FLUG REVUE June 2005: Engines for JSF undergo tests

June 2005

JSF POWERPLANT TEST PROGRESS

By Patrick Hoeveler

On a sunny spring day, an almost full Airbus A319 takes off from the runway in Fort Lauderdale. Only about 70km away, the F135 engine achieves virtually the same thrust output on Pratt & Whitney's open-air test facility as the airliner's two CFM56. But instead of providing the propulsion for an aircraft with 140 passengers on board, it will carry only a single pilot and his Lockheed Martin F-35 into the air. With a maximum thrust of 191.1kN, it is the most powerful fighter aircraft engine in the world. For the first time, the type is to be optimised both for conventional (Conventional Take-off and Landing/Carrier Variant, CTOL/CV) and short take-off and vertical landing (STOVL) missions. These requirements pose great challenges to the engineers working on the F135 and its rival, the F136, which is being developed by the GE Rolls-Royce Fighter Engine Team. Wherever they can, they are therefore using already proven components. Thus, the F135 is based on the F119 for the F/A-22 Raptor, combining the existing core with a new low pressure system, while the F136 possesses the slightly scaled down core of the YF120, the defeated rival of the F119.

F135 test stand

On the other hand, the designers are entering new territory with the LiftSystem, which Rolls-Royce is developing for both STOVL engines. Here an 1.83m long shaft connects the LiftFan to the main engine. This two-stage, counter-rotating fan converts the output of the shaft into thrust. During the transition to the hover, a clutch engages the constantly rotating shaft and is hydraulically interlocked. The loads are enormous. “With just under 30,000hp on the shaft from the low-pressure turbine, these are very different conditions than on conventional engines,” remarks Bob Griswold, President of the GE Rolls-Royce Fighter Engine Team, during an interview with FLUG REVUE. The turbines are identical on both CTOL and STOVL variants. “Although they are loaded quite differently, they are tailored to both situations. This poses a significant challenge for the designers.” At the same time as the clutch engages, the rear thrust nozzle rotates downwards with the aid of the 3 Bearing Swivel Module (3BSM). It can rotate by 95 degrees in only 2.5 seconds. While this is going on, the three-part system changes its cross-section, putting a lot of stress on the silicon nitride seals. The aircraft is now controlled by roll control nozzles in the wing, which are fed with bleed air from the main engine. As the STOVL version of the F-35 in particular suffered from weight problems the engine companies have appropriate weight reduction measures in sight. Thus, Rolls-Royce has already trimmed 170kg off the Lift Fan alone. „The F135 is more than 3% below the Not to Exceed weight for the STOVL configuration and nearly 6% below for the CTOL/CV version“, is the message from East Hartford. Furthermore, the core of the F135 is said to be able to accommodate further increases in thrust should they become future requirements.

The difficulties lie more in the area of control, which is supposed to be identical for both variants. “From the YF120 we have a lot of experience with regard to the turbomachinery. However, putting an integrated control system together for the STOVL that is also compatible to CTOL is a big challenge”, Griswold admits. This is even more important in the powered lift mode, because then the engine control takes over the control of the whole aircraft whose aerodynamic control surfaces are rendered ineffective. Besides, the intelligent control system is supposed to contain new prognosis systems which allow on-condition maintenance. “It tells well in advance when the engine will get sick,” says Griswold. This means that the engine can remain on wing a lot longer. The competition is also making low maintenance efforts a priority. The F135 is claimed to have 40 percent fewer parts than present engines and 50 percent lower logistics costs.

At Pratt & Whitney seven engines are currently involved in the test programme, three conventional (FX631, FX632, FX633) and four STOVL variants (FX640, FX641, FX642, FX643). During the system development and demonstration (SDD) phase, the programme recently successfully completed the post test Critical Design Review (CDR) by the JSF Joint Program Office (JPO). “The success of F135 testing is a reflection of the unmatched maturity of this engine. The design incorporates experience from more than 2,600 SDD test hours on top of 3,500 pre-SDD hours over the course of the development program so far”, claims Bill Gostic, Vice President, F135 Programs. He plans 2,000 additional SDD hours this year, in which two additional ground test engines will be added to the test cycle. According to Pratt & Whitney, the problems with erosion in the second stage of the low-pressure turbine of the STOVL variant been solved by a coating upgrade and by removing some blockage from the cooling air supply.

In general, the SDD engines differ from the earlier CDA (Concept Demonstration Aircraft) versions used on the X-35 mainly in size. As the F-35 evolved, the size of the engine increased to have more airflow and thrust capability. In detail as Pratt & Whitney officials explain, the LiftFan features aerodynamic refinements in the transition between the fan exit and the Variable Area Vane Box Nozzle (VAVBN) to improve performance. The lift fan clutch clamping structure is being revised to increase durability. Additionally, the main fan blade tip profiles are being altered on the first and second stage to eliminate vibratory excitation. The engineers also re-match the high and low pressure turbine controlling areas to improve performance. The turbine exhaust case has finally evolved to a variant common design for the CTOL/CV/STOVL applications. Finally, the exhaust nozzle for STOVL is being revised to increase performance in hover and to have a smaller outside diameter to match with the refined aircraft. “The production engines will likely have further refinement of the hot section and augmentor based on test learning”, says Pratt & Whitney.

The alternative powerplant Point F136 is just under two years behind the F135. At present negotiations are under way with the US Government regarding the start of the SDD phase (about 15 engines by 2012), which is expected for 1 August. “We are very optimistic,” says Griswold. The test results to date have apparently achieved all the objectives. The CTOL engine (002) completed trials on the GE altitude test facility in Evendale after about 150 hours and is currently being dismantled and inspected. The STOVL version (003) has completed some 50 hours to date and will undergo testing until May. Engineers were planning the first transition to the hover for shortly after this edition of FLUG REVUE went to press. A decision has also to be taken shortly over the final assembly location. The first engine will fly in 2008.

From the fourth production lot of F-35's, the F136 will enter competition with the F135 in a directed procurement by the government. Due to the earlier delay in the general programme (see FR 4/2005) GE and RR will only “lose” 75 aircraft. Only from the 6th production lot will there be a free competition. The total potential was originally around 5,000 engines, since then it has progressively become smaller. “But the requirement is still in the thousands,” the GE manager believes. Hence, to ensure that GE and RR do not miss out on the international decisions, he is not prepared to let Pratt & Whitney's lead get too great. Ultimately, he is expecting some benefits from being partnered with Rolls-Royce, and does not see big disadvantages from being second: “We had three more years to get the engine right,” he says. On the other hand, Pratt & Whitney has the advantage of the better learning curve resulting from longer experience with its product. But ultimately it is up to the customer to decide which argument is the more convincing.

From FLUG REVUE 6/2005

Last updated 13 May 2005

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