Nuclear Propulsion

Abstract

Nuclear propulsion utilizes nuclear reactions to produce energy, which is then used to propel a vehicle. There are three main forms of nuclear propulsion: naval nuclear propulsion, aero-nuclear propulsion, and space nuclear propulsion. Naval nuclear propulsion involves the use of nuclear reactors to power ships. This propulsion type provides several advantages over traditional propulsion methods, including increased range, speed, and maneuverability. The first nuclear-powered submarine, the USS Nautilus, was commissioned in 1955, and since then, many countries have developed nuclear-powered submarines and ships. Aero-nuclear propulsion involves the use of nuclear reactors to power aircraft or missiles. This concept has been the subject of much research and development. Space nuclear propulsion involves the use of nuclear reactors to power spacecraft. This type of propulsion provides several advantages over traditional propulsion methods, including increased speed and efficiency. The main challenge with space nuclear propulsion is developing extremely high-temperature reactors.

1. Introduction

The history of nuclear propulsion can be traced back to the mid-twentieth century when the development of the atomic bomb and the dawn of the nuclear age sparked a new era of technological innovation and exploration. As the world rapidly discovered nuclear technology’s tremendous power and potential, scientists and engineers began exploring new ways to harness this power for practical applications.

One of the earliest and most promising applications of nuclear technology was nuclear propulsion, which offered the potential to revolutionize space travel, naval operations, and air transportation. The first experimental reactors for nuclear propulsion were developed in the late 1940s and early 1950s. By the mid-1950s, the United States had launched the first nuclear-powered submarines, the USS Nautilus and the USS Seawolf [1].

The development of nuclear propulsion was driven by several critical factors, including the increasing demand for longer-range and more capable vessels, the growing interest in space exploration, and the need for a more efficient and reliable power source. Nuclear propulsion offers several key advantages over conventional propulsion systems, including higher energy density, greater operational efficiency, and reduced dependence on fuel resupply.

Despite its many benefits, nuclear propulsion also posed significant challenges and risks, including the need for safe and reliable power sources, effective shielding to protect against radiation exposure, and rigorous regulatory and licensing processes. These challenges led to the development of new technologies and processes for designing and operating nuclear propulsion systems. By the end of the twentieth century, nuclear propulsion had become a critical component of many military and space programs.

Today, nuclear propulsion plays a critical role in naval operations. Nuclear propulsion will play an essential role in space travel in the near future. Aero-nuclear propulsion continues to evolve and improve as new technologies and processes are developed. While the use of nuclear propulsion is still subject to challenges and risks, it remains one of the most promising and innovative applications of nuclear technology. It will likely continue to play a critical role in the world’s technological and scientific advancement for many years to come.

2. Naval nuclear propulsion

Naval nuclear propulsion refers to using nuclear reactors to power ships and submarines. Nuclear propulsion provides several advantages over conventional propulsion systems, including greater range, faster speeds, and reduced dependence on refueling.

Admiral Hyman G. Rickover played a crucial role in establishing the US nuclear navy. He was a naval engineer who saw the potential of nuclear power as a means of propulsion for ships. He pushed for and oversaw the development of the first nuclear-powered submarine, the USS Nautilus, which became operational in 1954. Rickover’s tireless work and leadership led to the widespread adoption of nuclear power in the US Navy, making it the world’s first nuclear navy and establishing the US as a leader in naval technology. He was known for his uncompromising standards and attention to detail, which helped ensure nuclear-powered vessels’ safe and reliable operation [1].

Naval nuclear reactors are similar in design to land-based reactors, but they must be highly compact and rugged to withstand the harsh conditions of maritime operations. The reactors use enriched uranium fuel rods to generate heat, producing steam that drives turbines and propellers. Unlike conventional diesel-electric propulsion systems, nuclear reactors do not require air to generate power, making them ideal for use in submarines that can remain underwater for extended periods.

The use of nuclear propulsion in the military is highly regulated and subject to strict safety and security standards. In addition, nuclear propulsion systems must be designed to withstand the impact of an enemy attack and to prevent the release of radioactive material in the event of an accident.

Many navies, including the United States and several other countries, have widely adopted naval nuclear propulsion. Nuclear propulsion has enabled these navies to maintain a strong and flexible presence at sea and respond to a wide range of military and humanitarian missions.

3. Aero nuclear propulsion

Aero nuclear propulsion refers to using nuclear reactors or radioisotope thermoelectric generators (RTGs) to power aircraft. While the use of nuclear power for aircraft propulsion has been researched for many years, it has never been widely adopted due to technical, regulatory, and public acceptance challenges.

In aero-nuclear propulsion, a nuclear reactor would be used to generate electricity, which could be used to electric power motors that drive the aircraft’s propellers or turbines. Alternatively, an RTG or NTP could be used to provide a direct heat source, which could produce high-speed exhaust to provide thrust.

The main advantage of aero-nuclear propulsion is that it offers a much higher energy density than conventional fuel sources, allowing for longer flight times and greater range. However, the use of nuclear power in aircraft also presents several challenges, including the need for a highly reliable and safe power source, effective shielding to protect against radiation exposure, and rigorous regulatory and licensing processes.

Despite these challenges, the concept of aero-nuclear propulsion continues to attract interest from researchers and military organizations, who see it as a potential solution to meet the growing demand for longer-range, higher-performance aircraft or missiles.

Aero nuclear propulsion can be combined with ramjet or scramjet technologies to increase thrust and specific impulse [2]. Ramjet and scramjet are two types of air-breathing propulsion systems that have the potential to revolutionize space and high-speed flight. Unlike traditional rocket propulsion systems that carry both fuel and oxidizer, ramjets and scramjets rely on the atmospheric air for oxygen, allowing them to be much lighter and more efficient [3].

3.1 Ramjet propulsion

A ramjet is a type of air-breathing propulsion system that uses the vehicle’s motion to compress incoming air, which is then mixed with fuel and burned to produce hot gases that are expelled through a nozzle to produce thrust. Ramjets are characterized by their simplicity and high performance at high speeds, making them well-suited for high-speed flight and space applications. The thrust of a ramjet is given by:

$$T=\dot{m}{v}_e,$$E1

where $T$ is the thrust, $\dot{m}$ is the mass flow rate of the propellant, and $v_e$ is the exhaust velocity.

