Nuclear Propulsion Systems: The Future of Interplanetary Spaceflight
A review of recent research, development, and the current and future state of nuclear propulsion technology
In July 2020, the Perseverance rover launched on an Atlas V rocket and started its journey to Mars. In February 2021 it arrived and landed on the surface.
Imagine a manned mission to Mars, where a group of people would have to spend 7 months in interplanetary space. They would need food, water, and electricity that entire time. They would also be bombarded by significant amounts of radiation from the sun.
Imagine if we could get from Earth to Mars in 3–4 months. What if we could get there in a few weeks? Nuclear propulsion could make that a reality.
There are three main types of nuclear propulsion being researched by NASA and other nuclear propulsion companies.
- Nuclear Thermal Propulsion
- NEP-Chemical Hybrid Propulsion
- Bimodal Propulsion
In this article, we’re going to explore how each type of nuclear propulsion works, the benefits and risks, and the recent research being done to improve the safety of the reactor — in particular, the intricate design of the cermet reactor fuel that will prevent a meltdown from occurring. We will also go over the main companies working on this technology, make some recommendations for future research, and make predictions for when this technology will be ready for flight.
How Nuclear Thermal Propulsion Works
First, you take a gas (usually hydrogen) and pump it around the nozzle. From the nozzle, it is pumped from the bottom upward around the outside of the reactor. The fuel then travels down through the core of the reactor, being heated up to over 2500K (2200C or 4000F) [1]. Some of this fuel is used to spin the turbopump, while the rest of it is shot out of the nozzle, generating thrust.
This way of generating thrust does 2 things.
- It heats the hydrogen before it even enters the reactor.
- It cools down the reactor and nozzle by transferring heat to the hydrogen.
When gas is heated, it expands. When the hydrogen is heated up by the nozzle and reactor, it expands, making it travel through the reactor faster, allowing it to absorb even more heat. When it leaves the nozzle, it’s hot enough to melt gold, platinum, titanium, and most other metals. The high exhaust velocity and heat allow NTP to perform much better than most rocket engines we have currently. NTP is capable of reaching an ISP of ~900s. [2]
How NEP/Chemical Hybrid Propulsion Works
Nuclear electric propulsion (NEP) is another recent development that shows promise. This form of propulsion is very similar to ionic propulsion systems. They both use electrical energy to propel noble gas particles, like Argon or Xenon. The key difference is that a normal ion drive uses energy from solar panels, while a NEP system would use energy from a small reactor. NEP is useful for missions to the outer planets and beyond where there is little sunlight. It also functions best on small payloads, like robotic missions or satellites.
However, if you combine an NEP system with a traditional chemical engine, you get the benefits of a high-thrust/low-ISP chemical engine (300–400s), but also the benefits of a low-thrust/high-ISP NEP engine (>10,000s). The chemical engine would perform major burns, such as orbital insertion burns. The NEP system would run continually for days or weeks on end while traveling through interplanetary space. By compounding the low acceleration over a long period, the NEP-chemical hybrid Mars Transfer Vehicle would gain a high velocity. This propulsion method could bring people to Mars in just 3–4 months, similar to NTP. [3]
How Bimodal Propulsion Works
Bimodal propulsion is a fusion of the two systems already being developed. Rather than combining NEP with a chemical rocket, it combines NEP with NTP. However, it’s been proposed to use a wave rotor (WR) in addition to the NTP and NEP to increase the specific impulse.
Bimodal implies there are two modes that the engine can run:
- NTP mode: The engine uses the heat of the reactor to expand liquid hydrogen and generate thrust
- NEP mode: The engine uses a wave rotor, a type of turbine, to convert the heat from the reactor into electricity that could power an electric thruster.
Integrating the wave rotor into the NTP system promises an increased ISP of 1400–2000s. Combining this with the NEP system, the whole bimodal system could achieve an ISP of 1800–4000s. These efficiencies could enable an Earth-Mars travel time of just 45 days! However, this technology is the least researched of the three and is the farthest from becoming a reality. [4]
How the Nuclear Fission Reactor Works.
The most important part of all three of these systems is the nuclear reactor itself. On a high level, Uranium 235 undergoes nuclear fission inside the reactor, generating heat that is used for heating hydrogen in NTP, or for generating electricity for NEP.
