In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter gaseous matter, in an accretion disk. Most astronomical objects, such as galaxies and planets, are formed by accretion processes; the accretion model that Earth and the other terrestrial planets formed from meteoric material was proposed in 1944 by Otto Schmidt, followed by the protoplanet theory of William McCrea and the capture theory of Michael Woolfson. In 1978, Andrew Prentice resurrected the initial Laplacian ideas about planet formation and developed the modern Laplacian theory. None of these models proved successful, many of the proposed theories were descriptive; the 1944 accretion model by Otto Schmidt was further developed in a quantitative way in 1969 by Viktor Safronov. He calculated, in detail, the different stages of terrestrial planet formation. Since the model has been further developed using intensive numerical simulations to study planetesimal accumulation.
It is now accepted. Prior to collapse, this gas is in the form of molecular clouds, such as the Orion Nebula; as the cloud collapses, losing potential energy, it heats up, gaining kinetic energy, the conservation of angular momentum ensures that the cloud forms a flatted disk—the accretion disk. A few hundred thousand years after the Big Bang, the Universe cooled to the point where atoms could form; as the Universe continued to expand and cool, the atoms lost enough kinetic energy, dark matter coalesced sufficiently, to form protogalaxies. As further accretion occurred, galaxies formed. Indirect evidence is widespread. Galaxies grow through smooth gas accretion. Accretion occurs inside galaxies, forming stars. Stars are thought to form inside giant clouds of cold molecular hydrogen—giant molecular clouds of 300,000 M☉ and 65 light-years in diameter. Over millions of years, giant molecular clouds are prone to fragmentation; these fragments form small, dense cores, which in turn collapse into stars.
The cores range in mass from a fraction to several times that of the Sun and are called protostellar nebulae. They possess diameters of 2,000–20,000 astronomical units and a particle number density of 10,000 to 100,000/cm3. Compare it with the particle number density of the air at the sea level—2.8×1019/cm3. The initial collapse of a solar-mass protostellar nebula takes around 100,000 years; every nebula begins with a certain amount of angular momentum. Gas in the central part of the nebula, with low angular momentum, undergoes fast compression and forms a hot hydrostatic core containing a small fraction of the mass of the original nebula; this core forms the seed of. As the collapse continues, conservation of angular momentum dictates that the rotation of the infalling envelope accelerates, which forms a disk; as the infall of material from the disk continues, the envelope becomes thin and transparent and the young stellar object becomes observable in far-infrared light and in the visible. Around this time the protostar begins to fuse deuterium.
If the protostar is sufficiently massive, hydrogen fusion follows. Otherwise, if its mass is too low, the object becomes a brown dwarf; this birth of a new star occurs 100,000 years after the collapse begins. Objects at this stage are known as Class I protostars, which are called young T Tauri stars, evolved protostars, or young stellar objects. By this time, the forming star has accreted much of its mass. At the next stage, the envelope disappears, having been gathered up by the disk, the protostar becomes a classical T Tauri star; the latter have accretion disks and continue to accrete hot gas, which manifests itself by strong emission lines in their spectrum. The former do not possess accretion disks. Classical T Tauri stars evolve into weakly lined T Tauri stars; this happens after about 1 million years. The mass of the disk around a classical T Tauri star is about 1–3% of the stellar mass, it is accreted at a rate of 10−7 to 10−9 M☉ per year. A pair of bipolar jets is present as well; the accretion explains all peculiar properties of classical T Tauri stars: strong flux in the emission lines, magnetic activity, photometric variability and jets.
