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As of 2015, the Chinese air-force has on its fleet, two dedicated aerial-refueling tanker aircraft: the Xian H-6U and the Ukrainian IL-78MP. The H-6U is represents the very first steps taken by the Chinese in developing a dedicated in-flight refueling tanker for its modernizing air-force. As such, it creates yet another branch in the already diverse modifications of the core Tupolev Tu-16 aircraft within the Chinese aerospace industry. With a first flight in 1988 and entering service in the early 1990s, the H-6U tanker has been in service with the Chinese air-force for twenty-five years. And it has provided significant user experience to the PLAAF on the concept of aerial-refueling.
Currently, there are about 20 H-6U aircraft distributed between the PLAAF and the PLAN. The IL-78MP, however, represents a newer capability even though the aircraft it is based on harkens back even more than the development of the tanker variant of the H-6. China is currently absorbing into its fleet, 3 former-Ukrainian IL-78MP aircraft. It remains unclear whether the variants obtained from Ukraine are the MRTT variants of the venerable IL-78 or the dedicated tanker “M” variant (or perhaps a mix of the two). However, considering that the 3 IL-78 aircraft have yet to be operationalized (except for publicity displays), the real tanker capability of the PLAAF resides with the H-6U fleet.
Note: an understanding of the design, inner-workings and performance of the IL-78 tanker can be found here.
Ground rules
Comparing fixed wing aircraft is an inherently complex process. As such, this effort will focus on two key features of the aircraft involved: time-on-station (TOS) / time-of-flight (TOF) and Transfer Fuel Load (TFL). The general flight performance of the aircraft involved is obtained from the manufacturer’s literature or the end-user literature (where applicable). The analysis is simplified to allow the reader to grasp the essentials of the performance metrics. The modeling of the performance of each of the aircraft considered is based on known metrics for the propulsion, structural weights and aerodynamics of the baseline vehicles upon which the tanker variants are based. Note, however, that simplifications made to the models for ease of interpretation mean that the numbers will have some error built into them. However, such error is fixed for all the aircraft compared so the relative trends of the aircraft will be similar.
The bomber-tanker?
The H-6U as an airplane design is long in the tooth. The H-6 is originally a Chinese copy of the Russian Tupolev Tu-16 bomber, itself a design from the early 1950s. Originally provided to the PLAAF as a bomber by the Soviet Union, the aircraft has since been built in China under the designation of the H-6. As a result, the H-6 has poor aerodynamic performance and still retains aerodynamic components from the late 1950s such as airflow fences etc. Propulsion performance is not much better, designed as it was closer to a turbojet engine system rather than a turbofan. And all components of the aircraft were designed from the standpoint of a high-speed bomber with emphasis on range. A tanker aircraft, however, requires high endurance and fuel efficiency in order to be able to transfer fuel to waiting fighters. All of these factors mean that the H-6 (and the Tu-16) are not designs suited for a dedicated aerial tanker.
However, necessity is the mother of invention. And the PLAAF needed a tanker to allow its fuel-thirsty fighters of the 1980s and 1990s to stay up long enough to be of use in a war. And no other aircraft was as well-known to the Chinese engineers as the H-6. If the fuel from the wing tanks could be transferred to fighters using wingtip hose-and-drogue refueling pods, then the H-6 could very well serve as a light tanker. An agreement between Beijing and commercial companies in the UK in 1986 allowed the influx of knowledge for the Chinese engineers to adapt the H-6 with two RDC-1 hose-and-drogue pods on the outer sections of each wing. They also replaced the payload bay in the fuselage of the H-6 with additional fuel tanks to increase the overall fuel carriage capacity close to the maximum MTOW of the aircraft. With the piping to connect these fuselage tanks with the external wing tanks, additional bomber-related equipment removed to save weight and simplified avionics, the H-6 was now a dedicated tanker. And it had a new name: the H-6U.
Performance comparisons
The performance of the Indian IL-78MKI, the A-330 MRTT, and the H-6U are presented below. The results are summarized in the form of TOF/Fuel plots. Because the vertical axis of the plots is TOF and the horizontal axis is the required fuel, the more vertical the curve for a particular aircraft gets, the more time on station it has. The flatter the curve, the lesser time on station it has for a given fuel amount. All TOF data is extracted at normal cruise speeds for the aircraft involved.
It is very clear that the design limitations on the H-6U as a result of its lineage is severe compared to its modern contemporaries. The H-6U has a maximum endurance (when using all internal wing fuel + fuselage fuel; theoretically) of about 7 hours at its normal cruise speed. By comparison, the Indian IL-78MKI has a maximum endurance of about 10.5 hours and the A-330 runs out of fuel after flying for about 15 hours at its normal cruise speed. Further, the maximum amount of fuel transfer available for the H-6U is about 35,000 kg when it is within 30 minutes flight time of its launch point. By comparison, the A-330 MRTT can offload 100,000 kg of fuel for the same distance/time and the IL-78MKI can offload 90,000 kg of fuel. In other words, for the same distance/time from the launch point, three H-6U aircraft are needed to match the capability of one IL-78MKI or one A-330 MRTT.
