By akademiotoelektronik, 24/03/2022

With the X-15, the rocket plane enters the space adventure

Eric BottlaenderSpace specialistOctober 17, 2021 at 5:17 p.m. The X-15 rocket plane in flight. NASA Credits

Its development launched even as the first rockets struggled to leave the atmosphere, the American experimental aircraft X-15 pushed the limits of high-speed manned flight, even after the passage into orbit of a certain Yuri Gagarin . And maybe, if the US Air Force had woken up a little earlier...

The pioneers moved fast. Very quickly.

From Mach 1 to… faster

If we know more about the space adventure under the aspect of research on ballistic missiles (which gave birth to the first satellite launchers), we should not forget that the post-war period constituted a fertile ground of innovations for the aviation, which very nearly arrived at the frontier of space as quickly as missiles... In the United States in 1945, the very young company Reaction Motors Inc. received a contract to design a rocket engine adapted to the Bell X-1. She then completed the design of the XLR-11 with ethanol and liquid oxygen, a small marvel for the time consisting of 4 combustion chambers that could be turned off or on as needed to vary the power. In October 1947, Chuck Yeager became the first man to exceed Mach-1.

Wind tunnel study for the release of the X-15 from a model of the B-52 bomber. NASA Credits

From the beginning of the 1950s, the US Air Force and NACA (the ancestor of NASA dedicated to aeronautical research) were quickly faced with a more complex challenge: while the first supersonic fighters were already pointing their noses, it was necessary to understand how to pass the next “wall”: that of hypersonics at Mach-4 and above. But the experimental aircraft in development (the X-2 and the X-3) do not have the power for that, and especially not the coating, because these flights in the upper atmosphere generate enormous heating. At this time, missiles do this very well (the heirs of the Germanic V2 have flourished on both sides of the Iron Curtain), but a manned aircraft would be a step forward, especially because it can conduct more missions. than "simply" bombing an opponent. Despite some budgetary opposition, in particular because military aircraft not dedicated to research already required a lot of money, the X-15 project was accepted in 1954. Objective? To be able to reach an altitude of 75 km, a speed of 7,200 km/h and withstand heating above 650°C.

The X-15, a resistant device (and a little maneuvering)

In November 1955, North American was chosen to produce the X-15, with propulsion entrusted shortly afterwards to Reaction Motors. However the NACA holds the file with an iron fist, and it is a question of designing exactly the plane requested! The X-15 is a 15.47 m long and 6.81 m wingspan aircraft, essentially consisting of a large tank with a pilot at one end, and a rocket engine at the other side. Which in itself doesn't take it too far from a “classic” launcher! So that it has a minimum of lift, a streamlined fuselage, two small wings and a set of four mobile fins at the rear are added to the central cylinder.

As the X-15 is transported under the wing of a B-52 bomber until it is dropped at an altitude of nearly 12 km, it also has a fin under the device, which must be dropped shortly before to land. To save space and reduce complexity, engineers replaced the rear axle with a couple of large skids, which were changed with each flight.

X-15 pilot turned legend (later), Neil Armstrong. NASA Credits

The X-15 is also equipped with numerous attitude thrusters, small jets which allow the aircraft to be oriented during flight phases in the upper atmosphere. This is a crucial point for controlling the crossing of the densest layers: calculations and simulations in the wind tunnel show that the ideal flight profile consists of "raising the nose" and presenting the belly of the X-15 to brake as much as possible and dissipate energy and heat. Which, in addition to control, implies very resistant materials. The shell is therefore not made of aluminum, but of nickel alloy (inconel, which has since become famous).

For the first flights, the engine is not ready!

As development progressed, and as the first unpowered gliding tests approached, it became apparent that the XLR-99 engine would not be ready in time. Never mind, it's possible to use a pair of XLR-11s instead… The same models that have, for more than a decade, propelled the X-1 beyond the sound barrier! On March 10, 1959, pilot James Crossfield flew the X-15 for the first time, but for the first tests, the aircraft remained under the wing of his carrier aircraft. It will be necessary to wait until June 8 for the teams to consider that it is time to carry out the first free flight. The motors are still not on, we must first check that the X-15 flies correctly. If it is not the ideal aircraft for aerobatics, its performance is consistent. A few days later, it was the second of the three aircraft which flew for the first time.

"Plans" view of the X-15. We see here that its rear axle is higher than the bottom wing, which had to be dropped before landing. NASA Credits

The first engine ignition in flight will not take place until September 17, 1959, but what a push! The pilot is obliged to reduce the possibilities of the engine to the maximum to control the plane, which rises all the same beyond Mach 2.1 and practically reaches 16 kilometers of altitude. This time, research teams have data to go further, higher and faster. An engine incident in early November with an XLR-11 led to a difficult landing and the fuselage of X-15 number 2 was damaged: the program was three months behind schedule. On February 11, 1960, James Crossfield reached 27 kilometers in altitude, while the two X-15s flew about once every 10 days. On May 12, Joe Walker reached Mach 3.2 for the first time…

Towards the border and beyond!

It should be noted that the X-15 program, managed by NASA and the US Air Force is then mainly seen as an aeronautical program, a formidable tool but... not a space plane. The American agency is in full preparation of its Mercury project and in the summer of 1960, the results are not conclusive. Perhaps there will remain some regrets for not having flown the first man to the frontier of space by plane, even if it remains obvious that the X-15 could never have targeted the 'orbit. Because yes, the new engine arrives in November 1960, and the XLR-99 allows new feats. Ah, if it had been delivered a few months earlier… On March 7, 1961, more than a month before Gagarin's flight, Robert White reached Mach 4.43, but only climbed to an altitude of 23.6 km. With a different profile, it would have been possible to aim for the “borders” of the 50 miles and the Karman Line. However, pilots and authorities do not yet know enough about the limits of the device.

The X-15 crews active in 1965. We can clearly see the cockpit optimized for very high speeds. NASA Credits

In 1961, X-15 pilots (including Neil Armstrong) exceeded Mach 5, and even Mach 6 (6,500 km/h) in the upper atmosphere! The data will be used both for NASA to study atmospheric re-entries and later, in the shuttle program... and for the US Air Force, which is preparing an ultra-fast observation plane: the SR-71 Blackbird (less fast, but with more interesting operational characteristics). The success of the Mercury program did not put an end to the flights of the X-15, on the contrary: a record number of tests took place in 1962 and 1963. On July 17, 1962, Robert White finally exceeded 50 miles in altitude, and became astronaut. 13 flights in the end until 1968 will “tick the box”, including two which will exceed in July and August 1963 the 100 kilometers of altitude. It then became the first space plane.

The rocket plane, a one-day story?

Nor is the X-15 program a model in terms of security. It pushes back the limits of aviation and validates new concepts, but it notably cost the life of Michael J. Adams on November 15, 1967, his aircraft having disintegrated at hypersonic speed. The second copy suffered two landing accidents, and NASA now considers that John McKay's injuries significantly reduced its life expectancy… Was this the price to pay for an innovative program in the 1960s? Most likely. During the last flights in 1968, many observers worried about whether the X-15 was ultimately useless, as no other device was considering such a flight envelope. But the work will actually be used a decade later, during the tests of the American STS shuttles.

Accident in October 1959, following a landing with too much fuel. Credits NASA/Air Force Museum

The rocket plane model is not dead today. Rockets still take off by being dropped from carrier planes (Pegasus, LauncherOne), and Virgin Galactic clings to the idea of ​​a tourist rocket plane! The latter has already made 4 flights beyond 50 miles (80 km) since 2018…