Fallout Propulsion — Leveraging Nuclear Thermal Propulsion to Revolutionize Deep Space Exploration

by Ishan Roy, Azam Lalani, and Sayf Al-Aziz Rashid

This article was written and published as part of a project to come up with and create a moonshot company called Fallout Propulsion

Image: NASA

In a world where resources are limited, humanity has always looked to the stars in search of new possibilities. Since well before the age of rockets and exploration, scientific discoveries made about stars and planets have expanded humanity’s knowledge of the universe. Now, thousands of years later, observations and research have shown us that asteroids contain large volumes of rare and useful materials and that it may be possible for humans to live on Mars. However, since Robert Goddard launched his first liquid-fueled rocket in 1926, the technology has not improved to the point where we can send large amounts of payload to Mars or deeper space. If we want to become a spacefaring civilization and harness the resources of our solar system, humanity needs a radical new solution…

The nuclear thermal propulsion system or NTP is Fallout Propulsion’s proposed propulsion system capable of efficient and cost-effective travel to deep space. NTPs were first proposed in 1945, but never really took hold until the 50s. In 1955, the United States Atomic Energy Commission launched Project Rover, a program which ran until 1973 with the goal of creating a nuclear thermal propulsion system. In 1961, under NASA’s direction, the renamed Project NERVA continued the development of all non-nuclear aspects of the engine. Nuclear aspects were kept compartmentalized under the control of the new Space Nuclear Propulsion Office. Many test engines were built over the duration of the project, many of which were actually fired. All the test articles were based on the solid core design using solid-fueled nuclear reactors to heat up the propellant. During the course of its run, NERVA test articles accumulated over 2 hours of ground testing time including 28 minutes where the engines were at full throttle. Although preliminary tests of the engine system were successful, the project was canceled during the overall downsizing of NASA after the Apollo program ended in 1972.

NERVA Test Articles. Image: NASA

The reason NASA and the U.S Government researched NTP engines is that they are more cost-effective, more efficient, and can propel spacecraft much farther distances much faster than traditional liquid-fueled rockets. To understand why nuclear thermal propulsion is superior to traditional liquid-fueled rockets, let’s take a look at how the latter works.

In rockets, thrust is produced by the rocket’s propulsion system. Propulsion systems for liquid-fueled rockets include a set of storage tanks where the propellants (fuel and oxidizer) are stored, and the rocket engine itself. The rocket engine has two major components: the combustion chamber (where the propellants are ignited), and the nozzle (where the exhaust gases from this combustion are accelerated to supersonic velocities). Pipes, pumps and control devices (valves) are used to regulate and modulate the propellant flow, and hence the thrust. The nozzle is the part an observer is able to see when looking at a rocket, and its design is arguably one of the most important parts of the engine.

Diagram of an open-cycle gas generator engine by Tim Dodd

The fuel is usually a highly refined jet fuel (RP-1), liquid hydrogen, or liquid methane. The oxidizer is almost always super-chilled liquid oxygen. The oxidizer is needed because there is very little outside oxygen available in the upper atmosphere and none at all in space.

Most rocket engines use asymmetrical, hourglass-shaped convergent-divergent nozzles. These were first developed in the 19th century for use in steam jet pumps and turbines and were first used in a rocket by Robert Goddard, who is credited with building the world’s first liquid-fueled rocket in 1926.

The working of a nozzle is remarkably simple. After the fuel and oxidizer are combined and ignited in a chemical reaction in the combustion chamber, the hot exhaust gas created by combustion enters the nozzle. As it travels through the converging section, it is pinched (“converges”) down to the minimum area, or throat, of the nozzle. The size of this throat is designed to restrict (“choke”) the flow and accelerate it to the speed of sound (Mach 1). It also determines the mass flow rate through the nozzle. When the exhaust gas expands (“diverges”) after reaching sonic velocity the throat, it expands to a supersonic speed. The expansion of this supersonic flow also causes the pressure and temperature to decrease. The ultimate exhaust velocity depends on the area ratio of the exit to the throat, as well as the pressure and temperature of the gas at the exit.

Now, let’s take a look at how a Nuclear Thermal Rocket propulsion system would work.

The obvious difference to begin with, is the presence of a nuclear reactor. A NERVA design utilizes a small, solid-core nuclear reactor fueled by 1.3 m uranium fuel rods. In an effort to save weight, the only coolant used is the working fluid, liquid hydrogen. The fission that occurs inside the reactor releases a tremendous amount of heat which is used to add energy to the propellant. The next main difference is the lack of oxidizer needed. Remember, in a liquid-fueled rocket, an oxidizer is needed as one of the elements for combustion. In an NTP, combustion is not necessary, and liquid hydrogen is used as the only fuel on board. The liquid hydrogen is stored in a tank as usual, and it flows into the engine assembly via two pipes. The hydrogen then flows through the reactor core. The nuclear fission occurring inside the reactor heats up the propellant to the point where it changes state into a high energy, fast-moving gas. The gaseous hydrogen enters the nozzle assembly in the same way a liquid-fueled rocket engine does and is accelerated out the back using the same converging-diverging nozzle set up.

These differences have a tremendous impact on a rocket’s performance when it comes to deep space exploration.

In order to reach long distances, sufficient thrust levels are required while also consuming the least amount of propellant as possible. This is typically quantified in terms of specific impulse (how long fuel can be burned to sustain a constant acceleration for a certain amount of propellant), which for chemical rockets, is less or close to 450 seconds. The specific impulse for an NTP engine, however, is 900 seconds. The fact that it is two times more efficient cuts down travel times to long distances like Mars by 25% while also using less fuel. The rocket’s dramatically increased energy density further allows for more enhancements in other parts of the rocket, and limitation of a crew’s exposure to radiation.

Clearly, NTP engines are hard to bet against for our future endeavors which is why we, Fallout Propulsion, seek to invest in this technology’s potential.

There are still points of improvement for NTP engines before we bring them to market. We plan to conduct interdisciplinary research in materials, nuclear, and fuel science. This includes finding small, yet safe nuclear reactors with current options including the molten-salt reactor as well as a gas-coolant system. Cooling technologies are also being analyzed to prevent overheating of rocket parts by the reactor. Finally, trials to find the most cost-effective fuel types will be run.

Once we do bring our engines to market, we expect a drastic change in the deep space industry. The creation of an NTP system would open up the space industry in ways we cannot even imagine. Morgan Stanley predicts that the space market will grow 285 percent to 1 trillion dollars by 2040. With the introduction of our highly capable system, it might even grow more than that. NTPs will make things like asteroid mining possible and deep space missions easier, faster, and more accessible. This shift is akin to the invention of the internal combustion industry, which revolutionized the transportation industry and opened up entirely new industries centered around transportation, logistics, and much more as well.

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