Graham Warwick/WASHINGTON DC
With ground tests of the engines for the rival concept demonstrator aircraft now under way, the Joint Strike Fighter (JSF) has taken a major step towards becoming a reality. With the first X-32 now taking shape at Boeing, and the first X-35 at Lockheed Martin, the maiden flights in early 2000 no longer seem so far away.
Both JSFs have already flown, and fought, in thousands of simulations designed to determine the combat effectiveness of the designs. Hundreds more simulations are planned before the JSF operational requirements are finalised in late 1999. If the services and industry get their way, this process could continue throughout development and production in an effort to preserve the affordability and commonality that are the hallmarks of the JSF programme.
Intended to meet the needs of the US Air Force, Navy and Marine Corps, as well as those of the UK Royal Navy and export customers, the JSF programme is unusual in that the operational requirements and the weapon-system design are being defined in parallel. The joint operational requirements document (JORD) is not scheduled to be finalised until December 1999, just months before the two teams submit their bids for development and production.
Until then, Boeing and Lockheed Martin are working with a series of joint interim requirements documents (JIRDs), the latest of which, JIRD III, is expected to be released in July. Each iteration is the result of extensive cost and operational performance trade studies to determine the affordability and combat effectiveness of changes to the JSF requirements.
It is a cumbersome, but powerful, process, say service chiefs and industry executives. What it does is make clear to the customer (the "warfighter" in US parlance) the cost of a particular design attribute. "A good way to tell how much you love something is to see the cost," says one service chief.
AFFORDABLE REQUIREMENT
"There are ways to make an aircraft more affordable, but if we do not start up front with a set of requirements focused on affordability, then we can only offset the high cost of performance," says JSF programme director Maj Gen Leslie Kenne. "If you have an affordable requirement, then you can't help but make significant progress in reducing the cost of the weapon system."
The requirements definition process involves extensive modelling and simulation at the JSF programme office and the service headquarters. "We have pilots and maintainers in the programme office," says Kenne, "and we have provided the services with a suite of models and simulations which they have agreed represent true operational effectiveness at campaign, mission and engagement level.
"Using those models, we input the attributes of legacy aircraft and look at the outcome, then input JSF with varying levels of attributes and look at the changes in outcome. We have done a tremendous amount of analysis, and we now understand the attributes that are the most leveraging" - the most effective, in other words.
Kenne cites as an example the manoeuvre load-factor capability: "From 8G to 9G there is little change in effectiveness, but below 7.5G we get problems." Another example is maximum speed, she says. "Going past Mach 1.5 is expensive. If it turns out that we need M1.2, with the ability to get to M1.5 quickly, that reduces overall cost.
"What is really different about this programme is that warfighters are worried to that level about cost-and that they now have the insight to make informed decisions to meet cost goals. This is the first time that warfighters have cost goals."
DESIGN EVOLUTION
Industry's role in the definition process is to evolve JSF designs to match each new version of the requirements, and to perform the trade studies that provide the cost and performance data used in the simulations that will determine the next iteration.
"With JIRD III," says Lockheed Martin JSF programme director Frank Cappuccio, "we will have 80-90% of the final definition." According to Kenne, work still to be completed includes finalising the avionics functionality. "Onboard versus offboard [sensors] trades have still to be made and the results will be in the draft JORD," she says.
According to industry, the latest iteration of requirements definition has driven down JSF costs, which were creeping up. "JIRD III will help push the cost of each variant down," says Frank Statkus, Boeing JSF programme manager. Changes have reduced the cost of the USAF's conventional take-off and landing (CTOL) version and reduced the weight of the US Marine Corps' short take-off and vertical landing (STOVL) variant.
"CTOL is the greatest cost challenge," says Statkus, "while STOVL is the most challenging on weight." The US Navy's carried-based (CV) CTOL variant has the most challenging operating environment, he says. The RN, meanwhile, describes its requirement as an "amalgam" of those of the USMC and USN.
The requirements definition process has proved effective, so far, in keeping the four services together on the programme. One reason, says Kenne, is that their needs do not have to be identical. "All the requirements are slightly different, but close enough to maintain a family of aircraft," she says.
Kenne likens this to the concept of "cousin" parts, which will be used in JSF manufacture to increase commonality. These have the same design, but are machined to different depths for each variant. Such parts cannot be too divergent, however, and the same goes for the services' requirements, she says.
"Nobody is looking for a high-end F-22, thankfully. They are looking for an F-16/F-18-class aircraft, but more survivable-and they are keeping the requirements in a box small enough to allow us to provide a family of aircraft."
CHALLENGES AHEAD
Challenges remain ahead, however. Air Force commanders want an advanced, integrated gun, but the other services are less keen. The USAF's requirement for a gun will be in JIRD III, says Kenne, but whether it makes it into the final JORD will depend on further trade studies and negotiations between the services.
"We think we need a gun in the aircraft, but we need the JSF first, so the programme must be successful," says a senior Air Force official, adding: "A lot of this is negotiable." It is not simply a matter of deleting the gun from the other versions, because there is a "scar weight" associated with the structural and other provisions required for an integrated gun.
A similar debate surrounds the capacity of the internal weapons bays. The Navy insists on the ability to carry two 900kg-class air-to-surface weapons internally, but the Marine Corps is concerned about the weight penalty on its STOVL variant of larger bomb bays. The debate could be overcome by developing smaller, higher yield, weapons, or improving targeting precision, but both could be expensive.
Lockheed Martin's Cappuccio believes that gun and bomb issues "-will bounce around for a while". The final decisions should be based on operational doctrine, he argues, and the Air Force says that, before making a decision, it will model the effectiveness of an advanced gun against the JSF target set.
Deliberations on weapon bay size, meanwhile, are likely to be influenced by requirements to accommodate future payload growth. "The reason for building a weapon system is to carry weapons," says a senior Navy official. "This is a core issue as we deal with the JIRD process - we must keep in mind why we are building the aircraft.
"The key to the trades process is lethality. We can't simply say that bigger is better; the trades process is much more sophisticated than that." Cappuccio says that the weapons bay size is "-not a technical issue, it's about commonality and dollars".
PROPULSION INTEGRATION
The biggest challenge ahead, Kenne says, continues to be flight control and propulsion system integration on the STOVL variant. The start of ground tests at Pratt & Whitney of the modified F119 engines for the CTOL/CV concept demonstrators is a big step forward. The next major milestone will come in the fourth quarter, when ground runs begin of JSF119 engines fitted with the lift-system hardware - direct-lift nozzles for Boeing and a shaft-driven lift fan for Lockheed Martin.
Statkus says the thrust level on Boeing's X-32B STOVL demonstrator will be "slightly less" than on the production aircraft. Inlet, fan and nozzle changes are planned for the definitive design, he says.
Cappuccio says design of Lockheed Martin's X-35 demonstrator is being driven by the STOVL configuration. The demonstrator will be "very representative" of the production design, he says, in the temperature and acoustic environment created by the lift system.
Design changes to the lift-fan inlet behind the cockpit and to the swivelling, "three-bearing", engine nozzle are planned for the production JSF.
With the start of engine runs, Kenne believes the JSF programme "-is in very good shape." Support from the services, allies and Congress is strong and "-we just have to deliver."
Source: Flight International
