Materials and To build a spacecraft, we must begin with materials. Sometimes the material choice is the solution. Other times, the design must Manufacturing accommodate the limitations of materials properties. The design of the Space Shuttle systems encountered many material challenges, such as

Introduction weight savings, reusability, and operating in the space environment. Gail Chapline NASA also faced manufacturing challenges, such as evolving federal Nondestructive Testing Innovations regulations, the limited production of the systems, and maintaining Willard Castner flight certification. These constraints drove many innovative materials Patricia Howell solutions. Innovations such as large composite payload bay doors, James Walker Friction Stir Welding Advancements nondestructive materials evaluation, the super lightweight tank, and Robert Ding the understanding of hydrogen effects on materials were pathfinders Jim Butler used in today’s industry. In addition, there were materials innovations Characterization of Materials in engineering testing, flight analysis, and manufacturing processes. in the Hydrogen Environment Jon Frandsen In many areas, materials innovations overcame launch, landing, and Jonathan Burkholder low-Earth orbit operational challenges as well as environmental Gregory Swanson challenges, both in space and on Earth. Space Environment: It’s More Than a Vacuum Lubert Leger Steven Koontz Chemical Fingerprinting Michael Killpack Environmental Assurance Anne Meinhold Unprecedented Accomplishments in the Use of Aluminum-Lithium Alloy Preston McGill Jim Butler Myron Pessin Orbiter Payload Bay Door Lubert Leger Ivan Spiker

200 Engineering Innovations Nondestructive inspection. In the industrial world, (detected by the human eye vs. an visual examination can be quite formal, electronic x-ray detector). Testing Innovations with complex visual aids, pass/fail Nondestructive testing is a routine criteria, training requirements, and part of a spacecraft’s life cycle. For the written procedures. Have you ever selected a piece of fruit reusable shuttle, nondestructive testing based on its appearance or squeezed Nondestructive testing depends on began during the manufacturing and it for that certain feel? Of course you incident or input energy that interacts test phases and was applied throughout have. We all have. In a sense, you with the material or part being examined. its service life. NASA performed performed a nondestructive test. The incident or input energy can be many such nondestructive tests on the Actually, we perform nondestructive modified by reflection from interaction shuttle vehicles and developed most testing every day. We visually examine within or transmission through the nondestructive testing innovations in or evaluate the things we use and buy material or part. The process of response to shuttle problems. to see whether they are suitable for their detection and interpretation of the purpose. In most cases, we give the modified energy is how nondestructive item just a cursory glance or squeeze; testing provides knowledge about the Quantitative Nondestructive however, in some cases, we give it material or part. Tests range from the Testing of Fatigue Cracks a conscious and detailed examination. simple detection and interpretation of One of the most significant We don’t think of these routine reflected visible light by the human eye nondestructive testing innovations examinations as nondestructive tests, (visual examination) to the complex was quantifying the flaw sizes that but they are, and they give us a sense of electronic detection and mathematical conventional nondestructive testing what nondestructive testing is about. reconstruction of through-transmitted methods could reliably detect. NASA Nondestructive testing is defined as the x-radiation (computerized axial used artificially induced fatigue cracks inspection or examination of materials, tomography [CAT] scan). From a to make the determination because parts, and structures to determine their nondestructive testing perspective, the such flaws were relatively easy to integrity and future usefulness without similarity between the simple visual grow and control, hard to detect, and compromising or affecting their examination and the complex CAT scan tended to bound the population of usefulness. The most fundamental is the input energy (visible light vs. flaws of interest. The need to quantify nondestructive test of all is visual x-rays) and the modified energy the reliably detectable crack sizes was

Two examples of the most basic nondestructive testing: Left, a gardener checks ripening vegetables. Right, Astronaut Eileen Collins, STS-114 (2005) mission commander, looks closely at a reinforced carbon-carbon panel on one of the wings of the Space Shuttle Atlantis in the Orbiter Processing Facility at Kennedy Space Center (KSC). Collins and the other crew members were at KSC to take part in hands-on equipment and Orbiter familiarization.

Engineering Innovations 201 mandated by a fracture control interest in having confidence in the starting crack size that could be used in Quantitative Nondestructive Testing fracture and life calculations. Although there was no innovation of any specific nondestructive testing method, quantifying—in a statistical way—the reliably detectable crack sizes associated with the conventional nondestructive evaluation methods was innovative and led the way to the adoption of similar Penetrant quantitative nondestructive evaluation InspectionInspection practices in other industries. Fatigue-cracked Panel Ultrasonic InspectionUltrasonic The quantification of nondestructive Inspection testing methods is commonly referred P X-ray Probability of Detection Curve InspectionX-ray Inspection to today as probability of detection. 100 The Space Shuttle Program developed 90 Eddy Current Inspection some of the earliest data for the 80 Eddy Current

) Inspection %

penetrant, x-ray, ultrasonic, and eddy (

70 n o i current nondestructive testing t c 60 e

t e

methods—the principal nondestructive D

f 50 o

y

testing methods used to inspect shuttle t i

l 40 i b a

components during manufacturing. b 30 o r P Data showed that inspectors certified to 20 aerospace inspection standards could, 10 on average, perform to a certain 0 probability of detection level defined Flaw Size Increasing as standard nondestructive evaluation.

Beyond standard nondestructive The special probability of detection nature, NASA examined a number of evaluation, NASA introduced a special specimen sets typically consisted of nondestructive testing methods. nondestructive evaluation level of 29 randomly distributed cracks of probability of detection wherein the approximately the same size. By Acoustic Emission Monitoring detection of cracks smaller than the detecting all 29 cracks, the inspector Late in the development of the shuttle standard sizes had to be demonstrated and the specific nondestructive Thermal Protection System and by test. Engineers fabricated fatigue- evaluation process were considered just before the first shuttle launch, cracked specimens that were used over capable of detecting the crack size to NASA encountered a major problem many years to certify and recertify, by a 90% probability of detection with with the attachment of the tiles to the test, the inspectors and their 95% confidence. nondestructive evaluation processes to Orbiter’s exterior skin. The bond the smaller, special nondestructive strength of the tile system was lower evaluation crack size. The size of the Nondestructive Testing of than the already-low strength of the fatigue cracks in the specimens was Thermal Protection System Tiles tile material, and this was not targeted to be a surface-breaking accounted for in the design. The low semicircular crack 0 .1 27 cm (0.050 in.) The development of Thermal Protection bond strength was due to stress long by 0.063 cm (0.025 in.) deep, a System tiles was one of the most concentrations at the tile-to-strain size that was significantly smaller than unique and difficult developments of isolation pad bond line interface. the standard nondestructive evaluation the program. Because of this material’s A Nomex ® felt strain isolation pad crack size of 0.3 81 cm (0 .1 50 in.) long “unknowns,” the tile attachment was bonded between each tile and by 0 .1 9 cm (0.075 in.) deep. scheme, and their extremely fragile the Orbiter skin to minimize the

202 Engineering Innovations lateral strain input to the tile from the aluminum skin. These stress concentrations led to early and Acoustic Emission Monitoring of Tiles During Proof Test progressive failures of the tile material at the tile-to-strain isolation Coating pad bond line interface when the tile was loaded. T Tile Attachment Scheme To determine whether low bond strengths existed, engineers resorted to Tile Body proof testing for each tile. This required Silicon Rubber (RTV 560) thousands of individual tile proof tests ® Nomex Strain Isolation Pad prior to first flight. Space Shuttle Silicon Rubber Columbia (Space Transportation (RTV 560) System [STS] -1 ) was at Kennedy Space Primer Center being readied for first flight Aluminum when NASA decided that proof testing Substrate was necessary. Since proof testing was not necessarily nondestructive and tiles could be damaged by the test, NASA sought a means of monitoring potential damage; acoustic emission Acoustic Tile Proof Test Emission nondestructive testing was an obvious Sensor Proof To Acoustic Emission Load choice. The acoustic signatures of a Signal Processing low bond strength tile or a tile damaged during proof test were determined through laboratory proof testing of Acoustic Emissions From Local Tile Failure Tile Body full-size tile arrays. T Stiff Spot / Stress Concentration

To say that the development and Felted Nomex® Pad Showing Stress implementation of acoustic emission Concentration in monitoring during tile proof testing Tile Bond Line Caused by Needling was done on a crash basis would be an understatement. The fast pace was Aluminum Substrate dictated by a program that was already behind schedule, and the tile bond acoustic emission monitoring. By the both tile density and strength. These strength problem threatened significant time the acoustic emission monitoring measurements could be used as a additional delay. At the height of the was phased out, NASA had performed quality-control tool to screen tiles for effort, 18 acoustic emission systems 20,000 acoustic emission monitored low density and low strength and could with fully trained three-person crews proof tests. also determine the orientation of the tile. were in operation 24 hours a day, 7 days a week. The effort was the Sonic Velocity Testing The sonic velocity technique input a largest single concentration of acoustic short-duration mechanical impulse into emission equipment at a single job Another early shuttle nondestructive the tile. A transmitting transducer and a site. As often happens with such testing innovation was the use of an receiving transducer, placed on opposite problems, where one solution can be ultrasonic test technique to ensure sides of the tile, measured the pulse’s overtaken and replaced by another, that the Thermal Protection System transit time through the tile. For the a tile densification design fix for the tiles were structurally sound prior Lockheed-provided tile material, low-strength bond was found and to installation. Evaluation of pulse LI-900 (with bulk density of 144 kg/m 3 implemented prior to first flight, thus or sonic velocity tests showed a [9 pounds/ft 3]), the average through-the- obviating the need for continued velocity relationship with respect to thickness sonic velocity was on the

Engineering Innovations 203 Sonic Velocity Tes ting of Tiles at K ennedy Space Center Thermal Protection System Facility The speed of sound through the tile is related to density and strength.

