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

Propellant

A propellant is a material that produces pressurized gas that:* can be directed through a nozzle, thereby producing thrust ;...

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

Fuel tank

A fuel tank is 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 propelled or released into an engine...

 in the case of many aircraft

Aircraft

An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

.

In rocket

Rocket

A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...

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

Rocket propellant tanks

Rocket propellant tanks are pressure vessel

Pressure vessel

A pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.The pressure differential is dangerous and many fatal accidents have occurred in the history of their development and operation. Consequently, their design,...

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

Aluminium alloy

Aluminium alloys are alloys in which aluminium is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silicon and zinc. There are two principal classifications, namely casting alloys and wrought alloys, both of which are further subdivided into the categories...

s, steels or carbon fibre wound tanks.

These kinds of tanks are usually constructed using monocoque

Monocoque

Monocoque is a construction technique that supports structural load by using an object's external skin, as opposed to using an internal frame or truss that is then covered with a non-load-bearing skin or coachwork...

 construction techniques. Balloon tank

Balloon tank

A balloon tank is a style of fuel tank used in the Atlas ICBM and Centaur upper stage that does not use an internal framework, but instead relies on a constant internal pressurization to keep its shape....

s are the most extreme of these, they are held rigid only by internal pressurisation, but are extremely lightweight.