Nuclear reactors power some of the world’s largest aircraft carriers and submarines, so one may argue that large orbital rockets will also benefit from this source of propulsion. Unfortunately, things are not that simple.

Rockets can be nuclear-powered but are primarily used in Space where their fuel efficiency allows them to reach other celestial bodies in less than half the time it takes other spacecraft. Their reduced mass flow rate compared to chemical rockets, though, makes them unsuitable for use during launch.

Readers may not be familiar with this, but the concept of a nuclear-powered rocket is not a new one. During the height of the Space Race between the United States and the former Soviet Union, nuclear-powered engines were successfully developed in the United States.

In 1961, a development program called NERVA (Nuclear Engine For Rocket Vehicle Application) was launched at the Los Alamos Scientific Laboratory in the United States to test the viability of nuclear-powered propulsion for spacecraft.

At least 20 reactors were tested, and several hours of successful test firing proved that nuclear propulsion was indeed possible, but due to budget constraints, the program was canceled in 1973.

At the time of writing, though, renewed goals of sending crewed missions to Mars by Space Agencies like NASA and SpaceX shifted the focus back to nuclear propulsion, with new government funding and more interest from the private sector.

This raises the question as to why nuclear propulsion is such an attractive and sought-after goal to achieve. With more affordable launch vehicles and reusable rockets on the increase, it may seem unnecessary.

NERVA Nuclear Engine
An example of one of the 20 thermonuclear rocket engines used in the 1960s as part of the NERVA program to test the feasibility of nuclear propulsion for spacecraft.

The following section highlights the importance of nuclear propulsion for spacecraft and why it is so crucial for interplanetary travel.

Why Nuclear Propulsion Is Needed

To date, the furthest humans ever traveled was to the Moon, approximately 400 000 km (248 500 miles), and the last time a crewed mission landed on its surface was in 1972, before the Apollo Program was canceled.

With a renewed effort to return to the lunar surface and even more ambitious plans to send crewed missions to Mars, there are a number of challenging obstacles that need to be addressed and overcome first.

Probably the most important of these obstacles is the vast distance spacecraft have to travel to reach other celestial bodies in our solar system and also the fuel required to get to these destinations.

Mars is approximately 64 million kilometers (39.77 million miles) from the Earth. The fastest travel time to Mars currently available is the Hohmann Transfer method, which opens up a launch window every 26 months, and takes roughly 243 days (8 months).

(Faster travel times are theoretically possible by carrying more fuel but at the expense of cargo capacity).

These times are too long for interplanetary travel for a variety of reasons. Long-term exposure to radiation in space, accompanied by the adverse effects of zero gravity, will have a huge negative impact on the human body, which may not be survivable.

Saturn V
The Saturn V, the only launch vehicle to ever carry humans to another celestial body to date, had to make use of three stages to enable the spacecraft to reach the Moon.

(The Saturn V used during the Apollo Program, the only launch vehicle ever to carry humans to another celestial body to date, had to make use of three stages to enable the spacecraft to reach the Moon. Learn more about rocket staging and how it works in this article.)

The long travel times, combined with a launch window only becoming available every 26 months, also makes planning a mission, making sure resupply missions reach their destination in time, and responding to possible emergencies very challenging.

This is due to the high fuel requirements of chemical rockets, which are necessary to enable them to push through the Earth’s thick atmosphere and break free from the planet’s large gravitational pull and enter orbit or travel further into space.

During launch, more than 85% of a rocket’s mass consists of fuel, of which the vast majority is expended just by reaching space. It leaves very little propellant left for large maneuvers like accelerating for extended periods, which are needed to reach other celestial bodies.

A much more fuel-efficient rocket engine is required to allow a vehicle to accelerate long enough to acquire the speeds needed to reach planets like Mars in a much shorter time. Specifically, an engine with a much higher Specific Impulse.

What Is Specific Impulse?

Specific Impulse is the Holy Grail for many rocket engineers and essentially refers to how efficiently a rocket engine can burn its fuel. Simply put, it refers to how long a certain amount of propellant can produce a certain amount of thrust.

To better understand Specific Impulse, one can compare it to how many miles per gallon a car can achieve. A more fuel-efficient vehicle will be able to travel much further than a vehicle with a much higher fuel consumption rate.

