Propulsive efficiency

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Propulsive_efficiency

In aerospace engineering, concerning aircraft, rocket and spacecraft design, overall propulsion system efficiency ${\displaystyle \eta }$ is the efficiency with which the energy contained in a vehicle's fuel is converted into kinetic energy of the vehicle, to accelerate it, or to replace losses due to aerodynamic drag or gravity. Mathematically, it is represented as ${\displaystyle \eta ={\eta }_{\mathrm{c}}{\eta }_{\mathrm{p}}}$, where ${\displaystyle {\eta }_{\mathrm{c}}}$ is the cycle efficiency and ${\displaystyle {\eta }_{\mathrm{p}}}$ is the propulsive efficiency.

The cycle efficiency is expressed as the percentage of the heat energy in the fuel that is converted to mechanical energy in the engine, and the propulsive efficiency is expressed as the proportion of the mechanical energy actually used to propel the aircraft. The propulsive efficiency is always less than one, because conservation of momentum requires that the exhaust have some of the kinetic energy, and the propulsive mechanism (whether propeller, jet exhaust, or ducted fan) is never perfectly efficient. It is greatly dependent on exhaust expulsion velocity and airspeed.

Cycle efficiency

Main article: Heat engine § Efficiency

Most aerospace vehicles are propelled by heat engines of some kind, usually an internal combustion engine. The efficiency of a heat engine relates how much useful work is output for a given amount of heat energy input.

From the laws of thermodynamics:

${\displaystyle dW=d{Q}_{\mathrm{c}}-(-d{Q}_{\mathrm{h}})}$

where

${\displaystyle dW=-PdV}$ is the work extracted from the engine. (It is negative because work is done by the engine.)

${\displaystyle d{Q}_{\mathrm{h}}=T_{\mathrm{h}}d{S}_{\mathrm{h}}}$ is the heat energy taken from the high-temperature system (heat source). (It is negative because heat is extracted from the source, hence ${\displaystyle (-d{Q}_{\mathrm{h}})}$ is positive.)

${\displaystyle d{Q}_{\mathrm{c}}=T_{\mathrm{c}}d{S}_{\mathrm{c}}}$ is the heat energy delivered to the low-temperature system (heat sink). (It is positive because heat is added to the sink.)

In other words, a heat engine absorbs heat from some heat source, converting part of it to useful work, and delivering the rest to a heat sink at lower temperature. In an engine, efficiency is defined as the ratio of useful work done to energy expended.

${\displaystyle {\eta }_{\mathrm{c}}=\frac{-dW}{-d{Q}_{\mathrm{h}}}=\frac{-d{Q}_{\mathrm{h}}-d{Q}_{\mathrm{c}}}{-d{Q}_{\mathrm{h}}}=1-\frac{d{Q}_{\mathrm{c}}}{-d{Q}_{\mathrm{h}}}}$

The theoretical maximum efficiency of a heat engine, the Carnot efficiency, depends only on its operating temperatures. Mathematically, this is because in reversible processes, the cold reservoir would gain the same amount of entropy as that lost by the hot reservoir (i.e., ${\displaystyle d{S}_{\mathrm{c}}=-d{S}_{\mathrm{h}}}$), for no change in entropy. Thus:

${\displaystyle {\eta }_{\text{cmax}}=1-\frac{T_{\mathrm{c}}d{S}_{\mathrm{c}}}{-T_{\mathrm{h}}d{S}_{\mathrm{h}}}=1-\frac{T_{\mathrm{c}}}{T_{\mathrm{h}}}}$

where ${\displaystyle T_{\mathrm{h}}}$ is the absolute temperature of the hot source and ${\displaystyle T_{\mathrm{c}}}$ that of the cold sink, usually measured in kelvins. Note that ${\displaystyle d{S}_{\mathrm{c}}}$ is positive while ${\displaystyle d{S}_{\mathrm{h}}}$ is negative; in any reversible work-extracting process, entropy is overall not increased, but rather is moved from a hot (high-entropy) system to a cold (low-entropy one), decreasing the entropy of the heat source and increasing that of the heat sink.

Propulsive efficiency

Propulsive efficiency is defined as the ratio of propulsive power (i.e. thrust times velocity of the vehicle) to work done on the fluid. In generic terms, the propulsive power can be calculated as follows:

${\displaystyle P_{\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{p}}=T\times v_{\mathrm{\infty }}}$

where ${\displaystyle T}$ represents thrust and ${\displaystyle v_{\mathrm{\infty }}}$, the flight speed.