Fission is when a neutron hits a uranium atom, splitting the uranium atom, and releasing a lot of heat and radiation. This also releases more neutrons. These neutrons can go on to hit more uranium atoms, which releases more neutrons. This chain reaction starts a feedback loop where the reactor generates a ton of heat.
This feedback loop can be problematic though — if the reactor is left to conduct fission reactions without stopping, it can cause a meltdown. A meltdown is when the reactor core gets so hot the uranium melts. This had numerous terrible consequences for the reactor, and its effects were seen in the Chernobyl disaster in 1986.
To stop the reactor from melting, we use something called control rods. These control rods are made of a combination of metals, such as boron, silver, indium, and cadmium. They can absorb a large amount of neutrons without going through fission themselves. If you adjust the height of the control rods, you can control the rate of fission occurring within the nuclear reactor.
Inside an NTP engine, the reactor looks a little different. There are many small holes for the hydrogen propellant to travel through. The control rods surround the nuclear fuel and can rotate. Part of the control rod usually has a material that can absorb neutrons, like Boron Carbide, while the other side has a material that will reflect the neutrons into the reactor, like Beryllium. If you rotate the section that absorbs neutrons toward the reactor, it will slow down the fission. The opposite occurs if you rotate that section away.
The Composition of the Reactor Fuel
The uranium used for the reactor fuel has changed over the years as this technology has developed. During the NERVA program in the ’70s, high-enriched uranium was used for ground tests. This provided high performance but was very dangerous due to the amount of radiation produced by highly enriched uranium
Recently though, the development of high assay low enriched uranium (HALEU) is helping NTP become a reality. High assay means a higher uranium 235 concentration (5%-20%) than natural uranium (0.7%). However, it is still low enriched because it is less than 20% enriched. For comparison, 0.7%–3.5% enriched uranium is used in most nuclear reactors.
The switch to HALEU makes the radiation output of the engine much lower than that of the NERVA program. In addition, studies show that liquid hydrogen is very effective at shielding against radiation [5]. Current design proposals for a nuclear-powered spacecraft include long liquid hydrogen tanks between the crew module and the reactor. This design choice makes the radiation shielding on the engine itself much lighter, and improves the feasibility of manned spaceflight on a nuclear-powered spacecraft.
A Big Problem: Heat
The main issue with getting NTP or NEP into space is the safety risks. If the nuclear reactor overheats, or has a meltdown, it could destroy the spacecraft and kill any astronauts onboard. Even testing it is dangerous. If a company tests out its nuclear reactor and it blows up, it will surely lose a ton of funding. Imagine the headlines: Nuclear Rocket Engine Explodes in Los Angeles!
The hydrogen can reach an exhaust heat of 2700K (~2400C). The reactor itself may have to withstand temperatures up to 2850K (~2550C). The whole engine becomes exponentially more expensive and complex if you need a large cooling system. Therefore, the reactor fuel needs to have high heat absorption capabilities to minimize the cooling required around the reactor.
Nuclear reactor fuel is usually a material called a cermet (ceramic-metal). The ceramic here is Uranium Oxide (UO₂), and the metal will be Tungsten (W). Why tungsten?
- Tungsten has the highest melting point of any element: ~3700K
To make the cermet fuel, the UO₂ powder must be coated in the tungsten. This will:
- Give the fuel much higher heat resistance
- Reduce corrosion from hot hydrogen
Coating the Uranium Dioxide in Tungsten
By using a technique called magnetron sputter deposition the UO₂ powder can be coated in tungsten, creating a core-shell nanoparticle powder. The core is the UO₂ particle, and the shell is the tungsten coating. Sputter deposition is a type of Physical Vapor Deposition (PVD) coating method.
Magnetron Sputter Deposition works like so:
- A magnetron generates strong magnetic fields.
- The magnetic fields are directed at the tungsten metal.
- Numerous ions hit the tungsten, knocking off individual tungsten atoms.
- These atoms will produce a metal vapor of tungsten.
- The tungsten vapor will cover the UO₂ powder and coat it uniformly.
This is done inside a vacuum chamber so the metal vapor is not absorbed by the air.
This method had not been done before because UO₂ is a powder, not a flat surface. Therefore, the tungsten vapor would not be able to reach every side of the powder. However, recently a method was developed to coat powders with sputter deposition. By spinning the powder inside the vacuum chamber, the uncoated sides of the grains and particles are exposed, allowing the tungsten vapor to coat it.
