Nuclear thermal rocket

What is a Nuclear Thermal Rocket (NTR)?

A nuclear thermal rocket (NTR) is a type of thermal rocket that uses heat from a nuclear reaction to propel a vehicle. Imagine a spacecraft as a race car, and the NTR as its supercharged engine – it’s designed to go faster and farther than any chemical rocket could ever dream of.

Why Are Nuclear Thermal Rockets More Effective?

Nuclear-powered thermal rockets are more effective because they can use low-molecular-mass propellants like hydrogen. This is akin to using a lighter fuel in your car – it burns cleaner and provides more power, making the vehicle go further with less fuel.

How Do NTRs Work?

NTRs work by heating a working fluid, usually liquid hydrogen, in a nuclear reactor and then expanding it through a rocket nozzle. It’s like using a blowtorch to heat up a balloon – once the air is heated, it expands and pushes against the balloon, making it fly.

History of NTRs

The history of NTRs dates back to 1944 when Stanisław Ulam and Frederic de Hoffmann contemplated controlling nuclear explosions for space vehicles. After World War II, research shifted towards NTRs, with over ten reactors built and tested but none having flown yet.

US Congress Approves Funding

In 2019, the US Congress approved funding for nuclear thermal propulsion rockets, with Lockheed Martin awarded the contract to build the spacecraft in 2023 and BWX Technologies developing the nuclear reactor. A launch is expected in 2027.

Types of NTRs

Different types of NTRs have been proposed over the years, each with its own unique challenges and benefits:

Solid Core Nuclear Reactors

Solid core nuclear reactors are a simple design that uses hydrogen as a propellant, delivering specific impulses of 850-1000 seconds. Over twenty-five years, US nuclear thermal rocket designs reached thrust-to-weight ratios of approximately 7:1, suitable for orbit outside Earth’s gravity well and avoiding radioactive contamination.

Particle-Bed Reactors

Particle-bed reactors could increase specific impulse to around 1000 seconds at the cost of increased complexity. These reactors transfer energy directly from neutrons to working mass, allowing exhaust temperatures to reach far beyond reactor core limits.

Liquid Core Engines

A liquid-core engine is proposed to operate at temperatures above the melting point of solid nuclear fuel and cladding. The maximum operating temperature would be expected to deliver specific impulse performance on the order of 1300-1500 seconds, making it a powerful contender in the race for space propulsion.

Challenges and Solutions

Liquid-core reactor design poses significant challenges due to the reaction time difference between the nuclear fuel and working fluid. Possible solutions include rotating the fuel/fluid mixture or using a nuclear salt-water rocket, which is simpler but generates massive radioactive waste.

Gas-Core Engines

The gas-core engine uses rapid circulation of the fluid to create a toroidal pocket of gaseous uranium fuel in the middle of the reactor. This design can achieve higher specific impulses than liquid-core engines, reaching 3000-5000 seconds.

Current Research and Development

Current research in the US since 2000 includes studying solid-core nuclear thermal rocket designs at Marshall Space Flight Center with Glenn Research Center. NASA continued research and development on NTRs in 2017 and included funding for a flight demonstration mission by 2024 in an appropriation bill passed by Congress in 2019.

Future Prospects

In addition to the US military, NASA administrator Jim Bridenstine has expressed interest in a project to demonstrate nuclear thermal propulsion for a future Mars mission. DARPA has awarded contracts to Gryphon Technologies and General Atomics for preliminary designs of a reactor. A partnership between NASA and DARPA was announced in January 2023, with Lockheed Martin awarded a $499 million contract to assemble an experimental nuclear thermal rocket engine.

While the risks associated with NTRs are significant, the potential benefits make them an exciting area of research for space exploration. As we continue to push the boundaries of what’s possible in space travel, NTRs could be the key to unlocking new frontiers and making interplanetary missions a reality.