Shuttle Undock...

[Chris Trott]

Here is a picture of the damage that the spacecraft sustained during launch from a foam strike:
http://www.nasa.gov/images/content/1872 ... pd2305.jpg

I would have thought they would have tried fixing it before coming down after what happened last time...

Most people don't watch the Mission Day briefings, but I caught a few of them and there were some numbers given out about damage to the shuttle in the past to the TPS (Thermal Protection System), including that NASA averages finding 300 impacts/locations of damage on the TPS after each flight.

After STS-107, much of the research done was to build a database of effects on the TPS using different sized and massed projectiles at different speeds to be able to compare impact damage observed during the scans and known impact data to be able to not only determine what hit, but how hard it hit, and how bad the damage really is because following the damage being done to the TPS test pieces, the information was then entered into the supercomputers at NASA and they ran thermal simulations to find out what would happen during re-entry with the known damage to the shuttle. They found that most of the TPS is very resiliant and entire tiles could be missing and still the shuttle be safe over most of the under surface of the shuttle. The only real vunerable parts to singular impact was the leading edges of the craft, exactly where the impact that destroyed Columbia occured. They also had the side effect of verifying the original tests done in the 1970s and early 1980s when the TPS was being developed for the Shuttle that said basically the same thing. The only difference between then and now is that they will know exactly what has been hit, how bad it's been hit, and exactly what will happen (via the thermal simulations that are done once the impacts have been located and measured) before reentry occurs. Before, they could only see the damage after reentry.

One thing I think most people forget - foam shedding didn't start happening until Greenpeace, the Sierra Club, and other "environmentalist" groups started complaining about how NASA applied the foam to the External Tank. After NASA changed the application method to be more "environmentally friendly", they started having problems with foam shedding and have been working ever since trying to figure out how to fix it. The only real fix is to go back to the old way, but some people think that the lives of people and millions of dollars worth or equipment aren't as important as nature. There's a line between responsible environmentalism and idiocy, and STS-107 was proof of what happens when that line is crossed because of political pressure being put on an agency or company to "go green" regardless of what might happen to safety because of it.

[jonesey2k]

As if the eco-friendly version will make any difference to the environment when the thing burns god knows much fuel on blast-off

[Chris Trott]

Something like a thousand gallons a minute, but the "fuel" is techincally eco-friendly. It is, after all, mainly Hydrogen and Oxygen. The "fuel propellant mixture in each SRB motor consists of an ammonium perchlorate (oxidizer, 69.6 percent by weight), aluminum (fuel, 16 percent), iron oxide (a catalyst, 0.4 percent), a polymer (a binder that holds the mixture together, 12.04 percent), and an epoxy curing agent (1.96 percent)." (to steal from NASA's web page on the SRB's)

BTW, the ET foam used CFC-14 in its composition when originally designed and did not have any problems with shedding foam or ice. In 1996 the EPA banned CFC-14 totally and NASA went to HCFC and has been having problems ever since. The amount of CFC-14 in the ET was less than 5% of the mixture and created less Ozone for application on the entire tank than a single can of aerosol spray that used CFC-14. Considering how infrequently the Shuttle flies, it's sad to see that an exception couldn't be granted as it is for other uses due to safety issues.

[Kevin]

but the "fuel" is techincally eco-friendly

Well, yes and no. It's the SRB propellants that are the environmental problem, not the Orbiter/ET's Main Propulsion System. The SRB's produce about 99 tons of HCl (hydrochloric acid) on each launch which is largely responsible for a 2000 km ozone hole which appars for a few hours after each Shuttle launch. There are also a lot of awkward chemicals resulting from the "binder" (which is basically a butyl rubber) being burned in the stratosphere.

BTW, the ET foam used CFC-14 in its composition when originally designed and did not have any problems with shedding foam or ice

Chris, you're right of course regarding the fact and date of the CFC change, but the ET has shed foam right from the first few flights. There have been many changes over the years to both materials and processes, but one constant is that the thing has always shed some foam. The sad difference nowadays is that up until 2003, no-one believed that the Orbiter would suffer anything more than "hangar rash" from a foam strike. Ice shedding, similarly: we always had criteria for allowable ice build-up (less than 1/16 th inch) and various allowables in bracket areas, 'doghouse' fairing, etc, but it was always there.

