New Materials for Aging Space Shuttle

On Feb. 1, 2003, the nation and the National Aeronautics & Space Administration suffered a tragic loss when space shuttle Columbia broke up during its reentry into Earth's atmosphere. An investigation concluded that the orbiter was doomed from the start of its mission when, just 82 seconds after launch, a less-than-2-lb piece of foam from the shuttle's large external fuel tank broke off and struck the leading edge of the wing, causing a breach in the Thermal Protection System (TPS).

To avoid a repeat of this accident, NASA has increased its focus on stopping the shedding of foam from the external tank during launch as well as on developing materials to repair the shuttle in the event that it suffers another TPS breach. Engineers addressed the foam issue by altering the foam application process in key areas of the external tank, while engineers and scientists worked to develop a mix of chemical and mechanical fixes for TPS damage.

The need for the chemical and mechanical fixes was reinforced this summer during the launch of the STS-114 mission flown by space shuttle Discovery. Tracking with an unprecedented set of cameras and imaging systems, NASA observed chunks of foam coming off the external tank during launch. Although the pieces of foam did not strike the orbiter, this event put the shuttle program on hold, sent engineers back to the drawing board, and increased the desire to have repair materials on board the shuttles ready to use.

Earlier this month, NASA announced that its engineers were beginning to have our hands well around the technical problems related to the foam shedding, and they are defining the fixes necessary to fly again. The agency is now targeting the next shuttle launch for sometime in early to mid-May.

As NASA and its observers anxiously await the next shuttle launch, engineers and researchers remain hard at work testing and improving repair materials that they hope will never have to be used. The development of these materials involves several NASA centers as well as numerous contractors, who have been working together on the projects for several years.

The repairs focus on two different TPS materials: thermal protective tiles and reinforced carbon-carbon (RCC) composite material. The protective thermal tiles are a silicon-based material that covers large areas of the orbiter including its entire underside where temperatures can reach 2,300 F. The RCC material is a laminate of graphite-infused rayon fabric used on areas that reach temperatures up to 3,000 F, such as the orbiter's nose cap and the leading edge of the wings.

One material that has been developed to repair cracks and nicks in the tiles is known as shuttle tile ablator 54 (STA-54). Its repair capability involves a chemical process developed by the Johnson Space Center (JSC) and Lockheed Martin Space Systems. The technique uses a silicone condensation reaction to create a silicone rubber, which is pyrolized during reentry to a ceramic state. The material is an ablator because the ceramic chars on reentry to dissipate heat and protect the orbiter from the high temperatures.

STA-54 is basically a mixture of a room-temperature-vulcanizing (RTV) rubber, glass microballoons, silicone oil, a fumed silica known as Cab-O-Sil, and a catalyst, explains Michael Fowler, technical leader for materials development at JSC. Although the exact materials used in the process are not publicly available, he notes that the RTV material which serves as the base material for the reaction is a common General Electric aerospace silicone rubber called RTV 511, and the catalyst used is (-aminopropyl)triethoxy-silane, which is produced by a GE subsidiary under the trade name A1100.

What happens in this process is that we actually take the RTV base material (part A) and mix it with a catalyst (part B), Fowler says. It becomes a silicone rubber with all of those fillers in it.

For use in space, the materials part A & B are loaded into two separate tubes housed in a caulk-gun-like applicator. The applicator is designed to mix the two parts as the astronauts dispense it into the damaged area. The material has been tested with successful results on damaged areas of about 6 inches in diameter by 0.5 inch deep.

The material actually starts to cure almost immediately, and within an hour it's set up to a point that from a visual inspection it will no longer change, says John Hodge, program manager for the tile repair kit at Lockheed Martin. Lockheed Martin produces the ablator repair materials for NASA. The final cure which happens in about 24 hours is to a Shore A40 hardness, which is like your pencil eraser, he notes.

Hodge points out that this process is not new. In fact, he says, it builds on a process developed for a ground-based program by modifying the materials so that the polymer will cure in a vacuum.

Once the process was adapted, it still faced some challenges. For example, the ablator was found to swell more than expected. If you heat anything up to the temperatures that we are talking about, it's going to expand, Fowler says. He explains that after studying the fillers, scientists took additional steps to reduce the swell to a manageable level.

Foam Problem

Application Or Composition?

