Nuclear Thermal Propulsion
July 26, 2024, Last Modified October 4, 2024
One method of propulsion for rockets that might rise in the future is nuclear thermal propulsion. In the early 1960s, the Nuclear Engine for Rocket Vehicle Applications (NERVA) initiative was created between NASA and the Atomic Energy Commission with thegoal to construct a nuclear engine that would exceed the capabilities of chemical engines. Their tests centered around the use of liquid hydrogen with a fission nuclear reaction. NERVA utilized liquid hydrogen because a fission reaction would allow them to use any propellant without worrying about an increase in its molecular weight; because of this, nuclear engines wouldn’t need to worry about the volume of propellant for their engines in comparison to chemical engines. NERVA’s testing revealed that nuclear engines were extremely efficient because of this ability to use lighter propellants, also highlighting the greater thrust velocity that nuclear thermal propulsion was capable of due to the lighter mass of the hydrogen molecules. They planned to have the nuclear engine sit on the upper stage of a rocket and ignite it only once it had exited the atmosphere. However, NERVA was shut down due to limited funding, and never ran such a test flight.
One of the reasons why nuclear engines are better than chemical rocket propulsion stems from something called Specific Impulse, which is the efficiency of a propulsion system's usage of propellant to produce thrust over a specific time interval. If two engines were to release 5lbs of propellant, the longer it takes would indicate a higher specific impulse (more time = more thrust = greater efficiency of propellant). Nuclear engine testing developed at NERVA doubled the specific impulse values generated by chemical rockets (due to lighte propellants, as stated above).
The basic design of a nuclear engine stems from a nuclear fission reaction. In this nuclear reaction, neutrons collide with uranium-235 nuclei inside a chamber which causes more uranium nuclei and neutrons to be released. This process will repeat in a chain reaction and generate extreme heat as uranium atoms are separated. As the fission reaction progresses, liquid hydrogen is passed through the reactor, turning into a gas, and pushed out the engine nozzle to generate thrust. The fission reaction is controlled with the use of cylindrical control rods made from beryllium and boron. Imagine the control rod in a vertical position with half the rod is coated with boron while the other half is made of beryllium. The boron absorbs neutrons which helps prevent the fission reaction from starting, or if the reaction has already begun, to slow the reaction and lower the heat; in contrast, the beryllium repels neutrons and allows the fission chain reaction to start and keep going. Even if the reaction is stopped, the engine will still be extremely radioactive (which is sort of a problem).
Image Courtesy of NASA
In one nuclear engine model, the core of the engine is a cylindrical chamber that hosts the nuclear fission reaction of uranium-235. Channels along the outsides of the chamber called fuel rods allow gases like the liquid hydrogen to heat up during the reaction before it enters the core, traveling through and exiting as stated above. Hydrogen is expelled out the nozzle after being transferred through the chamber, as shown, and stated above.
Image Courtesy of NASA
Like liquid fuel chemical propulsion systems, nuclear engines can also have liquid cores. One example is a centrifugal core where uranium is spun in a chamber at over 2000 RPM, causing it to cling to the sides of the chamber where hydrogen is then imported through the sides and falls through the middle and out the nozzle. The chamber has outer channels for the liquid hydrogen to help with cooling (similar to regenerative cooling).
However, many problems arise with this design, such as hydrogen exiting the chamber with uranium, causing the rocket engine to essentially be expelling radioactive material. A similar idea is the radiation liquid core that attempts to remedy this problem by channeling the liquid hydrogen down spinning tubes, yet problems continue to come up with each iteration.
Nuclear engines also come in gas core designs. A nuclear gas core open cycle is essentially a giant titanium alloy pressure shell with a uranium fission “plasma” in its center. Hydrogen is flowed around and out through a nozzle. A gas core closed cycle (not pictured), also known as the nuclear lightbulb, is made of a circular quartz wall that separates the hydrogen and uranium. Neon or argon fuel is pumped between the uranium core and fuel to protect the walls containing the fission reaction. However, the process of creating uranium plasma for the center of these engines is one of the many problems surrounding their designs. Additionally, the chamber needs to be made of materials capable of withstanding the uranium plasma of the fission reaction, which can reach temperatures above 50,000K. These engines also face similar problems as other nuclear engine designs: expelling radioactive material in addition to hydrogen.
Image Courtesy of Stanford University
Despite NERVA being canceled in 1973, nuclear thermal propulsion techniques like those mentioned above are continually worked on and studied. Working with the United States Defense Advanced Research Projects Agency (DARPA), NASA aims to send a spacecraft into space to test nuclear thermal propulsion. Their mission is known as Project DRACO (Demonstration Rocket for Agile Cislunar Operations), and will be expected to launch within the next 3-5 years. The engine is already equipped with multiple safety measures to protect the environment from its radioactivity; for instance, the engine will utilize a “poison wire” to absorb neutrons and prevent a fission reaction from spontaneously starting, similar to the beryllium-boron control rods inside the nuclear engine. Additionally, the spacecraft will only start its nuclear engine once it has reached orbit, and instead will be launched with the help of chemically-powered rocket engines (or be carried by a rocket). With this exciting launch, scientists will have a better understanding of the capabilities and improvements needed to make nuclear engines a reality–an outcome that would greatly benefit the future.