STS-118 returning
Published on Aug 20, 2007 at 11:59 pm.
4 Comments.
Filed under NASA, space shuttle.
In just over 12 hours from when I write this, the Space Shuttle Endeavour will return to Earth to finish the STS-118 mission. This has been another ISS construction mission. It was also the mission in which a teacher flew into space. Personally, I think that the teacher in space program is more public relations than educational. But, one of the biggest news stories has been damage to two of the Endeavour’s thermal protection tiles.
When a spacecraft returns from orbiting the Earth, it must slow from the terrific speed of orbit to spiral downwards towards Earth. Unfortunately, the speed of orbit is so great that when the spacecraft reaches lower altitudes, the air is so thick that the passage of the spacecraft through the air greatly compresses the gasses of the atmosphere. This heats the gasses to the point that those gasses begin to glow. This great temperature then causes the spacecraft to heat up. Without some sort of thermal protection, the spacecraft would heat to the point that astronauts on board would be cooked to death and the very structure of the spacecraft itself would soften and melt. The craft would then disintegrated in a giant fireball.
Most of the Space Shuttle is covered in heat resistant tiles that act as a thermal barrier to protect the craft from the heat of reentry into Earth’s atmosphere. The earliest space capsules used ablative heat shields that bubbled and boiled away during the heat of entry. This ablation carried the heat away from the capsule, protecting it from burning up. However, an ablative heat shield for a craft the size of the Space Shuttle was deemed impractical when the Space Shuttle was being designed. Nonetheless, work continued on designs for an ablative heat shield until the final designs for the shuttle were selected. Besides being impractical, there was the other problem that the ablative heat shields in use at the time were one shot affairs. You could use them one time, and then the capsule was gummed up too much to use again (there were other reasons that the capsules were one-shot craft, too). A ceramic reusable heat shield design was finally agreed upon as an acceptable thermal protection. But, a solid heat shield would never work. As the ceramic heated, it would expand. Ceramic isn’t very flexible, so the heat shield would simply crack and that could mean the end of the spacecraft. So, a decision was made to cover the Shuttle in tiles. As they expand, then they would close up the gaps between them. Not all parts of the spacecraft received equal heating. As the Shuttle enters the atmosphere, there is obviously more compressed air on the bottom than on the top (it hits the atmosphere with the bottom forward). Different portions of the Shuttle, therefore, needed different levels of protection, so different types of tiles were used. Eventually, in some areas that did not get very hot, felt blankets were deemed sufficient insulation. The tiles were used to protect the Shuttle where the heat was greatest. It sounded like a great idea, but the tiles have been a problem since the very first Space Shuttle flights.
First of all, the tiles are very lightweight and easily damaged. Part of what makes them work is that they are mostly empty space. That means that they are not strong. An impact with most anything at high speed can damage the tiles. Another problem is that they tiles have to be glued into position. As the tiles expand, that glue is stressed. Also, the vibrations and aerodynamic forces on the tiles during launch stress the glue. It is no great surprise that the first shuttle flights had a number of thermal protection tiles fall off of the Shuttle during launch. This was a great concern to NASA engineers at the time. Also, the Shuttle’s external tank is filled with cryogenic fluids and often accumulates ice. This ice can fall off and damage tiles, too, if it strikes them. To act as insulation for the external tank, it is coated in foam insulation. But, even that foam can fall off during launch and strike the Shuttle, damaging tiles. Again, this was a problem from the very early days of the Space Shuttle program in the early 1980’s.
But, the thermal tile heat shield turned out to be far more resilient to damage than critics first feared. The shuttles that had damaged tiles still returned to Earth safely. Even the shuttles that lost tiles managed to return to Earth. As the Shuttle hits the atmosphere, blunt side first, the air compresses and heats up. But, it also acts as a thick layer of gas between the Shuttle and additional air being compressed and heated up. This means that some of the highest heat loads occur near the leading edges of the wings, where the hot gasses blow past the wing instead for forming a buffer. That was what led to the destruction of the Columbia. When it was launched in January, 2003, foam (and perhaps ice) fell from the external tank and struck the leading edge of the Columbia’s wing. This broke some of the tiles there, and likely even caused a large hole, and a section of the tiles to detach. This was in a very vulnerable position on the Shuttle. As the spacecraft reentered on February 1, 2003, hot gasses made their way into the wing and began to soften and melt the wing structure until finally the wing collapsed, folded, and the shuttle went out of control and broke apart over Texas, raining debris from close to where I live all the way into Louisiana. That was a wakeup call.