The emission lines form as the accreted gas hits the "surface" of the star, which happens around its magnetic poles. The jets are byproducts of accretion: they carry away excessive angular momentum; the classical T Tauri stage lasts about 10 million years. The disk disappears due to accretion onto the central star, planet formation, ejection by jets, photoevaporation by ultraviolet radiation from the central star and nearby stars; as a result, the young star becomes a weakly lined T Tauri star, over hundreds of millions of years, evolves into an ordinary Sun-like star, dependent on its initial mass. Self-accretion of cosmic dust accelerates the growth of the particles into boulder-sized planetesimals; the more massive planetesimals accrete some smaller ones. Accretion disks are common around smaller stars, or stellar remnants in a close binary, or black holes surrounded by material, such as those at the centers of galaxies; some dynamics in the disk, such as dynamical friction, are necessary to allow orbiting gas to lose angular momentum and fall onto the central mas
An airfoil or aerofoil is the cross-sectional shape of a wing, blade, or sail. An airfoil-shaped body moved through a fluid process, produces an aerodynamic force; the component of this force perpendicular to the direction of motion is called lift. The component parallel to the direction of motion is called drag. Subsonic flight airfoils have a characteristic shape with a rounded leading edge, followed by a sharp trailing edge with a symmetric curvature of upper and lower surfaces. Foils of similar function designed with water as the working fluid are called hydrofoils; the lift on an airfoil is the result of its angle of attack and shape. When oriented at a suitable angle, the airfoil deflects the oncoming air, resulting in a force on the airfoil in the direction opposite to the deflection; this force can be resolved into two components: lift and drag. Most foil shapes require a positive angle of attack to generate lift, but cambered airfoils can generate lift at zero angle of attack; this "turning" of the air in the vicinity of the airfoil creates curved streamlines, resulting in lower pressure on one side and higher pressure on the other.
This pressure difference is accompanied by a velocity difference, via Bernoulli's principle, so the resulting flowfield about the airfoil has a higher average velocity on the upper surface than on the lower surface. The lift force can be related directly to the average top/bottom velocity difference without computing the pressure by using the concept of circulation and the Kutta-Joukowski theorem. A fixed-wing aircraft's wings and vertical stabilizers are built with airfoil-shaped cross sections, as are helicopter rotor blades. Airfoils are found in propellers, fans and turbines. Sails are airfoils, the underwater surfaces of sailboats, such as the centerboard and keel, are similar in cross-section and operate on the same principles as airfoils. Swimming and flying creatures and many plants and sessile organisms employ airfoils/hydrofoils: common examples being bird wings, the bodies of fish, the shape of sand dollars. An airfoil-shaped wing can create downforce on an automobile or other motor vehicle, improving traction.
Any object, such as a flat plate, a building, or the deck of a bridge, with an angle of attack in a moving fluid will generate an aerodynamic force perpendicular to the flow. Airfoils are more efficient lifting shapes, able to generate more lift than sized flat plates, to generate lift with less drag. A lift and drag curve obtained in wind tunnel testing is shown on the right; the curve represents an airfoil with a positive camber so some lift is produced at zero angle of attack. With increased angle of attack, lift increases in a linear relation, called the slope of the lift curve. At about 18 degrees this airfoil stalls, lift falls off beyond that; the drop in lift can be explained by the action of the upper-surface boundary layer, which separates and thickens over the upper surface at and past the stall angle. The thickened boundary layer's displacement thickness changes the airfoil's effective shape, in particular it reduces its effective camber, which modifies the overall flow field so as to reduce the circulation and the lift.
The thicker boundary layer causes a large increase in pressure drag, so that the overall drag increases near and past the stall point. Airfoil design is a major facet of aerodynamics. Various airfoils serve different flight regimes. Asymmetric airfoils can generate lift at zero angle of attack, while a symmetric airfoil may better suit frequent inverted flight as in an aerobatic airplane. In the region of the ailerons and near a wingtip a symmetric airfoil can be used to increase the range of angles of attack to avoid spin–stall, thus a large range of angles can be used without boundary layer separation. Subsonic airfoils have a round leading edge, insensitive to the angle of attack; the cross section is not circular, however: the radius of curvature is increased before the wing achieves maximum thickness to minimize the chance of boundary layer separation. This moves the point of maximum thickness back from the leading edge. Supersonic airfoils are much more angular in shape and can have a sharp leading edge, sensitive to angle of attack.
