Although the material composition of the rocket may vary, the basic function of a rocket engine, either nuclear or other, is to exert a force, F, for a time, t, on a body of mass, m, to change the velocity, v, of the body by an amount, v. The calculation of these parameters is what demonstrates the advantages of the nuclear rocket over the chemical one and determines the actual configuration of the rocket engine. The goal of the engine, which is to achieve v, is attained by expending a body of mass, m, of fuel from the vehicle.
The comparison of these quantities for both nuclear and chemical rockets shows the advantages of the former over the later. In comparison, a chemical rocket can achieve a specific impulse of approximately 450 s, while a solid core nuclear rocket can to date reach a specific impulse of approximately 900 s. The difference in specific impulse for these two types of rockets clearly shows that for long duration missions or manned missions to a planet, such as Mars, the nuclear engine is more advantageous to use than the chemical one.
The traditional and most practical approach to the design of a nuclear thermal propulsion (NTP) rocket is the use of a solid-core, heat-exchanger nuclear reactor. In this design the propellant, liquid hydrogen (H2(L)) is pumped through all extra-core components for cooling. After all the components have been cooled, the propellant is pumped through the reactor core to be heated to a temperature determined by the material limits of the core, and expanded through the nozzle to produce thrust. Typically the core material limits range from 2,500 to 3,000 K. The material temperature range is the limiting factor to the range of temperatures to which the propellant can be heated, and a limiting factor to the thrust of the rocket.
This traditional approach is in its specifics determined by the configuration of the reactor, which depends on the requirement for nuclear "criticality" of the core. This requirement leads to modification in the reactor configuration which tends towards minimizing the fixed mass of the reactor. The aspects of the fixed mass of the reactor that tend to be minimized are the neutron absorbing material of the core, neutron moderating components, highly enriched uranium fuel, complex fuel-loading regimes, a neutron-reflector to minimize neutron losses from the core, and the overall core dimensions.
At the same time that the configuration of the reactor is modified to meet criticality requirements, other technical aspects of the engine are also examined to maximize power density, minimize system mass, and integrate super light weight components. These technical aspects are solved by the consideration of neutronics issues, reactor control and dynamic requirements of the rocket.
Besides the technical aspects of the engine, which deal specifically to its physical configuration, one of the most important aspects of the engine is the fuel it uses. The main considerations to be taken into account in the selection of the reactor fuel material are that the material must have high temperature capability and adequate strength in the temperature range of 2,500 to 3,000 K. These two excluding characteristics are not the only ones desired in a reactor fuel. The fuel material should include a low neutron-absorption cross-section, high thermal conductivity, compatibility with a high-temperature uranium compounds, reasonable fabricability, compatibility with hot H2, and low mass and molecular weight. A reactor fuel material that exhibits all of the mentioned characteristics is considered to be ideal for an NTP engine. The only two classes of materials, which express most the desired characteristics are refractory metals such as tungsten, and carbon based materials such as graphite.
The main problem of NTP rockets, which is particular to this type of rocket alone, is the heating of engine components by nuclear radiation emanating from the core. Since the core power and power density are high and the system size/mass is minimized, the resulting neutron and x-ray leakages are high. These leakages are the cause of the heating of the components and can only be counter acted by substantial, efficient, and careful cooling of al components by the use of internal shields and of the propellant itself.
The NTP reactor must initiate and sustain a fission reaction, it is a high power heat exchanger, an intense source of nuclear radiation and mechanical structure with many tuype of loads under diverse and extreme temperature conditions.
Three fuel materials were developed at Loa Alamos before the program was terminated back in 1973 :
Bead Loaded Graphite: Consists of a graphite matrix containing fuel beads with a UC2 core coated with pyrocarbon to protect it from the humid atmosphere. Reactor tests showed this fuel to be capable of a Tc of 2,500 K for at least 1h.
Composite fuel : It consists of 30 to 35 % volume % UC .ZrC dispersed n graphite. This fuel is capable of a temperature of 2,700 K for at least 1h.
Carbide Fuel : A pure carbide mixture such as UC.ZrC would be required to maximise the time temperature performance of carbon based fules, although this material has poor thermal stress resistance, and it is difficult to fabricate. Although not enough testing was performed on this material to establish confidence in this fuel. Estimated performance is around 3,000 K (Isp 950 s)