Pulse Velocity Measurement Unit

Time Transmitter

Sound Waves Tile

Receiver

Tile

order of 640 m/sec (2 ,1 00 ft/sec), and Nondestructive Testing of External areas. The loss of foam applied to the the through-the-thickness flat-wise Tank Spray-on Foam Insulation large areas of the tank was not as much tensile strength was on the order of of concern because the automated Prior to the Columbia accident, no 1.69 kg/cm 2 (24 pounds/in 2). The LI-900 acreage spray-on process was better nondestructive testing methods were acceptance criterion for sonic velocity controlled, making it more unlikely to available for External Tank foam was set at 518 m/sec (1 ,700 ft/sec), come off. In the event it did come off, inspection, although NASA pursued which corresponded to a minimum the pieces would likely be small development efforts from the early strength of 0.9 1 kg/cm 2 (1 3 pounds/in 2). because acreage foam was relatively 1980s until the early 1990s. The foam Sonic velocity testing was phased thin. NASA’s intense focus resulted in was effectively a collection of small out in the early 1990s. the development and implementation air-filled bubbles with thin polyurethane of two methods for foam inspection— membranes, making the foam a thermal terahertz imaging and backscatter and electrical insulator with very high Post-Columbia Accident radiography—that represented new and acoustic attenuation. Due to these Nondestructive unique application of nondestructive properties, it was not feasible to inspect Testing of External Tank inspection methods. the foam with conventional methods A consequence of the Columbia such as eddy current, ultrasonics, or Terahertz Imaging (STS -1 07) accident in 2003 was the thermography. In addition, since the development of several nondestructive foam was considered nonstructural, Terahertz imaging is a method that innovations, including terahertz imaging problems of delaminations occurring operates in the terahertz region of the and backscatter radiography of External during foam application and foam electromagnetic spectrum between Tank foam and thermography of the popping off (“popcorning”) during microwave frequencies and far-infrared reinforced carbon-carbon—both on ascent were considered manageable frequencies. Low-density hydrocarbon orbit and on the ground—during vehicle through process control. materials like External Tank foam were turnaround. The loss of foam, reinforced relatively transparent to terahertz After the Columbia accident, NASA carbon-carbon impact damage, and radiation. Terahertz imaging used a focused on developing nondestructive on-orbit inspection of Thermal pulser to transmit energy into a testing methods for finding voids Protection System damage were all structure and a receiver to record the and delaminations in the thick, problems that could be mitigated to energy reflected off the substrate or hand-sprayed foam applications some extent through the application of internal defects. As the signal traveled around protuberances and closeout nondestructive testing methods. through the structure, its basic wave

204 Engineering Innovations

Terahertz Imaging System This system uses high-frequency electromagnetic pulses.

Transmitter Receiver 2 Transmitted 1 3 1 Foam-Air Pulse Re ection Time 2 Air-Metal Transmitted Re ection Pulse 3 Metal-Foam-Metal Time Re ection Foam-Air Re ection 1 indicates an air gap or delamination.

Insulating 1 Foam Air Gap 2 3 Air Gap

Aluminum Substrate properties were altered by the Backscatter Radiography that is scanned over the test object. The attenuation of the material and any backscattering of x-rays results from Backscatter radiography uses a internal defects. An image was made by the Compton effect—or scattering— conventional industrial x-ray tube to scanning the pulser/receiver in which absorption of the incident generate a collimated beam of x-rays combination over the foam surface and or primary x-rays by the atoms of the displaying the received signal. Probability of detection studies of Backscatter X-ray Imaging System inserted artificial voids showed around X-ray Tube 90% detection of the larger voids in simple geometries, but less than 90% detection in the more-complicated X-rays geometries of voids around protrusions. Collimator Detector Further refinements showed that delaminations were particularly difficult Collimated Backscatter to detect. The detection threshold for a X-ray Beam X-rays 2.54-cm- (1 -in.)-diameter laminar defect was found to be a height of 0.508 cm (0.2 in.), essentially meaning Insulating delaminations could not be detected. Foam The terahertz inspection method was used for engineering evaluation, and any defects found were dealt with by an InsuI lating foa m co vers the Aluminum Substrate External Ta nk. engineering review process. An irradiated column of foam that has voids produces less backscattered x-rays than a void-free column of foam.

Engineering Innovations 205 test material are reradiated at a lower Ground Turnaround Thermography transfer heat into the underlying energy as secondary x-rays in all material, and the surface temperature NASA selected infrared flash directions. The reradiated or would appear the same over the entire thermography as the method to backscattered x-rays were collected test surface; however, a delamination determine the structural integrity of the in collimated radiation detectors would prevent or significantly retard reinforced carbon-carbon components. mounted around the x-ray source. heat flow across the gap created by the Thermography was a fast, Voids or defects in the test material delamination, resulting in more-local noncontacting, one-sided application were imaged in backscatter radiography heat retention and higher surface that was easy to implement in the in the same manner as they were in temperature in comparison to the Orbiter’s servicing environment. conventional through-transmission material surrounding the delamination. radiography. Imaging of voids or Temperature differences were detected defects depended on less absorbing by the infrared camera, which provided material and less backscattered x-rays visual images of the defects. Electronic from the void. signals were processed and enhanced for easier interpretation. The heat pulse Since only the backscattered x-rays was provided by flashing xenon lamps were collected, the technique was in a hooded arrangement that excluded single sided and suited for foam ambient light. The infrared camera was inspection. The foam was well suited transported along a floor-mounted rail for backscatter radiography since system in the Orbiter Processing Facility Compton scattering is greater from low for the leading edge panel inspections, atomic number materials. The allowing full and secure access to all of technique was more sensitive to near the leading edge surfaces. After the surface voids but was unable to detect transport cart was positioned, the delaminations. Like terahertz imaging, camera was positioned manually via a backscatter radiography was used for grid system that allowed the same areas engineering evaluation, and defects to be compared from flight to flight. found were dealt with by an Infrared thermography inspection of the Orbiter nose captured at the instant of the engineering review process. xenon lamp flash. Kennedy Space Center Orbiter The thermography system was Processing Facility. validated on specimens containing flat Nondestructive Testing of bottom holes of different diameters The Thermographic Inspection Reinforced Carbon-Carbon System and depths. Validation testing confirmed System was an active infrared flash Components the ability of the flash thermography thermogaphy system. Thermographic system to detect the size holes that A recommendation of the Columbia inspection examined and recorded needed to be detected. Accident Investigation Board stated: the surface temperature transients “Develop and implement a of the test article after application of After the first Return to Flight comprehensive inspection plan to a short-duration heat pulse. The rate mission—STS -11 4 (2005)—the determine the structural integrity of all of heat transfer away from the test postflight thermography inspection Reinforced Carbon-Carbon (RCC) article surface depended on the thermal discovered a suspicious indication system components. This inspection diffusivity of the material and the in the joggle area of a panel. plan should take advantage of advanced uniformity and integrity of the test Subsequent investigation showed non-destructive inspection technology.” material. Defects in the material would that the indication was a delamination. To comply with this recommendation, retard the heat flow away from the This discovery set in motion an intense NASA investigated advanced surface, thus producing surface focus on joggle-area delaminations inspection technology for inspection of temperature differentials that were and their characterization and the reinforced carbon-carbon leading reflective of the uniformity of the consequence. Many months of further edge panels during ground turnarounds material and its defect content. A tests, development, and refinement and while on orbit. defect-free material would uniformly of the thermography methodology

