From Slashdot: BJ_Covert_Action writes:
“NASA engineers have finally discovered the root cause of the cracks that have been found on space shuttle Discovery’s main external tank. The main tank, one of the ‘Super Lightweight Tank’ models developed by Lockheed-Martin, employs an aluminum-lithium alloy developed by Lockheed-Martin specifically for this application.
The new alloy is used in various structural stringers throughout the SLWT design. Unfortunately, the batch of this alloy used in the tank that is currently mated with the Discovery shuttle appears to be of low quality. The alloy used in the stringers has a ‘mottled’ appearance, compared to the nominal appearance typically used in the main tank stringers (see picture in article).
This appearance is indicative of a fracture threshold that is significantly lower than typical. NASA has determined, through testing, that this low grade alloy has only 65% of the fracture strength of the nominal alloy typically used.
NASA engineers have devised a potential fix to the problem that they are currently testing to ensure the repair will cause no unintended consequences. NASA plans to have the Discovery shuttle ready to launch again by February 24th, 2011.”
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I’m reminded of another ship with low grade metal. Let’s all hope the solution they devise is sound, otherwise it may be a terrible ending for the shuttle program.
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Many years ago in a materials class, the professor mentioned about “Liberty Ships”, and how a few broke up in the N. Atlantic. Seems they also had a problem with the North Atlantic cold, causing the steel to change from ductile to brittle.
There is hardly anything that comes out of NASA that I accept without thinking about potential political angles.
I think you are rather unfair to the Titanic. It was actually an outstandingly safe ship.
Remember that it was:
– run at full speed (>20 knots) through a known iceberg area
– when a collision was inevitable, maneuvred in a way that was guaranteed to maximise damage
Even then it floated for four hours after the collision, allowing ample time for evacuation. However this, in line what had happened before, was handled with outstanding incompetence, causing many lifeboats to be launched not even half-full. Moreover most of these boats deliberately refrained from picking up survivors after the ship had sunk.
The Titanic disaster was caused by a combination of regulatory failure (specifying much smaller lifeboat capacity than needed) and an almost incredible lack of professionalism by the captain and crew.
I saw some Titanic show on the History Channel or something where they investigated the hull remains and concluded that manually formed cast-iron rivets in the lower bow of the ship (where the riveting machine couldn’t fit) failed, allowing the metal plates that run horizontally along the hull to buckle. The rest of the rivets were steel and machine-formed, and much stronger.
This subject of failure analysis of aluminum alloys is one that I once could claim some expertise. Base on what they have found and reported, I think this is a classic case of intergranular stress-corrosion cracking resulting from improper heat treatment. Ordinary tensile testing for quality control won’t show this. I doubt the metal orginally had the mottled appearance because that would have been a sign that there was something wrong with the heat treatment. They seem to have found a solution and didn’t blame CO2.
“Richard S Courtney says:
January 15, 2011 at 2:09 am”
Good one!
Appears to be possible surface corrosion which high strength aluminum alloys tend to do unless surface treated.
There was also some dissension on the Challenger disaster:
http://www.miaminewtimes.com/1990-08-22/news/bent-out-of-shape/
In addition, NASA management had predicted a mission failure rate of about 1:100,000, their engineers about 1:100 and empirical experience being about 1:35. The mishap appeared to fit the empirical data the best since the mishap occurred on the 31st flight (counting test flights) for this general design (not specific vehicle). The Columbia fared somewhat better, lasting until the 113th mission. How many would decline to fly knowing the actual odds?
Because of the predicted low failure rates, there also was never any rescue systems developed in case of problems while in orbit. I suspect that this is the real reason that NASA declined offered telescope examination of the Columbia tiles, as there was nothing they could do anyway except let everyone know in advance how doomed they were.
NASA management had also ignored engineering warnings about the use of 100% oxygen atmospheres in the Apollo, resulting in the Apollo-1 loss of crew.
Can’t wait to see if the space vehicles being developed for public use do any better.
Burma shave signs from a fighter pilots lounge: If you taxi without a care you may be buying a one way fare. Best they caught the problem before shuttle takeoff.
The Space Shuttle Program has turned into a “patching” project a long time ago.
Fortunately technology evolves, new materials have been developed and the time has come for these technologies to replace the old and they will.
NASA will make use of the new technologies and so will the new private space industry.
http://www.xcor.com/products/cryo_compatable_composites.html
Obama is yanking money from space programs and we go back 100 years to rail. Railroads don’t have this problem. Trains don’t go where we want to go. So it means they want us not to have cars but save energy.
Back in my flight instructor days, I had the dubious opportunity to use a new type of basic trainer. This aircraft made by a major manufacturer, was a two seat 112 hp
putt-putt that was supposed to give a “high performance” feeling to the flight training experience. It did. truly nasty stall/spin characteristics that rivaled that of a jet
fighter (notably the F-104) AND it required a level of skill to recognize the immediate
need to keep the heck out of a spin! On top of this, they were having ah, quality control
issues at the factory. Little things, like the rudder would jam hard over as the airplane
entered the spin and -stay there- enhancing the spin experience.Then, the wing spar
carry through it had several joints and bolts holding the wing on, except when they weren’t all put on at the factory. I had a demo flight with the dealer that consisted
of spins, (no locking rudder-found that out later) and wingovers, chandelles , etc.
then several hard landings (as part of the demo ) to show how rugged the aircaft
was for light training. We taxi up to the hangar, and the chief mechanic walked and said:”Heard from the factory gotta pull the inspection panel.” Three of the four bolts
that held the wing on were missing. We’d already paid for this airplane. Which would’ve been a great wind tee (direction pointer) and darn little else. took about
10 years to engineer that thing to where it is safe. The shuttle I feel is not safe..
