Katerina Swanson
Afterburners are used to increase the thrust of jet engines for short periods of time, providing greater aircraft take-off, climb, or combat performance. The process involves injecting additional fuel into a combustor in the jet pipe behind the turbine, reheating the exhaust gas. While this provides a significant increase in thrust without adding much weight or complexity to the engine, it also results in increased fuel consumption and decreased fuel efficiency. The exact amount of fuel used depends on various factors, including the aircraft, engine, fuel capacity, altitude, throttle setting, and temperature. For example, an F-16 at a service ceiling of FL500 can use full afterburner for up to 30 minutes, while smaller 4th-generation fighters like the Viper and Fulcrum can only maintain afterburner for around 5 minutes at 25,000 ft.
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What You'll Learn
Afterburner fuel flow rate
An afterburner is an additional combustion component used on jet engines, typically those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, take-off, and combat. Afterburning significantly increases thrust without adding much weight or complexity to the engine.
The afterburning process involves injecting additional fuel into a combustor or "burner" in the jet pipe behind the turbine, reheating the exhaust gas. This process consumes a lot of fuel and, therefore, most planes use afterburners sparingly. For example, a military jet would use its afterburners when taking off from a short runway on an aircraft carrier or during a high-speed manoeuvre in a dogfight.
The fuel flow rate and efficiency of an afterburner depend on several factors, including the aircraft, engine, fuel capacity, altitude, throttle setting, and temperature. For instance, the F-16 can reach Mach 2 with the help of afterburners, but it may only be usable for 30 minutes at a high altitude and with an efficient climb.
The efficiency of afterburners also declines at higher altitudes due to decreased inlet and tailpipe pressure. However, in some cases, such as the SR-71 aircraft, reasonable efficiency can be achieved at high altitudes due to high speed and correspondingly high pressure from ram intake.
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Fuel efficiency
Afterburners are a component used on jet engines, mostly those on military supersonic aircraft, to increase thrust for short periods of time. They are particularly useful for improving aircraft take-off, climb, or combat performance.
The afterburning process involves injecting additional fuel into a combustor in the jet pipe behind the turbine, which reheats the exhaust gas. This process significantly increases thrust without adding much weight or complexity to the engine. However, it comes at the cost of increased fuel consumption and decreased fuel efficiency. The exact amount of fuel used depends on various factors, including the aircraft, engine, fuel capacity, altitude, throttle setting, and temperature.
The inefficiency of afterburners is due to the fact that the exhaust gas from the main combustion process has a reduced oxygen content, and the fuel in the afterburner is not burning in a highly compressed air column. This limitation applies specifically to turbojets, where the gain in efficiency is limited to 50%. In contrast, military turbofan combat engines can achieve higher efficiency gains of up to 70% by adding bypass air into the exhaust, thereby increasing the core and afterburner efficiency.
Despite the fuel efficiency trade-off, afterburners are advantageous in certain scenarios. For instance, a military jet might use its afterburners during takeoff from a short runway, such as on an aircraft carrier, or during high-speed maneuvers in a dogfight. In these situations, the increased thrust provided by the afterburner outweighs the temporary decrease in fuel efficiency.
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Factors affecting fuel consumption
The fuel consumption of an afterburner is influenced by several factors, including the aircraft's design, engine specifications, fuel capacity, altitude, throttle setting, and temperature. These factors collectively determine the efficiency and duration of afterburner usage. Here are some key factors that affect fuel consumption in the context of afterburners:
- Aircraft and Engine Design: The type of aircraft and engine design play a significant role in fuel consumption. Different aircraft, such as the Tomcat and MiG, have varying fuel capacities, which directly impact the duration of afterburner usage. Additionally, the size and weight of the engine affect fuel efficiency. Larger engines may consume more fuel but can also provide greater thrust without relying on afterburners.
- Altitude: The efficiency of afterburners is influenced by altitude. As inlet and tailpipe pressure decrease with increasing altitude, afterburner efficiency declines, particularly in turbojets. In contrast, military turbofan combat engines may experience improved efficiency at high altitudes due to the addition of bypass air, which increases core and afterburner efficiency.
- Thrust Requirements: Afterburners are typically engaged to increase thrust during specific phases of flight, such as takeoff, climb, or combat manoeuvres. The amount of additional thrust required will impact fuel consumption. For example, during takeoff from a short runway or an aircraft carrier, afterburners may be used sparingly to provide the necessary thrust without consuming excessive fuel.
- Temperature: The temperature within the engine and the combustion chamber affects fuel consumption. Higher temperatures enable more efficient combustion, but if the temperature exceeds the engine's structural limits, it can weaken the internal components. Therefore, a balance must be maintained to optimize fuel efficiency while ensuring the safety of engine components.
