Faster and Safer Version of Zubrins Nuclear Salt Water Rocket

Nice bit of work that could be made to work.  Likely though it will be used to haul masws around the solar system as any trip to a star will with turn over still absorb a lifetime.

The solar system will need power tugs able to shift mile wide space stations about.  And time will not be an issue.

All this is known tech.

Faster and Safer Version of Zubrins Nuclear Salt Water Rocket- Practical Interstellar Travel for Humans

March 15, 2025 by Brian Wang

https://www.nextbigfuture.com/2025/03/faster-and-safer-version-of-zubrins-nuclear-salt-water-rocket-practical-interstellar-travel-for-humans.html

AI gives a thumbs up to Brett Bellmore innovative modifications to Robert Zubrin’s Nuclear Salt Water Rocket (NSWR). Switching from water to polyethene and storing in sausage strings could enhance its performance and safety, particularly focusing on avoiding criticality during storage, minimizing parasitic mass, and addressing practical challenges like micrometeorite protection and fuel state.

Robert Zubrin’s Nuclear Salt Water Rocket (NSWR) design is a rocket that uses known physics and engineering. My previous analysis shows that the first working prototype might be made in space with a 10-20 year development program for 10-30 billion. There are versions that could reach 7-8% of light speed. The use of low grade uranium enrichment for a more near term version is the one that is often described. However, if weapons grade uranium (90% enrichment) is used then he exhaust would be at 1.575% of the speed of light. A 30,000 ton ice asteroid and 7500 tons of uranium could propel a 300 ton payload including a crew to 7.62% of light speed.

One of key parts of the engineering is to use water to protect the nozzle from the intense heat of the system. A combination of the coatings and space between the pipes would prevent the solution from reaching critical mass until it was pumped into a reaction chamber. It would reach critical mass and it being expelled through a nozzle to generate thrust. The nozzle would be protected by running water.

A Nuclear Salt Polyethene Rocket might achieve 8.19% of light speed and could have better safety characteristics. The nuclear salt water rocket is one of the most buildable designs for transporting people to other star systems. It builds on decades of experience with nuclear fission. There is even experience working on nuclear rockets.

Navy and commercial ships regularly have 100,000 tons of displacement. The SpaceX Starship and Super Heavy booster could have fueled masses of 6000 tons. The sizes of the large 8.19% light speed nuclear rocket are very practical. 200 deliveries of Starship payloads could assemble what is needed. Over 50 space shuttle missions were needed to build the International Space Station.

Here is the step-by-step looking at the safety of polyethene, micrometeorite handling, and the performance of a gaseous uranium mixture.

Overview of the NSWR and the Goals

The NSWR is a theoretical propulsion system that uses a solution of enriched uranium salts in water to drive a continuous nuclear fission reaction, expelling high-velocity exhaust for thrust. It promises high specific impulse and thrust, potentially enabling rapid interplanetary or interstellar travel—Zubrin’s design suggests speeds up to 7.6% of light speed with a high mass ratio. However, storing the propellant safely (avoiding unintended criticality) while keeping the system lightweight is a key challenge, as parasitic mass (e.g., shielding or containment) reduces the achievable mass ratio and thus delta-V.

Modifications aim to:

Optimize storage to prevent criticality and reduce parasitic mass.

Enhance performance by adjusting the propellant composition.

Address practical concerns like thermal management, micrometeorite risks, and assembly in space.

First pass analysis of each modification:

1. Storing Propellant in “Sausage Strings” with Spectra Casing

Storing the uranium salt solution in segmented, sausage-like strings encased in Spectra (an oriented long-chain polyethylene), hanging behind the rocket. The engines would be mounted on booms, with the first section of the string shielded from neutron flux.

Feasibility and Safety

Criticality Prevention: The segmented geometry is a clever solution. By dividing the propellant into small, subcritical volumes, you avoid forming a critical mass during storage. Nuclear criticality depends on mass, geometry, and neutron moderation—here, the sausage-like segmentation ensures the uranium remains dispersed, preventing an unintended chain reaction. This aligns with established nuclear engineering principles for safe fuel storage.

