To: gaunty
To:
Regarding the Challenger explosion this is what the public was told. An “O” ring failed and allowed hot gasses and eventually a high velocity flame jet to escape past the defective “O” ring and impinge on the composite fuel and oxidizer tank resulting in a massive explosion. In addition the public was told that a contract engineer working for Thiokol prepared a Failure Mode Effects Analysis (FMEA) stating that if the “O” ring were exposed to very low temperatures it would most likely fail. He presented the FMEA to his company and they in turn sent it to the NASA Manager of the solid rocket booster branch at MSFC in Huntsville, Alabama for his vetting and final approval. He in fact did sign off on everything in the FMEA including the dire prediction about the failure of the “O” ring.
Just prior to the launch an inspection team went out to check the solid motors and the composite fuel tank. The lower part of the tank contained LOX (liquid Oxygen) which is about 260-degrees below zero F. This localized cryogenic temperature and the presence of high humidity caused ice to form around the solid rocket booster. This was noted and the launch crew was notified. Members of the Thiokol management team wanted to scrub the launch in order to wait for the humidity to drop or for better conditions. NASA engineer were reluctant to scrub for two reasons, 1) they didn’t want to miss their launch window and 2)They didn’t want to upset their preplanned successive launches of the other Space Shuttles. It became a pissing contest and NASA won. The person that was most vocal on the NASA launch team was the former manager of the solid motor branch at MSFC who had signed off on the FMEA. He had been promoted and transferred to the Cape.
The rest is history. The “O” ring failed and a jet of hot gasses escaped from the defective seal.
That is what the public was told at least they were told everything except who had signed off on the FMEA and the fact he was the most vocal member of the launch team to order
the launch contrary to what the FMEA had predicted.
Now, I’ll tell you what I think happened but first, I have to talk rocket science. There are several types of solid rocket motors. The most common is that used in small missiles. These are end grain or they burn on the exposed surface. When an end grain rocket burns, it burns like a cigarette. The interior volume of the motor case determines the internal pressure of the combustion. At ignition the interior exposed volume is small so that as the propellant is ignited the pressure is quite high. But, as the propellant burns the interior volume increases and the pressure drops.
On larger solid motors the propellant grain is cast with the center of the grain constructed in a geometric shape. The shape can be in the form of a cross or possibly like a star. In this type of motor the combustion takes place on the surface of the cast in geometric shape. Contrary to the drop in pressure on an end burning propellant grain the pressure remains constant (within design parameters). If you were to measure the perimeter of the cast in geometric shape you would find it equal to the inner circumference of the containing vessel. As the grain burns away the inner exposed surfaces will become less in surface area but it is now getting close to the inner walls of the rocket motor and if all goes well the rockets will be jettisoned just before burnout.
In many large motors such as those used on the shuttle there will be large chunks blown out during the ignition sequence thus increasing the exposed area that burns. This does two things, one, it creates a combustion instability and two, it increases the internal pressure because of the increase of exposed surface available to be burned.
The same thing can happen if there is a crack in the propellant grain. That is why they X-ray the grain prior to assembling the motor. If the grain cracks the surface available to burn is increased and in turn the internal pressure in creases and can be significant enough to blow the rocket motor to hell and back.
Now, a little bit of chemistry. When you were a kid you learned about the fire triangle in that you needed fuel, oxygen and an ignition source to have combustion. Oxygen by itself does not burn and it doesn’t explode. But if you had a combustible dust in the presence of oxygen and you struck a match the explosion would be violent. That’s what happens when a grain elevator explodes.
The solid rocket motor used on the shuttle is made up of segments that are loaded one on top of the other to create the propellant grain. They strap on a pointed end at the top and at the bottom they attach a steerable nozzle. At each segment attach point they install an “O” ring to maintain the internal pressure. I had suggested to NASA that they use non-combustible putty between each segment pair to create a complete seal. As the grain burned the putty close to the burning gasses would be consumed but the primary seal would remain intact. I never heard from them and, as far as I know only the "O" ring seal area has been redesigned and they don’t use the putty.
Now we get back to the Challenger as it reaches altitude. NASA says a jet of flame and hot gasses escaped past the defective seal and impinged on the oxidizer tank causing the explosion.
If the flames hit the Oxidizer tank and caused a rupture there is no combustible material that would burn that fast as to cause the violent explosion.
You also have to ask this question. If the flame got past the seal it had to come from the internal combustion within the motor. In order to get to the seal any propellant in the area would be burning and as a result, increasing the internal pressure high enough to cause the rocket motor to explode. And, that is what I think happened.
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The Cat
[This message has been edited by Lu Zuckerman (edited 06 January 2001).]
[This message has been edited by Lu Zuckerman (edited 06 January 2001).]