The alert call came at 12:25 a.m. on a black winter morning, with the temperature at 28 degrees below zero. Within 75 minutes, all 26 of the squadron's interceptors stood ready on a snowbound flight line, and soon the pilots were strapped in, waiting for one code word. When the radios barked “Flush” at 2:07 a.m., 25 jets roared to life and were airborne six minutes later. One cold-soaked jet burst its hydraulic lines and bled into the snow.

That is how retired US Air Force Colonel Jack Broughton remembered a no-notice exercise in the Convair F-106 Delta Dart, writing in Air Force Magazine in 2012. It captures the aircraft's whole reason for being: get airborne fast, climb high, and kill Soviet bombers before they reached North America.

The F-106 never fired a shot in anger and was never exported. Yet it guarded American skies from 1959 to 1988, set a world speed record, carried a nuclear rocket, and ended its days as a target drone and a NASA lightning magnet. Its pilots simply called it “the Six.”

From Deuce to Six: The Interceptor the F-102 Was Meant to Be

The F-106 began life as a fix. The F-102 Delta Dagger, winner of the US Air Force's 1954 interceptor program, had disappointed badly in early testing, and Convair had to rework it heavily before it was even supersonic. Convair's delta lineage ran back to the XF-92A research jet, but the Air Force wanted far more than an evolved prototype.

By December 1951 the service was already looking at an improved F-102B: a more powerful Wright J67 engine plus the advanced MX-1179 fire-control system, later called the MA-1. This was to be the “ultimate interceptor” the Air Force had wanted all along. The J67 fell a year behind schedule, so in early 1955 the program switched to the Pratt & Whitney J75.

The bigger engine needed more air, so the jet got new inlets and a variable-geometry duct, a longer and cleaner fuselage, a slightly larger wing and a redesigned fin. So much had changed that on 17 June 1956 the F-102B became the F-106A. Convair's chief test pilot Richard L. Johnson flew the first prototype at Edwards Air Force Base on 26 December 1956, a 20-minute flight cut short by mechanical problems.

Early test results were disappointing. Acceleration and top speed fell short of Convair's own estimates, the engine and avionics proved unreliable, and the program came close to cancellation. Air Defense Command fought for it. The Air Force had planned on 1,000 aircraft but ordered only 350, and Convair built 342 in total.

Dark Skies traces the F-106's development and service life in this documentary, one of the most-watched films on the type.

A Fighter Flown From the Ground

The heart of the F-106 was the Hughes MA-1 fire-control system. It could be linked to the Semi-Automatic Ground Environment, or SAGE, the continental air defence network of the time, so that controllers on the ground could steer the interceptor toward its target. It was ground-controlled interception taken to its logical extreme.

Paul Worcestor, who flew the Six with the Air National Guard's 102nd Fighter Wing from 1979 to 1987, told Hush-Kit that air defence ground crews “could provide targets electronically on the jet's display maps and actually fly the jet from their location on the ground via a data link.” Getting there was not painless: the MA-1 was so troublesome that it was upgraded more than 60 times in service.

Broughton described a cockpit built around the pilot, with vertical tape instruments at eye level and a television-like display showing the positions of friends and foes. If a radar lock left the pilot badly placed, he could press an autopilot takeover button and hang on while the jet flew itself to the firing point.

Four Falcons and a Nuclear Rocket

Like the F-102, the F-106 had no gun and no provision for bombs. Its weapons rode inside an internal bay, which kept the airframe clean for supersonic flight: four Hughes AIM-4 Falcon missiles, either infrared-guided AIM-4Gs or radar-guided AIM-4E/Fs, plus a single AIR-2 Genie.

The Genie was an unguided rocket with a 1.5-kiloton nuclear warhead, designed to be fired into a formation of enemy bombers. Afterburner has told the story of the five men who stood beneath a live Genie detonation in 1957. Worcestor's verdict on the weapon was blunt: “There was no beating the nuclear rocket, but the missiles were its challenge.”

The Falcons, he explained, had no proximity fuses and had to physically strike the target to detonate. Experience over Vietnam convinced the Air Force that fighters needed guns again, and from 1972 the Six Shooter upgrade gave the F-106A a frameless canopy, an optical gunsight and provision for an M61 Vulcan 20 mm cannon in place of the Genie. Most aircraft still flew operationally with the nuclear rocket.

