Eight top-secret aircraft that definitely aren't UFOs

Since its establishment in 1955, the Groom Lake airfield at Edwards Air Force Base—better known as Area 51—has hosted the development of some of the most exotic and advanced aircraft the world has ever seen. These so-called black projects, named for their ultra-classified nature, have produced planes like the SR-71 Blackbird, which is still the fastest and highest-operating aircraft ever built (that we know about); the F-117 Nighthawk, the world's first stealth attack aircraft; and the RQ-170, a mysterious and seldom-seen aerial reconnaissance UAV.

U-2 "Dragon Lady"

The Lockheed U-2 "Dragon Lady" is the elder statesman of Air Force reconnaissance planes. Along with other aeronautical heavyweights, like the B-52 Stratofortress, the U-2 is one of the only aircraft employed by the USAF for more than 50 years.

This single-seat, single-engine spy plane is capable of flying at 70,000 feet and can keep tabs on enemy activity through any weather conditions. However, the U-2's incredible capabilities came at a steep price. In order to reach 70,000 feet, the planes had to be flown within 12MPH of their stall speed. Traveling any slower would cause the plane to lose altitude, thereby increasing its chances of being detected; any faster and the plane could shake itself apart due to excessive stresses to its airframe. And since their cabins were only partially pressurized, U-2 pilots had to breath pure oxygen for an hour before they could take off, thereby removing all traces of nitrogen from their bodies; otherwise they risked crippling depressurization sickness.

A-12 "Oxcart"

The A-12 Oxcart was another high-altitude spy plane built by Clarence "Kelly" Johnson and his Lockheed Skunkworks team. Like the U-2 before it, the A-12 was designed to fly high and fast in order to stay out of the range of Soviet missile defenses.

Interestingly, the A-12 never served under the USAF. Instead, it flew missions for the Central Intelligence Agency, operating over the USSR and Cuba. Unlike the U-2, however, the A-12 flew for only five years, between 1963 and 1968, before being retired in favor of the larger and stronger SR-71 Blackbird. However, it wasn't until the mid-1990s that the CIA declassified the program. 

Fun fact: Col. Ken Collins, whom Engadget spoke to for this story, is credited with being the first pilot to lose an A-12 when his crashed near Wendover, Utah, in May of 1963. 

SR-71 "Blackbird"

The SR-71 Blackbird is arguably the finest aircraft ever produced by the United States, capable of reaching Mach 3 and flying at 70,000-plus feet. The SR-71 didn't evade surface-to-air missiles—it simply outran them. Despite retiring in the late 1990s, it still holds aeronautical records for highest and fastest flight by a USAF aircraft. Heck, the SR-71 is so awesome that even the X-Men have one.  

D-21

The Lockheed D-21 is one of the few machines on earth capable of outrunning an SR-71. The Air Force developed the D-21 specifically for missions deep within enemy airspace. This air-launched reconnaissance drone piggybacked on the M-12 (a variant of the A-12 Oxcart). When deployed, it flew at Mach 3.5—that's 2,685MPH—at altitudes in excess of 90,000 feet.

The D-12s could only be used once, however. They were designed to eject their camera payloads at the end of their missions and promptly self-destruct. Unfortunately, the D-12 program lasted only two years, between 1969 and 1971, before being grounded.

F-117 "Nighthawk"

If any aircraft to come out of Groom Lake is based on alien technology (as many conspiracy theorists believe), my money is on the F-117 Nighthawk—the world's first stealth attack jet. This aircraft is 66 feet long, with a 43-foot wingspan. But thanks to its origami-like shape and radar-absorbing paint job, it has the radar cross section of 0.001 square meters—that's roughly the size of your average bumblebee.

Tacit Blue

Tacit Blue, nicknamed "the Whale," never saw combat—nor was it ever meant to. Instead, the USAF designed this oddly shaped aircraft as a demonstration platform to showcase the branch's stealth technology as well as prove that such technology could be used to continually monitor a battlefield's front lines without being detected.

While Tacit Blue logged only 250 hours of flight time before being retired in 1985, many of the stealth technologies it tested have since made their way into modern warplanes. 

RQ-170 "Sentinel"

We know next to nothing about the Lockheed Martin RQ-170 Sentinel UAV. This aerial reconnaissance platform is used by both the USAF and the CIA, and has likely been deployed over both Pakistan and Iran. Some news outlets report that an RQ-170 helped track down Osama bin Laden.

In fact, one of the only reasons the Sentinel program was even revealed is because the USAF lost one over Iran in December of 2011 and had to publicly request that the Iranian government give back the wreckage.

B-2 "Spirit"

The B-2 Spirit is a long-range tactical stealth bomber capable of delivering a nuclear payload nearly anywhere on the planet without alerting enemy forces—at least until they notice the mushroom cloud. With an operational range of more than 6,000 miles (10,000 miles if it refuels in midair), the B-2 is capable of crossing the Pacific—twice—without ever putting its wheels down. What's more, it shares many of the same stealth technologies as the F-117 Nighthawk, but with a wicked-cool "flying wing" design. 

Though you won't ever see a B-2 coming, the USAF plans to keep its fleet of the $2.1 billion bombers in operation until at least 2058. 

It's not hard to see why many of these aircraft have been mistaken for unidentified flying objects. Their extreme performance is a direct result of their outlandish designs. Just look at the D-21. This Mach 3+ recon drone looks more like the A-12 Oxcart's engines that it rides upon than any conventional UAV.

"Some of the most exotic aircraft developed and/or test-flown at Area 51 can certainly be mistaken for UFOs and indeed were reported as such on occasion, especially before the existence of these aircraft was public knowledge," Nick Pope, a journalist and author who used to investigate UFOs for the British government, told Engadget. "Some of these sighting reports came from pilots who, when confronted by something flying at heights and speeds they thought impossible, didn't know what else to think."

Pope explains that these reports often worked to the benefit of the Defense Department, as such incredulous tales would prevent Soviet intelligence analysts from prying into its beyond-top-secret projects. "It's the ultimate irony that, far from covering up the truth about UFOs (as conspiracy theorists allege), the US government may have played a large part in creating the phenomenon."

Of course, the mystery surrounding many of these projects—not to mention their reputed links to unexplainable phenomena—aren't nearly as mind-boggling for the folks actually involved in their development. Retired Air Force colonel Kenneth Collins is one of a handful of pilots who flew the A-12 Oxcart during its testing phase at Groom Lake, but he doesn't remember much hubbub regarding UFO sightings near Area 51. "The Oxcart program and Area 51 were very classified and very secret; we pilots were only given the information needed to fly the aircraft," Col. Collins said. "We never received any outside reports ... I never heard anything on the news about lights in the sky or anything unusual."

"The strangest thing for me was," Col. Collins continued, "I was in the area near my quarters and got a call from operations. So I raced over [to the airfield], got into my pressure suit, and they launched me in the A-12 to go chase down a Russian surveillance balloon that had floated over. I don't know what they thought I was going to do at Mach 3 while chasing a balloon. I went by it so quick I wasn't able to see anything. But someone had the wild idea to try it, and I was more than happy to fly."