Sometimes proverbial smoke leads to fire. And in other cases, there can be far less to a story than some might infer.
Such is the case of “screech” in which certain pressure pulsations occur when operating in full afterburner mode. This phenomenon is not new to military aviation and therefore not unique to Pratt & Whitney’s F135 engine for the F-35 Joint Strike Fighter, although it was inaccurately reported as a “new” problem by the Air Force Times this week.
Nonetheless, Pratt & Whitney discovered screech around May 2009 during development testing at low altitude and high speed. It has already been addressed with minor hardware and software modifications that eliminate the issue. In fact, design of this fix benefitted directly from tools and processes developed for our F119, the only engine to power the F-22. Together, the F119 and F135 are the only two production engines to provide augmented stealth capability, particularly noteworthy at a time when would-be stealth aircraft elsewhere in the world have garnered a disproportionate amount of media attention.
Now that the modifications have been identified and implemented, the F135 provides maximum thrust throughout the flight envelope. Modified test aircraft have shown excellent results, while the production configuration is being validated this year for both the F135 Conventional Take-Off and Landing / Carrier Variant (CTOL/CV) and Short Take-Off and Vertical Landing (STOVL) model. After all, full thrust throughout the flight envelope was a prerequisite for the government granting Initial Service Release for the CTOL/CV engine in March 2010, followed by the STOVL one in December 2010. No other fifth generation propulsion system has even come close to certification, let alone production.
Therefore, a mention about screech contained in an annual report from the Pentagon’s Office of the Director, Operational Test & Evaluation hardly qualifies as breaking news or even news. We now return you to your regularly scheduled programming already in progress.
"Here’s some film of the second F-35B short take-off and vertical landing Joint Strike Fighter test plane making its first vertical landing on Jan. 10 at Naval Air Station Patuxent River in Maryland."
From Lockheed Martin: “Today’s vertical landing onto a 95-foot square pad showed that we have the thrust and the control to maneuver accurately both in free air and in the descent through ground effect,” said F-35 Lead STOVL Pilot Graham Tomlinson.
Tomlinson performed an 80-knot (93 miles per hour) short takeoff from Naval Air Station Patuxent River, Md., at 1:09 p.m. EDT. About 13 minutes into the flight, he positioned the aircraft 150 feet above the airfield, where he commanded the F-35 to hover for approximately one minute then descend to the runway.
“The low workload in the cockpit contrasted sharply with legacy short takeoff/vertical landing (STOVL) platforms,” said Tomlinson.
Perhaps the adage that March “comes in like a lion” applies to more than just weather. That’s certainly what happened the week of March 8. Seemingly overnight, the F-35 engine debate heated up, driven by significant budgetary and programmatic scrutiny.
The week began with a Monday, March 8th issue brief from Dr. Loren Thompson of the Lexington Institute, who encapsulated the issue of alternate F-35 engine funding by reinforcing at least five key points. First, $2.9 billion will be needed in the near term to develop the alternative F136 engine, while predicted savings over the long term are based on highly optimistic assumptions that may never pan out. Second, most modern military aircraft are developed with just one engine supplier and fleet-wide groundings almost never occur. Third, the still nascent F136 is the more likely of the two engines to suffer a significant problem, based on its protracted development and troubled performance to date. Fourth, two engines won’t bolster reliability because operations, maintenance and even future technology improvement funds would be split between two incompatible systems. Lastly, neither the taxpayer nor the F-35 customers would be well served to support two supply chains, when one has clearly outperformed the latter.
This last point takes us to a couple of mid-week articles that initially reported some inaccurate cost figures for the F135. The bottom line: there are no new cost overruns on the F135 engine, and any additional expenses are tied to ongoing restructuring of the entire F-35 program. The federal government continues to express confidence in Pratt & Whitney’s cost reduction strategy, which is illustrated by an 11% drop in the LRIP-4 contract offering. Additional double-digit cost reductions are projected for future lots as well.