In the same way, a rocket with a higher Specific Impulse can burn its thrusters longer, accelerate for extended periods, and as a result, cover much longer distances in a significantly shorter time.

Specific impulse is generally calculated by measuring the number of seconds a kilogram (or pound) of fuel lasts while producing a kilogram (or pound) of thrust. It is typically measured in seconds. (A very unscientific but practical explanation.)

As a result, a spacecraft with a Specific Impulse of 900 seconds is much more efficient than a spacecraft with a Specific Impulse of 450 seconds, allowing it to travel longer and further on the same amount of fuel.

However, Specific Impulse specifically refers to energy efficiency and not power or thrust. A launch vehicle with a high Specific Impulse may not necessarily have the mass flow rate (or thrust) to propel a rocket out of Earth’s atmosphere during launch.

This is where the benefits of nuclear propulsion become very clear. Due to its “almost limitless” power supply, it can continue to accelerate for extended periods, providing the continuous thrust to allow a spacecraft to reach its destination in a greatly reduced time.

However, as stated during the description of Specific Impulse, a thermonuclear propulsion system making use of hydrogen will be greatly beneficial in space but not practical for a rocket launch, literally lacking the explosive raw power of these orbital launch vehicles.

Thermonuclear Engine
Artist’s impression of a thermonuclear rocket engine, similar to the ones developed during the NERVA Program of the 1960s and 70s, with the cutaway showing the nuclear reactor and reflector.

Although it can steadily accelerate and propel a spacecraft for extended periods in space, it cannot achieve the same mass flow rate (or high thrust) achieved by chemical rockets, which accelerate millions of molecules through their nozzles in a very short period of time.

(There are also other drawbacks and dangers to nuclear propulsion, which will be discussed in more detail in a later section.)

A thermonuclear power rocket engine may look deceivingly similar to a conventional chemical rocket engine but differ entirely in the way they work. The following section describes how a nuclear propulsion system works on a spacecraft.

How Nuclear Propulsion Works On A Spacecraft

Visually, a thermonuclear rocket engine may look very similar to a conventional liquid-fueled rocket engine, but they differ fundamentally in a number of ways. In principle, the engine of a thermonuclear rocket works as follows:

Thermonuclear Rocket Engine

The engine consists of a cylindrical-shaped “solid core heat exchange type” nuclear reactor that powers the rocket. The core is surrounded by a reflector primarily made of beryllium.

A number of channels are created in the reactor core, running the length of the unit through which hydrogen is passed. The channel walls are coated with niobium carbide to protect the graphite against the highly corrosive hydrogen.

Uranium-235 pellets packed in graphite tubes make up the fuel rods that serve as the fuel source for the reactor. The reaction is started and kept running through fission of the Uranium-235 atoms.

The nuclear fission process is a nuclear radioactive decay process, initiated by propelling a neutron into a uranium atom, causing it to split into two. By splitting the atom, three more neutrons are released, starting a chain reaction as the neutrons hit and split more atoms.

The continuous reaction of splitting atoms and releasing more neutrons creates a large amount of heat, up to 2 200° Celsius (4 000° Fahrenheit). It is these high temperatures that make the whole nuclear propulsion system work.

Rotating Control Rods
On the left image, the rods are rotated to a position where the boron side faces the fuel elements inside the reactor, absorbing enough neutrons to prevent nuclear fission. In the image on the right, the rods are rotated so that the beryllium side faces the fuel elements, allowing enough neutrons to be reflected back and not absorbed, enabling the fission process to start.

To control the amount of heat generated by the reactor, the reflector surrounding the core contains rotating rods. The rods also consist of beryllium, but with one side coated with a material called boron, which has the ability to absorb neutrons.

During the initial phase of the fission process, the rods are rotated to a position where the boron side is facing the fuel elements inside the reactor, absorbing enough neutrons to prevent nuclear fission from taking place.

Rotating the rods so that the beryllium side faces the fuel elements allows enough neutrons to be reflected back and not absorbed, enabling the fission process to start. The amount of heat can be controlled by changing the position of the rods.

Liquid hydrogen serves as the propellant for the nuclear engine. The cryogenic fuel is stored at temperatures of approximately -253° Celsius (-423° Fahrenheit) to keep it in liquid form.

(Some chemical rockets also make use of hydrogen as fuel, but it is combined with liquid oxygen and combusted to provide the propulsion for an orbital rocket. Learn more about the different fuels launch vehicles use and their advantages & disadvantages in this article.)