Until the Columbia accident, it seems that some of the decision makers in NASA had begun to not take tile damage seriously. After all, so many flights had managed to safely return to Earth with damaged, or even missing, tiles. But, those tiles were in less critical places. In fact, a single tile missing from the underbelly of the Shuttle is unlikely to result in major damage unless it were in exactly the wrong place. The gasses would heat and likely damage the skin of the Shuttle behind where the tile were located, but it would be unlikely to burn through, and if it did, then it would be unlikely to result in loss of the spacecraft. Loss of tiles near the top of the spacecraft would have even less effect. But, the Columbia damage was not on top of the craft, and not on the underbelly. It was right at the leading edge of the wing, where the cushion of air caused by the craft hitting the atmosphere was blowing past the wing, not acting as a partial barrier to other hot gasses.
So, when shortly after launch, it became apparent that foam or ice may have struck the Endeavour, NASA engineers were somewhat concerned. Then, images like the one above showed substantial damage to two tiles. So, naturally, this brought back images of Columbia breaking up over Texas. So, when after careful analysis of the damage NASA decided to go ahead and land without trying to do repairs, a lot of people were understandably upset, given the news coverage. Well, just in case you are still wondering about NASA’s decision, I think that they may have made the right call. This damage is on the underbelly. If several tiles were missing, then that would be cause for alarm. However, the amount of damage here is less than if one tile were completely missing. And, the location of the damage is such that it is unlikely that a burn-through would occur, and if it did, it would be unlikely to result in loss of the spacecraft. This is damage not unlike what has been seen on previous Space Shuttle flights that safely managed to return to Earth.
Yes, this looks really dramatic in pictures, but it is probably just another in a long series of tile damage incidents that have been part of the Space Shuttle program since the first flights. Naturally, there will be some pretty tense moments 12 hours from now when the Shuttle returns, but I think that it will turn out just fine in the end.
But, you might wonder why NASA would chose to even risk reentry with damaged tiles when they could just do a spacewalk to repair them? Well, a spacewalk is not really a simple thing. Spacewalks can be quite dangerous. That is particularly true at the location of the damage. This would be like saying that in the event of a possible bird strike the pilot of an airliner should crawl out through the cockpit window and work his/her way along the fuselage to make some duct tape on the tail of the aircraft. Yes, I know some pilots that might try that if, and only if, it seemed the only way to make a safe landing. Already, a spacewalk had to be cut short as an astronaut’s glove was cut during a prior spacewalk. Also, there are no hand holds in this area of the Shuttle, and it is very possible than an astronaut might accidentally float out of control into other tiles, damaging them. So, a botched repair could make matters far, far worse. Therefore, since this really doesn’t seem to be any worse damage than previous shuttles that returned safely, it make sense to just leave it alone.
Of course, that doesn’t mean that there is no cause for some concern. Any time a spacecraft reenters the atmosphere, there is cause for concern. Space travel is not terribly safe just yet. But, the astronauts know this, and they accept the risks. We need to do so as well.
-Astroprof
Astrogeek on August 21, 2007 at 12:24 am: 1
Any ideas on what they are going to do for heat shields on whatever the replacement to the shuttle will be?
Astroprof on August 21, 2007 at 6:02 am: 2
I don’t think that they’ve settled on the final design for the CEV heat shield. However, what I hear is that it is probably going to be an ablative heat shield.
A Ler…-- Rastos de Luz on August 22, 2007 at 4:31 pm: 3
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Peter J on August 24, 2007 at 4:26 am: 4
Thank you for this! I was puzzled as to why the post-landing photo showed no severe burning within the damaged area, and neither NASA nor the mainstream media offered any answers, so your detailed post was very very welcome.