The fifties of the last century were a period of rapid development of nuclear technology. Superpowers built their nuclear arsenals, building nuclear power plants, icebreakers, submarines and warships with nuclear power plants along the way. New technologies held great promise. For example, the nuclear submarine did not have any restrictions on the cruising range in the submerged position, and the "refueling" of the power plant could be done every few years. Of course, nuclear reactors also had disadvantages, but their inherent advantages more than offset all the safety costs. Over time, the high potential of nuclear power systems interested not only the command of the navies, but also the military aviation. An aircraft with a reactor on board could have much better flight characteristics than its gasoline or kerosene counterparts. First of all, the military was attracted by the theoretical flight range of such a bomber, transport aircraft or anti-submarine aircraft.
In the late 1940s, the former allies in the war with Germany and Japan - the USA and the USSR - suddenly became bitter enemies. The geographical features of the mutual location of both countries required the creation of strategic bombers with an intercontinental range. The old technology was already unable to ensure the delivery of atomic ammunition to another continent, which required the creation of new aircraft, the development of rocket technology, etc. Already in the forties, the idea of installing a nuclear reactor on an aircraft was ripe in the minds of American engineers. Calculations of that time showed that an aircraft comparable in weight, size and flight parameters with a B-29 bomber could spend at least five thousand hours in the air at one refueling with nuclear fuel. In other words, even with the imperfect technologies of that time, a nuclear reactor on board with just one refueling could provide an aircraft with energy throughout its entire service life.
The second advantage of the hypothetical atomicolettes of that time was the temperatures reached by the reactor. With the correct design of a nuclear power plant, existing turbojet engines could be improved by heating the working substance with the help of a reactor. Thus, it became possible to increase the energy of the jet gases of the engine and their temperature, which would lead to a significant increase in the thrust of such an engine. As a result of all theoretical considerations and calculations, aircraft with nuclear engines in some heads have turned into a universal and invincible delivery vehicle for atomic bombs. However, further practical work cooled the ardor of such "dreamers".
NEPA program
Back in 1946, the newly formed US Department of Defense opened the NEPA (Nuclear Energy for the Propulsion of Aircraft) project. The goal of this program was to study all aspects of advanced nuclear power plants for aircraft. Fairchild was nominated as the lead contractor for the NEPA program. She was instructed to study the prospects of strategic bombers and high-speed reconnaissance aircraft equipped with nuclear power plants, as well as to shape the appearance of the latter. Fairchild employees decided to start work on the program with the most pressing issue: the safety of pilots and maintenance personnel. For this, a capsule with several grams of radium was placed in the cargo hold of the bomber used as a flying laboratory. Instead of part of the regular crew, the company's employees, "armed" with Geiger counters, took part in the experimental flights. Despite the relatively small amount of radioactive metal in the cargo compartment, the background radiation exceeded the permissible level in all the aircraft's habitable volumes. As a result of these studies, Fairchild employees had to sit down to the calculations and find out what protection the reactor would need to ensure proper safety. Already preliminary calculations have clearly shown that the B-29 aircraft simply will not be able to carry such a mass, and the volume of the existing cargo compartment will not allow the reactor to be placed without dismantling the bomb racks. In other words, in the case of the B-29, one would have to choose between a long flight range (and even then, in a very distant future) and at least some kind of payload.
Further work on the creation of a preliminary design of an aircraft reactor ran into new and new problems. Following the unacceptable weight and size parameters, difficulties appeared with the control of the reactor in flight, effective protection of the crew and structure, the transfer of power from the reactor to the propellers, and so on. Finally, it turned out that even with sufficiently serious protection, radiation from the reactor can negatively affect the power set of the aircraft and even the lubrication of the engines, not to mention the electronic equipment and the crew. According to the results of preliminary work, the NEPA program by 1948, despite the spent ten million dollars, had very dubious results. In the summer of 48, a closed conference was held at the Massachusetts Institute of Technology on the topic of the prospects for nuclear power plants for aircraft. After a number of disputes and consultations, the engineers and scientists participating in the event came to the conclusion that it was in principle possible to create an atomic aircraft, but its first flights were attributed only to the mid-sixties or even to an even later date.