206 Engineering Innovations determined that critical delaminations would be detected and sized by flash On-orbit Thermography thermography and provided the basis for flightworthiness. Processed On-orbit Thermography infrared images of reinforced The success of infrared thermography carbon-carbon test panels. for ground-based turnaround inspection of the wing leading edge panels and the extensive use of thermography during Return to Flight impact testing made Astronaut Thomas Reiter it the choice for on-orbit inspection of mounting pre-damaged the leading edge reinforced reinforced carbon-carbon test panels on the carbon-carbon material. A thermal International Space Station gradient through the material must exist during STS-121 (2006). to detect subsurface reinforced carbon-carbon damage with infrared thermography. A series of ground tests demonstrated that sunlight or solar heating and shadowing could be used to generate the necessary thermal gradient, which significantly simplified the camera development task. With the feasibility of on-orbit thermography demonstrated and with the spaceflight limitations on weight and power taken into account, NASA selected a commercial off-the-shelf microbolometer camera for modification and development into a space-qualified infrared camera for inspecting the reinforced carbon-carbon for impact damage while on orbit. The extravehicular activity infrared camera operated successfully on its three flights. Two reinforced carbon- carbon test panels with simulated damage were flown and inspected on STS -121 (2006). The intentional impact Extravehicular activity infrared damage in one panel and the flat bottom flight camera. holes in the other panel were clearly imaged. Engineers also performed a similar on-orbit test on two other intentionally damaged reinforced carbon-carbon test panels during a space station extravehicular activity with the

Engineering Innovations 207 same result of clearly imaging the damage. The end result of these efforts was a mature nondestructive inspection Friction Stir Welding Advancements technique that was transitioned and NASA invents welding fixture. demonstrated as an on-orbit nondestructive inspection technique.

Additional Nondestructive Testing Most nondestructive testing innovations resulted from problems that the shuttle encountered over the years, where nondestructive testing provided all or part of the solution. Other solutions worth mentioning include: ultrasonic extensometer measurements of critical shuttle bolt tensioning; terahertz imaging of corrosion under tiles; phased array Friction stir welding units, featuring auto-adjustable pin tools, welded External Tank barrel ultrasonic testing of the External sections at NASA’s Michoud Assembly Facility in New Orleans, Louisiana. The units measured Tank friction stir welds and the 8.4 m (27.5 ft) in diameter and approximately 7.6 m (25 ft) tall to accommodate the largest shuttle crawler-transporter shoes; barrel sections. thermographic leak detection of the In the mid 1990s, NASA pursued the implementation of friction stir welding main engine nozzle; digital technology—a process developed by The Welding Institute of Cambridge, England— radiography of Columbia debris; to improve External Tank welds. This effort led to the invention of an auto-adjustable surface replication of flow liner cracks; welding pin tool adopted by the Space Shuttle Program, the Ares Program (NASA- and the on-board wing leading edge health monitoring impact system. developed heavy launch vehicles), and industry. Standard fusion-welding techniques rely on torch-generated heat to melt and join the metal. Friction stir welding does not melt the metal. Instead, it uses a rotating pin and “shoulder” to generate friction, stir the metal together, and forge a bond. This process results in welds with mechanical properties superior to fusion welds.

Standard friction stir welding technology has drawbacks, however; namely, a non-adjustable pin tool that leaves a “keyhole” at the end of a circular weld and the inability to automatically adjust the pin length for materials of varying thickness. NASA’s implementation of friction stir welding for the External Tank resulted in the invention and patenting of an auto-adjustable pin tool that automatically retracts and extends in and out of the shoulder. This feature provides the capability to make 360-degree welds without leaving a keyhole, and to weld varying thicknesses.

During 2002-2003, NASA and the External Tank prime contractor, Lockheed Martin, implemented auto-adjustable pin tool friction stir welding for liquid hydrogen and liquid oxygen tank longitudinal welds. Since that time, these friction stir welds have been virtually defect-free. NASA’s invention was being used to weld Ares upper-stage cryogenic hardware. It has also been adopted by industry and is being used in the manufacturing of aerospace and aircraft frames.

208 Engineering Innovations Characterization of engines before they experienced Insights on Hydrogen the extreme loads of liftoff and Environment Embrittlement Materials in the flight; this is called internal hydrogen Hydrogen Environment embrittlement. Under the right NASA studied the effects of hydrogen conditions, internal hydrogen embrittlement in the 1960s. In the early 1970s, the scope of NASA-sponsored From the humid, corrosion-friendly embrittlement has the potential to research broadened to include hydrogen atmosphere of Kennedy Space Center, render materials too weak and brittle environment embrittlement effects on to the extreme heat of ascent, to the to survive high stresses applied later. fracture and fatigue. Engineers cold vacuum of space, the Space Alternatively, embrittlement can immersed specimens in hydrogen and Shuttle faced one hostile environment affect a material that is immersed in performed a battery of tests. They after another. One of those harsh hydrogen while the material is being applied repeated load cycles to environments—the hydrogen stressed and deformed. This specimens until they fatigued and broke environment—existed within the phenomenon is called hydrogen apart; measured crack growth rates in shuttle itself. Liquid hydrogen was environment embrittlement, which can cyclic loading and under a constant the fuel that powered the shuttle’s occur in pressurized hydrogen storage static load; and tested materials in complex, powerful, and reusable main vessels. These vessels are constantly high-heat and high-pressure hydrogen engine. Hydrogen provided the high stressed while in contact with environments. Always, results were specific impulse—the bang per pound hydrogen. Hydrogen environment compared for each material to its of fuel needed to perform the shuttle’s embrittlement can potentially reduce performance in room-temperature air. heavy-lifting duties. Hydrogen, ductility over time and enable however, was also a potential threat cracking, or hydrogen may simply During the early years of the Space to the very metal of the propulsion reduce the strength of a vessel until it Shuttle Program, NASA and contractor system that used it. is too weak to bear its own pressure. engineers made a number of key discoveries regarding hydrogen The diffusion of hydrogen atoms into Finally, hydrogen can react chemically environment embrittlement. First, a metal can make it more brittle and with elements that are present in a cracks were shown to grow faster when prone to cracking—a process called metal, forming inclusions that can loaded in a hydrogen environment. hydrogen embrittlement. This effect degrade the properties of that metal or This finding would have significant can reduce the toughness of carefully even cause blisters on the metal’s implications for the shuttle design, as selected and prepared materials. surface. This effect is called hydrogen fracture assessments of the propulsion A concern that exposure to hydrogen reaction embrittlement. In the shuttle’s system would have to account for might encourage crack growth was main engine components, the reaction accelerated cracking. Second, scientists present from the beginning of the Space between hydrogen and the titanium observed that hydrogen environment Shuttle Program, but the rationale for alloys occurred to internally form embrittlement could result in crack using hydrogen was compelling. brittle titanium hydrides, which was growth under a constant static load. most likely to occur at locations where This behavior was unusual for metals. The Challenge of the Hydrogen there were high tensile stresses in the Ductile materials such as metals tend part. Hydrogen reaction embrittlement Environment to crack in alternating stress fields, can affect steels when hydrogen not in fixed ones, unless a chemical Hydrogen embrittlement posed more atoms combine with the carbon atoms or an environmental cause is present. than a single engineering problem dissolved in the metal. Hydrogen Again, the design of the shuttle would for the Space Shuttle. This was partly reaction embrittlement can also blister have to account for this effect. Finally, because hydrogen embrittlement can copper when hydrogen reacts with the hydrogen environment embrittlement occur in three different ways. The internal oxygen in a solid copper piece, was shown to have more severe most common mode occurs when thereby forming steam blisters. effects at higher pressures. Intriguingly, hydrogen is absorbed by a material degradation of tensile properties was that is relatively unstressed, such as found to be proportional to the square the components of the shuttle’s main root of pressure.