The large, orange external fuel tank that houses the liquid oxygen and hydrogen used to propel the space shuttle into orbit has been the focus of a lot of research since the loss of Columbia. NASA engineers have determined that the foam-shedding problem, which doomed Columbia and poses a threat to future orbiters, is related to the process by which the foam is applied and not to its chemical makeup. Initially, however, there was concern that a recent change in foam composition may have contributed to the problem.

The external tank is covered by three types of spray-on foam: a polyurethane foam known as BX-250 and a pair of polyisocyanurate foams known as NCFI 24-124 and NCFI 24-57. NCFI 24-124 covers most of the liquid-oxygen and hydrogen tanks, and NCFI 24-57 is used on the lower hydrogen tank dome. Both of these foams are applied with the blowing agent HCFC-141b, a hydrochlorofluorocarbon. BX-250 found its use on the domes, ramps, and areas where foam is applied by handthe areas where the foam has been found to shed. The blowing agent for BX-250 is a chlorofluorocarbon, CFC-11.

The focus is on the blowing agents. In the early days of the shuttle program, all three foams were applied by using CFC-11, but after the 1987 adoption of the Montreal protocol, class I ozone-depleting compounds such as CFC-11 were to be phased out. In response to this rule, NASA did extensive tests and found that the blowing agent HCFC-141b was an acceptable replacement for applying the NCFI 24 foams. The agency also worked on finding a replacement for the BX-250 blowing agent but was allowed to continue using NASA's stockpiled CFC-11 until an alternative was certified for use.

In 1999, EPA looked to expand the rule dealing with CFCs. Under the new rule, NASA's use of CFC-11 containing BX-250 would come to an end. To avoid this situation, NASA successfully requested an exemption for use of foam containing CFC-11 for "applications associated with space travel." This exception gave the agency time to further develop a replacement, which led to the BX-265 foam applied with HCFC-141b.

The replacement BX-265 has been worked onto the external tank, and it was used on the external tank that launched Discovery this summer. The sections where BX-265 was used included the areas reworked as part of the return-to-flight requirements. A NASA team, however, concluded earlier this month that the foam piece that was seen to break off during the Discovery launch was not in a reworked area and, therefore, the switch in foam was not a factor.

Another challenge was dealing with bubbling that occurred when the material was extruded in a vacuum. Investigation revealed that the applicator allows air to leak into material, Fowler says. When the material is extruded in a vacuum, the air expands and creates nucleation sites, which are likely attracting the reaction's by-product ethanol, causing additional bubbling.

To test the impacts of the bubbling on the final product, Fowler and his team did some ground-based studies using vacuum chambers and an arc-jet facility. The arc jet produces conditions similar to what the shuttle experiences during reentry namely, the plasma that surrounds the shuttle, which generates temperatures approaching 3,000 F and creates a highly oxidative environment.

It turns out that, during our testing in the arc jet, the amount of bubbles that we were getting didn't really hurt us, Fowler notes. You can imagine that air or voids are pretty good insulators, too, he points out.

The challenge currently at hand for Fowler and Hodge relates to the shelf life of the catalyst material part B. Fowler says the microballoons, oil, and catalyst were starting to separate out in the application tube. It's not a problem from a curing standpoint, but it became a problem from a toxicity standpoint, he says.

As it turns out, the A1100 catalyst, when separated, is considered toxic. It reacts with water in the air, however, to form nontoxic materials. We've done some testing that shows that if you are 4 inches away from an A1100 source and there is any humidity, you can't even get the toxic material into your nose, Fowler explains, adding that he expects this toxicity to no longer be an issue once all of the paperwork is taken care of.

Although STA-54 is not currently flight qualified, it did fly on STS-114 this summer. According to Hodge, two dispensers of the materials flew on the recent mission in case some of the tiles were damaged. Because no damage occurred, the tubes were not taken out of the locker. The material is again scheduled to fly on the next shuttle mission, but this time the crew will do a test demonstration on a tile sample in the shuttle's cargo bay. The sample will then be returned to Earth for further testing using the arc jet.

Another repair material that flew on STS-114 is known as NOAX-nonoxide adhesive experimental. This material is designed to seal cracks of about 0.02 inch wide by 4 inches long in the RCC panels on the leading edge of the shuttle's wings.

Basically, we are looking at using a sealant type of material to essentially smear over the cracks in the wing leading edge, says Frank E. Ledbetter, chief of the Non-Metals Engineering Branch at Marshall Space Flight Center. Working in collaboration with researchers at JSC, Ledbetter and his team are using NOAX, developed by San Diego-based COI Ceramics.

