Rocket Fuel Combustion: Chemistry Behind Spaceflight

Burning rocket fuel is a chemical change. Rocket fuel is burned to produce power and propel vehicles forward. This process is called combustion, a chemical reaction in which a substance rapidly reacts with oxygen and releases heat. In rockets, the burning of fuel produces an exhaust, and the exhausted material is usually expelled as a propellant under pressure through a nozzle. The exhaust may be in the form of a gas, liquid, plasma, or a solid.

Solid rocket fuel

A simple solid rocket motor consists of a casing, nozzle, grain (propellant charge), and igniter. The casing must be designed to withstand the pressure and resulting stresses of the rocket motor and is often made from cardboard, aluminium, steel, or filament-wound graphite epoxy. The nozzle is constructed from heat-resistant carbon-based materials, such as amorphous graphite or reinforced carbon-carbon, and is designed to accelerate the exhaust gas out of the nozzle to produce thrust. The grain is the propellant charge, which burns in a predictable fashion to produce exhaust gases. The igniter starts the combustion process, after which the burning cannot be stopped in simpler solid rocket motors.

The burning rate of solid rocket fuel is influenced by various factors, including combustion chamber pressure, initial temperature of the propellant, velocity of combustion gases flowing parallel to the burning surface, local static pressure, and motor acceleration and spin. The burn rate is critical in the design of solid rocket motors, as it determines the thrust and overall performance of the rocket.

Liquid rocket fuel

Liquid propellants are desirable because they have reasonably high density and their combustion products have high specific impulse (Isp). This allows the volume of the propellant tanks to be relatively low. Liquid rockets can be monopropellant, bipropellant, or tripropellant systems, with monopropellant systems using a single type of propellant, bipropellant systems using two types, and tripropellant systems using three types.

Liquid bipropellant rockets use a liquid fuel and a liquid oxidizer. The fuel is a substance that burns when combined with oxygen, producing gas for propulsion. The oxidizer is an agent that releases oxygen for combination with a fuel. The ratio of oxidizer to fuel is called the mixture ratio.

Bipropellant liquid rockets can be cryogenic rocket engines, where the fuel and oxidizer are gases that have been liquefied at very low temperatures. Cryogenic propellants are liquefied gases stored at very low temperatures, most frequently liquid hydrogen (LH2) as the fuel and liquid oxygen (LOX) as the oxidizer. Hydrogen remains liquid at temperatures of -253°C (-423°F) and oxygen remains in a liquid state at temperatures of -183°C (-297°F). Cryogenic propellants are difficult to store over long periods and are therefore less desirable for military rockets that must be kept launch-ready.

Liquid bipropellant rockets can also be non-cryogenic/storable/hypergolic systems. Hypergolic propellants are fuels and oxidizers that ignite spontaneously on contact with each other and require no ignition source. The easy start and restart capability of hypergols make them ideal for spacecraft maneuvering systems. They are also ideal for military applications because they can be stored at normal temperatures and do not pose the storage problems of cryogenic propellants. However, hypergolic propellants are highly toxic and must be handled with extreme care.

Liquid rocket engines can be tested prior to use, whereas solid rocket motors require rigorous quality management during manufacturing to ensure high reliability. Liquid systems also enable higher specific impulse than solid and hybrid rocket motors and can provide very high tankage efficiency. Liquid rocket engines can usually be reused for several flights, and the flow of propellant can be throttled, allowing for control over the magnitude of thrust throughout the flight. Shutdown and restart capabilities allow for multiple burn cycles throughout a flight and provide an extra level of safety and mission abort capability in emergencies.

Liquid propellants are not without their issues, however. Because the propellant constitutes a very large proportion of the vehicle's mass, the center of mass shifts significantly rearward as the propellant is used, which can lead to a loss of control. Liquid propellants are also subject to slosh, which has frequently led to a loss of vehicle control. They can also suffer from pogo oscillation and vortexing within the tank, particularly towards the end of the burn. Liquid propellants can leak, especially hydrogen, possibly leading to the formation of an explosive mixture. Additionally, turbopumps for liquid propellants are complex to design and can suffer serious failure modes.

Thousands of combinations of fuels and oxidizers have been tried for liquid rocket fuel systems. Some of the more common and practical ones include:

• Liquid oxygen (LOX, O2) and liquid hydrogen (LH2, H2) – Used in the Space Shuttle main engines, Space Launch System core stage, and the BE-3 of Blue Origin's New Shepard, among others.
• Liquid oxygen (LOX) and liquid methane (CH4, liquefied natural gas, LNG) – Used in the Raptor (SpaceX) and BE-4 (Blue Origin) engines.
• Liquid oxygen (LOX) and RP-1 (kerosene) – Used in the Zenit rocket, R-7-derived vehicles including Soyuz, Delta, Saturn I, and Saturn IB first stages, and the Falcon 1 and Falcon 9, among others.
• Liquid oxygen (LOX) and alcohol (ethanol, C2H5OH) – Used in early liquid rockets, like the German (World War II) A4, aka V-2, and Redstone.
• Liquid oxygen (LOX) and gasoline – Used in Robert Goddard's first liquid rocket.
• Liquid oxygen (LOX) and carbon monoxide (CO) – Proposed for a Mars hopper vehicle.

Chemical reactions in rocket fuel

Burning rocket fuel is a chemical change. The combustion of rocket fuel involves a chemical reaction that releases energy, which makes things go. This energy release is accompanied by the formation of by-products.