Engineering Innovations 209 The overall approach to hydrogen experimented with coatings and plating These approaches—a combination of environment embrittlement research processes. The concept was to shield two or more hydrogen environment was straightforward. As a matter of vulnerable metal from any contact with embrittlement prevention methods— common practice, NASA characterized hydrogen. A thin layer of hydrogen were the practical solution for many of the strength and fracture behavior of its environment embrittlement-resistant the embrittlement-vulnerable parts of alloys. To determine how these alloys metal would form a barrier that the engines. For example, the most would tolerate hydrogen, engineers separated at-risk material from heavily used alloy in the engines was simply adapted their tests to include a hydrogen fuel. Inconel ® 71 8, an alloy known to be high-pressure hydrogen environment. affected by hydrogen environment Engineers concentrated their research After learning that high pressure embrittlement. Engineers identified an on coatings that had low solubility exacerbates hydrogen environment alternative heat treatment, different and low-diffusion rates for hydrogen embrittlement, they further adapted the from the one typically used, which at room temperature. Testing had tests to include a hydrogen pressure of limited embrittlement. But this alone demonstrated that hydrogen 703 kg/cm 2 (1 0,000 psi). Later in the was insufficient. In the most critical environment embrittlement is worst program, materials being considered locations, the alternative heat treatment at near-room temperature, so NASA for use in the main engine were tested was combined with copper plating and selected coatings based on their at a reduced pressure of 492 kg/cm weld overlays. effectiveness in that range. The most (7,000 psi) to be more consistent with efficient barrier to hydrogen, engineers A unique processing approach was also operation conditions. The difference found, was gold plating; however, the used to prevent embrittlement in the between room-temperature air material cost of developing gold plating engine’s main combustion chamber. property data and these new results was processes was a significant factor. This chamber was made with a highly a measurable effect of hydrogen Engineers observed that copper conductive copper alloy. Its walls environment embrittlement. Now that plating provided as much protection contained cooling channels that these effects could be quantified, the as gold, as long as a thicker and circulated cold liquid hydrogen and next step was to safeguard the shuttle. heavier layer was applied. kept the chamber from melting in the extreme heat of combustion. But the Protecting weld surfaces was often hydrogen-filled channels became Making Parts Resistant more challenging. The weld surfaces prone to hydrogen environment to Hydrogen Environment exposed to hydrogen fuel during flight embrittlement. These liquid hydrogen Embrittlement were typically not accessible to plating channels were made by machining slots after the weld was complete. One way to protect the main engines in the copper and then plated with Overcoming this problem required a from hydrogen environment nickel, which closed out the open slot more time-consuming and costly embrittlement was through materials and formed a coolant channel. The approach. Engineers developed weld selection. NASA chose naturally nickel plate cracked in the hydrogen overlays, processes in which hydrogen resistant materials when possible. There environment and reduced the pressure environment embrittlement-resistant were, however, often a multitude of capability of the channels. Engineers filler metals were added during a final conflicting demands on these materials: devised a two-part solution. First, they welding pass. These protective fillers they had to be lightweight, strong, developed an alternative heat treatment sealed over the weld joints and provided tough, well suited for the to optimize nickel’s performance in the necessary barrier from hydrogen. manufacturing processes that shaped hydrogen. Next, they coated the nickel NASA used overlays in combination them, weldable, and able to bear with a layer of copper to isolate it from with plating of accessible regions to significant temperature swings. The the liquid hydrogen. This two-pronged prevent hydrogen environment additional constraint of imperviousness strategy worked, and liquid hydrogen embrittlement in engine welds. to hydrogen environment embrittlement could be safely used as the combustion was not always realistic, so engineers chamber coolant.

210 Engineering Innovations Addressing Internal was repeatedly exposed to hydrogen internal hydrogen embrittlement. For Hydrogen Embrittlement in flight and after flight, at high instance, protective plating would temperatures and extreme pressure. operate on the same principle—the Whereas hydrogen environment The report suggested that in these creation of a barrier between hydrogen embrittlement was of great concern at exceptional heat and pressure conditions and a vulnerable alloy—whether NASA in the 1960s, internal hydrogen some engine materials might, in fact, hydrogen environment embrittlement or embrittlement was largely dismissed gather small amounts of hydrogen with internal hydrogen embrittlement was even through the early years of the each flight. Gradually, over time, these the chief worry. Continued testing of Space Shuttle Program. Internal materials could accumulate enough “charged” specimens would allow hydrogen embrittlement had never hydrogen to undermine ductility. quantification of internal hydrogen been a significant problem for the types embrittlement damage, just as hydrogen Engineers developed a special test of materials used in spaceflight immersion testing had enabled regimen to screen materials for hardware. The superalloys and measurement of hydrogen environment high-temperature, high-pressure particular stainless steels selected by embrittlement effects. NASA were thought to be resistant to hydrogen accumulation. Test specimens internal hydrogen embrittlement. were “charged” with hydrogen at Taking strategies generated to avoid 2 Engineers thought the face-centered, 649°C (1 ,200°F) and 3 51 .6 kg/cm hydrogen environment embrittlement cubic, close-packed crystal structure (5,000 psi). They were then quickly and refitting them to prevent internal would leave too little room for cooled and tested for strength and hydrogen embrittlement, however, often hydrogen to permeate and diffuse. ductility under normal conditions. required additional analysis. For Surprisingly, embrittlement by example, from the beginning of the Recall that internal hydrogen internal hydrogen embrittlement was Space Shuttle Program NASA used embrittlement occurs when hydrogen is observed to be as severe as by coatings to separate at-risk metals from absorbed before high operational hydrogen environment embrittlement. hydrogen. The agency intentionally stresses. Hydrogen enters into the metal As a subsequent string of fatigue tests chose these coatings for their and remains there, making it more confirmed this comparison, NASA performance at near-room temperature, brittle and likely to crack when extreme had to reevaluate its approach to when hydrogen environment service loads are applied later. It is the preventing hydrogen embrittlement. embrittlement is most aggressive. Tests accumulation of absorbed hydrogen, The agency’s focus on hydrogen showed the coatings were less effective rather than the immediate exposure at environment embrittlement had been a in the high heat that promotes internal the moment of high stress, that near-total focus. Now, a new awareness hydrogen embrittlement. New research compromises an internal hydrogen of internal hydrogen embrittlement and experimentation was required to embrittlement-affected material. When would drive a reexamination. prove that these protective coatings NASA initially designed the main were adequate—that, although they Fortunately, the process for calculating engine, engineers accounted for didn’t completely prevent the absorption design properties from test data had hydrogen absorbed during of hydrogen when temperatures and been conservative. The margins of manufacturing. Engineers, however, pressures were extreme, they did reduce safety were wide enough to bound the thought that the materials that were it to safe levels. formed and processed without combined effects of internal hydrogen collecting a significant amount of embrittlement and hydrogen hydrogen were not in danger of environment embrittlement. The wealth Special Cases: High-Pressure absorbing considerable amounts later. of experience gained in studying Fuel Turbopump Housing hydrogen environment embrittlement This notion about internal hydrogen and mitigating its effects also worked in NASA encountered a unique embrittlement was challenged during NASA’s favor. Some of the same hydrogen embrittlement issue during the preparation of an engine failure methodologies could now be applied to development testing of the main analysis document in 1988. The engine engine high-pressure fuel turbopump.

Engineering Innovations 211 High-Pressure Fuel and Oxidizer Turbopump Turbine Blade Cracks

After observing cracks on polycrystalline turbine blades, NASA redesigned the blades as single-crystal parts. First Stage Blade 42 Trailing Edge Root When tested in hydrogen, cracks were detected. Scientists used a Brazilian disc test to create the tensile and shear stresses that had caused growth. NASA resolved cracking in the airfoil with changes that eliminated stress concentrations and smoothed the flow of molten metal during casting. To assess cracking at damper contacts, scientists extracted test specimens from single crystal bars, machined contact pins from the damper material, and loaded two specimens. This contact fixture was supported in a test rig that allowed the temperature, loads, and load Schematic of Test Rig cycle rate to be varied. Solid Core Clamp Specimens were pre-charged Disc-shaped Specimens with hydrogen, tested at Clamped in Place elevated temperatures, and Contact Pin cycled at high frequency to actual operating conditions. Normal Force Normal Force Dynamic Dynamic Displacement Displacement

