A side-by-side comparison of the X-35 Joint Strike Fighter prototype with the F-35 Lightning II production aircraft underscores the huge strides the JSF program has made transitioning from demonstration to development. The X-35 demonstrators, built and flown during the concept development phase of the program in 2000–2001, validated specific propulsion and design features incorporated on today’s F-35 Lightning II.
The most important part of the demonstration for the X-35B (the short takeoff/vertical landing, or STOVL, variant shown in the illustrations here) was the integration and operation of the STOVL propulsion system. The X-35B proved the viability of the shaft-driven lift fan and the performance available in STOVL. It also proved the viability of up-and-away flight. The demonstration culminated in the successful completion of “Mission X” on 20 July 2001. This mission combined a short takeoff, acceleration to supersonic flight, and a vertical landing in a single flight. Mission X demonstrated the integration of a STOVL lift system into a high-performance fighter.
The X-35 demonstrators, by any standard, were much simpler than the F-35 Lightning IIs that will be rolling out of the Lockheed Martin factory in Fort Worth, Texas. The demonstrators, for example, used a number of off-the-shelf components to speed design and fabrication. The fast pace of the program meant that only essential functions and capabilities were provided and tested. For instance, the X aircraft did not carry or drop weapons or include mission avionics. The demonstrators also did not carry signature treatments.
Still, the production versions maintain a strong family resemblance to their predecessors. The basic shape and layout are the same. The wing sweep angles for the leading and trailing edges are identical. (The F-35C has a much larger wing to improve low-speed handling characteristics for carrier landings, but the sweep angle is the same. The X-35C also had larger wings and tails than the X-35A/B.) The fundamentals of the shaft-driven lift fan system for the STOVL variant remain unchanged as well, which includes keeping the original three-bearing swivel nozzle. However, at a more detailed level, the production versions are different in every respect—from the tips of their noses, to their internal structures, to the angular feathers on their engine nozzles.
The demonstrators have been transformed into operational fighters during the current System Design and Development phase of the program. The cockpit has been refined. Full avionics functionality has been added. Engine thrust has increased. Capability and durability of the lift system have been improved. Internal weapon bays, radar, targeting system, and low-observable treatments are functional. New technologies that didn’t actually fly on the demonstrators—such as the electrohydrostatic actuators, integrated power package, or IPP, and helmet-mounted display—now function as integral parts of the F-35.
Many changes aren’t visible in the accompanying graphics. The highly automated manufacturing process used for the production versions is entirely different from the mostly hand-built process used for the prototypes. The internal structure has been redesigned to reduce weight and accommodate operational systems. Internal systems themselves have been added, redesigned, and moved around to reduce weight and cost, improve survivability, and make the aircraft easier to maintain. The aircraft has integrated prognostic and health monitoring sensors built into critical systems. These systems are part of a larger autonomic logistics system that will be used to support the aircraft in the field.
We have chosen to compare the X-35B and F-35B aircraft to highlight the extensive refinements. Most of the changes described here, with the exception of those related to the vertical lift system, apply to the other Lightning II variants—the conventional takeoff and landing, or CTOL, F-35A and the carrier variant F-35C.
Eric Hehs is the editor of Code One.
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