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Is The Future of Rocket Propulsion Chemical or Nuclear?

Is The Future of Rocket Propulsion Chemical or Nuclear?

Introduction

In the late 1950s, with the launch of Russia’s Sputnik probe, we entered what has been known as the space age. It didn’t take long for the US to then put men on the Moon, and since then we have continued to endeavour into this new frontier. Now, with various organisations setting their sights for the Moon, Mars and beyond, it is worthwhile that we examine the propulsion of the future. Historically, chemical rockets have been the sole method of rocket propulsion, primarily due to the high thrust that they generate. This makes them the only currently available means of escaping the Earth’s gravity, as well as for performing manoeuvres on high-mass rockets. Yet once we have entered the realm of space, nuclear propulsion is certainly worth consideration, and may well find itself replacing chemical fuel as the future of rocket propulsion.

Nuclear vs Chemical

So let us begin our investigation with an analysis of how chemical and nuclear – including both ion and nuclear thermal – propulsion work. The traditional chemical rockets that we are most familiar with rely on two core components: fuel and oxidiser. In the engine, when both of these materials combine, the fuel is ignited and the exhaust is ejected through the nozzle of the rocket. This generates enormous levels of thrust, thus allowing rockets to escape Earth’s gravity and enter into space.

Ion Thruster - ICL
In regards to nuclear, we must split it into two distinct categories that warrant individual examination: nuclear ion and nuclear thermal. Nuclear ion fuel consists of noble gases, primarily xenon, which are then fired at by high-energy electrons. When these high-energy electrons hit the xenon atoms, they knock off some of their electrons, and consequently transform them into positive ions. A set of charged grids are then used to create a strong electric field in the engine, which then works to control the direction of the xenon atom’s ejection. More specifically, the positive ions are repelled by a positively charged grid and accelerated toward a grounded or negatively charged one, which further accelerates the ions and shoots them out the back of the engine to create thrust. However, this thrust is very minimal – often similar to the force of a piece of paper pushing against your hand. The reason for this is due to the fact that, although the exhaust velocity is roughly 10 times greater than a chemical rocket, the mass of the xenon gas is extremely small in comparison to chemical rockets, thus making them incapable of escaping Earth’s gravity. Nevertheless, in deep space where there is no air resistance (nor gravity that needs to be escaped), the rapid acceleration of the xenon ions does build up momentum over time, thus making it ideal for long-range (several months or years long) missions, where its efficiency is much desired.

Nuclear Thermal - Stack Exchange
On the other hand, nuclear thermal works by a very different process, relying on the heat generated in nuclear fission reaction. Just as in a nuclear reactor, the fuel consists of uranium, which is then shot with neutrons that cause the atom to split apart, generating substantial byproducts of thermal energy. This energy is then absorbed by the reactor wall, which is then transferred to the hydrogen propellant. As the hydrogen absorbs more heat it reaches extremely high temperatures (up to 3,000 K) and significantly increases in pressure, thus forcing the propellant to be ejected out of the nozzle at high thrusts. Again, however, these thrusts do not compete with that of chemical rockets, thus restricting their availability to outer space. Yet once in outer space, nuclear thermal rockets can be thought of, in many instances, as an ideal middle ground between chemical and ion rockets. Unlike chemical rockets, the high energy density and exhaust velocities lead to very high energy efficiency, and, unlike ion rockets, they generate enough thrust to be useful in shorter (maybe months long) missions, such as from Earth to Mars.

The Trajectory of Rocket Propulsion

Based on our previous analysis, it seems that we can well distinguish the optimal use cases of each of these three propulsion means: chemical for lift-off and escaping gravitational pull, nuclear thermal for medium-range deep space travel, and nuclear ion for long-range deep space travel. One may conceive of a future in which chemical rockets are used to transport from land to orbit (and vice versa), and once in orbit attach to some form of transit ship powered by nuclear thermal propulsion, designed to power the transportation between celestial bodies. Yet, although this appears to be the future, we are of course only in the early phases of space travel, and thus the reality is much more focused on what is available, rather than what is most ideal and efficient. The first person to land on Mars will almost certainly be transported there with 100% chemical propulsion, and that will likely be the primary rocket propulsion for at least the next fifty or so years. Nevertheless, R&D efforts are already in action for nuclear thermal rockets, with a partnership between DARPA and NASA aiming to test such a propulsion system in space by 2027 and, although this may be somewhat delayed, progress is being made. In terms of ion propulsion, they are already being used to power a range of probes and satellites, such as NASA’s Deep Space 1 as well as most modern commercial satellites. Additionally, ion propulsion could potentially be adopted for future use in long-distance cargo transport, where speed may not be paramount. Overall, it seems apparent that the future of space travel will be powered by a mix of both nuclear and chemical, and that it will stay like that for an extensive period of time, until the next great breakthrough.



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