Towards First Flight:

A Look at the STOVL Joint Strike Fighters

By Ian A. Maddock

Introduction

An operational supersonic Short Take-Off & Vertical Landing (STOVL) fighter has been the goal of several aircraft acquisition and development programs over the past forty years. Some of the most notable of these attempts have been the German EWR VJ 101C, the French Dassault Mirage III V and the Soviet Yakovlev Yak-141. Although each of these aircraft convincingly demonstrated vertical take-offs and supersonic speeds, none of them ever reached production, and history is littered with many attempts that either couldn't demonstrate supersonic STOVL capabilities or never left the drawing board. However, the desire for a supersonic STOVL fighter never ceased. Now, the vision of a supersonic STOVL fighter is finally materializing with the advent of the U.S.-led multi-national Joint Strike Fighter (JSF) program.

The JSF program began in 1994 as the Joint Advanced Strike Technology (JAST) program. JSF is developing an affordable family of next-generation strike fighters for the U.S. Navy, Air Force, and Marine Corps, the UK Royal Navy and Royal Air Force as well as the armed services of several other U.S. allies.

Initially, four contractors were involved: Boeing, Lockheed, McDonnell Douglas/British Aerospace and Northrop. In late 1994 Northrop joined the McDonnell Douglas/British Aerospace team. Based on concepts under study for the Defense Advanced Research Projects Agency (DARPA) Advanced STOVL program, the four contractors began developing concepts for the JSF STOVL variant. Boeing proposed using a vectored thrust system, somewhat similar to the one in use on the Harrier Jump Jet. Lockheed proposed using a shaft-driven lift fan. McDonnell Douglas/British Aerospace originally was studying a gas-driven lift-fan configuration. Northrop opted for a lift-plus-lift/cruise configuration similar to the one used on the Yak-141. When McDonnell Douglas/British Aerospace teamed with Northrop, they kept the lift-plus-lift/cruise configuration.

On 16 November 1996, the Secretary of Defense announced that Boeing and Lockheed Martin would continue into the Concept Demonstration Phase (CDP). Pratt & Whitney (P&W) also moved forward into CDP to develop the propulsion system, with General Electric (GE) developing an alternate engine.

Engine Development

The primary propulsion systems being designed for the JSF Program are derivatives of the F119-PW-100 engine that powers the F-22 Raptor. The propulsion system concepts for both Weapon System Contractors (WSCs) use a P&W F119 core (compressor, combustor and high pressure turbine). The F119, a low-bypass ratio turbofan, is in the 35,000 lb-thrust class. The propulsion system concepts for the Boeing and Lockheed Martin configurations utilize new fan and low-pressure turbine (LPT) designs, which are based on F119 designs, materials and processes. The JSF119 engines were based on the F119 for commonality and cost reduction issues. These F119-derivative engines are in the 40,000 lb-thrust class. For the demonstrator program, both concepts take advantage of F119 controls and externals as much as practical, but each aircraft will use new nozzles.

P&W began the CDP by completing the preliminary design of the two selected engines, the JSF119-PW-614 for Boeing and the JSF119-PW-611 for Lockheed Martin. P&W then began extensive facility modifications, altering test stands for carrier (CV) and conventional take-off and landing (CTOL) engines as well as for STOVL tests of the full-up propulsion systems (i.e., with the Roll-Royce (RR) and/or Rolls-Royce Allison lift components), at P&W's West Palm Beach, Florida plant. These facility modifications resulted in a total of four test stands for CV/CTOL tests and one for STOVL operations as well as two tests stands for STOVL operation with multi-component thrust measurement systems. Facility modifications were also required at the Air Force's altitude test facilities at the Arnold Engineering Development Center (AEDC) in Tullahoma, Tennessee, to accommodate the JSF119 engines, since even the CV/CTOL versions are much different than the F-22/F119 engine.

JSF119 engine fabrication began in March 1997 with the machining of the titanium billet that would become the first stage fan integrally bladed rotor (IBR). Engine assembly began when the first JSF compressor section was completed on 19 September 1997. The first JSF core was completed on 28 January 1998 at P&W's manufacturing plant in Middletown, Connecticut.

The engine test program is designed to clear the engines for flight in the shortest amount of time as possible. P&W is building two developmental engines, two qualification engines and two flight-test engines for the Boeing JSF design, and a similar program for Lockheed Martin, with well over 1000 hours of ground test time for each WSC. Engine testing began on 11 June 1998 with the commencement of testing on the Lockheed Martin SE611; this was followed 10 days later with the Boeing SE614. Although two months behind the extremely aggressive contract goal date, the two engine designs were brought to test within 18 months of contract award through close coordination between the WSCs, P&W, and their vendors.