The thrust can be computed from intake and exhaust massflows (${\displaystyle {\dot{m}}_{\mathrm{i}\mathrm{n}}}$ and ${\displaystyle {\dot{m}}_{\mathrm{e}\mathrm{x}\mathrm{h}}}$) and velocities (${\displaystyle v_{\mathrm{i}\mathrm{n}}}$ and ${\displaystyle v_{\mathrm{e}\mathrm{x}\mathrm{h}}}$):

${\displaystyle T={\dot{m}}_{\mathrm{e}\mathrm{x}\mathrm{h}}{v}_{\mathrm{e}\mathrm{x}\mathrm{h}}-{\dot{m}}_{\mathrm{i}\mathrm{n}}{v}_{\mathrm{i}\mathrm{n}}}$

${\displaystyle P_{\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{p}}=\left({\dot{m}}_{\mathrm{e}\mathrm{x}\mathrm{h}}{v}_{\mathrm{e}\mathrm{x}\mathrm{h}}-{\dot{m}}_{\mathrm{i}\mathrm{n}}{v}_{\mathrm{i}\mathrm{n}}\right)v_{\mathrm{\infty }}}$

The work done by the engine to the flow, on the other hand, is the change in kinetic energy per time. This does not take into account the efficiency of the engine used to generate the power, nor of the propeller, fan or other mechanism used to accelerate air. It merely refers to the work done to the flow, by any means, and can be expressed as the difference between exhausted kinetic energy flux and incoming kinetic energy flux:

${\displaystyle P_{\mathrm{e}\mathrm{n}\mathrm{g}}=\frac{1}{2}{\dot{m}}_{\mathrm{e}\mathrm{x}\mathrm{h}}{v}_{\mathrm{e}\mathrm{x}\mathrm{h}}^2-\frac{1}{2}{\dot{m}}_{\mathrm{i}\mathrm{n}}{v}_{\mathrm{i}\mathrm{n}}^2}$

${\displaystyle P_{\mathrm{e}\mathrm{n}\mathrm{g}}=\frac{1}{2}\left({\dot{m}}_{\mathrm{e}\mathrm{x}\mathrm{h}}{v}_{\mathrm{e}\mathrm{x}\mathrm{h}}^2-{\dot{m}}_{\mathrm{i}\mathrm{n}}{v}_{\mathrm{i}\mathrm{n}}^2\right)}$

The propulsive efficiency can therefore be computed as:

${\displaystyle {\eta }_{\mathrm{p}}=\frac{P_{\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{p}}}{P_{\mathrm{e}\mathrm{n}\mathrm{g}}}=2v_{\mathrm{\infty }}\frac{{\dot{m}}_{\mathrm{e}\mathrm{x}\mathrm{h}}{v}_{\mathrm{e}\mathrm{x}\mathrm{h}}-{\dot{m}}_{\mathrm{i}\mathrm{n}}{v}_{\mathrm{i}\mathrm{n}}}{{\dot{m}}_{\mathrm{e}\mathrm{x}\mathrm{h}}{v}_{\mathrm{e}\mathrm{x}\mathrm{h}}^2-{\dot{m}}_{\mathrm{i}\mathrm{n}}{v}_{\mathrm{i}\mathrm{n}}^2}}$

Depending on the type of propulsion used, this equation can be simplified in different ways, demonstrating some of the peculiarities of different engine types. The general equation already shows, however, that propulsive efficiency improves when using large massflows and small velocities compared to small mass-flows and large velocities, since the squared terms in the denominator grow faster than the non-squared terms.

The losses modelled by propulsive efficiency are explained by the fact that any mode of aero propulsion leaves behind a jet moving into the opposite direction of the vehicle. The kinetic energy flux in this jet is ${\displaystyle P_{\mathrm{j}\mathrm{e}\mathrm{t}}=1/2\left({\dot{m}}_{\mathrm{e}\mathrm{x}\mathrm{h}}{v}_{\mathrm{e}\mathrm{x}\mathrm{h}}^2-{\dot{m}}_{\mathrm{i}\mathrm{n}}{v}_{\mathrm{i}\mathrm{n}}^2\right)=P_{\mathrm{e}\mathrm{n}\mathrm{g}}-P_{\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{p}}}$ for the case that ${\displaystyle v_{\mathrm{i}\mathrm{n}}=v_{\mathrm{\infty }}}$.

Jet engines

Dependence of the energy efficiency (η) from the exhaust speed/airplane speed ratio (c/v) for airbreathing jets

The propulsive efficiency formula for air-breathing engines is given below. It can be derived by setting ${\displaystyle v_{\mathrm{i}\mathrm{n}}=v_{\mathrm{\infty }}=v_0}$ in the general equation, and assuming that ${\displaystyle {\dot{m}}_{\mathrm{e}\mathrm{x}\mathrm{h}}={\dot{m}}_{\mathrm{i}\mathrm{n}}}$. This cancels out the mass-flow and leads to:

${\displaystyle {\eta }_{\mathrm{p}}=\frac{2}{1+\frac{v_9}{v_0}}}$

where ${\displaystyle v_9}$ is the exhaust expulsion velocity  and ${\displaystyle v_0}$ is both the airspeed at the inlet and the flight velocity.

For pure jet engines, particularly with afterburner, a small amount of accuracy can be gained by not assuming the intake and exhaust massflow to be equal, since the exhaust gas also contains the added mass of the fuel injected. For turbofan engines, the exhaust massflow may be marginally smaller than the intake massflow because the engine supplies "bleed air" from the compressor to the aircraft. In most circumstances, this is not taken into account, as it makes no significant difference to the computed propulsive efficiency.

