Materials Selection for Space Shuttle Structures - Research Paper Example

Material Selection for Space Shuttle Structures and Payload. The increased space missions and space explorations demand safety and stability of the systems like the Space Transportation Systems (STS) and the payload. The safety of the structures and the proper operations of the STS and payload depend on the materials that are used in their manufacture. This research paper discusses the various materials that are used in the STS and payload based on their thermal and fractural characteristics. Materials like Reinforced Carbon Carbon (RCC), High temperature Reusable Surface Insulation (HRSI) , Low temperature Reusable Surface Insulation (LRSI), Fibrous Refractory Composite Insulation (FRCI ) , Advanced Flexible Reusable Surface Insulation (AFRSI) and Nomex blankets have been used for the STS according to the thermal characteristics of the STS during ascent and re-entry. For the payload construction, the types of materials used involve aluminum alloys, titanium alloys, filament – overwrapped composite materials, stainless steel. The choice of these materials have been based on the interaction of the space shuttle with the space environment. 1.0. Introduction : Space exploration, space missions and space tourism have necessitated safe and secured space transportation systems at low cost. The advent of Space Transportation System (STS) , generally called as the space shuttle by United States marked the revolution in space missions. This Space Transportation System is the world’s first reusable spacecraft. It is capable of carrying large satellites to and from the orbits. The space shuttle launches like a rocket, in the specified earth orbit it maneuvers as a spacecraft and while landing, it comes down as an airplane. According to NASA, the present space shuttle orbiters that are in operation include – the Discovery, the Atlantis and the Endeavour. These are designed for a life of ten years and at least 100 missions of flight. These space shuttles need to be designed with specific materials as they pass through various natural and man made disturbances in the space environment. Also the payload that is carried by the space shuttle needs to be designed with special materials that assure the safety of the system. Thus the space shuttle and the payload has to undergo efficient design, manufacturing and testing , judicious choice of materials, in depth analysis of the onboard operations with respect to safety, sufficient inspection and rigorous testing before flight. Also the commercialization of the space travel by companies like Scaled Composites , Virgin Galactic and others demands the perfect design of the space transportation systems with all safety aspects. This research analyses the design aspects, especially the choice of materials used in manufacture of the space transportation system and the payload. As the material selection should be based on the characteristics of the space environment , the effects of the space environment on the space shuttle and payload has also been discussed along with the critical operating and structural requirements. 2.0. Exploring the space environments in space missions : The space environments can be categorized as earth environments , near earth orbit environment and elevated space environment. The earth environment and near earth environment corresponds to the Low Earth Orbit ( LEO ) region. This region extends from the surface of earth to an altitude of 2000km. According to NASA (NASA Safety Standard 1740.14, 1995 ) this space environment extends exactly from 160 km to 2000 km. This region is characterized by severe atmospheric drag caused by gases in exosphere. Another major disturbance in this space region is the inner portion of the Van Allen radiation belt surrounding the earth. This radiation belt is due to the non uniform distribution of the earth’s geo magnetic field. This radiation belt is said to be closely related to polar aurora which occur due to collision of the charged particles in the magnetosphere of earth. These radiations and accumulated charges in this space region may cause failure of electronic components. However, due to the process of Geomagnetic Shielding, the effect of these radiations are a little less on near earth environment. Again this geo-magnetic activity is greatly influenced by solar storms. The geo-magnetic field is shown in fig. 1. Fig.1. Geo Magnetic field and the cluster of spacecrafts in the magnetosphere. The major impact of this near earth space environment on spacecrafts is due to the presence of atomic oxygen. The major concern, in earth environment is the heat generated due to atmospheric drag during the re-entry of the spacecraft. According to the article ‘Space Weather Impact on Columbia Studied’ (Leonard David, 2003) this heat generated can severely damage the structure of the spacecraft. In the near earth environment, space debris also pose a major threat to the spacecraft. These space junks are growing in numbers and are of serious concern with respect to space collisions. The elevated space environments, corresponds to the Medium Earth Orbit (MEO) extending from 2000 km to 35768 km and the Highly Elliptic Orbit (HEO) which is above the geostationary orbit of 35768 km. In the Geostationary orbit regions, the major space weather phenomenon seems to be the hot Plasma – ionized gas particles. These are hot electrons that are caused by disturbances in the solar wind. Another major disturbance in the geo-synchronous or elevated space environment are due to Asteroids, Comets, Meteoroid, Meteors, Meteorite and Micro - meteoroids. The micro – meteoroids have high velocities and may cause serious impacts when it encounters a spacecraft. 