Science - Space Shuttle, Nasa's Gamble on Safe Re-Entry at 3000 F (30,000 Tiles, K Each!)
Whereas the notion 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 security - for the envisaged Space transportation System, the designation "Space Shuttle" described the objective - a quick turnaround, with minimum refurbishment of traditional subsystems. To insulate the Orbiter from the searing heat of reentry, the idealized clarification is a "vacuum", which blocks all heat transference. The vacuum would be encased in a thin glass-wall box-like box - internal stabilization maintain 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 perform optimized thermal insulation) would be as thin as practical (and small in horizontal dimensions). This would adapt Orbiter outside curvatures - requisite to perform an widespread 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.
Science - Space Shuttle, Nasa's Gamble on Safe Re-Entry at 3000 F (30,000 Tiles, K Each!)
To perform the insulation properties desired, vacuum processing would be requisite (actually, a duplicate vacuum became part of the output manufacturing process, industrialized by Nasa and Lockheed Corporation). As every aspect theoretically feasible, the challenge was the smallest practical tile size that was producible in volume. The proper black (high-temperature-exposure) tile is highly light, six by six inches in size and about an inch and three-quarters in height. The tiles are bonded to the Orbiter buildings by proper Rtv (Room temperature Vulcanizing) adhesive.
Square and close-fitting to eliminate even narrow gaps between tiles, thus to prevent 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 succeed was clearly worth the exertion - along with the monumental cost. The Tps tile theory 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 succeed 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 minuscule speed-altitude transition period, and gently slows the Orbiter down, ultimately reaching ground level and landing speeds of a typical aircraft.
The Tps theory 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 right away 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 discussion table, seeing 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 right away linked 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 discussion 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 town 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 prominent 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 - assuredly due to stray bits of gravel, churned up from the concrete activate pad (despite rigorous vacuuming before each flight). The estimate of damaged tiles was apparently not too worrisome - however, anyone untoward about tiles (Thermal Protective System) had to be fully understood. The cost of the Tps was approximately 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 notion 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 request 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 accountability was the Orbiter car structure, all things 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 habitancy standing guard. I got out of the jeep and walked quickly to it. The sun was enthralling and I quickly saw what had been described, the line of damaged tiles - six by six inches of pure white, the compressed quartz fibers seeing 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 anyone 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 under the wing-fuselage - an approximately flat, huge expanse of solid black, not a singular damaged tile. As we walked nearby the car seeing anyone else looked unusual, Dr. Kraft said, "We've got to make an valid narrative to the Administrator, Congress and the press, but I want an engineering prognosis 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 potential 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 activate 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 buildings to spill overflowing rain-water onto the left wing - Columbia standing vertically in the activate mode. Rainwater, therefore, would have run along the prominent edge of the left wing. Occasionally hairline cracks were known to construct through the thin glass outer (black) coating of the tiles: whether 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 highly 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 frosty due to the requisite volumetric expansion from fluid to ice; during reentry, when the ice began to melt, becoming a gas, the drastic volume growth bursting the glass casing; when the car went through the sonic boom, the outside jolt of air pressure causing any remaining chunk of solidified filaments to be ejected from the top of the tile; or when the car landed, the vibration shaking any remaining ice-quartz chunks loose. Conclusion: this unusual set of circumstances caused unique damage to a small estimate of tiles - however, the "thermal protection" function was not compromised, in fact the continued functionality of the damaged tiles adds reliance to the reliability of the widespread Sts program. Recommendation: when rain precedes a launch, a tarpaulin should be used to safe the tiles.
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