How Rocket Fuel Tanks Stay Pressurized

Rocket fuel tanks are pressurised to maintain the tank's structural integrity and to ensure an uninterrupted and consistent propellant flow. There are two primary methods for pressurising a propellant tank: exogenous pressurisation and autogenous pressurisation. Exogenous pressurisation involves using an inert gas like helium or nitrogen, while autogenous pressurisation uses a small amount of the propellant itself, which is heated until it turns into gas and then fed back into the tank.

Maintaining Structural Integrity

Rocket propellant tanks need to be light enough to keep the launch vehicle's weight down, yet strong enough to maintain their structural integrity and remain rigid. This is achieved by slightly overpressurizing the tank. The thin walls of propellant tanks are surprisingly thin, especially considering their size and the amount of liquid they carry. For example, the walls of the external tank of the Space Shuttle were only about 2.5 mm thick.

To maintain their structural integrity and withstand the dynamic forces generated during launch, propellant tanks are slightly overpressurized. This strengthens the tank and makes it more rigid, allowing it to withstand the pressures of the first-stage boosters pushing from behind and the air pressure pushing against the front of the vehicle at supersonic speeds. Without this additional support, the forces during launch would cause the tanks to collapse in on themselves (a process known as buckling).

In some cases, the walls of propellant tanks are so thin that they will buckle and collapse without external support. These types of tanks are known as balloon tanks. An example of this occurred in 1963 when an Atlas-Agena D rocket collapsed on the launchpad due to a loss of pressure in the first-stage oxygen tank.

To maintain rigidity and structural integrity, it is crucial to keep the propellant tanks pressurized throughout transport, on the launchpad, and after launch. This is achieved through various methods, including exogenous and autogenous pressurization, which involve using inert gases or self-generated gaseous propellants, respectively.

By pressurizing the propellant tanks, the structural integrity of the rocket is ensured during launch, and proper propellant flow to the engine is maintained. This complex process is crucial for the optimal performance of the vehicle.

Ensuring Proper Propellant Flow

Maintaining proper propellant flow is a critical aspect of rocket engine design. To achieve the necessary thrust, the liquid-fuelled rocket engine must inject propellants at high velocities and extreme pressures into the combustion chamber. This rapid fuel depletion creates a vacuum that compromises the structural integrity of the tanks and disrupts the propellant flow.

To address this challenge, maintaining adequate pressure within the propellant tanks is essential. The pressure must exceed the combustion chamber pressure to ensure a consistent and uninterrupted propellant flow. This is achieved by pressurizing the propellant tanks, either through exogenous or autogenous methods.

Exogenous pressurization involves using an inert gas, typically helium, stored in separate smaller pressure vessels. During propellant depletion, helium is released into the propellant tanks to maintain the required pressure levels. This method ensures that the void left by the pumped-out propellants is filled, preventing a vacuum from forming.

On the other hand, autogenous pressurization employs a small amount of the propellant itself. In this process, a portion of the cryogenic propellant is heated until it evaporates, forming a gas that is then fed back into the propellant tank. This technique helps maintain the required pressure to feed the rocket's engines.

It is worth noting that both methods have their advantages and considerations. Autogenous pressurization, for instance, is advantageous for long-term spaceflight and interplanetary missions as it eliminates the need for inert gases, reducing weight and increasing payload capacity. However, it is prone to ullage collapse if the propellant sloshes during spacecraft maneuvers, leading to a sudden loss of pressure.

In summary, ensuring proper propellant flow in rocket engines relies on maintaining adequate pressure within the propellant tanks. This is achieved through pressurization methods that fill the void left by depleted propellants, preventing vacuum-induced disruptions to the propellant flow.

Exogenous Pressurization

The inert gas is released into the propellant tanks through feed lines when needed to maintain the required pressure levels. It is crucial to keep the pressurizing gas separate from the propellants, and mechanisms like a diaphragm, bladder, or bellows are used to achieve this. Exogenous pressurization offers a simpler design compared to pump-fed engines, as it does not require turbopumps.

The SpaceX Falcon 9 rocket utilizes exogenous pressurization, with helium stored in dark-colored cylindrical tanks situated within the liquid oxygen tank. This technique was also employed as early as the 1960s in the Saturn V rocket, where helium tanks kept the RP-1 tanks pressurized and were stored in LO2 tanks.

Autogenous Pressurization

The external tank of the Space Shuttle used autogenous pressurization, with liquid oxygen and hydrogen warmed by the shuttle's main engines and turned into gas, which was then fed back into their respective tanks. This method was also used in the Titan booster by 1968 and has been tested with the RL10 engine.

However, a major risk of autogenous pressurization is that it is prone to ullage collapse if the propellant sloshes. If the ullage gas mixes with the liquid propellant, it will be cooled and condense, causing a sudden loss of pressure. Therefore, this method is better suited for booster engines operating under constant acceleration in a single direction, rather than multiple engine burns separated by zero-g maneuvers.

Avoiding Excessive Cooling

When dealing with pressure-fed rocket engines, it is crucial to avoid excessive cooling of the pressurizing gas due to adiabatic expansion, especially during long burns. While cold helium won't liquify, it could negatively impact the overall performance of the rocket by freezing the propellant, decreasing tank pressures, or damaging components not designed for low temperatures.

To prevent these issues, engineers must take several factors into consideration. Firstly, they can introduce a warm pressurizing gas to compensate for the loss of liquid volume as the tank empties. This helps maintain the temperature and ensures that the propellant remains in its intended state.

Additionally, the choice of pressurizing gas is crucial. Gases like helium or nitrogen are commonly used, but they can cause challenges during storage due to their tendency to undergo adiabatic compression, which increases the temperature within the tank. This issue can be mitigated by using a separate gas supply, typically helium, stored in smaller, higher-pressure vessels like Composite Overwrapped Pressure Vessels (COPVs). These vessels can withstand extremely high pressures, preventing excessive temperature increases in the main fuel tanks.

It is also important to note that the use of liquid helium in pressure-fed rockets is generally avoided due to the associated complexities and challenges, such as the need for a heavy dewar as a storage tank and the occurrence of helium boil-off, which can lead to significant losses.

By carefully managing the pressurizing gas, introducing warm gas to compensate for volume loss, and selecting appropriate storage methods and gases, engineers can effectively avoid excessive cooling in rocket engine fuel tanks, ensuring optimal performance and minimizing potential issues during launch.

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