Ramjets are limited by their performance at low speeds, as they rely on the motion of the vehicle to compress the incoming air. To overcome this limitation, a ramjet can be combined with a conventional rocket propulsion system, allowing the vehicle to take off from a standstill and then transition to ramjet propulsion once it reaches high speeds [2].

3.2 Scramjet propulsion

A scramjet is a type of air-breathing propulsion system that is similar to a ramjet but operates at hypersonic speeds, typically above Mach 5. Unlike a ramjet, a scramjet uses shockwaves generated by the incoming air to compress and mix the air with fuel, allowing it to operate at much higher speeds than a ramjet. The thrust of a scramjet is given by the same formula as a ramjet.

Scramjets are well suited for high-speed flight and space applications, as they can provide high performance at hypersonic speeds with a relatively simple design. However, the development of scramjet technology is challenging, as it requires a deep understanding of high-speed aerodynamics and the ability to control and stabilize supersonic combustion [3].

4. Space nuclear propulsion

Space nuclear propulsion refers to using nuclear fission reactors or radioisotope decay to provide power for spacecraft propulsion and other space missions. Unlike conventional chemical propulsion systems, nuclear propulsion systems provide a much higher energy density, allowing faster, more efficient travel through space. There are two main types of space nuclear propulsion: nuclear thermal propulsion (NTP) and nuclear electric propulsion (NEP).

In NTP, a nuclear reactor is used to heat a propellant, such as hydrogen, to high temperatures, creating a high-speed exhaust that provides thrust. NTP offers higher thrust levels and specific impulses (a measure of propulsion efficiency) than conventional chemical propulsion systems. Although operational lifetime requirements are typically low (a few hours), the need for very high temperature (>2500 K) fuel and very high power density (>1 MW/L) for operational systems add design complexity.

On the other hand, NEP uses the electricity generated by a nuclear reactor or RTG to power an electric thruster (e.g., ion, Hall, MPD, VASIMR), which uses charged particles to produce thrust. NEP systems produce much lower thrust than NTP systems but are very efficient and well-suited for certain missions. Early NEP systems in the 10 kWe range may be well suited for deep space science missions, and advanced NEP systems in the 10 MWe range may enable fast (< 1 year) round-trip human Mars missions. NEP is less complex than NTP and offers high specific impulses but lower thrust levels. A primary challenge for NEP systems is ensuring that the specific mass (mass of integrated power/propulsion system divided by power into the propellant) is sufficiently low, typically <50 kg/kW, to enable deep space science missions and < 6 kg/kW to enable very fast human Mars missions.

Space nuclear propulsion has been used in a limited number of missions, including the Mars rovers, which relied on RTGs for power to turn the drive wheels. However, nuclear propulsion systems in space are still primarily limited due to technical and regulatory challenges. As of 2023, a multi-agency program is in place to demonstrate a nuclear thermal propulsion system in space.

4.1 Differences between chemical and nuclear thermal rocket propulsion

Chemical rocket propulsion and nuclear thermal propulsion are two types of rocket propulsion systems that differ in how they generate the necessary energy to produce thrust.

Chemical rocket propulsion involves chemical reactions between fuels and oxidizers to generate hot gases that are expelled through a nozzle to produce thrust. Two commonly used propellants in chemical rocket propulsion are liquid hydrogen and liquid oxygen, which produce high-speed exhaust gases with a high specific impulse (a measure of propellant efficiency). The mathematical formula for the rocket equation, which describes the change in velocity of a rocket, is given by:

$$\mathit{\Delta v}=v_e\mathit{Ln}\left(\frac{m_0}{m_f}\right),$$E2

where $\mathit{\Delta v}$ is the change in velocity, $v_e$ is the exhaust velocity, $m_0$ is the initial mass of the rocket, and $m_f$ is the final mass of the rocket.

On the other hand, nuclear thermal propulsion involves using a nuclear reactor to heat a propellant, which is then expelled through a nozzle to produce thrust. The main advantage of nuclear thermal propulsion over chemical rocket propulsion is its much higher specific impulse (a measure of propellant efficiency), which allows for much higher exhaust velocities and, thus, a much greater $\mathit{\Delta v}$. The mathematical formula for the specific impulse of a nuclear thermal rocket is given by:

$$I_{\mathit{sp}}=\frac{v_e}{g},$$E3

where I_sp is the specific impulse $v_e$ is the exhaust velocity, and $g$ is the acceleration due to gravity.

Nuclear thermal propulsion systems have the potential to be much more efficient than chemical rocket propulsion systems, but they also come with several challenges and limitations. For example, nuclear thermal propulsion systems require a reliable, safe, and efficient method of cooling the nuclear reactor and a means of containing and controlling the radioactive materials involved. Additionally, significant technical and regulatory challenges are associated with developing and operating nuclear thermal propulsion systems.

Chemical rocket propulsion and nuclear thermal propulsion are two different types of rocket propulsion systems that differ in how they generate the necessary energy to produce thrust. Chemical rocket propulsion relies on chemical reactions between fuels and oxidizers, while nuclear thermal propulsion relies on the heat generated by a nuclear reactor. While nuclear thermal propulsion has the potential to be much more efficient than chemical rocket propulsion, it also comes with several technical and regulatory challenges that must be overcome.

Typical values for the specific impulse of chemical propulsion rockets for Mars missions is ∼400 s, whereas the tested nuclear thermal propulsion rockets are in the 850 s range. Solid-fuel nuclear thermal propulsion rockets may be able to achieve 900 s I_sp (with hydrogen), and liquid-fuel nuclear thermal propulsion rockets may be able to achieve 1800 s I_sp with hydrogen and 1000 s I_sp with methane [4].

5. Technology considerations for space and aero-nuclear propulsion

Nuclear reactors are an essential component of nuclear propulsion systems and play a crucial role in generating heat and power for spacecraft. Over the years, several nuclear reactor designs have been developed and tested, each with its own unique features and capabilities. Below, we will take a closer look at six of the most significant nuclear reactor designs: KIWI, N.R.X., Phoebus, PEEWEE, XE-PRIME, and NF.

