**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 highspeed 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

### *Nuclear Propulsion DOI: http://dx.doi.org/10.5772/intechopen.110616*

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 highspeed 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].