3.0. Interaction of space environment with the shuttle : As discussed earlier, the various disturbances encountered in the space environment affect the structure and operation of the space transportation system. This necessitates certain critical operating and structural requirements during the design of the space transportation system. The various hazards caused by the interaction of the space environment with the structure and operational systems of the space transportation system can be seen in fig. 2. Fig.2 . Hazards due to the space environment. The figure shows the various disturbances in the form of gases, radiations , discharges, solar effects and particles like debris an meteors that occur in the space environment. Based on their impact ,the hazards produced by them are shown in different scales ranging from electron Volts (eV) to Giga electron Volts ( GeV ) . In the low earth region, the oxygen present causes erosion of the surfaces of the space craft. This oxygen in atomic state ( due to UV effect molecular oxygen is atomized) has less density but large flux. This large flux, elevates the oxygen to a highly reactive state that may cause serious erosions of surfaces through oxidation. When this oxidation combines with adverse thermal cycling of the spacecraft , the surface structural properties and the thermal control of the spacecraft are affected. The disturbances due to radiations from cosmos, sun flares, plasma produce electrons that get deposited on the spacecraft’s surface and induce electrostatic potentials in the range of kilovolts and Giga volts. These occur during sub storms and magnetospheric storms. These induced charges lead to Electro Static Discharges (ESD) that affect the structural components of the spacecraft. The presence of debris , meteors and micro – particles pose anther series of structural operational damage to space transportation system and the payload. When these space debris collide on functional satellites at orbital velocities, then the damage would produce more debris , which is referred to as Kessler Syndrome. These debris also affect the space walk of astronauts. The European Space Agency says that ‘The amount of damage depends on the mass of the particle and the relative velocity of the impact. Many small impacts are observed on the surfaces returned from LDEF, EURECA, and the Hubble Space Telescope (HST) Solar Array’(Micro-particle effects and analysis, 2007). 3.1. Critical operating requirements : The major sub systems involved in operational requirements of the space shuttle are main propulsion system, orbital maneuvering system, reaction control system, electrical power system, auxiliary power units, avionics system, etc. The main propulsion system provides the incremental velocity needed to place the shuttle in orbit. The required thrust for orbit circulation, orbit insertion and orbit transfer is provided by the orbital maneuvering system, the velocity changes in the orbit, thrust for pitch, roll and yaw are provided by the reaction control system. The electrical power system supplies the electrical power during the entire mission. The auxiliary power systems produce the pressure required for the hydraulic system of the vehicle. Finally the avionics system supports in the control of most of the shuttle systems. To protect all these operational systems from the extreme weather conditions of the space environments, the spacecraft engineers and scientists opt for solutions like hardening of the electronic systems, collision detection systems, shielding of the space shuttle and the payload. The proper operations of all the mechanical , electrical and electronic systems present in the space shuttle and the satellites depend on the maintenance of operating temperature range. The different regions that need to be heat shielded are shown in fig.3. as described by the APL laboratory (Bradley.G.B. , 2003) for the space craft to planet Mercury called MESSENGER which stands for Mercury Surface ( MES-) Space Environment (SEN-) Geochemistry and Ranging (GER). Fig.3. MESSENGER spacecraft showing critical portions that must be heat-shielded from radiation from the Sun and Mercury. 