**Figure 1.**

*Bubble-through liquid fuel reactor [20].*

**Figure 2.** *Radiation liquid fuel reactor [20].*

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].

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

**Figure 3.** *Particle or droplet liquid fuel reactor [20].*

**Figure 4.** *Propellant flow path in the CNTR (not to scale) [5, 6].*

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

**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].*

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.

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:


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 laserbased 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 matterantimatter 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 matterantimatter annihilation would likely be prohibitively high. The cost is a significant barrier to developing this technology for space applications.