By computing the exhaust velocity from the equation for thrust (while still assuming ${\displaystyle {\dot{m}}_{\mathrm{e}\mathrm{x}\mathrm{h}}={\dot{m}}_{\mathrm{i}\mathrm{n}}=\dot{m}}$), we can also obtain the propulsive efficiency as a function of specific thrust (${\displaystyle T/\dot{m}}$):

${\displaystyle {\eta }_{\mathrm{p}}=\frac{v_0}{v_0+\frac{1}{2}\frac{T}{\dot{m}}}}$

A corollary of this is that, particularly in air breathing engines, it is more energy efficient to accelerate a large amount of air by a small amount, than it is to accelerate a small amount of air by a large amount, even though the thrust is the same. This is why turbofan engines are more efficient than simple jet engines at subsonic speeds.

Dependence of the propulsive efficiency (${\displaystyle {\eta }_{\mathrm{p}}}$) upon the vehicle speed/exhaust speed ratio (v_0/v_9) for rocket and jet engines

Rocket engines

A rocket engine's ${\displaystyle {\eta }_{\mathrm{c}}}$ is usually high due to the high combustion temperatures and pressures, and the long converging-diverging nozzle used. It varies slightly with altitude due to changing atmospheric pressure, but can be up to 70%. Most of the remainder is lost as heat in the exhaust.

Rocket engines have a slightly different propulsive efficiency (${\displaystyle {\eta }_{\mathrm{p}}}$) than air-breathing jet engines, as the lack of intake air changes the form of the equation. This also allows rockets to exceed their exhaust's velocity.

${\displaystyle {\eta }_{\mathrm{p}}=\frac{2\frac{v_0}{v_9}}{1+(\frac{v_0}{v_9}{)}^2}}$

Similarly to jet engines, matching the exhaust speed and the vehicle speed gives optimum efficiency, in theory. However, in practice, this results in a very low specific impulse, causing much greater losses due to the need for exponentially larger masses of propellant. Unlike ducted engines, rockets give thrust even when the two speeds are equal.

In 1903, Konstantin Tsiolkovsky discussed the average propulsive efficiency of a rocket, which he called the utilization (utilizatsiya), the "portion of the total work of the explosive material transferred to the rocket" as opposed to the exhaust gas.

Propeller engines

Propulsive efficiency comparison for various gas turbine engine configurations

The calculation is somewhat different for reciprocating and turboprop engines which rely on a propeller for propulsion since their output is typically expressed in terms of power rather than thrust. The equation for heat added per unit time, Q, can be adopted as follows:

${\displaystyle 550P_{\mathrm{e}}=\frac{{\eta }_{\mathrm{c}}HhJ}{3600},}$

where H = calorific value of the fuel in BTU/lb, h = fuel consumption rate in lb/hr and J = mechanical equivalent of heat = 778.24 ft.lb/BTU, where ${\displaystyle P_{\mathrm{e}}}$ is engine output in horsepower, converted to foot-pounds/second by multiplication by 550. Given that specific fuel consumption is C_p = h/P_e and H = 20 052 BTU/lb for gasoline, the equation is simplified to:

${\displaystyle {\eta }_{\mathrm{c}}(\mathrm{\%}age)=\frac{12.69}{C_{\mathrm{p}}}.}$

expressed as a percentage.

Assuming a typical propeller efficiency ${\displaystyle {\eta }_{\mathrm{p}}}$ of 86% (for the optimal airspeed and air density conditions for the given propeller design), maximum overall propulsion efficiency is estimated as:

${\displaystyle \eta =\frac{10.91}{C_p}.}$

Oberth effect

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Oberth_effect 

 

In astronautics, a powered flyby, or Oberth maneuver, is a maneuver in which a spacecraft falls into a gravitational well and then uses its engines to further accelerate as it is falling, thereby achieving additional speed. The resulting maneuver is a more efficient way to gain kinetic energy than applying the same impulse outside of a gravitational well. The gain in efficiency is explained by the Oberth effect, wherein the use of a reaction engine at higher speeds (relative to any reference frame) generates a greater change in mechanical energy than its use at lower speeds. In practical terms, this means that the most energy-efficient method for a spacecraft to burn its fuel is at the lowest possible orbital periapsis, when its orbital velocity (and so, its kinetic energy) is greatest. In some cases, it is even worth spending fuel on slowing the spacecraft into a gravity well to take advantage of the efficiencies of the Oberth effect. The maneuver and effect are named after the Transylvanian Saxon physicist and a founder of modern rocketry Hermann Oberth, who first described them in 1927.