3.2. Critical structural requirements. The sub systems that are related to structural requirements are the Thermal Protection Systems (TPS) and Environmental Control and Life Support System (ECLSS). The Thermal Protection Systems is concerned about the usage of various materials to the outer structure to protect the space shuttle from excessive heat. The Environmental Control and Life Support System is concerned with the provision of cooling and heating of the system components, it is also concerned about the provision of habitable environment for the flight crew. The space craft design for these structural requirements are based on the models of the space environments including plasma interaction models, radiation belt models and models to predict atmospheric drag effects in lower orbits. After modeling they are subjected to failure and reliability analysis. These analysis include surface characterization, stress testing, image analysis, testing for mechanical, thermal , electrical , radiation effects. Careful design has to be emphasized for mechanical load sharing among components, transfer of loads through points of attachment with points. APL laboratory says that these structures have to be tested and that ‘Wind tunnel and rocket-assisted sled testing can then be used to fully address critical heat flux in dynamic air flow environments, as well as material degradation from rain erosion.’ 4.0. Material selection for space shuttle structures and payload : This section discusses the choice of materials for the space shuttle structures and the payload. The choice of the materials is crucial as the space shuttle and the payload has to and withstand all the disturbances in the mission. 4.1. The structure and components of space shuttle : Before going into the material selection , the basic structure and components of a shuttle need to be analyzed. Fig.4 shows the basic parts of the shuttle. They are the Orbiter, two numbers of Solid Rocket Boosters (SRB) and the External fuel tank. Fig.4. The integral parts of the Space Transportation System. The Solid Rocket Boosters ( SRB) : These SRBs along with the main engines provide the thrust needed to escape the earth’s gravitational pull. NASA says that the thrust of both the boosters is 5,300,000 lbs. After an altitude of 45km, these boosters are jettisoned from the orbiter and return back on parachutes. The components of the SRBs include large solid rocket motor , thrust vector control, separation, recovery and other instrumentation systems. The Orbiter : This Orbiter could be termed as the heart and brain of the STS, it looks similar to a DC-9 aircraft and is divided into three portions namely Forward fuselage, Mid fuselage and Aft fuselage. These three portions are shown in fig.5. , where the Forward fuselage houses the crew cabin, the Mid fuselage houses the payload and the Aft fuselage houses the main engines and the orbital maneuvering systems. Fig.5. The structure and components of Orbiter. The External Tank : The external tank is attached to the orbiter at the Aft fuselage via umbilical plates. These plates are subjected to pyrotechnics when the external tank separates. The external tank provides the fuel for the shuttle and is burnt away in the space after separation. 4.2. Materials used : With this knowledge of the structure of the STS, the choice of materials for the thermal protection systems and the Environmental Control and Life Support System needs to be analyzed. The Thermal Protection System (TPS), of the orbiter has be to designed according to the temperatures, launch acoustics, aerodynamic loads, space environments to which it would be exposed during the mission. The temperature during the ascent and re-entry phases is maintained at less than 177 degree C (350 degree F). The materials selection and location of the materials for the orbiter structure depends on the temperature capability of the material. Aluminum was the foremost choice as it has very less weight. The aero-thermal heating ( heating due to friction with air) experienced by the space shuttle during ascent and re-entry is enormous. The properties of aluminum do not permit the surface temperature of the shuttle to exceed 350°C. This clearly indicated the necessity for materials with insulating properties. The two major materials used for the orbiter include Reusable Surface Insulation (RSI) and Reinforced Carbon- Carbon (RCC). The RSI can be categorized into rigid ceramic tiles and flexible blankets. The locations for the RSI materials are based on the material reuse temperature and peak surface temperature. The RCC is a special structural material that is generally used in the regions like Wing Leading Edges (WLE), nose cap, area between nose cap and landing gear door, umbilical area where external tank is attached to the orbiter. These areas are characterized by very high temperatures. Aerothermodynamics plays an important role in the choice of materials, distribution of the materials and their specified thickness for the chosen materials. The heating environment for the orbiter has been said to follow geometric flow models. The peak design temperatures for the orbiter design are shown in fig.6. (Donald. M. C., 1993). Based on these temperature profile, the materials like RSI and RCC are distributed. These materials are capable of protecting the structure from high temperature, they have less weight and good thermal stability. The Reinforced Carbon- Carbon (RCC) material is made of laminates of carbon by a process called multiple pyrolysis and densification. By diffusion reaction process, an oxidation