CNN
Posted on 02/01/2003 10:18:41 AM PST by Timesink
Seven astronauts, including the first Israeli in space, were lost Saturday when the space shuttle Columbia broke apart in the skies of Texas. The incident occurred at an altitude of some 200,000 feet, shortly after reentry and 15 minutes before Columbia had been scheduled to land at Cape Canaveral. TIME science correspondent Jeffrey Kluger explains some of the possible causes and consequences of the accident:
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TIME.com: What are the possible scenarios that could have caused this disastrous accident on the shuttle's reentry into the Earth's atmosphere?
Jeffrey Kluger: There are three possible scenarios that explain this event. The first, which I believe is the likeliest explanation, would be an aerodynamic structural breakup of the shuttle caused by it rolling at the wrong angle. Remember, after reentry, the shuttle is descending without power, which means astronauts at the controls can't compensate for a loss of attitude by using the engines, they can only do so using the flaps. And that's extremely hard. Astronauts describe piloting the shuttle on reentry as like trying to fly a brick with wings. It's very difficult to operate, and even more so to correct any problems.
A second explanation might be a loss of tiles leading to a burn-through. (The shuttle is covered with heat-resistant tiles to protect the craft and those inside it from burning up in the scorching temperatures caused by the friction of reentry.) But I think that explanation is unlikely, because the tile-loss would have had to have been quite substantial for that to become possible. You'll hear a lot in the next few days about things falling off the shuttle during liftoff. But it often happens that they lose a few tiles, and I'd be surprised if it happened on a scale that could make an accident of this type possible.
The last option is some kind of engine failure leading to fuel ignition. Although the main tanks are mostly empty, there should still be fuel left in the maneuvering tanks. But probably not enough for an explosion that could have caused this breakup.
And just in case anybody was wondering, you can almost certainly rule out terrorism as a cause. This incident occurred well above the range of shoulder-fired missiles. And it would probably be easier to sneak a bomb onto Air Force One than to get one onto the shuttle.
TIME.com: So is reentry the Achilles heel of the shuttle program?
JK: No, the Achilles heel has always been liftoff, and the dangers posed by massive fuel load involved. Reentry has, of course, always been a difficult part of the space program. But this is, in fact, our first fatal accident on reentry. Apollo 13 is remembered as our most difficult ever reentry, but the ship and crew survived. The Soviets lost a crew on reentry in 1970 after an oxygen leak that caused the cosmonauts to suffocate on the way down. Reentry is a very difficult process, but the Russians mastered it in 1961 and we did the same a few years later.
TIME.com: Are shuttle crews trained to respond to the scenarios you've described?
JK: Yes, they're trained to deal with loss of attitude on reentry, and a range of other emergencies. But astronauts are not trained to deal with situations that result in certain death, because that would be a bit like training for what you might do if your car went over a cliff — in some situations there simply isn't anything you can do. One irony, though, is that NASA hadn't trained astronauts to deal with the sort of quadruple failure that occurred in Apollo 13, because they assumed that such a scenario would result in certain death. But the astronauts survived.
TIME.com: What are the immediate implications for the space program of Saturday's disaster?
JK: Following the precedent of the Challenger disaster in 1996, it's unlikely that NASA will undertake any further shuttle missions or any other manned space flights for the next two years. One immediate problem, though, is the International Space Station, which currently has a crew of three on board. They might consider one further flight to bring that crew home — the other option would be for them to return aboard a Russian Soyuz craft, which isn't the most comfortable or the safest ride. Beyond that, however, the space station is likely to be left unoccupied for a long time. NASA won't want to use the shuttle again until it can establish the cause of today's accident, and fix it. Now that we've lost two shuttles out of a fleet of five, it's even conceivable that the shuttle won't fly again. The shuttle was built as a space truck, and then the International Space Station was built to give it something to do. Both programs are likely to suffer as a result of this disaster.
The largest and heaviest (when loaded) element of the space shuttle, the ET has three major components: the forward liquid oxygen tank, an unpressurized intertank that contains most of the electrical components, and the aft liquid hydrogen tank. The ET is 153.8 feet long and has a diameter of 27.6 feet.
