Aircraft-specific energy is a form of

specific energy Specific energy or massic energy is energy per unit mass. It is also sometimes called gravimetric energy density, which is not to be confused with energy density, which is defined as energy per unit volume. It is used to quantify, for example, st ...

applied to aircraft and missile

trajectory A trajectory or flight path is the path that an object with mass in motion follows through space as a function of time. In classical mechanics, a trajectory is defined by Hamiltonian mechanics via canonical coordinates; hence, a complete tra ...

analysis. It represents the combined kinetic and potential energy of the vehicle at any given time. It is the total energy of the vehicle (relative to the Earth's surface) per unit weight of the vehicle. Being independent of the mass of the vehicle, it provides a powerful tool for the design of optimal trajectories. Aircraft-specific energy is very similar to

specific orbital energy In the gravitational two-body problem, the specific orbital energy \varepsilon (or specific ''vis-viva'' energy) of two orbiting bodies is the constant quotient of their mechanical energy (the sum of their mutual potential energy, \varepsilon ...

except that it is expressed as a positive quantity. A zero value of aircraft-specific energy represents an aircraft at rest on the Earth's surface, and the value increases as speed and altitude increases. As with other forms of specific energy, aircraft-specific energy is an

intensive property Physical or chemical properties of materials and systems can often be categorized as being either intensive or extensive, according to how the property changes when the size (or extent) of the system changes. The terms "intensive and extensive ...

and is represented in units of length since it is independent of the mass of the vehicle. That is, while the specific energy may be expressed in

joule The joule ( , or ; symbol: J) is the unit of energy in the International System of Units (SI). In terms of SI base units, one joule corresponds to one kilogram- metre squared per second squared One joule is equal to the amount of work d ...

per

kilogram The kilogram (also spelled kilogramme) is the base unit of mass in the International System of Units (SI), equal to one thousand grams. It has the unit symbol kg. The word "kilogram" is formed from the combination of the metric prefix kilo- (m ...

(J/kg), the specific energy height may be expressed in

meter The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of ...

s by the formula (v^2 / 2g) + h, where V is the airspeed of the aircraft, g is the acceleration due to gravity, and h is the altitude of the aircraft.

Applications

The field of

trajectory optimization Trajectory optimization is the process of designing a trajectory that minimizes (or maximizes) some measure of performance while satisfying a set of constraints. Generally speaking, trajectory optimization is a technique for computing an open-loop ...

has made use of the concept since the 1950s in the form of energy analysis. In this approach, the specific energy is defined as one of the dynamic states of the problem and is the slowest varying state. All other states such as altitude and flight path angle are approximated as infinitely fast compared to the specific energy dynamics. This assumption allow the solution of optimal trajectories in a relatively simple form. The specific energy is computed by the total energy (as defined above relative the Earth's surface) divided by the mass of the vehicle. It is a key element in performance of aircraft and rockets. For a rocket flying vertically (in a vacuum), it is the

apogee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

that the rocket would obtain. Aircraft-specific energy is used extensively in the energy–maneuverability theory governing modern aircraft dogfighting tactics. The primary goal of air combat manoeuvring is to maintain an optimal aircraft-specific energy. Speed allows an aircraft the ability to potentially outmaneuver adversaries, and altitude can be converted into speed, while also providing extended range for guided munitions (due to lower

air density The density of air or atmospheric density, denoted '' ρ'', is the mass per unit volume of Earth's atmosphere at a given point and time. Air density, like air pressure, decreases with increasing altitude. It also changes with variations in atmosph ...

and therefore lower drag at any given

velocity Velocity is a measurement of speed in a certain direction of motion. It is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of physical objects. Velocity is a vector (geometry), vector Physical q ...

). Aircraft such as the

F-16 Fighting Falcon The General Dynamics F-16 Fighting Falcon is an American single-engine supersonic Multirole combat aircraft, multirole fighter aircraft originally developed by General Dynamics for the United States Air Force (USAF). Designed as an air superio ...

were designed to be optimized in accordance to the energy-maneuverability theory, allowing for an aircraft to quickly gain aircraft-specific energy as fast as possible.

References

Aerodynamics {{aviation-stub