In the 50 s of the last century, the United States put forward the "three-in-one" strategic nuclear forces, that is, the nuclear strike capability of air-based, sea-based, and land-based missiles, and missile technology can be divided into ballistic missiles and cruise missiles.
However, there was a big problem in front of the designers at that time, that is, the traditional missile engine relied on violent combustion of fuel to provide the required thrust, which was a major annoyance for cruise missiles, because the low-altitude flight speed was very limited, which meant that it took more time to reach the target position, and the intercontinental flight required a lot of fuel, and the increase in fuel meant that the size and weight of the missile increased, which in turn required to increase the engine thrust. In this case, the United States proposed the "Pluto" program to develop a nuclear-powered ramjet engine.
The working principle of the ramjet engine is very simple, when the missile flies fast, a large amount of air pours in from the engine intake and completes the pressurization, without the need for a special compressor fan, the pressurized air moves backwards and ignites with the fuel mixture, forming a high-temperature gas to be sprayed backwards to produce propulsion, this kind of engine cannot be started at zero speed or low speed, because the air flow of the air intake is not enough. Since the role of ramjet engine combustion fuel is to produce high-temperature gas, then is there any other way to replace it, the designer thought of nuclear fuel, not directly burning nuclear fuel, but using the heat released by nuclear reactions to quickly heat the compressed air, so that it is heated and expands backwards to produce propulsion, which is different from the "boiling water" nuclear power used by submarines and aircraft carriers that we are familiar with.
Theoretically, a nuclear-powered ramjet engine is feasible, and it is estimated that 50 kg of nuclear fuel can allow the missile to fly continuously at a speed of Mach 3 for several days, enough to reach anywhere on the surface, and its daily maintenance is much simpler than that of strategic bombers and strategic nuclear submarines, and the cruise missile itself can be launched in a maneuver, which is more flexible than a ballistic missile fixed in a silos.
However, ramjet nuclear engines have many technical problems to overcome, in addition to miniaturized nuclear reactors, an important problem is how to withstand the rapid release of heat from nuclear reactions. The Lawrence Radiation Laboratory intends to build an experimental engine to verify the feasibility of certain key technologies, and the reactor, known as the Tory II-A, is designed to produce 155 kilowatts. Because it is a testing machine, it does not take into account the size and weight aspects, it looks like a locomotive, the interior is insulated with some kind of ceramic material, and uranium-235 is used as fuel. Because it was only tested on the ground, a large high-pressure air tank was built to simulate the air flow of high-speed flight.
The Tory II-A was tested several times in 1961 and there were slight cracks in the columnar nuclear fuel and extremely high radiation in the critical state of the reactor, but overall it was successful, and the air could be heated up to 1080 as it passed through the reactor, dissipating heat from the reactor by the way, and the tail nozzle ejected 320 kilograms of air per second.
With early success, a full-scale, practical Tory II-C was developed, with a design power of 500 kilowatts, and its high-pressure air tank was expanded tenfold to simulate aerial flight, taking only five days to fill up, but this air was only enough for the Tory II-C to run for five minutes. By March 5, 1964, it had carried out 82 tests of various kinds, including wind tunnel tests, and performed well in flight simulations, when the simulated missile was at an altitude of 3,000 meters at an altitude of 2Flying at a speed of Mach 8, the reactor reached a critical state, with a power of 750 kilowatts, an air flow of 570 kilograms per second, and a core temperature of 1242 .
At the same time, a corresponding cruise missile is being built, but it is much simpler, because there is no need to consider the change in center of gravity due to fuel consumption, and most of the structure is made of stainless steel because it needs to withstand the high temperatures that can be transmitted by the engine, as well as the high temperatures that can be caused by high-speed flight. The missile needs to be equipped with a solid-fuel rocket booster, which will initially launch the missile into the air and increase the speed to about Mach 1, at which point the ramjet nuclear engine can be started, but the missile needs to start the nuclear reactor before launch, which is a dangerous process for the human body, and it needs to be launched as soon as possible after launch.
The Pluto "program, as well as the intercontinental cruise missile program to which it belongs, looks promising, at least technically, it is already not too difficult, but it is at a cost of 2It was terminated after $600 million (July 1964), mainly because of the success of the Minuteman ICBM at that time, which was faster and had a lower unit cost.