Ultimate Nuclear Salt Water Rocket 7.6% of Light Speed

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Robert Zubrin’s Nuclear Salt Water Rocket (NSWR) design is a rocket that uses known physics and engineering . 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.

Scott Manley calls it a constant Chernobyl rocket. It is a nuclear reactor constantly at meltdown temperatures. The rocket was designed so that the liquid flow rate or velocity was what mattered most in the process, not what it was made out of. Robert Zubrin argued that if the proper velocity was chosen for the liquid traveling through the reaction chamber, the site of maximum fission release could then be located at the end of the chamber, thus allowing the system to remain intact and safe to operate.

The NSWR is a vehicle with ISP at the best ion drive levels or beyond and operating with hundreds of gigawatts of power.

The weaker NSWR engine analysis assumed a very low yield of 0.1% and a modest enrichment of 20%. The potential ultimate performance of a NSWR if more optimistic values are assumed.

An NSWR utilizing a 2% uranium bromide solution with 90% enriched U233, and obtaining a 90% fission yield. Assuming a nozzle efficiency of 0.9, the exhaust velocity of this system will be 4725 km/s, or about 1.575% of the speed of light (a specific impulse of 482,140 seconds). If the 300 tonne Titan mission spacecraft is endowed with 2700
tonnes of propellant (for a mass ratio of 10) a maximum velocity of 3.63% of speed of light could be obtained, allowing the ship to reach Alpha Centauri in about 120 years.

Deceleration could be accomplished without the use of substantial amounts of rocket propellant by using a magnetic sail (or “magsail”) to create drag against the interstellar
medium.

If a small 30,000 tons ice asteroid was used, then the final velocity could be 7.62% of the speed of light one could envisage a group of interstellar emigrants selecting a small ice asteroid with a mass of 30,000 tonnes and using it as propellant (together with 7,500 tonnes of uranium obtained elsewhere) for a 300 tonne spacecraft. In this case the ship could obtain a final velocity of about 7.62% light speed, and reach Alpha Centauri in about 60 years.

A group of astronauts who initiated the trip in their twenties would be alive a the end of the journey. The first generation children would still be in their prime.

I first wrote about it almost 10 years ago in 2016. Zubrin published the design in 1991.

It is a rocket that would be deployed in orbit or somewhere else off planet. NSWR is constructed of a bundle of boron-carbide coated pipes, each containing an aqueous solution of uranium or plutonium salt These pipes empty into a single long cylindrical plenum pipe of larger diameter, which terminates in a rocket nozzle. When the rocket is to be fired, the aqueous solution held in the pipe bundles is emptied into the plenum. When the plenum has filled to a certain point, the fluid assembly within it exceeds critical mass and goes prompt supercritical, with the neutron flux concentrated on the downstream end due to neutron convection. Enormous amounts of energy are generated within this region, flashing the solution to steam which then streams down the plenum pipe towards the nozzle, convecting the exponentially growing fission chain reaction with it. As the solution continues to pour into the plenum from the borated storage pipes, a steady-state condition of a moving detonating fluid can be set up within the plenum.

Assuming a solution of 2 atoms of 20% enriched uranium per 100 molecules of water and a fission yield of 0.1% is obtained, the specific impulse produced will be about 7,000 seconds. This is comparable to that available from electric propulsion (EP). However, unlike an EP system, a NSWR is not power limited (since waste heat is eliminated with the propellant) and systems with jet power ratings of thousands of megawatts are obtainable. The NSWR can thus deliver its very high specific impulse at thrust levels of the same magnitude as chemical engines, or about 4 orders of magnitude greater than that of a multimegawatt EP system. The NSWR system is simple and lightweight. Shielding considerations are minimized by the fact that almost no radioactive inventory travels with the vehicle. On the other hand, the exhaust stream from the NSWR is extremely
radioactive, which limits the use of the device to orbit transfer propulsion, despite its high thrust to weight.

Could the Reactions Be Improved?
The NSWR projects achieving a 90% fission yield. 90% of the uranium fissions in the chamber, an ambitious figure given the rapid expulsion of material.

