Nuclear Powered Spacecraft, Part 3

Nuclear Powered Spacecraft

Part 3

The primary differences between a nuclear reactor in a terrestrial power plant and one for a spacecraft lie in their size, power output, fuel type, cooling mechanism, and specific function (power vs. propulsion). Essentially, ground-based reactors are built for scale and efficiency on the ground, while spacecraft reactors are optimized for minimal mass, durability, and the unique challenges of the space environment.

Let's talk about NTP, Nuclear Thermal Propulsion again in more detail. 

Nuclear thermal propulsion, NTP for short, uses a nuclear reactor not to make electricity as Nuclear Electric Propulsion (NEP), but to make things extremely hot. That heat turns liquid hydrogen into a furious, expanding gas, which rushes out the nozzle and pushes the spacecraft forward.

Nuclear Thermal Propulsion is a high-efficiency rocket technology using a nuclear reactor to heat a propellant (like liquid hydrogen) to extreme temperatures, expelling it through a nozzle for thrust, offering faster trips (e.g., Mars in weeks, not months), reduced radiation exposure for crews, and greater mission flexibility compared to chemical rockets. NASA and other agencies are actively developing NTP, especially for crewed Mars missions, leveraging its high energy density for quicker, safer deep-space travel by significantly cutting transit times and improving abort capabilities. 

Sketch of a solid core fission nuclear thermal propulsion; source: Wiki.

But, let's go step by step:

1. The reactor wakes up

At the heart of the spacecraft is a compact nuclear fission reactor. Uranium fuel atoms split, releasing energy. Control rods slide in and out to keep everything  under control. This energy appears mainly as heat, not radiation blasting into space.

2. Hydrogen enters the furnace

Liquid hydrogen, stored in tanks, is pumped directly through channels in the reactor core. Hydrogen is chosen because it is the lightest element, which makes it perfect for propulsion, meaning its molecules can be accelerated to much higher exhaust velocities than heavier chemical byproducts like water vapor. As hydrogen passes through the reactor, it heats up to roughly 2,500–3,000 kelvin. It becomes a fast-moving gas. 

This results in a specific impulse roughly double that of the most efficient chemical engines -approximately 900 seconds compared to 450 seconds. It has also high efficiency, because lighter gases are easier to accelerate, NTP systems can travel farther and faster while using significantly less fuel mass. 

3. Expansion does the pushing

That super-hot hydrogen expands rapidly and is directed out through a rocket nozzle. Newton's law of motion takes the place here. Gas goes one way at extreme speed, spacecraft goes the other way.

4. Throttle, restart, repeat

Unlike chemical rockets, NTP engines can be throttled and restarted multiple times. You can burn, coast, adjust course, and burn again. 

It is a diagram illustrating the core concept of a nuclear thermal propulsion (NTP).

That's basically it. How simple. Isn't it?

Why bother with nuclear at all?

1. As already said, it has twice the efficiency of chemical rockets. Or you can call it specific impulse. NTP roughly doubles it, meaning you get much more push per kilogram of fuel.
2. It also means shorter travel times. Mars trips could drop from ~9 months to around 4–6 months, reducing radiation exposure and crew fatigue.
3. It can also contain heavier payloads, which means more science, more shielding, more snacks 😉.

What it is not, to remind:

1. It is not nuclear electric propulsion, that uses reactors to make electricity for ion engines.
2. It does not release radioactive exhaust in normal operation. The hydrogen stays hydrogen.
3. It is not a bomb strapped to a rocket. The reactor cannot explode like a weapon.

But of course, there is always a catch!

• Materials must survive extreme heat and radiation.
• Launch safety is politically sensitive topic.
• Ground testing is complex, expensive, and slow.

This is why NTP keeps coming back in waves. Development in the U.S. began in 1955 with Project Rover, managed by the Atomic Energy Commission (AEC) and the U.S. Air Force. After NASA was established in 1958, the project was transferred to the new agency, leading to the Nuclear Engine for Rocket Vehicle Application (NERVA) program in 1961. The NERVA program was cancelled in 1973 due to budget constraints, the conclusion of the Apollo program, and a lack of immediate need for a Mars mission. No nuclear thermal rocket ever flew in space during this era. 

The Soviet Union also had a parallel development program, successfully testing its own designs, such as the RD-0410. RD-0410 was a nuclear thermal rocket engine developed by the Chemical Automatics Design Bureau in Voronezh from 1965 through the 1980s using liquid hydrogen propellant.

Recently, NTP technology resurfaced in the 2010s for several key benefits: shorter trip times to Mars (reducing astronaut exposure to cosmic radiation in long run) and increased payload capacity. 

• DRACO Project: NASA and DARPA (Defense Advanced Research Projects Agency) collaborated on the Demonstration Rocket for Agile Cislunar Operations (DRACO) program to build and test an experimental NTP engine in orbit.
• Current Status (2025): While the DRACO flight demonstration faced delays and potential budget cuts, development efforts for future missions continue with the goal of integrating this powerful technology into future deep-space exploration programs. 

One interesting thing in the end!

With proper shielding and layout of the rocket, crew radiation exposure from an NTP engine is much lower than the radiation received from cosmic rays during a long Mars trip. Generally, the reactor is behind the crew, while space radiation comes from all directions.

So ironically, the nuclear engine is not the main villain. Space itself is.

The extreme environment of deep space, with its long distances, and harsh radiation exposure, means greater challenges than managing the nuclear reactor itself.