At the conference at MIT, it was announced the creation of two concepts for advanced nuclear engines, open and closed. The "open" nuclear jet engine was a kind of conventional turbojet engine, in which the incoming air is heated by a hot nuclear reactor. The hot air was thrown out through the nozzle, simultaneously rotating the turbine. The latter set in motion the compressor impellers. The disadvantages of such a system were immediately discussed. Due to the need for air contact with the heating parts of the reactor, the nuclear safety of the entire system caused special issues. In addition, for an acceptable layout of the aircraft, the reactor of such an engine had to be very, very small, which affected its power and level of protection.
A closed-type nuclear jet engine had to work in a similar way, with the difference that the air inside the engine would heat up on contact with the reactor itself, but in a special heat exchanger. Directly from the reactor, in this case, it was proposed to heat a certain coolant, and the air had to gain temperature upon contact with the radiators of the primary circuit inside the engine. The turbine and compressor remained in place and operated in exactly the same way as on turbojets or open-type nuclear engines. The closed circuit engine did not impose any special restrictions on the dimensions of the reactor and made it possible to significantly reduce emissions into the environment. On the other hand, a special problem was the selection of a coolant for transferring the reactor's energy to air. Various coolants-liquids did not provide the proper efficiency, and metal ones required preheating before starting the engine.
During the conference, several original methods were proposed for increasing the level of crew protection. First of all, they concerned the creation of load-bearing elements of an appropriate design, which would independently shield the crew from the radiation of the reactor. Less optimistic scientists have suggested not to risk pilots, or at least their reproductive function. Therefore, there was a proposal to provide the highest possible level of protection, and to recruit crews from elderly pilots. Finally, ideas appeared concerning equipping a promising atomic aircraft with a remote control system so that people during the flight would not risk their health at all. During the discussion of the last option, the idea came up to place the crew in a small glider, which was to be towed behind the atomic-powered aircraft on a cable of sufficient length.
ANP program
The conference at MIT, having served as a kind of brainstorming session, had a positive effect on the further course of the program for the creation of atomic-powered aircraft. In mid-1949, the US military launched a new program called ANP (Aircraft Nuclear Propulsion). This time, the work plan involved preparations for the creation of a full-fledged aircraft with a nuclear power plant on board. Due to other priorities, the list of enterprises employed in the program has been changed. Thus, Lockheed and Convair were hired as the developers of the airframe for a promising aircraft, and General Electric and Pratt & Whitney were tasked with continuing Fairchild's work on the nuclear jet engine.
In the early stages of the ANP program, the customer focused more on a safer enclosed engine, but General Electric conducted "outreach" to military and government officials. General Electric employees pressed for simplicity and, as a result, cheapness of an open engine. They managed to persuade those in charge, and as a result, the driving direction of the ANP program was divided into two independent projects: an "open" engine developed by General Electric and a closed circuit engine from Pratt & Whitney. Soon, General Electric was able to push through their project and achieve special priority for it and, as a result, additional funding.
In the course of the ANP program, another one was added to the already existing nuclear engine options. This time, it was proposed to make a motor, which in its structure resembles a nuclear power plant: the reactor heats the water, and the resulting steam drives the turbine. The latter transfers power to the propeller. Such a system, having a lower efficiency in comparison with others, turned out to be the simplest and most convenient for the fastest production. Nevertheless, this version of the power plant for atomic-powered aircraft did not become the main one. After some comparisons, the client and the ANP contractors decided to continue developing "open" and "closed" engines, leaving the steam turbine as a fallback.
