Elon Musk and Grimes' favorite plane is the SR-71 Blackbird, a legendary US spy plane that could fly at 3 times the speed of sound. Take a closer look.

• Elon Musk and Grimes recently said their favorite plane is the SR-71 Blackbird, citing its supersonic ability and lack of weapons.
• The revelation came as Grimes deciphered the meaning behind the name of her newborn baby, X Æ A-12, with "A-12" being the predecessor aircraft for the Blackbird.
• During its tenure for the US Air Force, the Blackbird excelled in high-altitude surveillance routinely reaching heights of over 70,000 feet and flying around three times the speed of sound.
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Elon Musk and girlfriend Grimes have given birth to their first child. While the name of the boy, X Æ A-12, has left most confused about how to pronounce it, the singer outlined how they arrived at the unique name.

In a tweet, Grimes explained the meaning behind each character and said that the last few characters "A-12" refers to the Lockheed A-12, a short-lived spy plane once employed by the US Central Intelligence Agency in the 1960s.

The A-12 was the precursor to the SR-71 Blackbird, the more well-known of the two that Grimes and Musk say is their favorite plane. The Blackbird became prominent during the Cold War and was purely for high-altitude surveillance as it had no offensive or defensive weapons.

It was the military equivalent of the Concorde as its two massive rocket-like engines could propel the bird into supersonic flight at some of the highest altitudes in Earth's atmosphere.

Take a closer look at the jet that the duo calls their favorite.

It's no surprise that the eccentric entrepreneur would consider such a revolutionary plane as his favorite.

An SR-71 Blackbird surveillance aircraft.      US Air Force

Musk is a noted space and aerospace enthusiast. His company SpaceX aims to build new rockets that will bring humans to space.

Elon Musk and SpaceX.      Kevork Djansezian/Getty Images

NASA also used the aircraft for testing, picking up where the Air Force left off until 1997, making it one of the longest-serving spy planes in the US' stable.

An SR-71 Blackbird surveillance aircraft.      Judson Brohmer/USAF

Source: NASA

The only weapons that Blackbird had were its cameras which provided invaluable intelligence that added troops and intelligence agencies on the ground.

An SR-71 Blackbird surveillance aircraft.      AP Photo/HO

Grimes wrote in her tweet that she and Musk are fans of the aircraft because it features no weapon. It was a completely harmless aircraft in and of itself.

An SR-71 Blackbird surveillance aircraft.      Imageforge / Shutterstock.com

Source: Twitter

Like most supersonic aircraft, it was ahead of its time. No other aircraft has managed to take its place.

An SR-71 Blackbird surveillance aircraft.      AP Photo/Mark Faram

The Blackbird was one of the most secretive aircraft in the Air Force's fleet as it soared at more than double the altitude of most airliners. Its capabilities, however, proved vital to the US intelligence community.

An SR-71 Blackbird surveillance aircraft.      Gary Cameron/Reuters

The first Blackbird flew in 1964 and began flying for the US Air Force in 1966, lasting nearly 30 years before retiring in 1990. The extensive costs associated with the aircraft enhances in satellite technology, and the impending collapse of the Soviet Union all contributed to its retirement.

An SR-71 Blackbird surveillance aircraft.      Gary Cameron/Reuters

Source: National Museum of the Air Force

Titanium was the material of choice for the aircraft as the altitude and speed conditions that the Blackbird would operate in would create temperatures levels in the thousands of degrees.

An SR-71 Blackbird surveillance aircraft.      Gary Cameron/Reuters

Source: Lockheed Martin

When it ran low on fuel, a tanker would be sent in to give it a top off, allowing it to stay in the air for long periods. Blackbirds would often depart with minimum fuel and then be refueled once airborne before heading to a mission zone.

An SR-71 Blackbird surveillance aircraft.      AP Photo/Walt Zeboski

Source: Lockheed Martin

The speed and height of the Blackbird meant that it could cross entire continents in a few hours. Pilots navigating by sight couldn't rely on ground features such as roads, depending instead on mountains and major coastlines.

An SR-71 Blackbird surveillance aircraft.      Gary Cameron/Reuters

Source: Lockheed Martin

The jet could fly at speeds of over 2,000 miles per hour, around three times the speed of sound, at altitudes greater than 80,000 feet. When in danger, the pilot could simply engage the afterburners and cruise away from any threat at supersonic speeds.

An SR-71 Blackbird surveillance aircraft.      Aaron Kohr/Shutterstock.com

Source: Lockheed Martin

Powering the spy plane were two Pratt & Whitney J58 engines that resembled rockets rather than airplane engines. They enabled the high altitude, supersonic fleets the Blackbird would undertake.

An SR-71 Blackbird surveillance aircraft.      Reuters

Source: Lockheed Martin and NASA

Here's the one-man cockpit of the high-speed spy plane.

An SR-71 Blackbird surveillance aircraft.      National Museum of the US Air Force

Two crew would command the bird, with only pilot in the front and a reconnaissance and navigation officer in the back. The crew would wear spacesuit-like uniforms as they embarked high in the upper altitudes.

An SR-71 Blackbird surveillance aircraft.      AP Photo/Itsuo Inouye

From the A-12, the two-seater SR-71 Blackbird was produced for the US Air Force. Compared to its predecessor, the Blackbird had more radar-reflecting surface and was less easily detected by enemy anti-aircraft.

An SR-71 Blackbird surveillance aircraft.      Dan Simonsen / Shutterstock.com

Source: Lockheed Martin

The jet was commissioned by the Central Intelligence Agency. Here's a model of it that sits outside its headquarters in Langley, Virginia.

An A-12 Oxcart surveillance aircraft.      AP Photo/Pablo Martinez Monsivais

Source: Central Intelligence Agency

First came the A-12 Oxcart, a single-seater spy plane that saw missions in Vietnam and North Korea. Though it was successful, fears arose that Soviet technology was catching up and could render the aircraft obsolete.

An A-12 Oxcart surveillance aircraft.      Reuters

Source: Central Intelligence Agency and Lockheed Martin

Defense aerospace manufacturer Lockheed was ultimately chosen to build the aircraft through Skunk Works, a code for the firm's Advanced Development Division. Aviation pioneer Clarence "Kelly" Johnson headed the project.

An SR-71 Blackbird surveillance aircraft.      Eugene Berman / Shutterstock.com

Source: Lockheed Martin and Central Intelligence Agency

That's where the SR-71 Blackbird came in, a supersonic spy plane with emphasis on speed and stealth.

An SR-71 Blackbird surveillance aircraft.      Eugene Berman / Shutterstock.com

The aircraft was extremely capable but had one problem, it could be shot down, as shown when Francis Gary Powers' U-2 was shot down in 1960 over Russia. President Dwight Eisenhower, recognizing this flaw, commissioned a new spy plane capable of evading enemy missiles.

A U-2 Dragon Lady surveillance aircraft.      Reuters

Source: Lockheed Martin

Before there were camera-equipped satellites, the US relied on airplanes to provide surveillance photos. One of the first aircraft built for the purpose was the U-2 Dragon Lady, capable of flying upwards of 70,000 feet.

A U-2 Dragon Lady surveillance aircraft.      US Air Force/Staff Sgt. Robert M. Trujillo