212 Engineering Innovations A leak developed during the test; Clearly, the combination of the Space Environment: this leak was traced to cracks in the hydrogen and steam mixture and mounting flange of the turbopump’s the uncommonly high stress It’s More Than housing. The housing was made from concentrations was promoting a Vacuum embrittlement-prone nickel-chromium hydrogen environment embrittlement ® ® alloy Inconel 71 8, and the cracks were in Inconel 100 at high temperatures. We know that materials behave found to originate in small regions of Resolving this issue required three differently in different environments highly concentrated stress. So, engineers modifications. First, detailed changes on Earth. For example, aluminum changed the material to a more- to the shape of the housing were made, does not change on a pantry shelf for ® hydrogen-tolerant alloy, Inconel 100, further reducing stress concentrations. years yet rapidly corrodes or degrades and they redesigned the housing to Second, gold plating was added to in salt water. reduce stress concentrations. This shield the Inconel ® 100 from the hot initially appeared to solve the problem. hydrogen and steam mixture. Finally, One would think that such material Then, cracks were discovered in other a manufacturing process called “shot degradation effects would be eliminated parts of the housing. Structural and peening” was used to fortify the by going to the near-perfect vacuum thermal analysis could not explain this surface of the housing against tensile of space in low-Earth orbit. In fact, cracking. The locations and size of the stresses by impacting it with shot, many of these effects are eliminated. cracks did not fit with existing fatigue determined to be promoting fracture, However, Orbiter systems produced gas, and crack-growth data. and therefore eliminated. particles, and light when engines, overboard dumps, and other systems To resolve this inconsistency, engineers operated, thereby creating an induced considered the service conditions of Summary environment in the immediate vicinity the housing. The operating environment The material characterization done in of the spacecraft. In addition, movement of the cracked regions was a mixture of of the shuttle through the tenuous high-pressure hydrogen and steam at the design phase of the main engine, and the subsequent anomaly resolution upper reaches of Earth’s atmosphere 149°C to 260°C (300°F to 500°F). (low-Earth orbit) at orbital velocity Generally, hydrogen environment during its development phase, expanded both the material properties produced additional contributions to embrittlement occurs near room the induced environment in the form temperature and would not be a database and the understanding of hydrogen embrittlement. The range of spacecraft glow and atomic oxygen significant concern at that level of heat; effects on certain materials. The however, because of the unexplained of hydrogen embrittlement data has been broadened from essentially interactions of spacecraft materials cracking, a decision was made to test with space environment factors like ® encompassing only steels to now Inconel 100 at elevated temperatures in solar ultraviolet (UV) light, atomic hydrogen and hydrogen mixed with including superalloys. It was also extended from including primarily oxygen, ionizing radiation, and steam. Again, the results were extremes of temperature can actually unexpected. Engineers observed a tensile properties to including extensive low-cycle fatigue and be detrimental to the life of materials pronounced reduction in strength and used in spacecraft systems. ductility in these environments at fracture-mechanics testing in elevated temperatures. Crack growth conditions favorable to internal For the Orbiter to perform certain occurred at highly accelerated rates— hydrogen embrittlement or hydrogen functions and serve as a platform for as high as two orders of magnitude environment embrittlement. The scientific measurements, the effects above room-temperature air when the resultant material properties database, of natural and Orbiter-induced — crack was heavily loaded to 30 ksi √i n now approaching 50 years of maturity, environments had to be evaluated and — (33 MPa √ m) and held for normal is valuable not only because these controlled. Payload sensitivities to these engine operating time. Moreover, crack materials are still being used, but also environmental effects varied, depending growth was driven by both the number because it serves as a foundation for on payload characteristics. Earth-based of load cycles and the duration of each predicting how other materials will observatories and other instruments are load cycle. Crack growth is typically perform under similar conditions—and affected by the Earth’s atmosphere in sensitive to the number and magnitude in the space programs of the future. terms of producing unwanted light of load cycles but not to the length of background and other contamination time for each cycle. effects. Therefore, NASA developed

Engineering Innovations 213 essential analytical tools for environment prediction as well as measurement systems for environment definition and performance verification, thus enabling a greater understanding of natural and induced environment effects for space exploration.

Induced Environment Characterization NASA developed mathematical models to assess and predict the induced environment in the Orbiter cargo bay during the design and development phase of the Space Shuttle Program. Models contained the vehicle geometry, vehicle flight attitude, gas and vapor emission source characteristics, and used low-pressure gas transport physics to calculate local gas densities, column densities (number of molecular species seen along a line of sight), as well as contaminant deposition effects on functional surfaces. Gas transport calculations were based on low-pressure The Atlantic Ocean southeast of the Bahamas is in the background as Columbia’s Shuttle Robotic Arm molecular flow physics and included and end effector grasp a multi-instrument monitor for detecting contaminants. The experiment, called scattering from Orbiter surfaces and the the Induced Environment Contaminant Monitor, was flown on STS-4 (1982). The tail of the Orbiter can be seen below. natural low-Earth orbit environment. The Induced Environment surfaces, quartz-crystal microbalances subsequent days. Shuttle flight attitude Contamination Monitor measured for deposited mass measurement, requirements could affect the cargo bay the induced environment on three a camera/photometer pair for particle gaseous environment via solar heating missions—Space Transportation measurement in the field of view, effects as well as the gases produced by System (STS)-2 (1 981 ), STS-3 (1 982), and a mass spectrometer. Additional engine firings. These gases could reach and STS-4 (1 982)—and was capable flight measurements made on the payload bay by direct or scattered of being moved using the Shuttle STS-52 (1 992) and many payloads flow. Frequently, specific payload or Robotic Arm to various locations for provided more data. shuttle system attitude or thermal control requirements conflicted with specific measurements. Most Before the induced environment the quiescent induced environment measurements were made during the measurements could be properly required by some payloads. on-orbit phase. This measurement interpreted, several on-orbit operational package was flown on the three aspects needed to be understood. With the above operational missions to assess shuttle system Because of the size of the vehicle and characteristics, data collected with the performance. Instruments included a its payloads, desorption of adsorbed monitor and subsequent shuttle humidity monitor, an air sampler for gases such as water, oxygen, and operations showed that, in general, the gas collection and analysis after nitrogen (adsorbed on Earth) took a measured data either met or were close return, a cascade impactor for fairly long time, the induced to the requirements of sensitive particulate measurement, passive environment on the first day of a payloads during quiescent periods. samples for optical degradation of mission was affected more than on A large qualification to this statement

214 Engineering Innovations had to be made based on a new payloads because of the release of Discovery of Effects understanding of the interaction of water over long periods of time. of Oxygen Atoms the natural environment with vehicle Other contamination-sensitive payloads After STS -1 (1 981 ) returned to Earth, surfaces. This interaction resulted in such as Hubble Space Telescope, researchers visually examined the significantly more light emissions and however, were not only successfully material surfaces in the payload bay material surface effects than originally delivered to space but were also for signs of contamination effects. expected. Data also identified an repaired in the payload bay. Most surfaces appeared pristine, additional problem of recontact of except for the exterior of the television particles released from the shuttle Unique Features Made camera thermal blankets and some during water dumps with surfaces in the It Possible painted surfaces. The outside surface payload bay. The induced environment of the blankets consisted of an organic control program instituted for the Space The Orbiter was the first crewed (polyimide) film that, before flight, Shuttle Program marked a giant step vehicle to provide protection of appeared gold colored and had a from the control of small free-flying instrumentation and sensitive surfaces glossy finish. After flight, most films instrument packages to the control of a in the payload bay during ascent were altered to a yellow color and no large and complex space vehicle with a and re-entry and allow exposure to longer had a glossy finish but, rather, mixed complement of payloads. This the low-Earth orbit environment. appeared carpet-like under high approach helped develop a system with Effects were observed without being magnification. Only the surfaces of good performance, defined the vehicle modified by flight heating or gross organic materials were affected; bulk associated environment, and facilitated contamination. Also, as part of the properties remained unchanged. effective communication between the induced environment control program, program and users. the entire payload bay was examined Patterns on modified surfaces indicated immediately on return. Because of directional effects and, surprisingly, The induced environment program these unique aspects, NASA was able the flight-exposed surfaces were found also showed that some attached to discover and quantify unexpected to have receded rather than having payloads were not compatible with interactions between the environment deposited contaminants. The patterns the shuttle system and its associated of low-Earth and the vehicle. on the surfaces were related to the

Atomic Oxygen Effects on Polymers and Plastics in low-Earth Orbit as Seen With the Scanning Electron Microscope; STS-46 (1992)

a b c

a) Scanning electron microscope image of a typical Kapto n ® polyimide plastic sheet. The various specs and bumps are from the inorganic filler used in plastic sheet manufacture. b) Scanning electron microscope image of a typical Kapto n ® polyimide plastic sheet after exposure to surface bombardment by atomic oxygen in low-Earth orbit. The rough surface is typical of atomic oxygen attack on plastics in low-Earth orbit and is the result of the strong dependence of chemical reaction on atom-surface collision energy. Note how some of the inorganic filler particles are standing on pedestals because they protect the underlying plastic from atomic oxygen attack. c) Scanning electron microscope image of a microelectron fabrication etching target also flown on STS-46 and exposed to low-Earth orbit atomic oxygen. The highly directional attack of low-Earth orbit atomic oxygen produced a clean, high-resolution removal of the unprotected plastic around the pattern of protective inorganic surface coatings. High-speed neutral atomic oxygen beams in ground-based production facilities may be a useful adjunct to microelectronic production as described in US Patent 5,271,800.

