Leading research, testing and analysis to support the development of nuclear thermal propulsion for spacecraft and vehicles.
What is Nuclear Thermal Propulsion?
Typically when the term “Nuclear Thermal Propulsion” (or NTP) is used, it is in reference to in-space propulsion systems that rely upon a low molecular weight (typically hydrogen) fuel that flows through a reactor to generate heat due to the nuclear fission processes and eventual thrust.
A low molecular weight propellant is desirable because for a given chamber temperature, the lower the molecular weight, the higher the resulting ISP (or Specific Impulse, a measure of rocket engine efficiency); e.g. H2 provides ~900 seconds ISP at a fuel temperature of ~2800K, while comparably using H2O in an NTP system would only result in ~375 seconds ISP at the same temperatures.
To maintain hydrogen in the liquid state, the hydrogen fuel must be stored at temperatures well below the normal freezing point of water (20K), and at those temperatures, they are considered “Cryogenic” propellants. This is different from “Electric Propulsion” where electric or magnetic fields are used to accelerate propellants (typically gases), or “Solid Propulsion” where a solid fuel/oxidizer blend is burned. Related to Nuclear Thermal Propulsion is Chemical Propulsion.
Description of the Nuclear Fission Process
To generate the heat and thrust for a thermal propulsion system like NTP, a source of energy is needed. In an NTP system, that heat is generated by flowing hydrogen through a reactor that is enriched with the fissile nucleus Uranium-235 in order to achieve fission. During the past NERVA Rover NTP development, highly enriched uranium (HEU) was used in order to achieve fission.
Current NTP designs are using low enriched uranium (LEU) or high-assay low enriched uranium (HALEU) for non-proliferation and security considerations while still achieving fission. The fission process occurs because incident neutrons are “absorbed” in a fissile nucleus, and an excited state is produced. This excited state makes the nucleus very unstable, and if the compounded nucleus is excited enough, a fission of the nucleus occurs (or splitting). When this fission occurs, new product nuclei (such as Sr-90 or Cs-137) are created such as illustrated in Figure 3 above as well as gamma rays, an average of ~2.5 neutrons, and a release of total energy averaging ~190 MeV to ~200 MeV (depending on the fission fragments, delayed/prompt particles and their kinetic energies).
This 190 MeV (megaelectronvolts) release of energy is what is utilized in the reactor to heat the hydrogen flowing through the reactor. If sufficient production of neutrons is achieved and they are moderated (or slowed) to the incident energies needed to achieve fission, as well as a sufficient enrichment of Uranium-235 in the reactor core, a nuclear chain reaction can be sustained and the resulting thrust due to this nuclear chain reaction will be maintained.
As seen in the above Figure 4, today’s best chemical propulsion systems can achieve ISPs of ~465 seconds, while NTP can achieve almost two times the ISP of ~900 seconds. In addition to the high ISP compared to other propulsion systems, NTP has an additional benefit of having a high thrust (10-15 klbf) to weight ratio so it dramatically reduces IMLEO (Initial Mass in Low Earth Orbit), the required number of SLS (Artemis’ Space Launch System) launches and enables “affordable Mars Missions” not possible using other propulsion options.
NTP will enable a much shorter round trip opposition-class Mars mission, or short “1-way” transit times (~4-6 months) to and from Mars using fast conjunction-class missions. Due to these shorter transit times, there is a significant reduction in crew exposure to space radiation and the debilitating physical effects of prolonged exposure to a zero-g environment (e.g., muscle and bone loss and visual impairment due to intracranial pressure).
The use of an NTP system allows vehicle reusability and component commonality (reduces development and recurring costs); also allows artificial gravity operation and increased abort capability. If an NTP system is used individually or in a clustered arrangement, it allows many other mission applications (e.g., reusable lunar cargo delivery, crewed lunar landing missions, crewed asteroid missions, a high energy injection stage for shortened robotic science missions to the outer planets) – can allow a “one size fits all” approach to engine development.
Hydrogen is stored at cryogenic temperatures (20 K) to maintain the propellant in a liquid state by Cryogenic Fluid Management Systems. Includes plumbing, valves, filters, and fluid management devices needed to ensure the propellant is adequately delivered to the reactor at the right conditions. The Turbopump includes turbomachinery/pumps needed to help push and condition the … Read the rest ⇢
Multiphysics analysis of the reactor (FEA/CFD/MCNP) combines neutronics, fluid, thermal, and structural simulations all coupled together, which captures design subtleties otherwise not seen without coupling the simulations. Numerical Propulsion System Simulation (NPSS) is an object-oriented, non-linear code originally developed at GRC in 1995 and is used to model full engine systems and integrate model details … Read the rest ⇢
Related Reading and Additional Images
Kilopower – A Fission Surface Power GRC led project
Artemis Program – NASA’s new lunar exploration program, which includes sending the first woman and the next man to land on the Moon. Through the Artemis program, NASA will use new technology to study the Moon in new and better ways, and prepare for human missions to Mars.
For further reading into the history of NTP development at GRC and various considerations that were made during development – see the following:
Affordable Development and Demonstration of a Small NTR Engine and Stage: How Small is Big Enough?
Borowski, Stanley K., Sefcik, Robert J., Fittje, James E., McCurdy, David R., Qualls, Arthur L., Schnitzler, Bruce G., Werner, James E., Weitzberg, Abraham, Joyner, Claude R.,
Space 2015, (Pasadena, CA)
Spacecraft Propulsion and Power, Spacecraft Design, Testing and Performance
The following list of NASA Special Publications (SP) provides design guidance for a number of rocket propulsion components and systems. Although somewhat dated (1970’s), much of the design guidance is still used in modern applications and developments.