Ablative shields

Carbon—carbon composites are used in high temperature service for aerospace and aircraft appHcations as weU as for corrosion-resistant industrial pipes and housings. AppHcations include rocket nozzles and cases, aircraft brakes, and sateUite stmctures. Carbonized phenoHc resin with graphite fiber functioned effectively as the ablative shield in orbital re-entry vehicles for many years (92). 

The overwhelming majority of commercial polymers are organic, that is, they are based on carbon chemistry, and they therefore possess poor heat resistance, so they can be ruled out from true high temperature applications, other than sacrificial ones such as ablative shields. But there is no reason why polymers should always be organic in future (the family of silicone rubbers provide an example of inorganic polymers). 

The most common end uses for these carbon fibers are ablative shields and high temperature packing materials, where current practice for the latter is to use the material in the form of 5, 10 or 20 ply yarns, with either S or Z twist. These values are dictated by the customary working practice used in the packing industry and interestingly, this does not follow the practice employed in the ropes industry, where 7 and 19 plies are used. An extra ply, as in 20 plies, would not be locked into the hexagonal structure and would be free to wander (Figure 6.6). 

The carbon-carbon composites are employed in rocket motors, as friction materials in aircraft and high-performance automobiles, for hot-pressing molds, in components for advanced turbine engines, and as ablative shields for re-entry vehicles. 

Launched ia 1959, N. A. Savannah operated very weU. Starting ia 1962, it made a goodwill voyage around the world. It was able to travel a distance of several times the earth s circumference on one fuel loading. However, the ship was not competitive economically with oil-powered merchant ships. The shielding was quite adequate, so that the reactor was safe. Nonetheless the vessel was opposed by antinuclear groups and the N.A. Savannah was eventually retired and put on display ia Charleston, South Carolina. In 1994, the ship was transferred to Norfolk, Virginia, to be held ia reserve. 

A mixture of PhenoHc MicrobaUoons and resin binder has a putty-like consistency. It can be molded to shape, troweUed onto surfaces, or pressed into a core. Curing gives a high strength, low density (0.144 g/cm ) foam free of voids and dense areas, and without a brittle skin. Syntactic foams are used in widely diverse appHcations, including boat flotation aids stmctural parts in aircraft, submarines, and missiles stmctural cores for waU panels and ablative heat shields for reentry vehicles and rocket test engines. 

One possible solution to the problem is to make greater use of intumescent materials which when heated swell up and screen the combustible material from fire and oxygen. Another approach is to try to develop polymers like the phenolic resins that on burning yield a hard ablative char which also functions by shielding the underlying combustible material. 

The presence of shielding compounds interferes with subsequent processes, as the formation of metal-support interactions is able to stabilize supported particles. Moreover, the shielding effect of the colloid protectors prevents the contact of metal particles with the reacting molecules, thus avoiding the use of unsupported colloidal particles as a catalytic system [11]. 

After the flue gas leaves the combustion chamber, most furnace designs extract further heat from the flue gas in horizontal banks of tubes in a convection section, before the flue gas is vented to the atmosphere. The temperature of the flue gases at the exit of the radiant section is usually in the range 700 to 900°C. The first few rows of tubes at the exit of the radiant section are plain tubes, known as shock tubes or shield tubes. These tubes need to be robust enough to be able to withstand high temperatures and receive significant radiant heat from the radiant section. Heat transfer to the shock tubes is both by radiation and by convection. After the shock tubes, the hot flue gases flow across banks of tubes that usually have extended surfaces to increase the rate of heat transfer to the flue gas. The heat transferred in the radiant section will usually be between 50 and 70% of the total heat transferred. 

A different pH-triggered deshielding concept with hydrophilic polymers is based on reversing noncovalent electrostatic bonds [78, 195, 197]. For example, a pH-responsive sulfonamide/PEl system was developed for tumor-specific pDNA delivery [195]. At pH 7.4, the pH-sensitive diblock copolymer, poly(methacryloyl sulfadimethoxine) (PSD)-hZocA -PEG (PSD-b-PEG), binds to DNA/PEI polyplexes and shields against cell interaction. At pH 6.6 (such as in a hypoxic extracellular tumor environment or in endosomes), PSD-b-PEG becomes uncharged due to sulfonamide protonation and detaches from the nanoparticles, permitting PEI to interact with cells. In this fashion PSD-b-PEG is able to discern the small difference in pH between normal and tumor tissues. 

Literally removal , but applied particularly in space technology to the process of using up the frictional heat developed on re-entry of the vehicle into the Earth s atmosphere by degradation of the heat shield. Certain thermoplastics, thermosetting resins and polytetrafluoroethylene have been evaluated as ablative materials. 

Documentation of the tests should also be provided by still photography, video camera/recorder systems, and high speed photography. The high speed photography with a minimum speed of 500 frames per second Is necessary to be able to see any flame front exiting a shield. A list of typical Instrumentation used on an operational shield test Is shown on Table II, (see reference A). 

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