The Patriot PAC-3 interceptor missile can target incoming ballistic missiles, which have a highly predictable trajectory. It is also capable against much slower maneuverable aircraft, though the older PAC-2 is better. It has little chance against a maneuvering hypersonic target, and only a little more against a violently maneuvering target at high supersonic speeds.

The Kh-47M2 Kinzhal is claimed to be highly maneuverable in all flight phases, including terminal, where the PAC-3 in Ukraine has intercepted 7/7.  Mooted theories:

• Treasonous Russian scientists supplied the U.S. with flight data that somehow obviated the alleged maneuverability. (CNN) Russian scientists criticize arrest of ‘brilliant’ hypersonic researchers on suspicion of treason.
• The Kinzhal claims are fraud, perpetrated  by Russia on us, or by the designers on the Russian military establishment.
• To these possibilities, we add a design flaw.

The Kinzhal is just an air-launched 9K720 Iskander, for which the same claims of maneuverability have been rumored since 2006. This is a long time for a missile to conceal flight characteristics from technical collection. Questions:

• Was the Iskander/Kinzhal ever maneuverable in the terminal phase?
• If so, did the missile lose this ability?

Imagine you are the pilot of a 1950’s era jet fighter. A late example is the F-104 Starfighter. These planes were much heavier than their prop predecessors, acquiring the vernacular reference “metal.” Jet engines of this era  produced much less thrust than modern examples. Since wings produce drag that reduces speed, and small wings have less drag, these aircraft had very small wings and tail.

You innocently decide to perform a barrel role. You neglect that the direction in which the aircraft points is a little different from the direction of travel. As you roll, the nose describes an innocent little circle. The circle widens as centrifugal force acts on the fuselage that cannot be countered by the tiny wings.

A moment later, your plane becomes a flying dumbbell, broadside to the wind. The g-force causes your head to impact the instrument panel with crushing force. That’s the end of your story. What happened to the plane is inertia coupling, caused by a maneuver that cannot be described as violent.

Now consider the Kinzhal. Unlike your plane, it doesn’t have separate wings and tail; only tiny fins smaller than either. The problem is magnified. The missile is expected to perform violent maneuvers, chancing turbulent flow around the fins. When that happens, the missile is toast, though the analog of inertia coupling can happen without this, resulting in toast. For intuition, think of a shopping cart with bad wheels.

This is a nonlinear control problem, and all such problems are hard. A  robust solution with those tiny fins would advance the state of the art. The designers may have been forced to adopt and advertise a solution which is not robust with respect to:

• Manufacturing tolerances.
• Substitution of materials, resulting in different weight distributions.
• Adaptation of the Kinzhal to air launch, also resulting in different weight distributions.
• Atmospheric variations of temperature and density.
• Something that worked in development that could not be reliably replicated in production –  possibly due to specification creep.

A legitimate solution is to disable the feature. The in-character response  of the Russian military establishment is to bury the shortfall in internal propaganda.