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 ⇢
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.
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
Project Rover and NERVA Programs – Historical Glenn Rockets Systems Areas
History of Nuclear Propulsion (air- and space-craft) – NESC Academy Video
For further reading on historical Rover/NERVA development at NASA see NTRS report “N92-11091” by S.K. Borowski
STMD (GCD) Nuclear Thermal Propulsion Video – An overview of a previous NTP program
Rocket Propulsion – An overview of rocket propulsion K-12
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)||Conference Paper||Spacecraft Propulsion and Power, Spacecraft Design, Testing and Performance||2015, September 1||NTRS|
|Modular Growth NTR Space Transportation System for Future NASA Human Lunar, NEA and Mars Exploration Missions||Borowski, Stanley K., McCurdy, David R., Packard, Thomas W.||AIAA Space 2012 Conference and Exposition, (Pasadena, CA)||Conference Paper||Spacecraft Propulsion and Power||2015, April 24||NTRS|
|2001: A Space Odyssey' revisited - The feasibility of 24 hour commuter flights to the moon using NTR propulsion with LUNOX afterburners||Borowski, Stanley, Dudzinski, Leonard||33rd Joint Propulsion Conference and Exhibit||Journal Article||Joint Propulsion||1997, July 6||DOI|
|Simulated LOX-augmented nuclear thermal rocket (LANTR) testing||Bulman, Melvin and Neill, Todd||36th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit||Journal Article||Joint Propulsion||2000, July 24||DOI|
|The Nuclear Thermal Propulsion Stage (NTPS): A Key Space Asset for Human Exploration and Commercial Missions to the Moon||Borowski, Stanley K., McCurdy, David R., Burke, Laura M.||Technical Memorandum||Lunar and Planetary Science and Exploration, Astronautics (General), Spacecraft Propulsion and Power||2014, October 29||DOI|
|High area ratio LOX-augmented nuclear thermal rocket (LANTR) testing||Melvin Bulman, D. Messitt, T. Neill and S. Borowski||37th Joint Propulsion Conference and Exhibit||Journal Article||Joint Propulsion||2012, August 22||DOI|
|Conventional and Bimodal Nuclear Thermal Rocket (NTR) Artificial Gravity Mars Transfer Vehicle Concepts||Borowski, Stanley K., McCurdy, David R., Packard, Thomas W.||AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Cleveland, Ohio||Technical Memorandum||Spacecraft Propulsion and Power, Aerospace Medicine||2016, December 22||DOI|
|A One-year Round Trip Crewed Mission to Mars using Bimodal Nuclear Thermal and Electric Propulsion (BNTEP)||Burke, Laura M., Borowski, Stanley K., McCurdy, David R. and Packard, Thomas||49th AIAA/ASME/SAE/ASEE Joint Propulsion Conference||Journal Article||Joint Propulsion||2013, July 14–17||NTRS|
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.
|Title||Author(s)||Type||Topic Area(s)||Date||Link/Report Number|
|Liquid rocket engine turbopump bearings - Space vehicle design criteria /chemical propulsion/||Butner, M. F., Keller, R. B., Jr.||Special Publication||Machine Elements and Processes||1971, March 1||NASA SP-8048 link to be added|
|Solid rocket motor igniters. NASA space vehicle design criteria, chemical propulsion||Special Publication||Propellants||1971, March 1||NTRS|
|Liquid rocket engine turbopump inducers||Jakobsen, J. K., Keller, R. B., Jr.||Book||Propulsion Systems||1971, may 1||NASA SP-8052 link to be added|
|Prevention of coupled structure-propulsion instability /pogo/, NASA space vehicle design criteria, structures||Special Publication||Space Vehicles||19070, October 1||NTRS|
|Liquid rocket pressure regulators, relief valves, check valves, burst disks, and explosive valves||Special Publication||Propulsion Systems||1973, March 1||NTRS|
|Liquid propellant gas generators||Special Publication||Propulsion Systems||1972, March 1||NASA SP-8081 link to be added|
|Liquid rocket engine fluid-cooled combustion chambers||Special Publication||Propulsion Systems||1972, April 1||NTRS|
|Liquid rocket metal tanks and tank components||Wagner, W. A., Keller, R. B.||Special Publication||Space Craft Propulsion and Power||1974, May 1||NTRS|
|Liquid rocket engine injectors||Gill, G. S., Nurick, W. H.||Special Publication||Spacecraft Propulsion and Power||1976, March 1||NASA SP-8089 link to be added|
|Liquid rocket actuators and operators||Special Publication||Auxuliary Systems||1973, May 1||NTRS|
|Liquid rocket valve components||Special Publication||Porpulsion Systems||1973, August 1||NTRS|
|Liquid rocket valve assemblies||Special Publication||Machine Elements and Processes||1973, November 1||NTRS|