The details of NOAX are proprietary, but Ledbetter describes it as a preceramic polymer that is loaded with several different silicone carbide powders used in ultra-high-temperature applications. Like STA-54, NOAX is dispensed with an applicator that is similar to a caulking gun. Once applied, the polymer cures within 48 hours, and the ceramic sintering (or pyrolizing) occurs during reentry.

NOAX also experiences some bubbling. We have shown that if we allow the material a few minutes to go through an initial bubbling and outgassing, then it really settles down to such a slow rate that the material is workable and applicable, Ledbetter explains.

The curing process also presented a challenge to getting NOAX ready for on-orbit use. Initially, Ledbetter explains, the curing process required an active heater, but carrying such a heater was deemed to be too cumbersome for the astronauts. Returning to the lab, he and his team studied whether the curing of the polymer needed to be complete.

Testing using the arc jet to simulate the plasma phase of reentry, Ledbetter says, shows that even if the polymer is uncured, it will go through an almost immediate cure and begin the sintering process when it hits the plasma.

To further evaluate the material, testing was done on-orbit. The results are still being analyzed, but so far the data validate ground-based testing of the material, Ledbetter says.

For larger damaged areas to RCC material, researchers at NASA's John H. Glenn Research Center (GRC) are working on a refractory metal overlay. This approach is attractive because it gives the agency a large degree of flexibility.

On the orbiter, there are 22 panels on each side each one with a totally different contour to it, points out Frank J. Ritzert, materials research engineer at GRC. It's very attractive to have some type of repair concept that would be versatile enough to repair any damage to any of the orbiter's panels, he says.

To that end, Ritzert was part of a team that developed a metallic overlay panel for the wing's RCC leading edge. Because of the high temperatures experienced at the leading edge, refractory metals-which are defined as having melting points above 2,000 C (3,632 F) were studied. After testing five or six of these metals, he explains, rhenium was selected for the job.

Although refractory metals provide the necessary temperature resistance, they do have one downside: These metals are highly susceptible to oxidation. This property is important because the shuttle's outer skin will be subjected to a highly oxidative environment during reentry.

With the help of James A. Nesbitt, materials research engineer at GRC, a silicone-based coating was selected to protect the metal. The coating is 60% silicon, 20% chromium, and 20% iron and is sold by the trade name R512E.

When the coating is applied to the refractory metal, it reacts with the base metal and forms a couple of layers of silicide, Ritzert says. Arc-jet testing on the rhenium coated with R512E has shown only positive results for the system, Ritzert says.

The overlay team is now working on scaling up the material to cover larger areas of damage with the target panel size of 24 by 24 inches. Currently, the panels have been tested for use on damaged areas up to about 16 inches in diameter, Ritzert notes.

As these research teams continue to improve their materials and design to fix damage that occurs to the shuttle, Dan Bell, thermal protection systems/subsystems manager at Boeing, notes that the company is producing new insulating tiles that are stronger than the current tiles and therefore less likely to be damaged by debris.

We're not just trying to prepare ourselves for damage; we're trying to reduce the likelihood of damage that would require a repair, Bell says. In fact, he notes that the repair materials may never be certified in the traditional context, but rather specific capabilities might be validated.

According to Bell, the new tiles Boeing has developed provide approximately an order of magnitude increase in impact capabilities over the current tiles and are already being applied to the space shuttles. These tiles, Boeing Rigid Insulation-18 (BRI-18), are made of silica and alumina fibers that have been heated to temperatures above 2,000 C.

The tiles are then coated with an impact-resistant material called toughened unipiece fibrous insulation. This coating composed of reactive boron oxide-containing borosilicate glass frit, a silicon tetraboride fluxing agent, and a molybdenum silicide emittance agent penetrates the tile material to form a solid lock with the base tiles, Bell explains. The tiles get a final coat of borosilicate glass frit and a silicon tetraboride fluxing agent to provide an impervious glass emittance layer on the tile surface.

With the shuttle scheduled to make several more flights before its 2010 retirement date, NASA will continue to test various repair materials as well as take advantage of new TPS materials.

At this point, the jury's still out on what type of repair we will baseline as the standard shuttle-repair method, says Kelly Humphries, public affairs specialist at JSC. He notes that NASA will continue to look at the various methods both chemical and mechanical until a firm decision is made.