The hydrogen is first relayed to the nozzle end and then pumped through openings in the nozzle walls and back up inside the reflector and reactor pressure shell. This keeps the nozzles, reflector, and pressure shell cool & prevents them from melting in the extreme heat.

Hydrogen Flow Through Reactor
Illustration of how hydrogen flows through channels in the 2 200° Celsius (4 000° Fahrenheit) warm reactor core before the pressurized gases are propelled through the nozzle.

The hydrogen, which is already warmed by this process, now passes back through the channels in the reactor core, where the fuel elements heat it to about 2 200° Celsius (4 000° Fahrenheit), turning it into gas and causing it to expand rapidly.

The rapidly expanding hydrogen gas is now expelled through the nozzle at a velocity of approximately 8 km/s (5 miles per second) or 17 900 mph. This is almost double the exhaust velocity of a conventional chemical rocket.

(Nuclear energy can also be used to provide the energy for another type of propulsion known as ion propulsion. An ion thruster is a form of electric propulsion that uses electricity to ionize atoms. Learn what ion propulsion is and how it works in this in-depth article.)

Advantages Of Nuclear Propulsion In Space

The numerous advantages of nuclear propulsion have already been highlighted throughout the article but can be summarized as follows:

  1. Nuclear propulsion provides two to three times the Specific Impulse of traditional chemical reactions, typically from 450 seconds (liquid hydrogen and liquid oxygen combination) to 900+ seconds for a thermonuclear powered engine.
  2. The higher Specific Impulse allows spacecraft to burn its thrusters and accelerate for substantially longer periods, dramatically reducing the travel time to other celestial bodies in our solar system like Mars missions, from 6-9 months to 3-4 months.

These are by no means the only advantages of nuclear power but are the most important, which will remove many of the limitations of current rocket propulsion technology.

Drawbacks & Obstacles In Nuclear Propulsion Development

Using nuclear power, though, comes with several dangers and additional challenges. Some viewers may still remember the devastation caused by the Chernobyl disaster in 1986, where a reactor explosion caused widespread devastation and radioactive contamination.

A similar event, the Fukushima nuclear disaster, caused by an earthquake and resulting tsunami, caused 154 000 people to be evacuated and contaminated large parts of the surrounding land and ocean.

The potential problems of sending a nuclear reactor into space in a chemical rocket with several thousand tons of highly-combustible propellants should be self-evident but are not the only obstacles that need to be overcome to make nuclear propulsion a reality:

  1. The potential radiation exposure to crew members is a primary concern. Several measures will have to be implemented to protect crew members during launch and while in space, even without any malfunctioning from the reactor side.
  2. In case of a failure/explosion, the widespread environmental impact will have to be taken into consideration, which will lead to costly additional measures in and around a launch facility. No-fly and other exclusion zones will also need to be expanded.
  3. The added weight of a nuclear reactor and the insulation needed to protect against radiation will dramatically impact fuel and cargo. To successfully launch such a heavy load into space, more propellant and precious cargo will have to be sacrificed.
  4. Ideally, a nuclear-powered spacecraft will stay in orbit around a planet if not traveling to another celestial body. It means the hydrogen onboard needs to be stored safely for sustained periods, which poses a number of challenges.
    Hydrogen is not only highly corrosive, but its molecules are so small they can easily slip through seemingly solid metals. Also, keeping it in liquid form at a temperature of -253° Celsius (-423° Fahrenheit) is also a potential problem. As a result, a safe long-term storage solution for liquid hydrogen needs to be developed.

Conclusion

As this article illustrated, for humans to travel safely and in a reasonable time to other planets in our solar system, a new form of propulsion is needed to allow spacecraft to accelerate for sustained periods and be much more fuel-efficient.

All current forms of space propulsion are simply not capable of providing the continuous thrust with the fuel they carry onboard after reaching space to make this possible. Fractional improvements can be made, but with great cost to cargo capacity.

Nuclear propulsion has already been ground tested successfully during the NERVA project of the 1960s, so it is possible. A lot of obstacles need to be overcome before it becomes a reality, but nuclear power currently remains our best bet for long-distance space travel.

This article was originally published on headedforspace.com. If it is now published on any other site, it was done without permission from the copyright owner.

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