resistant SiC coating can be formed, which helps in reduced oxidation of the space shuttle during missions. Further oxidation can be provided by treating the RCC with Tetra-Ethyl Ortho Silicate(TEOS) and applying a surface sealant which fills any micro cracks. This RCC is used in regions where the temperature exceeds 1260 degree C. The Reusable Surface Insulation (RSI) material could be categorized into High temperature Reusable Surface Insulation (HRSI) and Low temperature Reusable Surface Insulation (LRSI). These RSI tiles are made of high purity, low density Silica (99.8%) and Amorphous fiber insulation with ceramic bonding. The LRSI could be used in regions which have temperature in the range 371°C to 648°C and the HRSI is generally used in regions of temperature ranging from 648°C to 1260°C. The HRSI tiles have a surface coating of black reaction cured glass (RCG) with surface density of 144 kg/m3 or 352 kg/m3 and the LRSI tiles have a white surface coating of RCG with a density of 144 kg/m3 and are seen to be good in providing on-orbit thermal control. The RSI configurations are shown in fig.7. Fig.6. Peak Design Surface temperatures of Orbiter during ascent and re-entry . Other variants of RSI like the Fibrous Refractory Composite Insulation ( FRCI) could be used in regions of temperature less than 371°C, Advanced Flexible Reusable Surface Insulation (AFRSI) could be used for temperature ranges less than 816°C. the surface density of FRCI is 192 kg/m3 and that of AFRSI is 176 kg/m3. These tiles can be made to be strong, durable, less cracking tendency by the addition of aluminum-boro-silicate. The FRSI has a coating of white pigmented silicone, that provides the essential thermal and optical properties. The AFRSI is a batting of pure silica and amorphous silica fibers. This silica batting is sandwiched between an outer layer of high temperature silica and an inner layer of low temperature glass fabric. This sandwich is tightened by silica thread and coated with ceramic, which leads to water repelling capacity and durability. The AFRSI blankets are less in weight and hence they essentially replace LRSI tiles to have reduced installation time, reduced cost and fabrication. The FRSI tiles are used in regions adjacent to thermal barriers and penetrations and in aero-surface trailing edges. Fig.7. RSI material and configuration. All these ceramic RSI tiles use silicone adhesives and a layer of nylon felt to get bonded with the Orbiter. The RCC used in wing leading edge, chin panel and nose cap use discrete mechanical attachments like Inconel 718 and A-286 stainless steel fittings. These fittings provide thermal expansion, thermal isolation and accounts for structural displacements if any. The fitting of these to the wing leading edge are shown in fig.8. The RCC attachments are supported by cerachrome insulation of Inconel foil obtained from AB 312 ceramic cloth. This insulation provides protection from internal radiation and conduction. Fig.8. RCC materials in Wing Leading Edge (WLE). The properties of these can be studied by the material thermal limits and their structural design allowable. The material specifications for thermal limits of the above analyzed RCC and RSI material are shown in table in fig.9. Fig.9. Thermal limits of RCC and RSI materials. The allowable design values for the structural and thermal properties of RCC material are shown in fig.10. Fig.10. The allowable design values for the structural and thermal properties of RCC material The specific properties of the RSI material are shown in the table in fig.11. Fig.11. Specific properties of the RSI material. 4.3. Distribution of materials : Based on the analysis of the materials , the distribution of the materials over the surface or structure of the orbiter , according to NASA’s US Centennial of Flight Commission (Dwayne A.D., can be seen as in fig.12. The above mentioned seven different materials are used for the structural and operational requirements of the Space transportation System (STS). Though the structural skin is made of Aluminum, the other TPS materials are used to safeguard the shuttle in the temperatures ranging from -250°F in coldest regions of space to +3000°F during re-entry. These materials also sustain the deflection forces of the orbiter airframes as it responds to space environments. The internal temperatures of the orbiter are also maintained by proper insulation and heaters. RCC – could be used in nose cap, chin panel, wing leading edges, external tank attachment area. It gives protection when the temperature increases above 2300F. Black HRSI tiles could be used in areas of forward fuselage windows, the upper part of the shuttle, orbital maneuvering systems and reaction control system, wing glove areas, vertical stabilizers and body flap area in upper surface. These tiles are black in color to suit the entry emittance and they give thermal protection up to 2300F. Black colored FRCI tiles may be used instead of HRSI in areas where temperature rises above 2300F. Fig.12. Distribution of the materials over the surface or structure of the orbiter. White colored LRSI could be used in areas with temperature less than 1200F like vertical tail, maneuvering system pods, reaction control system pods, upper wing, etc. AFRSI , which has good durability and reduced weight . They could be used to replace LRSI in temperature ranges less than 1200F. White colored Nomex blankets , called FRSI could be used on payload doors, surfaces of upper wings, sides areas of Mid fuselage and Aft fuselage. These FRSI blankets are found to be suitable for temperatures less than 700F. Specific materials like thermal panes, elevon seals, silica thermal barriers, gap fillers could be used in specific areas as required. 