Rolls-Royce LiftFan Adds STOVL Operations to F-35's Capability

 - June 16, 2016, 12:36 PM

Assuming its planned appearance at the Farnborough International Airshow goes ahead, the Lockheed Martin F-35 will be the most significant new military aircraft to be seen in the skies of southern England for many years. It has been a long wait for British eyes, the nation being the first overseas partner to sign up to the Joint Strike Fighter program, as it became. Britain joined following the signing of a government-to-government memorandum of understanding way back in 1995.

It was a momentous decision for the UK government at the time, as the JSF design and configuration was yet to be confirmed, and experimental prototypes of the two rival projects, from Boeing and Lockheed Martin, would not fly for another five years.

For the UK, this would also prove to be an irreversible and game-changing decision, for the chosen JSF design, won by Lockheed Martin with the F-35, would be produced not only in conventional land-based and carrier based versions, but also with full Short Take-Off and Vertical Landing (STOVL) performance, in the form of the F-35B. This variant was destined to replace the Harrier and Sea Harrier with the U.S. Marine Corps and the UK’s Royal Air Force and Royal Navy.

The latter service will become totally dependent on the new aircraft to provide future air defence and attack capability for its new generation of super-carriers, which will not have catapults or deck arrester equipment. Meanwhile the U.S. Marine Corps will also be depending on the same V/STOL design (as it is known across the Atlantic) to operate from the short decks of its assault carriers and from short airstrips at forward operating bases.

UK engine maker Rolls-Royce was the key to providing the advanced technology propulsion solution that could be integrated into a supersonic, low observable airframe. By deciding to partner (through BAE Systems) with the U.S. JSF Program, the UK abandoned its own advanced Harrier replacement projects to cooperate in building a joint STOVL version of the JSF and to share development costs and benefit from the 3,000-plus production program envisioned for the new multi-role aircraft.

The F-35B had to incorporate as much commonality with the conventional F-35A and C models as possible, but the need to provide a safe and robust STOVL system within the tight physical constraints of a relatively small airframe posed a major design challenge for all concerned.

The chosen “LiftSystem” includes a center-mounted LiftFan and a downward-swivelling rear nozzle, with flight stabilization provided by ducted roll posts under each wing. Unlike early VTOL projects from the 1960s, which used separate lift fans for vertical thrust, the F-35B features a main engine, the Pratt & Whitney F135, connected to the LiftFan by a shaft and high-speed clutch, developed by Rolls-Royce.

Together, the LiftSystem components provide more than 40,000 pounds of downward thrust, and are activated by the pilot pushing a single button. This thrust is almost twice that of the Rolls-Royce Pegasus engine that powers the Harrier. The forward-mounted 50-inch diameter two-stage contra-rotating LiftFan delivers some 20,000 pounds of cold thrust and incorporates a host of experience from more than 60 years of STOVL technology, as well as more recent innovative Rolls-Royce powerplant developments, such as the latest hollow-blade disk technology and a thrust-vectoring variable-area vane-box nozzle.

The driveshaft and clutch deliver up to 29,000 shaft horsepower from the main engine to the LiftFan, while the hydraulically activated roll-post nozzles direct 1,950 pounds of bypass thrust from the main engine and provide roll control and stability. The main engine directs 18,000 pounds of thrust via a rear nozzle, which can rotate through 95 degrees in just 2.5 seconds. In conventional flight, the main engine can take the aircraft supersonic with afterburner (or “reheat” in the UK).

 15 Years of Development

Rolls-Royce first became involved in the F-35 System Development and Demonstration (SDD) phase of the program in December 2001, following the selection of the Lockheed Marin F-35 concept in October 2001. In April 2004, the first STOVL engine was tested and in November 2008 the company received its first production contract for the LiftSystem from Pratt & Whitney, with testing in a hover pit commencing in March 2009.

The story of the development of the F-35B’s propulsion system follows a similar path to that of the program as a whole, with many changes and delays along the way. Plans for an alternative main engine to the P&W F135 were later dropped on cost grounds. This despite the fact that the F136, which was being developed by GE and Rolls-Royce was showing signs of being a more capable main engine,.

As flight tests on the F-35B progressed, various over-heating issues arose that led to pauses, which in turn led to cost increases as temporary and then permanent fixes were identified, tested and finally adopted.

With such a complex aircraft, with so many highly innovative technical features, it was hardly surprising that development cost increases and delays would occur, and overcoming these many issues regularly brought the program under very close DoD scrutiny as the battle to keep the JSF budget affordable continued. The program became the subject of intense political debate.

However, while all the industrial partners struggled to stick to ever-extending block-capability clearance timescales, the sheer industry and service momentum that had built up started to turn the situation around, enabling the revised Initial Operational Capability goals to be reached.