A supercritical airfoil has its maximum thickness close to the leading edge to have a lot of length to shock the supersonic flow back to subsonic speeds. Such transonic airfoils and the supersonic airfoils have a low camber to reduce drag divergence. Modern aircraft wings may have different airfoil sections along the wing span, each one optimized for the conditions in each section of the wing. Movable high-lift devices and sometimes slats, are fitted to airfoils on every aircraft. A trailing edge flap acts to an aileron. A laminar flow wing has a maximum thickness in the middle camber line. Analyzing the Navier–Stokes equations in the linear regime shows that a negative pressure gradient along the flow has the same effect as reducing the speed. So with the maximum camber in the middle, maintaining a laminar flow over a larger percentage of the wing at a higher cruising speed is possible. However, some surface contamination will disrupt the laminar flow. For example, with rain on the wing, the flow will be turbulent.
Under certain conditions, insect debris on the wing will cause the loss of small regions of laminar f
Supersonic travel is a rate of travel of an object that exceeds the speed of sound. For objects traveling in dry air of a temperature of 20 °C at sea level, this speed is 344 m/s, 1,125 ft/s, 768 mph, 667 knots, or 1,235 km/h. Speeds greater than five times the speed of sound are referred to as hypersonic. Flights during which only some parts of the air surrounding an object, such as the ends of rotor blades, reach supersonic speeds are called transonic; this occurs somewhere between Mach 0.8 and Mach 1.2. Sounds are traveling vibrations in the form of pressure waves in an elastic medium. In gases, sound travels longitudinally at different speeds depending on the molecular mass and temperature of the gas, pressure has little effect. Since air temperature and composition varies with altitude, Mach numbers for aircraft may change despite a constant travel speed. In water at room temperature supersonic speed can be considered as any speed greater than 1,440 m/s. In solids, sound waves can be polarized longitudinally or transversely and have higher velocities.
Supersonic fracture is crack motion faster than the speed of sound in a brittle material. At the beginning of the 20th century, the term "supersonic" was used as an adjective to describe sound whose frequency is above the range of normal human hearing; the modern term for this meaning is "ultrasonic". The tip of a bullwhip is thought to be the first man-made object to break the sound barrier, resulting in the telltale "crack"; the wave motion traveling through the bullwhip is what makes it capable of achieving supersonic speeds. Most modern fighter aircraft are supersonic aircraft, but there have been supersonic passenger aircraft, namely Concorde and the Tupolev Tu-144. Both these passenger aircraft and some modern fighters are capable of supercruise, a condition of sustained supersonic flight without the use of an afterburner. Due to its ability to supercruise for several hours and the high frequency of flight over several decades, Concorde spent more time flying supersonically than all other aircraft combined by a considerable margin.
Since Concorde's final retirement flight on November 26, 2003, there are no supersonic passenger aircraft left in service. Some large bombers, such as the Tupolev Tu-160 and Rockwell B-1 Lancer are supersonic-capable. Most modern firearm bullets are supersonic, with rifle projectiles travelling at speeds approaching and in some cases well exceeding Mach 3. Most spacecraft, most notably the Space Shuttle are supersonic at least during portions of their reentry, though the effects on the spacecraft are reduced by low air densities. During ascent, launch vehicles avoid going supersonic below 30 km to reduce air drag. Note that the speed of sound decreases somewhat with altitude, due to lower temperatures found there. At higher altitudes the temperature starts increasing, with the corresponding increase in the speed of sound; when an inflated balloon is burst, the torn pieces of latex contract at supersonic speed, which contributes to the sharp and loud popping noise. To date, only one land vehicle has travelled at supersonic speed.