Extrapolated further, when the H-6U completely runs out of fuel at 7 hours, the A-330 can still offload 60,000 kg of fuel and the IL-78MKI can offload 40,000 kg of fuel. The H-6U is completely outclassed on all accounts by its regional competitors.
A practical scenario
The limitations of the H-6U tanker severely limit PLAAF options. A typical Su-30MKK Flanker requires about 10,000 kg of overall fuel in its tanks, but refueling with the tanker means that there is at least some fuel already existing in its tanks. If we assume that the Flanker is down to its last 10% fuel, meaning about 1,000 kg, it would need 9,000 kg of fuel to top up. At one hour flight time from the launch base, the H-6U is within 900 km of its base. At this point it has about 30,000 kg to offer to fighters. This means that a single H-6U can refuel 3 Su-30MKKs and must then immediately return to base to refuel. An IL-78MKI, however, could refuel almost 9 Su-30MKKs for the same performance. In this type of single-refuel mission mode, one IL-78MKI becomes the equivalent of three H-6Us.
Alternatively, one IL-78 could refuel the same three Flankers on their inbound and outbound trips without having to return to base. For all practical purposes, the same operation would require twice as many H-6Us.
Conclusions
It becomes apparent that the 20 or so H-6U tankers in the PLAAF fleet are rapidly used up so that for practical purposes, they are no more equivalent than 6-10 IL-78MKI or 6-8 A-330 MRTT. With the additional burden of high maintenance and general obsolescence in a network-centric environment, operating the H-6U becomes a greater liability than resource. It is no surprise that the PLAAF is looking at IL-78 replacements from Ukraine on an urgent basis. Further, unlike the IL-78MKI and the A-330, the H-6U is not a tanker-transport. It cannot carry any cargo or bombs. It is not multi-role. In its current shape and form, it represents a capability lesser than that offered by the 1960s era KC-135s. Currently, the H-6U flies with the original engines used in the bomber variants. But with the arrival of the H-6K with the D-30KP engines (same as that used in the IL-76MD), there is a possibility to improve the endurance and fuel-transfer limits on the H-6U. The H-6K, which is a cruise-missile carrier, utilizes the new engines. It is possible that in the coming years, the PLAAF may produce some H-6U(K) variants with the same high-bypass D-30KP engines. However, it is also possible that the IL-78MP may lead to the demise of the H-6U tanker given its general obsolescence. The PLAAF may also produce a tanker variant on its new Y-20 heavy-transport in a manner similar to the IL-78 variant of the Russian IL-76 transport design. Only time will tell on the route that Beijing will take.
Dr. Vivek Ahuja
Currently, there are about 20 H-6U aircraft distributed between the PLAAF and the PLAN. The IL-78MP, however, represents a newer capability even though the aircraft it is based on harkens back even more than the development of the tanker variant of the H-6. China is currently absorbing into its fleet, 3 former-Ukrainian IL-78MP aircraft. It remains unclear whether the variants obtained from Ukraine are the MRTT variants of the venerable IL-78 or the dedicated tanker “M” variant (or perhaps a mix of the two). However, considering that the 3 IL-78 aircraft have yet to be operationalized (except for publicity displays), the real tanker capability of the PLAAF resides with the H-6U fleet.
Note: an understanding of the design, inner-workings and performance of the IL-78 tanker can be found here.
Ground rules
Comparing fixed wing aircraft is an inherently complex process. As such, this effort will focus on two key features of the aircraft involved: time-on-station (TOS) / time-of-flight (TOF) and Transfer Fuel Load (TFL). The general flight performance of the aircraft involved is obtained from the manufacturer’s literature or the end-user literature (where applicable). The analysis is simplified to allow the reader to grasp the essentials of the performance metrics. The modeling of the performance of each of the aircraft considered is based on known metrics for the propulsion, structural weights and aerodynamics of the baseline vehicles upon which the tanker variants are based. Note, however, that simplifications made to the models for ease of interpretation mean that the numbers will have some error built into them. However, such error is fixed for all the aircraft compared so the relative trends of the aircraft will be similar.
The bomber-tanker?
The H-6U as an airplane design is long in the tooth. The H-6 is originally a Chinese copy of the Russian Tupolev Tu-16 bomber, itself a design from the early 1950s. Originally provided to the PLAAF as a bomber by the Soviet Union, the aircraft has since been built in China under the designation of the H-6. As a result, the H-6 has poor aerodynamic performance and still retains aerodynamic components from the late 1950s such as airflow fences etc. Propulsion performance is not much better, designed as it was closer to a turbojet engine system rather than a turbofan. And all components of the aircraft were designed from the standpoint of a high-speed bomber with emphasis on range. A tanker aircraft, however, requires high endurance and fuel efficiency in order to be able to transfer fuel to waiting fighters. All of these factors mean that the H-6 (and the Tu-16) are not designs suited for a dedicated aerial tanker.
