I still nowadays enjoy reading and going through old photos of the experimental planes NASA developed in the good old days of the aviation pioneers, the old days of the X-Plane program.

NASA started the X-Plane program — the X is for eXperimental — in 1946, and has since developed 57 prototypes — although not all of them have flown (like the X-27 High-Performance Fighter).

The objective of this program was to test new technologies, usually keeping their development secret.

Sadly, despite a long history of incredible achievements, the X-Plane program has remained inactive since 2013, although it has never been canceled — its budget was allocated to other NASA’s research topics that required some investment.

However, the good news is NASA has submitted recently a budget proposal aimed at reopening the X-Plane program which, over the years, has yielded some of the most spectacular and innovative aircraft in history.

Here you will find a collection of the most iconic concept planes developed jointly by NASA and the top-notch American aerospace companies, starting with the record-shatterer Bell X-1.

Bell X-1: The Pioneer

The pioneer of the series, the Bell X-1 had a rocket engine, straight wings, and was launched, already at altitude, from the belly of a B-29 bomber.

The X-1 was, in principle, a “bullet with wings”, its shape closely resembling a Browning 0.50-caliber (12.7mm) machine gun bullet, known to be stable in supersonic flight.

It conquered its place in aviation history as the first manned aircraft to break the sound barrier in level flight.

On October 14, 1947, 21 months after the maiden flight, the historic mark was obtained by the legendary pilot Chuck Yeager, the first human to break the mystical mark of Mach 1 (the speed of sound).

Bell X-2: The Big Brother

Eight years after the Bell X-1 had broken the sound barrier, the X-2 was built with new aerodynamic profiles optimized for supersonic flight and equipped with a powerful 15,000 pound-force Curtiss-Wright XLR25 rocket engine.

With optimized swept wings, this model reached the incredible mark of Mach 3.2 in September 1958, soaring at an altitude of 65,000 feet.

Douglas X-3 Stiletto: An Underpowered Dart

Built by Douglas to assess the feasibility of long-duration supersonic flight, the X-3 had a sharp nose and small 23-degree trapezoidal wings, with a 75% chord wingspan. It was the first X-plane to use turbojet engines: two Westinghouse XJ34-WE-17 with afterburners.

He also pioneered the use of titanium in major airframe components. But, due to the low thrust provided by the turbojet engines, the model was unable to overcome the Mach 1 barrier in level flight.

Its basic concept, however, served as a benchmark for the Lockheed F-104 Starfighter iconic project, which would exceed a flight speed of Mach 2.0 and, in some conditions, could support supersonic flight without the use of afterburners, a capability know as super-cruise.

Lockheed X-4 Bantam: The First Semi-Tailless Concept Plane

In 1948, Northrop was chosen to develop this concept aircraft, originally designed to study the loss of control problems of jet aircraft at transonic speeds, resulting from the interaction of the shock waves generated in the wings with the rear stabilizers (shock stall).

The X-4 didn’t have horizontal stabilizers on the tail and the changes in attitude (pitch and roll) were controlled by “elevons” — surfaces on the trailing edge of the wings, which act as elevators and ailerons at the same time.

The study created conditions for the emergence of the flying wing concept, but the non-existence at the time of electronic flight control systems to support piloting prevented great advances.

In spite of everything, the X-4 was the one who began to open the way for the technology that would be used later on in the Northrop XB-49, a pure flying wing bomber concept.

Bell X-5: Exploring the Variable Geometry Wings Technology

Also developed by Bell, responsible for much of the early X-Planes, the X-5 model flew in 1951 and aimed to test the feasibility of variable geometry wings. The concept had been developed previously during World War II by the German firm Messerschmitt with the P.1101 model, which was never tested in combat.

The X-5 was the first aircraft capable of altering the geometry of the aircraft in flight, adapting its arrow to speed and operating conditions. This technology was used in various military aircraft since the 1960s, one of them the iconic F-14 Tomcat.

Convair X-6 Crusader: The Nuclear-Powered Bomber

Probably one of the most exotic X projects, this model — developed by Convair from the gigantic B-36 bomber — tried to study the feasibility of using nuclear propulsion in aircraft.

The U.S.A. was looking for a means that would allow large groups of nuclear bombers to be kept in continuous flight for weeks, which would increase the response capability in case of war with the Soviet Union.

A new version of the B-36 bomber named NB-36H then received extensive modifications to accommodate a nuclear reactor and electric motors, but the plane never had a chance as the project was canceled before the reactor was completed.

