The History of Rocket Fuel

In the world of rocketry and space exploration, we often marvel at the incredible power and precision of rocket engines without understanding the origins of the propellants that drive them.

In this article, we will take a deep dive into the chemistry and processes behind the creation of rocket propellants.

From ethanol to cryogenic fuels like liquid oxygen, liquid hydrogen, and hydrazine, we’ll explore the fascinating world of rocket propellant production.

The Early Days: Ethanol Rockets

The history of rocket propulsion dates back to wartime Germany when the development of large rockets, such as the A4 (later known as the V2), relied on ethanol as the primary fuel.

Ethanol is a product of fermentation, where biological processes convert feedstock rich in starches and sugars into alcohol. In the case of rocketry, this alcohol had to be distilled to reach the high purity required for rocket engines.

This process closely resembles how we make beer, wine, and spirits, and it even had the unintended consequence of some of the rocket fuel finding its way into local speakeasies.

Moving on to RP-1: Highly Refined Kerosene

Modern rockets predominantly use RP-1, a highly refined form of kerosene, as their fuel source.

Kerosene is derived from crude oil, which contains a mixture of various hydrocarbon molecules with different carbon atom counts.

To create RP-1, the refining process narrows down the range of carbon atoms, aiming for molecules with approximately 12 carbon atoms.

Additionally, the fuel must be ultra-pure, and free from sulfur and unsaturated molecules, which are prone to polymerization.

A vital step in this refining process is hydrogenation, where hydrogen gas is used to remove sulfur and stabilize the fuel.

Cryogenic Propellants: Liquid Oxygen and Liquid Hydrogen

cyrogenic rocket fuel nasa
Credit: NASA

Liquid oxygen (LOX) is a key component of rocket engines, providing the oxygen needed for combustion in space.

LOX is created by cooling atmospheric oxygen to incredibly low temperatures, around -182°C (-297°F), through a process known as the Dual Thompson effect, which is part of the Hampson-Lin cycle.

This cooling allows oxygen to transition from a gas to a liquid state, making it suitable for rocket engines.

Another cryogenic propellant is liquid hydrogen (LH2), which is vital for high-performance spaceflight. Unlike LOX, LH2 doesn’t follow the Dual Thompson effect.

Instead, a two-stage cooling process is used, involving the initial cooling of hydrogen gas with liquid nitrogen. A subsequent compression and expansion process then produces liquid hydrogen.

However, LH2 poses a unique challenge due to its tendency to evaporate rapidly when stored.

Special catalysts are employed to convert the majority of the hydrogen from its higher-energy ortho state to the lower-energy para-state, allowing for safer storage and use.

The Origin of Hydrogen: Steam Reforming

spacex liquid hydrogen
Credit: SpaceX

The question of where hydrogen comes from in rocket propellants is an interesting one.

While electrolysis of water is an ideal method for producing hydrogen and oxygen, most rockets rely on hydrogen generated from steam reforming of fossil fuels.

In this process, high-temperature, high-pressure conditions are applied to a mixture of methane, steam, and suitable catalysts, resulting in the conversion of these components into hydrogen and carbon dioxide or carbon monoxide.

Although not environmentally friendly, this method remains practical for the relatively small-scale applications of rocketry.

Chemically Synthesized Propellants: Hydrazine and Beyond

Rocket propulsion also relies on chemically synthesized propellants like hydrazine.

Hydrazine, with its two nitrogen and four hydrogen atoms, is produced using the Olin-Raschig process, which starts with ammonia and involves the substitution of hydrogen atoms with chlorine, eventually yielding hydrazine, water, and sodium chloride.

Ammonia, a precursor chemical to many rocket propellants, is produced through the Haber-Bosch process, which combines nitrogen and hydrogen gasses under high pressure and temperature with the aid of catalysts.

Conclusion

Rocket propulsion is a complex and multifaceted field that depends on a range of propellants, each with its unique origins and production processes.

From the fermentation of ethanol to the cryogenic cooling of liquid oxygen and the chemical synthesis of hydrazine, understanding the chemistry behind rocket fuels helps us appreciate the extensive work carried out by the chemical industry to make spaceflight possible.

Rocketry is not just about reaching the stars; it’s a testament to human innovation and scientific ingenuity.

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