Answers Your Questions
| From: Amanda, of Sydney, Australia
To: Wayne Hale, flight director
Question: What are the safe re-entry angles when traveling back into the Earth's atmosphere, and why are they important?
Answer: Two angles are of primary importance during a safe reentry in Earth's atmosphere by the shuttle orbiter. The first is called "flight path angle," denoted by the Greek symbol gamma. Gamma measures the angle between the local horizontal plane and the orbiter's flight path or trajectory. If it's too shallow, the orbiter will skip back out of the atmosphere and miss its intended landing site. If it's too steep, the orbiter will descend into dense atmosphere layers too quickly and incinerate. Typical gamma at orbiter atmospheric entry ranges from minus 1 degree to minus 2 degrees.
The second angle of importance to a safe reentry is the orbiter's "angle of attack," denoted by the Greek symbol alpha. This is the angle between the orbiter's belly plane (where the black insulating tiles are located) and the direction of airflow. At atmospheric entry, the alpha needs to be in the range plus 37 degrees to plus 43 degrees, with the orbiter's nose pitched above the airflow. At significantly lower alpha, orbiter surfaces not covered by as much insulation as the belly would quickly be overheated by atmospheric friction at a speed of 7.9 kilometers per second. If it were possible for the orbiter to hold a higher gamma with its control jets and flaps (called elevons), two dangerous conditions would rapidly develop due to the increased belly cross-section exposed to the airflow. First, heating would exceed even the belly insulation's protective capacity. Second, the rate at which atmospheric friction slows the orbiter's speed would increase to the point where it would literally be torn to pieces.
Gamma remains nearly constant until the orbiter's speed has decreased to two or three times that of sound (approximately 0.8 kilometers per second). At speeds lower than these, the orbiter behaves more and more like a conventional aircraft, as opposed to a spacecraft returning from orbit, and lower alpha is permissible.