These proven savings stand in start contrast to the revisionist claims of those supporting a repeat of the “Great Engine War” of the 1980s. In a February 23rd memo to House Armed Services Committee Chairman Rep Ike Skelton, Deputy Secretary of Defense William Lynn wrote, “While much has been made of this example, the facts tell a more nuanced and inconclusive story. There were only minimal reductions in the acquisition unit cost price of the engines purchased for the F-15 and F-16 programs. Accordingly, it is difficult to cite this example to justify substantial savings due to competition.”
During a Wednesday, March 12th press conference, Undersecretary of Defense Ashton Carter reiterated the Defense Department’s concern regarding “hypothesized savings” for the F136 engine based on “some very optimistic assumptions that we don't think it is reasonable to accept.” And if recouping sunk costs doesn’t hold much allure, neither does the prospect of a second engine on technical grounds.
"We are not in the 1980s any longer, where high-performance engines had suspect reliability," Air Force Chief of Staff Gen. Norton A. Schwartz said. And he reiterated that only the U.S. Air Force is likely to buy from both engine manufacturers. "The Navy is not going to have two engines aboard ship," he said. "Our international partners are not going to have two engines. So the reality is that if we have an alternate engine and if there is a mandate for that, that obligation will ride primarily on the Air Force."
A disadvantageous “directed buy” scenario outlined by Undersecretary Carter and General Schwartz brings us to Gen. (Ret.) John Michael Loh, former commander of the Air Force’s Air Combat Command. In a Defense News editorial published on March 15th, General Loh brought up yet another important issue, stating “Air Combat Command will not receive its first operational F-35s with alternate engines until 2017. In my experience, putting an immature engine on a single engine fighter in high-rate production will result in many unnecessary, perhaps fatal, accidents while the new engine plods through its inevitable infant mortality phase.”
Above and beyond any potential safety of flight issues is the historical fact that two engines impact every facet of operations. “My firsthand experience with the F-16, after the introduction of the alternate engine, showed it limited operational flexibility significantly,” wrote General Loh. “It restricted decisions on basing because all F-16s at a base had to have the same engine, and it limited choices for deploying units for the same reason. Maintenance training and logistics support were more costly and complex.”
So, whether your primary concern is responsible government spending, empowering our military to accomplish their mission, or a combination of the two, the conclusion is the same: the F135 remains the best solution for the F-35 now and for the future.
Pratt & Whitney’s F135 propulsion system recently powered the Lockheed Martin F-35B Lightning II short takeoff/vertical landing (STOVL) fighter through the first in-flight engagement of its STOVL propulsion system. In the cockpit, at the controls during this historic flight was Graham Tomlinson, F-35 Lead STOVL Pilot and an employee of BAE Systems. Graham shared his thoughts on the significance of this major milestone, the performance of the aircraft and the performance of the Pratt & Whitney F135 engine.
1. As part of the first F-35B in-flight STOVL engagement, what specific criteria were you evaluating?
Graham Tomlinson: In the cockpit the task is to make sure that the “throttle and stick” responses are as predicted through these first conversions and our first experience of flight in STOVL mode. This really means comparing the responses to the training we’ve done in the simulators. All the responses did match extremely well – pitch, roll, yaw, and thrust - so it was a very easy and satisfying task for me. And the things the simulator can’t reproduce – noises, vibrations etc – were pleasantly subdued in the real thing.
But the key criteria were engineering responses, checked and analyzed on telemetry by the control room team. The pilot only gets a general impression; the engineers can check the detail. We were especially interested in the propulsion system response through the conversions, spooling up the liftfan and radically changing the thrust center of the engine and liftfan combo. We’ve done this static on the ground but never before with the aerodynamic effects of 200 knots (on the doors, on the intake flows, on the liftfan exhaust etc).
And after conversion in STOVL mode this again was our first flight opportunity to check the total thrust response and the balance of thrust from liftfan and core engine which are now not just thrust, but primary aircraft controls. These key parameters, plus the roll bleed posts off the core engine, are absolutely critical to STOVL flight control. The engineers were very pleased with the results.
2. Were there any surprises during the 14-minute STOVL segment of the test flight, not just in aircraft/engine performance, but also how it felt?
