Whereas the concept for shielding astronauts and their space vehicles from the extremes of reentry heat for the first vehicles in space, Mercury and Apollo, was "overkill" assurance, an excess of ablatable fiberglass and resin to insure safety - for the envisaged Space Transportation System, the designation "Space Shuttle" described the objective - a quick turnaround, with minimum refurbishment of primary subsystems. To insulate the Orbiter from the searing heat of reentry, the idealized solution is a "vacuum", which blocks all heat transference. The vacuum would be encased in a thin glass-wall box-like container - internal stabilization support of the thin, flat glass surfaces would be by quartz filaments (non-thermally conductive and light in weight) "scrunched" into the glass "box", which would then be vacuumized and sealed. Idealizing again, this "package" (tile - containing all functioning elements to achieve optimized thermal insulation) would be as thin as practical (and small in horizontal dimensions). This would accommodate Orbiter surface curvatures - necessary to achieve an overall aerodynamic shape so as to "fly" during the landing phase of a mission. Finally, the tile would be bonded (the cool - insulated bottom) to the vehicle's basic aluminum structure.
To achieve the insulation properties desired, vacuum processing would be essential (actually, a double vacuum became part of the production manufacturing process, developed by NASA and Lockheed Corporation). As every aspect theoretically feasible, the challenge was the smallest practical tile size that was producible in volume. The standard black (high-temperature-exposure) tile is extremely light, six by six inches in size and about an inch and three-quarters in height. The tiles are bonded to the Orbiter structure by standard RTV (Room Temperature Vulcanizing) adhesive.
Square and close-fitting to eliminate even narrow gaps between tiles, thus to preclude pockets of heated air, quartz-fabric "gap-fillers" are stuffed between tiles - a painstakingly tedious job. There are 30,000 black tiles (seen in all photos of a Space Shuttle). The end result was clearly worth the effort - including the monumental cost. The TPS tile system has proved its reliability, taming reentry heat (temperatures of from 2200 to 3000 degrees Fahrenheit).
The reentry heat develops from the compression of atmospheric gas caused by the Orbiter's speed, as it descends from the vacuum of outer space - it is not the result of friction. As the Orbiter speeds down (18,000 mph) into Earth's atmosphere, air molecules are impacted by the Orbiter, causing tiny pulses of heat and drag; this causes wing leading-edge temperatures to rise to from 2200°F to 3000°F (depending upon reentry angle) - during approximately a six minute speed-altitude transition period, and gradually slows the Orbiter down, eventually reaching ground level and landing speeds of a typical aircraft.
The TPS system made the Space Shuttle program possible.
The voice over the loudspeaker was concluding, "So congratulations to all of us on the Shuttle team, STS-2 appears to have been a great success, and - ", Another voice abruptly cut in, "Rockwell Space, Will Mr. (my name) please call Moser (my NASA counterpart) at the Cape."
At mention of my name, I rose from the conference table, looking at my boss, Chief Engineer, and his boss, president of Rockwell Space Division. The president signaled me, then pointed to the door to his office, which was open, his secretary standing in the doorway. She nodded to me and pointed to a phone. I made the call, the operator at the Cape was waiting, and promptly connected me. Our greetings were terse - Moser spoke for about five minutes, I asked a few questions. Then he said, "Cris Kraft is at Edwards, he wants you to drive up now, maybe we can get a head-start on what happened before they ferry Columbia back to the Cape." I said I'd be up at Edwards (Air Force Base) in about two hours.
Back in the conference room, I summarized the call, "Some tile damage was noted on the walk-around-inspection after landing. No indication of a TPS malfunction during reentry, but the tile damage looks unusual - Dr. Kraft (Director, Johnson Space Center at Houston) is up there - they want me to drive up now." The Chief Engineer nodded. "My secretary will call your wife."
I drove automatically, knowing well the freeway route - my mind reviewing what I'd been told. NASA had taken pictures of the damaged tiles on the walk-around - NASA wanted a knowledgeable "eyeball" look before the ferry flight home. The damage looked like chunks of the tile's insides, an inch or two, was missing. And all such damaged tiles seemed to be along the left wing leading edge, at varying distances apart. There was also other typical tile damage, the pits and break-throughs of the thin black glass, as after the STS-1 flight - undoubtedly due to stray bits of gravel, churned up from the concrete launch pad (despite careful vacuuming before each flight). The number of damaged tiles was apparently not too worrisome - however, anything untoward about tiles (Thermal Protective System) had to be thoroughly understood. The cost of the TPS was almost unbelievable, but their function, protecting against fiery reentry, as well as their compactness, permitting the Orbiter to function like an airplane - was what made viable the potential of the Space Shuttle program.
