In an experiment, one of space shuttle Discovery’s exterior heat tiles was deliberately altered and monitored during the shuttle’s high-speed return to Earth on Saturday. Data was collected to help understand airflow, and provided information for engineers designing the heat shield for the Orion spacecraft. One tile on the bottom of the shuttle’s left wing, about 10 feet behind the leading edge, included a 0.25-inch-high ridge that was expected to cause the airflow to go from smooth to turbulent, causing a rise in temperature of the tiles behind the altered tile. A Navy plane flew below Discovery as it returned home, monitoring the shuttle with an infrared camera. What did the experiment reveal?
Engineers were looking to better understand how smooth, laminar airflow, which provides a thin layer of insulation during peak heating, can change to the disturbed, turbulent flow — called boundary layer transition –which can cause downstream temperatures to climb, possibly affecting aerodynamics and causing damage.
A Navy P-3 Orion aircraft flying over the Gulf of Mexico captured infrared images of the shuttle’s underside when the orbiter had slowed to about 8.5 times the speed of sound. The temperature data and infrared imagery confirmed that the airflow was changed by the modified tile. The insulating layer of air changed to turbulent between mach 12 and 14. That, in turn, caused a rise in the temperatures of downstream tiles, but not as much as was expected.
The tiles in that area of the wing normally experience maximum temperatures of 815-870 degrees C (1,500 to 1,600 degrees F) when the re-entry airflow across the wing is smooth. The preliminary data that NASA received indicated the maximum temperature experienced by the “protuberance tile” was around 1090 degrees C (2,000 F), somewhat less than expected. If those measurements are confirmed, engineers may request a slightly higher protuberance for a future flight.
As for the large area of turbulence seen in the top P-3 image, engineers said said the image likely reflected a “typical” shuttle boundary layer transition during peak heating.
Engineers reported the protuberance tile looked normal and undamaged on the runway after Discovery’s landing, although superficial markings were seen on several downstream tiles. No such markings were seen on tiles behind a protruding gap filler that was spotted during an on-orbit inspection on one of Discovery’s elevons.
The shuttle’s transition from smooth to turbulent flow occurs naturally as the spacecraft slows down, typically at velocities around mach 8, or eight times the speed of sound, about 20 minutes after atmospheric entry begins. In some cases, however, the transition can occur earlier than that because of surface roughness or defects in the heat shield.
For example, a protruding tile spacer called a “gap filler” can shake loose during launch and extend up into the airflow, triggering an early, asymmetric boundary layer transition during re-entry. When that happens, the shuttle’s aerodynamics can be affected and downstream tiles can be subjected to more extreme heating.
During shuttle mission STS-28 in 1989, the boundary layer “tripped” 15 minutes after entry, at around mach 18. During shuttle mission STS-50 in 1992, one side of the shuttle’s belly went turbulent more than a minute ahead of the other side, causing a change in aerodynamics that prompted autopilot flight control inputs.
This experiment that could lead to improved heat shield designs for the Orion spacecraft which will bring astronauts to the ISS and the Moon, along with providing insights into shuttle aerodynamics.
Source: CBS News