Making Rocket Fuel: RP-1

Ethan Wong

August 9, 2024

One of the most well-known rockets in history is the German V-2 rocket which was developed during WWII. The V-2 had a liquid propulsion system of ethyl alcohol and liquid oxygen (LOX). The Germans even utilized regenerative cooling with the ethyl alcohol, in which the fuel was used to cool the engine by being passed through coolant channels surrounding the engine before being transferred into the combustion chamber. However, later fuels were used with LOX, such as RP-1 kerosene which was used in the Saturn V’s F-1 engines. But why is the kerosene fuel called RP-1 and how did they modify it?

Scientists decided to utilize kerosene as fuel because of its makeup of hydrocarbons, which are compounds that only have carbon and hydrogen atoms. Hydrocarbons are super efficient in rocket propulsion because they lack an oxygen atom; because of this, hydrocarbons were free of excess water molecules (H2O) that would’ve lowered the heat produced in the combustion, while also adding molecular weight to the overall reaction. 

Despite this, kerosene exhibited early problems when tested as a fuel in rocket engines. Under intense heating, the kerosene underwent thermal decomposition and polymerization. In other words, the heat caused the hydrocarbons in the kerosene to break down into smaller molecules (a process known as thermal decomposition), and some hydrocarbon molecules even went through pyrolysis, which is where their bonds broke and they were left with unshared electron pairs. These unshared electron pairs encouraged the formation of new molecules through covalent bonds, creating solid substances (a process known as polymerization). This process caused solid substances to create a blockage in the coolant channels of the engine, preventing the kerosene from cooling the hot engine walls. As a result, kerosene needed to be modified to be stronger with extreme temperatures. 

To solve the issue of having blockage in the coolant pipes of the rocket engine, scientists created a modified variant of kerosene known as RP-1, in which the hydrocarbons of kerosene were engineered to be more tolerant to heat, causing them to be less susceptible to thermal decomposition. Scientists did this by keeping various hydrocarbon structures (isomers) that are more heat-resistant and removing the ones that caused polymerization. Isomers are various structures for a compound that can have different rearrangements without changes in number/type of atoms. For instance, scientists kept ideal isomers of hydrocarbons that had carbon rings rather than a chain of carbon atoms (strength of structure = higher boiling points). The best isomer was polycyclics, which are essentially carbon rings that are meshed together to create stronger bonds. 

But as stated above, they also removed bad hydrocarbon structures such as alkenes (carbon atoms in double-bonded chains and aromatic structures (ring of carbon atoms that alternate in being single bonded or double bonded) because they were more likely to influence polymerization. For instance, alkene hydrocarbons’ double-bonded carbons would chain together into a giant molecule, which is not ideal. 

In addition to altering the presence of hydrocarbon isomers, the scientists made other slight changes to “clean” the new RP-1 fuel. For instance, they removed sulfur-containing compounds which can react with metals of the engine and also cause polymerization. However, it is still important to keep in mind that hydrogen-based fuels are more efficient. In a typical combustion reaction, water (H2O) and carbon dioxide (CO2) are produced. But with hydrogen, there’s no added CO2 which increases efficiency.