THE ORBITER "RAN INTO" THE FOAM

¶"How could a lightweight piece of foam travel so fast and hit the wing at 545 miles per hour?"

¶Just prior to separating from the External Tank, the foam was traveling with the Shuttle stack at about 1,568 mph (2,300 feet per second). Visual evidence shows that the foam debris impacted the wing approximately 0.161 seconds after separating from the External Tank. In that time, the velocity of the foam debris slowed from 1,568 mph to about 1,022 mph (1,500 feet per second). Therefore, the Orbiter hit the foam with a relative velocity of about 545 mph (800 feet per second). In essence, the foam debris slowed down and the

¶Orbiter did not, so the Orbiter ran into the foam. The foam slowed down rapidly because such low-density objects have low ballistic coefficients, which means their speed rapidly decreases when they lose their means of propulsion.

¶with very little component of velocity away from the wing.

¶Minimum Maximum Best Estimated Minimum Maximum Best Estimated

Impact Speed Impact (mph) Speed (mph) (mph)

During STS-107 375 654 — 477
After STS-107 528 559 — 528

¶Figure 3.4-5. The best estimates of velocities and volumes calculated during the mission and after the accident based on visual evidence and computer analyses. Information available during the mission was adequate to determine the foamʼs effect on both thermal tiles and RCC.

¶cameras are not operating or, as in the case of STS-107, out of focus. Launch Commit Criteria should include that sufficient cameras are operating to track the Shuttle from liftoff to Solid Rocket Booster separation.

¶Similarly, a developmental vehicle like the Shuttle should be equipped with high resolution cameras that monitor potential hazard areas. The wing leading edge system, the area around the landing gear doors, and other critical Thermal Protection System elements need to be imaged to check for damage. Debris sources, such as the External Tank, also need to be monitored. Such critical images need to be downlinked so that potential problems are identified as soon as possible.

¶Transport Analysis: Establishing Foam Path by Computational Fluid Dynamics

¶Transport analysis is the process of determining the path of the foam. To refine the Boardʼs understanding of the foam strike, a transport analysis team, consisting of members from Johnson Space Center, Ames Research Center, and Boeing, augmented the image analysis teamʼs research.

¶A variety of computer models were used to estimate the volume of the foam, as well as to refine the estimates of its velocity, its other dimensions, and the impact location. Figure 3.4-5 lists the velocity and foam size estimates produced during the mission and at the conclusion of the investigation.

¶The results listed in Figure 3.4-5 demonstrate that reason- ably accurate estimates of the foam size and impact velocity were available during the mission. Despite the lack of high- quality visual evidence, the input data available to assess the impact damage during the mission was adequate.

¶The input data to the transport analysis consisted of the computed airflow around the Shuttle stack when the foam was shed, the estimated aerodynamic characteristics of the foam, the image analysis teamʼs trajectory estimates, and the size and shape of the bipod ramp.

¶The transport analysis team screened several of the image analysis teamʼs location estimates, based on the feasible aerodynamic characteristics of the foam and the laws of physics. Optical distortions caused by the atmospheric density gradients associated with the shock waves off the Orbiterʼs nose, External Tank, and Solid Rocket Boosters may have compromised the image analysis teamʼs three position estimates closest to the bipod ramp. In addition, the image analysis teamʼs position estimates closest to the wing were compromised by the lack of two camera views and the shock

¶t Speed Volume Volume Volume mph) (cubic inches) (cubic inches) (cubic inches) 477 400 1,920 1,200 528 1,026 1,239 1,200

region ahead of the wing, making triangulation impossible and requiring extrapolation. However, the transport analysis confirmed that the image analysis teamʼs estimates for the central portion of the foam trajectory were well within the computed flow field and the estimated range of aerodynamic characteristics of the foam.

The team identified a relatively narrow range of foam impact velocities and ballistic coefficients. The ballistic coefficient of an object expresses the relative influence of weight and atmospheric drag on it, and is the primary aerodynamic characteristic of an object that does not produce lift. An object with a large ballistic coefficient, such as a cannon ball, has a trajectory that can be computed fairly accurately without accounting for drag. In contrast, the foam that struck the wing had a relatively small ballistic coefficient with a large drag force relative to its weight, which explains why it slowed down quickly after separating from the External Tank. Just prior to separation, the speed of the foam was equal to the speed of the Shuttle, about 1,568 mph (2,300 feet per second). Because of a large drag force, the foam slowed to about 1,022 mph (1,500 feet per second) in about 0.2 seconds, and the Shuttle struck the foam at a relative

Figure 3.4-6. These are the results of a trajectory analysis that used a computational fluid dynamics approach in a program called CART-3D, a comprehensive (six-degree-of-freedom) computer simulation based on the laws of physics. This analysis used the aerodynamic and mass properties of bipod ramp foam, coupled with the complex flow field during ascent, to determine the likely position and velocity histories of the foam.