KIWI (Kiwi Reactor) was a prototype nuclear reactor developed by the United States in the late 1950s. It was one of the first reactors developed specifically for use in spacecraft and was designed to be lightweight, compact, and highly efficient. KIWI used a solid core fuel design and employed a unique cooling system that used sodium as the coolant. The reactor was highly successful, and its design served as the basis for several other reactors developed later in the space program.

NRX (Nuclear Reactor Experiment) was a research reactor developed in the United States in the mid-1950s. It was used to test various reactor components and to study the behavior of materials in a nuclear environment. NRX was a small reactor that used a liquid core fuel design and was designed to be highly flexible, allowing for a wide range of experiments. The reactor was highly successful, and its design was later adapted for use in other reactors, including the KIWI and Phoebus reactors.

Phoebus (Phoebus Reactor) was a nuclear reactor developed in France in the mid-1960s. It was designed for use in space propulsion systems and was the first European reactor specifically developed for this purpose. PHOeBUS used a solid core fuel design and employed a unique cooling system that used helium as the coolant. The reactor was highly successful, and its design was later adapted for use in other reactors, including the XE-PRIME and NF reactors.

PEWEE was a small prototype reactor developed in the United States in the late 1950s. It was designed to be highly compact and lightweight and was used to test various components and materials for use in space reactors. PEEWEE used a solid core fuel design and was designed to be highly flexible, allowing for a wide range of experiments to be performed. The reactor was highly successful, and its design was later adapted for use in other reactors, including the KIWI and XE-PRIME reactors.

XE-PRIME (Experimental Prime Reactor) was a research reactor developed in the United States in the late 1960s. It was designed to test various components and materials for use in space reactors and to study materials’ behavior in a nuclear environment. XE-PRIME used a liquid core fuel design and was designed to be highly flexible, allowing for a wide range of experiments to be performed. The reactor was highly successful, and its design was later adapted for use in other reactors, including the Phoebus and NF reactors.

NF (Nuclear Furnace) was a nuclear reactor developed in France in the late 1970s. It was designed for space propulsion systems and was one of the first reactors to employ a new type of fuel known as particle bed fuel. NF used a solid core fuel design and employed a unique cooling system that used helium as the coolant. The reactor was highly successful, and its design was later adapted for use in other reactors, including the PHOEBUS and XE-PRIME reactors.

These six nuclear reactor designs represent some of the most significant advancements in the field of nuclear propulsion. We describe each in further detail. Table 1 shows the comparison of the different designs.

Design parameter/concept	Isp (s)	Thrust (kN)	Core thermal power (MW)	Core average temperature (°C)
ENABLER	825	333	1586	2427
SMALL ENGINE	780	73	367	2361
SNRE	1000	44	210	2227
710	873	444	2010	2371
CERMET	930	445	2000	2234
PBR 1	971	334	1945	2927
PBR 2	780	33	150	2477
PeBR	1000	315	1500	2727
LPNTR 1	1075	111	525	2927
LPNTR 2	1050	48	260	2927
WIRE CORE	930	914	4400	2727

Table 1.

Key design parameters for post-NERVA conceptual NTP reactors.

5.1 High-temperature materials

The core of a high-temperature nuclear reactor for nuclear thermal propulsion is where the fission reactions occur. It must be made of materials that can withstand the extremely high temperatures and radiation levels generated by these reactions. The melting point of the materials used in the reactor core is an essential factor in determining the safety and performance of the reactor. We will examine the melting points of several materials commonly used in high-temperature nuclear reactor cores. High-performance and temperature liquid-fueled systems are often proposed that use propellant flow to keep all core structural and moderator materials at reasonable temperatures (<800 K) while still allowing molten fuel to heat the propellant to a very high temperature before expansion through a nozzle. One potential concept is the Centrifugal Nuclear Thermal Rocket (CNTR) described in [5, 6].

For traditional solid-fuel NTP engines and the structural and moderator components of liquid-fueled engines, the first material we will consider is graphite, which has been used as a moderator in some high-temperature reactors. Graphite has a high melting point of around 3600°C and is an excellent thermal conductor, making it an ideal material for high-temperature reactors. However, graphite is also highly flammable and can become highly reactive in the presence of oxygen and high temperatures, making it less attractive for air-breathing high-temperature reactors.

Another commonly used material in high-temperature reactor cores is beryllium, which has a melting point of around 1278°C. Beryllium is a good thermal conductor with a high thermal expansion coefficient, making it well-suited for high-temperature reactors.

Tungsten is another material that is sometimes used in high-temperature reactors. Tungsten has a melting point of around 3410°C, making it an ideal choice for high-temperature reactors. Tungsten is also a good thermal conductor and is highly resistant to thermal shock, making it a good choice for high-temperature reactors. However, tungsten is also a very dense material, which can make it challenging to handle and can increase the weight of the reactor. Tungsten is also a strong neutron absorber, making it difficult to use in moderated systems fueled by high assay low enriched uranium (HALEU).

Hafnium is another material that is sometimes used in high-temperature reactors. Hafnium has a high melting point of around 2227°C and is highly resistant to thermal shock, making it well-suited for high-temperature reactors. However, hafnium is a highly reactive material that can be difficult to work with and has a high neutron absorption cross-section. Hafnium is also expensive, making it a less attractive choice for high-temperature reactors.

Molybdenum is another material that is sometimes used in high-temperature reactors. Molybdenum has a high melting point of around 2620°C and is a good thermal conductor, making it well-suited for use in high-temperature reactors. Molybdenum is also highly resistant to thermal shock, making it a good choice for use in high-temperature reactors. However, molybdenum is also a very dense material, which can make it difficult to handle and can increase the weight of the reactor.

The thermal properties of materials are critical in determining their suitability for use in high-temperature nuclear reactors. We will compare the thermal properties of five materials commonly used in high-temperature reactors: uranium dioxide (UO2), uranium carbide (UC), carbon, niobium carbide (NbC), and tungsten.

Uranium dioxide (UO2) is a commonly used fuel in nuclear reactors and has a melting point of around 2800°C. UO2 has a low thermal conductivity, meaning it does not conduct heat well and can lead to overheating in high-temperature reactors. Additionally, UO2 is highly reactive and can become unstable at high temperatures, which can pose a safety risk in high-temperature reactors [7].