It is ThrustSSC, driven by Andy Green, which holds the world land speed record, having achieved an average speed on its bi-directional run of 1,228 km/h in the Black Rock Desert on 15 October 1997. Richard Noble, Andy Green and a team of engineers are planning to break this record in 2019 at Hakskeen Pan in South Africa with the Bloodhound SSC hybrid jet- and rocket-propelled car. Supersonic aerodynamics is simpler than subsonic aerodynamics because the airsheets at different points along the plane cannot affect each other. Supersonic jets and rocket vehicles require several times greater thrust to push through the extra aerodynamic drag experienced within the transonic region. At these speeds aerospace engineers can guide air around the fuselage of the aircraft without producing new shock waves, but any change in cross area farther down the vehicle leads to shock waves along the body. Designers use the Supersonic area rule and the Whitcomb area rule to minimize sudden changes in size. However, in practical applications, a supersonic aircraft must operate stably in both subsonic and supersonic profiles, hence aerodynamic design is more complex.
One problem with sustained supersonic flight is the generation of heat in flight. At high speeds aerodynamic heating can occur, so an aircraft must be designed to operate and function under high temperatures. Duralumin, the traditional aircraft material, starts to lose strength and go into plastic deformation at low temperatures, is unsuitable for continuous use at speeds above Mach 2.2 to 2.4. Materials such as titanium and stainless steel allow operations at much higher temperatures. For example, the Lockheed SR-71 Blackbird jet could fly continuously at Mach 3.1 which could lead to temperatures on some parts of the aircraft getting above 315 °C. Another area of concern for sustained high-speed flight is engine operation. Jet engines create thrust by increasing the temperature of the air they ingest, as the aircraft speeds up, friction and compression heat this air before it reaches the engines; the maximum allowable temperature of the exhaust is determined by the materials in the turbine at the rear of the engine, so as the aircraft speeds up, the difference in intake and exhaust temperature that the engine can create decreases, the thrust along with it.
Air cooling the turbine area to allow operations at higher temperatures was a key solution, one that continued to improve through the 1950s and on to this day. Intake design
Interstellar Boundary Explorer
Interstellar Boundary Explorer is a NASA satellite in Earth orbit that uses Energetic Neutral Atoms to image the interaction region between the Solar System and interstellar space. The mission is part of NASA's Small Explorer program and was launched with a Pegasus-XL rocket on October 19, 2008; the mission is led by Dr. David J. McComas of the Southwest Research Institute and now with Princeton University; the Los Alamos National Laboratory and the Lockheed Martin Advanced Technology Center built the IBEX-Hi and IBEX-Lo sensors respectively. The Orbital Sciences Corporation manufactured the spacecraft bus and was the location for spacecraft environmental testing; the nominal mission baseline duration was two years after commissioning, the prime ended in early 2011. The spacecraft and sensors are still healthy and the mission is continuing in its extended mission. IBEX is in a Sun-oriented spin-stabilized orbit around the Earth. In June 2011, IBEX was shifted to a new much more stable orbit, it does not come as close to the Moon in the new orbit, expends less fuel to maintain its position.
The Interstellar Boundary Explorer mission science goal is to discover the nature of the interactions between the solar wind and the interstellar medium at the edge of our solar system. IBEX has achieved this goal by generating full sky maps of the intensity of ENAs in a range of energies every six months. Most of these ENAs are generated in the heliosheath, the region of interaction; the IBEX satellite was mated to its Pegasus XL rocket at Vandenberg Air Force Base and the combined vehicle was suspended below the Lockheed L-1011 Stargazer mother airplane and flown to Kwajalein Atoll in the Central Pacific Ocean. Stargazer arrived at Kwajalein on Sunday, October 12, 2008; the IBEX satellite was carried into space on October 2008, by the Pegasus XL rocket. The rocket was released from Stargazer, which took off from Kwajalein, at 17:47:23 UTC. By launching from this site close to the Equator, the Pegasus rocket lifted as much as 16 kg more mass to orbit than it would have with a launch from the Kennedy Space Center in Florida.
