X-15 and other space planes history

1942

Eve of concept

1947

First supersonic (Mach 1+) flight

1951

Corpus Camus

1953

Mach 2 / 20 km high

1959-1968

Mach 6.3 / 100 km high

1942
First US Jet, the Bell XP-59.
German rocket A4 (Hitler's V2 )
Eugen Sanger proposal of an intercontinental bomber, capable of a speed of Mach 24 and an altitude of 100 km (it's the founding study of space plane, of equal value that Tsiolkovsky's studies for rocket science.

1947
First flight going faster than Mach 1 limit's. This limit stand because the interaction between air and the plane can't be the same under and above the sound speed limit. Above the sound speed limit air's molecules doesn't act the same way than above. This is merely because the sound speed is linked to the molecule speed. Above the sound speed the molecules acted more sluggishly, and the usual aerodynamic laws do not apply.
Therefore as plane's concepts were based on those laws this sound's barrier was feared. Real problems only comes near Mach 2.
The first pilot to pass this limit was Chuck Yeager on a Bell X1 plane. His plane used a rocket motor as all succeeding rockets planes instead of jet motors.
This plane was launched from a B50 and landed horizontally like an ordinary plane.
That year also saw he discovery of the transistor effect (transfer resistor) this theoretical and practical work by Shockley, Brattain and Bardeen started the electronic miniaturization.

1951
Harvey J Allen find the "Corpus camus" theory about atmosphere reentry at survivable speed and temperature conditions. This started the lifting bodies experiences some years later which ultimately gave birth to the STS (space shuttle).

1953
Mach 2 and 20 km high with a D-558-2 skyrocket.
It was around this speed that an weird instability was discovered, which linked movements in one axis to the others. This instability exists because the plane geometry is not suited for supersonic aerodynamic laws.
The scientists considered one time the idea to have the same surface vertically than horizontally (in the wings).
This was rejected when someone pointed that a different geometry of the rear horizontal and vertical wings was enough to solve this problem (at least up to Mach 6). The section of the rear vertical wing must be at least 10°. Another new idea was to change direction with rockets (useful in void!).
The year later, NACA commanded a study to John V Becker about an evolution of the two first rockets planes (X1 and D-558).
The new plane must be able to go to Mach 6.3 and 100 km high.
As it was supposed to be launched from a B52 it's size was around 15 m and the motor thrust was 20 (metric) tons, temperature should be as high as 650° (Celsius) with local variations of 200°.
Reentry was supposed to use a wide angle to minimize the warming time (as Allen said it in 1951).
Thrust must be adjustable between 30% and 100% of full Thrust.
The mechanical and biological(!) parts of the plane must resist to variation of weight from -3g to +7g.

1959 / 1968
External parts of the plane X-15 was made in Inconel X (Nickel/chromium) with a width varying from 1mm to 2.3mm.
Internal parts was in Titanum and Aluminum alloy (2024-T4).
The External windshield was 9.5 mm large!
The ammonium/Lox motor XLR-99 weight finally 380kg instead of 250kg. The combustion chamber was a two stages igniting system with an exhaust nozzle 1 meter large. The turbo pump pump 5.9 metric tons per minute.
The 15 November 1960 one experimentation was done at half the specs by M. Crossfield. He reach Mach 3 and 3150 km/h in 137 seconds!
In 1962? M. White reach 95.9 km with a ballistic fly after having reached Mach 5.45 and 48km high in only 84seconds of thrust of the XLR-99 motor.
In the following years, X-15 was improved to flight at more than Mach 6.
As external temperature is a function proportional to the square of the speed, temperatures of 1300° may be reached.
No metal perform well at 1300°, in fact most melt under this temperature!
So an protection was put on the X-15 on the form of a material called MA-25S that was developed by the Martin Company, and it was sprayed onto the aircraft, much like a very heavy, thick paint (information given by the Nasa official X-15 official history writer, "Dennis R. Jenkins", try a search with this name on Amazon.com).
This was done by hand and required 700 hours of work to fix only 136 kg!