Do not pour me salt in a reactor or non-pulsed nuclear rocket engines
The idea of throwing atomic bombs over the stern in the Orion project turned out to be too brutal, but the volumes of energy that the nuclear fission reaction gives, not to mention fusion, are extremely attractive to space exploration. Therefore, many non-impulse systems were created, eliminating the problems of storing hundreds of nuclear bombs on board and cyclopean shock absorbers. We’ll talk about them today.
Nuclear physics on the fingers
What is a nuclear reaction? If the explanation is very simple, the picture will be approximately as follows. From the school curriculum, we remember that matter consists of molecules, molecules of atoms, and atoms of protons, electrons and neutrons (there are levels below, but this is enough for us). Some heavy atoms have an interesting property - if a neutron enters them, they decay into lighter atoms and release several neutrons. If these released neutrons get into other heavy atoms nearby, the decay will repeat, and we will get a nuclear chain reaction. The movement of neutrons at high speed means that this movement turns into heat when neutrons are slowed down. Therefore, a nuclear reactor is a very powerful heater. They can boil water, the resulting steam sent to the turbine, and get a nuclear power plant. And you can heat hydrogen and throw it out, getting a nuclear jet engine. The first engines were born from this idea - NERVA and RD-0410.
The formal authorship (patent) for the invention of the atomic rocket engine belongs to Richard Feynman, according to his memoirs "You are, of course, joking, Mr. Feynman." The book, by the way, is strongly recommended for reading. The Los Alamos Laboratory began developing nuclear rocket engines in 1952. In 1955, the Rover project was launched. At the first stage of the project, KIWI, 8 experimental reactors were built, and from 1959 to 1964, purge of the working fluid through the reactor core was studied. For temporary reference, the Orion project existed from 1958 to 1965. The Rover had the second and third stages, which studied reactors of higher power, but NERVA was based on Kiwi because of plans for the first test launch in space in 1964. Dates gradually moved out and the first ground start of the NERVA NRX / EST engine (EST - Engine System Test) was held in 1966. The engine successfully worked for two hours, of which 28 minutes was full throttle. The second NERVA XE engine was started 28 times and worked for a total of 115 minutes. The engine was found to be suitable for space technology, and the test stead was ready for testing new assembled engines. It seemed that NERVA had a bright future - a flight to Mars in 1978, a permanent base on the Moon in 1981, orbital tugs. But the success of the project caused a panic in Congress - the lunar program was very expensive for the United States, the Martian program would be even more expensive. In 1969 and 1970, space funding was seriously reduced - Apollo -18.19 and 20 were canceled. and huge amounts of money for the Martian program, no one would allocate. As a result, work on the project was carried out without serious replenishment of money and as a result, it was closed in 1972.
Hydrogen from the tank entered the reactor, heated there, and was thrown out, creating reactive thrust. Hydrogen was chosen as a working fluid because it has light atoms, and it is easier to disperse them to high speed. The higher the speed of the jet exhaust, the more efficient the rocket engine.
A neutron reflector was used to bring neutrons back into the reactor to maintain a nuclear chain reaction.
Control rods were used to control the reactor. Each such rod consisted of two halves - a reflector and a neutron absorber. When the rod was rotated by a neutron reflector, their flux in the reactor increased and the reactor increased heat transfer. When the rod was rotated by a neutron absorber, their flux in the reactor decreased, and the reactor reduced heat transfer.
Hydrogen was also used to cool the nozzle, and warm hydrogen from the nozzle cooling system rotated the turbopump to supply new portions of hydrogen.
The engine is in operation. Hydrogen was burned specifically at the exit of the nozzle in order to avoid the threat of an explosion; there would be no burning in space.
The NERVA engine created a thrust of 34 tons, about one and a half times less than the J-2 engine, which stood on the second and third stages of the Saturn-V rocket. The specific impulse was 800-900 seconds, which was two times more than the best engines on the oxygen-hydrogen fuel pair, but less than the electric propulsion engine or the Orion engine.
A bit about security
A newly assembled and not launched nuclear reactor with new fuel assemblies that have not yet worked is clean enough. Uranium is poisonous, therefore it is necessary to work with gloves, but no more. No remote controllers, lead walls and other things are needed. All radiating dirt appears after the start of the reactor due to scattering neutrons, "spoiling" the atoms of the body, coolant, etc. Therefore, in the event of a missile accident with such an engine, radiation contamination of the atmosphere and surface would be small, and of course, it would be much less than Orion's regular launch. In the case of a successful start, the infection would be minimal or completely absent, because the engine would have to start in the upper atmosphere or already in space.
The Soviet RD-0410 engine has a similar story. The idea of the engine was born in the late 40s among the pioneers of rocket and nuclear technology. As in the Rover project, the initial idea was an atomic jet engine for the first stage of a ballistic missile, then the development passed to the space industry. RD-0410 was developed more slowly, domestic developers were carried away by the idea of a gas-phase NRE (more on this below). The project was started in 1966 and lasted until the mid-80s. The mission "Mars-94" was called as a target for the engine - a manned flight to Mars in 1994.
The RD-0410 scheme is similar to NERVA - hydrogen passes through the nozzle and the reflectors, cooling them, are supplied to the reactor core, heated there and discharged.
According to its characteristics, RD-0410 was better than NERVA - the reactor core temperature was 3000 K instead of 2000 K for NERVA, and the specific impulse exceeded 900 s. RD-0410 was lighter and more compact than NERVA and developed ten times less thrust.