Engineering Innovations 215 vehicle velocity vector. When flights had the same limitations Researchers also evaluated coatings combining these data with the but supported the STS -1 data. that could be used to protect surfaces atmospheric composition and densities, Extrapolation of these preliminary from interaction with the environment. the material surface recession was recession data to longer-term missions Reaction rates were based on atomic caused by the high-velocity collision showed the potential for significant oxygen densities determined from of oxygen atoms with forward-facing performance degradation of critical long-term atmospheric density models, Orbiter surfaces leading to surface hardware, so specific flight potentially introducing errors in degradation by oxidation reactions. experiments were carried out to short-term experiment data. In addition, Oxygen atoms are a major constituent of quantify the recession characteristics researchers obtained very little insight the natural low-Earth orbit environment and rates for materials of interest. into the reaction mechanism(s). through which the shuttle flew at an orbital velocity of nearly 8 km/sec On-orbit Materials Behavior An additional flight experiment— (1 7,895 mph). The collision energy of Evaluation of Oxygen Interaction with Fifteen organizations participated in a oxygen atoms striking forward-facing Materials III—addressing both of these flight experiment on STS-8 (1 983) to shuttle surfaces in low-Earth orbit was questions was flown on STS-46 (1 992). understand materials behavior in the extremely high—on the order of 5 The primary objective was to produce low-Earth orbit environment. The electron volts (eV) —1 00 times greater benchmark atomic oxygen reactivity objective was to control some of the than the energy of atoms in typical data by measuring the atom flux parameters to obtain more-accurate low-pressure laboratory oxygen atom during material surface exposure. recession rates. The mission had a generators. The high collision energy of Secondary experiment objectives dedicated exposure to direct atom oxygen atoms in low-Earth orbit plays included: characterizing the induced impact (payload bay pointing in the an important role in surface reactivity environment near several surfaces; velocity direction) of 4 1.7 hours at an and surface recession rates. acquiring basic chemistry data related altitude of 225 km ( 121 nautical miles) to reaction mechanism; determining Material recession rates are determined resulting in the largest fluence of the the effects of temperature, mechanical by normalizing the change in sample early missions (3.5 x 10 20 atoms/cm 2). stress, atom fluence, and solar UV mass to the number of oxygen atoms Temperature control at two set points radiation on material reactivity; reaching the surface over the exposure was provided as well as instruments to and characterizing the induced and time (atoms/cm 2, fluence). Atom control UV and exposure to electrically contamination environments in the density is obtained from the standard charged ionospheric plasma species. shuttle payload bay. This experiment atmospheric density models used by The STS-8 experiment provided was a team effort involving NASA NASA and the Department of Defense. significant insight into low-Earth orbit centers, US Air Force, NASA Space Since oxygen atoms travel much environment interactions with Station Freedom team, Aerospace slower than the Orbiter, they impacted materials. Researchers established Corporation, University of Alabama the surfaces in question only when quantitative reaction rates for more in Huntsville, National Space Agency facing toward the vehicle velocity than 50 materials, and were in the of Japan, European Space Agency, and vector and had to be integrated over range of 2-3 x 10-24 cm 3/atom for the Canadian Space Agency. time and vehicle orientation. STS -1 hydrocarbon-based materials. recession data were approximate STS-46 provided an opportunity to Perfluorinated organic materials were because they had to be integrated make density measurements at several basically nonreactive and over changing vehicle attitude; had altitudes: 427, 296, and 230 km (2 31 , silicone-based materials stopped limited atom flux, uncontrolled 160, and 124 nautical miles). However, reacting after formation of a protective surface temperatures and solar UV the vehicle flew for 42 hours at 230 km silicon oxide surface coating. Material exposure; and predicted atom densities. (1 24 nautical miles) with the payload reaction rates, as a first approximation, Recession rates determined from bay surfaces pointed into the velocity were found to be independent of material samples exposed during the vector during the main portion of temperature, material morphology, and STS-5 (1 982) mission and Induced the mission to obtain high fluence. exposure to solar radiation or Environmental Contamination Monitor The mass spectrometer provided by the electrically charged ionspheric species.

216 Engineering Innovations US Air Force was the key component of the experiment and was capable of sampling both the direct atomic oxygen flux as well as the local neutral environment created by interaction of atomic oxygen with surfaces placed in a carousel. Five carousel sections were each coated with a different material to determine the material effects on released gases. Material samples trays, which provided temperature control plus instruments to control other exposure conditions, were placed on each side of the mass spectrometer/carousel. NASA achieved all of the Evaluation of Oxygen Interaction with Materials III objectives during STS-46. A well-characterized, short-term, high-fluence atomic oxygen exposure was provided for a large number of materials, many of which had never been exposed to a known low-Earth orbit atomic oxygen environment. The data provided a benchmark reaction rate Evaluation of Oxygen Interaction with Materials III flight experiment in the Orbiter payload bay of database, which has been used by the STS-46 (1992). Material exposure samples are located on both sides of the mass spectrometer gas evolution measurement assembly in the center. International Space Station, Hubble, and others to select materials and coatings to solar extreme UV radiation damage Intelsat 603 that was used to maintain ensure long-term durability. in increasing the generally low communications from a geosynchronous Reaction rate data for many of the surface reactivity of perfluorinated orbit. Failure of the Titan-3 upper stage materials from earlier experiments were organic materials. The mass left Intelsat 603 marooned in an confirmed, as was the generally weak spectrometer/carousel experiment unacceptable low-Earth orbit and dependence of these reaction rates on produced over 46,000 mass spectra subject to the effects of atomic oxygen temperature, solar UV exposure, providing detailed characterization degradation of its solar panels, which oxygen atom flux, and exposure to of both the natural and the induced could have rendered the satellite useless. charged ionospheric species. The role environment. The mass spectrometer NASA quickly advised the International of surface collision energy on oxygen database provided a valuable resource Telecommunications Satellite atom reactivity was quantified by for the verification of various models Organization (Intelsat) Consortium of comparing flight reaction rates of key of rarified gas and ionospheric plasma the atomic oxygen risk to Intelsat 603, Evaluation of Oxygen Interaction with flow around spacecraft. leading to the decision to place the Materials III experiment materials satellite in a configuration that was with reactivity measurements made in Intelsat Satellite expected to minimize atomic oxygen well-characterized laboratory oxygen damage to the silver interconnects on Knowledge gained from atomic atom systems with lower surface the solar panels. This was accomplished oxygen reactivity studies played a collision energies. This evaluation by raising the satellite altitude and key role in the STS-49 (1 992) rescue also provided an important benchmark changing its flight attitude so that of the communications satellite point for understanding the role of atomic oxygen fluence was minimized.

Engineering Innovations 217 NASA Discovers Light Emissions Spacecraft glow is caused by the interaction of high-velocity oxygen On the early shuttle flights, NASA atoms with nitrous oxide absorbed on observed another effect caused by the surfaces, which produces nitrogen the interaction between spacecraft dioxide in an electronically excited surfaces and the low-Earth orbit state. The excited nitrogen dioxide is environment. Photographs obtained released from the surfaces and emits by using intensified cameras and light as it moves away and decays conducted from the Orbiter cabin from its excited state. Some nitrous windows showed light emissions oxide on the surface and some of the (glow) from the Orbiter surfaces when released nitrogen dioxide result from in forward-facing conditions. the natural environment. The light The shuttle provided an excellent emission occurs on any spacecraft opportunity to further study this operating in low-Earth orbit; phenomenon. On STS-41D (1 984), however, the glow could be enhanced astronauts photographed various by operation of the shuttle attitude material samples using a special glow control engines, which produced spectrometer to obtain additional data nitrous oxide and nitrogen dioxide and determine if the glow was as reaction products. These findings dependent on surface composition. led to a better understanding of the These measurements, along with the behavior of spacecraft operating in material recession effects and data low-Earth orbit and improved accuracy obtained on subsequent flights, led to of instrument measurements. a definition of the glow mechanism. The Intelsat Solar Array Coupon flight experiment shown mounted on the Shuttle Robotic Arm lower arm boom and exposed to space environment conditions during STS-41 (1990).

To provide facts needed for a final decision about a rescue flight, NASA designed and executed the Intelsat Solar Array Coupon flight experiment on STS-4 1 (1 990). The experiment results, in combination with ground-based testing, supported the decision to conduct the STS-49 satellite rescue mission. On this mission, Intelsat 603 was captured and equipped with a solid re-boost motor to carry it to successful geosynchronous orbit.

STS-62 (1994) orbits Earth during a “night” pass, documenting the glow phenomenon surrounding the vertical stabilizer and the Orbital Maneuvering System pods of the spacecraft.

