|
|
| Fig. 1: A flowchart describing a hypothetical extraterrestrial process of turning water ice resources into methane fuel. (Image source: B. Wu.) |
In humanity's quest to explore the cosmos, the choice of rocket propellant plays a critical role in shaping the future of space exploration. In recent years, interest and activity in the launch and transportation side of the Space Industry has skyrocketed. This has been fueled primarily by the shift of rocket launches from Government-managed Space Agencies to the private sector, coupled with a rise in startups aspiring for a share of the pie of spacecraft launches by developing their own rocket technology.
The vast majority of these newly developed rockets rely on chemical propulsion. This offers several choices for fuel and oxidizer combinations, each with their advantages and disadvantages. Among all rockets, popular combinations of fuel and oxidizer include Hydrogen/Oxygen, Kerosene/Oxygen, Methane/Oxygen, and hypergolic propellants. The latter ignite upon contact and thus do not require the use of an igniter.
Due to the high level of toxicity of hypergolic propellants, as well as the lighter density/extremely low boiling point of Hydrogen fuel (thus necessitating the use of much larger and heavier tanks), new rocket designs almost exclusively use liquid Oxygen as an oxidizer and alternate between liquid Methane and highly-refined rocket-grade Kerosene (also known as RP-1) as fuel.
While the choice of an optimal propellant is likely to depend heavily on the application, Methane has the potential to offer many key advantages over potential competitors. For instance, Methane is a superior choice to Kerosene because of its clean-burning properties, greater combustion efficiency (when accounting for the similar boiling point of both propellants), and ease of manufacturing on extraterrestrial, resource-limited environments.
There is one particular scenario in which whether or not a propellant is clean- burning can make an outsized difference on the long-term cost and viability of operating a rocket that utilizes that propellant. In recent years, the aerospace industry largely shifting due to the breakthroughs in reusable rocketry and spacecraft pioneered by SpaceX has become more and more concerned with the long-term reusability of launch vehicles, a move that will drastically save manufacturing + resource costs and potentially enable greater access to Space. Even if a rocket or spacecraft is designed to be reusable, however, the cost (and length of time) it takes to refurbish the flight-ready components will be a deciding factor in how manufacturers design their next- generation aerospace systems. As detailed below, the amount of soot produced during the combustion process is directly correlated with refurbishment downtime and cost; methane is likely able to achieve this goal through a cleaner combustion process.
Combusting methane and oxygen gives off the products of carbon dioxide and hydrogen in gaseous forms, while combusting kerosene a much more complex hydrocarbon will result in an incomplete combustion process that produces soot and other particulate matter. Given that kerosene is primarily a mixture of high-boiling hydrocarbons containing 11-17 carbon atoms, the process of combustion will result in the hydrocarbons to first be cracked in order to form carbon and hydrogen; subsequently, each of these elements will interact with free oxygen atoms to form water and carbon oxides such as carbon monoxide and carbon dioxide. [1] As kerosene-fueled engines tend to burn fuel-rich (meaning that they burn kerosene at a higher stoichiometric ratio compared to the complete balanced combustion), the unbalanced production means that not all of the fuel will be completely burned, thus leaving significant carbon deposits on the surfaces on the engine itself. [2] These reaction byproducts result in soot formation on any surface that comes into contact with the combustion byproducts, with the combustion chamber of a kerosene-burning rocket engine harboring the highest soot deposits. [3] These must be cleaned off in between launches, resulting in potentially lengthy processing times if a rocket is designed to be reusable, as well as a higher risk of soot deposits making future combustion unstable. By contrast, carbon dioxide is unlikely to leave large amounts of residual soot on the surface of an engine combustion chamber making Methane-powered engines easier to maintain and giving it the ability to be serviced faster between flights.
One further note: While hypergolic propellants are less common today than they were at the onset of the Space Age, they are still the propellant of choice amongst a few launch vehicles (mostly China and Russia), as well as a plethora of upper stages that operate after orbital velocity has been achieved. While these propellants appear to be far less energy dense compared to their hydrocarbon counterparts (approximately 1/3 of the energy density of kerosene), their low cost due to the property that they are stored at room temperature means that the usage of such fuels have not been entirely eliminated, particularly for lower-energy applications in Space. [4,5]
Specific Impulse (also denoted as Isp) is often the primary metric by which the efficiency of a rocket engine is evaluated. The specific impulse is given in units of seconds, and a higher figure indicates a higher efficiency. The equation for calculating specific impulse is
| Isp | = | F ṁ g0 |
Where F is defined as the thrust that the rocket engine provides (in Newtons), ṁ is the mass flow rate of the propellants (in kg/s), and g0 is the standard gravitational constant (defined in standard units as 9.8 meters per second squared). The specific impulse is calculated by taking the thrust of the engine divided by the mass flow rate of propellants and standard gravity. Since the mass flow rate is inversely proportional to the specific impulse, an optimal propellant would contain a relatively high energy density. The intuition behind this is that for the same thrust level, less propellant will flow through the combustion chamber, thus enabling the engine to run longer and increase efficiency. [4] The energy density of Liquified Natural Gas, the closest analogue to pure liquid Methane, is 22.2 MJ/L, while the energy density of jet fuel (closes analogue to RP-1) is approximately 35 MJ/L. [6,7] Although the energy density of Kerosene is slightly higher, most of the advantage that it harbors could potentially be later offset due to the fact that the difference in boiling points between Methane and Oxygen is smaller than the difference in boiling points between Kerosene and Oxygen (which has implications on the construction of the rocket body), as discussed further below. Even very small changes in weight carried onboard the rocket can lead to significant differences in fuel efficiency. Furthermore, evaluating the specific impulse of methane rocket engines and kerosene rocket engines under standard conditions (assuming atmospheric pressure at the exit of the nozzle and an engine chamber pressure of 1000 pounds per square inch), methane rocket engines achieve approximately 299 seconds in specific impulse while kerosene engines achieve roughly 289 seconds in specific impulse. [5] While this signifies that methane engines have a slight efficiency advantage over kerosene engines under standard conditions, it should be noted that one might expect to see higher specific impulse numbers should an engine be able to handle extremely high chamber pressure, for instance. It is thus entirely likely that a kerosene engine could be more efficient than a methane fueled engine when evaluated under this metric; the specific details of which are left to manufacturer-specific designs and considerations.
