RP-1
Developed in the 1950s, RP-1 is outwardly similar to other kerosene-based fuels like Jet A and JP-8 used in turbine engines but is manufactured to stricter standards.
While RP-1 is widely used globally, the primary rocket kerosene formulations in Russia and other former Soviet countries are RG-1 and T-1, which have slightly higher densities.
RP-1 is a fuel in the first-stage boosters of the Electron, Soyuz, Zenit, Delta I-III, Atlas, Falcon, Antares, and Tronador II rockets.
[2][needs update] During and immediately after World War II, alcohols (primarily ethanol, occasionally methanol) were commonly used as fuels for large liquid-fueled rockets.
Their high heat of vaporization kept regeneratively-cooled engines from melting, especially considering that alcohols would typically contain several percent water.
The cycle rapidly escalates (i.e., thermal runaway) until an engine wall rupture or other mechanical failure occurs, and it persists even when the entire coolant flow consists of kerosene.
These include tighter density and volatility ranges, along with significantly lower sulfur, olefin, and aromatic content.
Unsaturated compounds (alkenes, alkynes, and aromatics) are also held to low levels, as they tend to polymerize at high temperatures and long periods of storage.
Just as cyclic and branched molecules improve octane rating in petrol, they also significantly increase thermal stability at high temperatures.
In contrast, the main applications of kerosene (aviation, heating, and lighting), are much less concerned with thermal breakdown and therefore do not require stringent optimisation of their isomers.
Any petroleum can produce RP-1 with enough refining, though real-world rocket-grade kerosene is sourced from a small number of oil fields with high-quality base stock, or it can be artificially synthesized.
This, coupled with the relatively small demand in a niche market compared to other petroleum users, drives RP-1's high price.
While the Soviets would eventually abandon chilling their kerosene, decades later SpaceX would revisit the idea for their Falcon 9 rocket.
As such, it can be beneficial to use less energy overall in exchange for lower-molecular-mass exhaust, meaning that chemical rocket engines achieve their peak efficiency at non-stoichiometric ratios.
As such, methalox has made a resurgence in popularity in 21st century rockets, at the expense of kerolox (better efficiency) and hydrolox (better handling).
Rocket engines have cycle lifetimes measured in minutes or even seconds, preventing truly heavy deposits.
Recent heavy-hydrocarbon engines have modified components and new operating cycles, in attempts to better manage leftover fuel, achieve a more-gradual cooldown, or both.
The breakdown products of both molecules are also gases, with fewer problems due to phase separation, and much less likelihood of polymerization and deposition.
However, in flight the kerosene tank needs a separate pressurization system to replace fuel volume as it drains.
A few highly volatile propellant designs do not even need the gas loop; some of the liquid automatically vaporizes to fill its own container.
While the material price of such a highly refined hydrocarbon is still less than many other rocket propellants, the number of RP-1 suppliers is limited.
While superior to RP-1, it was never adopted for use – its formulation was not finished before development of Atlas and Titan I (designed around RP-1) leading to RP-1 becoming the standard hydrocarbon rocket fuel.
