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In engineering, a heat shield is a component designed to protect an object or a human operator from being burnt or overheated by dissipating, reflecting, and/or absorbing heat. [1] The term is most often used in reference to exhaust heat management and to systems for dissipating frictional heat. Heat shields are used most commonly in the automotive and aerospace industries.
Heat shields protect structures from extreme temperatures and thermal gradients by two primary mechanisms. Thermal insulation and radiative cooling, respectively isolate the underlying structure from high external surface temperatures, while emitting heat outwards through thermal radiation. To achieve good functionality the three attributes required of a heat shield are low thermal conductivity (high thermal resistance), high emissivity, and good thermal stability (refractoriness). [2] Porous ceramics with high emissivity coatings (HECs) are often employed to address these three characteristics, owing to the good thermal stability of ceramics, the thermal insulation of porous materials and the good radiative cooling effects offered by HECs.
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Due to the large amounts of heat given off by internal combustion engines, [3] heat shields are used on most engines to protect components and bodywork from heat damage. As well as protection, effective heat shields can give a performance benefit by reducing engine bay temperatures, therefore reducing the temperature of the air entering the engine. [4] Heat shields vary widely in price, but most are easy to fit, usually by stainless steel clips, high temperature tape or specially designed metal cable ties. There are three main types of automotive heat shield:
Heat shields are often fitted by both amateur and professional personnel during the optimization phase of engine tuning. Heat shields are also used to cool engine mount vents. When a vehicle is at higher speed there is enough ram air to cool the under the hood engine compartment, but when the vehicle is moving at lower speeds or climbing a gradient there is a need of insulating the engine heat to get transferred to other parts around it, e.g. Engine Mounts. With the help of proper thermal analysis and use of heat shields, the engine mount vents can be optimized for the best performances. [6]
Some aircraft at high speed, such as the Concorde and SR-71 Blackbird, must be designed considering similar, but lower, overheating to what occurs in spacecraft. In the case of the Concorde the aluminum nose can reach a maximum operating temperature of 127 °C (which is 180 °C higher than the ambient air outside which is below zero); the metallurgical consequences associated with the peak temperature were a significant factor in determining the maximum aircraft speed.
Recently new materials have been developed that could be superior to RCC. The prototype SHARP (Slender Hypervelocity Aerothermodynamic Research Probe) is based on ultra-high temperature ceramics such as zirconium diboride (ZrB2) and hafnium diboride (HfB2). [7] The thermal protection system based on these materials would allow to reach a speed of Mach number 7 at sea level, Mach 11 at 35000 meters and significant improvements for vehicles designed for hypersonic speed. The materials used have thermal protection characteristics in a temperature range from 0 °C to + 2000 °C, with melting point at over 3500 °C. They are also structurally more resistant than RCC, so they do not require additional reinforcements, and are very efficient in re-irradiating the absorbed heat. NASA funded (and subsequently discontinued) a research and development program in 2001 for testing this protection system through the University of Montana. [8] [9]
The European Commission funded a research project, C3HARME, under the NMP-19-2015 call of Framework Programmes for Research and Technological Development in 2016 (still ongoing) for the design, development, production and testing of a new class of ultra-refractory ceramic matrix composites reinforced with silicon carbide fibers and carbon fibers suitable for applications in severe aerospace environments. [10]
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Spacecraft that land on a planet with an atmosphere, such as Earth, Mars, and Venus, currently do so by entering the atmosphere at high speeds, depending on air resistance rather than rocket power to slow them down. A side effect of this method of atmospheric re-entry is aerodynamic heating, which can be highly destructive to the structure of an unprotected or faulty spacecraft. [11] An aerodynamic heat shield consists of a protective layer of special materials to dissipate the heat. Two basic types of aerodynamic heat shield have been used:
With possible inflatable heat shields, as developed by the US (Low Earth Orbit Flight Test Inflatable Decelerator - LOFTID) [21] and China, [22] single-use rockets like the Space Launch System are considered to be retrofitted with such heat shields to salvage the expensive engines, possibly reducing the costs of launches significantly. [23] On November 10, 2022, LOFTID was launched using an Atlas V rocket and, then, detached in order to reenter the atmosphere. [24] The outer layer of the heat shield consisted of a silicon carbide ceramic. [25] The recovered LOFTID had minimal damage. [24]
Passive cooled protectors are used to protect spaceships during atmospheric entry to absorb heat peaks and subsequently radiate heat to the atmosphere. Early versions included a substantial amount of metals such as titanium, beryllium and copper. This greatly increased the mass of the vehicle. Heat absorption and ablative systems became preferable.
