Sylvie Willis
The Mikoyan-Gurevich MiG-21 is a Soviet aircraft that has been flown by thousands of pilots. It has a fuel capacity of 2,600 liters (690 US gallons), but 210 gallons of this cannot be used, giving it a usable fuel capacity of 475 gallons. The MiG-21's fuel consumption is impacted by its engine power—more powerful engines tend to burn fuel faster. The placement of its internal fuel tanks ahead of the center of gravity also affects its endurance, which is only 45 minutes in clean condition. This issue has been somewhat addressed in Chinese and Soviet variants, which feature improved internal fuel tank layouts and larger external fuel tanks.
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What You'll Learn
- The Mig-21 has a fuel capacity of 2,600 liters, but 210 gallons cannot be used
- The plane becomes unflyable when 210 gallons of fuel remain
- The plane's centre of gravity shifts as fuel is consumed, making it unstable
- The plane has an endurance of 45 minutes in a clean condition
- The Tumansky R-25 jet engine has an emergency mode that increases fuel consumption by 50%
The Mig-21 has a fuel capacity of 2,600 liters, but 210 gallons cannot be used
The Mikoyan-Gurevich MiG-21 is a Soviet-designed aircraft that has been flown by thousands of pilots. It has a fuel capacity of 2,600 liters, or about 690 US gallons. However, 210 gallons of this fuel cannot be used, which means the MiG-21 has a usable fuel capacity of 475 gallons. This is because the internal fuel tanks are positioned ahead of the center of gravity, and as fuel is consumed, the center of gravity shifts rearward. Once the fuel level reaches 210 gallons, the center of gravity passes the rear limit, and the aircraft becomes unflyable. This issue was somewhat addressed by carrying fuel in external tanks closer to the center of gravity, as seen in Chinese and second-generation Soviet variants, which also had larger external fuel tanks.
The MiG-21's short endurance and low fuel capacity led to the development of the MT and SMT variants, which had greater fuel capacity. The MiG-21's fuel capacity and endurance were also impacted by its delta wing design, which while excellent for fast climbing, resulted in a rapid loss of speed during turning combat. This caused issues with reaching the aircraft's theoretical maximum performance, as pilots struggled to reach Mach 2 before running out of fuel.
The MiG-21's performance at high altitudes was also affected by its fuel load. It struggled to achieve its maximum Mach number at high altitudes before expending its modest fuel load. This was less of an issue at low altitudes, where the MiG-21 performed well in the Israeli-Arab wars and in Vietnam. Nowadays, most MiG-21s are employed at low altitudes for ground attack missions, delivering bombs and rockets against ground targets.
The MiG-21's fuel capacity and endurance limitations have been well-documented, and its design has evolved to address these issues. The development of new variants with improved fuel capacity and the use of external fuel tanks have extended the aircraft's range and endurance. However, the MiG-21's fuel-related challenges have been a persistent aspect of its operational history.
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The plane becomes unflyable when 210 gallons of fuel remain
The Mikoyan-Gurevich MiG-21 is a Soviet aircraft that has been flown by thousands of pilots of all calibers. It is easy to service and reliable, but it has a short range due to the poor placement of its internal fuel tanks. The MiG-21's total fuel capacity is 2,600 liters or about 690 US gallons. However, 210 gallons of this fuel is unusable as the aircraft becomes unflyable when the center of gravity passes out of the rear limit.
As the MiG-21 consumes fuel, its center of gravity moves aft. This shift in the center of gravity can make the aircraft statically unstable and difficult to control. When 210 gallons of fuel remain, the aircraft becomes unflyable as the center of gravity passes out of the acceptable range. At this point, the aircraft may nose up into a steep climb and stall, as described by one pilot. This issue is not unique to the MiG-21, with Egyptian pilots reporting similar problems with the Su-7 and Soviet Tu-154 airliner.
The MiG-21's short range and endurance of only 45 minutes in clean conditions have been noted as limitations. To counter this issue, fuel can be carried in external tanks closer to the center of gravity. Chinese variants of the MiG-21 improved the internal fuel tank layout and also utilized larger external fuel tanks. Additionally, the use of a more powerful engine can increase fuel consumption but maintain the same flight time while boosting overall flight performance.
The MiG-21's performance at high altitudes is impacted by its modest fuel load. It may struggle to reach its theoretical maximum Mach number before running out of fuel. Pilots have reported difficulty in reaching Mach 2 and achieving the claimed altitude of 60,000 feet, with a maximum of 46,000 feet being more typical. The MiG-21's fuel consumption can be further impacted by the use of its emergency power rating (EPR) booster feature, which increases thrust but also increases fuel consumption by 50% over the rate in full afterburner.
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The plane's centre of gravity shifts as fuel is consumed, making it unstable
The Mikoyan-Gurevich MiG-21 is a fighter jet with a Tumansky R-25 engine. It has a fuel capacity of 2,600 litres (690 US gallons), but 210 gallons of this cannot be used. The MiG-21's engine is located at the extreme tail, while its fuel tanks are positioned ahead of the centre of gravity, in the wings and fuselage. This design causes the plane's centre of gravity to shift rearward as fuel is consumed, moving beyond the acceptable parameters.