These initial tests were conducted at P&W's West Palm Beach facility and included component performance evaluations, compression system stability demonstrations, vibration surveys, operating system functional verification, and control software verification. The JSF119 engines were then instrumented for simulated altitude testing at AEDC. In over 120 hours of combined initial testing, the Boeing JSF119-614 and Lockheed Martin JSF119-611 engines demonstrated component efficiencies higher than anticipated, turbine temperatures lower than predicted, and very low vibration levels.

The STOVL engines began testing in mid-November 1998 on P&W's unique test stands that have been designed to measure forces in all directions to properly characterize STOVL engine operation and its relationship with the aircraft vehicle management system. By the end of 1998, P&W had four JSF119 engines under test, having accumulated a total of approximately 200 hours. Testing has continued with approximately 2000 hours of combined ground testing completed as of early 2000.

Boeing Propulsion System

Pratt & Whitney is developing the Boeing X-32 propulsion system. RR is responsible for the component design, development, fabrication, and testing of the lift system and spool duct. P&W is responsible for the component design, development, fabrication and testing of the engine; including the low-pressure spool, the remote augmentor, the 2-D convergent/divergent nozzle, the jet screen off-take, the primary engine controls (hardware and software), and the engine externals and accessories. P&W is under contract for the integration and qualification of all engine system components, including the RR developed items. Boeing is responsible for the component design, fabrication and testing of all airframe propulsion components. Boeing is also responsible for physically and functionally integrating the engine system provided by P&W and RR with the aircraft, thus forming the propulsion system. Boeing is then responsible for certifying the aircraft for flight. The Boeing X-32A aircraft will demonstrate CTOL and CV capabilities while the X-32B will demonstrate the STOVL capabilities.

The primary vertical lift of the X-32B propulsion system is from the two lift nozzles located between the turbine exhaust case and the augmentor of the SE614 engine, just aft of the aircraft center of gravity. The lift module consists of two vectoring lift nozzles with internal butterfly shutoff valves. The spool duct extends from the back of the lift module/transition duct to the augmentor. The nozzles can be rotated through a 55º arc from 45º aft of vertical to 10º forward of vertical. The lift nozzles are contained within the airframe near its center of gravity. When these nozzles are in operation, the main cruise nozzle is in the fully closed position.

The lift module consists of a double walled offtake case, two butterfly shutoff valves and two vectoring, fixed area convergent/divergent lift nozzles. The lift nozzles vector by rotating on bearings in a manner similar to the Harrier. The lift nozzles are stored at the 45º position behind STOVL bay doors when not in use. When the aircraft is hovering close to the ground, the engine inlet is shielded from the effects of hot gas ingestion by a curtain of cool air from the jet screen nozzle, which is located on the bottom of the fan duct.

Pitch and yaw control during STOVL operations is maintained by separate auxiliary nozzles located in the aft section of the aircraft. Roll control is maintained through similar nozzles located in the wing tips, which, like the other auxiliary nozzles, are supplied by fan duct air. During conventional (i.e. wing-borne) flight, the lift system and ACS are not required. The butterfly valves on the lift module are closed and the air is directed to the cruise nozzle; the lift system nozzles and ACS nozzles are covered by actuated doors to reduce the drag on the air vehicle and to reduce the low observable signature.

Located just in front of the 2-D cruise nozzle are two twin roll tubes protruding from either side of the propulsion system. At the end of these roll tubes are the roll nozzles, which help to control the aircraft during semi-jet-borne and jet-borne (vertical/transitional) flight. Below the 2-D nozzle is a single pitch nozzle. The pitch and yaw nozzles are combined on the Preferred Weapon System Concept (PWSC) -374 design (i.e. the production configuration). The cruise exhaust nozzle is a structurally integrated 2D design derived from the F119/YF119; the convergent flaps control the nozzle throat and fully close during jet-borne operations. Besides conventional throat and exit area control, the nozzle provides ±20º pitch thrust vectoring during conventional operation. All of the STOVL specific hardware on the X-32B weighs approximately 600 pounds and is eliminated on the X-32A and PWSC CTOL and CV variants of the aircraft.

The inlet system is diverterless and bleedless. The "bump" that is located on the top of the inlet is designed to provide compression of the supersonic flow ahead of the inlet cowl. The entire inlet is a single molded composite piece, which was manufactured using automated fiber placement technologies developed at Boeing's Phantom Works in St. Louis, Missouri. The cowling translates forward for the high airflow demands of jet borne operations.

In December 1995, Boeing successfully completed three months of engine and hover tests using a 94%-scale Large-Scale Powered Model (LSPM) at Boeing facilities in Washington. These LSPM tests verified the Boeing STOVL propulsion system and provided valuable data. During the following years, thousands of hours of sub-scale testing on both low-speed and high-speed aspects of the Boeing propulsion system - as well as STOVL ground effects testing - were also completed.