In modern vehicles, passive cooling can be found as reinforced carbon–carbon material instead of metal. This material constitutes the thermal protection system of the nose and the front edges of the Space Shuttle and was proposed for the vehicle X-33. Carbon is the most refractory material known with a sublimation temperature (for graphite) of 3825 °C. These characteristics make it a material particularly suitable for passive cooling, but with the disadvantage of being very expensive and fragile. Some spacecraft also use a heat shield (in the conventional automotive sense) to protect fuel tanks and equipment from the heat produced by a large rocket engine. Such shields were used on the Apollo Service Module and Lunar Module descent stage. The Parker Solar Probe, designed to enter the corona of the Sun, experiences a surface temperature of 2,500 °F. [26] To withstand this temperature without damage to its body or instruments, the spacecraft uses a heat shield using a carbon-carbon ceramic with a layer of carbon foam in between. [27] The probe was launched into space on August 18, 2018. [28]
Heat shields are often affixed to semi-automatic or automatic rifles and shotguns as barrel shrouds in order to protect the user's hands from the heat caused by firing shots in rapid succession. They have also often been affixed to pump-action combat shotguns, allowing the soldier to grasp the barrel while using a bayonet.[ citation needed ]
Heat shields are used in metallurgical industry to protect structural steel of the building or other equipment from the high temperature of nearby liquid metal.[ citation needed ]
The Space Shuttle is a retired, partially 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. Its official program name was Space Transportation System (STS), taken from the 1969 plan led by U.S. Vice President Spiro Agnew for a system of reusable spacecraft where it was the only item funded for development.
A nuclear thermal rocket (NTR) is a type of thermal rocket where the heat from a nuclear reaction replaces the chemical energy of the propellants in a chemical rocket. In an NTR, a working fluid, usually liquid hydrogen, is heated to a high temperature in a nuclear reactor and then expands through a rocket nozzle to create thrust. The external nuclear heat source theoretically allows a higher effective exhaust velocity and is expected to double or triple payload capacity compared to chemical propellants that store energy internally.
A spacecraft is a vehicle that is designed to fly and operate in outer space. Spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, planetary exploration, and transportation of humans and cargo. All spacecraft except single-stage-to-orbit vehicles cannot get into space on their own, and require a launch vehicle.
Atmospheric entry 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. There are two main types of atmospheric entry: uncontrolled entry, such as the entry of astronomical objects, space debris, or bolides; and controlled entry of a spacecraft capable of being navigated or following a predetermined course. Technologies and procedures allowing the controlled atmospheric entry, descent, and landing of spacecraft are collectively termed as EDL.
Thermal insulation is the reduction of heat transfer between objects in thermal contact or in range of radiative influence. Thermal insulation can be achieved with specially engineered methods or processes, as well as with suitable object shapes and materials.
In the study of heat transfer, radiative cooling is the process by which a body loses heat by thermal radiation. As Planck's law describes, every physical body spontaneously and continuously emits electromagnetic radiation.
A waverider is a hypersonic aircraft design that improves its supersonic lift-to-drag ratio by using the shock waves being generated by its own flight as a lifting surface, a phenomenon known as compression lift.
A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordance with Newton's third law. Most rocket engines use the combustion of reactive chemicals to supply the necessary energy, but non-combusting forms such as cold gas thrusters and nuclear thermal rockets also exist. Vehicles propelled by rocket engines are commonly used by ballistic missiles and rockets. Rocket vehicles carry their own oxidiser, unlike most combustion engines, so rocket engines can be used in a vacuum to propel spacecraft and ballistic missiles.
VentureStar was a single-stage-to-orbit reusable launch system proposed by Lockheed Martin and funded by the U.S. government. The goal was to replace the Space Shuttle by developing a re-usable spaceplane that could launch satellites into orbit at 1/10th of the cost. While the requirement was for an uncrewed launcher, it was expected to carry passengers as cargo. The VentureStar would have had a wingspan of 68 feet (20.7 m), a length of 127 feet (38.7 m), and would have weighed roughly 1000 t.
