Physics dictate that a tactical fighter-sized stealth aircraft must be optimized to defeat higher-frequency bands such the C, X and Ku bands, which are used by fire control radars to produce a high-resolution track. Industry, Air Force and Navy officials all agree that there is a “step change” in an LO aircraft’s signature once the frequency wavelength exceeds a certain threshold and causes a resonant effect—which generally occurs at the top part of the S-band.Russian air defenses may appear formidable as part of Moscow’s increasingly sophisticated anti-access/area denial (A2/AD) capability, but areas protected by these systems are far from impenetrable bubbles or 'Iron Domes’ as some analysts have called them.While it is true that a layered and integrated air defense may effectively render large swaths of airspace too costly—in terms of men and materiel—to attack using conventional fourth generation warplanes such as the Boeing F/A-18E/F Super Hornet or Lockheed Martin F-16 Fighting Falcon, these systems have an Achilles’ Heel. Russian air defenses will still struggle to effectively engage fifth-generation stealth aircraft such as the Lockheed Martin F-22 Raptor or F-35 Joint Strike Fighter.(This first appeared in August 2016.)