Introduction
Components such as heat shields for spacecraft and for turbochargers are very challenging. They are subjected to very high operating temperatures and especially heat shields should never fail. The recent crash of the space shuttle Challenger happened because the heat shields failed and as a result, the catastrophic failure and crash of the space shuttle occurred. Turbochargers are fitted on engines of high-performance cars, trucks, diesel engine generators, ships, and electric generators. Any failure of the turbocharger can result in severe accidents with the engine exploding or going up in flames and both the driver and bystanders can receive severe injuries or even perish. The paper analysis the service requirements for these components and attempts to find the group of material that would suit these products. Further, additional sources are used to find the exact material specifications for these applications.
About CES
CES provides a summary of material properties and process attributes. There are a number of charts given that give a brief commentary about the use. The material charts map the areas of property space occupied by each material class. The charts can be used as a data source or as a selection tool. Sequential application of several charts allows several design goals to be met simultaneously.
The charts have different families of materials with similar behavior and characteristics. The families are metals, polymers, elastomers, ceramics, and technical ceramics, glasses, and hybrids. Each member of the family would have vastly different properties. The charts give broad and early stages of material selection and are not to be used for obtaining precise values of properties needed for the actual component production. Once the approximate material to be used is selected, then external sources have to be used to find the exact alloy and grade to be used (CES, 2009).
Heat Shields for SpaceCraft Reentry
Heat shields are used in different types of spacecraft that would be expected to reenter the earth’s atmosphere. Some types of spacecraft include space shuttles, space exploration vehicles, navigation and communication satellites and other such objects. The heat shields are required when the spaceship is planned to reenter the atmosphere and not when it is taking off and going into orbit (PAO, 2009).
How the component operates
Reusable spacecraft such as the space shuttles and ballistic missiles that enter space and reenter the earth’s atmosphere is subjected to a number of forces. One of the forces is the speed of the craft and the high kinetic energy they would have when they are captured by the earth’s gravity. The earth’s gravitational force captures these objects and pulls them at great speeds that often reach speeds of 11 km per second. The temperature in outer space is sub-zero but when the spacecraft starts moving into the earth’s far atmosphere, the air in front of touching the nose of the craft starts to compress and the temperature would start to rise rapidly.
Temperatures would reach more than 2600 degrees centigrade. Since the vessel is moving very rapidly, there would be no time left for heat to be transferred from the skin to the surrounding air and the temperature does not drop at all. When the spacecraft enters the farther reaches of the atmosphere, the thin air tries to slow down the vessel by rubbing against the skin. This rubbing action creates and the air compression creates a further rise in temperature. The high speed means that the heat is not transferred to the atmosphere and the air that would have acted as a heat conductor becomes ineffective and instead serves to increase the temperature.
As the spacecraft progresses inside the atmosphere, the speed will have slightly reduced to about 8 kilometers per hour and the denser air keeps rubbing and takes the temperature to 1800 degrees centigrade. Such a high temperature would melt metals such as steel, nickel, and iron. If the spacecraft is longer or has a wider cross-section, then the heat buildup gradient would vary but the nose and other parts would still be at 1800 degrees centigrade.
The skin of the spacecraft protected by the heat shield must be such that it is able to withstand all these thermal shocks and yet remain intact. Even a tiny pinprick of a gap in the heat shield can cause the spacecraft to disintegrate. This happened to the space shuttle Columbia that lost a heat shield tile on the exterior of the spacecraft and this resulted in the fiery crash of the spacecraft, killing all the crew members (KSC, 2008).
Service Requirement
Heat shields are fitted very carefully on the external frame of the spacecraft and are used along with Mercury-based cooling systems to protect the spacecraft during reentry. It must be understood that heat shields on their own cannot protect the spacecraft, as they would burn off after a few milliseconds of being exposed to high temperatures. Hence, cooling systems are critical and these cooling systems take away the heat from the heat shields so that the heat shields can continue to function normally.
