Aerodynamic heating is the heating of a solid body produced by its high-speed passage through air. In science and engineering, an understanding of aerodynamic heating is necessary for predicting the behaviour of

meteoroids A meteoroid ( ) is a small rocky or metallic body in outer space. Meteoroids are distinguished as objects significantly smaller than ''asteroids'', ranging in size from grains to objects up to wide. Objects smaller than meteoroids are classifie ...

which enter the Earth's atmosphere, to ensure spacecraft safely survive

atmospheric reentry Atmospheric entry (sometimes listed as Vimpact or Ventry) is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. Atmospheric entry may be ''uncontrolled entry ...

, and for the design of high-speed

aircraft An aircraft ( aircraft) is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravity by using either Buoyancy, static lift or the Lift (force), dynamic lift of an airfoil, or, i ...

and missiles. "For high speed aircraft and missiles aerodynamic heating is the conversion of kinetic energy into heat energy as a result of their relative motion in stationary air and the subsequent transfer through the skin into the structure and interior of the vehicle. Some heat is produced by fluid compression at and near stagnation points such as the vehicle nose and wing leading edges. Additional heat is generated from air friction along the skin inside the boundary layer". These two regions of skin heating are shown by van Driest. Boundary layer heating of the skin may be known as kinetic heating.

The effect of skin heating on aircraft wing design

The effects of aerodynamic heating on the temperature of the skin, and subsequent heat transfer into the structure, the cabin, the equipment bays and the electrical, hydraulic and fuel systems, have to be incorporated in the design of

supersonic Supersonic speed is the speed of an object that exceeds the speed of sound (Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately . Speeds greater than five times ...

and

hypersonic aircraft Hypersonic flight is flight through the atmosphere below altitudes of about at Hypersonic speed, speeds greater than Mach 5, a speed where Dissociation (chemistry), dissociation of air begins to become significant and high heat loads exist. Spe ...

and missiles. One of the main concerns caused by aerodynamic heating arises in the design of the wing. For subsonic speeds, two main goals of wing design are minimizing weight and maximizing strength. Aerodynamic heating, which occurs at supersonic and hypersonic speeds, adds an additional consideration in wing structure analysis. An idealized wing structure is made up of

spars SPARS was the authorized nickname for the United States Coast Guard (USCG) Women's Reserve. The nickname was derived from the USCG's motto, "—"Always Ready" (''SPAR''). The Women's Reserve was established by law in November 1942 during Wor ...

, stringers, and skin segments. In a wing that normally experiences subsonic speeds, there must be a sufficient number of stringers to withstand the axial and bending stresses induced by the lift force acting on the wing. In addition, the distance between the stringers must be small enough that the skin panels do not buckle, and the panels must be thick enough to withstand the

shear stress Shear stress (often denoted by , Greek alphabet, Greek: tau) is the component of stress (physics), stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross secti ...

and shear flow present in the panels due to the lifting force on the wing. However, the weight of the wing must be made as small as possible, so the choice of material for the stringers and the skin is an important factor. At supersonic speeds, aerodynamic heating adds another element to this structural analysis. At normal speeds, spars and stringers experience a load which is a function of the lift force, first and second moments of inertia, and length of the spar. When there are more spars and stringers, the load in each member is reduced, and the area of the stringer can be reduced to meet critical stress requirements. However, the increase in temperature caused by energy flowing from the air (heated by skin friction at these high speeds) adds another load factor, called a thermal load, to the spars. This thermal load increases the force felt by the stringers, and thus the area of the stringers must be increased in order for the critical stress requirement to be met. Another issue that aerodynamic heating causes for aircraft design is the effect of high temperatures on common material properties. Common materials used in aircraft wing design, such as aluminum and steel, experience a decrease in strength as temperatures get extremely high. The

Young's Modulus Young's modulus (or the Young modulus) is a mechanical property of solid materials that measures the tensile or compressive stiffness when the force is applied lengthwise. It is the modulus of elasticity for tension or axial compression. Youn ...

of the material, defined as the ratio between stress and strain experienced by the material, decreases as the temperature increases. Young's Modulus is critical in the selection of materials for wing, as a higher value lets the material resist the yield and shear stress caused by the lift and thermal loads. This is because Young's Modulus is an important factor in the equations for calculating the critical buckling load for axial members and the critical buckling shear stress for skin panels. If the Young's Modulus of the material decreases at high temperatures caused by aerodynamic heating, then the wing design will call for larger spars and thicker skin segments in order to account for this decrease in strength as the aircraft goes supersonic. There are some materials that retain their strength at the high temperatures that aerodynamic heating induces. For example, Inconel X-750 was used on parts of the airframe of the

X-15 The North American X-15 is a Hypersonic speed, hypersonic rocket-powered aircraft which was operated by the United States Air Force and the National Aeronautics and Space Administration (NASA) as part of the List of X-planes, X-plane series of ...

