Nuclear powered spacecraft

Nuclear Powered Spacecraft

Part 1

Nuclear Electric Propulsion (NEP) is a spacecraft propulsion system that uses a nuclear reactor to generate electricity, which then powers electric thrusters for propulsion, as we were reading in previous article Ion thrusters. This process creates continuous, low thrust but highly efficient acceleration, ideal for long-duration deep-space missions to planets like Mars or Saturn where solar power is insufficient. NEP systems convert a reactor's heat into electricity, which is used to ionize a propellant and accelerate it to produce thrust, offering much higher efficiency than chemical rockets but less thrust than Nuclear Thermal Propulsion (NTP). 

Nuclear Thermal Propulsion (NTP) uses a reactor's heat to directly heat a propellant, which is then expelled through a nozzle for high-thrust, high-efficiency propulsion, ideal for faster transit times to Mars. In contrast, Nuclear Electric Propulsion (NEP) converts the reactor's heat into electricity to power electric thrusters (like ion engines), accelerating propellant electromagnetically for lower thrust but exceptional long-duration efficiency.

Sketch of a nuclear thermal rocket; Source Wiki

As thermal rockets, nuclear thermal rockets work almost exactly like chemical rockets: a heat source releases thermal energy into a gaseous propellant inside the body of the engine, and a nozzle at one end acts as a very simple heat engine: it allows the propellant to expand away from the vehicle, carrying momentum with it and converting thermal energy to coherent kinetic energy. The specific impulse (Isp) of the engine is set by the speed of the exhaust stream.

Because chemical rockets and nuclear rockets are made from refractory solid materials1, they are both limited to operate below 3,000 °C, by the strength characteristics of high-temperature metals. Chemical rockets use the most readily available propellant, which is waste products from the chemical reactions producing their heat energy. Most liquid-fueled chemical rockets use either hydrogen or hydrocarbon combustion. Nuclear thermal rockets using gaseous hydrogen propellant therefore have a theoretical maximum specific impulse that is 3 to 4.5 times greater than those of chemical rockets.

Through Project Rover, Los Alamos National Laboratory began developing nuclear thermal engines as soon as 1955 and tested the world's first experimental nuclear rocket engine, KIWI-A, in 1959.[32] This work at Los Alamos was then continued through the NASA's NERVA program (1961–1973). NERVA achieved many successes and improved upon the early prototypes to create powerful engines that were several times more efficient than chemical counterparts. However, the program was cancelled in 1973 due to budget constraints. To date no nuclear thermal propulsion system has ever been implemented in space.

Nuclear Electric Propulsion (NEP) (more properly nuclear electric propulsion) systems convert heat from the fission reactor to electrical power, much like nuclear power plants on Earth. This electrical power is then used to produce thrust through the acceleration of an ionized propellant2.

The terminology is slightly inconsistent because the "electric" part is powered by electricity, which can be generated from a nuclear reactor, but could also be generated by solar panels, making the "rocket" technically non-nuclear. This is distinct from a "nuclear thermal rocket," where a nuclear reactor directly heats a propellant for expulsion. 

Nuclear electric propulsion subsystems and conceptual design. SOURCE: Briefing to the committee by Lee Mason, NASA, June 8, 2020. 

The key elements to NEP are:

• A compact reactor core
• An electric generator
• A compact waste heat rejection system such as heat pipes
• An electric power conditioning and distribution system
• Electrically powered spacecraft propulsion

SNAP-10A (Systems for Nuclear Auxiliary Power), launched into orbit by USAF in 1965, was the first use of a nuclear reactor in space and of an ion thruster in orbit.

The world's first nuclear reactor power plant to operate in space, SNAP 10A, was launched into Earth orbit on April 3, 1965. Source: Wiki.

US-A (Upravlyaemy Sputnik Aktivnyy) was satellite series of 33 Soviet reconnaissance satellites, launched between 1967 and 1988 to monitor NATO and merchant vessels using radar, the satellites were powered by nuclear reactors. The US-A program was responsible for orbiting a total of 33 nuclear reactors, 31 of them BES-5 types with a capacity of providing about two kilowatts of power for the radar unit. In addition, in 1987 the Soviets launched two larger TOPAZ nuclear reactors (six kilowatts) in Kosmos satellites (Kosmos 1818 and Kosmos 1867) which were each capable of operating for six months. 