As I drove, I thought of Dr. Chris Kraft and the near-disaster of Apollo 13. His was the voice of calm authority that had buoyed up America and the desperate astronauts aboard, during the hectic days when no-one knew whether the damaged capsule could be "jerry-rigged" for a safe return to Earth. There was a degree of satisfaction in his asking for me - but then NASA had given me the chore of presenting the TPS briefings at all NASA Flight Readiness Reviews for two years. As Assistant Chief Engineer, my responsibility was the Orbiter vehicle structure, everything but the engines and electronic systems: Design, Stress, Aero, Thermo, Dynamics, Weights, and Materials and Processes, which included the tiles.
I turned into Edwards Air Force Base, parked my car, then saw a jeep speeding towards me. The driver was an engineer I recognized, and a security guard. I climbed in and we took off. The Orbiter was where it had landed, roped off, with a half-dozen security people standing guard. I got out of the jeep and walked quickly to it. The sun was bright and I quickly saw what had been described, the line of damaged tiles - six by six inches of pure white, the compressed quartz fibers looking like Styrofoam - in the midst of solid arrays of black tiles.
"So, what do you think?" The voice was familiar. I turned and said, "Hi, Dr. Kraft" We shook hands.
"Never saw anything like it," I said, "nothing like it in all our tests - it's as if a chunk of the quartz filaments broke away from the inside." The engineer handed me a set of photos of the damaged tiles. Together we walked along the wing, then ducked under the ropes to look beneath the wing-fuselage - an almost flat, huge expanse of solid black, not a single damaged tile. As we walked around the vehicle seeing whatever else looked unusual, Dr. Kraft said, "We've got to make an official report to the Administrator, Congress and the press, but I want an engineering analysis first. Send us a top tile engineer to be at the Cape day after tomorrow - we'll get a NASA man also, then, plus you and Moser, I want an internal layout of possible causes in a week."
That's what happened. The investigation became a "detective" scenario, puzzling out clues, conjecturing what could have happened, verification by test. A key factor was the photos of the Shuttle on the launch pad with its adjacent work platform - the night before launch, as a rainstorm drenched platform and Shuttle.
A week later, a briefing was given to the NASA Base Directors and Administrator:
- The rainstorm was from a direction which caused a platform on the work-assembly structure to spill overflowing rain-water onto the left wing - Columbia standing vertically in the launch mode. Rainwater, therefore, would have run along the leading edge of the left wing.
- Occasionally hairline cracks were known to develop through the thin glass outer (black) coating of the tiles: either during manufacture; during the bonding process of attaching each tile to the airframe structure; when stuffing "gap-fillers"; or possibly even during the prior STS-1 flight (close packing of tiles causing occasional excess pressure on the thin glass walls during the vibration of landing).
- If, therefore, some of the running rainwater encountered a tile with a hairline crack - during the rainy night, some water therefore entered the tile.
- During the flight mission, the left wing was pointed to deep outer space for many hours, the temperatures of the wing tiles therefore dropping to extremely low values, approaching absolute zero. Water in a tile would therefore hard-freeze into ice, becoming a clump of filamentary quartz - as the ice formed from liquid water, the volumetric expansion would cause internal pressure to be exerted upon the encasing of thin glass.
- The ice-filamentary clump could therefore crack the glass casing and be ejected from the tile - occurring during one of the following events: upon freezing due to the significant volumetric expansion from fluid to ice; during reentry, when the ice began to melt, becoming a gas, the drastic volume increase bursting the glass casing; when the vehicle went through the sonic boom, the surface jolt of air pressure causing any remaining chunk of solidified filaments to be ejected from the top of the tile; or when the vehicle landed, the vibration shaking any remaining ice-quartz chunks loose.
- Conclusion: this unusual set of circumstances caused unique damage to a small number of tiles - however, the "thermal protection" function was not compromised, in fact the continued functionality of the damaged tiles adds confidence to the reliability of the overall STS program.
- Recommendation: when rain precedes a launch, a tarpaulin should be used to protect the tiles.
Aaron Kolom qualifies as a "rocket scientist" with over 50 years aerospace engineering: Stress Analyst to Chief of Structural Sciences on numerous military aircraft, to Corp. Director Structures and Materials, Asst. Chief Engineer Space Shuttle Program through first three flights (awarded NASA Public Service Medal), Rockwell International Corp.; Program Manager Concorde SST, VP Engineering TRE Corp.; Aerospace Consultant.
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