3.6 DE-ORBIT/RE-ENTRY

¶As Columbia re-entered Earthʼs atmosphere, sensors in the Orbiter relayed streams of data both to entry controllers on the ground at Johnson Space Center and to the Modular Auxiliary Data System recorder, which survived the breakup of the Orbiter and was recovered by ground search teams. This data – temperatures, pressures, and stresses – came from sensors located throughout the Orbiter. Entry controllers were unaware of any problems with re-entry until telemetry data indicated errant readings. During the investigation data from these two sources was used to make aerodynamic, aerothermal, and mechanical reconstructions of re-entry that showed how these stresses affected the Orbiter.

¶The re-entry analysis and testing focused on eight areas:

Analysis of the Modular Auxiliary Data System re-

corder information and the pattern of wire runs and sensor failures throughout the Orbiter.

Physical and chemical analysis of the recovered de-

bris to determine where the breach in the RCC panels likely occurred.

Analysis of videos and photography provided by the general public.
Abnormal heating on the outside of the Orbiter body.

Sensors showed lower heating and then higher heating than is usually seen on the left Orbital Maneuvering System pod and the left side of the fuselage.

Early heating inside the wing leading edge. Initially,

heating occurred inside the left wing RCC panels before the wing leading edge spar was breached.

Later heating inside the left wing structure. This analy-

sis focused on the inside of the left wing after the wing leading edge spar had been breached.

Early changes in aerodynamic performance. The Or-

biter began reacting to increasing left yaw and left roll, consistent with developing drag and loss of lift on the left wing.

Later changes in aerodynamic performance. Almost

600 seconds after Entry Interface, the left-rolling tendency of the Orbiter changes to a right roll, indicating an increase in lift on the left wing. The left yaw also increased, showing increasing drag on the left wing.

¶For a complete compilation of all re-entry data, see the

¶entry Timeline (Appendix D.9). The extensive aerothermal calculations and wind tunnel tests performed to investigate the observed re-entry phenomenon are documented in NASA report NSTS-37398.

¶Re-Entry Environment

¶In the demanding environment of re-entry, the Orbiter must withstand the high temperatures generated by its movement through the increasingly dense atmosphere as it deceler- ates from orbital speeds to land safely. At these velocities, shock waves form at the nose and along the leading edges of the wing, intersecting near RCC panel 9. The interaction between these two shock waves generates extremely high temperatures, especially around RCC panel 9, which experiences the highest surface temperatures of all the RCC panels. The flow behind these shock waves is at such a high temperature that air molecules are torn apart, or "dissoci- ated." The air immediately around the leading edge surface can reach 10,000 degrees Fahrenheit; however, the boundary layer shields the Orbiter so that the actual temperature is only approximately 3,000 degrees Fahrenheit at the leading edge. The RCC panels and internal insulation protect the aluminum wing leading edge spar. A breach in one of the leading-edge RCC panels would expose the internal wing structure to temperatures well above 3,000 degrees Fahrenheit.

¶In contrast to the aerothermal environment, the aerodynamic environment during Columbiaʼs re-entry was relatively be- nign, especially early in re-entry. The re-entry dynamic pressure ranged from zero at Entry Interface to 80 pounds per square foot when the Orbiter went out of control, compared with a dynamic pressure during launch and ascent of nearly 700 pounds per square foot. However, the aerodynamic forces were increasing quickly during the final minutes of Columbiaʼs flight, and played an important role in the loss of control.

¶Orbiter Sensors

¶The Operational Flight Instrumentation monitors physical sensors and logic signals that report the status of various Orbiter functions. These sensor readings and signals are telemetered via a 128 kilobit-per-second data stream to the Mission Control Center, where engineers ascertain the real-time health of key Orbiter systems. An extensive review of this data has been key to understanding what happened to STS-107 during ascent, orbit, and re-entry.

¶The Modular Auxiliary Data System is a supplemental instrumentation system that gathers Orbiter data for processing after the mission is completed. Inputs are almost exclusively physical sensor readings of temperatures, pressures, mechanical strains, accelerations, and vibrations. The Modular Auxiliary Data System usually records only the missionʼs first and last two hours (see Figure 3.6-1).

¶The Orbiter Experiment instrumentation is an expanded suite of sensors for the Modular Auxiliary Data System that purposes. Because Columbia was the first Orbiter launched, was installed on Columbia for engineering development

¶Panel 10 Panel 9

¶Figure 3.6-1. The Modular Auxiliary Data System recorder, found near Hemphill, Texas. While not designed to withstand impact damage, the recorder was in near-perfect condition when recovered on March 19, 2003.

¶engineering teams needed a means to gather more detailed flight data to validate their calculations of conditions the vehicle would experience during critical flight phases. The instrumentation remained on Columbia as a legacy of the development process, and was still providing valuable flight data from ascent, de-orbit, and re-entry for ongoing flight analysis and vehicle engineering. Nearly all of Columbiaʼs sensors were specified to have only a 10-year shelf life, and in some cases an even shorter service life.