However, heat shields are vital for the safety of the spacecraft since they directly take the heat and thermal shocks. The spacecraft would have heat gradients along different regions. The highest temperature would happen along with the nose and the front edges of the spacecraft wings. The temperature in these zones would be about 1650 degrees centigrade. The rear part of the spacecraft would have temperatures of 350 to 700 degrees centigrade. Therefore it is possible to use different materials for the regions to reduce the cost of the heat shields. However, there should be thermal compatibility between the materials and heat must be allowed to flow between them.
Designers also tend to use the lower heated areas as heat sinks to transfer the heat from the high heat areas to the lower heated areas. The service requirements of the shields are that they should be reusable for at least 100 cycles; be able to withstand variations of temperatures ranging from 1800 to 150 degrees centigrade. They should have a very low thermal conductivity of 0.06 w/mk and about 0.12 W/mk at 1100 degrees centigrade. The coefficient of dilation should be 0.000000007 while the density of the material should be less than 0.15 grams per cubic centimeter. In addition, these tiles have to be uniform in size and have a contour on them and the gap between the tiles should be less than 0.3 millimeters so that hot air can flow (KSC, 2008).
Selection of Material
The materials that are required should be very lightweight since heavy and dense materials would increase the weight of the spacecraft to beyond acceptable limits (Timoshenko, 1997). Please refer to the CES Material Properties charts for strength and maximum service temperature. As per the chart, only technical ceramics are capable of meeting the requirement of being able to withstand temdegreesres of 1500 degree centigrade and yet have the strength and low weight.
Young’s Modulus measures the stiffness of the material. The material that is used should be able to deflect slightly when heat and shock loads are applied but have less density. With reference to Young’s modulus and density, Titanium and its alloys would be suitable but one has to consider while Ti and its alloys would be able to withstand the strain, the density, cost, and fabrication would be very difficult and again technical ceramics suit the requirement.
With reference to the strength over maximum service temperature, the material should be able to resist high temperatures yet retain its strength and form. Aluminum oxide and stainless steel can be considered but Al2O3 is very brittle and would disintegrate at the slight application of shock loads and stainless steel alloys would be very heavy. Hence, the SiC grade is better suited. With reference to the thermal expansion and thermal conductivity, the material should not expand when subjected to high temperature and yet be able to conduct heat from the surface to the internal cooling systems. While some types of heat-resisting steel and NI steels are available, the weight however would be very high when compared to the service requirements (CES, 2009).
Based on the above finding and as per the recommendations given by ASTM for spacecraft heat shields, the material recommended is silicon di oxide technical ceramic (ASTM, 2009). This material is also regarded as a hybrid composite since it is possible to use glass filler and bonding agents to incr the strength.
Method of manufacture
Silicon Carbide granules are prepared in the special furnace by using sub-micron powders and these granules are powdered in special mills to create very fine powders. The powders are then mixed with additives and fillers of glass to increase the thermal resistance. The powders are then sintered and then extruded to create very thin fibers of less than a micron in size. Some bonding additives are added and the fibers are loaded in specially contoured dies to create a rough tile. The tiles are then given the required shape using CBN wheels, proper edges with chamfer, and bevels provided. The products are ceramic tiles that are then carefully assembled by experts on the skin of the spacecraft. These heat shields are specially made and cannot be bought in the market (KSC, 2008).
CES Results with Other Sources
It is found that CES does not give the exact material composition to be used. It gives a family of materials and further reference from external sources is required to get the exact material specification.
Turbochargers
Turbochargers are external accessories that are fitted on engines to increase the power output of the engines. These devices are used on petrol, diesel and gas engines and fitted on trucks, cars, motorcycles, ship engines, defense vehicles, buses an,d other types of automotive engines (Garrett, 2009).