The IBEX satellite launched into a highly-elliptical transfer orbit with a low perigee, used a solid fuel rocket motor as its final boost stage at apogee, in order to raise its perigee and to achieve its desired high-altitude elliptical orbit. IBEX is in a highly-eccentric elliptical terrestrial orbit, which ranges from a perigee of about 86,000 km to an apogee of about 260,000 km, its original orbit was about 7,000 by 320,000 km —that is, about 80% of the distance to the Moon—which has changed due to an intentional adjustment to prolong the spacecraft's useful life. This high orbit allows the IBEX satellite to move out of the Earth's magnetosphere when making scientific observations; this extreme altitude is critical due to the amount of charged-particle interference that would occur while taking measurements within the magnetosphere. When within the magnetosphere of the Earth, the satellite performs other functions, including telemetry downlinks. In June 2011 IBEX shifted to a new orbit; the new orbit has a period of one third of a lunar month, with the correct phasing, avoids taking the spacecraft too close to the Moon, whose gravity can negatively affect IBEX's orbit.
Now the spacecraft uses less fuel to maintain a stable orbit, increasing its useful lifespan to more than 40 years. The heliospheric boundary of the Solar System is being imaged by measuring the location and magnitude of charge-exchange collisions occurring in all directions; the satellite's payload consists of IBEX-Hi and IBEX-Lo. Each consists of a collimator that limits their fields-of-view, a conversion surface to convert neutral hydrogen and oxygen into ions, an electrostatic analyzer to suppress ultraviolet light and to select ions of a specific energy range, a detector to count particles and identify the type of each ion. Both of these sensors are a single-pixel camera with a field-of-view of 7° x 7°; the IBEX-Hi instrument is recording particle counts in a higher energy band than the IBEX-Lo energy band. The scientific payload includes a Combined Electronics Unit that controls the voltages on the collimator and the ESA, it reads and records data from the particle detectors of each sensor.
Compared to other space observatories, IBEX has a low data transfer rate due to the limited requirements of the mission.... IBEX data transfer rates are slow compared with other telescopes due to the nature of the data it collects. IBEX does not need a "high speed" connection, since it only has the opportunity to collect up to a few particles per minute. Communication from the satellite to the ground is 20 times slower than a typical home cable modem and from the ground to the satellite only 2,000 bits per second, 250 times slower! Once the signal is collected by the receivers on Earth it is carried over the internet to Mission Control Center in Dulles, VA and to the IBEX Science Operation Center in San Antonio, TX." IBEX is collecting energetic neutral atom emissions that are traveling through the Solar System to Earth that cannot be measured by conventional telescopes. These ENAs are created on the boundary of our Solar System by the interactions between solar wind particles and interstellar medium particles.
On the average IBEX-Hi detects about 500 particles per day, IBEX-Lo, less than 100. By 2012, o
The Whitcomb area rule called the transonic area rule, is a design technique used to reduce an aircraft's drag at transonic and supersonic speeds between Mach 0.75 and 1.2. This is one of the most important operating speed ranges for commercial and military fixed-wing aircraft today, with transonic acceleration being considered an important performance metric for combat aircraft and dependent upon transonic drag. At high-subsonic flight speeds, the local speed of the airflow can reach the speed of sound where the flow accelerates around the aircraft body and wings; the speed at which this development occurs varies from aircraft to aircraft and is known as the critical Mach number. The resulting shock waves formed at these points of sonic flow can result in a sudden increase in drag, called wave drag. To reduce the number and power of these shock waves, an aerodynamic shape should change in cross sectional area as smoothly as possible; the area rule says that two airplanes with the same longitudinal cross-sectional area distribution have the same wave drag, independent of how the area is distributed laterally.
Furthermore, to avoid the formation of strong shock waves, this total area distribution must be smooth. As a result, aircraft have to be arranged so that at the location of the wing, the fuselage is narrowed or "waisted", so that the total area does not change much. Similar but less pronounced fuselage waisting is used at the location of a bubble canopy and the tail surfaces; the area rule holds true at speeds exceeding the speed of sound, but in this case, the body arrangement is in respect to the Mach line for the design speed. For example, consider that at Mach 1.3 the angle of the Mach cone formed off the body of the aircraft will be at about μ = arcsin = 50.3°. In this case the "perfect shape" is biased rearward. A classic example of such a design is the Concorde; when applying the transonic area rule, the condition that the plane defining the cross-section meets the longitudinal axis at the Mach angle μ no longer prescribes a unique plane for μ other than the 90° given by M = 1. The correct procedure is to average over all possible orientations of the intersecting plane.