There were two serious limiters for the nuclear-powered plane concept: the first was the risk of an accident with a nuclear-powered aircraft; the second, even more obvious, the difficulty in keeping crew members flying for weeks. The plane would need huge adaptations, including bedrooms, kitchen, large water reservoirs, among other things. In the end, it seemed not a bright idea to continue exploring the concept.

Ryan X-13 Vertijet: Paving the Way to VTOL Aircraft

The X-13 Vertijet aimed to demonstrate the feasibility of a jet aircraft capable of taking off vertically (like a rocket), flying overhead, transitioning to horizontal flight, and at the end of the mission, also landing vertically.

The VTOL (Vertical Takeoff and Landing) aircraft concept was very popular in the 1950s. Two prototypes were built and, in July 1957, an X-13 took off from Washington DC, crossed the Potomac River, and landed in front of the Pentagon.

But the USAF chose to cancel the project, claiming that an operational requirement was missing to justify its existence. Ironically, a few years later the VTOL concept became a requirement for navies in various countries.

North American X-15: The Hypersonic Pioneer

The X-15 is one of the most incredible, and perhaps the best known, of the rocket aircraft in the X-plane family.

Created to study the characteristics of hypersonic flight, it was powered by a Reaction Motors XLR99-RM-2 rocket engine, generating no less than 70,400 pound-force.

The model was flown by legendary names in aviation, such as Neil Armstrong, William “Bill” Dana, and William “Pete” Knight. The latter, in October 1967, performed a flight where he reached the astonishing speed of Mach 6.7 (approximately 7,274 km/h), a world record that remains standing to this day.

You can read more about other curious facts and figures of this marvelous engineering-jewel here:

Hiller X-18: The Tilt-Wing Technology Demonstrator

Created in the late 1950s, the X-18 was intended to validate the V/STOL (Vertical / Short Takeoff and Landing) concept with the use of tilting wings. The model was equipped with two turboprop engines in the wings, which rotated on the horizontal axis during takeoffs and landings.

Although it had a short operational life, the X-18 assisted in the development of various technologies that helped advance the concept of tilt-wing aircraft, such as the current Bell V-22 Osprey.

Bell X-22: The Ducted Fan Pioneer

Still, within the V/STOL program, the X-22 model evaluated the use of ducted fans, which was considered promising, although it did not have any technical viability in the mid-60s.

Surprisingly, this concept has been recently studied again for the new Urban Air Mobility demonstrators and should be the basis of the new e-VTOL aircraft generation, which should start flying commercially before the end of the 2020s.

Bell X-14: The F-35B Precursor

When the X-13 was canceled, Bell was already flying the X-14, another VTOL-capable model, but based on a different concept: thrust-vectoring by means of deflectors. Unlike the X-13, the X-14’s vertical takeoff and landing operations were performed with the aircraft already in a straight and wing-leveled attitude.

The X-14 was one of the longest-serving X-planes, having flown between 1957 and 1981. In that period, it received a series of improvements, such as a new fly-by-wire control, which allowed the manufacturer to expand the test envelope of this embryonic technology, which is similar to that used today by the Harrier and the Lockheed F-35 fighter.

Martin Marietta X-24: The First Lifting Body

Included in the list of X-Planes, the X-24 models were conceived within a special NASA program intended to verify the characteristics and viability of the so-called “lifting-body”: a wingless aircraft that generates lift by the aerodynamic configuration of their fuselage alone.

The program produced the X-24A, X-24B, M2-F3, and HL-10 variants. These last two were left out of the X-Plane family, but the quartet was essential for NASA to advance the studies that made the Space Shuttle feasible.

The X-31: Pushing the Limits of Flight

Created in the early 1990s from a partnership between NASA, Darpa, and DLR (German space agency), the X-31 had as its mission the concepts of vectored thrust and super maneuverability in combat aircraft.

By far, it was the largest “Frankstein” among the X-Planes, with parts and components from the German prototype TKF-90 (one of the predecessors of the Eurofighter Typhoon), the F-16, the F-16XL, the F/A-18, the B-1 Lancer bomber, the V-22 Osprey tilt-rotor, and even the F-20 Tigershark prototype, as well as the Cessna Citation executive jet.

The objective was to reduce the costs and risks of the project. Its engine was the established General Electric F404 turbofan, which equipped the first versions of the F/A-18.

The X-32 and the X-35: The Joint Strike Fighter Contenders

The Boeing X-32 and Lockheed Martin X-35 were the only two cases of X-Plane projects with a view to developing operational aircraft.