Possible improvements include:
Increasing Fission Yield:
Higher Concentration: The 2% uranium bromide solution could be increased, raising energy density and potentially exhaust velocity. However, this risks criticality in storage or injection, necessitating neutron absorbers or precise geometry control.

Neutron Reflectors: Surrounding the reaction chamber with materials like beryllium or graphite could reflect neutrons back, boosting the fission rate. This could push the yield closer to 100%, though 90% is already near the practical limit given the continuous-flow design.

Longer Residence Time: Retaining the solution in the chamber longer might increase fission, but this could reduce thrust or complicate the design.

Exhaust Velocity Boost
Exhaust velocity depends on temperature and molecular mass.

The fission energy (~200 MeV per event) already drives the water to plasma temperatures (millions of Kelvin), dissociating it into H and O ions. Lighter propellants (e.g., hydrogen) could raise velocity, but uranium salts don’t dissolve in hydrogen, and water’s abundance (e.g., from ice asteroids) is a key advantage.

Enhancing plasma conditions or using magnetic fields to accelerate ions might increase velocity but this requires advanced technology.

The 90% fission yield and 4,725 km/s exhaust velocity are already highly optimized. Marginal gains might be possible with reflectors or concentration tweaks.

The main challenge for NSWR is the engineering to build and safely operate it.

Project Roadmap for NSWR Development (about 15-24 Years)

Phase 1: Bench-Scale Experiments (Non-Nuclear)
Objective: Validate the basic engineering principles of the NSWR using safe, non-nuclear materials.

Key Activities:
Storage and Injection Testing: Build small-scale models of the storage tanks and injection systems using surrogate solutions (e.g., non-radioactive heavy metal salts) that mimic the chemical properties of uranium salts. Test the ability to store and move these solutions without issues.

Criticality Control: Use neutron-absorbing materials (e.g., boron or cadmium surrogates) and specific tank designs to prevent simulated “criticality.” Compare results to computer models to ensure accuracy.

Nozzle Cooling: Test a small water-cooling system for the rocket nozzle using high-temperature heat sources (like plasmas) to mimic the heat of a nuclear reaction. Check if it can handle the heat without breaking.

Duration: 1-2 years

Outcome: Proof that the storage, injection, and cooling systems work as expected in a safe, non-nuclear setup.

Phase 2: Subcritical Nuclear Tests
Objective: Test the system with real nuclear materials, but in a way that avoids a full nuclear reaction (subcritical state).

Key Activities:
Subcritical Testing: Use low-concentration uranium salt solutions (e.g., natural uranium) in designs that can’t go critical. Measure how neutrons behave to confirm our models are correct.

Safety Checks: Test safety features like neutron absorbers and emergency stop systems to ensure no accidental reactions happen.

Injection Precision: Fine-tune the injection system to control the flow of nuclear solution accurately with real materials.

Duration: 2-3 years

Outcome: Confidence that the system behaves predictably with nuclear materials and stays safe under subcritical conditions.

Phase 3: Criticality Tests in a Controlled Environment
Objective: Achieve a controlled nuclear reaction (criticality) with a tiny amount of material in a secure setting.

Key Activities:
Small Reaction Chamber: Build a miniature reaction chamber using a small amount of highly enriched uranium (e.g., a few grams). Test it in a remote facility with strong safety measures.

Controlled Reaction: Slowly adjust the solution or design to start a fission reaction, then stop it safely. Monitor the energy and stability closely.

Data Refinement: Use the results to improve our models of how the reaction works.

Duration: 2-3 years

Outcome: Proof that we can start and stop a nuclear reaction safely on a small scale.

Phase 4: Integrated System Tests

Objective: Combine the reaction chamber, nozzle, and cooling into a working ground-based prototype.

Key Activities:
Prototype Build: Connect the reaction chamber to a nozzle and cooling system, adding controls for injection and safety.

Low-Power Tests: Run the system at low power (e.g., lower uranium concentration) to check thrust and cooling. Measure how much push it generates and if it stays intact.

