The pace of JSF engine development is picking up, as one team gets ready to assemble the first flight-test powerplant and the second enters the system development and demonstration phase
It is one of the most complex and ambitious military propulsion development efforts of modern times. Two teams are marching in parallel to develop two basic engines and four derivative propulsion systems for three versions of the Lockheed Martin F-35 Joint Strike Fighter (JSF). The potential numbers are huge, with a projected market of 5,000 aircraft over the next 20 years, making it easily the most significant manned fighter programme of the 21st century.
As developer of the lead engine on the F-35, Pratt & Whitney’s F135 programme team is clear about its pivotal role in helping the JSF achieve its goals. P&W is the prime contractor with responsibility for the main engine and overall system integration, with Rolls-Royce providing the lift fan, three-bearing swivel module and roll posts for the short take-off and vertical landing (STOVL) version. Hamilton Sundstrand is providing the control system, external accessories and gearbox.
Leading the effort is Bill Gostic, vice-president of F135 programmes at P&W, who says the post-test critical design review in March was “a major milestone for us as it allowed us to assess if we were still on the right track to achieve our initial performance specification as we go into initial low-rate production”. The week-long event uncovered no “show-stoppers”, says Gostic, although “we do have a few things to work on”.
Based on the F119 powering the Lockheed/Boeing F/A-22 Raptor, the F135 combines the six-stage high-pressure compressor and single-stage high-pressure turbine of the air-superiority fighter’s engine with a new low-pressure (LP) spool. It builds on the JSF119 derivative engine that powered all the JSF concept demonstration aircraft (CDA), accumulating almost 200 flight hours in the process, in addition to more than 3,600 ground test hours.
The current F135 engine effort is being funded under a $4.8 billion, 10-year system development and demonstration (SDD) contract awarded in October 2001. The programme has so far seen three conventional take-off and landing/carrier version (CTOL/CV) and four STOVL engines delivered to ground test. The development phase takes the propulsion system through flight clearance and flight test all the way to qualification for low-rate initial production (LRIP), beginning in 2009.
Some 1,363h have been amassed on the CTOL engines and 1,807h on the STOVL engines, for a total of 3,170h so far. “The focus is to mature the engines as much as possible through the development tests. There is no substitute for actually running the engine and flying it,” says Gostic, who adds that assembly of the first flight-test CTOL engine is due to begin at the company’s Middletown, Connecticut site in late August. “We plan to deliver that no later than the end of December, and just as soon as possible to give Lockheed Martin some schedule margin.” Initial flight release is set for January 2006, with the first flight pencilled in August 2006.
STOVL flights
P&W will also deliver two spare CTOL engines by year-end. The first STOVL F135 flight-test engine is due for delivery to Lockheed in February 2007, with the first flight set for the end of that year. In all, the company is scheduled to supply 21 engines, of which 14 are CTOL/CV configured and seven STOVL. Of the group of 14 conventional engines, six will be allocated to the CV tests.
“By the time we get to the initial flight qualification of the CTOL engine later this year, we will have around 4,000h of test time,” says Gostic. “That begins to rise even more steeply as we start to fly, and we expect to have around 50,000h by the time the first STOVL flies.” The shared heritage with the F119 also contributes valuable lessons to the JSF effort, he says. “There are things we are still learning, either for performance or in terms of upgrades we can make to the design. There are some synergies there, and we believe it is absolutely crucial to the operability of the engine and single-engine safety.”
For example, sustained high-g manoeuvres performed by the F/A-22 in flight tests at Edwards AFB, California revealed the need to tighten the oil sealing around the LP turbine when oil leaks appeared in the No 5 bearing compartment. “When we compared the models of the structure versus the actual event of sustained high g, we discovered we needed to make some changes,” says Gostic. The turbine exhaust casing section in both the F119 and F135 has been stiffened, also resulting in less “rubbing” of the turbine blades. “It’s probably something we would have found [in the F135] five or six years from now, and would have to fix.”
It has not all been plain sailing with the F135. Outstanding issues emerging from the critical design review are “relative to optimising the engine/airframe combination for vulnerability [ballistics], low-observable properties and managing the thermal loads”, Gostic says. “That’s important, the F-35 being a stealth-type aircraft, and all the heat loads for the electronics are absorbed through the fuel. If the fuel gets to too high a temperature, we run into coking of the combustor fuel nozzles.”
Coking, or the depositing of carbon particles on the burners, produces inefficient, non-uniform combustion. This disturbs the thermal profile through the turbine and raises operating and maintenance costs – critical issues in the JSF.
The ground tests have also pointed to the need for minor component modifications to counter “additional development issues”, says Gostic. These range from tightening attachments or modifying components to counter potential high- cycle fatigue issues spotted during vibration tests, to optimisation of cooling holes in the turbine or afterburner liner. “That’s what the initial test phase is for – to identify any issues and make sure you eliminate them before you go to flight test.”