Kapton is a polyimide film used in flexible printed circuits and space blankets, which are used on spacecraft, satellites, and various space instruments. Invented by the DuPont Corporation in the 1960s, Kapton remains stable across a wide range of temperatures, from 4 to 673 K. Kapton is used in electronics manufacturing, space applications, with x-ray equipment, and in 3D printing applications. Its favorable thermal properties and outgassing characteristics result in its regular use in cryogenic applications and in situations where high vacuum environments are experienced.
Carbon fibre reinforced carbon (CFRC), carbon–carbon (C/C), or reinforced carbon–carbon (RCC) is a composite material consisting of carbon fiber reinforcement in a matrix of graphite. It was developed for the reentry vehicles of intercontinental ballistic missiles, and is most widely known as the material for the nose cone and wing leading edges of the Space Shuttle orbiter. Carbon-carbon brake discs and brake pads have been the standard component of the brake systems of Formula One racing cars since the late 1970s; the first year carbon brakes were seen on a Formula One car was 1976.
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 which enter the Earth's atmosphere, to ensure spacecraft safely survive atmospheric reentry, and for the design of high-speed aircraft and missiles.
The Space Shuttle thermal protection system (TPS) is the barrier that protected the Space Shuttle Orbiter during the extreme 1,650 °C (3,000 °F) heat of atmospheric reentry. A secondary goal was to protect from the heat and cold of space while in orbit.
A reentry capsule is the portion of a space capsule which returns to Earth following a spaceflight. The shape is determined partly by aerodynamics; a capsule is aerodynamically stable falling blunt end first, which allows only the blunt end to require a heat shield for atmospheric entry. A crewed capsule contains the spacecraft's instrument panel, limited storage space, and seats for crew members. Because a capsule shape has little aerodynamic lift, the final descent is via parachute, either coming to rest on land, at sea, or by active capture by an aircraft. In contrast, the development of spaceplane reentry vehicles attempts to provide a more flexible reentry profile.
Multi-layer insulation (MLI) is thermal insulation composed of multiple layers of thin sheets and is often used on spacecraft and cryogenics. Also referred to as superinsulation, MLI is one of the main items of the spacecraft thermal design, primarily intended to reduce heat loss by thermal radiation. In its basic form, it does not appreciably insulate against other thermal losses such as heat conduction or convection. It is therefore commonly used on satellites and other applications in vacuum where conduction and convection are much less significant and radiation dominates. MLI gives many satellites and other space probes the appearance of being covered with gold foil which is the effect of the amber-coloured Kapton layer deposited over the silver Aluminized mylar.
An aeroshell is a rigid heat-shielded shell that helps decelerate and protects a spacecraft vehicle from pressure, heat, and possible debris created by drag during atmospheric entry. Its main components consist of a heat shield and a back shell. The heat shield absorbs heat caused by air compression in front of the spacecraft during its atmospheric entry. The back shell carries the load being delivered, along with important components such as a parachute, rocket engines, and monitoring electronics like an inertial measurement unit that monitors the orientation of the shell during parachute-slowed descent.
A reusable spacecraft is a class of spacecraft that have been designed with repeated launch, orbit, deorbit and atmospheric reentry in mind. This contrasts with conventional spacecraft which are designed to be expended after use. Examples of reusable spacecraft are spaceplanes and space capsules like the SpaceX Dragon. Such spacecraft need mechanisms to prevent the disintegration of the spacecraft and its occupants/cargo during reentry.
AVCOAT 5026-39 is a NASA code for a specific ablative heat shield material created by Avco . It is an epoxy novolac resin with special additives in a fiberglass honeycomb matrix. In fabrication, the empty honeycomb is bonded to the primary structure and the resin is gunned into each cell individually.
In spacecraft design, the function of the thermal control system (TCS) is to keep all the spacecraft's component systems within acceptable temperature ranges during all mission phases. It must cope with the external environment, which can vary in a wide range as the spacecraft is exposed to the extreme coldness found in the shadows of deep space or to the intense heat found in the unfiltered direct sunlight of outer space. A TCS must also moderate the internal heat generated by the operation of the spacecraft it serves.
Silicone Impregnated Refractory Ceramic Ablator, or SIRCA, is a lightweight ceramic ablative material, often used in thermal protection systems to protect parts of launch vehicles and spacecraft from very high temperature heat sources.
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