Uranium carbide (UC) is a relatively new material that is being investigated for use in high-temperature reactors. UC has a high melting point of around 2900°C and higher thermal conductivity than UO2. However, UC is also highly reactive and can become unstable at high temperatures, which can pose a safety risk in high-temperature reactors [8].

Carbon is a common material used in high-temperature reactors as a moderator and reflector. Carbon has a high melting point of around 3600°C and is a good thermal conductor, making it well-suited for use in high-temperature reactors. However, carbon is also highly flammable and can become highly reactive in the presence of high temperatures, which can pose a safety risk in high-temperature reactors.

Niobium carbide (NbC) is a refractory material that is being investigated for use in high-temperature reactors. NbC has a high melting point of around 3300°C and is highly resistant to thermal shock, making it well-suited for use in high-temperature reactors. However, NbC is also a highly reactive material that can be difficult to work with, which can pose a challenge in high-temperature reactors [9].

Tungsten is another material that is commonly used in high-temperature reactors. Tungsten has a high melting point of around 3410°C and is a good thermal conductor, making it well-suited for use in high-temperature reactors. Tungsten is also highly resistant to thermal shock, making it a good choice for use in high-temperature reactors. However, tungsten is also a very dense material, which can make it difficult to handle and can increase the weight of the reactor.

In conclusion, the thermal properties of the materials used in high-temperature reactors are critical in determining their suitability for use in these reactors. Uranium dioxide, uranium carbide, carbon, niobium carbide, and tungsten all have unique thermal properties that make them suitable for different applications in high-temperature reactors. The choice of material for use in a high-temperature reactor will depend on the specific requirements of the reactor, including the desired thermal conductivity, resistance to thermal shock, and ease of handling.

The choice of material for use in a high-temperature reactor will depend on the specific requirements of the reactor, including the desired thermal conductivity, resistance to thermal shock, and ease of handling.

Table 2 shows the melting points of some high-temperature reactor materials of interest.

Application	Material	Melting temperature (°C)
Fuel	Uranium	1132
Fuel	Uranium dioxide	2865
Fuel	Uranium carbide	3900
Fuel	Uranium nitride	2000
Structures/refractory metal	Tungsten	3422
Structures/refractory metal	Molybdenum	2610
Structures/refractory metal	Rhenium	3180
Structures/refractory metal	Tantalum	2996
Structures/cladding refractory non-metal	Carbon	3600
Structures/cladding refractory non-metal	Niobium carbide	4500
Structures/cladding refractory non-metal	Zirconium carbide	3300
Structures/cladding refractory non-metal	Hafnium carbide	3900
Structures/cladding refractory non-metal	Tantalum carbide	4215

Table 2.

Melting points of some nuclear reactor materials of interest in nuclear thermal propulsion.

5.2 The ROVER program

The Rover Nuclear Rocket Engine Program was a research and development program aimed at developing a nuclear-powered rocket engine for space exploration and interplanetary missions. The program was run by the US Atomic Energy Commission (AEC) and the National Aeronautics and Space Administration (NASA) from 1955 to 1973. It was part of a larger effort to develop new technologies for space exploration [10].

The main objective of the Rover program was to develop a nuclear-powered rocket engine that could provide a high specific impulse (a measure of fuel efficiency) and high thrust, enabling spacecraft to reach high speeds and travel longer distances than was possible with chemical propulsion systems. The program was focused on developing a new type of engine, known as a nuclear thermal rocket engine, which used nuclear reactors to heat a propellant to provide thrust.

The Rover program consisted of several research and development phases, including laboratory experiments, component testing, and full-scale engine testing. During the laboratory phase, researchers conducted experiments to study the behavior of various materials and components in a nuclear environment, including the heat transfer and cooling systems, the fuel elements, and the reactor core [11].

During the component testing phase, individual components of the engine were tested to validate their performance and to identify any problems. This phase included testing the heat exchangers, the pumps, and the fuel elements.

The full-scale engine testing phase involved the development and testing of prototype engines to validate the performance of the engine and to demonstrate its feasibility. The tests were conducted at various facilities, including the Nevada Test Site and the Marshall Space Flight Center. The engines were operated at full power during these tests, and the performance was measured and analyzed.

The Rover program was highly successful and produced several important breakthroughs in the field of nuclear propulsion. The program demonstrated the feasibility of nuclear thermal rocket engines and showed that they could significantly improve specific impulse and thrust over chemical propulsion systems. The program also produced a wealth of data and information on the behavior of materials and components in a nuclear environment, which has been invaluable for future research and development in this field.

However, despite its many successes, the Rover program was eventually terminated in 1973 due to a change in priorities and a shift in focus toward other areas of space exploration. The program was never fully operational, and no nuclear-powered spacecraft were ever built or flown as a result of the Rover program. Table 3 shows the timetable of the nuclear thermal propulsion tests.

Reactor	Date	Maximum power (MWth)	Maximum runtime (s)	Net Isp (s)	Exit T (°C)
KIWI-A	7/1/1959	70	300
KIWI-A’	7/8/1960	88	307
KIWI-A3	10/19/1960	112.5	259
KIWI-B1A	12/7/1961	225	36
KIWI-B1B	9/1/1962	880	Multiple
KIWI-B4A	12/30/1962	450	Multiple
KIWI-B4D	5/1/3/1964	990	40
KIWI-B4E	8/28/1964	937	480
KIWI-B4E	9/10/1964	882	150
NRX-A2	9/24/1964	1096	40
NRX-A3	4/23/19656	1093	210
NRX-A3	5/20/1965	1072	792
PHOEBUS-1A	6/25/1965	1090	630
NRX-A4/EST	March 1966	1055	1740	820	2127
NRX-A5	June 1966	1120	1776
PHOEBUS-1B	2/23/1967	1450	1800	835	2171
NRX-A6	10/15/1967	1125	3720	847	2282
PHOEBUS-2A	6/25/1968	4082	750	805	2037
PEEWEE-1	Fall 1968	514	2400	845	2266
XE-PRIME	7/1/1969	1140	210	710	2127
NF-1	July 1972	44	6528	830	2277

Table 3.

