Page 180 - Understanding Flight

CH06_Anderson 7/25/01 8:59 AM Page 167

High-Speed Flight 167

Skin Heating

Thermal protection requirements of hypersonic aircraft are also affected
by the dissociation of the air. Vehicles traveling at high Mach numbers
will experience extremely hot gases. Some of this is due to temperature
increases across shock waves and some is from skin friction. The high-
temperature air will burn right through any normal material. The Space
Shuttle uses ceramic tiles for thermal protection. The dissociation of air
molecules actually helps in keeping the vehicle cooler. It takes energy
from heat to break the chemical bonds of the molecules. Thus heat
energy is converted to chemical energy and the surface temperatures do
not get as hot as would otherwise be predicted. However, they still get
very hot, so the surface must be protected.
Extremely high-speed flight is experienced during reentry to the
atmosphere. The Space Shuttle, as well as Apollo and Soyuz capsules,
must all endure very high heat upon reentry. When the Space Shuttle
first hits the atmosphere, it is traveling at approximately 14,000 mi/h
(23,000 km/h). The thin air that slams into the nose of the Space Shuttle
converts kinetic energy to heat. In theory, the air that impacts the nose
of the Space Shuttle will reach over 36,000°F (20,000°C), which is about
four times the temperature of the sun! Can this really happen?
When the air reaches high temperatures, it goes through complex
changes. Some of the energy of impact goes into breaking chemical
bonds rather than creating heat. Oxygen dissociates and ionizes. The
impact is so great, in fact, that the ionized gas that develops around a
vehicle reentering the atmosphere prevents radio communication with
the outside. This is what is known as the reentry blackout
On Oct. 3, 1967, the X-15A-2
experienced by all spacecraft since the first successful
(Figure 6.16) was outfitted with
atmospheric reentry (the Russian Sputnik). Rather than
an experimental ramjet. The heat
having skin temperatures reaching 36,000°F (20,000°C) the
was so intense, three of four
temperature is closer to one-fourth that value, but still sunlike
explosive mounting bolts
temperatures.
exploded and the fourth failed,
With these high temperatures most things that reenter the
causing the dummy ramjet to
atmosphere burn up. “Shooting stars,” or meteors, are nothing
separate from the aircraft.
more than small meteorites, which burn up as they skip