Power Increase: Gradually ramp up the power by tweaking the solution or flow, watching for any problems with heat or structure.

Duration: 3-4 years

Outcome: A fully working ground prototype that produces thrust and handles heat as expected.

Phase 5: Prototype Flight Tests
Objective: Fly a small NSWR prototype in space to test it in real conditions.

Key Activities:
Flight Design: Build a lightweight version of the NSWR that fits on a small spacecraft with basic systems (e.g., power and communication).

Short Flights: Launch it on suborbital or short orbital missions. Test short bursts of the engine in space, focusing on starting and stopping it safely.

Performance Check: Measure thrust, efficiency (specific impulse), and how it works in a vacuum and microgravity.

Duration: 3-5 years

Outcome: Proof that the NSWR works in space, with data to guide improvements.

Phase 6: Scaling Up to Advanced Versions
Objective: Build bigger, more powerful NSWRs for real space missions.

Key Activities:
Design Upgrades: Use flight data to design a larger NSWR with higher thrust and longer runtime. Improve materials or layouts for better performance.

Full-Scale Testing: Test the big version on the ground, running it for long periods to simulate deep space trips. Check its durability and safety.

Mission Prep: Attach it to a spacecraft with mission gear (e.g., crew areas or science tools) and test the whole setup.

Duration: 5-7 years

Outcome: A ready-to-use NSWR for advanced missions, like trips to Mars or beyond.

Safety and Oversight
Safety First: Every phase needs backup systems, emergency stops, and remote testing where possible. Nuclear phases (3-6) especially need strong containment and monitoring.

Regulations: Work with agencies like the Nuclear Regulatory Commission (NRC) or International Atomic Energy Agency (IAEA) to follow rules on nuclear safety and prevent misuse.

Environment: Plan flight tests to avoid harming Earth’s environment, like keeping nuclear materials contained during launch and reentry.

This roadmap takes the NSWR from a risky idea to a proven technology in a careful, step-by-step way. Starting with safe experiments, moving to controlled nuclear tests, and building up to flight-ready prototypes ensures we solve problems early and keep risks low. If successful, the NSWR could revolutionize space travel with its high thrust and efficiency, opening the door to faster missions across the solar system and beyond. The whole process might take 15-24 years, depending on funding, collaboration, and test results.

Speeding Up Development by 5 Years or More

1. Parallelization of Tasks
What It Means: Conduct overlapping activities where possible. For example:
In Phase 1 (Bench-Scale Experiments), storage and injection testing can run concurrently with nozzle cooling tests.

During Phase 4 (Integrated System Tests), design work for Phase 5 (Prototype Flight Tests) can begin early.

Impact: Overlapping tasks within and between phases could save 1-2 years per transition, potentially reducing the total timeline by up to 5 years across the six phases.

2. Increased Resource Allocation
What It Means: Boost funding and personnel, such as hiring more engineers, scientists, and technicians, and investing in advanced testing equipment.

Impact: More resources can shorten design, testing, and iteration cycles, cutting each phase’s duration by 20-30%.

3. Advanced Simulation and Modeling
What It Means: Use high-fidelity computer simulations to predict system behavior, reducing reliance on physical tests, especially in early phases.

Impact: Faster design refinement and fewer experimental iterations can shave months to years off phases like bench-scale and subcritical testing.

4. Regulatory Streamlining
What It Means: Engage regulatory bodies (e.g., the Nuclear Regulatory Commission or International Atomic Energy Agency) early to align with safety and compliance requirements.

Impact: Preemptive collaboration can minimize approval delays, particularly in nuclear phases, saving significant time.

5. International Collaboration
What It Means: Partner with other countries or space agencies to share expertise, facilities, and funding.

Impact: Additional resources and knowledge can accelerate development across all phases.

6. Modular Design
What It Means: Develop the NSWR in separate, testable components (e.g., storage tanks, injection systems) that can be integrated later.

Impact: Parallel component testing speeds up the process, reducing delays in later integration phases.

