NASA Logo - Web Link to

+ Text Only Site
+ Non-Flash Version
+ Contact Glenn

Photo of the Space Shuttle
 with some of its characteristics

As an aircraft moves through the air, the air molecules near the aircraft are disturbed and move around the aircraft. Exactly how the air re-acts to the aircraft depends upon the ratio of the speed of the aircraft to the speed of sound through the air. Because of the importance of this speed ratio, aerodynamicists have designated it with a special parameter called the Mach number in honor of Ernst Mach, a late 19th century physicist who studied gas dynamics.

As a spacecraft re-enters the earth's atmosphere, it is traveling very much faster than the speed of sound. The aircraft is said to be hypersonic. Typical low earth orbit re-entry speeds are near 17,500 mph and the Mach number M is nearly twenty five, M < 25. The chief characteristic of re-entry aerodynamics is that the temperature of the flow is so great that the chemical bonds of the diatomic molecules of the air are broken. The molecules break apart producing an electrically charged plasma around the aircraft. The air density is very low because re-entry occurs many miles above the earth's surface. Strong shock waves are generated on the lower surface of the spacecraft.

The only manned aircraft to fly in this regime is the Space Shuttle. The figure shows the Shuttle as it passes through the re-entry regime. The Shuttle used a rocket propulsion system to get into orbit, but during re-entry the aircraft is actually a glider. Small rockets are used for maneuvering because the low density of the air at altitudes above 50 miles makes aerodynamic surfaces ineffective. The heat is so great during re-entry that a special thermal protection system must be used to keep the aircraft intact. On the Shuttle, special silicon tiles are placed on the aluminum skin to insulate the skin. On the leading edge of the wings, carbon-cabon composite material is used to withstand the heat. The high forces and high heat dictate that the Shuttle has short, blunt wings. The Shuttle flies at a high angle of attack during re-entry to generate drag to dissipate speed. It executes some hypersonic split-S maneuvers to kill off speed during re-entry. The lift of the wings is only important in the final flare maneuver at touchdown.


Guided Tours

Navigation ..

Button to Display Propulsion Index
Beginner's Guide Home Page


     First Gov Image

+ Inspector General Hotline
+ Equal Employment Opportunity Data Posted Pursuant to the No Fear Act
+ Budgets, Strategic Plans and Accountability Reports
+ Freedom of Information Act
+ The President's Management Agenda
+ NASA Privacy Statement, Disclaimer,
and Accessibility Certification


Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: May 13 2021

+ Contact Glenn