As more service test-pilots and future instructors sampled the F-35B, they became very enthusiastic advocates of the aircraft, happily extolling the outstanding flight stability in the hover and at slow forward speeds, especially transitioning to a landing. With a lift thrust that allows a payload in excess to that carried by the Harrier, and the ability to bring home unused ordnance in a rolling vertical landing, the F-35B’s unique STOVL capabilities, combined with stealth and high speed, promise to bring a step change in air operations from carriers and land bases.

The LiftSystem has now been evaluated in severe conditions, including operating with wind speeds as high as 250 knots across the LiftFan, while still keeping within stability requirements. Early issues involving the robustness of some components have been overcome, and revised cooling solutions have solved the over-heating problems.

Speaking recently to the author, the UK’s Chief of Staff for the joint RAF/RN Lightning Force and designated future Commander of the famous 617 Squadron, Wing Commander John Butcher, confirmed that the F-35B is very easy to fly in the transition and landing/take-off modes. In his opinion, it will allow pilots, whether from the air force or navy, to adapt easily to carrier deck or traditional land-based operations. Butcher has considerable Harrier experience and said that the highly automated design of the STOVL version of the Lightning II will indeed enable pilots to fully exploit all its unique features, offering an unbeatable combination of stealthy operations, extreme situational awareness and the ease of STOVL handling.  

 Recent Progress

As the first appearance of the F-35 in UK skies approached, AIN sought the views of Rolls-Royce’s Jarrett Jones, v-p F-35B LiftSystem, concerning the program’s progress since the last Farnborough show. Asked what have been the main challenges to overcome, he replied, “We’ve introduced some improvements to the durability of the clutch, which provides enough torque to turn the London Eye, through new materials, which has given us full engagement life for the clutch plates. We have also improved the interstage vanes, which has cleared the full environmental envelope.”

Asked what the main advances have been in the past two years, he said, “In partnership with Pratt & Whitney, we’ve passed some major test milestones over that period which have paved the way to unrestricted operations. These include the successful completion of the Eglin climatic testing, which subjected the LiftSystem to a full range of hot, cold, icing, humidity, solar and rain conditions. We also carried out successful rain and water-ingestion testing on the LiftFan.

“Another successful completion was the second aircraft sea trial [DT-2] and Operational Test [OT-1] on the USS Wasp, demonstrating operational capability.” He added that full envelope flight tests were successfully completed to validate robust mechanical and control systems functionality, together with incorporating block upgrades to interstage vanes.

Asked if the flight tests were reflecting accurately what was expected in simulations of the control and performance aspects of the propulsion program, Jones confirmed that the LiftSystem operation and performance had met or exceeded predictions throughout the flight envelope.

One of the issues that has been subject to some speculation concerning F-35B operations has been linked to landings on ship decks and runway surfaces. Asked if tests had indicated that ingestion or surface damage had required any new attention or changes, Jones said, “Both shipboard deployments were completed without any surface damage to the USS Wasp landing surfaces and no deterioration has been accumulated on the PAX [Patuxent River test facility] runways during flight testing. We’ve also not encountered any LiftFan damage due to ingestion of surface material during the flight test program.”

The UK has said that rolling vertical landings will be a feature of operations from its new RN aircraft carriers. Jones underlined the fact that the LiftSystem was designed for such rolling vertical landing capability and this had been successfully demonstrated during the flight test program, so it won’t make any difference to operating flexibility or require any design changes in the system, he said.

Another subject that often surfaces when looking at future F-35B operations by the UK services concerns what is being put in place for the shipboard support of the LiftSystem aboard the new carriers. Jones said, “Ski-jump takeoffs have been demonstrated during the flight test program in support of the UK aircraft carrier application, and maintenance trials will be conducted during the third aircraft carrier sea trial scheduled for later this year. Our plans for supporting both the USMC and UK Royal Navy shipborne operations are well developed and we are also working on our support capability plan for the overall UK fleet.”

Completion of the SDD phase for the LiftSystem is on schedule, and the sand ingestion test is the last one left to complete. It will demonstrate the durability of the LiftSystem when operated in remote or sandy environments.

Jones said that as well as providing actual hardware and testing activities, Rolls-Royce has pursued more than 150 cost-cutting initiatives to further improve the value of the system to its F-35B customers.

There can be no doubt that, for Rolls-Royce, the success of this most challenging program has been an important achievement in its ongoing engagement in top-end military engine technology. Delivering the lift system for such a key application has become one of the most essential elements in the company’s long-term future.

Being responsible for the world’s only vertical lift technology in production—and for the world’s premier next-generation multi-role combat aircraft—is certainly keeping Rolls-Royce at the forefront of defence air systems innovation.