Thermal protection and control

5.2.2.1 Thermal protection

Due to the extreme conditions in space, thermal protection is an important topic. By improved thermal protection systems for re-usable spacecrafts the costs in space transportation could be lowered, and moreover, a higher mission flexibility and security in manned space travel could be obtained. In the range of thermal protection systems, in particular, ceramic materials for protective layers or fiber composites are important. Ceramic fiber composites for example can be used for re-usable, high temperature components such as nozzles or combustion chambers of rocket engines or heat shields of reentry space systems. Like past applications, such as:

• Substrate foils from oxide ceramics for reflector layers (e.g. internal multi-screen insulation, which was developed for the orbital glider HERMES on basis of a sol gel procedure)

• Formation of ceramic matrix from silicon-organic oligo- and polymer precursors for complex structures

• Nanopowder (SiC, Al2O3) as a matrix component

• Nanostructured ceramic fibers

• Fiber coatings with nanoscale texture show, nanotechnology could be used favourably in the areas of thermal protection and hot structures for future reuseable space transportation systems (see Muehlratzer 2001). For a long-term exposition at temperatures above 1400 °C however, rather single-crystal oxide fibers are favored such as sapphire, while for a temperature range from 1100 to 1400 °C in particular siliconborcarbonitride (SiBN3C)-fibres and oxygen-poor SiC

Preceramic polymers for fibers as well as high temperature stable interfaces are important (Sporn the production of nano- 2002) structured ceramic fiber composites In Germany in this context, the joint project „ceramic fiber composites for high temperature engines in space" is promoted by the Bavarian research foundation with the participation of Astrium, four other companies and some scientific research institutions. Among other things oxidation protection procedures are to be developed by application of preceramic polymer precursors for carbon-based construction units. Thus the cooling effort should be reduced and the application temperature of the materials should be increased to maximum 2000 degrees Celsius.

Also nanostructured heat-insulating layers are suitable as thermal protection for combustion chambers in space propulsion systems. By means of PLD (pulse laser deposition) methods, nano-structured heat-insulating layers can be manufactured as interior coatings of combustion chamber components and be specifically adapted to the requirements (adhesion layers, sealing layers, active layers etc.). Appropriate heat-insulating layers on the basis of ZrO2 have been developed in co-operation with FhG IWS, TU Dresden and Astrium and have been tested successfully under working conditions (Gawlitza 2002, see chapter 5.3.2).29

5.2.2.2 Thermal control

Thermal control of space systems is a further topic of high relevance. This concerns, among other things, the protection of sensitive electronics against large variations in temperature. This comprises for example an efficient radiation of electronic power dissipation, which in particular represents a problem within the miniaturization of satellites. Nano-

29 IDW-news from 04.07.2002: „Keramische Faserverbundstoffe für bessere Raketenan triebe" (http://idw-online.de)

Magnetic fluids for a high precise temperature control of miniaturi-

materials offer different approaches for an improved thermal monitoring of space travel systems. For example, nanostructured diamond-like carbon layers can improve thermal control systems of nanosatellites, since they possess approx. four times a higher thermal conductivity than copper (Rossoni et al. 1999). Beyond that, diamond-like-carbon layers offer also corrosion protection, e.g. against atomic oxygen and are stable in a wide temperature range (see section 5.3). Another approach for the thermal control of miniaturized satellites are MEMS-based micro cooling loops (Birur et al. 2001). Also magnetic fluids possess application potentials in thermal control systems. Magnetic fluids are concentrated, sedimentation-stable dispersions of ultrafine ferromagnetic particles in almost arbitrary dispersing mediums (carrier liquids). The agglomeration of zed electronic compo the magnetic particles is prevented by a nm thick polymer coating. Due nents to the small dimension of the dispersed particles, magnetic fluids behave usually superparamagnetic. Average particle sizes are between 5 to 50 nm. From a technological point of view, magnetic fluids are of interest, because the pressure, the viscosity, the electrical and thermal conductivity can be controlled by external magnetic fields. Magnetic fluids at present are used mainly as sealing and damping media. In the future there might be applications in space technology in highly precise thermal control systems for miniaturized electronic components or as freefloating self-lubricating bearing for micromechanical components (IWGN 1999).

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