A superficially related concept is the Sears–Haack body, the shape with the minimum wave drag for a given length and a given volume. However, the Sears–Haack body shape is derived starting with the Prandtl–Glauert equation which governs small-disturbance supersonic flows, but this equation is not valid for transonic flows. So although the Sears–Haack body shape, being smooth, will have favorable wave drag properties according to the area rule, it is not theoretically optimum; the area rule was discovered by Otto Frenzl when comparing a swept wing with a w-wing with extreme high wave drag while working on a transonic wind tunnel at Junkers works in Germany between 1943 and 1945. He wrote a description on 17 December 1943, with the title "Arrangement of Displacement Bodies in High-Speed Flight"; the results of this research were presented to a wide circle in March 1944 by Theodor Zobel at the Deutsche Akademie der Luftfahrtforschung in the lecture "Fundamentally new ways to increase performance of high speed aircraft."Subsequent German wartime aircraft design took account of the discovery, evident in slim mid-fuselage of aircraft including the Messerschmitt P.1112, P.1106 and Focke-Wulf 1000x1000x1000 type A long-range bomber, but apparent in delta wing designs including the Henschel Hs 135.
Several other researchers came close to developing a similar theory, notably Dietrich Küchemann who designed a tapered fighter, dubbed the "Küchemann Coke Bottle" when it was discovered by US forces in 1946. In this case Küchemann arrived at the theory by studying airflow, notably spanwise flow, over a swept wing; the swept wing is an indirect application of the area rule. Wallace D. Hayes, a pioneer of supersonic flight, developed the transonic area rule in publications beginning in 1947 with his Ph. D. thesis at the California Institute of Technology. Richard T. Whitcomb, after whom the rule is named, independently discovered this rule in 1952, while working at the NACA. While using the new Eight-Foot High-Speed Tunnel, a wind tunnel with performance up to Mach 0.95 at NACA's Langley Research Center, he was surprised by the increase in drag due to shock wave formation. Whitcomb realized that, for analytical purposes, an airplane could be reduced to a streamlined body of revolution, elongated as much as possible to mitigate abrupt discontinuities and, hence abrupt drag rise.
The shocks could be seen using Schlieren photography, but the reason they were being created at speeds far below the speed of sound, sometimes as low as Mach 0.70, remained a mystery. In late 1951, the lab hosted a talk by Adolf Busemann, a famous German aerodynamicist who had moved to Langley after World War II, he talked about the behavior of airflow around an airplane as its speed approached the critical Mach number, when air no longer behaved as an incompressible fluid. Whereas engineers were used to thinking of air flowing smoothly around the body of the aircraft, at high speeds it did not have time to "get out of the way", instead started to flow as if it were rigid pipes of flow, a concept Busemann referred to as "streampipes", as opposed to streamlines, jokingly suggested that engineers had to consider themselves "pipefitters". Several days Whitcomb had a "Eureka" moment; the reason for the high drag was
In aerodynamics, a hypersonic speed is one that exceeds the speed of sound stated as starting at speeds of Mach 5 and above. The precise Mach number at which a craft can be said to be flying at hypersonic speed varies, since individual physical changes in the airflow occur at different speeds; the hypersonic regime is alternatively defined as speeds where Cp and Cv are no longer able to be reasonably considered constant. While the definition of hypersonic flow can be quite vague and is debatable, a hypersonic flow may be characterized by certain physical phenomena that can no longer be analytically discounted as in supersonic flow; the peculiarity in hypersonic flows are as follows: Shock layer Aerodynamic heating Entropy layer Real gas effects Low density effects Independence of aerodynamic coefficients with Mach number. As a body's Mach number increases, the density behind a bow shock generated by the body increases, which corresponds to a decrease in volume behind the shock due to conservation of mass.
