24 October 2015

Fox Two! The Birth of the AIM-9 Sidewinder Missile

An early Sidewinder test round- note the early constant chord fins
At the end of the Second World War, the Navy had been focusing its weapons development work at the remote Naval Ordnance Test Station Inyokern, California (today known as China Lake). Three hours northeast of Los Angeles in the high Mojave Desert and with 1,200 square miles of land to blow things up, the site tended to attract some of the more innovative if not unconventional minds in weapons design. A CalTech graduated named Bill McLean was working on a few ideas to improve the lethality of unguided air-to-air rockets. At the time, outside of guns, the only other air-to-air option in widespread use were rockets which were notoriously difficult to employ even against ground targets. One USAF Air Defense Command pilot once remarked that "the only thing you were sure to hit with air-to-air rockets was the ground."(In 1956 two USAF F-89 Scorpions tried to down an errant drone, firing over 200 rockets and failing to score a single hit.) The best options of the day were radar guidance, but the technology of the late 1940s and early 1950s meant a radar-guided missile would be heavy. McLean wanted something lightweight and simple that could be bought by the thousands. He reasoned that a self-contained guidance system would simplify the use of such a missile and he and a group of volunteers in their free time worked on infrared guidance- the missile could home in on the hot jet exhaust. 

At the time the Navy had been using 5-inch rockets since the Second World War in the air-to-ground role. Work on refining the 5-inch rocket had continued at China Lake (one result was the Zuni air-to-ground rocket still in use today). McLean wanted to combine the simplicity of a 5-inch rocket with infrared guidance. A rotating, gyroscopically-stabilized mirror looking through a transparent nose dome would reflect heat energy to a lead-sulfide photocell. Rather than focus on the target, the detector looked at the target's change in position and this way it automatically "led" the target and could hit from angle instead of just a tail chase which would require a significant speed advantage. This sort of guidance is called "proportional pursuit". Using his free time and the volunteer help of his fellow engineers, McLean developed a way to translate the seeker's findings to control inputs to the missile's fins. Another one of the team's ingenious solutions for the missile was a simple way to stabilize the missile so it didn't rotate about its long axis and complicate the work of the infrared seeker. Discs with spurs to catch the airstream to spin like roller wheels were installed on the edges of the main fins and were called "rollerons". By spinning up in the air stream, the rollerons acted like gyroscopes, keeping the missile from rotating on its long axis. It was an elegantly simple solution to a complex flight control problem. 

In 1950, the name for the missile was adopted, Sidewinder- after the predatory snake common in the Mojave Desert that used heat to sense its prey. Other names had been mooted, one of which was "Low IQ Homing Head" in reference to the missile's simplicity. By August 1952, the first aerial test shot was ready to take place. Future astronaut and Navy test pilot Wally Schirra fired the Sidewinder prototype from a Douglas AD-4 Skyraider at a Grumman F6F Hellcat drone. And it missed. And so did the next eleven test shots. Finally on 11 September 1953 after many fixes and revisions, the Sidewinder finally scored a proximity hit on its thirteenth test shot against a Hellcat drone. Four months later it scored a direct hit on a QB-17 drone square in its No. 1 engine. On 17 February 1954 McLean's small cadre scored another victory when another Sidewinder prototype destroyed another QB-17 that was thought to be indestructible because it had survived so many missile hits. 

The early test Sidewinder shots required the test pilot to monitor a small voltmeter in the cockpit to determine if the missile seeker was properly sensing the target's heat source. Realizing this was an unnecessary distraction in combat, McLean's small team came up with another simple solution- with just one additional wire from the missile, they could generate a tone that could be heard in the pilot's headset to let them know the seeker had the target- the famous "Sidewinder growl". The Navy was enamored with the Sidewinder's simplicity- at the time the US Air Force was bringing the Hughes AIM-4 Falcon into operational use. Developed by a vast engineering team at Hughes' southern California facilites, it was a much more complex weapon despite also being infrared guided. In fact, in 1956, a Navy team came to Holloman AFB in New Mexico (which sits astride the White Sands Missile Range) to prove to the USAF that an Air Force pilot who had never fired a Sidewinder before could destroy a target drone. It was an official shoot-out between the Navy's Sidewinder and the Air Force's Falcon missile and it was wildly successful for the Navy team- the story goes that the Navy test pilot on the team bet everyone in the test teams that the Sidewinder would work as advertised. Despite this, the USAF deployed the Falcon anyway and it proved to be dismal failure in the skies of Vietnam, quickly getting replaced by the Sidewinder in what was called the "Falcon Fiasco".

Evolution of the Sidewinder family
The first production Sidewinders (AIM-9A and AIM-9B) refined the aerodynamics from the straight, constant-chord fins of the prototypes. Still retaining the 5-inch diameter body, the seeker would feed commands to the forward set of fins while the larger aft set of fixed fins housed the rollerons, a layout that would be retained to this day until the debut in 2001 of the AIM-9X variant. The Sidewinder would have its combat debut in the 1958 Taiwan Straits Crisis when PLAAF MiG-17s used their high altitude advantage and cannon weaponry against the Taiwanese F-86 Sabres. In a secret program, several Taiwanese Sabres were quickly modified to fire the Sidewinder (again, reflecting the simplicity of the missile) and were used 24 September 1958 to ambush the Chinese MiGs. 

The Sidewinder missile so significantly changed the nature of aerial combat that it was even copied by the Soviet Union as the Vympel K-13 (NATO designation AA-2 "Atoll". It wasn't until the end of the Cold War that Russian designers admitted to what had been widely suspected for years. Even the parts numbers of the Sidewinder were replicated on the K-13! Hundreds of thousands of missiles have been built for United States military but also under license in Europe. China even copied the K-13 for its own use. The US Army fielded a surface-launched version that mounted four AIM-9D missiles on a tracked vehicle and was called the Chaparral. Older AIM-9C missiles pulled from service were converted in the 1990s into lightweight anti-radar missiles designated AGM-122 Sidearm. And the basic principles of the Sidewinder have influenced a large number of other infrared-guided missiles from around the world, from Israel's Python family to the French Magic family.

Navy deck crew lift a Sidewinder onto a Hornet's missile rail
The first Sidewinder missile's electronics consisted of only seven vacuum tubes and five moving parts. Over the years since then, the missile has retained the 5-inch diameter but has gotten longer from 109 inches to 119 inches as well as lighter, from 155 lbs to 118 lbs. The latest production model of the Sidewinder is the AIM-9X, the first missile to change the basic layout and seeker function completely. Instead of the rotating mirror, the seeker has a staring focal plane array using a pixel-based sensor derived from digital camera technology. The rollerons are gone and the fins are considerably smaller, now only making the -9X lighter but also making internal carriage easier. Jet vanes are now in the motor exhaust at the tail end to give it thrust vectoring which makes the rollerons unnecessary, makes the fins smaller and lighter, and gives the missile tremendous maneuverability. 

Not bad for missile that started out as a free time project with less than 25 engineers!

Source: Air & Space Smithsonian, November 2010, Volume 25, Number 5. "Sidewinder: The Missile That Has Rattled Enemy Pilots Since 1958" by Preston Lerner, p54-61. Photos: Wikipedia

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