Beginning with the STS-6 mission, a lightweight ET was introduced. Although future tanks may vary slightly, each will weigh approximately 66,000 pounds inert. The last heavyweight tank, flown on STS-7, weighed approximately 77,000 pounds inert. For each pound of weight reduced from the ET, the cargo-carrying capability of the space shuttle spacecraft is increased almost one pound. The weight reduction was accomplished by eliminating portions of stringers (structural stiffeners running the length of the hydrogen tank), using fewer stiffener rings and by modifying major frames in the hydrogen tank. Also, significant portions of the tank are milled differently to reduce thickness, and the weight of the ET's aft solid rocket booster attachments were reduced by using a stronger, yet lighter and less expensive titanium alloy. Earlier several hundred pounds were eliminated by deleting the anti-geyser line. The line paralleled the oxygen feed line and provided a circulation path for liquid oxygen to reduce accumulation of gaseous oxygen in the feed line while the oxygen tank was being filled before launch. After propellant loading data from ground tests and the first few space shuttle missions was assessed, the anti- geyser line was removed for STS-5 and subsequent missions. The total length and diameter of the ET remain unchanged.
The ET is attached to the orbiter at one forward attachment point and two aft points. In the aft attachment area, there are also umbilicals that carry fluids, gases, electrical signals and electrical power between the tank and the orbiter. Electrical signals and controls between the orbiter and the two solid rocket boosters also are routed through those umbilicals.
The liquid oxygen tank is an aluminum monocoque structure composed of a fusion-welded assembly of preformed, chem-milled gores, panels, machined fittings and ring chords. It operates in a pressure range of 20 to 22 psig. The tank contains anti-slosh and anti-vortex provisions to minimize liquid residuals and damp fluid motion. The tank feeds into a 17-inch- diameter feed line that conveys the liquid oxygen through the intertank, then outside the ET to the aft right-hand ET / orbiter disconnect umbilical. The 17-inch-diameter feed line permits liquid oxygen to flow at approximately 2,787 pounds per second with the SSMEs operating at 104 percent or permits a maximum flow of 17,592 gallons per minute. The liquid oxygen tank's double-wedge nose cone reduces drag and heating, contains the vehicle's ascent air data system (for nine tanks only) and serves as a lightning rod. The liquid oxygen tank's volume is 19,563 cubic feet. It is 331 inches in diameter, 592 inches long and weighs 12,000 pounds empty.
The intertank is a steel / aluminum semimonocoque cylindrical structure with flanges on each end for joining the liquid oxygen and liquid hydrogen tanks. The intertank houses ET instrumentation components and provides an umbilical plate that interfaces with the ground facility arm for purge gas supply, hazardous gas detection and hydrogen gas boiloff during ground operations. It consists of mechanically joined skin, stringers and machined panels of aluminum alloy. The intertank is vented during flight. The intertank contains the forward SRB-ET attach thrust beam and fittings that distribute the SRB loads to the liquid oxygen and liquid hydrogen tanks. The intertank is 270 inches long, 331 inches in diameter and weighs 12,100 pounds.
Each propellant tank has a vent and relief valve at its forward end. This dual-function valve can be opened by ground support equipment for the vent function during prelaunch and can open during flight when the ullage (empty space) pressure of the liquid hydrogen tank reaches 38 psig or the ullage pressure of the liquid oxygen tank reaches 25 psig.
The liquid oxygen tank contains a separate, pyrotechnically operated, propulsive tumble vent valve at its forward end. At separation, the liquid oxygen tumble vent valve is opened, providing impulse to assist in the separation maneuver and more positive control of the entry aerodynamics of the ET.
There are eight propellant-depletion sensors, four each for fuel and oxidizer. The fuel-depletion sensors are located in the bottom of the fuel tank. The oxidizer sensors are mounted in the orbiter liquid oxygen feed line manifold downstream of the feed line disconnect. During SSME thrusting, the orbiter general-purpose computers constantly compute the instantaneous mass of the vehicle due to the usage of the propellants. Normally, main engine cutoff is based on a predetermined velocity; however, if any two of the fuel or oxidizer sensors sense a dry condition, the engines will be shut down.
