Rockets had their humble start in 13th century China: bamboo tubes filled with gunpowder, attached to arrows, to be shot at Mongol invaders. Seven hundred years later, similar, albeit slightly bigger, tubes were filled with liquid hydrogen and oxygen, attached to a lunar module, and shot at the moon. Now, in the 21st century, we’re seeing another evolution, this time with Direct Fusion Drive (DFD). But the more things change, the more they stay the same, and the fundamental concepts behind rocketry that repelled the Mongols at the Battle of Kai Keng are the same fundamentals that are going to propel us to Pluto and beyond in record time.
Rocket Science
Well, how do rockets work anyway? On a theoretical level, they work thanks to Newton’s Third Law of Motion, which holds that every action has an equal and opposite reaction. Propellant is mixed and burned in a combustion chamber. The gas produced goes through a nozzle and accelerates to supersonic speeds (think about how partially covering the outlet of a hose with your thumb sends the water shooting out). The gas shooting out the other end necessitates an equal reactive force in the opposite direction on the engine, a force called thrust. This thrust is what makes the rocket fly, and since the rocket carries its own fuel and oxidizer (the chemical that fuel needs to burn), it doesn’t need oxygen from the environment to conduct combustion like a car engine does, making it ideal for traveling through the vacuum of space.
Chemical Rockets
Since their creation, conventional rockets have depended on exothermic chemical reactions to create the necessary hot, propelling gas. An exothermic reaction is a chemical reaction that produces heat. These rockets come in 2 major forms: solid-fuel rockets and liquid fuel. The fuel and oxidizer grains are mixed from the beginning, and to cause combustion, the surface of the mixture is ignited. Liquid fuel rocket fuel and oxidizer are originally separate and then injected into the combustion chamber, where they mix and burn.
How DFD Works
These two methods have sent countless astronauts into space, but in the coming decades, DFD may supplant them. DFD aims to use the power of its fusion reactions to not only propel the module but to generate its electricity.
At its core, DFD is a fusion reaction powering a plasma rocket. Powerful rings of magnets are lined up to form something resembling a cylinder. Smaller but stronger magnets are situated at both ends. In the center of this configuration, helium-3, a stable helium isotope with only 1 neutron, is put in a fusion reaction with deuterium, a hydrogen isotope with roughly double the mass of ordinary hydrogen (isotopes are atoms of the same element that have the same number of protons but a different number of neutrons). These reactions and some side reactions create a fusion region the length of a surfboard. Cool plasma flows around it to extract energy (plasma is gas-like matter made up of ions and electrons; the presence of these electrons makes plasma really good at conducting electricity and easily influenced by electromagnetic fields). Antennae surrounding the engine create a rotating magnetic field that induces a current in the plasma. The plasma ions get more and more energized until they fuse, and the fusion products get shot out the back of the engine like hot gases in a conventional rocket, creating thrust.
This process produces tons of excess heat, which is used in a Brayton Engine to create more than 200 kW of electrical power. All remaining heat gets rejected to space, the ultimate thermal reservoir.
How DFD Measures Up to Conventional Propulsion
Conventional rockets have many advantages: high thrust and (relatively) low cost. They’re a reliable mechanism that has been used for centuries. What makes DFD so attractive is its compact design and efficiency. An important quantity in rocketry is specific impulse—impulse per unit of propellant or how efficiently a rocket creates thrust. The higher the specific impulse (often measured in seconds), the less propellant required for the desired impulse. Solid and liquid fuel rockets top out with a specific impulse of around 500 seconds, but a DFD engine boasts a specific impulse of 10,000 to 20,000 seconds, all while being no more than 2 meters in diameter.
With these numbers, NASA predicts that a 10,000 kg mission with DFD could get to Pluto in 4 years, which is much shorter than the 9.5 years it took 478 kg New Horizons on hydrazine propulsion, with more capabilities to do scientific work thanks to its increased power supply.
The Future
All in all, NASA is very excited about the DFD concept, as it can easily fit into NASA’s current launch infrastructure and would be an ultra-low radiation fusion option; however, there’s still a lot of work to be done. For example, while the Brayton Engine has been developed and the ion-energizing process is being developed, the crux of the concept, nuclear fusion, still has to be demonstrated. DFD design is going into its second phase, which seeks to improve its plasma heating system and shielding capabilities. According to the current timeline, actual flights are planned for after 2030 and, in the future, could power orbital platforms, interplanetary travel, and even bases on the Moon and Mars.
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Sources:
Paluszek, M., Pajer, G., Razin, Y., Slonaker, J., Cohen, S., Feder, R., … Walsh, M. (2014). (rep.). Direct Fusion Drive for a Human Mars Orbital Mission (pp. 1–9). Princeton, New Jersey: Princeton Plasma Physics Laboratory Office of Reports and Publications.
Princeton Satellite Systems. (2018). Technical Video of Dfd Engine. YouTube. https://www.youtube.com/watch?v=hggqvB5I95I.
Thomas, S. (n.d.). Direct Fusion Drive: Enabling Rapid Deep Space Propulsion. Lecture.
Benson, T. (2014, June 12). Brief History of Rockets. NASA. https://www.grc.nasa.gov/www/k-12/TRC/Rockets/history_of_rockets.html.