RP-1 Kerosene is a highly refined form of kerosene similar to jet fuel that is the most common variation of refined Kerosene used in rocket launches. While it retains the properties of being stable at room temperature and lower risk of explosion, RP-1 must undergo a lengthy and expensive refinement process after being extracted from the ground as crude oil. Methane, on the other hand, can easily be produced via the Sabatier Reaction, which produces water and methane given the inputs of hydrogen and carbon dioxide along with a methanation catalyst. [8] The relative simplicity of the Sabatier process (depicted in the below equation) compared to refining crude oil indicates that the production of Methane has the potential to be significantly cheaper than the production of Kerosene.
| CO2 + 4H2 → CH4 + 2H2O |
Moreover, this process has significant advantages during interplanetary expeditions because the Sabatier process does not need to be executed on Earth; it could be conducted anywhere in which the raw ingredients of Hydrogen and Carbon Dioxide are readily available. [9] An example of such a scenario is the surface of Mars; a spacecraft carrying Hydrogen and a methanation catalyst can mix the local carbon dioxide-rich atmosphere in order to produce Methane, with Fig. 1 depicting a hypothetical process of the steps that need to be taken in order for Methane fuel to be produced from water ice on the Martian surface. By contrast, Kerosene first requires that raw oil can be extracted. This is currently only possible on the surface of the Earth, and given the (relatively) small size of the customer segment, supply constraints are generally much more severe. Furthermore, the refinement process that transforms crude oil into RP-1 requires large-scale equipment that is likely not feasible to be transported aboard spacecraft.
That being said, it must be noted that the above argument is less relevant for today, especially for Earth-based applications. This is because there remain significant deposits of natural gas and petroleum that can be refined into liquified natural gas and RP-1 kerosene fuel, respectively. This argument, however, becomes extremely relevant when considering the implications of interplanetary travel: if there is an immediate need for fuel on the Lunar or Martian surfaces, for instance, then the cost of delivering the fuel will likely be prohibitive: a spacecraft has to carry enough fuel to accelerate itself to escape velocity, which will decrease the amount of payload available and thus incur a higher cost for the customer. Furthermore, the time that it takes for fuel to be transported from Earth, especially for Martian destinations, will likely mean that those who wish to establish outposts on Earths celestial neighbors must consider local alternatives for fuel production, in which case Methane has a clear advantage over Kerosene due to the Sabatier process itself.
The case for using Methane over Kerosene as the fuel for next-generation chemical rocket propulsion is compelling: Methane's clean-burning properties due to burning at a more complete stoichiometric ratio compared to kerosene in most applications not only simplify engine maintenance but also reduce the risk of soot buildup, making it a practical choice for reusable rocket engines. Furthermore, while current production of rocket propellants largely rely on refining existing resources on Earth, the ease of manufacturing methane through the Sabatier Reaction makes it a cost-effective and versatile option, particularly during interplanetary missions where it's infeasible to transport raw materials constantly from Earth which is not possible with Kerosene. Despite the fact that Methane exhibits a slightly lower energy density when compared to kerosene, forward-thinking engineers are already integrating Methane-based propulsion systems on next- generation reusable launch vehicles and interplanetary spacecraft.
As we stand on the precipice of a new era of space exploration marked by increased private sector involvement and ambitious missions, the choice of rocket fuel for chemical propulsion will play a pivotal role in shaping the future of space travel. Methane offers the promise of cleaner, more efficient, and economically viable propulsion systems for humanity's spacefaring destiny.
© Brian Wu. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] D. Xie et al., "Effect of Kerosene Combustion Atmosphere on the Mild Steel Oxide Layer," Sci. Rep. 12, 379 (2022).
[2] D. Lee, K. H. Shin, and C. Lee, "Fuel-Rich Combustion Characteristics of Hybrid Rocket Engines," American Institute of Aeronautics and Astronautics, AIAA 2015-3828, 23 Jul 15.
[3] M. Ross and J. A. Vedda, "The Policy and Science of Rocket Emissions," The Aerospace Corporation, April 2018.
[4] J. D. Clark and I. Asimov, Ignition!: An Informal History of Liquid Rocket Propellants (Rutgers University Press, 2018).
[5] J. R. Rumble, Ed., CRC Handbook of Chemistry and Physics, 103rd Ed. (CRC Press, 2022).
[6] "Fuel Properties Comparison," U.S. Office of Energy Efficiency and Renewable Energy, DOE/GO-102021-549B, January 2002.
[7] "Handbook of Products," Air BP, 2000.
[8] C. Vogt et al., The Renaissance of the Sabatier Reaction and Its Applications on Earth and in Space," Nat. Catal. 2, 188 (2019).
[9] J. C. Navarro et al., "Policies and Motivations For the CO2 Valorization Through the Sabatier Reaction Using Structured Catalysts. A Review of the Most Recent Advances," Catalysts 8, 578 (2018).