Protection thermal

Fig. 2. A simplified material thermal performance analysis for a reentry vehicle thermal protection system where = density x surface recession thickness = total aerodynamic heat/heat of ablation ...

The practice of employing reusable thermal protection systems for reentry is becoming more common. These are essentially ablative materials exposed to environments where veryHtde ablation actually occurs. Examples iuclude the space shuttle tiles and leading edges, exhaust no22le flaps for advanced engines, and the proposed stmctural surface skin for the National Aerospace plane. 

The thermal protection system of the space shutde is composed mainly of subliming or melting ablators that are used below their fusion or vaporization reaction temperatures (42). In addition to the carbon-carbon systems discussed above, a flexible reusable surface insulation composed of Nomex felt substrate, a Du Pont polyamide fiber material, is used on a large portion of the upper surface. High and low temperature reusable surface insulation composed of siHca-based low density tiles are used on the bottom surface of the vehicle, which sees a more severe reentry heating environment than does the upper surface of the vehicle (43). 

H. N. KeUey and G. L. SEAh, Assessment of Alternate Thermal Protection Systemsfor the Space Shuttle Orbiter (AIAA/ASME 3rd Joint Thermophysics, Eluids, Plasma and Heat Transfer Conference, June 7—11, 1982, St. Louis, Mo., AIAA-82-0899, 1982. 

A. Balkrishnan, W. Nicolet, S. Sandhu, and J. Dodson, Galileo Probe Thermal Protection Entry Heating Environments and Spallation Experiment Design,... 

C. WiUiams and L. RonquiUo, Thermal Protection System for the Space Shuttle External Tanks, 6th SPI Inti., Tech. /Mark. Conf, 1983, pp. 90—100. 

Vaulted. Vaulted tanks are installed inside a concrete vault. The vault, itself a Hquid-tight compartment, reduces the fire protection requirements as the NFPA and the International Fire Code Institute (IFCI) recognize these tanks as fire-resistant aboveground storage tanks. The vault provides a two-hour fire wall, thermal protection that minimizes tank breathing losses and pollution, secondary containment, and baUistic protection. 

Carbon—graphite foam is a unique material that has yet to find a place among the various types of commercial specialty graphites. Its low thermal conductivity, mechanical stabiHty over a wide range of temperatures from room temperature to 3000°C, and light weight make it a prime candidate for thermal protection of new, emerging carbon—carbon aerospace reentry vehicles. 

Operating conditions that may overload a machine and raise its temperature beyond permissible limits may be called unfavourable. This overheating, however, will be gradual (exponential), unlike rapid (adiabatic) heating as caused during a locked rotor condition. The machine now follows its own thermal curve and therefore a conventional thermal protection device can be used to protect it from such conditions. These conditions may arise due to one or more of the following ... 

These are conditions in which overheating of the machine may not trace back to its own thermal curves as in the first case. The temperature rise may now be adiabatic (linear) and not exponential and hence rapid. Now a normal thermal protection device may not be able to respond as in the previous case. Some conditions causing overheating may not necessarily be fault conditions. Nevertheless, they may require fast tripping, and hence are classilled in this category for more clarity. Such conditions may be one or more of the following ... 

Rotating electrical machines. Built-in thermal protection, rules for protection of rotating electrical machines — BS 4999-11 1/1987... 

Kolfertz, G., Full thermal protection with PTC thermistors of three-phase squirrel cage motors, Siemens Review, 32, No. 12 (1965). 

When polyethylene is to be used in long-term applications where a low power factor is to be maintained and/or where it is desired to provide thermal protection during processing, antioxidants are incorporated into the polymers. These were discussed extensively in Chapter 7 but a few particular points with regard to their use in polyethylene should be made. Although amines have been used widely in the past phenols are now used almost exclusively. 

Since 1950, plastics have been development for uses in very high temperature environments. By 1954, it was demonstrated that plastic materials were suitable for thermally protecting structures during intense propulsion heating. This discovery, at that time, became one of the greatest achievements of modern times, because it essentially initially eliminated the thermal barrier to hypersonic atmospheric flight as well as many of the internal heating problems associated with chemical propulsion systems. 

Thermal protection The design of vehicles for hypersonic atmospheric flight represents a compromise between the intended... 

Ablative systems are not limited by the heating rate or environmental temperature, but rather by the total heat load. In spite of this limitation, however, the versatility of ablation has permitted it to be used on various hypervelocity atmospheric vehicles. No single, universally acceptable ablative material has been developed. Nevertheless, the interdisciplinary efforts of materials scientists and engineers have resulted in obtaining a wide variety of ablative compositions and constructions. These thermally protective materials have been arbitrarily categorized by their matrix composition, and typical materials are given in Table 2-9. 

A form of cooling, and the one of prime interest, concerns ablative cooling. It is essentially a heat and mass transfer process in which mass is expended to achieve thermal dissipation, absorption, and blocking. The process is passive in nature, serves to control the surface temperature, and greatly restricts the flow of heat into the material substrate. As a result of these desirable attributes, ablative cooling (includes use of plastic compositions) has been widely used for thermal protection of solid propellant motors and less extensively in liquid propellant motors. 

Figure 166. Wrist-cooler, neck-cooler, cap and vest with PCM for thermal protection (pictures from Climator)...

Finally, we can report that a shelter for telecommunications with over 300 kg of PCM for thermal protection is commercialized in India. 

Structural applications include thermal protection shingles for spacecraft and exhaust nozzles for turboprop engines. The exhaust nozzle was successfully tested in flight with the Do 228 airplane. The flight tests are now continuing to get experience with the long term use of CMC s. 

For thermal protection during reentry from space, Dornier manufactured some model shingles from SiC/SiC and C/SiC. Fig. 13 shows two shingles each reinforced with two beads. For these parts a special prepreg was developed which had a 2.5 D fiber structure and enabled easy forming by autoclave tools to be carried out. 

Fig. 13. Shingle from C/SiC for thermal protection systems of reentry vehicles... 

A cold cap is usually mounted on the top of the pump assembly to prevent vapour from reaching the vacuum chamber. A thermal protection switch is often used. The maximum working pressure of a diffusion pump is about 10-3 torr. The ultimate pressure of a diffusion pump can be around 10-9 torr and heavily depends on the oil vapour pressure (p < 10-8 torr at room temperature for very good oils). Pumps with very large pumping speed (up to 104l/s) are commercially available. 

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