I want to ride my atomic airliner to the Space Port-to go to the Moon then Mars!
I’m at a loss here to understand how this could happen without gross corruption in the process.
I’ve manufactured flight grade parts, both for aircraft and space vehicles. The pedigree for the metals (“Certs”, as they’re commonly called), the manufacturing processes themselves, almost to the chain of custody, it all is tracked and documented. Inspectors will come on site and check your documentation, and they’ll check your inspection tools for calibration. You guys think science is rigorous??
The quality of the parts should never be in question. The ONLY questions that remain are:
1. The inherent design, and
2. The integrity of the manufacturing chain.
#1…Well, we get what we get. But #2? A failure there points to corruption, either for political or financial gain. In the case of the former, it’s usually tied to the latter.
So I’m puzzled here because I know the process, and I don’t understand how such an obvious flaw could slip by the cadre of inspectors. There’s got to be that faint scent of cover-up at Canaveral.
Now that they have discovered the problem. They must inspect all critical components for any sign of cracks. The probability of fatigue cracking failure at launch (from high vibrational stress at the bottom of the crack) is too high. Pilots and mechanics inspect for cracks. I know because I was a navy pilot and did research at the Naval Air Materials Center specifically with aluminum alloys. http://www.stormingmedia.us/68/6848/0684884.html
“Then why didn’t they consider delaying the launch of Challenger in 1982 in freezing weather (& after warning of danger from engineers) for just a few days?”
There’s no schedule pressure anymore. NASA knows there’s only a few missions left and no-one really cares about them other than NASA workers and astronauts… delaying the launch just means they have jobs for longer.
And that’s probably the root cause of this problem. When people discover they’ll be losing their jobs, the usual reaction is for the better ones to leave so the project is left in the hands of the few, if any, who are totally dedicated and the ones who can’t get a job elsewhere.
In the Challenger case I seem to remember they did look at a graph of O-ring leaks vs temperatures, but it only showed the temperature range where there were problems so there was no obvious correlation. Combine that with the pressure to launch ASAP and ‘go fever’ pushing people to allow a flight in conditions where things had gone wrong before but no-one had died yet and that explains pretty much everything.
>>The Titanic disaster was caused by …. an almost incredible
>>lack of professionalism by the captain and crew.
You mean that they did as they were told by management. A bit like the Challenger disaster, where management said ‘launch no matter how cold it is’. So were the Challenger crew culpable, or NASA management? Were the Titanic crew culpable, or the shipping-line management?
.
I wonder how much of the fact that once they engineers who built the tank are done they are out of a job. How much effort goes into work when you have been given your two week notice. I hope not but it could be there. Why go over the pieces without the the normal microscopic check when it worked before and it is the last one to make.
Amen.
Richard S Courtney, re railroad gauges.
New Zealand railroads have an exceptionally narrow guage, designed to facilitate getting trains around bends in mountainous terrain. It tends to limit speed somewhat, as locos will ‘rock’ themselves right off the rails if speeds are too high. If you are interested, look up NZ’s ‘Raurimu Spiral’, which winds up something like 3000 feet from it’s start and is an interesting daylight journey. This solved a huge construction problem – the originator was an engineering apprentice!
Now they find it? Good timing NASA
http://www.itl.nist.gov/div898/handbook/pmc/section2/pmc231.htm
http://www.samplingplans.com/aqlprimer.htm
http://www.siliconfareast.com/ltpd_aql2.htm
Re: NASA and allowing 1% defective – never happened – see above.
At least for semiconductor components consumer product manufacturers required an AQL of .065% for eg transistors, way back in the 1960s. MIL specs were tighter. By 1995 Bosch required AQL less than 3 ppm for complex integrated circuits for use in ABS assemblies. I have no NASA experience, but would expect inferred AQLs in the few ppm range. However for the shuttle program no one made more than a few thousand of anything for them, so for sure 100 % testing and inspection was employed with large guard bands and zero defect requirements. The issue then becomes MTBF. Its conceivable but unlikely, that they never expected the shuttles to fly this long so clould have been light on MTBF requirements.
Douglas DC, that’s either the Grumman AA1A Yankee or the Piper Tomahawk.
My wife and I had the misfortune to do our power flight training in a Tomahawk. Both of us survived. I had the advantage of over 2000 hours in gliders.
But, this thing *should* have been so overengineered, 65% should still get it done comfortably.
Safety margins for flight capable vehicles are in the 10% range. Safety is maintained by continual inspection and appropriate repairs. So if the max rated G load is 3 Gs the aircraft will be designed to about 3.3 Gs. Now some of the safety margin is in the G load. But still.
For aircraft everything structural is tested to destruction. To make sure it meets the design criteria. Everything is weighed. And documented.
We have a rule in aerospace that “no aircraft flies until the weight of the paperwork equals the weight of the aircraft.”
A lot of someones didn’t do their job on this one.
Nothing like strapping yourself onto four million pounds of explosives built by the LOW BIDDER…
There is no engineering patch for inferior material.
Gerry says:
January 15, 2011 at 2:19 pm
Nothing like strapping yourself onto four million pounds of explosives built by the LOW BIDDER…
Kinda like using the Met Office’s supercomputers to predict climate change, eh…