How the component operates
The turbocharger uses the exhaust gas of the engines to drive a small compressor wheel that rotates at high speed to pump more fresh air into the engine. In IC engines, the piston serves to compress the air and the fuel is then injected inside, causing ignition. Ignition occurs from either a sparkplug in petrol engines or by compression in diesel engines. The amount of compression that a piston can achieve is limited by the cylinder bore but if the air is compressed more, then the expansion caused when firing occurs is more intensive.
The turbocharger helps in this aspect by supplying highly compressed air into the cylinder where the piston further compresses it so that more energy is released. There are two sides to the turbocharger, the exhaust side, and intake side. The exhaust side sits on the exhaust manifold of the engine while the intake side pumps fresh air into the intake manifold. A common shaft with two wheels is mounted in bearings in the housing.
The turbine wheel is exposed to exhaust gases while the compressor wheel is exposed to fresh air. When the exhaust gases are directed through a variable section housing that looks like a trumpet, to the turbine wheel and it starts rotating along with the shaft. A turbine wheel is mounted on the intake side and this wheel starts rotating. The vanes on the wheels are given a specific geometry and direction so that the turbine wheel directs the exhaust gases outside when it rotates while the compressor wheel directs fresh air into the cylinder (Garrett, 2009).
Service Requirement
The speed of rotation of the wheels can be as high as 150,000 rpm and exhaust side temperatures can rise to more than 800degreese centigrade. In addition, the operating pressure inside the housing is often more than 30 pounds per square inch. When the hot exhaust gas flow inside, a thermal gradient is created. In cold weather areas, the outside temperature may be sub zero while the inside temperature is more than 800 degree centigrade.
With such a huge temperature gradient, the component should be able to withstand creep and fatigue load and it must not disintegrate or crack. The intake side of the turbocharger is exposed to only the ambient temperature but since air is being compressed very rapidly, there is a sudden increase of temperature to about 400 degree centigrade. In addition to the temperature difference, the material has to withstand abrasive particles. The compressor wheel has another requirement that it should have a low inertia and density and must be able to rotate freely (Garrett, 2009).
Selection of Material and Manufacturing Process
The turbocharger is made of a number of components that are manufactured separately and then assembled. The mains parts that would be considered are: turbine housing, turbine wheel, compressor wheel and compressor housing.
Turbine Housing
The turbine housing is a casting would sit on the exhaust side of the turbocharger and it encloses the turbine wheel, bearings and other parts. As per the CES chart, the casting material has to be light, stiff, be easily machinable and must have high fracture toughness. While Tungsten and Titanium would be ideal, they are very expensive and cannot be considered since costs of the turbocharger would be very high. As noted in the service requirements, the material needs to have a high hot hardness and good thermal conductivity.
They should also not deform easily so the Young’s to density ratio can be high since weight and density is not a primary concern as the housing is stationary and does not rotate. The material should have a high conductivity and diffusivity since heat generated must not be retained but transferred to the atmosphere. The material should also be able to have a high strength over the maximum service temperature and it should be remembered that the device would be running continuously over a few hours (CES, 2009).
Considering the requirements and the material specifications and also referring to the actual practice in the filed, steel alloys are considered. It is recommended that steel of HERCUNETE-S A3N having a material composition of 20%Cr-10%Ni-3%W-2%Nb is recommended. This steel can withstand temperatures of 900 degree centigrade, can withstand high internal stress and has a good thermal conductivity. The casting is manufactured by using green casting process or even investment casting. After fettling and cleaning, the casting is machined on the mounting face and the mounting bores on CNC machines. Additional drill tap is done on a radial machine or even on machining centers (Timoshenko, 1997).
Turbine Wheel
The turbine wheel is exposed to hot and abrasive exhaust gases when it is drievn by them. The wheel has a complicated vane profile with variable thickness and it must be medium weight and not having a lot of inertia. The wheel is expected to run at a high speeds of 150,000 rpm and it should have a high machinability and casting would have to take complex forms. The Young’s Modulus versus density curve means that while the material should be able to withstand strain, it should have a lower density than steel.