Because the vehicle remains near periapsis only for a short time, for the Oberth maneuver to be most effective the vehicle must be able to generate as much impulse as possible in the shortest possible time. As a result the Oberth maneuver is much more useful for high-thrust rocket engines like liquid-propellant rockets, and less useful for low-thrust reaction engines such as ion drives, which take a long time to gain speed. Low thrust rockets can use the Oberth effect by splitting a long departure burn into several short burns near the periapsis. The Oberth effect also can be used to understand the behavior of multi-stage rockets: the upper stage can generate much more usable kinetic energy than the total chemical energy of the propellants it carries.

In terms of the energies involved, the Oberth effect is more effective at higher speeds because at high speed the propellant has significant kinetic energy in addition to its chemical potential energy. At higher speed the vehicle is able to employ the greater change (reduction) in kinetic energy of the propellant (as it is exhausted backward and hence at reduced speed and hence reduced kinetic energy) to generate a greater increase in kinetic energy of the vehicle.

Explanation in terms of work and kinetic energy

Because kinetic energy equals mv²/2, this change in velocity imparts a greater increase in kinetic energy at a high velocity than it would at a low velocity. For example, considering a 2 kg rocket:

• at 1 m/s, the rocket starts with 1² = 1 J of kinetic energy. Adding 1 m/s increases the kinetic energy to 2² = 4 J, for a gain of 3 J;
• at 10 m/s, the rocket starts with 10² = 100 J of kinetic energy. Adding 1 m/s increases the kinetic energy to 11² = 121 J, for a gain of 21 J.

This greater change in kinetic energy can then carry the rocket higher in the gravity well than if the propellant were burned at a lower speed.

Description in terms of work

The thrust produced by a rocket engine is independent of the rocket’s velocity relative to the surrounding atmosphere. A rocket acting on a fixed object, as in a static firing, does no useful work on the rocket; the rocket's chemical energy is progressively converted to kinetic energy of the exhaust, plus heat. But when the rocket moves, its thrust acts through the distance it moves. Force multiplied by displacement is the definition of mechanical work. The greater the velocity of the rocket and payload during the burn the greater is the displacement and the work done, and the greater the increase in kinetic energy of the rocket and its payload. As the velocity of the rocket increases, progressively more of the available kinetic energy goes to the rocket and its payload, and less to the exhaust.

This is shown as follows. The mechanical work done on the rocket (${\displaystyle W}$) is defined as the dot product of the force of the engine's thrust (${\displaystyle \overrightarrow{F}}$) and the displacement it travels during the burn (${\displaystyle \overrightarrow{s}}$):

${\displaystyle W=\overrightarrow{F}\cdot \overrightarrow{s}.}$

If the burn is made in the prograde direction, ${\displaystyle \overrightarrow{F}\cdot \overrightarrow{s}=\Vert F\Vert \cdot \Vert s\Vert =F\cdot s}$. The work results in a change in kinetic energy

${\displaystyle \mathrm{\Delta }{E}_k=F\cdot s.}$

Differentiating with respect to time, we obtain

${\displaystyle \frac{\mathrm{d}{E}_k}{\mathrm{d}t}=F\cdot \frac{\mathrm{d}s}{\mathrm{d}t},}$

or

${\displaystyle \frac{\mathrm{d}{E}_k}{\mathrm{d}t}=F\cdot v,}$

where ${\displaystyle v}$ is the velocity. Dividing by the instantaneous mass ${\displaystyle m}$ to express this in terms of specific energy (${\displaystyle e_k}$), we get

${\displaystyle \frac{\mathrm{d}{e}_k}{\mathrm{d}t}=\frac{F}{m}\cdot v=a\cdot v,}$

where ${\displaystyle a}$ is the acceleration vector.

Thus it can be readily seen that the rate of gain of specific energy of every part of the rocket is proportional to speed and, given this, the equation can be integrated (numerically or otherwise) to calculate the overall increase in specific energy of the rocket.

Impulsive burn

Integrating the above energy equation is often unnecessary if the burn duration is short. Short burns of chemical rocket engines close to periapsis or elsewhere are usually mathematically modeled as impulsive burns, where the force of the engine dominates any other forces that might change the vehicle's energy over the burn.

For example, as a vehicle falls toward periapsis in any orbit (closed or escape orbits) the velocity relative to the central body increases. Briefly burning the engine (an "impulsive burn") prograde at periapsis increases the velocity by the same increment as at any other time (${\displaystyle \mathrm{\Delta }v}$). However, since the vehicle's kinetic energy is related to the square of its velocity, this increase in velocity has a non-linear effect on the vehicle's kinetic energy, leaving it with higher energy than if the burn were achieved at any other time.

Oberth calculation for a parabolic orbit

If an impulsive burn of Δv is performed at periapsis in a parabolic orbit, then the velocity at periapsis before the burn is equal to the escape velocity (V_esc), and the specific kinetic energy after the burn is

${\displaystyle \begin{array}{rl}{e}_k& =\frac{1}{2}{V}^2\\ & =\frac{1}{2}(V_{\text{esc}}+\mathrm{\Delta }v{)}^2\\ & =\frac{1}{2}{V}_{\text{esc}}^2+\mathrm{\Delta }v{V}_{\text{esc}}+\frac{1}{2}\mathrm{\Delta }{v}^2,\end{array}}$

where ${\displaystyle V=V_{\text{esc}}+\mathrm{\Delta }v}$.

