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 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 and is the concept used on all tested NTRs. 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. Early publications were doubtful of space applications for nuclear engines. In 1947, a complete nuclear reactor was so heavy that solid core nuclear thermal engines would be entirely unable to achieve a
thrust-to-weight ratio of 1:1, which is needed to overcome the
gravity of the
Earth at launch. Over the next twenty-five years, U.S. nuclear thermal rocket designs eventually reached thrust-to-weight ratios of approximately 7:1. This is still a much lower thrust-to-weight ratio than what is achievable with chemical rockets, which have thrust-to-weight ratios on the order of 70:1. Combined with the large tanks necessary for liquid hydrogen storage, this means that solid core nuclear thermal engines are best suited for use in orbit outside Earth's
gravity well, not to mention avoiding the
radioactive contamination that would result from atmospheric use) One way to increase the working temperature of the reactor is to change the nuclear fuel elements. This is the basis of the particle-bed reactor, which is fueled by several (typically spherical) elements that "float" inside the hydrogen working fluid. Spinning the entire engine could prevent the fuel element from being ejected out of the nozzle. This design is thought to be capable of increasing the specific impulse to about 1000 seconds (9.8 kN·s/kg) at the cost of increased complexity. Such a design could share design elements with a
pebble-bed reactor, several of which are currently generating electricity. From 1987 through 1991, the
Strategic Defense Initiative (SDI) Office funded
Project Timberwind, a non-rotating nuclear thermal rocket based on particle bed technology. The project was canceled before testing.
Pulsed nuclear thermal rocket In a conventional solid core design, the maximum exhaust temperature of the working mass is that of the reactor, and in practice, lower than that. That temperature represents an energy far below that of the individual
neutrons released by the fission reactions. Their energy is spread out through the reactor mass, causing it to thermalize. In power plant designs, the core is then cooled, typically using water. In the case of a nuclear engine, the water is replaced by hydrogen, but the concept is otherwise similar. Pulsed reactors attempt to transfer the energy directly from the neutrons to the working mass, allowing the exhaust to reach temperatures far beyond the melting point of the reactor core. As
specific impulse varies directly with temperature, capturing the energy of the relativistic neutrons allows for a dramatic increase in performance. To do this, pulsed reactors operate in a series of brief pulses rather than the continual
chain reaction of a conventional reactor. The reactor is normally off, allowing it to cool. It is then turned on, along with the cooling system or fuel flow, operating at a very high power level. At this level the core rapidly begins to heat up, so once a set temperature is reached, the reactor is quickly turned off again. During these pulses, the power being produced is far greater than the same sized reactor could produce continually. The key to this approach is that while the total amount of fuel that can be pumped through the reactor during these brief pulses is small, the resulting efficiency of these pulses is much higher. Generally, the designs would not be operated solely in the pulsed mode but could vary their
duty cycle depending on the need. For instance, during a high-thrust phase of flight, like exiting a
low Earth orbit, the engine could operate continually and provide an Isp similar to that of traditional solid-core design. But during a long-duration cruise, the engine would switch to pulsed mode to make better use of its fuel.
Liquid core Liquid core nuclear engines are fueled by compounds of
fissionable elements in
liquid phase. A liquid-core engine is proposed to operate at temperatures above the melting point of solid nuclear fuel and cladding, with the maximum operating temperature of the engine instead of being determined by the reactor pressure vessel and
neutron reflector material. The higher operating temperatures would be expected to deliver specific impulse performance on the order of 1300 to 1500 seconds (12.8-14.8 kN·s/kg). A liquid-core reactor would be extremely difficult to build with current technology. 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. One possible solution is to rotate the fuel/fluid mixture at very high speeds to force the higher-density fuel to the outside, but this would expose the reactor pressure vessel to the maximum operating temperature while adding mass, complexity, and moving parts. 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. Nuclear fuel is not retained, which drastically simplifies the design. However, the rocket would discharge massive quantities of extremely
radioactive waste and could only be safely operated well outside the Earth's
atmosphere and perhaps even
magnetosphere.
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 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. Although less efficient than the open-cycle design, the closed-cycle design is expected to deliver a specific impulse of about 1500 to 2000 seconds (15 to 20 kN·s/kg). == Solid core fission designs in practice ==