Strength-to-Weight Ratio: Spectra is an excellent choice. As a high-strength polyethylene, it excels in tension (unlike compression, where buckling is a concern), making it ideal for a trailing string design. This minimizes parasitic mass, preserving the mass ratio critical for high delta-V.

Exhaust Velocity: Spectra’s lower average atomic weight (carbon and hydrogen, ~6–7 g/mol per atom) compared to water (oxygen at 16 g/mol, hydrogen at 1 g/mol, averaging ~6 g/mol per atom in H₂O) means that feeding the casing into the fuel stream could maintain or slightly boost exhaust velocity. Lighter exhaust particles increase specific impulse, a key NSWR advantage.

The original exhaust velocity 4,725 km/s assumes nearly pure hydrogen exhaust (M ≈ 1–2 g/mol). To approach or exceed this, the polyethene must dissociate fully, and the exhaust must remain hydrogen-rich. If the Spectra casing boosts hydrogen content and reduces parasitic losses (e.g., neutron absorption by oxygen), we might achieve a 5–10% increase over the baseline.

Ultimate velocity ≈ 24,562 km/s

Fraction of light speed ≈ 8.19%

Implementation Details

Segmentation: Dividing the string into sausage-like sections prevents hydraulic pressure buildup along its length, a practical engineering benefit for stability in space.

Booms and Shielding: Positioning engines on booms and shielding the initial string section from neutron flux helps manage radiation and heat, protecting the spacecraft and fuel feed system from the intense exhaust environment.

This approach seems feasible and aligns with your goal of minimizing parasitic mass while ensuring safety.

2. Switching to Uranium-Doped Polyethylene (Skipping Water)

You suggest replacing the water-based uranium salt solution with uranium-doped polyethylene, citing thermal management benefits and potential performance gains.

Performance Benefits

Atomic Weight: Carbon (12 g/mol) has a lower atomic weight than oxygen (16 g/mol). In the NSWR, exhaust velocity scales inversely with the square root of the average molecular weight of the exhaust products. Replacing water (H₂O, ~18 g/mol) with polyethylene (repeating -CH₂-, ~14 g/mol per unit, plus uranium) slightly reduces the average molecular weight, potentially increasing specific impulse.

High-Temperature Behavior: At the extreme temperatures of the NSWR reaction (thousands of degrees Kelvin), both water and polyethylene decompose or vaporize. Water breaks into hydrogen and oxygen, while polyethylene decomposes into carbon and hydrogen fragments. The physical state at room temperature (liquid water vs. solid polyethylene) becomes irrelevant, supporting your hypothesis.

Thermal Management

Advantages in Space: Water poses challenges in space—freezing in the cold vacuum or boiling if exposed to sunlight. Polyethylene, a solid at room temperature, simplifies storage by avoiding phase changes, reducing the need for active thermal control systems (more parasitic mass).

Safety of Polyethylene

Material Properties: Spectra, a form of ultra-high-molecular-weight polyethylene, is renowned for its strength (used in bulletproof vests) and durability. It’s chemically stable under normal conditions and could encase uranium safely during storage.

Extreme Conditions: In the NSWR’s reaction chamber, polyethylene will decompose under intense heat and radiation. This is intentional if fed into the fuel stream, but its behavior under neutron bombardment and high temperatures needs testing to ensure no unwanted byproducts (e.g., carbon residues) clog the system.

This modification could enhance performance and simplify storage, though engineering the uranium doping process and validating decomposition behavior would be critical next steps.

3. Criticality in the Feed Mechanism

The proposal to allow a controlled “smidge” of criticality in the feed mechanism to convert the fuel into a high-pressure gas for injection, powering secondary systems like fuel pumps (analogous to the Raptor engine’s turbopumps).

Feasibility

Controlled Reaction: Initiating a small, controlled fission reaction in the feed system is plausible. By carefully designing the geometry and neutron moderation (e.g., with hydrogen in polyethylene), you could generate heat and pressure to gasify the fuel, driving it into the combustion chamber.

Secondary Power: Using this energy to power systems like pumps or actuators mimics staged combustion cycles in chemical rockets, improving efficiency and reducing reliance on external power sources.

Safety

Control is Key: The transition from subcritical storage to controlled criticality must be precise. A feedback system (e.g., neutron-absorbing control rods or dynamic geometry adjustments) would be essential to prevent runaway reactions.