First samples
In 1951-52, the ANP program approached the possibility of building the first prototype aircraft. The Convair YB-60 bomber, which was being developed at that time, was taken as a basis for it, which was a deep modernization of the B-36 with a swept wing and turbojet engines. The P-1 power plant was specially designed for the YB-60. It was based on a cylindrical unit with a reactor inside. The nuclear installation provided a thermal power of about 50 megawatts. Four GE XJ53 turbojet engines were connected to the reactor through a piping system. After the engine compressor, the air passed through the pipes past the reactor core and, heating up there, was thrown out through the nozzle. Calculations showed that air alone will not be enough to cool the reactor, so tanks and pipes for boron water solution were introduced into the system. All power plant systems connected to the reactor were planned to be mounted in the rear cargo compartment of the bomber, as far as possible from the habitable volumes.
YB-60 prototype
It is worth noting that it was also planned to leave the native turbojet engines on the YB-60 aircraft. The fact is that open-circuit nuclear motors pollute the environment and no one would allow this to be done in the immediate vicinity of airfields or settlements. In addition, the nuclear power plant, due to technical features, had poor throttle response. Therefore, its use was convenient and acceptable only for long flights at cruising speed.
Another precautionary measure, but of a different nature, was the creation of two additional flying laboratories. The first of them, designated NB-36H and proper name Crusader ("Crusader"), was intended to check the safety of the crew. On the serial B-36, a twelve-ton cockpit assembly was installed, assembled from thick steel plates, lead panels and 20-cm glass. For additional protection, a water tank with boron was located behind the cab. In the tail section of the Crusader, at the same distance from the cockpit as on the YB-60, an experimental ASTR reactor (Aircraft Shield Test Reactor) with a capacity of about one megawatt was installed. The reactor was cooled with water, which transferred the heat of the core to heat exchangers on the outer surface of the fuselage. The ASTR reactor did not perform any practical task and worked only as an experimental radiation source.
NB-36H (X-6)
Test flights of the NB-36H laboratory looked like this: the pilots lifted an aircraft with a damped reactor into the air, flew to the test area over the nearest desert, where all the experiments were carried out. At the end of the experiments, the reactor was turned off, and the plane returned to base. Along with the Crusader, another B-36 bomber with instrumentation and a transport with Marine paratroopers took off from Carswell airfield. In the event of a crash of a prototype aircraft, the marines were to land next to the wreckage, cordon off the area and take part in eliminating the consequences of the accident. Fortunately, all 47 flights with a working reactor did without a forced rescue landing. Test flights have shown that a nuclear powered aircraft does not pose any serious environmental hazard, of course, provided it is operated correctly and there are no incidents.
The second flying laboratory, designated X-6, was also to be converted from the B-36 bomber. They were going to install a cockpit on this aircraft, similar to the unit of the "Crusader", and mount a nuclear power plant in the middle of the fuselage. The latter was designed on the basis of the P-1 unit and equipped with new GE XJ39 engines, created on the basis of the J47 turbojets. Each of the four engines had a thrust of 3100 kgf. Interestingly, the nuclear power plant was a monoblock designed to be mounted on an aircraft just before the flight. After landing, it was planned to drive the X-6 into a specially equipped hangar, remove the reactor with engines and put them in a special storage facility. At this stage of the work, a special purge unit was also created. The fact is that after stopping the compressors of jet engines, the reactor ceased to be cooled with sufficient efficiency, and an additional means of ensuring the safe shutdown of the reactor was required.
Pre-flight check
Before the start of flights of aircraft with a full-fledged nuclear power plant, American engineers decided to conduct appropriate research at ground-based laboratories. In 1955, an experimental installation HTRE-1 (Heat Transfer Reactor Experiments) was assembled. The fifty-ton unit was assembled on the basis of a railway platform. Thus, before starting the experiments, it could be taken away from people. The HTRE-1 unit used a shielded compact uranium reactor using beryllium and mercury. Also, two JX39 engines were placed on the platform. They were started using kerosene, then the engines reached operating speed, after which, at the command from the control panel, the air from the compressor was redirected to the working area of the reactor. A typical experiment with the HTRE-1 lasted several hours, simulating a long flight of a bomber. By the middle of 56, the experimental unit reached a thermal capacity of over 20 megawatts.