Another vital series of tests is being conducted on the redesigned STOVL lift fan nozzle, which is now smaller than the CDA version. The nozzle was reduced in size to cut drag and weight, and will be tested later this year along with the associated variable-angle-vane box nozzle (VABN). The area of the box had to be changed to adapt to the revised packaging of the surrounding weapons bays, as well as to suit revisions to the centre of gravity of the production-standard aircraft. “This is the first time we’ve taken a design like that to production in which the packaging is different in the SDD aircraft to the CDA,” says Gostic. “We have to maximise the efficiency of the flow and reduce the losses out of the lift fan as it enters the VABN. Rolls-Royce is integral in that.”
The competing General Electric Rolls-Royce Fighter Engine Team (FET) is also at a critical point in its efforts to bring the rival F136 to market. After its first STOVL test this year, the team delivered the formal proposal for the F136 SDD phase to the JSF joint programme office on 2 May, and expects to be under contract by 1 August. The SDD contract will be worth about $2.4 billion between 2005 and 2013, and takes in release of engines for flight test.
The award will be a huge boost for the engine team, which has often battled against funding odds to survive. “In 1997 there wasn’t an F136 programme of any kind,” says Tom Hartmann, vice-president for JSF programmes at R-R. “Now the award of SDD comes as an incredible relief. It’s marvellous because we have had to endure some heart-stopping moments.”
The run-up to this August’s SDD award began with successful core and fan rig-testing in 2000, leading to the first full engine test in July 2004. GE, responsible for 60% of the programme, is developing the five-stage all-blisk high-pressure compressor at the heart of the engine as well as the controls and accessories, structures and afterburner. Stages three to five of the compressor are inertia-welded together, with all five stages having forward-swept aerofoils.
The GE-developed single-stage high-pressure turbine is combined with the first of the three low-pressure turbine stages in a coupled, vaneless counter-rotating system. R-R, with 40% of the programme, is responsible for the three-stage fan, single annular Lamilloy combustor, three-stage low-pressure turbine and gearboxes.
SDD includes the production and qualification of 14 engines, seven of which are for flight-test aircraft and seven for ground tests. First engine to test will be around mid-2008, but earlier tests are due to begin in the second quarter of 2007 to address some risk-reduction items on the afterburner using one of the original test engines. Flight tests are expected to start in 2009, with production engines available for the F-35 Lot 4 initial production batch in 2012.
FET president Bob Griswold says the STOVL test marked the effective completion of the third pre-SDD phase, which began in 2002. The first phase, covering engine definition, ran from 1995 to 1997, and the second phase, including coreand fan testing, ran to 2001. The SDD phase will involve at least 12,000h of engine tests.
“From 1 August we will be launching head-over-heels into preliminary design of the SDD motor,” says Hartmann. The SDD engine will feature “tweaks” to suit the higher power requirement of the production-standard F-35, says Griswold. “We’ve been following that closely and matching the engine characteristics. You don’t like being second, but, if there’s an advantage, hopefully we’ve done a better job of matching performance with the aircraft.” Hartmann adds: “We took the time, as the aircraft design matured, to optimise the engine design and we think that’s going to allow for future capability. We have an engine that matches the current specification with some margin.”
Improvements include upping the fan flow and slightly adjusting the core size. “We’re looking at trying to get a bit more flow through the fan area and the front frame by a small change in fan diameter and recontouring of the struts,” Hartmann says. Compared with the original YF120-based engine sized for the higher thrust requirement of Boeing’s direct-lift JSF contender, the pre-SDD F136 engine was 92.5% scale. This is now being adjusted upward to about 95% scale for the SDD phase. In terms of airflow, the inlet adjustment will increase flow capacity from 163kg/s (362lb/s) to 172kg/s for the SDD engine. Tests of the revised fan will take place in a test cell on a pre-SDD engine, rather than on a rig, to “get a bigger bang for the buck”, says Hartmann.
Reducing risks
Over the December-January period, FET plans to perform tests on another pre-SDD engine fitted with revised control system software algorithms. “We need cool weather for this work, which is more for control system verification and shakedown,” says Hartmann. “We want to reduce the risk of the control system for SDD.” Another milestone will be the F136 initial baseline review to be held with the JSF joint programme office. “That’s the first major event, and we’re going to be collecting data to have at that review.”
FET is keen to make the best of both its later development cycle position to tailor its design, and of what it sees as its single-engined operating heritage. “Right now I’m more interested in holding operating costs,” says Hartmann, “but between GE and R-R we have put together a team that has unique STOVL and single-engine operations experience between the Pegasus and the F-16. We are going to ‘roll out those smarts’ and build the best engine that is going to meet the three very different operational needs of CV, CTOL and STOVL. We think we have a big advantage.”
Guy Norris/Los Angeles
Source: Flight International