Space Assembly: Building and testing this in space, as you suggest, avoids gravity-related complications and contains the hazardous exhaust beyond Earth’s magnetosphere.

This idea could work, enhancing both propulsion and system integration, but requires meticulous design to balance safety and functionality.

4. Micrometeorite Issues

You ask how polyethylene (Spectra) would handle micrometeorite impacts, given the long strings trailing behind the rocket.

Polyethylene’s Strengths

Resilience: Spectra’s high tensile strength and flexibility make it more resistant to micrometeorite punctures than brittle materials. Small impacts might deform rather than shatter it, reducing catastrophic failure risks.

Self-Containment: If a segment is breached, the sausage-like design limits propellant loss to that section, preserving the rest of the string.

Challenges and Solutions

Vulnerability: In space, micrometeorites travel at kilometers per second, and no material is immune. A 1-mm particle could puncture Spectra, releasing uranium and compromising the string.

Mitigation:

Additional Shielding: Whipple shields (thin, spaced layers) around the strings could dissipate micrometeorite energy.

Redundancy: Multiple strings ensure that losing one doesn’t cripple the system.

Self-Healing: Incorporating self-sealing polymers could plug small breaches, though this adds mass.

Polyethylene is a strong starting point, but micrometeorite protection requires supplementary measures to ensure reliability over long missions.

5. Performance of a Gas Uranium Mixture

Looking at replacing the liquid uranium salt solution with a gaseous uranium mixture.

Performance Considerations

Density and Feed Rate: Gases have lower densities than liquids, potentially reducing the mass flow rate into the reaction chamber. For the NSWR’s fission reaction to sustain itself, sufficient uranium must be present. A high-pressure gas (e.g., uranium hexafluoride, UF₆, vaporized) could compensate, but the system might need larger volumes or higher compression, adding complexity.

Exhaust Dynamics: A gaseous fuel could alter the reaction kinetics and exhaust composition. Lighter exhaust products (e.g., hydrogen or carbon fragments) might boost specific impulse, but efficiency depends on maintaining the nuclear chain reaction, which thrives on dense fuel concentrations.

Comparison to Liquid: The original liquid design leverages water as a neutron moderator and coolant. A gas might require alternative moderation (e.g., graphite or hydrogen gas), impacting performance.

Feasibility

Advantages: Gas simplifies injection (no pumps needed if pressurized) and avoids liquid-specific issues like freezing.

Trade-Offs: Lower density could reduce thrust unless offset by high pressure or flow rates. Detailed modeling is needed to assess if the reaction sustains itself as effectively as with a liquid.

A gaseous mixture could work, potentially offering design simplicity, but might trade thrust for specific impulse unless optimized carefully.

Additional Thoughts: Assembly and Refueling

Space-Based Construction: Assembling this in space is ideal. Gravity complicates handling long strings, and the radioactive exhaust makes ground testing impractical and costly. Beyond the magnetosphere, testing is safer and more representative of operational conditions.

Refueling: Bringing lightweight mining and enrichment gear to harvest uranium from asteroids or Oort Cloud objects is ambitious but plausible for interstellar missions. Polyethylene could be synthesized from carbon-rich sources, enhancing sustainability.

Promising Modifications to the Nuclear Salt Water Rocket

The modifications to the NSWR are creative and promising:

Sausage Strings with Spectra: A lightweight, safe storage solution that prevents criticality and boosts mass ratio.

Uranium-Doped Polyethylene: Improves performance (via lower atomic weight) and simplifies thermal management, with Spectra being reasonably safe but needing validation under reaction conditions.

Feed Mechanism Criticality: A clever efficiency boost, feasible with careful control.

Micrometeorite Handling: Polyethylene is robust, but additional shielding or redundancy is necessary.

Gas vs. Liquid: A gaseous uranium mixture could simplify design but may compromise thrust unless pressurized effectively.

These ideas enhance the NSWR’s potential for high delta-V missions, though they require detailed engineering and space-based testing to confirm feasibility. The shift to polyethylene and segmented storage particularly stands out as a practical evolution of Zubrin’s concept, balancing safety, performance, and mass efficiency. Building and refining this “fun system” in space could indeed push the boundaries of nuclear propulsion!