When the vehicle leaves the gravity field, the loss of specific kinetic energy is

${\displaystyle \frac{1}{2}{V}_{\text{esc}}^2,}$

so it retains the energy

${\displaystyle \mathrm{\Delta }v{V}_{\text{esc}}+\frac{1}{2}\mathrm{\Delta }{v}^2,}$

which is larger than the energy from a burn outside the gravitational field (${\displaystyle \frac{1}{2}\mathrm{\Delta }{v}^2}$) by

${\displaystyle \mathrm{\Delta }v{V}_{\text{esc}}.}$

When the vehicle has left the gravity well, it is traveling at a speed

${\displaystyle V=\mathrm{\Delta }v\sqrt{1+\frac{2V_{\text{esc}}}{\mathrm{\Delta }v}}.}$

For the case where the added impulse Δv is small compared to escape velocity, the 1 can be ignored, and the effective Δv of the impulsive burn can be seen to be multiplied by a factor of simply

${\displaystyle \sqrt{\frac{2V_{\text{esc}}}{\mathrm{\Delta }v}}}$

and one gets

${\displaystyle V}$ ≈ ${\displaystyle \sqrt{2V_{\text{esc}}\mathrm{\Delta }v}.}$

Similar effects happen in closed and hyperbolic orbits.

Parabolic example

If the vehicle travels at velocity v at the start of a burn that changes the velocity by Δv, then the change in specific orbital energy (SOE) due to the new orbit is

${\displaystyle v\mathrm{\Delta }v+\frac{1}{2}(\mathrm{\Delta }v{)}^2.}$

Once the spacecraft is far from the planet again, the SOE is entirely kinetic, since gravitational potential energy approaches zero. Therefore, the larger the v at the time of the burn, the greater the final kinetic energy, and the higher the final velocity.

The effect becomes more pronounced the closer to the central body, or more generally, the deeper in the gravitational field potential in which the burn occurs, since the velocity is higher there.

So if a spacecraft is on a parabolic flyby of Jupiter with a periapsis velocity of 50 km/s and performs a 5 km/s burn, it turns out that the final velocity change at great distance is 22.9 km/s, giving a multiplication of the burn by 4.58 times.

Paradox

It may seem that the rocket is getting energy for free, which would violate conservation of energy. However, any gain to the rocket's kinetic energy is balanced by a relative decrease in the kinetic energy the exhaust is left with (the kinetic energy of the exhaust may still increase, but it does not increase as much). Contrast this to the situation of static firing, where the speed of the engine is fixed at zero. This means that its kinetic energy does not increase at all, and all the chemical energy released by the fuel is converted to the exhaust's kinetic energy (and heat).

At very high speeds the mechanical power imparted to the rocket can exceed the total power liberated in the combustion of the propellant; this may also seem to violate conservation of energy. But the propellants in a fast-moving rocket carry energy not only chemically, but also in their own kinetic energy, which at speeds above a few kilometres per second exceed the chemical component. When these propellants are burned, some of this kinetic energy is transferred to the rocket along with the chemical energy released by burning.

The Oberth effect can therefore partly make up for what is extremely low efficiency early in the rocket's flight when it is moving only slowly. Most of the work done by a rocket early in flight is "invested" in the kinetic energy of the propellant not yet burned, part of which they will release later when they are burned.

Essentialism

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Essentialism 

Essentialism is the view that objects have a set of attributes that are necessary to their identity. In early Western thought, Platonic idealism held that all things have such an "essence"—an "idea" or "form". In Categories, Aristotle similarly proposed that all objects have a substance that, as George Lakoff put it, "make the thing what it is, and without which it would be not that kind of thing". The contrary view—non-essentialism—denies the need to posit such an "essence". Essentialism has been controversial from its beginning. In the Parmenides dialogue, Plato depicts Socrates questioning the notion, suggesting that if we accept the idea that every beautiful thing or just action partakes of an essence to be beautiful or just, we must also accept the "existence of separate essences for hair, mud, and dirt".

Older social theories were often conceptually essentialist. In biology and other natural sciences, essentialism provided the rationale for taxonomy at least until the time of Charles Darwin. The role and importance of essentialism in modern biology is still a matter of debate. Beliefs which posit that social identities such as race, ethnicity, nationality, or gender are essential characteristics have been central to many discriminatory or extremist ideologies. For instance, psychological essentialism is correlated with racial prejudice. Essentialist views about race have also been shown to diminish empathy when dealing with members of another racial group. In medical sciences, essentialism can lead to a reified view of identities, leading to fallacious conclusions and potentially unequal treatment.