218 Engineering Innovations Chemical from lot to lot and that supplier integrity over time. Central to the process changes or even contaminated solution was both a foolproof analysis Fingerprinting material can appear to be “in spec” process and an electronic data repository but actually contain subtle, critical for benchmarking and monitoring. Comprehensive Electronic differences. This situation has the System for Greater Flight Safety potential to cause significant problems with hardware performance. A Chemical “Fingerprint” A critical concern for all complex Just as fingerprints are a precise manufacturing operations is that NASA needed a system to readily detect method to confirm an individual’s contaminants and material changes over those subtle yet potentially detrimental identity, the solid rocket motor project time can creep into the production material variances to ensure the employed chemical “fingerprints” to environment and threaten product predictability of material properties and verify the quality of an incoming raw quality. This was the challenge for the the reliability of shuttle reusable solid material. These fingerprints comprised solid rocket motors, which were in rocket motors. The envisioned solution a detailed spectrum of a given production for 30 years. was to pioneer consistent and repeatable analytical methods tailored to specific, material’s chemical signature, which It is possible that vendor-supplied raw critical materials that would yield could be captured digitally and verified materials appear to meet specifications accurate assessments of material using a combination of sophisticated laboratory equipment and custom analytical methods. The challenge was to accurately Environmental establish a baseline chemical fingerprint of each material and develop Assurance reproducible analytical test methods to monitor lot-to-lot material variability. Reuseable Solid Rocket Motor A further objective was to gain a TCA* Reduction History greater understanding of critical reusable solid rocket motor materials, such as insulation and liner ingredients, many of which were the same materials used since the Space Shuttle Program’s inception. New analytical techniques such as the atomic force microscope were used to assess materials at fundamental chemical, molecular, and mechanical levels. These new techniques provided the high level of detail sought. Because of unique attributes inherent in each * 1,1,1 trichloroethane material, a one-size-fits-all analysis method was not feasible. During the Space Shuttle Program’s operation, issues arose regarding the use of substances that did not meet emerging environmental regulations and current To facilitate documentation and data industry standards. NASA worked to develop chemicals, technologies, and processes sharing, the project team envisioned a comprehensive electronic database to that met regulatory requirements, and the agency strove to identify, qualify, and provide ready access to all relevant replace materials that were becoming obsolete as a result of environmental issues. data. The targeted level of background The stringent demands of human spaceflight required extensive testing and detail included everything from where qualification of these replacement materials. and how a material was properly used to details of chemical composition.

Engineering Innovations 219 researchers acquired test samples (usually three to five lots of materials) Tools for Materials Evaluation and developed reliable test methods. Atomic Force Microscope Images of Metal Surface Because of the unique nature of each material, test methods were tailored to Image 3-D Plot each of the 14 materials. A “material” site in the project database was designed to ensure all data were properly logged and critical reports were written and filed. Once the team agreed sufficient data had been generated, a formal report was drafted and test methods were selected to develop new standard acceptance Grit Blasted procedures that would ultimately be 1 µm used by quality control technicians to certify vendor materials. The framework developed to package the wide-ranging data was termed the Fingerprinting Viewer. Program data were presented through a series of cascading menu pages, each with increasing levels of detail.

Polished 1 µm The Outcomes The atomic force microscope affords a visual evaluation of surface preparation processes Beyond meeting the primary program to improve understanding of their effects on bonding. The top panel represents topography objectives, a number of resulting of a grit blast surface for comparison to a highly polished one. The atomic force microscope uses an extremely fine probe to measure minute interactions with surface features even benefits were noted. First, through down to an atomic scale. The maps at left are scaled from black at the bottom of valleys to increased data sharing, employees

white at the tops of peaks within the scanned area. The 3-D projections at right are on a .

d communicated more effectively, both e v

common height scale. The grit blast surface clearly offers greatly increased surface area r e

s internally and with subtier suppliers. e

and mechanical interlocking for enhanced bonding. Beyond simple topography, the probe r

s t The powerful analytical methods interactions with atomic forces can also measure and map properties such as microscopic h g i r

l hardness or elastic modulus on various particles and/or phase transitions in a composite l employed also added to the suppliers’ A

. K

material, which in turn can be correlated with chemical and physical properties. T materials knowledge base. Subtle A

© materials changes that possibly resulted from process drift or changes The ideal system would enable a The Fingerprinting Process at subtier suppliers were detectable. qualified chemist to immediately Eight subtier suppliers subsequently The chemical fingerprinting program, examine original chemical analysis data implemented their own in-house which began in 1998 with a prioritized for the subtle yet significant differences chemical fingerprinting programs to list of 14 critical materials, employed between the latest lot of material and improve product consistency, recertify a team approach to quantify and previous good or bad samples. material after production changes, document each material. The or even help develop key steps in To develop such a system, commercially interdisciplinary team included design the manufacturing process to ensure available hardware and software were engineering, materials and processes repeatable quality levels. used to the greatest extent possible. engineering, procurement quality Since an electronic framework to tie engineering, and analytical chemistry. Additionally, engineers could now the data together did not exist, one was Each discipline group proposed test accurately establish shelf-life designed in-house. plans that included the types of testing extensions and storage requirements to be developed. Following approval,

220 Engineering Innovations Unprecedented Accomplishments in the Use of Aluminum-Lithium Alloy

NASA was the first to use welded aluminum-lithium alloy Al 21 95 at cryogenic temperatures, incorporating it into the External Tank under circumstances that demanded innovation. From the beginning of the Space Shuttle Program’s launch phase, NASA .

d sought to reduce the weight of the e v r e

s original tank, thereby increasing e r

s t

h payload capacity. Since the tank was g i r

l l

A carried nearly to orbit, close to 100% of

. K T

A the weight trimmed could be applied to

© the payload. NASA succeeded in This high-performance liquid chromatography/mass spectometry is employed to document minute implementing numerous weight-saving details of a material’s chemical and molecular composition. Through the chemical fingerprinting system, seemingly minuscule discrepancies raise red flags that trigger investigations and preclude measures, but the biggest challenge was defective materials from reaching the production floor. Dr. Ping Li shown here at ATK in Utah. to incorporate a lightweight aluminum alloy—aluminum-lithium Al 21 95— for stockpiled materials. The ability to material, the team could look deeper to into the tank structure. This alloy had store greater amounts of materials over find the true root cause and implement never been used in welded cryogenic longer periods of time was valuable in proper corrective actions. environments prior to NASA’s cases where new materials needed to be initiative. Several challenges needed to certified to replace existing materials be overcome, including manufacturing that had become obsolete. From Fingerprints to the aluminum-lithium tank components, Flight Safety welding the alloy, and repairing the Finally, investigators were able to solve welds. NASA and the External Tank The overarching value of the chemical production issues with greater prime contractor broke new ground in fingerprinting program was that it efficiency. Comprehensive database the use of aluminum-lithium to produce provided greater assurance of the safety features, including standardized test the “super lightweight tank.” methods and the extensive online and reliability of critical shuttle flight reference database, provided resources hardware. The fundamental The original tank weighed 34.500 needed to resolve production issues in a understanding of critical reusable solid metric tons (76,000 pounds) dry. matter of days or even hours—issues rocket motor materials and improved By the sixth shuttle mission, the tank’s that otherwise would have required communications with vendors reduced weight had been reduced to 29.900 major investigations. In some cases, the occurrence of raw materials issues. metric tons (66,000 pounds). This fingerprinting was also used to indicate NASA will implement chemical configuration was referred to as the that a suspect material was actually fingerprinting methods into the “lightweight tank.” acceptance testing of raw materials within required specifications. These The real challenge, however, was still used in future human space exploration materials may have been rejected in to come. In 1993, the International endeavors. The full benefits of the previous cases but, by using the Space Station Program decided to program will continue to be realized in fingerprinting database to assess the change the station’s orbital inclination years to come.

