In 1954, the National Advisory Committee for Aeronautics - which later evolved into NASA - sponsored a joint research project that involved the US military and the private sector. The aim of the research was simple: discover how aircraft material, structures, and control surfaces would fare at very high altitudes and hypersonic speeds. A bidding process was carried out, and North America emerged winners ahead of Bell, Republic, and Douglas.

The program was a huge success, and the North American X-15 was born. On October 3, 1967, the X-15 became the fastest airplane ever flown when Major William "Pete" Knight steered the aircraft to 4,520 mph, Mach 6.72. Since then, the X-15 has partaken in 199 research flights and helped pave the way for spaceflight. The flights were typically short, lasting around ten minutes each. While the X-15 was and is still remembered for its incredible speed, here are some other cool facts about the world's fastest jet.

10 Half of its Bulk Consists of Fuel Tanks

North American X-15
Via NASA

Would you believe that half of the bulk of the North American X-15 is actually a good number of fuel tanks?

Since the X-15 needed a huge amount of energy for hypersonic flights, the 15-meter (49 feet) long jet has half of this height carrying propellant tanks for powering its rocket.

North-American-X-15-1
Via AirforceMuseum

The amazing part about it is that the fuel burns for at most two minutes and then goes off. The jet receives all the energy it needs in those precious two minutes. A miscalculation of time (down to the second) would be an extra-large error.

9 It Travels At 4520mph

North-American-X-15-2
Via AirForceMuseum

The first auto to achieve hypersonic speed was the X-1 (which went roughly a bit above the speed of sound). The X-15 was created to beat that record six times over, and it did. In all its 199 flights, the X-15 dared speeds so great to the point it moved at the blazing speed of 4520mph (Mach 6.7).

North_American_X-15
Clemens Vasters

That set the world record for speed till this very day. Such controlled rocket speed is yet to be found again (in the same conditions). Perhaps the X-15's speed would be toppled in the distant future, but definitely not so soon.

RELATED: Here's How The Fastest Soviet Fighter Jets Compare Against American

8 Only One Casualty

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350z33

In all the 199 flights, which the X-15 featured at very high altitudes with the very unfavorable conditions above, there was only one casualty recorded as all other pilots made it back safe whether complications were developed or not.

North-American-X-15-4
Mytwocents

This unfortunate pilot was Air Force Major Michael Adams, who died in 1967 when the jet tore apart at a hypersonic speed upon re-entry. So, it could be said that he did not even have a living chance with the way things went on air that day. However, his sacrifice is not forgotten, as many lessons have been learned from the events that caused that tragedy.

7 Pilots Were Regarded As Astronauts

North-American-X-15-5
Bubba73

The first thing anybody remembers the X-15 for is that its speed record is yet to be broken. Being the pioneer rocket jet that moved in the thinnest of air, it serves as a kind of prototype for modern space flight engines and high altitude jets.

North-American-X-15-6
Ragnhild&Neil Crawford

It was even the X-15 that first brought about the need for pressure suits. Furthermore, the X-15 was the first propulsion craft to actualize very high altitudes, with 13 of those flights exceeding the altitude of 80km, which was above the Airforce's spaceflight range. As a result, the pilots who manned those flights were all declared astronauts.

RELATED: NASA Testing For Near Silent Supersonic X-59 Flight Using F-15s

6 Design and Structure

North-American-X-15-7
Via AirForceMuseum

A jet that can be dropped from space (close to the earth's stratosphere) and survive extreme heat (up to a thousand Fahrenheit) is definitely a structural marvel. But how did the X-15 achieve this? Its body was made with an extremely strong nickel alloy - which gave it a kind of gun-metal black color - called Inconel X.

North-American-X-15-8
Via AirForceMuseum

This alloy was able to survive such high temperatures without melting. The way the wings and nose of the jet were designed is a technique that is still used for most space crafts as it was not designed with the traditional thrusters but in such a way that it could navigate its way through space where necessary.

5 Unconventional Landing Gear

North-American-X-15-9
Via AirForceMuseum

The X-15 was structured to land on dry lake beds. As such, the retractable landing gear was actually a nose-wheel carriage with two rear skids. This was very different from other models.

North-American-X-15-16
KLaRock

These skids stopped quite at the ventral fin of the jet; hence the pilot always had to jettison the lower fin (which was recovered via parachute) to land successfully. This peculiar landing gear was meant to reduce weight and increase simplicity, and it achieved that successfully.

RELATED: These Are The Fastest Fighter Jets In The World

4 More Heat, Less Speed

North-American-X-15
NASA

With the program's success and relatively safe landings, new ways were being sought to take the craft to higher altitudes and speed. But, the problem with more speed is the fact that there will be an increase in heat. Now everything, including the Inconel X, has a melting point.

North-American-X-15
via airandspacemagazine

This was when the jet became a somewhat whitish object as an ablative coating was applied to help with the heat. It seemed to be quite effective as it held its ground against temperatures of up to two thousand degrees (surface temperature) and ended up being discolored only in a few places after its first flight.

3 It Is Launched From Above

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Airforcefe

To conserve the X-15's fuel, it was usually taken high and dropped from a Boeing B-52 Stratofortress. The B-52 often carried it a little above 40,000 feet before letting go.

North-American-X-15
via wired

This was because the rocket engine was powered by anhydrous ammonia and liquid oxygen which when fired, burned real fast. Hence, the ideal method was to take it high and then let it move from there rather than waste fuel starting the flight from the ground base.

RELATED: Here Are The 15 Fastest Jets Ever Used By Military Forces

2 Independent Ejection Seat

North-American-X-15-13
Via AirForceMuseum

The seat of the X-15 was the most peculiar for a craft of its time. To begin with, it was literally a tractor seat. The seat was fashioned after a tractor seat because findings revealed that the tractor seat was more suited for the human spine. That is just one of many examples where tractors were very useful outside a farm.

North-American-X-15-14
Via AirForceMuseum

The seat was made in such a way that it could be ejected even at very high altitudes and still land safely by itself in the event of a casualty or accident. However, it was never used in actual flight.

1 Bullet Shape

North-American-X-15-10
Mys 721tx

Having to glide through very high altitudes so close to space (sometimes just about in space), the X-15 needed a body that would aid the whole process without providing some sort of structural hindrance to flight.

North-American-X-15-12
Tony Hisgett

And that it did; starting from its nose which was tailored down to an altitude sending ball with thrusters about it. Its entire body was made as compact as could be and this, coupled with its color gave it the look of a giant bullet.

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