Engine test. The side torch in the lower left sets fire to hydrogen to prevent explosion.
The development of solid-phase NRE
We remember that the higher the temperature in the reactor, the greater the flow rate of the working fluid and the higher the specific impulse of the engine. What prevents to increase the temperature in NERVA or RD-0410? The fact is that in both engines the fuel elements are in a solid state. If you raise the temperature, they will melt and fly out with hydrogen. Therefore, for higher temperatures, it is necessary to come up with some other way of implementing a nuclear chain reaction.
Nuclear fuel salt engine
In nuclear physics there is such a thing as a critical mass. Remember the chain nuclear reaction at the beginning of the post. If the fissile atoms are very close to each other (for example, they were squeezed by pressure from a special explosion), then an atomic explosion will result - a lot of heat in a very short time. If the atoms are not squeezed out so densely, but the flux of new neutrons from fission increases, a thermal explosion will result. A conventional reactor under these conditions will fail. Now imagine that we take an aqueous solution of fissile material (for example, uranium salts) and feed them continuously into the combustion chamber, providing there more mass than critical. It will produce a continuously burning nuclear “candle”, the heat from which accelerates the reacted nuclear fuel and water.
The idea was proposed in 1991 by Robert Zubrin and, according to various estimates, promises a specific impulse from 1300 to 6700 s with traction, measured in tons. Unfortunately, such a scheme has disadvantages:
- The complexity of fuel storage - it is necessary to avoid a chain reaction in the tank, placing the fuel, for example, in thin tubes from a neutron absorber, so the tanks will be complex, heavy and expensive.
- A large consumption of nuclear fuel - the fact is that the efficiency of the reaction (the number of decayed / the number of spent atoms) will be very low. Even in the atomic bomb, fissile material does not “burn out” completely, most of the valuable nuclear fuel will be wasted there and then.
- Ground tests are almost impossible - the exhaust of such an engine will be very dirty, even dirtier than the Orion.
- There are some questions about the control of a nuclear reaction - not the fact that a simple verbal description of the scheme will be easy to implement.
The next idea is what if we create a vortex of the working fluid in the center of which there will be a nuclear reaction? In this case, the high temperature of the core will not reach the walls, being absorbed by the working fluid, and it can be raised to tens of thousands of degrees. Thus, the idea of an open-cycle gas-phase NRE was born: a
gas-phase NRE promises a specific impulse of up to 3000-5000 seconds. In the USSR, the gas-phase NRE project (RD-600) was launched, but it did not even reach the mock-up stage.
"Open cycle" means that nuclear fuel will be thrown out, which, of course, reduces efficiency. Therefore, the following idea was invented, which dialectically returned to solid-phase NRE - let's surround the region of the nuclear reaction with a sufficiently heat-resistant substance that will transmit radiated heat. Quartz was proposed as such a substance, because at tens of thousands of degrees the heat is transmitted by radiation and the container material must be transparent. The result is a closed-cycle gas-phase NRE, or “nuclear light bulb”:
In this case, the limitation on the core temperature will be the thermal strength of the “bulb” shell. The melting point of quartz is 1700 degrees Celsius, with active cooling, the temperature can be increased, but, in any case, the specific impulse will be lower than the open circuit (1300-1500 s), but nuclear fuel will be spent more economically and the exhaust will be cleaner.
In addition to the development of solid-phase NRE, there are also original projects.
Fission fragment engine
The idea of this engine is the lack of a working fluid - it is emitted spent nuclear fuel. In the first case, subcritical discs are made from fissile materials, which do not start a chain reaction on their own. But if the disk is placed in a reactor zone with neutron reflectors, a chain reaction will start. And the rotation of the disk and the absence of a working fluid will cause the decayed high-energy atoms to fly away into the nozzle, generating thrust, and not the decayed atoms will remain on the disk and get a chance at the next disk rotation: An
even more interesting idea is to create a dusty plasma (remember “plasma crystal ” on the ISS) from fissile materials, in which the decay products of nanoparticles of nuclear fuel are ionized by an electric field and are thrown out, creating a traction:
They promise a fantastic specific impulse of 1,000,000 seconds. Enthusiasm is cooled by the fact that the development is at the level of theoretical research.
In an even more distant future, the creation of nuclear fusion engines. Unlike nuclear decay reactions, where nuclear reactors were created almost simultaneously with a bomb, thermonuclear reactors still have not moved from tomorrow to today, and fusion reactions can only be used in the Orion style - throwing thermonuclear bombs.
Nuclear photon rocket
Theoretically, it is possible to warm up the core to such an extent that traction can be created by reflecting photons. Despite the lack of technical limitations, such engines at the current level of technology are disadvantageous - the traction will be too small.
It will be fully operational rocket heating the working fluid from the RTG. But the RTG emits relatively little heat, so this engine will be very inefficient, although very simple.
At the current level of technology, you can assemble a solid-state NREV in the style of NERVA or RD-0410 - the technologies are mastered. But such an engine will lose a bunch of "nuclear reactor + ERE" in specific impulse, winning in traction. And more advanced options are still only on paper. Therefore, I personally think the “reactor + ERD” combination is more promising.
Sources of information
The main source of information is the English Wikipedia and the resources indicated in it as links. Paradoxically, there are curious articles on NRE on the Tradition - solid - phase NRE and gas-phase NRE . An article about fission fragments and dusty plasma engines .