Created within the Joint Strike Fighter program, the two aircraft competed for the multi-million dollar contract for a multi-mission combat aircraft with stealth characteristics, meeting the requirements of the US Air Force, Navy, and Marines.

The relatively ugly X-32 ended up being canceled, especially for featuring a large radar cross-section signature (at which point Boeing claimed its aircraft would be unbeatable). Nevertheless, the efforts were not in vain, as part of the technologies was reused later in the development of the F/A-18E Super Hornet.

The Complete List of NASA’s X-Planes

If you are still interested to know more about the whole series of X-planes, here you will find the links to all of them, enjoy!

• Bell X-1: sound barrier
• Bell X-2: Mach 2–3 supersonic flight
• Douglas X-3 Stiletto: sustained supersonic flight
• Northrop X-4 Bantam: tailless aircraft
• Bell X-5: variable-sweep wing
• Convair X-6: nuclear reactor test aircraft (for future nuclear-powered aircraft)
• Lockheed X-7: “flying stovepipe” unmanned ramjet and guidance test missile
• Aerojet General X-8: sounding rocket (became Aerobee)
• Bell X-9 Shrike: guided missile
• North American X-10: unmanned missile technology demonstrator
• Convair X-11: single engine missile testbed for Atlas
• Convair X-12: three engine missile testbed for Atlas
• Ryan X-13 Vertijet: tail sitter VTOL jet
• Bell X-14: thrust vectoring VTOL jet
• North American X-15: hypersonic rocket-powered research aircraft (Mach 6)
• Lockheed X-17: research rocket testing high Mach re-entry
• Hiller X-18: tilt-wing STOVL cargo aircraft
• Curtiss-Wright X-19: VTOL tiltrotor
• X-20 Dyna-Soar spaceplane program (cancelled)
• Northrop X-21 laminar flow wing
• Bell X-22: ducted fan V/STOL
• Martin Marietta X-23 Prime: lifting body reentry
• Martin-Marietta X-24: lifting body test aircraft
• Bensen X-25: single-seat autogyro
• Schweizer X-26 Frigate: sailplane
• Lockheed X-27 Lancer: proposed high-performance technology demonstrator based on Lockheed CL-1200 (only mockup built)
• Pereira X-28A Sea Skimmer: single-seat flying boat evaluated by US Navy
• Grumman X-29: forward-swept wing test aircraft
• Rockwell X-30: single-stage-to-orbit spacecraft (cancelled)
• Rockwell-MBB X-31: extreme angle of attack test aircraft
• Boeing X-32: Joint Strike Fighter Program entry (lost to X-35)
• Lockheed Martin X-33: unmanned scale demonstrator for VentureStar single stage to orbit spacecraft
• Orbital Sciences X-34: reusable launch vehicle testbed (only unpowered prototype built)
• Lockheed Martin X-35 Joint Strike Fighter Program: entry winner; became F-35 Lightning II
• McDonnell Douglas X-36: tailless fighter research agility aircraft
• Boeing X-37: reusable unmanned spacecraft
• NASA X-38: crew return vehicle for International Space Station (cancelled after tests)
• X-39: reserved for USAF/DARPA program use
• Boeing X-40 Space Maneuver Vehicle: testbed X-37 guidance and flight characteristics
• X-41 Common Aero Vehicle: classified DARPA/NASA maneuvering re-entry vehicle
• X-42 Pop-Up Upper Stage: classified rocket upper stage
• NASA X-43 Hyper-X: hypersonic scramjet
• Lockheed Martin X-44 MANTA: multi-axis tailless aircraft concept
• Boeing X-45: unmanned combat air vehicle
• Boeing X-46: unmanned combat air vehicle (proposal only)
• Northrop Grumman X-47 Pegasus: unmanned combat air vehicle
• Boeing X-48: blended wing aircraft
• Piasecki X-49: compound helicopter technology demonstrator
• Boeing X-50 Dragonfly: gyrodyne unmanned aerial vehicle
• Boeing X-51: Mach 5+ scramjet missile demonstrator
• X-53 Active Aeroelastic Wing: wing-warping flight demonstrator
• Gulfstream X-54: supersonic boom intensity research and demonstration aircraft
• Lockheed Martin X-55: advanced composites technology demonstrator
• Lockheed Martin X-56: flutter suppression and gust load testing unmanned testbed

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Rodney Rodríguez Robles is an aerospace engineer, cyclist, blogger, and cutting edge technology advocate, living a dream in the aerospace industry he only dreamed of as a kid. He talks about coding, the history of aeronautics, rocket science, and all the technology that is making your day by day easier.

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