The locations of the liquid oxygen sensors allow the maximum amount of oxidizer to be consumed in the engines, while allowing sufficient time to shut down the engines before the oxidizer pumps cavitate (run dry). In addition, 1,100 pounds of liquid hydrogen are loaded over and above that required by the 6-1 oxidizer / fuel engine mixture ratio. This assures that MECO from the depletion sensors is fuel-rich; oxidizer-rich engine shutdowns can cause burning and severe erosion of engine components.
Four pressure transducers located at the top of the liquid oxygen and liquid hydrogen tanks monitor the ullage pressures.
Each of the two aft external tank umbilical plates mate with a corresponding plate on the orbiter. The plates help maintain alignment among the umbilicals. Physical strength at the umbilical plates is provided by bolting corresponding umbilical plates together. When the orbiter GPCs command external tank separation, the bolts are severed by pyrotechnic devices.
The ET has five propellant umbilical valves that interface with orbiter umbilicals: two for the liquid oxygen tank and three for the liquid hydrogen tank. One of the liquid oxygen tank umbilical valves is for liquid oxygen, the other for gaseous oxygen. The liquid hydrogen tank umbilical has two valves for liquid and one for gas. The intermediate-diameter liquid hydrogen umbilical is a recirculation umbilical used only during the liquid hydrogen chill-down sequence during prelaunch.
The ET also has two electrical umbilicals that carry electrical power from the orbiter to the tank and the two SRBs and provide information from the SRBs and ET to the orbiter.
A swing-arm-mounted cap to the fixed service structure covers the oxygen tank vent on top of the ET during the countdown and is retracted about two minutes before lift- off. The cap siphons off oxygen vapor that threatens to form large ice on the ET, thus protecting the orbiter's thermal protection system during launch.
Various parameters are monitored and displayed on the flight deck display and control panel and are transmitted to the ground.
The contractor for the external tank is Martin Marietta Aero space, New Orleans, La. The tank is manufactured at Michoud, La. Motorola, Inc., Scottsdale, Ariz., is the contractor for range safety receivers.
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You may have a piece of SIRCA from the leading edge. I know that NASA was experimenting with it a few years ago. SIRCA = Silicon Impregnated Reusable Ceramic Ablative. It is not a true "ablative" -- When brand new it is white. After its first use the surface layer heats up and ablates to form a hard black surface which provides a standoff protecting the rest of the leading edge tile.
In all likelihood, every bit and byte of data, up to the moment of explosion, should be recovered and serve as the most reliable source for nailing down the cause of this tragedy.
And what a wonderful address just now by the President...what a man.
Thanks! My specialty is mission control/operations, not thermal design. It was given to me a few years ago.
Amen to that. "We know that they didn't return to Earth but they did return safely home." Maybe I screwed that up a little but that is a beautiful thought.
The External Tank (ET) is the only non-reusable major component of the Space Shuttle system that consists of the ET, the Orbiter and the two Solid Rocket Boosters. The ET is the single largest element at 154 feet long and 27.6 feet in diameter, and during launch it serves as the structural backbone of the shuttle, absorbing most of the six million pounds of thrust generated during flight and providing propellant to the Orbiter’s three main engines.
A new version of the ET, the Super Lightweight Tank (SLWT), is 7,500 pounds lighter than the previous Lightweight Tank design. The reduced weight is a result of component redesign and the use of Weldalite™, an aluminum-lithium (Al-Li) alloy developed by Lockheed Martin. The Al-Li alloy is 30 percent stronger and five percent less dense than the aluminum alloy previously used in manufacturing ETs.
The first SLWT flew as part of STS-91 on June 2, 1998.
Weighing 58,500 pounds empty and 1.6 million pounds when filled with cryogenic propellants, the ET supplies over 535,000 gallons of liquid oxygen and liquid hydrogen to the Orbiter's engines. The 7,500-pound weight savings resulting from the SLWT increases shuttle payload capacity by a similar amount. The performance increase is vital to the building and supplying of the International Space Station.