- Fuel Injection and Combustion: Afterburners work by injecting additional fuel into the combustor or "burner" located behind the turbine. The efficiency of this fuel injection process influences fuel consumption. Additionally, the method of combustion, such as mixing the bypass and turbine streams before fuel injection or stabilizing the flame individually in each stream, can impact fuel efficiency.
- Operational Considerations: The use of afterburners has operational implications that affect fuel consumption decisions. For example, engaging afterburners increases an aircraft's visibility, making it detectable by IR sensors and more vulnerable to IR missiles. Therefore, pilots may choose to minimize afterburner usage to maintain tactical advantage and operational flexibility.
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Afterburner vs main combustion
An afterburner, or "reheat" in British English, is an additional combustion component used on some jet engines, primarily those on military supersonic aircraft. The purpose of an afterburner is to increase thrust, usually for supersonic flight, takeoff, and combat. The afterburning process involves injecting extra fuel into a combustor (burner) in the jet pipe behind the turbine, reheating the exhaust gas. This significantly increases thrust, providing an alternative to using a bigger engine, which would add weight. However, this comes at the cost of increased fuel consumption and decreased fuel efficiency, limiting its use to short periods.
The highest temperature in a jet engine occurs in the combustion chamber, where fuel is burned at a rate of approximately 8,520 lb/h (3,860 kg/h) in a relatively small proportion of the air entering the engine. The combustion products are diluted with air from the compressor to reduce the gas temperature to the Turbine Entry Temperature (TET) of 1,570 °F (850 °C), ensuring the turbine's longevity. The need to significantly lower the temperature of the combustion products is a primary limitation on how much thrust can be generated.
In contrast, the afterburner combustor reheats the gas to a much higher temperature than the TET. As a result, the gas is accelerated by the heat addition, known as Rayleigh flow, and then by the nozzle to a higher exit velocity than without the afterburner. Burning all the oxygen delivered by the compressor stages would create temperatures high enough to weaken the engine's internal structure. However, by mixing the combustion products with unburned air from the compressor, a substantial amount of oxygen remains available for burning large quantities of fuel in the afterburner.
The afterburner is generally inefficient compared to the main combustion process due to the exhaust gas's reduced oxygen content from previous combustion. Additionally, the fuel does not burn in a highly compressed air column. The afterburner's efficiency declines further at higher altitudes as inlet and tailpipe pressure decreases, although this limitation primarily applies to turbojets. In military turbofan combat engines, adding bypass air into the exhaust can increase core and afterburner efficiency by up to 70%, depending on the bypass ratio.
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Use cases
The use of afterburners is generally limited to short periods due to their high fuel consumption. Afterburners are used to increase thrust for improved aircraft take-off, climb, or combat performance. For instance, a military jet would use its afterburners when taking off from a short runway on an aircraft carrier, or during a high-speed manoeuvre in a dogfight.
Afterburners are also used when a jet needs to reach high speeds. For example, the F-16 can reach Mach 2 with the help of a full afterburner. The F-15Es and F-16s with ground-attack loadouts need to use full afterburner to get into the air. The B-1 can accelerate from .8 to .95 mach in a few seconds with the help of a full afterburner, but due to the high visibility of afterburners, they are not used often.
Afterburners are also useful when a jet needs to climb to high altitudes. At the F-16 service ceiling of FL500, a full afterburner will use less fuel than military thrust at sea level. Thus, at high altitudes, a full afterburner might be usable for up to 30 minutes.
Afterburners are inefficient at high altitudes due to the decrease in inlet and tailpipe pressure with increasing altitude. However, the SR-71 is a counterexample to this limitation as it had reasonable efficiency at high altitudes due to its high speed and correspondingly high pressure from ram intake.
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Frequently asked questions
An afterburner uses a lot of fuel to generate power, which is why planes use them sparingly. The exact amount of fuel used depends on the aircraft, the engine, the fuel capacity, the altitude, the throttle setting, and the temperature.
Afterburners significantly increase thrust, which requires a lot of fuel. They can increase thrust without adding much weight or complexity to the engine.
Afterburners are used when taking off from a short runway, such as on an aircraft carrier, or during high-speed manoeuvres in a dogfight.
An afterburner injects additional fuel into a combustor in the jet pipe behind the turbine, reheating the exhaust gas. This increases thrust without adding much weight to the engine.
Aside from using a lot of fuel, afterburners make aircraft highly visible at night and easy to spot by IR sensors and lower-tech IR missiles.