Cost Breakdown by Phase
Phase 1: Bench-Scale Experiments (Non-Nuclear)
Description: Small-scale models using non-nuclear materials.

Estimated Cost: $50-100 million.

Phase 2: Subcritical Nuclear Tests
Description: Tests with nuclear materials in a subcritical state, requiring specialized facilities and safety measures.

Estimated Cost: $200-500 million.

Phase 3: Criticality Tests in a Controlled Environment
Description: Controlled fission reactions with stringent safety protocols.

Estimated Cost: $500 million – $1 billion.

Phase 4: Integrated System Tests
Description: Ground-based prototype construction and testing.

Estimated Cost: $1-2 billion.

Phase 5: Prototype Flight Tests
Description: Spacecraft design, launch, and orbital testing.

Estimated Cost: $2-5 billion.

Phase 6: Scaling Up to Advanced Versions
Description: Full-scale NSWR development for deep space missions.

Estimated Cost: $5-10 billion.

Total Cost Estimate
Original Cost Range: $8.75 – $18.6 billion (sum of phase estimates without acceleration).

Accelerated Cost Range: Speeding up by 30% could increase costs by 50% due to parallel work and resource scaling, resulting in $13.125 – $27.9 billion. A more conservative range, accounting for efficiencies, is $10-20 billion.

$10-30 Billion Project – Cheaper Than the SLS Project

The NSWR development roadmap can be accelerated from 16-24 years to 10-15 years by leveraging parallel task execution, increased funding, advanced simulations, regulatory collaboration, international partnerships, modular design, and robust risk management. This acceleration raises the projected development cost from $8.75-18.6 billion to approximately $10-20 billion, reflecting the intensified resource demands. These estimates are rough and subject to uncertainties like regulatory hurdles or technical challenges, but they provide a practical framework for planning this ambitious project.

12 thoughts on “Ultimate Nuclear Salt Water Rocket 7.6% of Light Speed”

  2. There’s another very similar concept called the Fission Fragment rocket, which has been purported to have a maximum speed of 5% C. IMO you should do an article on that. No expert on it but it might avoid some of the issues of this.

  3. Michael Scarangella comment-

    There’s a concept called buckling, and rare gaseous reactor cores would have trouble doing it, even on ‘tens of meters scale’. That’s the math that isn’t math thing with 2 fuel atoms per 100 moderator atoms at rarified conditions while feeding enough through the wall to protect the structure with boundary layer, while fuel residence time long enough to achieve this astronomically high 90% burn up.

    Normally the fuel throughput on a reactor is zero as in it’s not being ejected.

  4. A 300 ton ship is NOT for a human “interstellar” trip. That is a very small mass for even some interplanetary trips. It could work for a probe. People forget the human physiological needs and logistical needs, which will add mass. (lots of mass, as in compatable to the fuel load). First (and the two are intricately related) is Artificial Gravity and Radiation Protection. Neither is optional. Forget this Fission Fragment Salt Water Rocket accelerating at 1 g to provide gravity. At 3.5 or 7 % of light speed, your fuel limited and done at about 2.4 weeks at 1g to 3.5 % C. Double that for 7 %. A rotating structure NOT LESS than 100 meters in radius spinning at 3 rpm WILL be needed for 1 g. In fact, 18 months and beyond in zero g is basically a DEATH sentence. No one would survive after 18 straight months in microgravity. Additionally one of the many deliterious health effects of microgravity is a progressive significant reduction to human radiation tolorance.
    For a probe this could work, but not in the next 50 years. The heavy lift and asteriod part for water to build this and have thousands of tons of fissionable material isn’t realistic considering that we are about ten years behind where Soace X should have been for even 150 tons to low Earth Orbit. Testing is still underway, 10 years later. This kind of nuclear fission powered spacecraft for an interstellar mission even for a probe is NOT realistic THIS CENTURY.

