Page 181 - Understanding Flight

CH06_Anderson 7/25/01 8:59 AM Page 168

168 CHAPTER SIX

Fig.6.16. The rocket-propelled X-15. (Photo courtesy of NASA and the U.S.Air Force.)

across the Earth’s atmosphere. Old satellites usually never get as far as
the ground before completely burning. So, how do the Space Shuttle
and the command modules reenter at equivalent speeds?
The Apollo command modules had a special carbon-based surface
on its base. This surface burned off slowly as the craft reentered the
atmosphere. The burning heat shield resulted in two effects. The first
was that the burning consumes energy, and thus heat, from the air.
The second is that the by-products were swept away, taking heat
along with them. The astronauts were thus kept cool behind this heat
shield. This form of skin cooling is called ablation. The problem with
ablation is that it is not reusable.
Special tiles were designed for the Space Shuttle that are extremely
poor heat conductors. The tiles absorb heat very slowly. When
the surface of the tiles reaches the high reentry temperature,
Hypersonic vehicles do not have
the tiles radiate heat out to maintain a constant surface
sharp noses and wings because
temperature. But the longer the Shuttle experiences the heat,
there must be enough material
the deeper the heat will penetrate into the tile. So the tiles
to absorb the heat. Sharp objects
must be thick enough to prevent the heat from reaching the
will burn off at high speeds.
aluminum skin before the heat load is removed on landing.
Note that airplanes that fly at slower supersonic speeds
experience substantial skin heating. The Concorde fuselage expands
as much as 10 inches due to heat from skin friction in cruise. The
SR-71 experiences such heat that the top of the wing is corrugated on