¶At 22 years old, the majority of the Orbiter Experiment instrumentation had been in service twice as long as its specified service life, and in fact, many sensors were already failing. Engineers planned to stop collecting and analyzing data once most of the sensors had failed, so failed sensors and wiring were not repaired. For instance, of the 181 sensors in Columbiaʼs wings,⁵⁵⁵⁵ President Ronald Reagan, "Message to the Congress on Americaʼs Volume 1, July 1994, p. 7. CAIB document CAIB015-0161. Agenda for the Future," February 6, 1986, Public Papers of the 78 Beth Dickey, "The Few, the Tired," Government Executive, April 2001, p. Presidents of the United States: Ronald Reagan: Book I-January 1 to 71. June 27, 1986 (Washington, DC: U.S. Government Printing Office, 1982- 79 Brewer, "Perfect Places," pp. 159. 1991), p. 159.Show full note had already failed or were producing questionable readings before STS-107 was launched.

¶Re-Entry Timeline

¶Times in the following section are noted in seconds elapsed from the time Columbia crossed Entry Interface (EI) over the Pacific Ocean at 8:44:09 a.m. EST. Columbiaʼs destruction occurred in the period from Entry Interface at 400,000 feet (EI+000) to about 200,000 feet (EI+970) over Texas. The Modular Auxiliary Data System recorded the first indications of problems at EI plus 270 seconds (EI+270). Because data from this system is retained onboard, Mission

¶Control did not notice any troubling indications from telemetry data until 8:54:24 a.m. (EI+613), some 10 minutes after

¶Entry Interface.

¶Left Wing Leading Edge Spar Breach (EI+270 through EI+515)

¶At EI+270, the Modular Auxiliary Data System recorded the first unusual condition while the Orbiter was still over the Pacific Ocean. Four sensors, which were all either inside

Sensor 1 WLE WLEStrain Strain- -V12G9921A V12G9921

Sensor 4 WLE WLESpar SparTemp Temp- V09T9895A - V09T9895

Sensor 2 WLE WLEClevis Clevis- -V09T9910A V09T9910

Sensor 3 Looking Aft AftPanel Panel99Lower LowerSurface SurfaceTemp Temp- -V09T9666A V09T9666 Forward

¶Figure 3.6-2. Location of sensors on the back of the left wing leading edge spar (vertical aluminum structure in picture). Also shown are the round truss tubes and ribs that provided the structural support for the mid-wing in this area.

¶or outside the wing leading edge spar near Reinforced Carbon-Carbon (RCC) panel 9-left, helped tell the story of what happened on the left wing of the Orbiter early in the re-entry. These four sensors were: strain gauge V12G9921A (Sensor 1), resistance temperature detector V09T9910A on the RCC clevis between panel 9 and 10 (Sensor 2), thermocouple V07T9666A, within a Thermal Protection System tile (Sensor 3), and resistance temperature detector V09T9895A (Sensor 4), located on the back side of the wing leading edge spar behind RCC panels 8 and 9 (see Figure 3.6-2).

¶V12G9921A – Left Wing Leading Edge Spar Strain Gauge

¶48:39

¶STS - 107

STS - 073 STS - 090

¶Strain (micro-in./in.)

¶STS - 109

¶-250

-500 First off nominal indication -750

¶-1000

0 100 200 300 400 500 600 700 800 900 — 1000

44:09 59:09 Time (seconds from EI)

¶Figure 3.6-3. The strain gauge (Sensor 1) on the back of the left wing leading edge spar was the first sensor to show an anomalous reading. In this chart, and the others that follow, the red line indicates data from STS-107. Data from other Columbia re-entries, similar to the STS-107 re-entry profile, are shown in the other colors.

¶V07T9910A – Left Wing Leading Edge Spar Temperature

¶600 48:59

STS - 073 STS - 090

¶Temperature (0F)

¶-100

-200 First off nominal indication -300

0 100 200 300 400 500 — 600

44:09 Time (seconds from EI) 59:09

¶Figure 3.6-4. This temperature thermocouple (Sensor 2) was mounted on the outside of the wing leading edge spar behind the insulation that protects the spar from radiated heat from the RCC panels. It clearly showed an off-nominal trend early in the re-entry sequence and began to show an increase in temperature much earlier than the temperature sensor behind the spar.

¶Sensor 1 provided the first anomalous reading (see Figure 3.6-3). From EI+270 to EI+360, the strain is higher than that on previous Columbia flights. At EI+450, the strain reverses, and then peaks again in a negative direction at EI+475. The strain then drops slightly, and remains constant and negative until EI+495, when the sensor pattern becomes unreliable, probably due to a propagating soft short, or "burn-through" of the insulation between cable conductors caused by heating or combustion. This strain likely indicates significant damage to the aluminum honeycomb spar. In particular, strain rever- sals, which are unusual, likely mean there was significant high-temperature damage to the spar during this time.

¶At EI+290, 20 seconds after Sensor 1 gave its first anomalous reading, Sensor 2, the only sensor in the front of the

70 Clevis Temperatures 10" Hole with Sneak Flow