To understand the chemical reactions in rocket fuel, let's delve into the components of rocket fuel and the types of rocket fuel available.

Components of Rocket Fuel:

Rocket fuel consists of two primary components: fuel and an oxidizer. The fuel is the substance that burns, while the oxidizer facilitates the burning process. In simple terms, the oxidizer provides the oxygen necessary for combustion, even at high altitudes where oxygen levels are low.

Types of Rocket Fuel:

Rocket fuels can be broadly categorized into solid, liquid, gas, and hybrid fuels. Each type has unique characteristics and is suitable for specific applications.

Solid Rocket Fuel:

Solid rocket fuel is the oldest form of rocket fuel, dating back to ancient Chinese fireworks. It is a mixture of fuel and oxidizer combined in a solid form. For example, aluminum powder can serve as the fuel, while ammonium perchlorate, a powerful oxidizer, is used in the boosters. When ignited, this mixture produces aluminum oxide, aluminum chloride, water vapour, and nitrogen gas, and a significant amount of energy. Solid rocket fuels are simpler to handle and store than liquid fuels, making them ideal for military and space applications.

Liquid Rocket Fuel:

Liquid-fuelled rockets use propellants in a liquid state. An example of a liquid fuel is liquid hydrogen (LH2), while liquid oxygen (LOX) serves as the oxidizer. Liquid-fuelled rockets have the advantage of being throttleable, meaning they can be shut down and restarted. They also offer higher specific impulse, which measures the efficiency of the rocket fuel. However, they require complex valves, seals, and turbopumps, making them more costly.

Gas Rocket Fuel:

Gas fuel rockets utilise propellant in the gas phase. This type of rocket fuel is less common and may include gases like oxygen or nitrogen collected from the upper atmosphere.

Hybrid Rocket Fuel:

Hybrid rocket fuels combine a solid fuel with a liquid or gaseous oxidizer. This combination offers the advantages of both liquid and solid fuels. Hybrid rockets can be safer than solid rockets as they may use less toxic oxidizers. However, they face challenges in efficiently mixing the propellants during combustion, which can limit their performance.

Now, let's delve into the specific chemical reactions that occur during the combustion of rocket fuel:

Liquid Hydrogen and Liquid Oxygen Reaction:

In this reaction, liquid hydrogen (LH2) combines with liquid oxygen (LOX) to form water (H2O) and release a significant amount of energy. This reaction is often referred to as the "green" reaction due to its environmentally friendly nature.

Aluminum and Ammonium Perchlorate Reaction:

In solid rocket boosters, aluminum powder reacts with ammonium perchlorate, a powerful oxidizer. This reaction produces aluminum oxide, aluminum chloride, water vapour, and nitrogen gas, and a substantial amount of energy. The high temperatures generated by this reaction heat the inside of the solid rocket boosters, causing the rapid expansion of water vapour and nitrogen, which creates thrust and lifts the rocket.

In summary, chemical reactions in rocket fuel involve the combustion of fuel and oxidizer, resulting in the release of energy and the formation of various by-products. The specific chemical reactions depend on the type of rocket fuel used, such as solid, liquid, or hybrid fuels. These reactions are essential for generating the thrust necessary to propel rockets forward, following Newton's Third Law of motion.

Oxidising agents

Burning rocket fuel is a chemical change. In chemical rockets, both an oxidising agent and a reducing agent (fuel) must be present in the mixture.

Ammonium perchlorate is a common oxidising agent used in solid rocket fuel. It is a powerful oxidiser and highly explosive. When combined with aluminium powder, it produces aluminium oxide, aluminium chloride, water vapour, nitrogen gas, and a lot of energy. This reaction is used in the Space Launch System's solid rocket boosters.

Liquid oxygen (LOX) is another important oxidising agent used in liquid-fuelled rockets. It is often paired with liquid hydrogen to create a high-performance, high-energy propellant system. This combination is used in the Space Launch System's core and upper stages.

Other oxidising agents include nitrogen tetroxide (N2O4), hydrogen peroxide (H2O2), and liquid ozone (O3). These agents have their own advantages and disadvantages in terms of boiling point, density, heating capacity, and reactivity.

Additionally, there are ongoing experiments with oxidising agents such as O4 (disuperoxide) and ionic liquids to improve rocket performance and address safety and environmental concerns.

The choice of oxidising agent depends on the specific requirements of the rocket, such as thrust, specific impulse, burn time, and environmental impact.

The exhaust produced

In the case of a hydrogen-carbon-based fuel, like gasoline, the exhaust includes water (from the combination of hydrogen and oxygen) and carbon dioxide (from the combination of carbon and oxygen). However, the exhaust can also contain chemical by-products from the oxidizer alone. For example, if gasoline is burned in air, which contains nitrogen, the exhaust may include nitrous oxides.

The temperature of the exhaust is high due to the heat generated during combustion. This high temperature ensures that the exhaust usually occurs as a gas, although liquid or solid exhaust products can also be formed. Soot, for instance, is a type of solid exhaust that occurs in certain combustion processes.

Additionally, the type of exhaust produced can vary depending on the type of rocket fuel used. Solid rocket fuels, for instance, typically result in higher thrust and shorter burn times compared to liquid fuels. Moreover, the exhaust composition can be adjusted by modifying the mixture ratio of the fuel and oxidizer during launch to optimize either thrust or efficiency, depending on the stage of the flight.

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