The TERM (Transport and Energy Module) project is a Russian nuclear electric spacecraft concept designed for high-efficiency, long-term power in space, particularly for tasks like orbital tug services and deep space cargo delivery. Developed through a collaboration between Roscosmos and Rosatom, it features a gas-cooled megawatt-class nuclear reactor to power ion engines. While initially proposed for crewed Mars missions, its most common application is as an orbital tug, but it is also being considered for other civil and military applications. 

Nuclear spacecraft propulsion consists of a nuclear reactor, a propellant system, and a nozzle. The reactor generates heat, which can either directly heat a propellant (Nuclear Thermal Propulsion or NTP) or convert to electricity to power an accelerator (Nuclear Electric Propulsion or NEP). Both systems use a working fluid (propellant) accelerated to create thrust according to Newton's third law.3 

Nuclear Thermal Propulsion (NTP)

• Nuclear Reactor: A fission reactor that produces a large amount of heat.
• Propellant: Typically a lightweight, low-molecular-weight gas like hydrogen, which is pumped through the reactor core.
• Nozzle: The hot propellant is expanded and accelerated through a nozzle to create thrust.
• Key components: Include the reactor, pump to move the propellant, heat exchangers, and the nozzle. 

Nuclear Electric Propulsion (NEP)

• Nuclear Reactor: Generates heat from fission.
• Power Conversion System: Converts the reactor's thermal energy into electricity.
• Propellant: An inert gas, such as xenon or krypton, is used.
• Ion Thruster: An electromagnetic field is used to accelerate the ionized propellant, creating a small but continuous thrust.
• Radiators: Remove waste heat from the system. 

Nuclear powered spacecraft are a crucial technology for future deep space missions to Mars and beyond. They offer more power, efficiency, and speed then conventional systems. It enables faster and more economical space missions, advantages of long lasting and reliable energy and high thrust propulsion. Nuclear power in space exploration brings new solutions for next era of space missions.  

Advantages of nuclear propulsion and power

• Higher energy density: Nuclear power has a higher energy density than chemical or solar power, which is essential perspective for deep space missions.
• Greater speed and efficiency: Nuclear propulsion can significantly reduce mission travel times by providing higher thrust and specific impulse compared to chemical rockets.
• Solar independence: Nuclear systems provide a reliable, continuous power source regardless of solar source, which is vital for missions far from the Sun or in sunless areas.
• Enables future missions: This technology is key to achieving ambitious goals like human missions to Mars and establishing permanent bases on the Moon. 

Of course, there are many challenges and considerations to take into account. We will hopefully come back to this topic. 

Conclusion

Despite the challenges, nuclear powered spacecraft represent a necessary and practical solution for overcoming the limitations of current propulsion and power systems. Continued development of this technology is fundamental for humanity to fulfill its potential for interplanetary and deep space missions. 

Bimodal nuclear thermal rocket concept, which uses a controlled nuclear fission reaction to heat a liquid hydrogen propellant, similar to the reactors used in nuclear power plants and submarines, but adapted for space propulsion. The energy is used to heat the propellant, which is then expelled to create thrust, a system being designed for advanced upper stages like the Copernicus for the Space Launch System. Source: Wiki. 

Notes and References:

1: Refractory solid materials are non-metallic, inorganic substances that can withstand high temperatures without deforming, melting, or losing strength. These materials are crucial for industrial applications like furnaces, kilns, and reactors, as they are used to line the interiors of high-temperature equipment, holding and processing materials like molten metal. Key properties include high heat resistance, resistance to chemical attack, and the ability to maintain structural integrity at extreme temperatures. 

2: National Academies of Sciences, Engineering, and Medicine. 2021. Space Nuclear Propulsion for Human Mars Exploration. Washington, DC: The National Academies Press. https://doi.org/10.17226/25977.

3: https://www.ansto.gov.au/our-science/nuclear-technologies/reactor-systems/nuclear-propulsion-systems