4.4. The payload structure and materials : The structural design of the payload carried by STS are defined in the NASA handbook (NHB 1700.7., 1992 ) The major factors that are crucial in the design are the pressure vessels, fracture control mechanisms, materials used for structures, propellant tanks for the payload propulsion system. The most common types of materials used in these systems involve aluminum alloys, titanium alloys, filament – overwrapped composite materials, stainless steel. The fabrication methods involved are spin forming, forging, hydroforming, machining, and cryoforming. According to NASA technical standard ( NASA-STD-5003, 1996) , ‘ For payloads using the Space Shuttle, NASA requires full assurance of system safety. This is accomplished through good design, manufacturing, test, and operational practices, including the judicious choice of materials, detailed analysis, appropriate factors of safety, rigorous testing, control of hardware, and adequate inspection. For payloads carried on the Space Shuttle, it is specifically required that design shall be based on fracture control procedures when failure of structure can result in a catastrophic event’. The key factors in fracture mechanics describe the behavior of crack-like flaws in materials when subjected to stress. These factors that determine material selection are tensile strength, hazardous fluid, fracture toughness, leak before burst, load limit, maximum design pressure, type of pressure vessel, payload organization and factors related to non destruction evaluations. The material selection for payload has been governed by MIL–HDBK–5, Metallic Materials and Elements for Aerospace Vehicle Structures and MIL–HDBK–17, Plastics for Aerospace Vehicles. NASDA ( SSP 30559 , 2000) quotes that ‘ A - ALLOWABLES - Material “A” or equivalent allowable values shall be used in all applications where failure of a single load path could result in a loss of structural integrity in primary structure. B and S ALLOWABLES - Material “B” or “S” or equivalent allowable values may be used in redundant structure in which the failure of a component would result in a safe redistribution of applied loads to other load–carrying structure’. These allowables are materials like aluminum alloys, titanium alloys, filament – overwrapped composite materials. 5.0. Future direction : With the growing number of space missions and commercialization of space transport, the need for safety of the STS system and the crew increases. Also new mission to other planets demand reliability and durability of the structural components of the STS and Payload. According to new generation space craft and payload manufacturers, (Bradley.G.B. , 2003), advanced and composite materials which give reduction in launch volume, mass and cost are proposed to be used. These new technologies include lightweight composite radiators, better thermal coatings, thermal switch technology and they aim to reduce the thermal control system’s resources. The materials under investigation are ‘lightweight composite laminates using pitch-based fibers that produce a mass saving of three over conventional radiators. The combination of boron nitride powder plus a three dimensional lay-up significantly increases the thermal efficiency of pitch fiber laminates…a new coating technique that will provide tailored optical properties to increase environmental stability and electrical conductivity’ (Bradley.G.B. , 2003). They also propose new Micro and Nanotechnology for ‘variable emissivity of thermal control of spacecraft using Micro Electro Mechanical Systems (MEMS) … the magnetometer-assisted MEMS-based inertial measurement units capable of being used in smart munitions and mechanical mixing filter operating in microwave frequencies’. With respect to payload design the APL laboratory has deign new generation solar panels that are light in weight and could withstand very high temperatures. The new material is proposed to have good stiffness to weight ratios and is made of aluminum honeycomb as core material and an outer layer of Kapton insulator. The use of carbon fiber cynate ester gives good thermal shock absorption. Meanwhile , scientists and engineers at the Air Force Research Laboratory (AFRL) have developed ‘a new class of wear-resistant materials, comprised of very hard 3-5-nm grains of carbides or oxides embedded in an amorphous matrix of either diamond-like carbon or a metal/ceramic mixture, that could improve the performance and durability of aircraft engines. During the material characterization stage, the materials exhibited an unusual combination of high hardness (exceeding that of ceramics) and fracture strength (similar to that of tough metal alloys)’ as quoted by a technical of Aero Space America (Gregory.A.S. and Edward.H.G., 2005). 