Delays to the Joint Strike Fighter program and uncertainty over acquisition and support costs have ruled out one-for-one replacement of inventory fighters in the U.S. fleet for well over a decade. Half of the U.S. Air Force fighter force in 2030 will comprise conventional aircraft, according to current fleet plans outlined to Congress, and slated F-35C numbers and acquisition rates are being questioned at the highest levels of the U.S. Navy.

Outside the U.S., South Korea appears likely to select the F-15 Silent Eagle for its next fighter buy this summer, deferring stealth to Block 2 of its planned indigenous KF-X fighter. The most recent U.K. National Audit Office report on the British carrier and JSF program suggests that the plan to replace the Tornado with 90 F-35s (on top of the 48 aircraft that will be acquired to arm the nation's new aircraft carriers) has been slipped well past 2030.

When the Joint Strike Fighter program was kicked off in late 1996, it was intended not only to be stealthy, but to match or surpass any other across the full range of fighter missions (with the exception of the F-22 in air dominance) while costing less to acquire and support than the F-16. The business plan envisioned a near-complete takeover of the global fighter market with an initial operational capability (IOC) in or soon after 2010.

Competitors are preparing marketing offensives on the basis of a different vision of how stealth should be used, based on the fact that three classes of stealth technology have been defined.

The F-35, F-22, Sukhoi T-50, Chengdu J-20 and Shenyang J-31 occupy a middle range, designed to present a low radar cross-section (RCS) to most airborne radars and many surface-based acquisition and tracking radars, with the lowest RCS from the frontal aspect and the highest at the tail. The F-35 and F-22, at least, make minimal if any use of active electronic warfare (EW).

The second class includes almost all current combat aircraft. They feature RCS-reduction measures including fundamental shaping attributes (the Eurofighter Typhoon, for instance, has full line-of-sight protection of its engine faces with serpentine ducts), details such as the Rafale's saw-tooth appliques and RCS-tailored refueling probe, and radar-absorbent materials. They also include jamming systems.

The third group is an expanding range of prototypes and developmental aircraft with ultra-low RCS over a broad bandwidth and full range of aspect angles. These include the Northrop Grumman X-47B, the classified intelligence, surveillance and reconnaissance UAV being developed by the same company for the U.S. Air Force, Europe's Taranis and Neuron, and the Chinese UCAV seen in ground tests in May.

It is no accident that most of these aircraft resemble one another or that all (except the future U.S. Long Range Strike Bomber) are unmanned. No shape other than a tailless flying wing provides the desired stealth, and incorporating a cockpit is difficult except in a large vehicle.

All three classes of stealth air vehicle are likely to be operational in 2030, regardless of the JSF program. Three areas of uncertainty will define the actual make-up of air forces worldwide.

The first concerns the ability of RCS-managed conventional aircraft, as part of a system of systems, to operate against improved threats such as Russia's S-400/500 surface-to-air missile (SAM) systems. Boeing is promoting RCS reductions on the Eagle and Super Hornet, along with improved EW, and the U.S. Navy is funding “significant” classified upgrades. The Navy Hornet/Growler community envisions “rolling back” SAM threats with jamming and standoff weapons, and the service's vision for future anti-surface warfare is based on weapons (initially Tomahawk and Joint Standoff Weapon derivatives) rather than stealthy penetrating platforms. European fighter developers are studying RCS reductions for post-2020 versions of current aircraft.

The second unpredictable factor is the rate at which the use of very stealthy unmanned aircraft expands. Many U.S. commentators and think tanks have pushed for a greater role for survivable UAVs and bombers. Designed to survive by stealth rather than speed or agility, these aircraft offer longer range than tactical fighters, which creates options as the U.S. looks at the military balance in the Western Pacific. The emerging threat of a reconnaissance-strike complex aimed at U.S. aircraft carriers has pushed the Navy to the front of this trend.

The third consideration is the performance of the JSF program and its Russian and Chinese analogs, in cost, timeline and flexibility. The pacing item for the JSF is clearly software integration and testing; the U.S. Air Force has accepted that the aircraft will enter service with a limited weapons.

Not coincidentally, the upgrading of the F-22 Raptor, which has also failed to keep pace with aspirations, is dominated by software rather than hardware changes. These challenges are not going to be easier for developers in China, Russia or South Korea.

This is because a stealthy aircraft intended for a full range of fighter missions presents high hurdles in two inseparable software-dominated areas: sensor fusion and emissions control (Emcon). It is also a problem from the viewpoint of networking.

Sensor fusion is common to modern fighters. In principle, it means displaying data from active and passive radio-frequency (RF) sensors, infrared and optical devices, networks and databases as single targets and tracks. On a stealth aircraft, sensor fusion also supports Emcon, minimizing and managing RF energy to avoid detection.

The “fusion engine”—as the software package is termed on the F-35—is complex and critical. Testing is challenging because even the best ground-based systems-integration laboratory may not address the phenomenology of sensors in a real-world. One engineer describes a sensor-fused air combat avionics system as “having a nervous breakdown” when confronted with dense European air traffic. It is also crucial in preventing blue-on-blue attacks and collateral damage.

Sensor fusion is useful for a conventional fighter and can be phased in via upgrades, but it is essential at IOC if stealth is a primary goal.

A major impediment to flexibility in stealth aircraft is networking. Emcon paradoxically complicates networking while depending on it. One powerful Emcon tool is sharing active RF functions among aircraft in a fighting unit. The F-22 and F-35 were specified to have low-probability-of intercept (LPI) data links to connect a flight of aircraft.

LPI data links, however, use individual pencil beams, low-sidelobe antennas and power management to make detection difficult, which limits users and range. The F-22 can transmit only non-stealth voice radio outside the normal four-ship unit, while the F-35's Multifunction Advanced Data Link (MADL) will not talk to F-22s, and the aircraft will have non-LPI Link 16 for communication with other aircraft.

An effort to retrofit stealth aircraft with MADL has been deferred, but the Air Force funded Northrop Grumman to demonstrate a system known as JetPACK (Joint Strike Fighter Enterprise Terminal) that connects F-22s and F-35s with the rest of the force via an airborne gateway that communicates with their intraflight data links.

Missing from the baseline Block 3F service-entry standard is compatibility with Rover (remote video receiver) systems, which is essential for close air support. (Rover did not exist when the F-35 requirement was written.)

South Korea's approach to the KF-X is another way forward. The Block 1 variant is expected to have a low-observable airframe, with semi-conformal and external weapons and sensors. The Block 2 will be fully stealthy, with a conformal weapons bay. This gives South Korean industry the option to develop sensor fusion and Emcon incrementally and provides options for customers, who could operate a mix of versions, avoiding the total costs of an all-stealth force and diseconomies of a mixed force. But it blurs the line between stealth and conventional aircraft.

Tap the icon in the digital AW&ST Defense Technology edition for an interactive feature on stealth and counterstealth in the 2020s, or go to AviationWeek.com/stealth