Rajan Wilcox
Rocket propellant is used as a reaction mass ejected from a rocket engine to produce thrust. Modern rockets used for space travel primarily use a type of rocket engine called a liquid-propellant rocket engine, which requires an oxidizer to combust. The oxidizer and fuel are kept in two separate tanks within the rocket's main structure, with the oxidizer above the fuel. This arrangement ensures that there is enough oxidizer available to last the entire mission and allows for the adjustment of the oxidizer-to-fuel ratio during flight to maximize system performance.
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What You'll Learn
Oxidizer and fuel are kept in separate tanks
The oxidizer and fuel are kept in separate tanks in a rocket due to the nature of the oxidizer, which is typically liquid oxygen. This liquid oxygen must be kept at extremely low temperatures of -183° Celsius (-297° Fahrenheit) to be stored in a rocket's oxidizer tank. The oxidizer tank can also house the helium tanks used to pressurize a rocket's propellant tanks.
The oxidizer and fuel are combined in the combustion chamber under high pressure during the launch, providing the necessary thrust for the rocket to propel forward. This process is facilitated by turbopumps, which pump the fuel and oxidizer from their respective tanks into the combustion chamber.
Keeping the oxidizer and fuel separate until combustion is essential because the oxidizer, in this case, liquid oxygen, needs to be kept at cryogenic temperatures. This is achieved by turning gaseous oxygen into a liquid state through cooling, which allows for more efficient storage and a greater amount of oxygen to be carried, ensuring sufficient supply for the entire mission.
Additionally, the separation of oxidizer and fuel allows for dynamic mixture ratios during launch. By adjusting the oxidizer-to-fuel ratio, engineers can maximize overall system performance. For instance, during lift-off, thrust is prioritized over specific impulse, so a careful adjustment of the ratio can allow for higher thrust levels.
Furthermore, the use of liquid propellants, including liquid oxygen as the oxidizer, offers advantages over solid propellants. Liquid-fueled rockets have higher specific impulse and are capable of being throttled, shut down, and restarted. The combustion chamber in liquid-fueled rockets can withstand high combustion pressures and temperatures, and cooling can be achieved regeneratively with the liquid propellant.
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Oxidizer is stored as a liquid at extremely low temperatures
Oxidizers, such as liquid oxygen, dinitrogen tetroxide, and hydrogen peroxide, are commonly used in rocket tanks and are stored as liquids to maximize their density and minimize the bulk of the tanks. Liquid oxygen, in particular, is a popular choice due to its high density compared to liquid hydrogen and kerosene, which means it requires smaller tanks. However, to maintain its liquid state, it must be stored at extremely low temperatures, specifically -253 °C for liquid hydrogen. This cryogenic storage presents challenges as it requires heavy insulation and makes handling more hazardous. The low temperature also increases the brittleness of the tank materials, similar to how the Titanic's steel hull became brittle at -2 °C.
To address the embrittlement issue, the SpaceX Starship utilizes a specific type of stainless steel that maintains ductility even at low temperatures. While liquid hydrogen requires cryogenic temperatures and has a low density, it offers high performance due to its low molecular weight. This makes it suitable for upper-stage use, where high impulse is prioritized over thrust-to-weight ratios. In contrast, rocket-grade kerosene, or RP-1, burned with liquid oxygen is more convenient as it can be stored at room temperature and has a higher density, reducing tank weight.
The choice between liquid hydrogen and RP-1/LOX combinations depends on the specific requirements of the rocket application. For instance, RP-1/LOX is commonly used in launch vehicles due to its higher density and ease of storage. The oxidizer-to-fuel ratio can also be adjusted throughout a flight to maximize performance. During lift-off, when thrust is more critical, tuning the ratio can enable higher thrust levels. Once the rocket is airborne, the ratio can be adjusted for higher efficiency.
While liquid oxidizers offer advantages, they also come with challenges. Storable oxidizers like nitric acid and nitrogen tetroxide are highly toxic and reactive. Additionally, the use of liquid hydrogen as fuel presents difficulties in terms of storage temperature, density, and design complexity, impacting the overall performance of the vehicle. Despite these challenges, liquid propellants, including liquid oxidizers, remain popular in orbital launch vehicles due to their high performance and the potential for using local resources for propellant production in future planetary missions.
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Oxidizer and fuel are pumped under high pressure into a combustion chamber
The majority of modern rockets used for space travel are liquid-propellant rocket engines. These engines require both an oxidizing agent and a reducing agent (fuel) to be present in the mixture for combustion to occur. The oxidizer and fuel are kept in two separate large pressurised tanks within the rocket's main structure. During a rocket launch, the fuel and oxidizer are pumped under high pressure from their respective tanks into a combustion chamber. This process is facilitated by turbopumps.