By August 1997, Boeing had completed several major nozzle tests of their STOVL propulsion system. The first test, conducted at the Boeing Nozzle Test facility in Seattle, evaluated the RR lift components. The 17% scale model was used to assess the performance and operability of the lift system and spool duct during conventional flight, STOVL operations, and transition from one flight mode to the other. The full range of JSF nozzle pressure ratios, mass flows and lift module positions were also evaluated. In wind tunnel tests at AEDC, Boeing conducted a three-week-long evaluation of the performance of the high-speed inlet/forebody compression system. The tests employed a 13% scale model and encompassed the full range of JSF flight speeds and attitudes.

On 28 February 2000, Boeing announced that the first flight of the X-32B would be delayed, this delay is due to technical challenges encountered in integrating the STOVL propulsion system with the flight control system. The strike by the Society of Professional Engineering Employees in Aerospace that ran for 40 days may also impact some JSF activities.

On 4 November 1999, the first flight test engine assembly was completed. Boeing received the first flight test engine from P&W on 6 March 2000. Boeing currently expects the first flight of the X-32B to take place in the fall of 2000.

Lockheed Propulsion System

Lockheed Martin is developing a STOVL lift system that uses a vertically oriented Lift Fan. As with the Boeing system, both P&W and RR are developing the X-35 propulsion system. The responsibilities of P&W, RR and Lockheed are also similar. A two-stage low-pressure turbine on the P&W SE611 engine delivers the horsepower to drive a new, larger fan than the one on the F119 and also powers the STOVL Lift Fan. The Lift Fan provides up to 18,500 lb of thrust, using variable inlet guide vanes to modulate the airflow and therefore the thrust. The Lift Fan has a clutch that engages for X-35C STOVL operations and a telescoping "D"-shaped nozzle to provide thrust deflection; the D nozzle consists of four sections with the final part containing fixed vanes.

The Allison Lift Fan is located behind the cockpit in a bay with upper and lower clamshell doors. When operating at normal speeds, the Lift Fan is capable of supporting nearly half of the weight of the X-35. Another STOVL-unique feature on the X-35 is the auxiliary inlet for the main engine, located above the fuselage and behind the lift fan; this is used for the high air flow demands of hover.

The engine exhausts through a three-bearing swivel nozzle (3BSN) that can deflect the thrust from horizontal to just forward of vertical. Two roll ducts supplied by engine fan air provide roll control. Yaw control is through swivel nozzle yaw. Pitch control is effected via Lift Fan/engine thrust split.

For conversion to short take-off mode, the Lift Fan inlet and exhaust doors open, the inlet guide vanes are closed down to minimize air flow, and the clutch is engaged. As the clutch plates synchronize, the Lift Fan gear drive accelerates and is brought up to the input shaft speed. A mechanical lock-up device then assures that the clutch does not slip once the Lift Fan is fully engaged. The inlet guide vanes are then opened to bring the Lift Fan up to speed and the D nozzle is rotated down to vector the Lift Fan thrust aft; with the main engine thrust, this helps accelerate the aircraft forward and upward. After transitioning to wing-borne flight, the inlet guide vanes are again closed down to reduce the air flow through the Lift Fan, the clutch is disengaged, the nozzle is retracted, and the inlet and exhaust doors are closed.

For the conversion to vertical landing mode, the aircraft decelerates and the Lift Fan inlet and exhaust doors open. The Lift Fan is then brought up to speed as described above, but the D nozzle is left retracted to its fully vertical position.

The clutch is designed to engage in 3-7 seconds. With the variable geometry vanes closed and the engine speed reduced to 80-85%, horsepower during engagement is reduced to about 4,000 hp. After engagement, it transmits approximately 28,000 hp at 8,500 rpm. The clutch plates absorb energy during engagement and then dissipate it before the next engagement via cooling air.

Simple configuration changes enable the conversion of the SE611 from a CTOL/CV to a STOVL engine. Engine controls and software will differ among the various configurations. For the STOVL variant, the fan duct incorporates a bypass offtake system for aircraft roll control. A shaft is attached to the engine's low-pressure rotor. The axisymmetric nozzle is replaced with the 3BSN.

The 3BSN nozzle, developed by Rolls-Royce, was patterned along the lines of the exhaust system on the Yakovlev Yak-141 STOVL prototype that last flew at the 1992 Farnborough air show. A US Navy program also developed swivel nozzles in the late 1960s and was proposed for a supersonic STOVL design by Convair (one of the Lockheed Martin heritage companies) in the early 1970s.