Engineering Innovations 221 to 57 degrees (a “steeper” launch Since the two propellant tanks were stress at cryogenic temperature. inclination), allowing Russian vehicles proof tested at room temperature and This cycle was repeated three more to fly directly to the station. That flown cryogenically, this fracture times to meet a four-mission-life change cost the shuttle 6 ,1 23 kg toughness ratio was a crucial factor. program requirement with the exception (1 3,500 pounds) of payload capacity. that, on the fourth cycle, the sample A simulated service test requirement The External Tank project office was stressed to failure and had to was imposed as part of lot acceptance proposed to reduce the dry weight of exceed a predetermined percent of for all aluminum-lithium material the tank by 3,402 kg (7,500 pounds). the flight stress. Given the size of the used on the tank. The test consisted of barrel plates for the liquid hydrogen The Space Shuttle Program sought applying room temperature and and liquid oxygen tanks, only one barrel to incorporate lightweight cryogenic load cycles to a cracked plate could be made from each lot of aluminum-lithium Al 21 95 into the sample to evaluate the ability of the material. As a result, this process was majority of the tank structure, replacing material to meet the fracture toughness adopted for every tank barrel plate— the original aluminum-copper alloy requirements. Failure resulted in the 32 in each liquid hydrogen tank and Al 2 21 9; however, NASA first plate being remelted and reprocessed. four in each liquid oxygen tank—and needed to establish requirements for Implementation of simulated service implemented for the life of the program. manufacturing, welding, and repairing testing as a lot acceptance requirement aluminum-lithium weld defects. Another challenge was related to the was unique to the aluminum-lithium aluminum-lithium weld repair process NASA started the super lightweight material. Testing consisted of cropping on compound curvature parts. The tank program in 1994. During the two specimens from the end of each effect of weld shrinkage in the repairs early phase, advice was sought from plate. Electrical discharge machining caused a flat spot, or even a reverse welding experts throughout the United (a process that removes metal by curvature, in the vicinity of the repairs States and the United Kingdom. discharging a spark between the tool and contributed to significant levels of The consensus: it was virtually and the test sample) was used to residual stress in the repair. Multiple impossible to perform repairs on introduce a fine groove in each sample. weld repairs, in proximity, showed the welded aluminum-lithium. The samples were then cyclically propensity for severe cracking. After loaded at low stresses to generate a The aluminum-lithium base metal examination of the repaired area, it was sharp fatigue crack that simulated also presented challenges. Lockheed found that welding aluminum-lithium a defect in the material. Martin worked with Reynolds resulted in a zone of brittle material Aluminum to produce the aluminum- The first sample was stressed to failure; surrounding the weld. Repeated repairs lithium base metal. One early problem the second sample was stressed to near caused this zone to grow until the was related to aluminum-lithium failure and then subjected to cyclic residual stress from the weld shrinkage material’s fracture toughness—a loading representative of load cycles exceeded the strength of the weld measure of the ability of material with the tank would see on the launch pad repair, causing it to crack. a defect to carry loads. Although during tanking and during flight. The technique developed to repair material was screened, flight hardware In the second sample, initial loading these cracks was awarded a US Patent. requirements dictated that structures was conducted at room temperature. The repair approach consisted of must have the ability to function in This simulated the proof test done on alternating front-side and back-side the event a defect was missed by the the tank. Next, the sample was grinds as needed to remove damaged screening process. The specific stressed 13 times (maximum tanking microstructure. It was also found that difficulty with the aluminum-lithium requirement) to the level expected aluminum-lithium could not tolerate was that the cryogenic fracture during loading of propellants at as much heating as the previous toughness of the material showed cryogenic temperatures and, finally, aluminum-copper alloy. This required little improvement over the stressed to maximum expected flight increased torch speeds and decreased room-temperature fracture toughness.

222 Engineering Innovations The use of aluminum-lithium AI 2195 in manufacturing major External Tank components, such as the liquid hydrogen tank structure shown above, allowed NASA to reduce the overall weight of the External Tank by 3,402 kg (7,500 pounds). The liquid hydrogen tank measured 8.4 m (27.5 ft) in diameter and 29.4 m (96.6 ft) in length. Photo taken at NASA’s Michoud Assembly Facility in New Orleans, Louisiana. fill volumes to limit the heat to which comparable; however, in the case of aluminum-lithium panels together the aluminum-lithium was subjected. the aluminum-lithium alloy repair, the and simulating a weld repair in the strengths were lower. center of the original weld. The panel Additional challenges in implementing was then loaded to failure. The test effective weld repairs caused NASA to Past experience and conventional that was supposed to indicate better reevaluate the criteria for measuring the thinking was that in the real hardware, strength behavior than the excised strength of the welds. In general, weld where the repair is embedded in a repair material actually failed at a repair strengths can be evaluated by long initial weld, the repaired weld lower stress level. excising a section of the repaired will yield and the load will be material and performing a tensile test. redistributed to the original weld, To understand this condition, an The strength behavior of the repaired resulting in higher capability. To extensive test program was initiated material is compared to the strength demonstrate this assumption, a tensile to evaluate the behavior of repairs behavior of the original weld material. test was conducted on a 43-cm- on a number of aluminum-copper In the case of the aluminum-copper (1 7-in.)-wide aluminum-lithium panel alloy (Al 2 21 9) and aluminum- alloy Al 2 21 9, the strengths were that was fabricated by welding two lithium alloy (Al 21 95) panels.

Engineering Innovations 223 Orbiter Payload Bay Door One of the largest aerospace composite applications of its time.

With any space vehicle, minimum weight is of critical importance. Initial trade studies indicated that using a graphite/epoxy structure in place of the baselined aluminum structure provided significant weight savings of about 408 kg (900 pounds [4,000 newtons]), given the large size and excellent thermal-structural stability. Two graphite/epoxy composite materials and four structural concepts—full-depth honeycomb sandwich, frame-stiffened thin sandwich, stiffened skin with frames and stringers, and stiffened skin with frames only— were considered for weight savings and manufacturing producibility efficiency. These studies resulted in the selection of the frame-stiffened thin sandwich configuration, and component tests of small specimens finalized the graphite fiber layup, matrix material, and honeycomb materials. Graphite/epoxy properties at elevated temperatures are dependent on moisture content and were taken into account in developing mechanical property design allowables. Additionally, NASA tracked the moisture content through all phases of flight to predict the appropriate properties during stiffness. Mechanical fasteners were used for connection re-entry when the payload bay doors encountered maximum of major subassemblies as well as final assembly of the doors. temperatures of 177°C (350°F). All five Orbiter vehicles used graphite/epoxy doors, one of the Payload bay doors were manufactured in 4.57-m (15-ft) largest aerospace composite applications at the time, and sections, resulting in two 3 x 18.3 m (10 x 60 ft) doors. performance was excellent throughout all flights. Not only was The panel face sheets consisted of a ± 45-degree fabric ply the expected weight saving achieved and thermal-structural imbedded between two 0-degree tape plies directed normal to stability was acceptable, NASA later discovered that the the frames and were pre-cured prior to bonding to the Nomex ® graphite/epoxy material showed an advantage in ease of repair. honeycomb core. A lightweight-aluminum wire mesh bonded Ground handling damage occurred on one section of a door, to the outside of face sheets provided lightning-strike resulting in penetration of the outer skin of the honeycomb core. protection. Frames consisted primarily of fabric plies with the The door damage was repaired in 2 weeks, thereby avoiding interspersions of 0-degree plies dictated by strength and/or significant schedule delay.

224 Engineering Innovations Test panels were covered with a difficulty in making and planishing photo-stress coating that, under multiple repairs, a verification polarized light, revealed ground rule was established that every the strain pattern in the weld repair. “first repair of its kind” had to be The Al 2 21 9 panel behaved as replicated on three wide tensile panels, expected: the repair yielded, the loads which were then tested either at redistributed, and the panel pulled well room temperature or in a cryogenic over the minimum allowable value. environment, depending on the In aluminum-lithium panels, however, in-flight service condition expected the strains remained concentrated in for that part of the tank. the repair. Instead of the 2 21 MPa All these measures combined (32,000 pounds/in 2) failure stress accomplished the first-ever use of obtained in the initial welds, the welded aluminum-lithium at cryogenic welds were failing around 172 MPa temperatures, meeting the strict (1 8,000 pounds/in 2). These lower demands of human spaceflight. The failure stress values were problematic super lightweight tank incorporated due to a number of flight parts 20 aluminum-lithium ogive gores that had already been sized and (the curved surfaces at the forward machined for the higher 2 21 MPa end of the liquid oxygen tank), four (32,000 pounds/in 2) value. liquid oxygen barrel panels, 32 liquid Based on this testing, it was determined hydrogen barrel panels, 12 liquid that weld shrinkage associated with the oxygen tank aft dome gores, 12 liquid repair resulted in residual stresses in hydrogen tank forward dome gores, the joint, reducing the joint capability. and 11 liquid hydrogen aft dome gores. To improve weld repair strengths, Through this complex and innovative engineers developed an approach to program, NASA reduced the 29,937-kg planish (lightly hammer) the weld bead, (66,000-pound) lightweight tank by forcing it back into the joint and another 3,40 1.9 kg (7,500 pounds). spreading the joint to redistribute and The 26,560-kg (58,500-pound) super reduce the residual stresses due to lightweight tank was first flown on shrinkage. This required scribing and Space Transportation System (STS)-9 1 measuring the joint before every repair, (1 998), opening the door for the making the repair, and then planishing shuttle to deliver the heavier the bead to restore the weld to its components needed for construction previous dimensions. Wide panel test of the International Space Station. results and photo-stress evaluation of planished repairs revealed that the newly devised repair procedure was effective at restoring repair strengths to acceptable levels. Testing also revealed that planishing of weld beads is hard to control precisely, resulting in the process frequently forming other cracks, thus leading to additional weld repairs. Because of the

Engineering Innovations 225