Jet blast deflector
Encyclopedia
A jet blast deflector (JBD) or blast fence is a safety device that redirects the high energy exhaust from a jet engine
to prevent damage and injury. The structure must be strong enough to withstand heat and high speed air streams as well as dust and debris carried by the turbulent air. Without a deflector, jet blast
can be dangerous to people, equipment and other aircraft.
Jet blast deflectors range in complexity from stationary concrete, metal or fiberglass fences to heavy panels that are raised and lowered by hydraulic arms and actively cooled. Blast deflectors can be used as protection from helicopter
and fixed-wing aircraft
propwash and from rocket engine
exhaust. At airport
s and jet engine service centers, jet blast deflectors can be combined with sound-deadening
walls to form a ground run-up enclosure within which a jet aircraft engine can safely and more quietly be tested at full thrust.
A jet blast deflector is often called simply a "blast deflector", however, this term has other uses. In gunnery, the term "blast deflector" refers to a device which protects the gun crew from the muzzle blast of a gun. In small arms, a "blast deflector" is another name for a muzzle brake
which directs muzzle blast to the sides and upward to prevent the muzzle from climbing during automatic fire.
's two General Electric GE90
engines combine to create a thrust of approximately 200000 pound-force—this level of force is high enough to kill. To prevent these problems, jet blast deflectors redirect the air stream in a non-dangerous direction, frequently upward.
and MD-11
with engines mounted in the tail above the fuselage. Airports often place their blast fences at the beginnings of runways, especially when roadways or structures are adjacent. Airports that are in dense urban areas often have blast fences between taxiways and airport borders. Jet blast deflectors usually direct exhaust gases upward. However, a low-pressure zone can form behind the blast fence, causing ambient air and debris to be drawn upward with the jet exhaust, and hot, toxic gases to circulate behind the blast fence. Jet blast deflectors have been designed to counteract this problem by using multiple panels and various angles, and by using slotted panel surfaces.
s use jet blast deflectors at the rear of aircraft catapult
s, positioned such that other aircraft may be otherwise damaged by exhaust blast. Jet blast deflectors are made of heavy duty material that is raised and lowered by hydraulic cylinder
s or linear actuators. The jet blast deflector lies flush with and serves as a portion of the flight deck
until the aircraft to be launched rolls over it on the way to the catapult. When the aircraft is clear of the deflector, the heavy panel is raised into position to redirect the hot jet blast. As soon as the deflector is raised, another aircraft can be brought into position behind it, and flight deck personnel can perform final readiness duties without the danger of hot, violent exhaust gases. Such systems were installed on aircraft carriers in the late 1940s and early 1950s, as jet-powered aircraft began to appear in navies.
Jet blast deflectors aboard aircraft carriers are placed in very close proximity to the 2300 °F temperatures of modern jet fighter exhaust. The non-skid decking surface of the deflector suffers heat damage and requires frequent maintenance or replacement. Additionally, the hot deflector surface cannot be used as normal decking until it has cooled enough to allow aircraft tires to roll over it. To mitigate the heat problem, active cooling systems were installed in the 1970s, tapping the fire mains (fire suppression water systems) to use seawater circulating through water lines within the deflector panel. However, the water cooling system adds more complexity and failure points, and requires additional maintenance. The most recent method tried by the United States Navy
for solving the heat problem was introduced in 2008 with USS George H.W. Bush which uses heavy-duty metal panels covered in heat-dissipating ceramic tiles similar to those used on the Space Shuttle
. The tiled panels can be changed quickly and easily—the ship carries a large replacement supply. Without active water lines, the passively-tiled deflector is expected to require much less maintenance.
Most deflectors direct the jet blast upward, but the hot gases can instead be directed downward and channeled below the deck to vent harmlessly out of the side of the aircraft carrier. This solution offers two benefits: The top surface of the deflector does not overheat and is immediately available to serve as decking after a launch, and the air above the deflector is not made hazardous to aircraft flying through the hot gases.
Jet blast deflectors at airports have been identified as a potential place to mount wind turbine
s intended to recover some of the energy of the takeoff thrust in the form of electricity.