Timeline of nuclear thermal propulsion tests.

The Rover Nuclear Rocket Engine Program was a highly successful research and development program that made significant contributions to the field of nuclear propulsion. The program demonstrated the feasibility of nuclear thermal rocket engines and produced valuable data and information for future research and development. Although the program was eventually terminated, its legacy continues to influence the development of new technologies for space exploration.

Table 1 shows key design parameters for the tested nuclear thermal propulsion systems. These reactors were fueled with UC₂ particles with a diameter range of 50–150 μm. The fuel elements were hexagonal in shape, and the propellent was H₂. The matrix material in the fuel elements was graphite.

5.3 Conceptual designs of nuclear thermal propulsion reactors

Many conceptual designs were created during and after the NERVA program. A summary of some of those reactor concepts is provided here [12].

ENABLER (Economical Nuclear Auxiliary Booster Launch Engine for Reentry): this was a conceptual design for a smaller NTP that could be used as an auxiliary engine for space missions. The design was based on a liquid-core concept, which was more compact than the solid-core design used by NERVA-1.

SMALL ENGINE: this was a conceptual design for a compact NTP that would be suitable for use in small spacecraft. The design was based on a liquid-core concept and was intended to be smaller and lighter than NERVA-1 [13].

SNRE (Space Nuclear Rocket Engine): This was a conceptual design for a compact NTP that would be suitable for use in small spacecraft. Like SMALL ENGINE, it was based on a liquid-core design [14].

710: This was a conceptual design for a compact NTP that would be suitable for use in small spacecraft. The design was based on a liquid-core concept and was intended to be smaller and lighter than NERVA-1.

CERMET (Ceramic-Metal) Nuclear Rocket Engine: This was a conceptual design for an NTP that would use ceramic-Metal fuel instead of traditional nuclear fuel. The design was based on a liquid-core concept, and the fuel was intended to provide a higher power density than traditional nuclear fuel.

PBR #1 (Phoebus-1 Reactor): PBR #1 was a conceptual design for a compact NTP that would be suitable for use in small spacecraft. The design was based on a liquid-core concept and was intended to be smaller and lighter than previous NTPs [15].

PBR #2 (Phoebus-2 Reactor): PBR #2 was a conceptual design for an improved version of PBR #1, with a higher power density and improved efficiency.

PeBR (Phoebus-Electron Beam Reactor): PeBR was a conceptual design for an NTP that would use an electron beam instead of a traditional nuclear reactor. The design was based on a liquid-core concept and was intended to be more efficient than traditional NTPs.

LPNTR#1 (Low-Power Nuclear Thermal Rocket): LPNTR#1 was a conceptual design for a compact NTP that would be suitable for use in small spacecraft. The design was based on a liquid-core concept and was intended to be smaller and lighter than previous NTPs [16].

LPNTR#2 (Low-Power Nuclear Thermal Rocket 2): LPNTR#2 was a conceptual design for an improved version of LPNTR#1, with a higher power density and improved efficiency [16].

MARS WIRE CORE: MARS WIRE CORE was a conceptual design for an NTP that would use a wire-wrapped fuel element instead of traditional fuel rods. The design was based on a liquid-core concept and was intended to provide a higher power density than traditional NTPs.

Table 1 shows a comparison of the various conceptual designs for NTPs.

5.4 The particle bed reactor (PBR)

Dr. James Powell was an American scientist and engineer who made significant contributions to the development of nuclear thermal propulsion (NTP) technology. He is well known for his work on the Particle Bed Reactor (PBR), a conceptual design for a compact NTP that would be suitable for use in small spacecraft.

Dr. Powell began his work on NTP technology in the mid-1960s when he was part of a team at the Lewis Research Center (now the John H. Glenn Research Center) in Cleveland, Ohio. His work aimed to develop a compact NTP that would be suitable for use in small spacecraft. The Phoebus-1 Reactor was one of several NTP concepts developed during this period, and it was notable for its small size and high power density.

The PBR design was based on a radial inflow particle bed concept, in which the reactor would heat a working fluid, typically hydrogen, to extremely high temperatures. The hot propellant would then be expelled through a nozzle, producing high-speed exhaust that would generate thrust. The radial inflow geometry allowed the bulk of the system to remain relatively cool, with only the fuel particles and an inner “hot frit” required to operate at a very high temperature. The design was intended to be more compact and lightweight than other NTP concepts, making it well-suited for use in small spacecraft.

Although the PBR was never built or tested, Dr. Powell’s work on NTP technology paved the way for further research and development in this field. Today, NTP remains a topic of ongoing research and development, with the goal of developing safe and effective propulsion systems for use in space.

In recognition of his contributions to NTP technology, Dr. Powell was awarded several patents for his work on the PBR and other NTP concepts. He remains an important figure in the history of space propulsion and continues to be remembered for his innovative work on nuclear thermal propulsion.

In 1982, Dr.Powell, while at Brookhaven National Laboratory, presented his work to Grumman representatives, and that got the attention of many people concerning the promise of the PBR and its potential capabilities. In 1987, with $200 million in funding from the strategic defense initiative (SDI) program, a three-phase program was initiated to develop a dramatically higher-performance propulsion engine based on the PBR concept. Phases I and II of the program were to further design and develop the concept and plan comprehensive testing, beginning in Phase III. Unfortunately, the program funding was not continued past 1992. Although zero-power critical experiments were performed, an environmental impact statement (EIS) was completed for a ground test facility, and in-pile fuel testing was completed, the effort did not result in the actual testing of a PBR engine [17, 18].

5.5 Comparison of space chemical and nuclear Propulsion

Space propulsion refers to the technology used to propel spacecraft and other space vehicles. There are two main space propulsion types: chemical and nuclear. Both of these technologies have their advantages and disadvantages, and the choice between them depends on the specific requirements of each mission.

Chemical propulsion is the most widely used technology for space propulsion and has been used for many years to launch and propel spacecraft into orbit. Chemical propulsion works by burning propellant to generate high-speed exhaust, which provides thrust to the spacecraft. The propellant is stored in tanks onboard the spacecraft and is burned in a controlled manner to produce the required thrust.

