anyway).
The data for comparison is obtained by radar and magnetic mapping from satellites in orbit. From this data, an elevation and magnetic map is then created. The elevation map is used for flight over land, magnetic for flight over sea.
Missiles using TERCOM usually do not include a map of the entire Earth, but only the area around the projected trajectory, plus the data from TERCOM often serves only as a correction for the internal INS. I couldn’t find any information about how the SLAM handled it. Personally, I think that the missile could have contained a larger map, since the weight of the other equipment did not play a big role and there was plenty of room in the 26 meters. This is a big difference from conventional missiles, where every gram counts because of the limited amount of fuel.
Although the SLAM was never built and all that’s left are the prototypes, TERCOM is example of a technology that has found its use elsewhere. It is still used today, along with other systems, for example in the Tomahawk missile as well as pretty much in all other cruise missiles.
Unfortunately, I have not been able to find out much about the electronics used in SLAM. The only thing that is certain is that it was specially developed to withstand high radiation. Some components were made to be radiation resistant, others were shielded.
It is quite likely that the technologies used in SLAM electronics were later used in the space, where radiation is also considerable.
I haven't been able to find out the details about the onboard computer, the type of memory used, the buses, etc. All I know for sure is that there was a computer because TERCOM couldn't operate without it. In addition to navigation, the computer supposedly could receive command updates via radio link, with the possibility of self-destruct, or guidance to another target. However, from a certain time of flight, or after a certain command, it switched to autistic mode so that it no longer responded to commands from outside, and the missile couldn't be taken by the enemy.
SLAM is considered by many to be the most deadly weapon ever developed. When I first came across it three months ago, I was fascinated. What is so frightening about SLAM is not the ability to carry hydrogen bombs, but the unstoppability combined with the sheer ruthlessness of a drive that burned everything it passed over with neutron beams from an unshielded reactor.
It was only then that I realized how desperate people must have been in the Cold War. Spending millions of dollars and a lot of manpower on a weapon in a desperate attempt to relieve the pressure of a gun on your head, by pointing a bigger gun at your opponent's head. Duel which couldn't be won.
What was going on in the minds of the scientists and engineers who, with unlimited money, were building a device so deadly that it still has no competition in the world today? They must have been delighted with the progress they were making because most of the technology used to make it wasn't even covered by the science fiction writers at the time. Looking at those photos, I realize that anything was possible, there were no barriers, no unbridgeable boundaries. It must have been spectacular on one hand, terrifying on the other, like the whole concept of SLAM. I can't quite imagine the moral dilemma of working at the very cutting edge of technology that could have destroyed the world.
I think we can only be glad that SLAM was never completed. Its very existence it would have changed the balance of fighting forces on one side, which could have led to either war or even more intense and reckless armament.
- Pluto/Slam - bisbos.com - site of the author, who kindly allowed me to use 3D illustration
- Project Pluto on Wikipedia - information about the reactor
- CRUISE MISSILES - detailed article about cruise missiles, which contains detailed information about TERCOM
- SLAM Supersonic Low-Altitude Missile - very detailed article about SLAM
- SLAM voughtaircraft.com - quite a lot of information on the website of one of the companies involved in SLAM
- The Little Crowbar That Could - a brief summary about SLAM
- Vought SLAM (Pluto) - a brief article, nice table in European units
- The Missile From Hell - some popular science type of information
- Cruise Missiles Of The 1950s & 1960s - at the very bottom there is some information and a picture showing the shadow shape of the missile
- Pluto (cruise missile) - an article summarizing most of the information
- Supersonic Low Altitude Missile - some strange presentation read by a computer
- www.nv.doe.gov/library/factsheets/DOENV_763.pdf - two-page PDF about SLAM, mostly basic information
Some photos and videos
- The Flying Crowbar - article containing video from reactor test
- Supersonic Low Altitude Missile - Spanish article with neat photos
- Project Pluto - SLAM - two photos scanned from somewhere
- Project Pluto on Wikimapia - some interesting photos and a map of the area where Project Pluto (reactor) was developed
- Project Pluto (SLAM) - beautiful explanatory picture
- VT-858 The Big Stick - not sure where to put this video, it’s basically SLAM, but the labels and company names don’t fit
Plastic model of SLAM
- http://www.fantastic-plastic.com/ProjectPlutoPage.htm - a lot of photos of the model and some chatting
- http://www.fantastic-plastic.com/ProjectPlutoCatalogPage.htm - you can order it here for $125 + postage
Discovery Channel - Nuclear Airplane
- http://www.youtube.com/watch?v=0TzCPZyAmpQ - 3/5 (information about atomic engines)
- http://www.youtube.com/watch?v=Dp-0Y8lQ-JQ - 5/5 (from the second minute information about SLAM)
Discovery Channel - Project Pluto (documentary about SLAM)
Missiles of war
- MISSILES - MACHINES OF WAR [ NATIONAL GEOGRAPHIC ] PART 3 OF 4 (detailed information about TERCOM and some information about SLAM from the second minute)