In 2016, Falcon 9 rocket unexpectedly exploded on the launchpad due to a breach in the helium system used to pressurize the liquid propellant tanks, raising the question of why rocket propellant tanks need to be pressurized.
Rocket propellant tanks are primarily pressurized to maintain the tank’s structural integrity by keeping it rigid, as well as replacing the void created by propellants being pumped out of the tank at high velocities, ensuring an uninterrupted and consistent propellant flow.
Orbital rockets are highly complex machines that can consist of several million parts. For example, the Saturn V that launched astronauts to the Lunar Surface had approximately three million moving parts.
The number of parts and the level of complexity are not only limited to an orbital launch vehicle’s rocket engine, structure, and navigational system but also to its fuel and oxidizer tanks (collectively known as propellant tanks).
In many cases, they need to be able to store liquid propellants at cryogenic temperatures. They also need to be light enough to help reduce weight yet be strong enough to maintain the vehicle’s structural integrity and withstand the dynamic forces generated during launch.
As the following sections will illustrate, pressurizing rocket propellant tanks is crucial for any liquid-fueled rocket for a number of reasons, and there is also more than one way of pressurizing fuel and oxidizer tanks.
Why Rocket Fuel Tanks Are Pressurized
Rocket propellants make up more than 85% of an orbital launch vehicle’s total mass. As a result, the fuel and oxidizer tanks also occupy the largest part of the vehicle’s structure.
Pressurizing these large vessels primarily serves two purposes:
- Maintain The Structural Integrity Of Propellant Tanks
- Maintain Proper Propellant Flow
1) Maintain The Structural Integrity Of Propellant Tanks
As mentioned during the introduction, rocket propellant tanks need to be light enough to keep the launch vehicle’s weight down yet be strong enough to maintain their structural integrity and remain rigid. This is done by slightly overpressurizing the tank.
To keep weight down, propellant tank walls are surprisingly thin, especially considering their size and the amount of liquid they carry. For example, the walls of the 46.9 meters (153.8 feet) long external tank of the Space Shuttle were only about 2.5 mm (0.098 inches) thick.
And even though most propellant tanks are structurally reinforced to support their own weight and stay fairly rigid, the forces they are subjected to during a launch will cause them to collapse in on themselves (a process known as buckling) without additional support.
By slightly overpressurizing it, a propellant tank is significantly strengthened and made more rigid, which allows 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.
In some cases, the walls of propellant tanks are so thin they will literally buckle and collapse in on themselves if they are not supported by external structures. These types of tanks are known as balloon tanks.
They were used in early versions of the Atlas rocket and are still used by the Centaur upper stage, used by the Delta IV Heavy, Atlas V, and early versions of the SLS launch system. The walls of its propellant tanks are a mere 0.36 – 0.41 mm (0.014 – 0.016 inches) thick.
The only way to allow these tanks to maintain their structural integrity and remain rigid during transport, on the launchpad, and after launch is to overpressurize them and keep them pressurized throughout this period.
(Although rocket propellant tanks technically form part of a rocket’s propulsion system, it also a crucial part of the structural system, as illustrated in this article. Learn more about the four primary systems an orbital rocket consists of and their characteristics in this article.)
2) Maintain Proper Propellant Flow
In order for a liquid-fueled rocket engine to produce the large amount of thrust needed to accelerate a launch vehicle through our dense atmosphere and into orbit around the Earth, an immense amount of pressure needs to be generated in its combustion chamber.
This is achieved by using turbopumps to inject the propellants at high velocities & extreme pressures into the combustion chamber. For example, the first stage of the Saturn V rocket alone used fuel at a rate of approximately 12 700 kg per second (28 000 pounds/second).
At this rate, both fuel and oxidizer tanks are emptied rapidly, which will create a vacuum, that will not only cause the tanks to lose structural integrity and buckle but also interfere with and severely disrupt the propellant flow.
To prevent this, the tank needs to remain pressurized to fill the void left by the propellants being pumped out, as well as maintain a positive pressure to allow the turbopumps to inject the fuel and oxidizer at the required speeds into the combustion chamber.
Although most liquid-fueled rocket engines use turbopumps to help achieve the right speeds and pressures to achieve the necessary thrust to propel a launch vehicle forward, another type of engine called a pressure-fed engine solely relies on the pressure inside its tanks.