How do I know this? For ten years I was a Senior/Principal Propulsion Systems Engineer for Martin Marietta on Shuttle & the early development of the Space Station. I was also the propulsion lead on the post-Challenger study for a liquid booster as a direct one-to-one replacement for the SRB: we proved it was possible, attractive and affordable, and then NASA sabotaged the study result to protect the solid motor lobby. It left an nasty taste, but that's another topic.

Kevin

[TSR2]

Thanks for the insight Kevin. Its always good to hear from someone "at the coal face" with real first hand experience.

[Chris Trott]

Kevin, I echo Ben on the thanks for the insight. I spent time with some of the crew at LMSS @ Waterton Canyon when I was living in Denver and got to meet several of the guys who'd moved from STS to Atlas or Centaur and got some great insights from them as a follow-on to my trips to Birmingham for Space Camp and Space Academy (although I've heard that both SC and SA have been "dumbed down" since I was there) and my general interest in spaceflight at that time of my life. They had indicated that the ET shedding issue wasn't a problem until after the changeover from CFC-14 and I interpreted that as meaning that if any foam was shed, it wasn't much or very large, hence not being "a problem". After the change to HCFC, they said that they started seeing large chunks of foam, which they felt to be a "problem" and thus major fixes were being contemplated even during the late 1990s which lead to the SLWT, although it came a bit too late.

[Kevin]

Chris,

Yes, we had very different perceptions of the nature/existence of the problem over the years.

The TPS foam has always been a poor engineering material, though. When I joined the Martin Company in '81 the then Chief of Thermal Engineering described the thermal properties of the foam (then known as NCFI - North Carolina Foam Insulation) as "a barrel of s**t". This was based on the fact that thermal properties of a given batch could vary by up to a factor of SIX :o and mechanical properties were highly variable: this is in spite of the fact that some 40% of the manufacturing cost of the tank has always been in the TPS, so we were spending a lot of money to apply it in the best, most repeatable way. One of the best design engineers I've ever worked with led a team to develop a new type of spraygun to apply the two-part foam (it reacts chemically in the gun and air & sets as it hits the tank). Many people spent years trying to make it better and it's certainly true, as you say, that the freon issue made things worse.

Looking back, we probably should have abandoned the externally-applied TPS and gone for an internal insulation system as Douglas used on the S-IVB lunar stage; we did look at it, but the recurring cost always ruled it out. A more elegant solution overall, which we discussed with (then) Rockwell - I was in Advanced Programs by then - would have been to go for a "Shuttle 2", where we kept all the stuff which worked well, and eliminated that which didn't. Top of the list for replacement was the Orbiter TPS black tile setup, which could have been replaced by a single-skin RCC sheet, much like a giant Airfix wing. Also on the list was the wing leading edge, where nobody really liked the existing multi-panel RCC elements (which are held on by light brackets and terribly vulnerable to distortion by impact, as probably happened to Columbia); the only reason these are multi-panel and supported in this way is because the metal wing structure expands differentially to the RCC leading edge, but if we redesigned the wing to be all-carbon, this design wouldn't be necessary. The Shuttle 2, which we studied in '86-'87 was much liked by the Contractor engineers, but NASA wanted to go for something all-new, basically to keep their Field Center research base busy, so they started first the NASP (National Aero Space Plane, a single-stage-to-orbit airbreather) and later, when that failed they started the X-33. Both of these failed for fundamentally the same reason: the technology required to build them simply wasn't there. Although the X-33 was not as advanced (read 'full of unknowns') as the NASP, it had many complexities of its own but was eventually doomed by a surprising error: the Lockheed Skunk Works, who originated the basic design concept, were not launch vehicle/space people, and got the cg in the wrong place. It proved impossible to locate equipment/tankage in such a way as to get a flyable vehicle, payload went negative and it turned into an embarassing white elephant.

Now NASA has perhaps at last turned in the right direction in trying to reinvent Apollo with the Orion program, although the US aerospace technical base is now pitifully small compared with previous eras. It's going to be a lot harder than they thought at first (example - the Apollo Command Module heatshield material, for which all the technical/manufacturing details are apparently available, cannot be replicated in the lab and will probably not be available for Orion) but I wish them well.

Kevin