The distance between the bow shock and the body decreases at higher Mach numbers. As Mach numbers increase, the entropy change across the shock increases, which results in a strong entropy gradient and vortical flow that mixes with the boundary layer. A portion of the large kinetic energy associated with flow at high Mach numbers transforms into internal energy in the fluid due to viscous effects; the increase in internal energy is realized as an increase in temperature. Since the pressure gradient normal to the flow within a boundary layer is zero for low to moderate hypersonic Mach numbers, the increase of temperature through the boundary layer coincides with a decrease in density; this causes the bottom of the boundary layer to expand, so that the boundary layer over the body grows thicker and can merge with the shock wave near the body leading edge. High temperatures due to a manifestation of viscous dissipation cause non-equilibrium chemical flow properties such as vibrational excitation and dissociation and ionization of molecules resulting in convective and radiative heat-flux.
Although "subsonic" and "supersonic" refer to speeds below and above the local speed of sound aerodynamicists use these terms to refer to particular ranges of Mach values. This occurs because a "transonic regime" exists around M=1 where approximations of the Navier–Stokes equations used for subsonic design no longer apply because the flow locally exceeds M=1 when the freestream Mach number is below this value; the "supersonic regime" refers to the set of Mach numbers for which linearised theory may be used. NASA defines "high" hypersonic as any Mach number from 10 to 25, re-entry speeds as anything greater than Mach 25. Among the aircraft operating in this regime are the Space Shuttle and various developing spaceplanes. In the following table, the "regimes" or "ranges of Mach values" are referenced instead of the usual meanings of "subsonic" and "supersonic"; the categorization of airflow relies on a number of similarity parameters, which allow the simplification of a nearly infinite number of test cases into groups of similarity.
For transonic and compressible flow, the Mach and Reynolds numbers alone allow good categorization of many flow cases. Hypersonic flows, require other similarity parameters. First, the analytic equations for the oblique shock angle become nearly independent of Mach number at high Mach numbers. Second, the formation of strong shocks around aerodynamic bodies means that the freestream Reynolds number is less useful as an estimate of the behavior of the boundary layer over a body; the increased temperature of hypersonic flows mean that real gas effects become important. For this reason, research in hypersonics is referred to as aerothermodynamics, rather than aerodynamics; the introduction of real gas effects means that more variables are required to describe the full state of a gas. Whereas a stationary gas can be described by three variables, a moving gas by four, a hot gas in chemical equilibrium requires state equations for the chemical components of the gas, a gas in nonequilibrium solves those state equations using time as an extra variable.
This means that for a nonequilibrium flow, something between 10 and 100 variables may be required to describe the state of the gas at any given time. Additionally, rarefied hypersonic flows do not follow the Navier–Stokes equations. Hypersonic flows are categorized by their total energy, expressed as total enthalpy, total pressure, stagnation pressure, stagnation temperature, or flow velocity. Wallace D. Hayes developed a similarity parameter, similar to the Whitcomb area rule, which allowed similar configurations to be compared. Hypersonic flow can be separated into a number of regimes; the selection of these regimes is rough, due to the blurring of the boundaries where a particular effect can be found. In this regime, the gas can be regarded as an ideal gas. Flow in this regime is still Mach number dependent. Simulations start to depend on the use of a constant-temperature wall, rather than the adiabatic wall used at lower speeds; the lower border of this region is around Mac
A jet engine is a type of reaction engine discharging a fast-moving jet that generates thrust by jet propulsion. This broad definition includes airbreathing jet engines. In general, jet engines are combustion engines. Common parlance applies the term jet engine only to various airbreathing jet engines; these feature a rotating air compressor powered by a turbine, with the leftover power providing thrust via a propelling nozzle – this process is known as the Brayton thermodynamic cycle. Jet aircraft use such engines for long-distance travel. Early jet aircraft used turbojet engines which were inefficient for subsonic flight. Most modern subsonic jet aircraft use more complex high-bypass turbofan engines, they give higher speed and greater fuel efficiency than piston and propeller aeroengines over long distances. A few air-breathing engines made for high speed applications use the ram effect of the vehicle's speed instead of a mechanical compressor; the thrust of a typical jetliner engine went from 5,000 lbf in the 1950s to 115,000 lbf in the 1990s, their reliability went from 40 in-flight shutdowns per 100,000 engine flight hours to less than 1 per 100,000 in the late 1990s.