These engines have a much simpler design since they don’t use any turbopumps. Slightly overpressurizing the tanks is critical since they rely solely on the pressure from an inert gas like helium (housed in a separate tank) to push the propellants to the combustion chamber.
(It has to be noted that pressure-fed engines are much less powerful than pump-fed engines due to the absence of turbopumps. As a result, they are primarily used in Reaction Control Systems and Orbital Maneuvering Systems of spacecraft.)
How Rocket Fuel Tanks Are Pressurized
Pressurizing the propellant tanks of a rocket can be done in a number of ways. The type of method and substances used to pressurize them can also depend on the type of propellant used. However, the two primary methods for pressurizing a propellant tank are:
- Exogenous Pressurization
- Autogenous Pressurization
1) Exogenous Pressurization
Exogenous pressurization refers to the pressurization of propellant tanks by using an inert gas like helium or nitrogen (mostly helium), which are contained in separate smaller pressure vessels and released into the propellant tanks to retain the required pressure.
The inert gases are kept in smaller higher-pressure vessels like COPV (Composite Overwrapped Pressure Vessels) tanks that can withstand pressures of up to 60 MPa. These smaller tanks are typically housed within the liquid oxygen tank for several reasons.
The freezing temperatures at which liquid oxygen is stored (below ‑183° Celsius or ‑297° Fahrenheit) allow the helium tanks to be smaller since the lower temperatures make it possible for more helium to fit inside their tanks.
Other advantages of placing the helium tanks in the liquid oxygen tanks include the ability to pressurize them to higher levels, and the cryogenic temperatures also allow the use of thinner walls, which helps to keep the overall weight of the launch vehicle down.
(Placing the cold helium tanks near or in the RP-1 fuel tanks, which are stored at room temperature, can pose a threat and result in freezing the RP-1 fuel and is generally avoided.)
This method of storing helium tanks in liquid oxygen was used as early as the 1960s in the Saturn V rocket, where the helium tanks that kept the RP-1 tanks pressurized were stored in LO2 tanks. Modern launch vehicles like SpaceX’s Falcon 9 rocket still use this technique.
Once pressurization of a tank is needed as propellants are being pumped to the combustion chamber or the pressure drops below accepted levels for other reasons, helium is released through feed lines to keep the pressure inside the propellant tanks at the appropriate levels.
It is crucial to keep the pressurizing gas from mixing with the propellants in the tanks they are pressurizing, and mechanisms like a diaphragm, bladder, or bellows are used to keep the helium and propellant separate under dynamic conditions.
2) Autogenous Pressurization
Autogenous pressurization refers to the pressurization of propellant tanks by using a small amount of the cryogenic propellants themselves by allowing a small amount of it to heat up and evaporate and feeding the gaseous propellant back into the tank to pressurize it.
Typically, a small amount of cryogenic propellant is released from the tank and passed through the engine or a heat exchanger to warm it up until it evaporates and forms a gas, which is fed back into the propellant tank to pressurize it.
The large external propellant tank used by the Space Shuttle, which housed both liquid oxygen (LO2) and liquid hydrogen (LH2) tanks that were used to feed propellant to the orbiter’s main engines during a launch, is a perfect example of autogenous pressurization.
(Rockets use a variety of different propellants, apart from the liquid hydrogen, RP-1, and liquid oxygen already mentioned in this article. Learn more about the different fuels orbital rockets use in this article.)
During the launch, liquid oxygen was flowed from the tank through the external tank & orbiter umbilical into the Space Shuttle Main Engines (SSME). Some of the gaseous oxygen was then relayed back through the external gaseous oxygen umbilical to the liquid oxygen tank to pressurize it.
Similarly, liquid hydrogen was flowed from the tank through the external tank & orbiter liquid hydrogen umbilical into the Space Shuttle Main Engines (SSME). Some of the gaseous hydrogen was then relayed back through the gaseous hydrogen umbilical to the hydrogen tank to pressurize it.
In both cases, the cryogenic propellants were warmed by the shuttle’s main engines, which caused them to warm up and turn into a gas, which was fed back to each tank to maintain adequate pressure.
As this article clearly illustrated, propellant tank pressurization in an orbital rocket is a crucial part of making the vehicle perform optimally. Not only does it ensure the structural integrity of the rocket during launch, but it also helps to maintain proper propellant flow to the engine.
Although there are a number of different ways in which fuel and oxidizer tanks can be pressurized, the two primary methods of pressurizing are known as exogenous pressurization and autogenous pressurization.
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