HTRE-1
Subsequently, the HTRE-1 unit was redesigned in accordance with the updated project, after which it was named HTRE-2. The new reactor and new technical solutions provided a power of 14 MW. However, the second version of the experimental power plant was too large for installation on airplanes. Therefore, by 1957, the design of the HTRE-3 system began. It was a deeply modernized P-1 system, adapted to work with two turbojet engines. The compact and lightweight HTRE-3 system provided 35 megawatts of thermal power. In the spring of 1958, tests of the third version of the ground test complex began, which fully confirmed all the calculations and, most importantly, the prospects for such a power plant.
Difficult closed circuit
While General Electric was prioritizing open circuit engines, Pratt & Whitney wasted no time in developing its own version of a closed nuclear power plant. At Pratt & Whitney, they immediately began investigating two variants of such systems. The first implied the most obvious structure and operation of the facility: the coolant circulates in the core and transfers heat to the corresponding part of the jet engine. In the second case, it was proposed to grind the nuclear fuel and place it directly into the coolant. In such a system, the fuel would circulate along the entire coolant circuit, however, nuclear fission would occur only in the core. It was supposed to achieve this with the help of the correct shape of the main volume of the reactor and pipelines. As a result of the research, it was possible to determine the most effective shapes and sizes of such a system of pipelines for circulating the coolant with fuel, which ensured the efficient operation of the reactor and helped to provide a good level of protection from radiation.
At the same time, the circulating fuel system proved to be too complex. Further development mainly followed the path of "stationary" fuel elements washed by a metal coolant. As the latter, various materials were considered, however, difficulties with the corrosion resistance of pipelines and the provision of circulation of liquid metal did not allow us to dwell on the metal coolant. As a result, the reactor had to be designed to use highly superheated water. According to calculations, the water should have reached a temperature of about 810-820 ° in the reactor. To keep it in a liquid state, it was necessary to create a pressure of about 350 kg / cm2 in the system. The system turned out to be very complex, but much simpler and more suitable than a reactor with a metal coolant. By 1960, Pratt & Whitney had completed work on their nuclear power plant for aircraft. Preparations began for testing the finished system, but in the end these tests did not take place.
Sad end
The NEPA and ANP programs have helped create dozens of new technologies as well as a number of interesting know-hows. However, their main goal - the creation of an atomic aircraft - even in 1960 could not be achieved within the next few years. In 1961, J. Kennedy came to power, who immediately became interested in advances in nuclear technology for aviation. Since those were not observed, and the costs of the programs reached completely obscene values, the fate of ANP and all atomic-powered aircraft turned out to be a big question. Over a decade and a half, more than a billion dollars were spent on research, design, and construction of various test units. At the same time, the construction of a finished aircraft with a nuclear power plant was still a matter of the distant future. Of course, additional expenditures of money and time could bring the atomic aircraft to practical use. However, the Kennedy administration decided differently. The cost of the ANP program was constantly growing, but there was no result. In addition, ballistic missiles have fully proven their high potential. In the first half of the 61st, the new president signed a document according to which all work on atomic-powered aircraft had to be stopped. It is worth noting that not long before, in the 60th year, the Pentagon made a controversial decision, according to which all work on open-type power plants was stopped, and all funding was allocated to “closed” systems.
Despite some success in the field of creating nuclear power plants for aviation, the ANP program was considered unsuccessful. For some time, at the same time as ANP, nuclear engines were developed for promising missiles. However, these projects did not give the expected result. Over time, they were also closed, and work in the direction of nuclear power plants for aircraft and missiles completely stopped. From time to time, various private companies tried to conduct such developments on their own initiative, but none of these projects received government support. The American leadership, having lost faith in the prospects for atomic-powered aircraft, began to develop nuclear power plants for the fleet and nuclear power plants.