Archive film of an AIR-2 Genie test, posted by the Nevada Aerospace Hall of Fame.

Mach 2.31 Over Edwards

On 15 December 1959, Major Joseph W. Rogers, the Air Force's F-106 project officer at Convair, took F-106A 56-0467 up over Edwards. At about 40,000 feet he made two passes over the straight course, one in each direction, averaging about 2,455 km/h (roughly 1,525 mph), or Mach 2.31. It was a new world speed record.

Rogers broke the mark set two years earlier by Major Adrian Drew in a modified F-101A Voodoo, and earned the Distinguished Flying Cross and the Thompson Trophy. He had first tried with another jet, 56-0459, but it developed uncontrollable compressor stalls, so 56-0467 took its place.

The Six entered service in June 1959, with deliveries to the squadrons starting that October, and early operations were rough. Generator defects, cold-weather fuel problems and a canopy that jettisoned in flight all caused trouble, and the whole fleet was briefly grounded in December 1959.

The worst problem was the ejection seat. Accounts of the toll differ: the early seats are generally said to have killed the first 12 pilots who used them, while Broughton counted 13 fatal ejections. A Weber rocket-catapult zero-zero seat was retrofitted after 1965 and finally fixed it.

Thirty Years on Alert

Once the teething problems were solved, pilots came to love the Six. It served across the contiguous United States, Alaska and Iceland, and briefly in Germany and South Korea, replacing older interceptors such as the F-86 Sabre Und F-89 Scorpion. It was considered for Vietnam but never sent.

In Operation Highspeed, a fly-off against the Navy's F4H-1 Phantom, the F-4 Phantom II was judged the better all-rounder. The Phantom was needed for other roles first, so the F-106 kept guarding North America for two more decades. Worcestor's unit had NORAD responsibility for the busy northeast corridor, and he intercepted pairs of Soviet Tu-95 Bear bombers three times: once in September 1981 and twice in April 1982.

PeriscopeFilm archive footage of the US Air Force's 1965 William Tell weapons meet, featuring the F-106 Delta Dart.

Der F-15 Eagle began replacing the Six in some units in 1981, with many jets passing to the Air National Guard. Between June 1983 and August 1988 the Delta Darts were withdrawn and flown to storage in Arizona, and the type left service in 1988.

The Six's strangest moment, a pilotless jet that landed itself in a Montana field in 1970, has its own story on Afterburner: Der Maisfeldbomber: Die F-106, die selbst landete.

Target Drones and Lightning Bolts

Retirement was not the end. From 1986, 194 surplus airframes were converted into QF-106A target drones under the Pacer Six program. They could still be flown with a pilot for ferry flights and fly uncrewed for missile tests. The last QF-106 shot down fell at Holloman Air Force Base on 20 February 1997, and the last one was destroyed in January 1998.

NASA found its own uses. Its Langley Research Center flew an F-106B two-seater from 1979 to 1991, and from 1980 to 1986 the Storm Hazards program deliberately flew it into thunderstorms. According to NASA's final report, the jet made 1,496 storm penetrations and took 714 direct lightning strikes, with the strike rate peaking at pressure altitudes between 38,000 and 40,000 feet.

On one hour-long flight in 1984, lightning struck the aircraft 72 times. The program helped improve lightning protection standards for commercial and military aircraft. The goal, Fisher said, was certification and test standards that would keep lightning from affecting newer technologies such as computerised cockpit displays and composite skins.

The final chapter was the Eclipse project of 1997 and 1998, in which a USAF C-141A Starlifter towed a modified QF-106 into the air on a cable to test whether a space launcher could be towed aloft and released. Four decades after its first flight, the ultimate interceptor ended its career as a glider on a rope.

It never fought a war, but for almost 30 years it was the jet Soviet bomber crews would have met first.

Sources: Wikipedia (Convair F-106 Delta Dart), This Day in Aviation, Air & Space Forces Magazine (Jack Broughton, “Pain and Gain in the Century Series”), Hush-Kit (Interview with Convair F-106 Delta Dart pilot), NASA Technical Reports Server (Final results of the NASA storm hazards program), NASA Langley release via SpaceNews