Lockheed Martin is under contract to the NASA Marshall Space Flight Center to assemble ETs at Michoud Assembly Facility in New Orleans through 2001. Negotiations are currently underway for the production of additional ETs to support the Space Shuttle program.
External Tank Components 
The liquid oxygen tank is an aluminum monocoque structure composed of a fusion-welded assembly of preformed, chem-milled gores, panels, machined fittings and ring chords. It operates in a pressure range of 20 to 22 pounds per square inch. The tank contains anti-slosh and anti-vortex provisions to minimize liquid residuals and damp fluid motion. The tank feeds into a 17-inch-diameter feed line that conveys the liquid oxygen through the intertank, then outside the ET to the aft right-hand ET / orbiter disconnect umbilical. The 17-inch-diameter feed line permits liquid oxygen to flow at approximately 2,787 pounds per second with the Space Shuttle Main Engines operating at 104 percent or permits a maximum flow of 17,592 gallons per minute. The liquid oxygen tank's nose cone reduces drag and heating and serves as a lightning rod. The liquid oxygen tank's volume is 19,563 cubic feet. It is 331 inches in diameter and 592 inches long.
The liquid hydrogen tank is an aluminum semimonocoque structure of fusion-welded barrel sections, five major ring frames, and forward and aft ellipsoidal domes. Its operating pressure range is 32 to 34 pounds per square inch. The tank contains an anti-vortex baffle and siphon outlet to transmit the liquid hydrogen from the tank through a 17-inch line to the left aft umbilical. The liquid hydrogen feed line flow rate is 465 pounds per second with the Space Shuttle Main Engines at 104 percent or a maximum flow of 47,365 gallons per minute. At the forward end of the liquid hydrogen tank is the ET / orbiter forward attachment pod strut, and at its aft end are the two ET / orbiter aft attachment ball fittings as well as the aft solid rocket booster-ET stabilizing strut attachments. The liquid hydrogen tank is 331 inches in diameter, 1,160 inches long, and has a volume of 53,518 cubic feet.
The intertank is a steel / aluminum semimonocoque cylindrical structure with flanges on each end for joining the forward liquid oxygen and aft liquid hydrogen tanks. The intertank houses ET instrumentation components and provides an umbilical plate that interfaces with the ground facility arm for purge gas supply, hazardous gas detection and hydrogen gas boiloff during pre-launch operations. It consists of mechanically joined skin, stringers and machined panels of aluminum alloy. The intertank is vented during flight. The intertank contains the forward solid rocket booster (SRB)-ET attach thrust beam and fittings that distribute the SRB loads to the liquid oxygen and liquid hydrogen tanks. The intertank is 270 inches long and 331 inches in diameter.
To prevent the super cold liquid oxygen (-297 degrees F) and liquid hydrogen (-423 degrees F) from forming ice on the outside surfaces of the ET, a multi-layered thermal protection coating approximately one inch thick is applied. The insulation allows the ET to withstand the extreme internal and external temperatures generated during prelaunch, launch and flight.
At launch, propellants are pressure-fed at a combined rate of 1,035 gallons per second through 17-inch diameter feed lines to the Orbiter’s engines. Eight and one-half minutes into flight, with the Orbiter and ET at an altitude of about 71 nautical miles, the main engines are cut off and the ET is jettisoned. The tank slowly tumbles, reenters the atmosphere and burns up, with small surviving parts safely falling into remote areas of the Pacific or Indian Oceans.
ET Weight Savings Means More Payload
The first Space Shuttle powered by an External Tank flew on April 12, 1981. The version of the ET used on the initial launches weighed 76,000 pounds. A subsequent redesign program netted a 10,000-pound weight savings on the ET. Because the ET and orbiter have virtually reached gravitational escape velocity when the Et is jettisoned, every pound reduced from the ET results in another pound that can be taken to orbit. Thus the 66,000-pound Lightweight Tank, introduced on the sixth Space Shuttle mission in 1983, resulted in substantial improvements to shuttle payload performance.