While this is a complex mechanism, especially inside a vacuum chamber, it allows for much higher-quality coatings. A study proved that tungsten could be used to coat yttria-stabilized zirconia (YSZ) powder [6]. The YSZ was used as a placeholder for real UO₂ because they have similar properties, such as their lattice structure. Additionally, the YSZ powder particles are usually spherical, making it easier to apply a uniform coating.
The Results of the Research
The researchers were successfully able to apply a conformal tungsten core-shell coating to the YSZ powder. They were able to reach a thickness of 70–540nm for the coating. [7]
Previous studies have shown that conformal coatings can increase the density of the cermet, thus reducing the weight of the heat-resistant metals and the amount of neutrons the metal absorbs. [8]
This means:
- The metal will not disturb the fission reactions as much.
- The reduced weight of the fuel will keep the rocket engine lighter
- The increased density of the tungsten will reduce the amount of hydrogen diffusing through the shell into the UO₂ core of the nanoparticles.
Hydrogen diffusion is one of the main ways hot hydrogen corrodes the reactor core. This core-shell tungsten coating could hypothetically reduce the amount of corrosion while still keeping the refractory (heat-resistant) properties of the tungsten. However, additional testing must be done to confirm this.
Biggest Companies in the Space
NASA is likely the biggest player in this industry and has partnered with DARPA to develop two types of nuclear propulsion: NTP and NEP. NASA & DARPA are also partnering with some private companies, such as:
- Lockheed Martin
- BWX Technologies
- Aerojet Rocketdyne
- Ultra Safe Nuclear Corporation
There are some international companies making progress too, such as Roscosmos and the Chinese National Nuclear Corporation. A notable company is Pulsar Fusion, located in the UK, working towards electric and fusion propulsion.
NASA is also funding research at major research labs and universities, such as:
- Massachusetts Institute of Technology
- University of Alabama, Huntsville
- University of South Carolina
- Idaho National Laboratory
Future Work and Predictions
The next step for this research is to put the fake cermet fuel with YSZ-W core-shell nanoparticles through hot hydrogen testing to see how heat-resistant + anticorrosive the solution is in practice. After the method is established and repeatable, we could begin experimenting with making tungsten-coated UO₂ cermets.
The next big milestone for this technology is to run actual tests of the engines. Developing highly accurate simulations, and safe, rigorous ground testing methods will be crucial for this technology to develop in the next few years. NASA’s goal is to test NTP as soon as 2027. However, the field of aerospace is prone to significant delays. We should expect some ground testing and significant development by 2030, and a first flight likely before 2035. If we’re lucky, we might see a manned Mars mission during the 2030s, but if not during the 2030s, there is a very high chance of a Mars mission occurring during the 2040s.
There are millions of dollars being poured into nuclear propulsion development internationally. Whether it’s at major space agencies, private companies, or major research labs/universities. This field of technology will grow significantly in the next 5–10 years.
Takeaways/Summary
- There are 3 types of nuclear propulsion feasible for manned spaceflight: NTP, NEP-chemical hybrid, and Bimodal NTP/NEP.
- NTP uses the thermal energy from a reactor to heat liquid hydrogen and generate thrust
- NEP generates electricity with the reactor and uses that to power a traditional electric thruster.
- NTP and a NEP-chemical hybrid rocket could both enable 3–4 month Mars transfer times
- A Bimodal NTP/NEP rocket could theoretically enable travel times of 45 days, though the technology has not had much development
- The nuclear reactor undergoes fission of U²³⁵ atoms to generate heat
- The reactor fuel being used is HALEU, which is much safer than the HEU used in the 70s
- The reactor fuel is a cermet made with Uranium Oxide and Tungsten.
- Recent studies showed that a core-shell coating of UO₂ and W could improve the corrosion resistance, and increase the melting point of the cermet fuel, while also keeping the fuel lightweight.
- A technique called magnetron sputter deposition was used to coat YSZ powder in tungsten inside a rotating vacuum chamber.
- Future testing should include hot hydrogen tests of the YSZ-W cermet and the transition to creating UO₂-W cermets.
- Ground testing capabilities and high-accuracy simulations are both crucial for the continued development of this technology
- NTP testing could happen as soon as 2027, and manned missions using this tech could happen in the 2030s or 2040s.
- The field of nuclear space propulsion will likely explode in the next 5–10 years as research by NASA and private companies continues.
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