One of the main advantages of chemical propulsion is its simplicity and low cost. Chemical propulsion systems are relatively easy to design, build, and operate and can be manufactured using existing technologies and materials. In addition, chemical propulsion systems are widely available and can be easily adapted for use in various spacecraft and missions.

However, chemical propulsion also has several disadvantages. The main disadvantage of chemical propulsion is its low specific impulse, which limits the maximum speed that can be achieved and the total mission time. This makes chemical propulsion less effective for missions requiring high speeds or long durations, such as interplanetary missions. In addition, chemical propulsion systems are limited by the amount of propellant that can be carried on board, which restricts the range and capabilities of the spacecraft.

On the other hand, nuclear propulsion is a more advanced technology that offers several advantages over chemical propulsion. Nuclear propulsion uses nuclear reactors or radioisotope thermoelectric generators (RTGs) to generate heat, producing high-speed exhaust to provide thrust. Nuclear propulsion systems offer a much higher specific impulse than chemical propulsion systems, allowing for higher speeds and longer mission times.

One of the main advantages of nuclear propulsion is its higher energy density, which enables spacecraft to carry more payload and reach higher speeds. This makes nuclear propulsion more suitable for missions requiring high speeds or long durations, such as interplanetary missions. In addition, nuclear propulsion systems are not limited by the amount of propellant that can be carried on board, which eliminates the range restrictions of chemical propulsion systems.

However, nuclear propulsion also has several disadvantages. The main disadvantage of nuclear propulsion is its complexity and high cost. Nuclear propulsion systems are much more challenging to design, build, and operate than chemical propulsion systems, requiring specialized materials and technologies. In addition, the use of nuclear power in spacecraft is subject to strict safety and security regulations, which can add significant cost and complexity to the development and operation of nuclear propulsion systems.

Both chemical propulsion and nuclear propulsion have their advantages and disadvantages, and the choice between them depends on the specific requirements of each mission. Chemical propulsion is a simpler and less expensive technology that is well-suited for missions requiring moderate speeds and short durations. Nuclear propulsion is a more advanced technology that offers higher performance but is more complex and expensive to develop and operate. However, space exploration to Mars and beyond is much more likely with nuclear thermal propulsion, including future fusion reactors [19].

6. Current and near-term nuclear thermal Propulsion programs

Several National Aeronautics and Space Administration (NASA) programs and the United States Department of Energy (DOE) focus on developing PBRs for near-term missions and technology demonstrations. The key technology used in the PBRs is the triso fuel. Triso (tri-structural isotropic) nuclear fuel particles are small, spherical fuel elements used in high-temperature gas-cooled reactors (HTGRs) and PBRs. The fuel particles consist of a central kernel of fissile material, typically enriched uranium or plutonium, surrounded by several layers of materials that provide structural support and containment for the fissile material.

The outermost layer of the fuel particle is made of a ceramic material that provides containment for the fissile material and acts as a barrier to releasing radioactive gases. The ceramic layer is highly porous, which allows gas to flow through it, and is designed to retain its integrity even under extreme conditions, such as high temperatures and pressures.

The next layer is made of a layer of carbon, which acts as a moderator to slow down fast neutrons and increases the probability of a fission reaction. The carbon layer also provides structural support for the fuel particle and helps to prevent it from disintegrating during high-temperature operation.

The innermost layer of the fuel particle is the fuel coating, which consists of a mixture of fissile material and other materials, such as pyrolytic carbon and silicon carbide, that help to retain the fissile material and prevent it from spreading.

Triso fuel particles are used in PBRs because they provide a highly-efficient and safe means of containing and controlling the fissile material. The fuel particles are designed to withstand high temperatures and pressures and to provide a high degree of containment for the fissile material. Additionally, the fuel particles are small and can be easily packaged, making them well-suited for compact PBRs.

NASA, the US DOE, and the Defense Advanced Research Projects Agency (DARPA) are supporting research and development programs to develop, test, and demonstrate the feasibility of the triso fuel and the PBR concept. General Atomics, BWX Technologies, Inc. (BWXT), and Ultra Safe Nuclear are some companies currently developing the PBR concept, with technology demonstrations planned for the 2025–2027 timeframe.

BWXT’s TRISO fuel is a type of fuel used in high-temperature gas-cooled nuclear reactors. TRISO stands for “Tri-Isotropic” and refers to the unique microstructure of the fuel particles. The fuel particles are coated with multiple layers of carbon and ceramic materials, which helps to retain the fissile material even in the event of a loss of coolant accident. This makes TRISO fuel ideal for use in advanced reactors, such as high-temperature gas-cooled reactors, requiring fuel that can withstand high temperatures and high radiation levels. TRISO fuel is manufactured through a complex process that involves forming, coating, and sintering the fuel particles. The result is a highly durable fuel that offers superior performance and improved safety compared to traditional nuclear fuel.

6.1 The centrifugal nuclear thermal rocket (CNTR)

Liquid-fuel nuclear rocket engines have been envisioned since at least 1954, and various liquid-fuel NTP design concepts were proposed in the 1960s. Liquid fuel nuclear rocket engine concepts described to date employ one of three basic design approaches: (1) the bubble-through reactor, (2) the radiation reactor, and (3) the particle or droplet reactor. These reactor concepts are illustrated in Figures 1–3, respectively. The bubble-through reactor design features a reactor fuel that is rotated at high speed to maintain a layer of liquid fuel annulus around the inner cylindrical surface of the fuel. As the hydrogen propellant is bubbled through this liquid fuel, it is heated to the temperature of the liquid fuel. The hot hydrogen then exits the engine through the nozzle to produce a thrust. The radiation reactor design features a rotating cylinder to maintain the liquid fuel but flows hydrogen axially down the center of rotating tubes where it is heated by radiation and surface convection from the liquid. In the particle or droplet reactor design, fuel particles or liquid droplets are continuously recirculated through the propellant stream in the activity zone of the reactor [21].

Figure 1.

Bubble-through liquid fuel reactor [20].

Figure 2.

Radiation liquid fuel reactor [20].

Figure 3.

Particle or droplet liquid fuel reactor [20].

