Nuclear-Thermal Thruster

A Nuclear-Thermal Thruster (NTT) is a type of thermal rocket where the heat from a nuclear reaction, often nuclear fission, replaces the chemical energy of the propellants in a chemical rocket. A working fluid, usually liquid hydrogen, is heated to a high temperature in a nuclear reactor and then expands through a rocket nozzle to create thrust. The external nuclear heat source theoretically allows a higher effective exhaust velocity and is expected to double or triple payload capacity compared to chemical propellants that store energy internally.

Nuclear fuel types

A nuclear thermal rocket can be categorized by the type of reactor, ranging from a relatively simple solid reactor up to the much more difficult to construct but theoretically more efficient gas core reactor. As with all thermal rocket designs, the specific impulse produced is proportional to the square root of the temperature to which the working fluid (reaction mass) is heated. To extract maximum efficiency, the temperature must be as high as possible. For a given design, the temperature that can be attained is typically determined by the materials chosen for reactor structures, the nuclear fuel, and the fuel cladding. Erosion is also a concern, especially the loss of fuel and associated releases of radioactivity.

Solid core

A NERVA solid-core design Solid core nuclear reactors have been fueled by compounds of uranium that exist in solid phase under the conditions encountered and undergo nuclear fission to release energy. Flight reactors must be lightweight and capable of tolerating extremely high temperatures, as the only coolant available is the working fluid/propellant. A nuclear solid core engine is the simplest design to construct.

A solid core reactor's performance is ultimately limited by the material properties, including melting point, of the materials used in the nuclear fuel and reactor pressure vessel. Nuclear reactions can create much higher temperatures than most materials can typically withstand, meaning that much of the potential of the reactor cannot be realized. Additionally, with cooling being provided by the propellant only all of the decay heat remaining after reactor shutdown must be radiated to space, a slow process that will expose the fuel rods to extreme temperature stress. During operation, temperatures at the fuel rod surfaces range from the 22 K of admitted propellant up to 3000 K at the exhaust end. Taking place over the 1.3 m length of a fuel rod, this is certain to cause cracking of the cladding if the coefficients of expansion are not precisely matched in all the components of the reactor.

Using hydrogen as a propellant, a solid core design would typically deliver specific impulses (Isp) on the order of 850 to 1000 seconds, which is about twice that of liquid hydrogen-oxygen designs such as the Space Shuttle main engine. Other propellants have also been proposed, such as ammonia, water or LOX, but these propellants would provide reduced exhaust velocity and performance at a marginally reduced fuel cost. Yet another mark in favor of hydrogen is that at low pressures it begins to dissociate at about 1500 K, and at high pressures around 3000 K. This lowers the mass of the exhaust species, increasing Isp.

Pulsed nuclear thermal rocket

The pulsed nuclear thermal rocket (not to be confused with nuclear pulse propulsion, which is a hypothetical method of spacecraft propulsion that uses nuclear explosions for thrust) is a type of solid nuclear thermal rocket for thrust and specific impulse (Isp) amplification. In this concept, the conventional solid fission NTT can operate in a stationary as well as in a pulsed mode. Because the residence time of the propellant in the chamber is short, an important amplification in energy is attainable by pulsing the nuclear core, which can increase the thrust via increasing the propellant mass flow. However, the most interesting feature is the capability to obtain very high propellant temperatures (higher than the fuel) and then high amplification of exhaust velocity. This is because, in contrast with the conventional stationary solid NTT, propellant is heated by the intense neutron flux from the pulsation, which is directly transported from the fuel to the propellant as kinetic energy. By pulsing the core it is possible to obtain a propellant hotter than the fuel. However, and in clear contrast with classical nuclear thermal rockets (including liquid and gas nuclear rockets), the thermal energy from the decay of fission daughters is unwanted.[citation needed]

Very high instantaneous propellant temperatures are hypothetically attainable by pulsing the solid nuclear core, only limited by the rapid radiative cooling after pulsation.

Liquid core

Liquid core nuclear engines are fueled by compounds of fissionable elements in liquid phase. A liquid-core engine operates at temperatures above the melting point of solid nuclear fuel and cladding, with the maximum operating temperature of the engine instead being determined by the reactor pressure vessel and neutron reflector material.

One major issue is that the reaction time of the nuclear fuel is much longer than the heating time of the working fluid. If the nuclear fuel and working fluid are not physically separated, this means that the fuel must be trapped inside the engine while the working fluid is allowed to easily exit through the nozzle.

An alternative liquid-core design is the nuclear salt-water rocket. In this design, water is the working fluid and also serves as the neutron moderator. The nuclear fuel is not retained which drastically simplifies the design. However, the rocket discharges massive quantities of extremely radioactive waste and can only be safely operated well outside planetary atmospheres and perhaps even entirely outside planetary magnetospheres.

Gas core

The final fission classification is the gas-core engine. This is a modification to the liquid-core design which uses rapid circulation of the fluid to create a toroidal pocket of gaseous uranium fuel in the middle of the reactor, surrounded by hydrogen. In this case the fuel does not touch the reactor wall at all, so temperatures could reach several tens of thousands of degrees, which would allow specific impulses of 3000 to 5000 seconds (30 to 50 kN·s/kg). In this basic design, the "open cycle", the losses of nuclear fuel would be difficult to control, which has led to studies of the "closed cycle" or nuclear lightbulb engine, where the gaseous nuclear fuel is contained in a super-high-temperature quartz container, over which the hydrogen flows. The closed cycle engine actually has much more in common with the solid-core design, but this time is limited by the critical temperature of quartz instead of the fuel and cladding.