Graham Tomlinson: I hadn’t realized some of the limitations of motion simulators. When we make small acceleration requests the simulator gives a modest kick-in-the-pants. In flight, the kick-in-the-pants feels stronger (which all pilots will love!).
The other expected effect, but still nice to confirm in flight, was the smoothness of the ride. The liftfan is right behind your back, and you can hear/feel it humming away, but both the noise levels and the vibration levels were just fine. It’ll undoubtedly get louder as we slow down to the hover, but I was very pleased with the subdued levels at the speeds we’ve tested so far.
3. How would you characterize the performance of the Pratt & Whitney F135 engine and how does its performance differ from other fighter aircraft engines?
Graham Tomlinson: So far in the JSF test program the F135 has lived up to our lofty expectations; it has been there when we want it, and has been no trouble at all. That’s a remarkable achievement for an engine which has to reconfigure from a 40K pound thrust category in Max AB conventional mode, to a 40K pound thrust category with the liftfan spinning in STOVL mode. For me the best thing has been that we simply haven’t had to worry about it!
4. What was going through your mind when you engaged that clutch for the first time?
Graham Tomlinson: The flight test team is super-professional. We’re all thoroughly practiced and trained; we can do the job in our sleep. And that means that for this first button press there was zero stress, we were just going the through a routine test we’d seen a hundred times before. And that’s a good thing too, as pilots’ brains are well known to shrivel to the size of a pea in flight!
5. What is the best part of your job as the F-35 test pilot?
Graham Tomlinson: Being part of the team, bringing back the results and the data we need, and the satisfying feeling when the team makes progress. We’ve just made a huge step forward.
By now, most of you will have viewed the video posted here and elsewhere on the Web showing the first in-flight F-35 engagement of the Short Take Off Vertical Landing (STOVL) Lift Fan propulsion system. Days later, I still get goose bumps watching the F-35 Lightning II, powered by the Pratt & Whitney F135 engine, successfully achieve such a crucial milestone. And while that video engenders excitement and gratification throughout the Pratt & Whitney family as well as among our teammates, it also reminds us that there are numerous milestones ahead as the F135 continues to power Lockheed Martin’s F-35 flight test program while the engine also advances toward full production.
First, however, let’s remember how far we’ve come. Roughly a decade ago, Pratt & Whitney evolved the proven F119 engine (the exclusive propulsion system for the F-22 Raptor) to power both Joint Strike Fighter finalists. Following Lockheed Martin’s selection as the winning prime contractor, Pratt & Whitney began testing and manufacturing 18 flight test engines, the last of which will be delivered shortly. More than 126 test flights and 13,000 test hours later, the F135 remains the only engine to power an F-35, an accomplishment that will continue for quite some time to come.
In the world of jet engine test, there is a metric that is used to determine how the engine is progressing through System, Development and Demonstration. The metric is referred to as risk retirement. As the risk in the program is retired, the engine progresses in maturity. It’s worth noting that 90 percent of the F135’s program risk has now been retired through testing and other verifiable processes. And that number will be even higher by year-end. The most visible benefit of this engineering rigor is that despite the occasional challenge inherent with such a sophisticated propulsion system, the F135 has never caused a flight test delay or in-flight incident.
Pratt & Whitney will deliver its very first production F135 in the coming weeks. Also expected in the near future, and arguably most noteworthy is achievement of Initial Service Release for the Conventional Take Off and Landing/Carrier Variant (CTOL/CV) F135 engine, a key U.S. government endorsement that will mean the F135 meets all safety, reliability and performance requirements for operational use in the field. Simply put, the F135 is about to become the most advanced fighter engine ever certified for the defense of America’s freedom. Stand by for even more goose bumps.
Pratt & Whitney’s F135 propulsion system powers the new advanced, single-engine tactical fighter, the F-35 Lightning II, developed by Lockheed Martin. The F-35 has unique capabilities for conventional takeoff and landing (CTOL), carrier-variant (CV), and short takeoff and vertical landing (STOVL). The same F135 engine will power all three variants. Visit http://www.f135engine.com to learn more.