Although the radiation reactor engine concept is the simplest, it suffers from the fact that the greatest heat generation occurs at the outer boundary of the liquid uranium, which is the inner wall of the rotating cylinder, and containment materials are not yet known which maintain needed structural characteristics when operating at these temperatures. For the droplet reactor, the neutronics modeling is intractable with the current neutronics modeling tools and techniques. Hence research efforts presently focus on the bubble-through concept due to more tractable thermodynamics and neutronics.

A liquid fuel reactor concept presently under study by a NASA-sponsored university team employs a bubble-through reactor design while utilizing multiple elongated Centrifugal Fuel Elements (CFEs). A nineteen CFE engine configuration is illustrated in Figures 4 and 5. Like solid fuel NTP systems, propellant from the propellant tank (not shown) passes through the neutron reflector, a regeneratively cooled section of the nozzle, neutron moderator, and structure before entering the fueled region. This propellant flow configuration assures that all moderators and structural materials within the engine remain at a relatively low temperature (< 800 K). In Figure 4, the propellant enters through the porous rotating cylinder wall at ∼800 K, passes radially through the molten uranium fuel annulus, and exits axially through the bore into a common plenum before being accelerated through a converging/diverging nozzle. Liquid uranium near the inner cylinder wall of each CFE is maintained at ∼1500 K by the inflowing propellant. The uranium temperature near the center of the rotating cylinder could reach 5500 K but only contacts the propellant and does not contact any structural material. The system operates at high pressure (>3.5 MPa) to avoid bulk boiling of the uranium metal.

Figure 4.

Propellant flow path in the CNTR (not to scale) [5, 6].

Figure 5.

Full-core slice of the engine shown in Figure 4 at the axial midplane (top) and a close-up of a single CFE (bottom) [5, 6].

While modeling and analysis efforts as of this writing continue to support the viability of this concept, several engineering challenges must be addressed before the engineering feasibility of this concept can be established. These challenges include:

1. Adequate heat transfer between the metallic liquid uranium and the propellant must be demonstrated.

2. A porous rotating cylinder wall must be developed that allows propellant to flow into the Centrifugal Fuel Element (CFE) while not allowing molten uranium to be forced out (by the centrifugal force) through the propellant flow passages. The porous wall needs to help ensure adequate mixing between the propellant and uranium by finely distributing the inflowing propellant and by distributing the propellant flow to match the axial power profile within the rotating cylinder.

3. A coating must be developed for the inside of the rotating cylinder wall that is compatible with liquid uranium and all potential propellants at ∼1500 K.

4. The rotating cylinder itself must be designed and fabricated, with transpiration and film cooling as needed to avoid potential hot spots.

5. Reliable methods for rotating the cylinders at several thousand RPM must be developed, and methods for accommodating the failure of individual cylinders must be devised.

6. Methods for accommodating transients, including startup and shutdown, must be devised to minimize the loss of uranium fuel and avoid vibrational instabilities.

7. The reactor and cylinder exit must be designed to ensure that the uranium loss rate from the system is acceptable, with a High Assay Low Enriched Uranium (HALEU) loss goal of <0.01% of the propellant mass.

8. Methods for controlling reactivity as needed for startup/shutdown and due to burnup or entrainment of Uranium in the propellant must be designed.

9. The neutronic design of the fuel must be optimized. Experience from previous (lower temperature) liquid reactor development programs should be used to ensure stable operation during startup, operation, and shutdown.

10. Methods for incorporating the CNTR reactor into an NTP engine must be devised. The CNTR uses a moderator block approach. Methods used for incorporating traditional NTP reactors into an NTP engine with a moderator block may be directly applicable.

This list of engineering challenges is daunting and reveals why a liquid-fuel nuclear thermal rocket engine has not yet been developed or prototyped. But the challenges all derive from the high temperatures, which yield the high-performance potential – 1800 s specific impulse with a thrust-to-weight ratio comparable to solid fuel NTP engines [22, 23].

Currently, scientific missions to the outer planets of the Solar System require planetary flyby trajectories so that velocity is gained from the respective planets along the way to the destination. Such trajectories result in infrequent and narrow launch windows and transit times from Earth to the destination planet that is typically double that of a direct trajectory. Unfortunately, chemical propulsion systems lack the performance needed to enable direct trajectories to the Solar System outer planets.

Mission analyses have shown that solid fuel NTP enables direct trajectory rendezvous missions to Jupiter and Saturn, with launch windows occurring approximately annually and using commercial heavy-lift rockets. Preliminary results of similar mission analyses have shown that liquid fuel NTP enables direct trajectories as far as selected Kuiper Belt objects, including Pluto and Quaoar. In addition to opening up the Solar System to scientific exploration, liquid fuel NTP can significantly reduce travel times for human exploration of the Solar System since using trajectories other than minimum energy trajectories becomes feasible for many planetary destinations. It is this potential that warrants research into liquid fuel NTP [24].

6.2 Nuclear fusion propulsion

Nuclear fusion is a promising technology for space propulsion that has been under investigation for several decades. The idea is to harness the energy released during the fusion of atomic nuclei to propel spacecraft through space. The fusion process involves merging two or more atomic nuclei to form a heavier nucleus and release a large amount of energy. This energy is produced as a result of the strong nuclear force that holds the protons and neutrons in the nucleus together. The energy released by fusion reactions is much greater than that of chemical reactions, making it a potential source of clean, safe, and sustainable energy for space exploration.

The key challenge in developing nuclear fusion for space propulsion is to achieve sustained, controlled fusion reactions. The high temperatures and pressures required to initiate and maintain fusion reactions are difficult to achieve and maintain in a controlled manner. In addition, the fuel used in fusion reactions, typically hydrogen isotopes such as deuterium and tritium, must be heated to tens of millions of degrees Celsius to achieve the conditions required for fusion. Also, 80% of the energy released in a standard deuterium-tritium fusion reaction is in the neutron that is produced, and if that neutron is, in turn, used to produce tritium (replacing the tritium that is consumed), then the neutron’s energy is essentially converted into heat which severely limits performance. A neutronic fusion (such as p-11B) appears to have much greater performance potential. Still, it is much more difficult to achieve high “Q” (energy out/energy in) values than D-T.

