Aerospace Propulsion
Aerospace propulsion is the technology and science behind generating thrust to move aircraft and spacecraft through air and space. This field has evolved since the early 20th century, starting with powered airplanes that utilized combustion engines and propellers for lift and thrust. Key advancements occurred during and after World War II, leading to the development of more sophisticated propulsion systems, including rockets, which expanded flight capabilities and facilitated space exploration.
Aerospace propulsion systems can be broadly categorized into propellers and jet propulsion, with various subtypes optimized for specific applications. For instance, jet engines, such as turbojets and ramjets, rely on air for combustion, while rocket propulsion ejects propellant independently of the atmosphere, making it essential for space travel. Other innovative propulsion methods include nuclear and solar propulsion, which offer potential for improved efficiency and reduced environmental impact.
Solar sails represent a groundbreaking approach, leveraging solar wind and radiation pressure for propulsion without the need for traditional fuel. As aerospace propulsion technology continues to advance, it holds promise for deeper space exploration and the development of greener alternatives to conventional propellants.
Aerospace Propulsion
FIELDS OF STUDY: Spacecraft Propulsion; Aerospace Engineering; Space Technology
ABSTRACT: Propulsion is the system creating force for movement. Aerospace propulsion encompasses everything involving the movement of aerospace vehicles. This includes aircraft engine design as well as launching an object into outer space and directing travel through space. Aerospace propulsion is based on the laws of motion developed by English physicist Isaac Newton. While different types of propulsion systems exist, each has the same purpose: to create a thrust to move the object. Scientists are developing new technology to create more efficient, ecofriendly propulsion systems.
The Beginnings of Aerospace Propulsion
Propulsion is the way in which a force is created to cause movement. The word "propulsion" is derived from the Latin words pro, which means before or forward, and pellere, which means to drive. There are many forms of propulsion, from biological processes to complex mechanical inventions. Aerospace propulsion involves those systems used to move aircraft and spacecraft.
The birth of modern aerospace propulsion technology dates back to the early twentieth century with the advent of powered airplanes. These early flying devices used engines and one or more propellers to generate thrust and gain lift off. Most were powered by combustion, or burning fuel, although electric motors and other systems were also developed. Propeller technologies were refined over the years, including the use of turbomachinery such as turboprops, and remained in use with certain types of aircraft.
The creation of more powerful propulsion technology accelerated through the 1930s, pushed by the formation of what in 1943 would become the Jet Propulsion Laboratory (JPL) in the United States. It built on the early development of rocket technology by engineer Robert Goddard (1882–1945). JPL applied and adapted rockets to help aircraft achieve liftoff and refined rocket designs. World War II further spurred advances in rocketry in Germany, the Soviet Union, and the United States. The first fully rocket-powered aircraft were tested and expanded the capabilities of flight.
Rocket technology opened up the possibility of space travel. After World War II, the space race broke out between the United States and the Soviet Union as the superpowers competed technologically. The Soviet Union was the first to reach space with the launch of Sputnik in 1957. A year later, the United States launched the unmanned satellite Explorer 1. The same year, the United States formed the National Aeronautics and Space Administration (NASA) to continue its focus on aerospace research. JPL joined NASA in 1958, and both groups continued to work on new propulsion methods to allow further exploration. As of 2015, rockets remained the chief form of spacecraft propulsion technology, but alternative forms have the potential to allow cheaper, more efficient space travel.
How Does Aerospace Propulsion Work?
All propulsion is based on the work of English physicist Sir Isaac Newton (1642–1727), specifically his second and third laws of motion. The third law states that every action has an equal and opposite reaction. This means that a pair of forces (push and pull) exist that act when any two objects interact. The sizes of the two forces are always the same. The direction of the force on the first object is always directly opposite the direction of the force on the second object. If a person or thing pushes on something, it pushes back on the person or thing.
For example, if a person sits in a chair, the person exerts a downward force on the chair, while the chair exerts an upward force on the person. The result from the interaction is two forces: one pushing on the chair and one pushing on the person. The chair and person stay in one place only because of the restrictive force of friction and comparatively powerful force of Earth’s gravity acting on both.
In propulsion, acceleration and mass also play a role. Newton’s second law states that the acceleration of an object is directly proportional to the magnitude of the net force exerted on the object. It is inversely proportional to the object’s mass. A bird flapping its wings exerts a force on the air, and the air exerts a force against the bird. Because the mass of the bird is relatively small, the acceleration produced by the interaction overcomes friction and allows the bird to take off.
These examples can be applied to how aircraft and spacecraft move. Engines power the propulsion system, whether a propeller or jet. This creates a thrust force, typically by accelerating a gas’s or other propellant’s mass. The engine forces the propellant out one end, forcing the craft itself in the opposite direction. The expelled matter and the craft exert equal and opposite forces on each other. If the thrust force is large enough compared to the mass of the craft and surrounding friction, or drag, the craft will move.
Airplanes and launch rockets operate in the atmosphere, and many aerospace propulsion systems, such as propellers, rely on air. They also must overcome the air’s friction to remain in flight. For propulsion in space, where there is no air to push against, rockets are suitable because they push against their own exhaust gases.