, a North American aircraft that flew at hypersonic speeds in 1958. Titanium is another high-strength material, even at high temperatures, and is often used for wing frames of supersonic aircraft. The

SR-71 The Lockheed SR-71 "Blackbird" is a retired Range (aeronautics), long-range, high-altitude, Mach number, Mach 3+ military strategy, strategic reconnaissance aircraft developed and manufactured by the American aerospace company Lockheed Co ...

used titanium skin panels painted black to reduce the temperature and corrugated to accommodate expansion. Another important design concept for early supersonic aircraft wings was using a small thickness-to-chord ratio, so that the speed of the flow over the airfoil does not increase too much from the free stream speed. As the flow is already supersonic, increasing the speed even more would not be beneficial for the wing structure. Reducing the thickness of the wing brings the top and bottom stringers closer together, reducing the total moment of inertia of the structure. This increases axial load in the stringers, and thus the area, and weight, of the stringers must be increased. Some designs for hypersonic missiles have used liquid cooling of the

leading edge The leading edge is the part of the wing that first contacts the air;Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 305. Aviation Supplies & Academics, 1997. alternatively it is the foremost edge of an airfoil sectio ...

s (usually the fuel en route to the engine). The Sprint missile's heat shield needed several design iterations for Mach 10 temperatures.

Reentry vehicles

Heating caused by the very high reentry speeds (greater than

Mach The Mach number (M or Ma), often only Mach, (; ) is a dimensionless quantity in fluid dynamics representing the ratio of flow velocity past a Boundary (thermodynamic), boundary to the local speed of sound. It is named after the Austrian physi ...

20) is sufficient to destroy the vehicle unless special techniques are used. The early space capsules such as used on Mercury, Gemini, and

Apollo Apollo is one of the Twelve Olympians, Olympian deities in Ancient Greek religion, ancient Greek and Ancient Roman religion, Roman religion and Greek mythology, Greek and Roman mythology. Apollo has been recognized as a god of archery, mu ...

were given blunt shapes to produce a stand-off

bow shock In astrophysics, bow shocks are shock waves in regions where the conditions of density and pressure change dramatically due to blowing stellar wind. Bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby fl ...

, allowing most of the heat to dissipate into the surrounding air. Additionally, these vehicles had ablative material that sublimates into a gas at high temperature. The act of sublimation absorbs the thermal energy from the aerodynamic heating and erodes the material rather than heating the capsule. The surface of the heat shield for the Mercury spacecraft had a coating of aluminium with glassfiber in many layers. As the temperature rose to the layers would evaporate and take the heat with it. The spacecraft would become hot but not harmfully so. The

Space Shuttle The Space Shuttle is a retired, partially reusable launch system, reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. ...

used insulating

tile Tiles are usually thin, square or rectangular coverings manufactured from hard-wearing material such as ceramic, Rock (geology), stone, metal, baked clay, or even glass. They are generally fixed in place in an array to cover roofs, floors, wal ...

s on its lower surface to absorb and radiate heat while preventing conduction to the aluminium

airframe The mechanical structure of an aircraft is known as the airframe. This structure is typically considered to include the fuselage, undercarriage, empennage and wings, and excludes the propulsion system. Airframe design is a field of aeros ...

. Damage to the heat shield during liftoff of

'' Columbia'' contributed to its

destruction Destruction may refer to: Concepts * Destruktion, a term from the philosophy of Martin Heidegger * Destructive narcissism, a pathological form of narcissism * Self-destructive behaviour, a widely used phrase that ''conceptualises'' certain kin ...

upon reentry.

References

{{Reflist

The effect of skin heating on aircraft wing design

Reentry vehicles

See also

References

Further reading