The wheel would be exposed to constant high exhaust temperature so the thermal expansion should be low while the thermal conductivity should be high. Again the curve for strength versus the maximum service temperature means that the wheel should retain the vane forms even at high temperatures. As per CES, Nickel steel is best suited for the component since these steels can withstand high temperatures and thermal stresses.
Based on the above requirements and material specifications and as per the practices used by Garret, the material used is a super alloy Inconel 713C of Nickel with a composition of Ni-2Nb 12.5 Cr-4.2 MO-0.8 Ti-6.1 Al-0.12 C-0.012 B-0.1Zr. This material can withstand high temperatures of 1000 degree centigrade besides being able to stand high thermal shocks and stresses (Timoshenko, 1997). The wheels are manufactured by casting and the variable profiles of the vanes have to be maintained in the casting process. Further machining is done on the bore and mounting face and a light-grinding cut on the OD of the vanes may be taken to maintain the gap and clearance.
Compressor Housing
The compressor housing sits on the intake side of the engine and houses the compressor wheel and other components. The part is not subjected to hot gases but the compression caused by the compressor wheel causes the temperature to rise to more than 100 degree centigrade. The material should be easily machinable, be a good conductor of heat, and have high fracture toughness. The pressure inside the diffuser area would be high and the material should have a Youngs Modulus versus density curve that allows the material to take extra strain without deforming.
The temperature rise would not be as high as the turbine side but the compressed air would cause a rise in the temperature inside the diffuser so the strength versus the max service temperature can be in the median range. The thermal expansion and the thermal conductivity curve would suggest that the material should be able to give away the heat and yet not expand excessively at higher temperatures. CES suggests that Nickel steel alloys can be used (CES, 2009).
As per the recommendations made by Garret, the material recommended is Inconel 601 alloy having a composition of Ni 61 Fe Bal Cr 23 Al 1.4 C 0.10 Mn 1.0 S 0.015 Si 0.5 (Timoshenko, 1997). The casting is prepared by using die casting process since this process gives a uniform material flow and allows intricate shapes to be cast. The casting is them machined on machining centers and drill and tapped.
Compressor Wheel
The wheel must be lightweight with low inertia and the material must be capable of being cast into thin and complex sections. The temperature at the intake side is not very high and pressure build up occurs inside the housing. According to CES, aluminum alloy is suited for this application. The part needs to have complex profile so casting into complex shapes with variable vane thickness is required. So the Youngs modulus and desnsity curve would mean a component that can withstand the strain buy yet be light. The thermal conductivity and expansion are not very crucial since the part is not exposed to high temperatures at the air intake side. However, since the component would rotate at very high speeds of 150,000 rpm, the fracture toughness and Young’s modulus would have to be high (CES, 2009).
As per the recommendations and practices of Garret, the material used is 354-T6 Aluminum Alloy with composition of Si 7 Cu 1.8 Mg 0.5 Zn 1 Ti 0.1 Fe 0.2 (Timoshenko, 1997). The wheel is cast using investment casting and then machined on the bore and mounting face on CNC lathes.
Conclusion
The paper has examined the product performance and requirement for heat shields for spacecrafts and also for turbochargers. Using CES and external sources, the material specification and manufacturing process have been examined.
References
ASTM, 2009. Annual Book of ASTM Standards. Published by ASTM International, USA.
Ashby MF, 2005. Material selection in mechanical design, 3rd Edition. Elsevier-Butterworth Heinemann.
CES, 2009. The CES EduPack Resource Booklet 2: Material and Process Chart. CES, Granata Design.
Garrett, 2009. How a Turbo System Works. Web.
Faraday, 2009. Advanced Turbocharger Designs: Materials and Modeling. Web.
KSC, 2008. Space Shuttle Orbiter Systems: Thermal Protection System. Web.
PAO, 2009. Orbiter Thermal Protection System. Web.
Timoshenko. Stephen. 1997. Mechanics of Materials, Fourth Edition. NY, PWS Publishing Company.