Jet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...
to prevent damage and injury. The structure must be strong enough to withstand heat and high speed air streams as well as dust and debris carried by the turbulent air. Without a deflector, jet blast
Jet blast
Jet blast is the phenomenon of rapid air movement produced by the jet engines of aircraft, particularly on or before takeoff.A large jet-engined aircraft can produce winds of up to 100 mph up to 200 feet behind it at 40% maximum rated power...
can be dangerous to people, equipment and other aircraft.
Jet blast deflectors range in complexity from stationary concrete, metal or fiberglass fences to heavy panels that are raised and lowered by hydraulic arms and actively cooled. Blast deflectors can be used as protection from helicopter
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...
and fixed-wing aircraft
Fixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...
propwash and from rocket engine
Rocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
exhaust. At airport
Airport
An airport is a location where aircraft such as fixed-wing aircraft, helicopters, and blimps take off and land. Aircraft may be stored or maintained at an airport...
s and jet engine service centers, jet blast deflectors can be combined with sound-deadening
Soundproofing
Soundproofing is any means of reducing the sound pressure with respect to a specified sound source and receptor. There are several basic approaches to reducing sound: increasing the distance between source and receiver, using noise barriers to reflect or absorb the energy of the sound waves, using...
walls to form a ground run-up enclosure within which a jet aircraft engine can safely and more quietly be tested at full thrust.
A jet blast deflector is often called simply a "blast deflector", however, this term has other uses. In gunnery, the term "blast deflector" refers to a device which protects the gun crew from the muzzle blast of a gun. In small arms, a "blast deflector" is another name for a muzzle brake
Muzzle brake
Muzzle brakes and recoil compensators are devices that are fitted to the muzzle of a firearm or cannon to redirect propellant gases with the effect of countering both recoil of the gun and unwanted rising of the barrel during rapid fire...
which directs muzzle blast to the sides and upward to prevent the muzzle from climbing during automatic fire.
Purpose
High energy jet engine exhaust can cause injury and damage. Jet blast has been known to uproot trees, shatter windows, overturn automobiles and trucks, flatten poorly-made structures and injure people. Other aircraft in the jet blast, especially lightweight ones, have been blown around and damaged by jet exhaust. Hurricane-force air streams moving at speeds up to 100 knots (54.4 m/s) have been measured behind the largest jet-powered aircraft at distances of over 200 foot. A Boeing 777Boeing 777
The Boeing 777 is a long-range, wide-body twin-engine jet airliner manufactured by Boeing Commercial Airplanes. It is the world's largest twinjet and is commonly referred to as the "Triple Seven". The aircraft has seating for over 300 passengers and has a range from , depending on model...
's two General Electric GE90
General Electric GE90
General Electric GE90 is a family of high-bypass turbofan aircraft engines built by GE Aviation for the Boeing 777, with thrust ratings ranging from 74,000 to 115,000 lbf . It was first introduced in November 1995 on British Airways' 777s, and is available only on the 777...
engines combine to create a thrust of approximately 200000 pound-force—this level of force is high enough to kill. To prevent these problems, jet blast deflectors redirect the air stream in a non-dangerous direction, frequently upward.
Airports
Jet blast deflectors began to appear at airports in the 1950s. Airports in the 1960s used jet blast deflectors with a height of 6 to 8 ft (1.8 to 2.4 ), but airports in the 1990s needed ones that were twice as high, and even up to 35 feet (10.7 m) high for jet airliners such as the McDonnell Douglas DC-10McDonnell Douglas DC-10
The McDonnell Douglas DC-10 is a three-engine widebody jet airliner manufactured by McDonnell Douglas. The DC-10 has range for medium- to long-haul flights, capable of carrying a maximum 380 passengers. Its most distinguishing feature is the two turbofan engines mounted on underwing pylons and a...
and MD-11
McDonnell Douglas MD-11
The McDonnell Douglas MD-11 is a three-engine medium- to long-range widebody jet airliner, manufactured by McDonnell Douglas and, later, by Boeing Commercial Airplanes. Based on the DC-10, it features a stretched fuselage, increased wingspan with winglets, refined airfoils on the wing and smaller...
with engines mounted in the tail above the fuselage. Airports often place their blast fences at the beginnings of runways, especially when roadways or structures are adjacent. Airports that are in dense urban areas often have blast fences between taxiways and airport borders. Jet blast deflectors usually direct exhaust gases upward. However, a low-pressure zone can form behind the blast fence, causing ambient air and debris to be drawn upward with the jet exhaust, and hot, toxic gases to circulate behind the blast fence. Jet blast deflectors have been designed to counteract this problem by using multiple panels and various angles, and by using slotted panel surfaces.
