Five X-planes ahead of their time

You first test new flight techniques with a test model, a so-called X-plane. These American test aircraft have set records for 75 years.

October 14, 1947: Seven kilometers above the Mojave Desert, a 9-meter arrow falls from the hold of a B-29 bomber. Pilot Chuck Yeager ignites the rocket propulsion of his Bell X-1 and becomes the first human to break the sound barrier at an altitude of 13 kilometers. Yeager’s ‘Glamorous Glennis’ – named after his wife – is the first in the US X-Series of experimental aircraft. KIJK lists five impressive successors.

1) Hypergreen: electrically powered NASA X-57 Maxwell

Not nearly as fast as the X-1, but unsurpassed green: NASA’s X-57 Maxwell was to fly completely electrically on electric motors and lithium batteries. During take-off, the X-57 (based on an Italian two-engine Tecnam P2006T) uses all fourteen engines, once up to speed, turning only the large ones at the ends of the wings. The lightweight X-57 should be able to handle flights of up to an hour, with a maximum range of 160 kilometers and a top speed of 282 kilometers per hour. The green plane makes its maiden flight in the spring.

The artist’s impression of the fully electrically powered two-seater NASA X-57 Maxwell. © NASA

2) Nuclear bomber Convair X-6

According to the new sustainability guide from the EU, the Convair X-6 could probably also be called green, because the sustainable ‘umbrella’ also includes natural gas and nuclear energy. X-6 (invented in atomic age from the 1940s and 1950s) would fly on nuclear-powered turbojet aircraft. With a 3 MW reactor on board, this bomber could stay in the air for up to a week to be able to strike back hard in the event of a nuclear war with the Soviet Union. A thick layer of lead between the cockpit and the reactor should protect the crew.

The X-6 itself was never built, but the (space) airline Convair conducted test flights between 1955 and 1957 with a rebuilt B-36 bomber equipped with a rotating nuclear reactor, which otherwise did not yet provide propulsion. This was never possible due to the high cost and security risks. The Russian nuclear-powered Tupolev Tu-119 bomber did not come off the drawing board either.

X-6 X-plan
An NB-36H bomber, the basis of the nuclear-powered X-6. © US Air Force

3) Hypersonic with Boeing X-51 Waverider

Yeager’s rocket-powered X-1 actually reached Mach 1.06 – the equivalent of 1,100 kilometers per hour. It was very impressive 75 years ago. Today, the U.S. Air Force raises the bar even higher. Hypersonic is the dream, with unmanned aerial vehicles capable of hitting anywhere in the world with more than Mach 5 in an hour.

The Boeing X-51 Waverider is such a hypersonic design: this scramjet swallows air from outside, heats it up and expels it with minimal internal resistance, so the airflow in the engine remains above the speed of sound. The design owes its nickname to the way it generates lift; the unmanned scramjet surfs on a shock wave of its own production. During a test flight on May 1, 2013, Waverider reached speeds above Mach 5 in 210 seconds, still the record for the longest powered hypersonic flight.

X-51 Wave Rider
Artistic impression of the Boeing X-51A Waverider. © US Air Force

4) Unmanned naval fighter X-47B

A drone, but different. The Northrop Grummans X-47B is an unmanned jet that can take off and land on an aircraft carrier. This would allow aircraft carriers to patrol the deployment area without endangering the pilots. The tailless X-47B, of which two protesters were built, flew in 2012 from the aircraft carrier George W. Bush. The plane could even refuel during the flight – a world first. Still, the test models have been put in mothballs as the fleet project’s requirements for unmanned combat aircraft were changed. The successor X-47C could become the first operational naval fighter aircraft in the UCLASS project.

X-47B X-plan
Test model of the unmanned naval aircraft X-47B. © US Navy

5) Flying Wing X-48

The X-planes are not just about taller, faster and longer: sometimes they are test models for a new, economical design. The Boeing X-48, for example, has wings that flow smoothly into the stretched, triangular hull. Similarly, the torso is on one mixed wingdesign for the lift that holds the aircraft in the air. And it could easily be more economical than the existing fuselages. Because the same surface can hold much more payload, such a mixed blade uses up to 20 percent less fuel according to calculations.

X-48 X-plan
Thanks to its elongated hull that provides lift just like the wings, the Boeing X-48 would consume much less fuel than traditional designs. © Tony Landis / NASA

The question is whether you will ever be able to use such a mixed wing yourself. Boeing tests showed that passengers did not like sitting in a kind of theater setting on a plane. A safe evacuation plan also proved to be a major challenge. This did not prevent KLM and TU Delft, together with Airbus, from coming up with their own mixed wing design: Flying V. The design was conceived by TU Berlin-educated Justus Benad and is being tested with scale models. Do we really get an X-plane from European soil?

Sources: Wikipedia, NASA, Delft University of Technology

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