Thread by @John_A_Ridge

Reports that 🇷🇺 Kh-22s are exploding mid-flight are the least surprising news I've heard all war. A 🧵with some thoughts. 1/13

The Kh-22 [AS-4 Kitchen] is a supersonic air-launched cruise missile originally developed as an anti-shipping missile. First entering service in 1962, it is the oldest standoff munition in 🇷🇺 inventory. Production is estimated to have ceased around 1988. 2/13

The Kh-22 is carried exclusively by 🇷🇺 Tu-22M3 [Backfire-C] strategic bombers. The Tu-22M3 can carry up to 3 Kh-22s, though it is common for them to carry one or two at a time. 3/13

The most interesting and relevant technical aspect of the Kh-22 is its propulsion system. Unlike other cruise missiles that typically use jet engines, the Kh-22 uses an actual liquid-propellant rocket engine. 4/13

The Kh-22 is powered by a single R-201-300 engine developed by the Isayev Design Bureau (OKB-2). The R-201-300 uses TG-02 (also known as Tonka or Tonka-250) as the fuel and AK-27I, a formulation of inhibited red-fuming nitric acid (IRFNA), as the oxidizer. 5/13

TG-02 is a ~50/50 mixture of triethylamine and various isomers of xylidine. AK27I consists of ~73% nitric acid, ~27% dinitrogen tetroxide (NTO), and an iodine-based inhibitor. The inhibitor is present to prevent the nitric acid from rapidly corroding the oxidizer tanks. 6/13

The combination of TG-02 and AK27I is hypergolic, meaning they ignite immediately upon contact with each other in the absence of an ignition source. While this simplifies engine design by not requiring a separate ignition system, it complicates propellant handling. 7/13

The Kh-22's propellants, combined with the age of the missiles, themselves likely goes a long way to explain these in-flight failures. The newest units are almost 35 years old. The R-201-300 is a maze of pumps and propellant lines that have valves, seals, etc. 8/13

A single eroded seal could lead to a propellant leak that would easily cause an explosive engine failure, esp. since the propellants are hypergolic. For example, the Cygnus Orb-3 mission failed when one of Antares' 35-year-old NK-33 engines exploded shortly after launch. 9/13

Deviations in propellant composition due to improper storage could likewise cause a violent engine failure. For example, the nitric acid in IRFNA decomposes over time to nitrogen dioxide, water, and oxygen with exposure to light or elevated temperatures. 10/13

Despite the presence of an inhibitor, IRFNA will still slowly corrode its storage tank. Over time, this will leech material from the tank walls and contaminate the propellant. Either of these effects could produce a significant deviation in propellant composition. 11/13

These deviations could easily produce temperatures or pressures that exceed design parameters or potentially cause unexpected combustion instabilities leading to violent vibrations capable of tearing the engine and missile apart. 12/13

These are just a few examples of the innumerable failure modes that are possible when working with aged (and likely improperly stored/maintained) liquid rocket engines and propellants (that are also hypergolic). 13/13