Ground run-up enclosure
After a jet engine has been overhauled or has undergone the replacement of parts, it is normal to run the engine up to full thrust to test it. Rural airports rarely provide more than a distant portion of the airfield within which to test engines at full thrust, but urban airports surrounded by residential areas often specify that engine tests be conducted within a ground run-up enclosure, so that the engine noise can be reduced for residents.Aircraft carriers
Aircraft carrierAircraft carrier
An aircraft carrier is a warship designed with a primary mission of deploying and recovering aircraft, acting as a seagoing airbase. Aircraft carriers thus allow a naval force to project air power worldwide without having to depend on local bases for staging aircraft operations...
s use jet blast deflectors at the rear of aircraft catapult
Aircraft catapult
An aircraft catapult is a device used to launch aircraft from ships—in particular aircraft carriers—as a form of assisted take off. It consists of a track built into the flight deck, below which is a large piston or shuttle that is attached through the track to the nose gear of the aircraft, or in...
s, positioned such that other aircraft may be otherwise damaged by exhaust blast. Jet blast deflectors are made of heavy duty material that is raised and lowered by hydraulic cylinder
Hydraulic cylinder
A Hydraulic cylinder is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in engineering vehicles.- Operation :...
s or linear actuators. The jet blast deflector lies flush with and serves as a portion of the flight deck
Flight deck
The flight deck of an aircraft carrier is the surface from which its aircraft take off and land, essentially a miniature airfield at sea. On smaller naval ships which do not have aviation as a primary mission, the landing area for helicopters and other VTOL aircraft is also referred to as the...
until the aircraft to be launched rolls over it on the way to the catapult. When the aircraft is clear of the deflector, the heavy panel is raised into position to redirect the hot jet blast. As soon as the deflector is raised, another aircraft can be brought into position behind it, and flight deck personnel can perform final readiness duties without the danger of hot, violent exhaust gases. Such systems were installed on aircraft carriers in the late 1940s and early 1950s, as jet-powered aircraft began to appear in navies.
Jet blast deflectors aboard aircraft carriers are placed in very close proximity to the 2300 °F temperatures of modern jet fighter exhaust. The non-skid decking surface of the deflector suffers heat damage and requires frequent maintenance or replacement. Additionally, the hot deflector surface cannot be used as normal decking until it has cooled enough to allow aircraft tires to roll over it. To mitigate the heat problem, active cooling systems were installed in the 1970s, tapping the fire mains (fire suppression water systems) to use seawater circulating through water lines within the deflector panel. However, the water cooling system adds more complexity and failure points, and requires additional maintenance. The most recent method tried by the United States Navy
United States Navy
The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...
for solving the heat problem was introduced in 2008 with USS George H.W. Bush which uses heavy-duty metal panels covered in heat-dissipating ceramic tiles similar to those used on the Space Shuttle
Space Shuttle thermal protection system
The Space Shuttle thermal protection system is the barrier that protects the Space Shuttle Orbiter during the searing heat of atmospheric reentry...
. The tiled panels can be changed quickly and easily—the ship carries a large replacement supply. Without active water lines, the passively-tiled deflector is expected to require much less maintenance.
Most deflectors direct the jet blast upward, but the hot gases can instead be directed downward and channeled below the deck to vent harmlessly out of the side of the aircraft carrier. This solution offers two benefits: The top surface of the deflector does not overheat and is immediately available to serve as decking after a launch, and the air above the deflector is not made hazardous to aircraft flying through the hot gases.
Other uses
Blast deflectors have been used in the testing of rocket engines and in the launching of rockets. Rocket blast deflectors differ from jet blast deflectors in that they must withstand tremendously high pressures and temperatures. Rocket blast deflectors are often made of concrete.Jet blast deflectors at airports have been identified as a potential place to mount wind turbine
Wind turbine
A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...
s intended to recover some of the energy of the takeoff thrust in the form of electricity.
See also
- Etihad Airways accident—a jet accident within a ground run-up enclosure