Liquid

In the aerospace context, liquid refers to a substance that flows and takes the shape of its container. It is used as a propellant, coolant, or lubricant in many aerospace systems and vehicles.

Examples of liquids used in aerospace include:

• Rocket propellants: Rockets use liquid fuels and oxidizers to generate the high-speed thrust needed to lift off from the Earth's surface. Examples include liquid hydrogen and liquid oxygen used in the Space Shuttle's main engines, or kerosene and liquid oxygen used in the first stage of the SpaceX Falcon 9 rocket.
• Coolants: Liquid coolants are used to remove heat from various systems in an aircraft or spacecraft. For example, liquid hydrogen is used as a coolant in the cryogenic fuel tanks of the Space Shuttle and the International Space Station.
• Lubricants: Liquids such as oil are used to lubricate moving parts in aircraft and spacecraft engines and other mechanical systems, reducing friction and wear.

• Hydraulic fluids: Many aerospace systems, such as landing gear and flight control surfaces, are operated using hydraulic systems. Liquids such as hydraulic oil and Skydrol are commonly used as hydraulic fluids.

• Cleaning agents: Aerospace equipment requires regular cleaning and maintenance to ensure that it continues to function properly. Liquids such as isopropyl alcohol and acetone are commonly used as cleaning agents in aerospace applications.

Additionally, liquid hydrogen and liquid oxygen are also used as rocket propellant in combination to make a rocket engine work, this combination is known as cryogenic propellant. These cryogenic liquids are stored in the rocket at very low temperatures and require specialized insulation and handling equipment.

One of the most common liquids used in the aerospace industry is liquid hydrogen (LH2). LH2 is used as a fuel in rockets, and it has the highest specific impulse of any known chemical propellant, making it a highly efficient option for space travel. However, it requires extremely low temperatures to remain in a liquid state, which presents significant engineering challenges.

Another commonly used liquid in the aerospace industry is liquid oxygen (LOX), which is often used in combination with LH2 for rocket propulsion. LOX is also used in aircraft as a component of the air conditioning system and in life support systems.

Overall, the use of liquids is essential in the aerospace industry to achieve the high levels of performance and efficiency required for space travel and aircraft operations. The development and use of specialized liquids continues to be an important area of research and innovation in aerospace engineering.

Similar things to liquids in the aerospace context include gases, such as the various types of propellant gases used in rocket engines, and solid materials, such as the various types of rocket solid propellants used in space launch vehicles. Additionally, the use of supercritical fluids, which have properties between those of a liquid and a gas, is an area of active research in the aerospace industry, particularly for use in advanced propulsion systems.