Several approaches are being explored to achieve controlled nuclear fusion for space propulsion, including magnetic confinement, inertial confinement, and laser-based fusion. Magnetic confinement fusion involves confining the plasma in a magnetic field, while inertial confinement fusion involves rapidly compressing the fuel to initiate fusion. Laser-based fusion involves using laser beams to heat and compresses the fuel.

Magnetic confinement fusion is the most mature of the fusion technologies. It has been the focus of several large-scale fusion experiments, including the International Thermonuclear Experimental Reactor (ITER) being built in France. ITER aims to demonstrate the feasibility of commercial fusion power and develop the technologies required for fusion-based space propulsion.

Inertial confinement fusion is a newer approach that has the potential to achieve fusion in a much smaller and simpler device than magnetic confinement fusion. However, it is still in the early stages of development and has yet to demonstrate sustained fusion reactions.

Laser-based fusion is another promising approach that has shown great promise in recent years. The technology involves high-powered laser beams to heat and compresses the fuel to achieve fusion conditions. Laser-based fusion has the potential to achieve fusion in a smaller and simpler device than either magnetic confinement or inertial confinement fusion. It has the added advantage of responding quickly to changes in power demand, making it well-suited for use in space propulsion systems.

Despite the promise of nuclear fusion for space propulsion, many technical challenges must be overcome. The tritium fuel used in fusion reactions must be carefully managed to ensure that it does not pose a threat to the environment or human health. In addition, the high temperatures and pressures required for fusion reactions must be carefully controlled to ensure the safety of the spacecraft and its crew.

Despite these challenges, the potential benefits of nuclear fusion for space propulsion make it an area of intense research and development. The technology has the potential to revolutionize space exploration by providing a clean, safe, and sustainable source of energy for spacecraft propulsion. This could enable missions to be conducted more efficiently and with greater payloads, opening up new frontiers in space exploration and enabling humanity to expand its presence in the universe.

Nuclear fusion has the potential to be a game-changer for space propulsion. The technology offers the promise of clean, safe, and sustainable energy for spacecraft propulsion, which could enable a new era of space exploration. With continued investment and innovation, nuclear fusion may become the power source of the future.

6.3 Matter-antimatter nuclear propulsion

Antimatter propulsion using positron-electron or proton-antiproton annihilation is a theoretical form of propulsion that has the potential to revolutionize the way we explore the universe. In this form of propulsion, energy is produced by the annihilation of matter and antimatter, specifically positrons and electrons or protons and antiprotons. This energy can then be harnessed to propel a spacecraft to extremely high speeds, making it possible to explore the universe much faster than with current propulsion methods.

The concept of antimatter propulsion is based on the principle of matter-antimatter annihilation. When a particle of matter and its corresponding antiparticle collide, they annihilate each other, releasing a large amount of energy in the form of gamma rays. The idea is to somehow harness that energy in a way that produces a powerful, highly efficient propulsion system for space applications.

One of the key advantages of antimatter propulsion using positron-electron or proton-antiproton annihilation is its potential for extremely high energy output. If the reaction products can be efficiently directed, then that energy could propel a spacecraft to extremely high speeds, approaching 10% of the speed of light, making it possible to reach the nearest star in just a few decades. This is a significant improvement over current propulsion methods, which would take thousands of years to reach the same destination.

In addition to its high energy output, antimatter propulsion using positron-electron or proton-antiproton annihilation is also highly efficient. Unlike traditional propulsion methods that rely on chemical reactions to produce energy, this form of propulsion uses the energy produced by matter-antimatter annihilation to propel the spacecraft. This means that a much smaller amount of fuel is required to achieve the same level of performance as traditional propulsion methods, assuming the reaction products can be directly used as a propellant without creating significant waste heat.

However, despite its many advantages, several challenges are associated with developing antimatter propulsion using positron-electron or proton-antiproton annihilation. One of the biggest challenges is the production of large quantities of antimatter. Currently, scientists can only produce tiny amounts of antimatter in particle accelerators, making it difficult to develop a practical and scalable propulsion system.

Another challenge is the containment of the antimatter. Antimatter is highly reactive and dangerous and must be kept away from normal matter to prevent accidental annihilation. This requires the development of highly specialized and advanced electromagnetic containment systems, which must withstand the intense energy produced by annihilating matter and antimatter.

Another challenge is directing the matter/antimatter reaction products in a way that efficiently produces thrust without producing a significant amount of waste heat.

Finally, there is also the issue of cost. Antimatter production is extremely expensive, and the cost of developing an antimatter propulsion system using matter-antimatter annihilation would likely be prohibitively high. The cost is a significant barrier to developing this technology for space applications.

7. Conclusions

Nuclear thermal propulsion is a promising technology that has the potential to revolutionize space travel and make interplanetary missions more feasible and efficient. Unlike conventional chemical rocket propulsion systems, nuclear thermal propulsion systems use the heat generated by a nuclear reactor to heat a propellant, allowing for much higher specific impulses and faster interplanetary travel times.

The benefits of nuclear thermal propulsion for space travel are numerous and far-reaching. One of the most significant advantages is its much higher specific impulse, which allows for much higher exhaust velocities and, thus, a much greater change in velocity (Δv). This means that spacecraft equipped with nuclear thermal propulsion systems can reach their destinations much faster than those equipped with conventional chemical rocket propulsion systems.

Another advantage of nuclear thermal propulsion for space travel is its increased payload capacity. Since nuclear thermal propulsion systems have a much higher specific impulse, they require less propellant to achieve the same Δv, allowing for increased payload capacities and more extensive missions.

Nuclear thermal propulsion also offers better reliability and safety than conventional chemical rocket propulsion systems. Nuclear thermal propulsion systems are less susceptible to the problems associated with chemical reactions, such as explosions or leaks, and they can operate for extended periods without refueling.

In conclusion, nuclear thermal propulsion is a game changer for space travel, offering improved performance, increased payload capacities, and improved reliability and safety compared to conventional chemical rocket propulsion systems. While significant technical and regulatory challenges must be overcome, the potential benefits of nuclear thermal propulsion make it a promising technology for the future of space travel.

Acknowledgments

The authors are grateful for the technical advice and review by Dr. Michael Houts, Nuclear Research Manager at NASA Marshall Space Flight Center.