This, combined with decreased fuel consumption, permitted routine transatlantic flight by twin-engined airliners by the turn of the century, where before a similar journey would have required multiple fuel stops. Jet engines date back to the invention of the aeolipile before the first century AD; this device directed steam power through two nozzles to cause a sphere to spin on its axis. It was seen as a curiosity. Jet propulsion only gained practical applications with the invention of the gunpowder-powered rocket by the Chinese in the 13th century as a type of firework, progressed to propel formidable weaponry. Jet propulsion technology stalled for hundreds of years; the earliest attempts at airbreathing jet engines were hybrid designs in which an external power source first compressed air, mixed with fuel and burned for jet thrust. The Caproni Campini N.1, the Japanese Tsu-11 engine intended to power Ohka kamikaze planes towards the end of World War II were unsuccessful. Before the start of World War II, engineers were beginning to realize that engines driving propellers were approaching limits due to issues related to propeller efficiency, which declined as blade tips approached the speed of sound.
If aircraft performance were to increase beyond such a barrier, a different propulsion mechanism was necessary. This was the motivation behind the development of the gas turbine engine, the commonest form of jet engine; the key to a practical jet engine was the gas turbine, extracting power from the engine itself to drive the compressor. The gas turbine was not a new idea: the patent for a stationary turbine was granted to John Barber in England in 1791; the first gas turbine to run self-sustaining was built in 1903 by Norwegian engineer Ægidius Elling. Such engines did not reach manufacture due to issues of safety, reliability and sustained operation; the first patent for using a gas turbine to power an aircraft was filed in 1921 by Frenchman Maxime Guillaume. His engine was an axial-flow turbojet, but was never constructed, as it would have required considerable advances over the state of the art in compressors. Alan Arnold Griffith published An Aerodynamic Theory of Turbine Design in 1926 leading to experimental work at the RAE.
In 1928, RAF College Cranwell cadet Frank Whittle formally submitted his ideas for a turbojet to his superiors. In October 1929 he developed his ideas further. On 16 January 1930 in England, Whittle submitted his first patent; the patent showed a two-stage axial compressor feeding a single-sided centrifugal compressor. Practical axial compressors were made possible by ideas from A. A. Griffith in a seminal paper in 1926. Whittle would concentrate on the simpler centrifugal compressor only. Whittle was unable to interest the government in his invention, development continued at a slow pace. In 1935 Hans von Ohain started work on a similar design in Germany, both compressor and turbine being radial, on opposite sides of same disc unaware of Whittle's work. Von Ohain's first device was experimental and could run only under external power, but he was able to demonstrate the basic concept. Ohain was introduced to Ernst Heinkel, one of the larger aircraft industrialists of the day, who saw the promise of the design.
Heinkel had purchased the Hirth engine company, Ohain and his master machinist Max Hahn were set up there as a new division of the Hirth company. They had their first HeS 1 centrifugal engine running by September 1937. Unlike Whittle's design, Ohain used hydrogen as fuel, supplied under external pressure, their subsequent designs culminated in the gasoline-fuelled HeS 3 of 5 kN, fitted to Heinkel's simple and compact He 178 airframe and flown by Erich Warsitz in the early morning of August 27, 1939, from Rostock-Marienehe aerodrome, an impressively short time for development. The He 178 was the world's first jet plane. Heinkel applied for a US patent covering the Aircraft Power Plant by Hans Joachim Pabst von Ohain in May 31, 1939. Austrian Anselm Franz of Junkers' engine division introduced the axial-flow compressor in their jet engine. Jumo was assigned the next engine number in the RLM 109-0xx numbering sequence for gas turbine aircraft powerplants, "004", the result was t