The 1950s was a game-changing decade for the world of military aviation, especially with the rise of jet propulsion in place of propeller engines. It was in this decade that the U.S. Air Force came close to fielding one of the most advanced (for its time) "super fighters" ever conceived.
Known as the XF-108 Rapier, and designed by North American – also known for producing the widely successful P-51 Mustang – it was equipped to fly at more than three times the speed of sound for long distances, hunting down and destroying marauding Soviet bombers out to drop nuclear weapons on the US and its allies during the Cold War.
The XF-108 would also be able to complement its larger sibling, the XB-70 Valkyrie bomber, which itself was capable of achieving speeds in the Mach 3 range. Though the Valkyrie's insane speed would be too much for Soviet air defense systems to handle, it wouldn't hurt having a smaller fighter escort nearby which could keep up with the bomber, just in case enemy fighters made it too close for comfort.
An XB-70 Valkyrie prototype before its first flight (Photo US Air Force)
North American sought to make its newest fighter extremely deadly, adding a Hughes AN/ASG-18 radar that was coupled to an infrared search and tracking system. Together, this sensor suite would be able to lock onto and track targets at long ranges with solid accuracy.
The XF-108's primary weapon was the GAR-9 missile, which had a range of over 100 miles, and could fly at more than six times the speed of sound on its way to its target. The Rapier would carry three of these on an internal rotary launcher in its belly.
To give it speed, the XF-108 would use two General Electric afterburning turbojets that could pump out over 29,300 pounds of force apiece with the afterburners lit. In comparison, an F/A-18 Super Hornet's two GE F414 turbofan engines can put out only 22,000 pounds of force apiece in burner!
An AIM-47 Falcon waiting to be loaded into a YF-12 interceptor prototype. The Falcon was the direct successor to the GAR-9 (Photo US Air Force)
However, the Rapier lost its mission rapidly with the advent of intercontinental ballistic missiles and guided cruise missiles — the latter of which could be launched from surface warships and submarines floating off the coast of the United States. Instead of sending swarms of bombers, all of which could hypothetically be shot down if responded to quickly enough, the USSR could mail nuclear weapons to the US with ICBMs, which were much more difficult to destroy.
Sadly, the Rapier never had a chance to even fly.
In late 1959, by the end of the decade, the program was shelved altogether, having only made it to the mockup stage. In a matter of months, the XF-108 was no more. Its sister plane, the XB-70 would meet a somewhat similar fate in the next decade, and would be retired from flight testing after a fatal and highly publicized accident.
An A-5A Vigilante prototype (Photo US Navy)
Some of the technology and lessons learned in designing the XF-108 were put to use with a later North American product – the A-5/RA-5 Vigilante, which bears a considerable resemblance to its predecessor. The GAR-9 was also developed further into the AIM-47 Falcon missile, which would later be matured into the AIM-54 Phoenix, deployed with the F-14 Tomcat.
Mach 3-capable fighter jets have long since been relegated to the history books as an unnecessary hope of the past, but with improvements in propulsion technology, it's very possible that future fighters fielded by the US military might be blessed with such incredible speed.