The liquid propellant and oxygen are carefully managed to ensure sufficient oxygen availability for the entire mission. To achieve this, the oxygen is cooled down to an extremely low temperature of -183° Celsius (‑297° Fahrenheit), transforming it into a liquid state that can be stored in the rocket’s oxidizer tank. The liquid oxidizer tank also houses the helium tanks used to pressurise the rocket’s propellant tanks.
The oxidizer and fuel are pumped under high pressure into the combustion chamber, where they mix and ignite, producing combustion gases. These gases are expelled at high velocity through a nozzle, creating thrust. The thrust produced is directly related to the mass flow rate of the propellants and their exhaust velocity, also known as specific impulse.
The performance of rocket engines is optimised by adjusting the oxidizer-to-fuel ratio, which can be done dynamically during launch. For instance, during lift-off, thrust is prioritised over specific impulse, so the O/F ratio is adjusted to maximise thrust. Once the rocket has cleared the launchpad, the engine O/F ratio can be fine-tuned for higher efficiency.
Additionally, the majority of rocket engines are designed to run fuel-rich, as fuel-rich combustion products are less chemically reactive (corrosive) than oxidizer-rich combustion products. This design consideration helps to ensure the longevity and reliability of the rocket engine.
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Oxidizer-to-fuel ratio can be adjusted for efficiency
The oxidizer-to-fuel ratio in rocket tanks can be adjusted for efficiency and to maximise overall system performance. This adjustment can be made throughout the flight. For instance, during lift-off, thrust is more valuable than specific impulse, and tuning the oxidizer-to-fuel ratio may allow for higher thrust levels. Once the rocket is away from the launchpad, the ratio can be adjusted for higher efficiency.
The combustion rate of the fuel is largely determined by the oxidizer flux and exposed fuel surface area. This combustion rate is typically insufficient for high-power operations such as boost stages unless the surface area or oxidizer flux is increased. However, increasing the oxidizer flux too much can lead to flooding and loss of flame-holding, which locally extinguishes combustion.
The choice of propellant also affects the exhaust velocity and specific impulse (the total energy delivered per unit of propellant mass consumed). Solid-fuel rockets have a lower specific impulse than liquid-fuel rockets, which can be throttled, shut down, and restarted. Liquid-fuel rockets are used in most orbital launch vehicles due to their higher specific impulse and the ability to throttle, shut down, and restart.
Liquid propellants also offer the advantage of high-performance oxidizers, such as liquid oxygen, dinitrogen tetroxide, and hydrogen peroxide, which have better specific impulses than the ammonium perchlorate used in solid rockets. The optimum oxidizer-to-fuel mass ratio varies depending on the exact engine design.
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Oxidizer and fuel are needed for combustion
Oxidizers and fuels are required for combustion, a high-temperature exothermic redox chemical reaction that produces oxidized, often gaseous products. In the context of rocket fuel, an oxidizer is a substance that oxidizes another substance, initiating or promoting combustion in other materials. It is a crucial ingredient of propellants that releases oxygen to enable the combustion of a fuel.
The oxidizer and fuel must be in the proper proportion for combustion to occur. The oxygen balance, or the percentage excess or deficiency of oxygen present in a compound for complete oxidation, is an important aspect for an oxidizer. Incomplete combustion occurs when there is not enough oxygen to allow the fuel to react completely, producing carbon and carbon monoxide instead of carbon dioxide.
The oxidizer and fuel molecules must contain bonds with small bond energies to maximize energy output. This ensures that the combustion temperature is as high as possible. The heat of formation depends on the bond energy between atoms in various compounds.
Additionally, the type of oxidizer used is important. Perchlorates and nitrates, for example, are better oxidizers than nitrites and chlorates. These oxidizers provide the oxygen required for the combustion of the fuel binder, releasing the maximum possible energy.
In summary, oxidizers and fuels are necessary for combustion, and the specific proportions and types of oxidizers and fuels used are critical for maximizing energy output and ensuring complete combustion.
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Frequently asked questions
All rocket fuels require an oxidizer to ignite and combust. As a rocket travels higher, there is less oxygen available to support engine combustion. Therefore, rockets need to carry oxidizers to provide enough oxygen for fuel to combust internally.
The oxidizer, stored in the oxidizer tank, provides oxygen to combine with the fuel during combustion. This combustion produces high-pressure gases that accelerate the rocket by expelling mass rearward at high velocity.
Having the oxidizer above the fuel ensures that the oxidizer, which is crucial for combustion, is readily available to mix with the fuel. This arrangement also helps maintain the desired oxidizer-to-fuel ratio, which is critical for optimizing thrust and engine cooling during different stages of flight.