6.0. Conclusion : Space missions and space tourisms have led to the innovation of new types of space crafts and payloads. These new space shuttles or Space Transportation System ( STS ) and the payloads or satellites have been constructed with new materials that assure safety of crew and the systems. This research paper has discussed the various space environments to which the space transportation system has to be exposed. These space environments, namely the earth environment , near earth environment and the elevated earth environment have been explained in detail along with their impact on the Space Transportation System ( STS ) and the payloads. Based on these impacts of the space environment, the critical requirements for the proper structure and operation of the Space Transportation System ( STS ) and the payloads have been discussed in detail. Major emphasis of the research has been given to the materials used for the structures of the Space Transportation System ( STS ) and the payloads. The analysis of different materials has been based on the structure and components of the space shuttle like the external tank, the Solid Rocket Boosters ( SRB) and the Orbiter. The materials for these structures are generally based on Aluminum but to withstand the excessive heat in the space environment, other materials that are used for the orbiter include Reusable Surface Insulation (RSI) and Reinforced Carbon- Carbon (RCC). Based on the aerothermodynamics, geometric heat flow models and peak design temperatures for the orbiter , the materials like RSI and RCC are distributed. These materials are capable of protecting the structure from high temperature, they have less weight and good thermal stability. The RCC is a special structural material that is generally used in high temperature regions like Wing Leading Edges (WLE), nose cap, area between nose cap and landing gear door, umbilical area where external tank is attached to the orbiter. It gives protection when the temperature increases above 2300F. It has been explained that Black HRSI tiles could be used in areas of forward fuselage windows, the upper part of the shuttle, orbital maneuvering systems and reaction control system to give thermal protection up to 2300F. Black colored FRCI tiles may be used instead of HRSI in areas where temperature rises above 2300F. White colored LRSI could be used in areas with temperature less than 1200F. AFRSI , which has good durability and reduced weight . They could be used to replace LRSI. Nomex blankets , called FRSI could be used for temperatures less than 700F. The most common types of materials used in payload systems involve aluminum alloys, titanium alloys, filament – overwrapped composite materials, stainless steel and their fabrication methods involve spin forming, forging, hydroforming, machining, and cryoforming. The design of this payload system has been based on fracture control procedures when failure of structure can result in a catastrophic event. The material selection for payload has been based on MIL–HDBK–5 - Metallic Materials and Elements for Aerospace Vehicle Structures and MIL–HDBK–17 - Plastics for Aerospace Vehicles. According to these norms, ‘A’ allowables have been used for primary structures and ‘B’ or ‘S’ allowables are used for secondary or redundant structures. The research has also highlighted the future directions in the fields of Micro and Nano technologies that aim at developing composite materials that have good thermal stability and fracture stability. References: Bradley. G. Boon, Materials and Structures Research and Development at APL John Hopkins APL Technical Digest, Volume 24, Number 1, 2003. Donald.M.Curry., Space Shuttle Orbiter Thermal Protection System Design and Flight Experience, NASA Technical Memorandum 104773, July 1993. Dwayne A. Day, Shuttle Thermal Protection System (TPS). NASA’s US Centennial of Flight Commission http://www.centennialofflight.gov/essay/Evolution_of_Technology/TPS/Tech41G2.htm. Accessed 25th April 2009. European Space Agency , Micro-particle effects and analysis , 2nd October 2007. http://www.esa.int/TEC/Space_Environment/SEM9H5T4LZE_0.html Gregory.A.S. and Edward.H.G., STRUCTURES, DESIGN AND TEST, Aerospace America , December 2005. Leonard David , Space Weather Impact on Columbia Studied , 3rd March 2003. www.space.com/technology NASA Safety Standard – 1740.14, Guidelines and Assessment Procedures fro Limiting Orbital Debris , Office of Safety and Mission Assurance. 1st August 1995. http://www.orbitaldebris.jsc.nasa.gov/library/NSS1740_14/nss1740_14-1995.pdf.  NASA technical standard, FRACTURE CONTROL REQUIREMENTS FOR PAYLOADS USING THE SPACE SHUTTLE , NASA-STD-5003, October 7, 1996. NASDA , Structural Design and Verification Requirements, National Space Development Agency of Japan. SSP 30559 Revision C, September 29, 2000. NASA Handbook NHB 1700.7, Safety Policy and Requirements f or Payloads Using the Space Transportation Systems (STS). 1992 Read More