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STS-87 is Home! The Post-Flight Inspection Begins
by Greg KatnikDecember 23, l997
STS-87 rolled to a stop; the mission was complete! That statement would be true for the flight of the Columbia, however a new mission began when the wheels of the Columbia came to a stop -- the post flight inspections. My division is responsible for the overall analysis of these inspections and we insure that all changes made, due to these inspections, do not affect other areas that may jeopardize the flight-worthiness of the shuttle. This division does not focus on one specific area, but analyzes all information and ensures that all aspects are kept in balance.
Immediately after the Columbia rolled to a stop, the inspection crews began the process of the post flight inspection. As soon as the orbiter was approached, light spots in the tiles were observed indicating that there had been significant damage to the tiles. The tiles do a fantastic job of repelling heat, however they are very fragile and susceptible to impact damage. Damage numbering up to forty tiles is considered normal on each mission due to ice dropping off of the external tank (ET) and plume re-circulation causing this debris to impact with the tiles. But the extent of damage at the conclusion of this mission was not "normal."
The pattern of hits did not follow aerodynamic expectations, and the number, size and severity of hits were abnormal. Three hundred and eight hits were counted during the inspection, one-hundred and thirty two (132) were greater than one inch. Some of the hits measured fifteen (15) inches long with depths measuring up to one and one-half (1 1/2) inches. Considering that the depth of the tile is two (2) inches, a 75% penetration depth had been reached. Over one hundred (100) tiles have been removed from the Columbia because they were irreparable. The inspection revealed the damage, now the "detective process" began.
During the STS-87 mission, there was a change made on the external tank. Because of NASA's goal to use environmentally friendly products, a new method of "foaming" the external tank had been used for this mission and the STS-86 mission. It is suspected that large amounts of foam separated from the external tank and impacted the orbiter. This caused significant damage to the protective tiles of the orbiter. Foam cause damage to a ceramic tile?! That seems unlikely, however when that foam is combined with a flight velocity between speeds of MACH two to MACH four, it becomes a projectile with incredible damage potential. The big question? At what phase of the flight did it happen and what changes need to be made to correct this for future missions? I will explain the entire process.
The questions that needed to be answered were:
- what happened?
- what phase of flight did it happen in?
- why did it happen?
- what corrective action is required?
At this point, virtually every inch of the orbiter was inspected and all hits were documented and mapped to aid in visualizing the damage. Maps were constructed of the lower surface, the left and right surfaces and the top surface of the orbiter. At this point, a "fault tree" was created. The fault tree provides a systematic approach in considering all possibilities of what may have happened. Everything that is on the fault tree is considered to be legitimate until it is totally ruled out. Some of the considerations were where the damage occurred -- in the OPF, in the VAB, or on the pad before launch. These were quickly eliminated because an inspection at T-3 ("t minus three") hours takes place on each mission and everything was normal.
After these and many other considerations were eliminated, the focus was placed on the ascent, orbit and re-entry phase of the mission. Because of the fore and aft flow characteristics of the damage sites, and the angle of penetration, the ascent phase seemed most likely. The orbit phase of flight was eliminated because the characteristics of these types of hits (most likely meteorites or space debris) occur in a random pattern and direction. Re-entry was eliminated because the "glazing and re-glassifying" of the tiles due to heat upon re-entry (a normal process) indicated that the damage had occurred prior to this phase. The fault-tree was now pointing to the ascent phase.
The pictures that were taken by cameras mounted in the orbiter umbilical began to give the first clues. These cameras are designed to turn on during the solid rocket booster (SRB) separation, and turn off after the separation is complete, thereby recording the event. This process occurs once again when the external tank separates from the orbiter. The initial review of these photographs did not reveal any obvious damage to the external tank. No foam missing, no "divots" (holes) and no material loss. Everything appeared normal.
The SRBs were then focused on for the answers. After inspection of the SRBs, no clues were found. In fact, the solid rocket boosters looked to be in great condition. Where to now? The external tank photographs were magnified and reviewed once again. This time some material loss was noted, but not in a significant degree. The attention was now focused on the crew cabin cameras. These cameras gave more of a side view of the external tank as it tumbled back to Earth. These photographs revealed massive material loss on a side of the external tank that could not be viewed by the umbilical cameras!
Where are we now? One of the questions had now been answered. The ascent phase of flight was when the damage occurred. With the information provided by the photography and the mapped flow of damage, a logical reason could be established as to "what" happened. It was determined that during the ascent, the foam separation from the external tank was carried by the aerodynamic flow and pelted the nose of the orbiter and cascaded aft from that point. Once again, this foam was carried in a relative air-stream between MACH two and MACH 4!
Now the big question -- why? The evidence of this conclusion has now been forwarded to Marshall Space Flight Center (MSFC) because this is the design center for the external tank. MSFC will pursue the cause of damage. Here are some descriptions of some of the considerations:
- The primer that bonds the tank foam to the metal sub-stream was defective and did not set properly. This was eliminated as a cause because the photography indicated that the areas of foam loss (divots) did not protrude all the way down to the primer.
- The aerodynamics of the roll to "heads up." The STS-87 mission was the first time this maneuver had ever been completed.
- The STS-86 mission revealed a similar damage pattern but to a much lesser degree than STS-87. The STS-86 tile damage was accepted ruled as an unexplained anomaly because it was a night launch and did not provide the opportunity for the photographic evidence the STS-87 mission did. A review of the records of the STS-86 records revealed that a change to the type of foam was used on the external tank. This event is significant because the pattern of damage on this flight was similar to STS-87 but to a much lesser degree. The reason for the change in the type of foam is due to the desire of NASA to use "environmentally friendly" materials in the space program. Freon was used in the production of the previous foam. This method was eliminated in favor of foam that did not require freon for its production. MSFC is investigating the consideration that some characteristics of the new foam may not be known for the ascent environment.
- Another consideration is cryogenic loading, specifically hydrogen (-423 degrees Fahrenheit) and oxygen (-297 degrees Fahrenheit). These extreme temperatures cause the external tank to shrink up to six (6) linear inches while it is on the pad prior to launch. Even though this may not seem much when compared to the circumference of the external tank, six inches of shrinkage is significant.
As this investigation continues, I am very comfortable that the questions will be answered and the solutions applied. In fact, some of the solutions are already in progress. At present the foam on the sides of the tank is being sanded down to the nominal minimum thickness. This removes the outer surface, which is tougher than the foam core, and lessens the amount of foam that can separate and hit the orbiter.
