Why JF-17 Thunder Fighter Jet Used Russian RD-93 Engine?

Why JF-17 Thunder Fighter Jet Used Russian RD-93 Engine?

The JF-17 Thunder combat aircraft was jointly developed by the Pakistan Aeronautical Complex and Chengdu Aircraft Industry Corporation (PAC/CAC).

Pakistan and China initiated a project for jointly developed fighter aircraft and they signed a memorandum of understanding (MoU) in 1995 for joint design and development of a new fighter. In June 1999, the contract to jointly develop and produce the Chengdu FC-1/Super 7 was signed. The project was to be a 50:50 partnerships.

In 2005 China signed a $267 million contract with Russia for 100 RD-93 turbofan engines, a variant of the RD-33, to power the JF-17 Thunder fighter aircraft. Engine deliveries from Klimov and Chernyshev were to begin in 2006. It may be mentioned that the RD-93 engine is a variant of the RD-33 engine developed to power the MiG-29 fighter jet.

Many of us wondered that why does the JF-17 fighter jet use an RD-93 engine and not a more powerful NPO Saturn engine?

Usama Ahmad, a former Employee in the Govt. of Pakistan answers the question on Quora.

There are 4 key elements influencing the JF-17’s design which lead to the RD-93 being picked as the engine of choice for the JF-17.

  1. Doctrine
  2. Aerodynamics and Technical Restraints
  3. Modularity
  4. International politics and Engine availability

Doctrine

The PAF, like the rest of the armed forces, has practiced a primarily defensive doctrine ever since the 1990s and the advent of the nuclear age in South Asia. As the conventional disparity between India and Pakistan grows greater in favor of India, the Pakistani armed forces act more as a holding force designed to hold off Indian pressure until Foreign powers intervene to end the conflict and bring about a negotiated settlement of disputes before war escalates to the point where one side (Pakistan basically) uses nuclear weapons. This defensive mentality reflects in the JF-17’s design.

There are two points which need to be noted in doctrine:

  1. PAF fighters must prioritize Interception capabilities (Rapid interception, strong dogfight capability) overstrike capability (long-range missions)
  2. PAF Fighters must exhibit high availability (low maintenance time and long amounts of time between engine overhaul).PAF fighters must prioritize Interception capabilities (Rapid interception, strong dogfight capability) overstrike capability (long-range missions)

The JF-17 is designed primarily as a defensive, light interceptor aircraft with a secondary strike/close air support capability.

As you might know RD-93 is smokey and inefficient. What it does have is significant thrust though with Afterburners.

This enables the JF-17 to bring more power to a dogfight and engage enemy aircraft in a shorter response time which is what the JF-17 is designed for (Interception).

But it seriously reduces the JF-17’s performance in long range strike missions which is acceptable to the PAF since again, its designed as a light weight interceptor and Air superiority fighter, fighting in conjunction with more high-tech fighters like the F-16.

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One problem the PAF is trying to address is the JF-17’s low loiter time and long duration CAP missions. The JF-17’s low loiter time stems from it’s fuel inefficient RD-93 engine which reduces it CAP time. This is the primary reason why the PAF is considering switching from the RD-93 to the more fuel efficient FADEC RD-33MK.

Fun fact: RD-93 is an RD-33 with gearbox moved to allow it to fit into JF-17 and with maximum wet thrust increased slightly at the expense of a slight reduction in mean time between overhaul.

PAF Fighters must exhibit high availability (low maintenance time and long amounts of time between engine overhaul)

This is the primary reason the WS-13A from China was not used in the PAF version of the JF-17. The WS-13A is known for having maintenance issues. The mean time between overhaul is too low for it meaning you would have Engine overhaul facilities like Kamra filled constantly with JF-17s in need of maintenance.

For a small airforce like the PAF which is already outnumbered by the Indian Air Force, having a large number of aircraft in maintenance during times of tension, mobilization etc is a big No-No. The IAF can match us in strength with even 50-60% Aircraft availability. The PAF would be in serious trouble with even 70-80% availability.

This hasn’t completely ruled out the WS-13A. The PAF is waiting for improvements in it’s design and we’ll have to see how it performs on the FC-31.

Aerodynamics and Technical Restraints

Here's Why IAF Shouldn’t Underestimate The PAF JF-17 Thunder Fighter Jet

The first thing to note is that the RD-93 is a smaller class engine compared to the Al-31 series. 1 Al-31F engine is significantly larger than 1 RD-93 engine and the difficulty of fitting an engine much larger than the RD-93 on the JF-17, (a small, light interceptor which was smaller than even the F-16) was prohibitive.

Some of the comparable engines to the RD-93 engines are the French Snecma M88 from the Rafale which has less thrust than the RD-93 and the Eurofighter’s EJ-2000 which again has lower WET thrust than the RD-93. Another engine worth mentioning here is the WS-13A which has a higher wet thrust but was a relatively untested Chinese engine back when the JF-17 was being designed (Late 90s and Early 2000s).

You may have noted that all of these engines are from Twin Engine Aircraft and the JF-17 uses just ONE of them. That should tell you a lot about the JF-17’s size. In fact let me show you some images to compare the JF-17 size with the Su-27 (the Su-27 utilizes the Al-31 series engine.

Here’s image depicting the size comparison:

Why JF-17 Thunder Fighter Jet Used Russian RD-93 Engine??

As you can see, the JF-17 is significantly smaller than the Al-31F powered Su-27 but somewhat close in size to the RD-93 powered Mig-29. The Mig-29 is still significantly larger and heavier than the JF-17 though.

The JF-17 has 1 RD-93. Su-27 has two AL-31 engines. An AL-31 class engine would rip apart JF-17’s airframe.

