Pratt & Whitney F135

F135 is an advanced turbofan engine developed by the U.S.-based Pratt & Whitney for fifth-generation fighter aircraft. This engine powers all variants of the F-35 Lightning II joint strike fighter family and was developed as a derivative of the F119 engine. The F135 is available in different configurations to accommodate both conventional takeoff and landing (CTOL/CV) and short takeoff and vertical landing (STOVL) aircraft.

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The F135 engine was developed under the Joint Strike Fighter (JSF) program based on the F119 engine. The development process included a series of testing and validation phases spanning from conceptual design to operational capability. By 2008, more than 10,000 flight test hours had been completed and the engine was successfully deployed on F-35 test aircraft.

The F135 engine has two main configurations:

• Maximum thrust: 43,000 lb (191.3 kN)
• Intermediate thrust: 28,000 lb (128.1 kN)
• Engine length: 220 in (5.59 m)
• Inlet diameter: 43 in (1.09 m)
• Bypass ratio: 0.57
• Overall pressure ratio: 28

• Short takeoff thrust: 40,740 lb (181.2 kN)
• Hover (loiter) thrust: 40,650 lb (180.8 kN)
• Main engine thrust: 18,680 lb (83.1 kN)
• Lift fan thrust: 18,680 lb (83.1 kN)
• Roll post thrust: 3,290 lb (14.6 kN)
• Engine length: 369 in (9.37 m)
• Bypass ratio (in high-lift mode): 0.51
• Overall pressure ratio: 29

The F135 engine consists of a three-stage fan, a six-stage high-pressure compressor, a two-stage low-pressure turbine, and an exhaust system with variable geometry. In the STOVL variant, additional components include a lift fan, a three-bearing swivel nozzle (3BSD), and roll post exhaust outlets.

Key engine components are equipped with design features that prevent failures and facilitate maintenance. These systems include advanced prognostic health management (PHM) sensors, vibration analyzers, temperature and pressure sensors, and integrated life management algorithms.

The F135 is designed to operate in conjunction with the F-35’s Autonomic Logistics Information System (ALIS). Data from sensors on the engine are transmitted to ground stations for post-flight analysis. This architecture supports timely maintenance, fault prediction, and inventory management.

Current use of the F135 engine has revealed limitations in its cooling capacity. According to the GAO’s 2023 report, increased power and cooling demands under the Block 4 modernization program have forced the engine to operate beyond its design limits, reducing engine life and increasing maintenance costs. An additional $38 billion in engine maintenance costs is projected.

In response, modernization of the engine’s power and thermal management systems has become a priority. However, the GAO report notes that the requirements for this modernization have not yet been clearly defined and that the program has not sufficiently transparently reported the causes of its cost increases. This situation hinders Congress’s ability to adequately oversee the process.