In philosophy

An essence characterizes a substance or a form, in the sense of the forms and ideas in Platonic idealism. It is permanent, unalterable, and eternal, and is present in every possible world. Classical humanism has an essentialist conception of the human, in its endorsement of the notion of an eternal and unchangeable human nature. This has been criticized by Kierkegaard, Marx, Heidegger, Sartre, Badiou and many other existential, materialist and anti-humanist thinkers. Essentialism, in its broadest sense, is any philosophy that acknowledges the primacy of essence. Unlike existentialism, which posits "being" as the fundamental reality, the essentialist ontology must be approached from a metaphysical perspective. Empirical knowledge is developed from experience of a relational universe whose components and attributes are defined and measured in terms of intellectually constructed laws. Thus, for the scientist, reality is explored as an evolutionary system of diverse entities, the order of which is determined by the principle of causality.

In Plato's philosophy, in particular the Timaeus and the Philebus, things were said to come into being by the action of a demiurge who works to form chaos into ordered entities. Similarly, many definitions of essence hark back to the ancient Greek hylomorphic understanding of the formation of the things as articulated, for example, by Aristotle. According to that account, the structure and real existence of any thing can be understood by analogy to an artefact produced by a craftsperson. The craftsperson requires hyle (timber or wood) and a model, plan or idea in their own mind, according to which the wood is worked to give it the indicated contour or form (morphe). Aristotle was the first to use the terms hyle and morphe, developing an account indebted to Plato's. According to Aristotle's explanation, all entities have two aspects: "matter" and "form". It is the particular form imposed that gives some matter its identity—its quiddity or "whatness" (i.e., "what it is"). Plato was one of the first essentialists, postulating the concept of ideal forms—an abstract entity of which individual objects are mere facsimiles. To give an example: the ideal form of a circle is a perfect circle, something that is physically impossible to make manifest; yet the circles we draw and observe clearly have some idea in common—the ideal form. Plato proposed that these ideas are eternal and vastly superior to their manifestations, and that we understand these manifestations in the material world by comparing and relating them to their respective ideal form. Plato's forms are regarded as patriarchs to essentialist dogma simply because they are a case of what is intrinsic and a-contextual of objects—the abstract properties that make them what they are. One example is Plato's parable of the cave. Plato believed that the universe was perfect and that its observed imperfections came from man's limited perception of it. For Plato, there were two realities: the "essential" or ideal and the "perceived".

Aristotle (384–322 BC) applied the term essence to that which things in a category have in common and without which they cannot be members of that category (for example, rationality is the essence of man; without rationality a creature cannot be a man). In his critique of Aristotle's philosophy, Bertrand Russell said that his concept of essence transferred to metaphysics what was only a verbal convenience and that it confused the properties of language with the properties of the world. In fact, a thing's "essence" consisted in those defining properties without which we could not use the name for it, rather than those properties without which a thing would not be what kind of thing it actually is. Although the concept of essence was, according to Bertrand Russell, "hopelessly muddled" it became part of every philosophy until modern times. The Egyptian-born philosopher Plotinus (204–270 AD) brought idealism to the Roman Empire as Neoplatonism, and with it the concept that not only do all existents emanate from a "primary essence" but that the mind plays an active role in shaping or ordering the objects of perception, rather than passively receiving empirical data.

Examples

Naturalism

Dating back to the 18th century, naturalism is a form of essentialism in which social matters are explained through the logic of natural dispositions. The invoked nature can be biological, ontological or theological. It is opposed by antinaturalism and culturalism.

Human nature

See also: Philosophical anthropology

In the case of Homo sapiens, the divergent conceptions of human nature may be partitioned into essentialist versus non-essentialist (or even anti-essentialist) positions. Another established dichotomy is that of monism versus pluralism about the matter.

Monism will demand that enhancement technologies be used to create humans as close as possible to the ideal state. [...] The Nazis would have proposed the list of characteristics for admission to the SS as the universal template for enhancement technologies. Hedonistic utilitarianism is a less objectionable version of monism, according to which the best human life is one that contains as much pleasure and as little suffering as possible – but like Nazism, it leaves no room for meaningful choice about enhancement.

— Nicholas Agar

Biological essentialism

Main article: Species § The species problem

Before evolution was developed as a scientific theory, the essentialist view of biology posited that all species are unchanging throughout time. The historian Mary P. Winsor has argued that biologists such as Louis Agassiz in the 19th century believed that taxa such as species and genus were fixed, reflecting the mind of the creator. Some religious opponents of evolution continue to maintain this view of biology.

Work by historians of systematic biology in the 21st century has cast doubt upon this view of pre-Darwinian thinkers. Winsor, Ron Amundson and Staffan Müller-Wille have each argued that in fact the usual suspects (such as Linnaeus and the Ideal Morphologists) were very far from being essentialists, and that the so-called "essentialism story" (or "myth") in biology is a result of conflating the views expressed and biological examples used by philosophers going back to Aristotle and continuing through to John Stuart Mill and William Whewell in the immediately pre-Darwinian period, with the way that biologists used such terms as species.

