At present, there are four main steps in the upgrade of WS-13.
1. 011 replaced the four-stage fan of RD-33 with the three-stage fan of the original WS-12. The new fan increase the flow rate by 6 kilograms per second, and raised the maximum thrust to 9.06 tons (88.94KN).
2.Using a brand-new 011 and a low-pressure turbine jointly developed by Klimov, the working temperature of the new low-pressure turbine can be increased by 70 degrees, and its working efficiency is also higher. With this turbine, the potential of the new three-stage fan can be better exploited. After the improvement, the maximum thrust reaches 9.28 tons (91.1KN).
3. 011 redesigned the backward combustion chamber of the RD-33, significantly reducing the problem of black smoke from the RD-33's oil-rich combustion,Greatly reduced fuel consumption and increased service life
4.011 replaced the original directionally solidified turbine blades with DD6 single crystal turbine blades, which raised the operating temperature of the turbine by another 50-70 degrees and increased the engine thrust to 9.5 tons
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This is the development history from WS-13 to WS-21 that I know of.