¶on-board engineering quality imaging from the

¶F3.4-4

¶Velocity (ft/sec)

¶F3.4-5

¶F3.4-6

0.5 1 1.5 2 2.5 Ballistic Number (psf)

¶R3.4-1 Figure 3.4-7. The results of numerous possible trajectories based on various assumed sizes, shapes, and densities of the foam. Either the foam had a slightly higher ballistic coefficient and the Orbiter struck the foam at a lower speed relative to the Orbiter, or the foam was more compact and the wing struck the foam at a higher speed. The "best fit" box represents the overlay of the data from the image analysis with the transport analysis computations. This data enabled a final selection of projectile characteristics for impact testing. R3.4-2 speed of about 545 mph (800 feet per second). (See Ap- R3.4-3 pendix D.8.)

¶The undetermined and yet certainly irregular shape of the foam introduced substantial uncertainty about its estimated aerodynamic characteristics. Appendix D.8 contains an independent analysis conducted by the Board to confirm that the estimated range of ballistic coefficients of the foam in Figure 3.4-6 was credible, given the foam dimension results from the image analyses and the expected range of the foam density. Based on the results in Figure 3.4-7, the physical dimensions of the bipod ramp, and the sizes and shapes of the available barrels for the compressed-gas gun used in the impact test program described later in this chapter, the Board and the NASA Accident Investigation Team decided that a foam projectile 19 inches by 11.5 inches by 5.5 inches, weighing 1.67 pounds, and with a weight density of 2.4 pounds per cubic foot, would best represent the piece of foam that separated from the External Tank bipod ramp and was hit by the Orbiterʼs left wing. See Section 3.8 for a full discussion of the foam impact testing.

¶Findings:

¶F3.4-1 Photographic evidence during ascent indicates

the projectile that struck the Orbiter was the left bipod ramp foam.

¶F3.4-2 The same photographic evidence, confirmed by

independent analysis, indicates the projectile struck the underside of the leading edge of the left wing in the vicinity of RCC panels 6 through 9 or the tiles directly behind, with a velocity of approximately 775 feet per second.

¶Shuttle during launch and ascent.

¶F3.4-4 The current long-range camera assets on the Kennedy Space Center and Eastern Range do not provide best possible engineering data during Space

¶Shuttle ascents.

¶F3.4-5 Evaluation of STS-107 debris impact was hampered by lack of high resolution, high speed cameras (temporal and spatial imagery data). F3.4-6 Despite the lack of high quality visual evidence,

the information available about the foam impact during the mission was adequate to determine its effect on both the thermal tiles and RCC.

¶Recommendations:

¶R3.4-1 Upgrade the imaging system to be capable of

providing a minimum of three useful views of the Space Shuttle from liftoff to at least Solid Rocket Booster separation, along any expected ascent azimuth. The operational status of these assets should be included in the Launch Commit Criteria for future launches. Consider using ships or aircraft to provide additional views of the Shuttle during ascent.

¶R3.4-2 Provide a capability to obtain and downlink high-

resolution images of the External Tank after it separates.

¶R3.4-3 Provide a capability to obtain and downlink high-

resolution images of the underside of the Orbiter wing leading edge and forward section of both wingsʼ Thermal Protection System.

3.5 ON-ORBIT DEBRIS SEPARATION – THE "FLIGHT DAY 2" OBJECT

¶Immediately after the accident, Air Force Space Command began an in-depth review of its Space Surveillance Network data to determine if there were any detectable anomalies during the STS-107 mission. A review of the data resulted in no information regarding damage to the Orbiter. However, Air Force processing of Space Surveillance Network data yielded 3,180 separate radar or optical observations of the Orbiter from radar sites at Eglin, Beale, and Kirtland Air Force Bases, Cape Cod Air Force Station, the Air Force Space Commandʼs Maui Space Surveillance System in Hawaii, and the Navy Space Surveillance System. These observations, examined after the accident, showed a small object in orbit with Columbia. In accordance with the International Designator system, the object was named 2003- 003B (Columbia was designated 2003-003A). The timeline of significant events includes:

January 17, 2003, 9:42 a.m. Eastern Standard Time:

Orbiter moves from tail-first to right-wing-first orientation

January 17, 10:17 a.m.: Orbiter returns to tail-first orientation
January 17, 3:57 p.m.: First confirmed sensor track of object 2003-003B
January 17, 4:46 p.m.: Last confirmed sensor track for

¶F3.4-3 There is a requirement to obtain and downlink this date

January 18: Object reacquired and tracked by Cape

¶Cod Air Force Station PAVE PAWS

January 19: Object reacquired and tracked by Space

¶Surveillance Network

January 20, 8:45 – 11:45 p.m.: 2003-003B orbit decays. Last track by Navy Space Surveillance System

¶Events around the estimated separation time of the object were reviewed in great detail. Extensive on-board sensor data indicates that no unusual crew activities, telemetry data, or accelerations in Orbiter or payload can account for the release of an object. No external mechanical systems were active, nor were any translational (forward, backward, or sideways, as opposed to rotational) maneuvers attempted in this period. However, two attitude maneuvers were made: a 48-degree yaw maneuver to a left-wing-forward and payload-bay-to-Earth attitude from 9:42 to 9:46 a.m. EST), and