The roll control ducts are located on either side of the SE611 engine and are also produced by Rolls-Royce. These roll control ducts extend out to the point of the wing fold and are supplied with their thrust with the air from the engine fan. The ducts on the end of the post open and close differentially for roll control.

The Shaft Driven Lift Fan (SDLF) concept, patented by Lockheed Martin, was successfully demonstrated through their Large Scale Powered Model (LSPM) in 1995-96. According to Lockheed Martin, they selected the SDLF propulsion system for three primary reasons: the STOVL Lift Fan thrust can be de-coupled from the P&W cruise engine, thereby enabling the cruise engine to be appropriately sized for conventional flight; the significant amount of thrust augmentation obtained from the Lift Fan greatly exceeds the additional weight incurred; and the lower exhaust jet temperature and pressures result in a more benign ground environment during hover than that produced by direct lift.

During the summer of 1997, Allison conducted testing of a model of the Lift Fan nozzle at the NASA Lewis Powered Lift Facility in Ohio. The test results validated the computational fluid dynamics predictions of exhaust nozzle performance. B.F. Goodrich conducted testing of the Lift Fan clutch being developed under a subcontract to Allison. Testing demonstrated high-speed clutch engagements that were representative of the X-35 STOVL operating conditions. A favorable clutch plate wear rate translated into a clutch plate life of over four times the X-35 flight demonstration requirement.

The SE611 STOVL engine first drove the Lift Fan on 10 November 1998 and it operated at 100% power for the first time on 22 November. By the end of 1998, the Lockheed Martin SE611 STOVL engine had operated with the Rolls-Royce Allison Lift Fan both engaged and disengaged with the three bearing swivel nozzle actuated from 0-90° at full STOVL power.

P&W delivered a flight engine to Lockheed in December 1999 for a fit check. On 9 December, Lockheed successfully installed the JSF119-611 flight engine on the X-35A demonstrator aircraft at Lockheed Martin Skunk Works, Palmdale, Calif. Installation was completed in only three hours, including the time spent confirming procedures and documenting interfaces. Following the check, the engine was returned to P&W. Following acceptance testing, the flight test engines were shipped to Lockheed for CTOL flight tests, Lockheed took possession of the CTOL flight test engine on 13 February 2000.

On 28 January 2000 the FX662 STOVL engine exceeded its vertical lift operational thrust requirements. However, on 8 February a bracket supporting the No. 3 bearing inside the Lift Fan gearbox failed; the failure was not catastrophic and was limited to the bearing housing. Prior to its failure, this Lift Fan had run for 67 hours, which is equivalent to more than 18 months of operational service. Testing is scheduled to resume following an investigation of the failure. Lockheed Martin is currently estimating the first flight of their STOVL JSF to take place during the fourth quarter of 2000.

The Alternate Engine Program

The goal of the JSF propulsion system development is to produce two competitive, multi-service, physically and functionally interchangeable propulsion turbomachinery designs, both compatible with the Rolls-Royce STOVL components. To this end, General Electric (GE) received a contract to begin the preliminary design of the JSF F120 core turbomachinery in November 1995. This core is now being developed and components tested, with a full-scale core engine test scheduled for late 2000.

GE, Rolls-Royce, and Allison Advanced Development Company (AADC), teamed to produce the F120 alternate engine for JSF. Rolls-Royce's responsibility in this program is to design and develop the low-pressure fan, which will employ a development of the hollow titanium fan technology on Rolls-Royce large commercial engines. AADC (part of Rolls-Royce) is responsible for the combustor and low-pressure turbine. GE will lead the team and build the high-pressure compressor and high-pressure turbine.

Starting in 2001, after the final downselect for EMD, the Alternate Engine Program will develop the appropriate concept-unique low-pressure components. EMD of the Alternate Engine is currently planned for 2004-2011. Competition with the P&W engine will then commence.

The Future

Over the course of the next year, both the X-32 and the X-35 will perform flight tests and another supersonic STOVL fighter will have taken to the skies. However, the JSF will be the first supersonic STOVL aircraft to enter production and serve with the military.

The JSF Program Office will decide which aircraft design will prevail following the evaluation of information produced during the flight test program. There has been much talk about whether or not the Pentagon will award a winner-take-all contract for the JSF. It is quite possible that the production of the fighter may be split between the two contractors in order to sustain the industrial base. Whatever the outcome of the selection, within the next decade there will be another supersonic STOVL fighter serving the US and her allies.

About the author

Ian Maddock works for Analytic Services, Inc. (ANSER) in Arlington, VA as a Military Systems Analyst in the Joint Technology Division. Mr. Maddock has a M.S. in Defense and Strategic Studies from Southwest Missouri State University. He currently supports the Joint Strike Fighter propulsion management team and has served as the author/co-author of several studies.