Anti-essentialists contend that an essentialist typological categorization has been rendered obsolete and untenable by evolutionary theory for several reasons. First, they argue that biological species are dynamic entities, emerging and disappearing as distinct populations are molded by natural selection. This view contrasts with the static essences that essentialists say characterize natural categories. Second, the opponents of essentialism argue that our current understanding of biological species emphasizes genealogical relationships rather than intrinsic traits. Lastly, non-essentialists assert that every organism has a mutational load, and the variability and diversity within species contradict the notion of fixed biological natures.

Gender essentialism

Main article: Gender essentialism

In feminist theory and gender studies, gender essentialism is the attribution of fixed essences to men and women—this idea that men and women are fundamentally different continues to be a matter of contention. Gay/lesbian rights advocate Diana Fuss wrote: "Essentialism is most commonly understood as a belief in the real, true essence of things, the invariable and fixed properties which define the 'whatness' of a given entity." Women's essence is assumed to be universal and is generally identified with those characteristics viewed as being specifically feminine. These ideas of femininity are usually biologized and are often preoccupied with psychological characteristics, such as nurturance, empathy, support, and non-competitiveness, etc. Feminist theorist Elizabeth Grosz states in her 1995 publication Space, time and perversion: essays on the politics of bodies that essentialism "entails the belief that those characteristics defined as women's essence are shared in common by all women at all times. It implies a limit of the variations and possibilities of change—it is not possible for a subject to act in a manner contrary to her essence. Her essence underlies all the apparent variations differentiating women from each other. Essentialism thus refers to the existence of fixed characteristic, given attributes, and ahistorical functions that limit the possibilities of change and thus of social reorganization."

Gender essentialism is pervasive in popular culture, as illustrated by the #1 New York Times best seller Men Are from Mars, Women Are from Venus, but this essentialism is routinely critiqued in introductory women's studies textbooks such as Women: Images & Realities. Starting in the 1980s, some feminist writers have put forward essentialist theories about gender and science. Evelyn Fox Keller, Sandra Harding, and Nancy Tuana argued that the modern scientific enterprise is inherently patriarchal and incompatible with women's nature. Other feminist scholars, such as Ann Hibner Koblitz, Lenore Blum, Mary Gray, Mary Beth Ruskai, and Pnina Abir-Am and Dorinda Outram have criticized those theories for ignoring the diverse nature of scientific research and the tremendous variation in women's experiences in different cultures and historical periods.

Racial, cultural and strategic essentialism

Main articles: Race (human categorization) and Strategic essentialism

Cultural and racial essentialism is the view that fundamental biological or physical characteristics of human "races" produce personality, heritage, cognitive abilities, or 'natural talents' that are shared by all members of a racial group. In the early 20th century, many anthropologists taught this theory – that race was an entirely biological phenomenon and that this was core to a person's behavior and identity. This, coupled with a belief that linguistic, cultural, and social groups fundamentally existed along racial lines, formed the basis of what is now called scientific racism. After the Nazi eugenics program, along with the rise of anti-colonial movements, racial essentialism lost widespread popularity. New studies of culture and the fledgling field of population genetics undermined the scientific standing of racial essentialism, leading race anthropologists to revise their conclusions about the sources of phenotypic variation. A significant number of modern anthropologists and biologists in the West came to view race as an invalid genetic or biological designation.

Historically, beliefs which posit that social identities such as ethnicity, nationality or gender determine a person's essential characteristics have in many cases been shown to have destructive or harmful results. It has been argued by some that essentialist thinking lies at the core of many simplistic, discriminatory or extremist ideologies. Psychological essentialism is also correlated with racial prejudice. In medical sciences, essentialism can lead to an over-emphasis on the role of identities—for example assuming that differences in hypertension in African-American populations are due to racial differences rather than social causes—leading to fallacious conclusions and potentially unequal treatment. Older social theories were often conceptually essentialist.

Strategic essentialism, a major concept in postcolonial theory, was introduced in the 1980s by the Indian literary critic and theorist Gayatri Chakravorty Spivak. It refers to a political tactic in which minority groups, nationalities, or ethnic groups mobilize on the basis of shared gendered, cultural, or political identity. While strong differences may exist between members of these groups, and among themselves they engage in continuous debates, it is sometimes advantageous for them to temporarily "essentialize" themselves, despite it being based on erroneous logic, and to bring forward their group identity in a simplified way to achieve certain goals, such as equal rights or antiglobalization.

Machine learning

Pelillo argues that traditional machine learning techniques often align with an essentialist paradigm by relying on features - properties assumed to be essential for classification tasks. For instance, pattern recognition, which attempts to extract essential attributes from data, is described as inherently essentialist since it presupposes that objects have stable, identifiable essences that define their categories. This perspective extends to similarity-based approaches, which use prototype theory to establish relationships within data by grouping instances around central prototypes that exhibit the "essence" of a category.

Expanding on this, Pelillo and Scantamburlo highlight that certain machine-learning scenarios, such as when data is highly dimensional or features are poorly defined, challenge the essentialist framework. They advocate for alternative paradigms that consider relational and contextual information instead of isolated feature analysis. This relational focus aligns with anti-essentialist stances, which view categories as dynamic and context-dependent rather than fixed.