As the centre of gravity moves towards the rear of the plane, the aircraft becomes unstable and difficult to control. This instability is caused by the changing weight distribution, which affects the plane's balance and lift. The centre of gravity is crucial to achieving longitudinal stability, and when it shifts too far aft, the aircraft may assume a nose-up attitude, making it challenging to control. This phenomenon is not unique to the MiG-21; other aircraft, such as the Su-7, have exhibited similar issues.
The MiG-21's endurance is limited to approximately 30-33 minutes due to its fuel capacity and consumption rate. This short range is further exacerbated by the centre of gravity issue. To counter this problem, external fuel tanks can be used, positioned closer to the centre of gravity. Chinese and second-generation Soviet variants of the MiG-21 improved the internal fuel tank layout and utilised larger external fuel tanks. Additionally, the MiG-21's engine has an emergency mode that boosts its performance but also increases fuel consumption by 50%.
The instability caused by fuel consumption is a critical factor in the MiG-21's performance and endurance. The shifting centre of gravity not only affects the aircraft's stability but also the pilot's ability to control it effectively. This design challenge has been addressed through modifications and the use of external fuel tanks to improve the aircraft's range and stability.
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The plane has an endurance of 45 minutes in a clean condition
The Mikoyan-Gurevich MiG-21 is a Soviet-designed aircraft that has been flown by thousands of pilots of all calibers. While it is easy to service and reliable, it has a short range due to the poor placement of its internal fuel tanks. As fuel is consumed, the center of gravity shifts rearward, making the plane unstable and difficult to control. This issue is exacerbated when the plane carries less than 210 gallons of fuel, at which point the aircraft becomes unflyable.
The MiG-21's Tumansky R-25 jet engine includes a second fuel pump in the afterburning stage, which can be activated in emergency mode to provide a temporary power boost. This feature increases fuel consumption by 50% over the rate in full afterburner and can only be used for a maximum of 2 minutes before the engine overheats.
The plane's endurance is further impacted by its performance at high altitudes. It struggles to reach its theoretical maximum Mach number before expending its modest fuel load. Pilots have reported difficulty in reaching Mach 2 before running out of fuel, and the plane's climb rate falls short of the brochure's claims.
To counter the limited fuel capacity, some variants of the MiG-21, such as the Chinese and second-generation Soviet versions, introduced improvements to the internal fuel tank layout. They also carried significantly larger external fuel tanks, which helped extend the plane's endurance beyond 45 minutes in a clean condition.
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The Tumansky R-25 jet engine has an emergency mode that increases fuel consumption by 50%
The Tumansky R-25 jet engine, designed as a replacement for the Tumansky R-13 in MiG-21 fighters, features an emergency mode that increases fuel consumption by 50%. This mode, known as ЧР (Russian for "чрезвычайный режим" or Emergency Power Rating), involves the activation of a second fuel pump in the afterburning stage, resulting in a significant increase in thrust.
Under 2,000 meters (6,600 ft) of altitude, the engine can produce 97.4 kilonewtons (21,896 lbf) of thrust with the emergency mode engaged. The engine's rpm increases by 2.5%, and the compression ratio rises, leading to an increase in exhaust temperature. However, the use of this mode is limited to a maximum of 2 minutes in both practice and wartime scenarios due to the risk of engine overheating.
The Tumansky R-25 engine's emergency mode provides a substantial boost in performance for the MiG-21, improving its climbing rate and thrust-to-weight ratio. This temporary power enhancement can be crucial in close-quarters dogfights or when rapid manoeuvring is required.
The MiG-21, despite its reputation for modest fuel capacity and endurance, benefits from the R-25 engine's emergency mode in specific scenarios. The ability to temporarily increase fuel consumption and thrust can provide the necessary boost for the aircraft to excel in certain combat situations. However, pilots must carefully manage this trade-off between fuel consumption and enhanced performance, ensuring efficient utilization of the emergency mode to maintain overall mission effectiveness.
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Frequently asked questions
The MiG-21's fuel consumption depends on the variant. The SMT variant consumes more fuel than the MF variant because it has a more powerful engine. The MiG-21's Tumansky R-25 jet engine also includes a second fuel pump in the afterburning stage, which, when activated, increases fuel consumption by 50% over the rate in full afterburner.
The MiG-21 carries 2,600 liters of fuel, or about 690 U.S. gallons. However, 210 gallons of this fuel cannot be used, as the aircraft becomes unflyable when the center of gravity passes out of the rear limit. Therefore, the MiG-21's usable fuel capacity is 475 gallons.
The MiG-21 has a short range due to its limited fuel capacity. At high altitudes, it struggles to reach its theoretical maximum Mach number before expending its modest fuel load. Additionally, the poor placement of the internal fuel tanks ahead of the center of gravity affects the aircraft's stability and endurance. As fuel is consumed, the center of gravity shifts rearward, making the plane difficult to control.
