Heat shields for spacecraft re-entry and turbochargers for engines

1.1. How the Component Operates

When spacecraft, space ships and missiles renter the earths atmosphere, they are acted by their own speed of descent that may be powered or in free fall, the high kinetic energy they have and the earths gravitational force that would pull these objects and add to their velocity. Typically, spacecrafts attain speeds of 11 kilometres per second when they re enter the earth’s atmosphere. Just before the ship enters the outer atmosphere, the speed will have increased and there is rapid air compression and the temperature starts rising and temperatures of 2649 degree centigrade would be reached. This temperature is sufficient to melt all metals or at least to turn them into malleable form that would easily flow and have a high level of plasticity. This happens even though the temperature in outer space is less than minus 150 degree centigrade (NASA, 2008).
As the spacecrafts begins to enter the outer reaches of the atmosphere, the speed will slightly reduce. The earth’s atmosphere acts as a blanket and as the passing body passes through the atmosphere, the air starts rubbing against the exterior skin of the spacecraft, creating a tremendous amount of friction. Under normal condition, as the spacecraft passes through the atmosphere, the air would have acted as a heat conductor and taken away the generated heat. However, since the speed is very high, there is no time for the heat to be removed from the skin and the heat starts building up. Speed of the vehicle would have reduced to about 7.5 kilometres per second while the temperature would be around 1871 degree centigrade. Such high temperatures are still enough to cause melting of nickel and iron. Length, size and shape of the spacecraft would be additional variables. The heat the is generated on the outside skin must in no case be allowed to enter the aircraft and if there is even a hairline gap in the heat shield, the whole spacecraft will explode in a ball of flame. This is what happened to the ill fated space shuttle Columbia that burnt down when it was attempting to renter the Earth’s atmosphere from a low level space mission. Failure of a heat shield was the main cause for the tragic fire and crash of the vehicle and all the crewmembers on board were killed besides causing billions of dollars loss for the crashed vehicle (ScienceDaily, 7 October 2008).

1.2. Service Requirements

Heat Shields form one part of the cooling system that are used for spacecraft protection. By just having heat shields, heat will be blocked for a few milliseconds but afterwards, the heat shields will burn off and the aircraft will quickly burn up. Heat shields are actually used along with cooling systems that remove the heat from the skin and the heat shields. Heat distribution is not the same throughout the spacecraft but there are heat differentials. A simulation of the heat transfer is shown as below.
As seen in the above figure, the maximum temperature occurs on the nose and along the leading edges of the wings where temperatures can be as high as 1650 degree centigrade. These areas are protected ceramic tiles made of silicon dioxide fibre. Superfine fibres are used in the bonding process and diameter of the fibres is as low as 1.5 microns. When the spacecraft is re-entering the atmosphere, it undergoes a high heating that occurs due to the bow shock effect. The shock is due to the high speed of the aircraft that would be coming in at speeds of more than Mach 5. The shock causes a very rapid compression of the gas in front, of more than 100 times and the temperature then rises to more than 3000 degrees. The hot gas that is created would then strike the frontal side of the vehicle and the heat is quickly transferred through the rest of the airframe. Following figure illustrates the heat gradients through different regions of the spacecraft (Buran, 2008).
In the earlier era, ablation method was used to melt away small part of the flux and the heat shield, thus carrying away the heated bits. However, with the advent of space shuttles that had to be reused, ceramic tiles were brought in instead. The service requirements of the tiles were that they should be reusable for at least 100 cycles; deal with variations of temperatures ranging from 1800 to 150 degree centigrade; have very low thermal conductivity of 0.06 w/mk and about 0.12 W/mk at 1100 degree centigrade. The coefficient of dilation was to be 0.000000007 while the density of the material should be less than 0.15 grams per cubic centimetre. In addition, these tiles had to be uniform in size or as per the drawing, have a contour on the underside and the play or gap between the tiles should be less that 0.3 millimetre so that hot air could flow. The gap between the tiles was filled by quartz fibre. There were some additional requirements that the tiles should not shear off during entry due to the high thermal and axial loads and they had to retain their form. Even if one tile broke off, the spacecraft would explode into flames. This is what happened to the Columbus space shuttle that was lost because one tile was lost during re entry (NASA, 13 May 2008).

1.3. Material Selection

The material that is selected should be able to meet all the service requirements as given in the previous section. According to CES, the material that closely meets the requirements is silicon dioxide fibre. The material is able to meet all the requirements of high temperature resistance, thermal conductivity, high fracture resistance, low Young’s Modulus and creep strength. In addition, quartz fibres are also used as filler material between the tiles. The manufacturing process suggested is (CES, 2008).

1.4. Manufacturing Process

According to the American Ceramic Society (AcerS, 2008), the fibres are by taking sub micron powders of silicon carbide and these are fabricated in an Acheson furnace to produce granules. These granules are further pulverised in special rolling mills to produce very fine powders. These powders are then mixed with additives to further increase the strength and thermal stability and quartz fine ground particles are added along with bonding materials. The whole mixture is then sintered to create fine fibres that are extruded using micro filters. Additional bonding materials are added to create composite fibres. The fibres are then placed into specially made dies that have a very detailed geometry and fused to produce the required tiles. The tiles are then chamfered, bevelled, ground and lapped to create the special tiles, Additionally, colour agents and dyes are also added. The tiles are then bonded to an organic felt cloth and mounted very carefully on the fuselage of the spacecraft. These tiles are made to order for a specific spacecraft and cannot be bough commercially (AcerS, 2008). Following table gives the physical properties of the material.

1.5. Comparison of Results with Real Situation

After comparing the results with studies from NASA, the components and manufacturing process of CES and real situations was found to be similar. This would suggest that CES is up to date with what is happening in the industry.

2.1. Service Requirement

Turbochargers are exposed to high temperatures of more than 1000 degree centigrade and very high operating pressure of more than 30 pounds per square inch. The turbocharger shaft would be rotating at speeds of 200,000 rpm. In addition, the exhaust side of the device is subjected to constant flow of hot exhaust gases that create a thermal gradient. When operating in cold temperature on in snowy regions, the inside portion of the turbocharger would be exposed to hot gases while the outside portion is exposed to sub zero temperatures. This kind of temperature difference means that the components have to withstand creep and fatigue loads with a very high temperature differential. The intake side of the turbocharger that takes in fresh air would be subjected to extreme pressures that would cause a rise in the temperature. To offset pressure eddies and for a gradual flow of air, a variable cross section like a venturi is provided. The variable cross section looks like a horn and allows both exhaust gases and fresh air to flow without harmful turbulence (Holset, 2009).

2.2. Material Selection and Manufacturing process

The turbocharger is made up of a number of sub components, each with its own requirement. The following table provides some details of the materials selected and the manufacturing process for each. It must be understood that the turbocharger is a very high performance device and every automobile has to have a custom made turbocharger designed and manufactured. In addition, once an engine is fitted with a turbocharger, the engine would develop more power and consequently, the stress on the engine parts would be much higher. Automobile companies design new parts with different materials for a turbocharged engine than when compared to a normally aspirated engine. It is not advisable to take any engine and fit a bought out turbo as this can result in serious explosion of the engine and kill people (Duffy, 1994).

2.3. Comparison of Results with Real Situation

After comparing the results with studies from Holset and APS Ltd., the components and manufacturing process of CES and real situations was found to be similar. This would suggest that CES is up to date with what is happening in the industry.