Propulsion Systems
Several types of aerospace propulsion systems exist, but each basically has the same purpose. They use different sources and move to propel an object through air or space. The two main categories are propellers and jet propulsion, with subcategories of each providing different advantages. Some spacecraft employ other systems, generally relying on solar power. The major difference between various propulsion systems is the amount of thrust generated. The thrust depends on the mass flow through the engine and the exit velocity of the gas. Some aircraft and spacecraft need high thrust to accelerate quickly, while others need less to fly along at a constant speed. The amount of thrust also dictates fuel efficiency.
A spinning propeller provides propulsion through the air and was the first means of propelling aircraft. Propellers may be powered by combustion or electric engines and may take several forms. A propeller may be powered by turbomachinery to create a turboprop or a propfan. One form of duct propulsion involves a fan propeller surrounded by a duct, which enhances certain operating qualities.
Jet propulsion involves an engine that expels a jet of matter to achieve thrust. Some jets use turbines and duct propulsion to rely on air. The oxygen gained from the air flow is used to burn the fuel and create exhaust propellant. Some examples of duct propulsion jet engines include turbojets and ramjets. Duct propulsion engines give a high thrust-to-weight ratio and a high thrust to the frontal area of an aircraft or spacecraft. This allows longer flight ranges in low altitudes. The turbojet is the most common of type of duct propulsion engine, while the ramjet is used for high-speed flight within the atmosphere.
Rocket propulsion systems are a class of jet propulsion in which the thrust is produced by ejecting a stored propellant without taking in air. Rockets are the most powerful but least efficient type of jet engine. They are used to launch spacecraft into orbit as well as to direct most types of spacecraft. They can be classified in different ways, such as by size, propellant type (liquid, solid, or gas), basic function, vehicle type (aircraft or spacecraft), or energy source (chemical, nuclear, or electric).
Chemical propulsion systems use a chemical reaction to propel an object. They are classified by the type of propellant used: liquids, solids, gases, or a combination of liquids and solids. Bipropellant systems contain two liquids, an oxidizer such as liquid oxygen and a fuel such as kerosene. Monopropellants use a single liquid that contains a mixture of both an oxidizer and a fuel. Solid propellant engines use solids called grain. Gaseous propellant engines use high-pressure gas such as nitrogen or helium. Because of the heaviness of the tanks for gaseous propellants, however, few spacecraft use these. Hybrid propellant engines use both liquid and solid propellants, but they are less common than liquid or solid propellant engines.
Nuclear propulsion engines use a nuclear reactor to generate heat to propel an object. They work much like chemical liquid propellant engines. Nuclear propulsion engines are a type of electric propulsion engine. In electric propulsion engines, energy comes from a separate source than the propellants. (In chemical propulsion systems, the power comes from the propellants themselves.) Several theoretical types of nuclear spacecraft propulsion have been proposed.
Solar propulsion refers to the use of solar energy to propel a craft. The simplest form is an airplane with an electric engine powered by solar panels. However, some spacecraft use solar-generated electricity to power electromagnetic propulsion. These include electrostatic ion propulsion (EIP) and Hall effect drives, which accelerate ions to generate thrust. Other electromagnetic systems, such as the variable specific impulse magnetoplasma rocket (VASIMR), are under development. Solar sails may also be considered a type of solar propulsion.
Solar Sails
The advancement of aerospace propulsion technology not only applies to aeronautics but also extends to other fields. This technology could help the environment in the long term. Scientists have the potential to devise propellants that have less of a negative impact on Earth. They even have the ability to work on new technologies that could eliminate the need for propellants. One of these is the solar sail, which has been in development for many years. It uses the pressure of solar wind and solar radiation to propel a vehicle through space without ejecting any mass.
A solar sail was first successfully used by the Japanese Interplanetary Kite-Craft Accelerated by Radiation of the Sun (IKAROS) satellite that launched in May 2010. The United States launched the NanoSail-D in November 2010, which initially had some issues. However, by January 2011, it had released from its larger satellite and deployed its solar sail. In May 2015, the Planetary Society successfully launched the solar sail satellite LightSail aboard the Atlas V rocket and deployed into space. In 2022, NASA was conducting the third phaase of its Innovative Adanced Concepts (NIAC) program. In the past, solar sails were refractive and large. NASA was developing a diffractive solar sail that was smaller and more versatile.
In future, advancements in aerospace propulsion technology may increase the speed of space vehicles while reducing fuel consumption. This would allow humans to probe deeper into outer space and travel greater distances to celestial bodies.
PRINCIPAL TERMS
- chemical propulsion: the use of a chemical reaction to propel an object.
- duct propulsion: a form of propulsion that uses a duct to direct air or fluid into an engine, increase its momentum, and expel it to generate thrust.
- jet propulsion: the means of propelling an object by discharging a jet of ejected matter in the opposite direction of the intended motion.
- nuclear propulsion: the use of a nuclear reactor to propel an object.
- propulsion: the way of creating force to move an object.
- rocket propulsion: a class of jet propulsion in which the thrust is produced by ejecting a propellant such as fuel; used as the main form of spacecraft propulsion because it does not need air.
- solar propulsion: propulsion using solar energy, whether collected by solar panels to power an electric engine (ion propulsion) or harnessed directly to push a solar sail.
- turbomachinery: devices that use an engine with rotating blades to generate or transfer energy from a fluid.
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