The weight-savings continued in 1998 with the Super Lightweight Tank weighing another 7,500 pounds less. This opened the door for the Space Shuttle to carry the heavier components of the International Space Station.
Related NASA's Fact Sheet site: A Walk Around the Space Shuttle for information on the other components (Orbiter and Solid Rocket Boosters) of the Space Shuttle.
All of the huge "S" turns which reduce speed are executed over the western states and are completely computer controlled.
Pilot control returns at subsonic speeds for landing.
The computer program that operates this speed reduction might have been sabotaged.
I don’t think so. I personally can’t even speculate what happened.
NEWS RELEASE Office of Public Affairs
United States Air Force
Air Force Materiel Command
Arnold Engineering Development Center
100 Kindel Drive
Arnold AFB, TN 37389-2213
(931) 454-5586
http://www.arnold.af.mil
Writer: Danette Duncan
Date: March 19, 1999
Release # 99-041
Photo # none
AEDC Performs Shuttle Materials Test for NASA/Lockheed Martin
ARNOLD AFB, Tenn.—Arnold Engineering Development Center is assisting the National Aeronautics Space Administration with improvements in existing Space Shuttle materials.
According to NASA, during several previous Space Shuttle flights, including the shuttle launched Nov. 29, 1998, the shuttle external tank experienced a significant loss of foam from the intertank. The material lost caused damage to the thermal protection high-temperature tiles on the lower surface of the shuttle orbiter. The loss of external tank foam material and subsequent damage to reentry tiles is a concern because it causes tile replacement costs to significantly increase,,u. however, it is not a flight safety issue. As a result, NASA-Marshall Space Flight Center selected AEDC to perform flight hardware materials tests on the shuttle’s external tank panels in the center’s von Karman Facility Supersonic Tunnel A. The purpose was to establish the cause of failure for the tank thermal protection materials at specified simulated flight conditions. "NASA chose AEDC due to its technical expertise and historical program successes," Steve Holmes, a NASA-MSFC technical coordinator, said.
The Lockheed Martin-manufactured non-reusable external tank, the largest element of the Space Shuttle, fuels the shuttle orbiter during powered flight and is comprised of three components—a liquid oxygen tank, a liquid hydrogen tank and an intertank assembly that connects the two propellant tanks. At the full capacity of 528,600 gallons of propellant, the external tank weighs 1.6 million pounds. The tank is covered with a multi-layered, spray-on foam insulation that provides thermal insulation for the tank against the extreme internal and external temperatures generated during prelaunch, launch and flight.
Wayne Hawkins, Sverdrup project engineer, explained the foam system is exposed to multiple forces, causing difficulty in determining the actual failure of the thermal protection system. "Multiple forces act on the foam system," Hawkins said. "The environmental factors include thermal protection system cell expansion, aerodynamic loading, highly variable local flow conditions, oscillating shocks, vibration, temperature and main external tank substrate flexure."
Although NASA and other facilities have performed a number of tests in an attempt to define the underlying root cause of this foam loss, they were not successful. At one time, the center’s 4-foot and 16-foot transonic aerodynamic wind tunnels were possibilities for the test, but Tunnel A’s ability to closely duplicate flight conditions and control both ambient pressure and test sample immersion time made it the facility of choice. Tunnel A is a continuous flow-variable density wind tunnel with an automatically driven flexible-plate nozzle and a 40- by 40-inch test section and can cover the Mach number range of 1.5 to 5.5.
"The ideal success for the test is the generation of foam loss on a consistent basis with simulated flight conditions," Hawkins said.
Although the AEDC Tunnel A tests did not replicate the in-flight failures, they did provide detailed measurements to better understand the flight environment and fundamental failure mode. From these tests, NASA determined the failure is caused principally by foam cell expansion due to external heating at approximately Mach 4 combined with pressure change and aerodynamic shear. Specialized miniature shear gages and other instrumentation were installed during the test to measure these forces. The customer and sponsor were pleased with the AEDC test results. "No other facility can test with articles/models as large as AEDC with conditions that can match flight," Holmes said.
Yes, there are tape recorders. It is highly unlikely that they survived re-entry intact.
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