    • Total mass per person (excluding fuel/propulsion) for a 50-60 year mission:
      Living quarters: ~10 tons (structure, furnishings)

      Agriculture: ~20 tons (hydroponics, soil, plants)

      Water + systems: ~2.5 tons

      Shielding & misc.: ~20–50 tons

      Water and shielding could use the water for the NSWR propulsion. so 20-30 tons per person.
      500 people at 30 tons would be 15,000 tons for the crew. So a 50X bigger ship.
      If there was topor (keeping rotating crew in long sleep phases) and/or cryonics the mass might be 3-5 times less.

      Gravitational rotation and other structures could push this to 30,000 to 50,000 tons or so. 100 times larger than the 300 ton – crew payload.

      There is loads of fresh water needed to be stored to protect the reaction chamber and the nozzles.

  5. The math really isn’t mathing in a lot of different directions. 7500 tons of U @ 5% enrichment is more fissile material than a 4GW LWR uses in 30 years. Increase that enrichment to 20% or 93% per the various sections of the article and we’re exceeding output of multiple 4GW units over a century. Sure that’s the type of energy that would be needed to achieve 2-7% seven percent of the speed of light, but you don’t get 90% fuel depletion with to fissile atoms per 100 moderator molecules with the propellant feed rate you’d see in this rocket.

    The uncanny thing is that my coworker forwarded a research paper on this topic to me the other day. We were talking about NTP in general at the end of the day – nuke fuel nerd smalltalk.

  6. The real challenge for high delta v with the NSW rocket is avoiding criticality during storage. The more parasitic mass required to accomplish this, the lower the limit on mass ratio is, and your 7.6% of light speed assumes a fairly high mass ratio.

    I’d propose storing the propellant in the form of one or more long “sausage strings” of the uranium salt solution with an oriented long chain polyethene (Spectra) casing, hanging behind the rocket. (Segmented like a sausage to prevent the hydraulic pressure from adding up.) The actual engines could be off to either side on booms, and the first section of the string shielded against the neutron flux from the exhaust. (Yes, I have this engineer’s tendency to immediately start thinking about implementation details…)

    Geometry would prevent criticality. Structures in tension have the best strength to weight ratio, as they’re not subject to buckling, so this would minimize parasitic mass. And Spectra has a lower average atomic weight than water, it wouldn’t compromise exhaust velocity any to feed ground up casing into the fuel stream to boost thrust a bit. Really, the only limit on mass ratio for this approach is the characteristic support length of the strings.

    You could, hypothetically, skip the water, (It introduces thermal management problems storing the fuel in space.) and just use uranium doped polyethene; You could see higher performance this way on account of carbon having a slightly lower atomic weight than oxygen; At the temperatures we’re talking about, the fact that water is a liquid at room temperature, and polyethene a solid, would scarcely matter.

    Analogous to the way the fuel pumps work in a Raptor engine, you could allow just a smidge of criticality in the feed mechanism for the strands, to convert them into a moderately high pressure gas you could inject into the ‘combustion’ chamber, while actually powering secondary systems.

    It would be a fun system to design, but all the work would have to be done in space, it really wouldn’t be practical to assemble such a system in a gravity field, and the nature of the exhaust is such that dealing with it in a ground based lab would be more expensive than doing your testing out beyond the magnetosphere.

  7. Absolutely awesome concept. Getting above 7% light speed. A loss of cooling accident will cause a meltdown or explosion. I could favor it for interstellar exploration but regular use in the solar system for interplanetary travel might pollute the Earth’s atmosphere. I invented nuclear fission rockets that have no radioactive exhaust. The space force told me my nuclear tug design wasn’t a nuclear tug. Somebody is lying.

  8. The older I get the less feasible I see space travel in this way possible, the Iss constantly gets hit by micro meteorites, at these speeds above collisions are most often fatal, just small debris.

  9. This concept really needs to be developed and tested in space.

    Given it would be expelling radiation as a fine mist, if it’s expanding through interplanetary space away from a planet, the better.

    And its uses are not just for interstellar missions, but for outer solar system trips in less than a few decades. Trips of 1 year to Jupiter and 2-3 years to Saturn, Uranus and Neptune seem pretty feasible, maybe less time depending on Isp and power.

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