The Nuclear Salt Water Rocket Engine: A New Way to Fly

Today, we’re venturing into the intriguing world of the nuclear saltwater rocket engine. In the realm of science fiction, we’ve often marveled at the advanced propulsion systems depicted in series like “The Expanse.”

The fusion drives featured in this show are a rare gem within the genre, as they align surprisingly well with the laws of physics.

They offer powerful thrust comparable to our conventional chemical rockets but with the remarkable ability to operate continuously for days or even weeks, much like the ultra-efficient ion propulsion systems utilized on deep space probes.

But what if we could imagine such advanced propulsion systems in real life? Well, that’s where the brilliance of some of the sharpest minds in the field comes into play.

In this article, we’ll delve into the captivating concept of the nuclear saltwater rocket, a brainchild of the renowned space advocate Robert Zubrin, who is also known for his groundbreaking Mars Direct concept, a cornerstone of plans to send humans to the Red Planet.

The Basics of Rocket Science

Let’s start with a quick refresher on how rocket engines work. Rocket propulsion operates on the principle of conservation of momentum.

Essentially, for a rocket to move in one direction, it expels mass in the opposite direction.

This expulsion of mass results in thrust, propelling the rocket forward. However, this process demands a significant amount of energy.

In traditional chemical rockets, this energy is primarily derived from the combustion of fuel.

In contrast, ion thrusters draw their power from solar panels, converting sunlight into electricity to accelerate charged particles for propulsion.

Both of these methods, while impressive, pale in comparison to the energy density offered by nuclear reactions.

Nuclear Thermal Engines: A Promising Start

Before we dive into the nuclear saltwater rocket concept, let’s explore nuclear thermal engines briefly. These engines leverage a nuclear reactor to heat a propellant by passing it over the reactor.

By maintaining a relatively high propellant temperature, these engines can operate continuously with ample propellant reserves.

While this may sound like science fiction, it’s worth noting that real-world tests of such engines have been conducted, notably at a facility amusingly named Jackass Flats.

However, the development of nuclear thermal engines faced significant challenges and roadblocks, including political and financial constraints.

It’s believed that their capabilities could have played a pivotal role in extending the Apollo program to Mars, but the government hesitated to allocate resources to such an extravagant endeavor.

Gas-Core Nuclear Reactors: A Brave New Frontier

One fascinating avenue within nuclear propulsion research is the exploration of gas-core nuclear reactors.

In this innovative approach, the reactor takes the form of a superheated plasma containing fissile materials like uranium or plutonium.

While the technical challenges are substantial, the potential benefits are tantalizingly futuristic, but that’s a topic for another discussion.

Enter the Nuclear Saltwater Rocket

nuclear salt water rocket engine
Credit: Simple Rockets

Now, let’s shift our focus to the star of the show: the nuclear saltwater rocket.

This unique propulsion system operates by sustaining a nuclear reaction within the propellant as it flows through the engine.

In many ways, it mirrors the operation of a chemical rocket, with one crucial distinction – it harnesses vastly more energy.

While the exhaust of a chemical rocket reaches temperatures in the thousands of degrees, the nuclear saltwater rocket takes it to a whole new level, potentially reaching hundreds of thousands of degrees.

The propellant for this rocket is a mixture of water and a uranium or plutonium salt, typically a two percent uranium tetrabromide mixture with regular water.

The uranium is enriched to approximately 20% uranium-235, qualifying as reactor-grade uranium. This concoction contains all the elements necessary to initiate a chain reaction.

1. The Complex Challenge of Critical Mass

Achieving a critical mass is a pivotal challenge in this design.

If a sufficient amount of the propellant mixture accumulates in the right place, a neutron-driven reaction will run away exponentially, causing the water to explode into hot plasma.

To prevent this, the fuel tank must incorporate copious amounts of neutron-absorbing materials like boron, strategically distributed throughout its interior.

In case of a fuel leak, the suppression of excess neutrons is essential to avoid unintended reactions.

2. Powering the Reaction

In simple terms, the nuclear saltwater rocket operates by pumping the saltwater mixture down boron-lined pipes into the combustion chamber.

By controlling the flow speed meticulously, engineers aim to achieve a highly energetic, self-sustaining nuclear reaction, resulting in the generation of incredibly energetic plasma, the force behind the thrust.

It’s important to note that this isn’t your typical slow chain reaction as seen in conventional nuclear reactors. Instead, the criticality is maximized to burn as much fuel as possible before it expands into space.

This approach could be likened to the prompt criticality conditions observed in the Chornobyl reactor excursion, maintained continuously and harnessed for propulsion.

3. The Role of Water

Water plays a pivotal role in this process as both a moderator and a neutron “lifeguard.”

Enriched uranium alone cannot sustain a chain reaction because the highly energetic neutrons released during fission events have a low probability of triggering subsequent fission events.

Slower neutrons, on the other hand, are more likely to induce further fissions. Water acts as a moderator, slowing down the highly energetic neutrons until they reach the right speed to sustain the chain reaction.

This intricate dance of neutrons within the water molecules is a key element of the design.

4. Managing Neutron Flow

Controlling the highly energetic reaction is essential to prevent catastrophic outcomes.

Neutrons, in their energetic state, tend to bounce around in the water and get carried along by the flow, potentially running upstream into unwanted areas of the engine.

To address this, the water flow must reach a certain speed to ensure the majority of the reaction occurs precisely where intended, rather than causing unintended reactions in the engine’s plumbing.


In conclusion, the nuclear saltwater rocket concept represents a remarkable fusion of science fiction and reality, promising a future where humanity could harness the incredible energy of nuclear reactions for efficient space travel.

While this technology is still in the realm of theoretical exploration and faces numerous challenges, it offers the tantalizing prospect of unlocking the boundless potential of propulsion systems that could rival the impressive capabilities depicted in series like “The Expanse.”

As we continue to push the boundaries of space exploration, concepts like the nuclear saltwater rocket remind us that our journey to the stars is limited only by our imagination and determination to make the impossible possible.

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top