A propellant tank is a container which is part of a vehicle, where

propellant A propellant (or propellent) is a mass that is expelled or expanded in such a way as to create a thrust or other motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, the ...

is stored prior to use. Propellant tanks vary in construction, and may be a

fuel tank A fuel tank (also called a petrol tank or gas tank) is a safe container for flammable fluids. Though any storage tank for fuel may be so called, the term is typically applied to part of an engine system in which the fuel is stored and propelle ...

in the case of many

aircraft An aircraft is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravity by using either Buoyancy, static lift or by using the Lift (force), dynamic lift of an airfoil, or in ...

. In

rocket A rocket (from it, rocchetto, , bobbin/spool) is a vehicle that uses jet propulsion to accelerate without using the surrounding air. A rocket engine produces thrust by reaction to exhaust expelled at high speed. Rocket engines work entir ...

vehicles, propellant tanks are fairly sophisticated since weight is on a premium.

Rocket propellant tanks

Rocket propellant tanks are

pressure vessel A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure. Construction methods and materials may be chosen to suit the pressure application, and will depend on the size o ...

s where liquid fuels are stored prior to use. They have to store the propellant, while minimizing slosh and particularly when the tank is nearly empty, minimizing vortexing. Rocket propellant tanks are often constructed of materials such as

aluminium alloy An aluminium alloy (or aluminum alloy; see spelling differences) is an alloy in which aluminium (Al) is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silicon, tin, nickel and zinc. There are two pr ...

s, steels or carbon fibre wound tanks. These kinds of tanks are usually constructed using

monocoque Monocoque ( ), also called structural skin, is a structural system in which loads are supported by an object's external skin, in a manner similar to an egg shell. The word ''monocoque'' is a French term for "single shell". First used for boats, ...

construction techniques.

Balloon tank A balloon tank is a style of propellant tank used in the SM-65 Atlas intercontinental ballistic missile (ICBM) and Centaur upper stage that does not use an internal framework, but instead relies on a positive internal pressurization to keep i ...

s are the most extreme of these, they are held rigid only by internal pressurization, but are extremely lightweight. Rocket propellant tanks are of many shapes but the optimum shape of a tank is spherical, because for given volume it results in a tank with least weight. Normally, propellant in the tank is stored at a pressure of about 1-4 bar, if the system uses turbopump to deliver high pressure to the combustion chamber. This method reduces the wall thickness and hence the weight of the tank. If the propellant in the tank is stored at very high pressure, then the wall thickness of the tank is increased and hence the weight of the tank. Aircraft fuel system components Fuel containers {{aircraft-component-stub