Now it should be noted that the Pakistan Air Force is currently mulling over getting a new engine for JF-17 but AL-31 is not it.

The engines under consideration is the RD-33MK, a FADEC controlled version of RD-33 which is more efficient.

The EJ-2000, an engine with great export potential, has higher dry thrust but lower WET thrust. The Pakistan Air Force designers wanted more maximum thrust (in line with the emphasis on interception and air-to-air combat) and were less interested with efficient long term cruising apparently (since strike capability is secondary).

Another design choice being implemented in the JF-17 is the use of more composite materials in its air frames to allow a significant weight reduction in the JF-17. This would give the PAF more bang for it’s buck, enabling an even lighter fighter and better fuel efficiency. This is especially important when you consider that the JF-17, while it has room to support a variety of engines in the same class as the RD-93, doesn’t have the room or the air frame for an engine larger than the RD-93 (the AL-31F) to give more thrust or give more efficiency in future JF-17 Blocks.

Its rumored that the JF-17 Block 3 series will be implementing significant amounts of composite materials in it’s air frame.

The AL-31 F simply doesn’t fit into a small, light aircraft like the JF-17 so it was never really a contender in the JF-17 design. Its a great engine, used on the J-10 and J-20 but the JF-17, an aircraft smaller even than the already small F-16, simply couldn’t accommodate it in either its fuselage or bear the stress on its airframe.

Modularity

The point about modularity is placed here merely to emphasize that the RD-93 in no way represents the end of the line for all future versions of the JF-17. As mentioned before, the PLAAF flies its JF-17 version with the WS-13A and if the mean time between engine overhauls is increased for the WS-13A, the PAF would be happy to use it on their future JF-17 versions. The WS-13A will be used to power the FC-31 and the PAF is waiting to see how it performs and how the Chinese will fine tune it, resolve its maintenance issue etc before the PAF adopts it.

Historically speaking, the JF-17 owes a lot to the old PAF Saber 2 design and something called the Super Seven design I think. These were PAF attempts to design its own Fighter aircraft which would basically be a redesigned and updated version of the F-7 which was itself a Chinese version of the Mig-21.

The Mig-21 was one of the most capable interceptor aircraft of its time, holding its own against several American fighters if piloted well and a worthy adversary in dogfights. Its primary drawback was its non-modular design which prevented upgrades and customization. For example, it had a very tight nose cone section that prevented its radar from being upgraded with a larger, more capable radar resulting in the Mig-21 becoming rapidly obsolete in the 1980s as more fighters with more capable, long range radars and missiles took to the field.

The JF-17 was designed to be a leap forward with the old Mig-21 design. One of the primary goals of the JF-17 design was to fix the old Mig-21’s lack of modularity. This would enhance its export potential but it would also enable the PAF to customize and upgrade the JF-17 well into the 2020s and even 2030s if possible.

This modular approach extended to the JF-17s engine fuselage. Which is why it is perfectly possible to see future JF-17 versions powered by the French Snecma M88 from the Rafale, the European Eurofighter’s EJ-2000 or the WS-13A from China.

So this permits the PAF to swap out the RD-93 for another engine should the need arise. In face the PLAAF is already flying fighters with the WS-13 engine in China.

International politics and Engine availability

There’s a story that the older Kamra engineers tell. Back when the JF-17 was being designed Pakistan was under heavy sanctions. Recall that this was the 1998-2002 period when Pakistan was under the Pressler amendment sanctions and the sanctions imposed after the Atomic weapons tests conducted at Chagai in 98. The PAF was seriously boxed in and had a very limited plethora of high-tech suppliers. The face that President Musharraf was pressing the PAF to diversify its suppliers and buy more from China was also a big factor back then.

There was so much uncertainty back then over what the JF-17’s engine would be, who would be willing to supply us with an engine etc that the then Air Chief Marshall Mushaf Ali Mir ordered that the rest of the JF-17 design be decoupled from engine choice. The JF-17 would be designed with a modular engine compartment which would have enough room to accommodate a variety of RD-93 class engines (The Russian RD-33 and French Snecma M88 were possible choices back then) so that later on, whichever engine was available to the PAF would be able to fitt into the JF-17.

This was keeping in line with the rest of the JF-17’s modular design which permits swapping equipment and components in and out of the JF-17 in order to permit upgrades or meet export order customer requirements. this is why the JF-17 has a somewhat roomier backside compared to other aircraft.

Conclusion

The RD-93 choice stems from a variety of reasons, and i have done my best to cover as much of them as possible to present a holistic view as to why the RD-93 was chosen over not just the AL-31F but also other comparable engines in the PAF’s version of the JF-17.

The reasons stem from the need for an aircraft that prioritize interception and air-to-air combat over long range strike missions, the availability of international suppliers willing to sell us engines, the aerodynamic and technical restraints (Al-31F too large, would dangerously stress the JF-17 air frame) and other related design choices.

Future trends might store some interesting possible developments in the JF-17 engine sage:

  • Newer Block versions of the JF-17 are inducted with more composites reducing airframe weight which might cement the RD-93 as the JF-17’s permanent engine
  • The FADEC controlled of RD-33MK becomes the mainstay of future JF-17 blocks with its higher fuel efficiency permitting more loiter time for the JF-17 and longer CAP missions
  • The WS-13A performs well in the FC-31 and the Chinese fine-tune it, resolve its low meantime between engine overhaul resulting in it becoming the main engine of choice for future JF-17 blocks.
  • Indian pressure on Russia to cancel RD engine supplies to Pakistan resulting in the WS-13A being adopted with more composite materials in the JF-17 airframe.

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