However, necessity is the mother of invention. And the PLAAF needed a tanker to allow its fuel-thirsty fighters of the 1980s and 1990s to stay up long enough to be of use in a war. And no other aircraft was as well-known to the Chinese engineers as the H-6. If the fuel from the wing tanks could be transferred to fighters using wingtip hose-and-drogue refueling pods, then the H-6 could very well serve as a light tanker. An agreement between Beijing and commercial companies in the UK in 1986 allowed the influx of knowledge for the Chinese engineers to adapt the H-6 with two RDC-1 hose-and-drogue pods on the outer sections of each wing. They also replaced the payload bay in the fuselage of the H-6 with additional fuel tanks to increase the overall fuel carriage capacity close to the maximum MTOW of the aircraft. With the piping to connect these fuselage tanks with the external wing tanks, additional bomber-related equipment removed to save weight and simplified avionics, the H-6 was now a dedicated tanker. And it had a new name: the H-6U.
Performance comparisons
The performance of the Indian IL-78MKI, the A-330 MRTT, and the H-6U are presented below. The results are summarized in the form of TOF/Fuel plots. Because the vertical axis of the plots is TOF and the horizontal axis is the required fuel, the more vertical the curve for a particular aircraft gets, the more time on station it has. The flatter the curve, the lesser time on station it has for a given fuel amount. All TOF data is extracted at normal cruise speeds for the aircraft involved.
It is very clear that the design limitations on the H-6U as a result of its lineage is severe compared to its modern contemporaries. The H-6U has a maximum endurance (when using all internal wing fuel + fuselage fuel; theoretically) of about 7 hours at its normal cruise speed. By comparison, the Indian IL-78MKI has a maximum endurance of about 10.5 hours and the A-330 runs out of fuel after flying for about 15 hours at its normal cruise speed. Further, the maximum amount of fuel transfer available for the H-6U is about 35,000 kg when it is within 30 minutes flight time of its launch point. By comparison, the A-330 MRTT can offload 100,000 kg of fuel for the same distance/time and the IL-78MKI can offload 90,000 kg of fuel. In other words, for the same distance/time from the launch point, three H-6U aircraft are needed to match the capability of one IL-78MKI or one A-330 MRTT.
Extrapolated further, when the H-6U completely runs out of fuel at 7 hours, the A-330 can still offload 60,000 kg of fuel and the IL-78MKI can offload 40,000 kg of fuel. The H-6U is completely outclassed on all accounts by its regional competitors.
A practical scenario
The limitations of the H-6U tanker severely limit PLAAF options. A typical Su-30MKK Flanker requires about 10,000 kg of overall fuel in its tanks, but refueling with the tanker means that there is at least some fuel already existing in its tanks. If we assume that the Flanker is down to its last 10% fuel, meaning about 1,000 kg, it would need 9,000 kg of fuel to top up. At one hour flight time from the launch base, the H-6U is within 900 km of its base. At this point it has about 30,000 kg to offer to fighters. This means that a single H-6U can refuel 3 Su-30MKKs and must then immediately return to base to refuel. An IL-78MKI, however, could refuel almost 9 Su-30MKKs for the same performance. In this type of single-refuel mission mode, one IL-78MKI becomes the equivalent of three H-6Us.
Alternatively, one IL-78 could refuel the same three Flankers on their inbound and outbound trips without having to return to base. For all practical purposes, the same operation would require twice as many H-6Us.
Conclusions
It becomes apparent that the 20 or so H-6U tankers in the PLAAF fleet are rapidly used up so that for practical purposes, they are no more equivalent than 6-10 IL-78MKI or 6-8 A-330 MRTT. With the additional burden of high maintenance and general obsolescence in a network-centric environment, operating the H-6U becomes a greater liability than resource. It is no surprise that the PLAAF is looking at IL-78 replacements from Ukraine on an urgent basis. Further, unlike the IL-78MKI and the A-330, the H-6U is not a tanker-transport. It cannot carry any cargo or bombs. It is not multi-role. In its current shape and form, it represents a capability lesser than that offered by the 1960s era KC-135s. Currently, the H-6U flies with the original engines used in the bomber variants. But with the arrival of the H-6K with the D-30KP engines (same as that used in the IL-76MD), there is a possibility to improve the endurance and fuel-transfer limits on the H-6U. The H-6K, which is a cruise-missile carrier, utilizes the new engines. It is possible that in the coming years, the PLAAF may produce some H-6U(K) variants with the same high-bypass D-30KP engines. However, it is also possible that the IL-78MP may lead to the demise of the H-6U tanker given its general obsolescence. The PLAAF may also produce a tanker variant on its new Y-20 heavy-transport in a manner similar to the IL-78 variant of the Russian IL-76 transport design. Only time will tell on the route that Beijing will take.
Dr. Vivek Ahuja