Šekrst and Skansi build on these ideas, noting that supervised learning, by utilizing labeled datasets, reflects essentialist tendencies since it relies on predefined human-defined categories. However, they argue that this does not commit machine learning to an ontological stance on essentialism. Instead, they propose that the categories used in supervised learning are human-constructed in feature selection processes and reflect epistemological practices rather than metaphysical truths. Similarly, unsupervised learning's clustering and similarity-based approaches often resemble prototypical reasoning but do not inherently affirm or deny essentialism, focusing instead on pragmatic task performance.

In historiography

Essentialism in history as a field of study entails discerning and listing essential cultural characteristics of a particular nation or culture, in the belief that a people or culture can be understood in this way. Sometimes such essentialism leads to claims of a praiseworthy national or cultural identity, or to its opposite, the condemnation of a culture based on presumed essential characteristics. Herodotus, for example, claims that Egyptian culture is essentially feminized and possesses a "softness" which has made Egypt easy to conquer. To what extent Herodotus was an essentialist is a matter of debate; he is also credited with not essentializing the concept of the Athenian identity, or differences between the Greeks and the Persians that are the subject of his Histories.

Essentialism had been operative in colonialism, as well as in critiques of colonialism. Post-colonial theorists, such as Edward Said, insisted that essentialism was the "defining mode" of "Western" historiography and ethnography until the nineteenth century and even after, according to Touraj Atabaki, manifesting itself in the historiography of the Middle East and Central Asia as Eurocentrism, over-generalization, and reductionism. Into the 21st century, most historians, social scientists, and humanists reject methodologies associated with essentialism, although some have argued that certain varieties of essentialism may be useful or even necessary. Karl Popper splits the ambiguous term realism into essentialism and realism. He uses essentialism whenever he means the opposite of nominalism, and realism only as opposed to idealism. Popper himself is a realist as opposed to an idealist, but a methodological nominalist as opposed to an essentialist. For example, statements like "a puppy is a young dog" should be read from right to left as an answer to "What shall we call a young dog", never from left to right as an answer to "What is a puppy?"

In psychology

Paul Bloom attempts to explain why people will pay more in an auction for the clothing of celebrities if the clothing is unwashed. He believes the answer to this and many other questions is that people cannot help but think of objects as containing a sort of "essence" that can be influenced.

There is a difference between metaphysical essentialism and psychological essentialism, the latter referring not to an actual claim about the world but a claim about a way of representing entities in cognition. Influential in this area is Susan Gelman, who has outlined many domains in which children and adults construe classes of entities, particularly biological entities, in essentialist terms—i.e., as if they had an immutable underlying essence which can be used to predict unobserved similarities between members of that class. This causal relationship is unidirectional; an observable feature of an entity does not define the underlying essence.

In developmental psychology

Essentialism has emerged as an important concept in psychology, particularly developmental psychology. In 1991, Kathryn Kremer and Susan Gelman studied the extent to which children from four–seven years old demonstrate essentialism. Children believed that underlying essences predicted observable behaviours. Children were able to describe living objects' behaviour as self-perpetuated and non-living objects' behavior as a result of an adult influencing the object. Understanding the underlying causal mechanism for behaviour suggests essentialist thinking. Younger children were unable to identify causal mechanisms of behaviour whereas older children were able to. This suggests that essentialism is rooted in cognitive development. It can be argued that there is a shift in the way that children represent entities, from not understanding the causal mechanism of the underlying essence to showing sufficient understanding.

There are four key criteria that constitute essentialist thinking. The first facet is the aforementioned individual causal mechanisms. The second is innate potential: the assumption that an object will fulfill its predetermined course of development. According to this criterion, essences predict developments in entities that will occur throughout its lifespan. The third is immutability. Despite altering the superficial appearance of an object it does not remove its essence. Observable changes in features of an entity are not salient enough to alter its essential characteristics. The fourth is inductive potential. This suggests that entities may share common features but are essentially different; however similar two beings may be, their characteristics will be at most analogous, differing most importantly in essences. The implications of psychological essentialism are numerous. Prejudiced individuals have been found to endorse exceptionally essential ways of thinking, suggesting that essentialism may perpetuate exclusion among social groups. For example, essentialism of nationality has been linked to anti-immigration attitudes. In multiple studies in India and the United States, it was shown that in lay view a person's nationality is considerably fixed at birth, even if that person is adopted and raised by a family of another nationality at day one and never told about their origin. This may be due to an over-extension of an essential-biological mode of thinking stemming from cognitive development. Paul Bloom of Yale University has stated that "one of the most exciting ideas in cognitive science is the theory that people have a default assumption that things, people and events have invisible essences that make them what they are. Experimental psychologists have argued that essentialism underlies our understanding of the physical and social worlds, and developmental and cross-cultural psychologists have proposed that it is instinctive and universal. We are natural-born essentialists." Scholars suggest that the categorical nature of essentialist thinking predicts the use of stereotypes and can be targeted in the application of stereotype prevention.