Heat shield
In space travel, the heat shield is a layer on a spacecraft that is intended to protect it from the heat generated when it enters an atmosphere . The basic development took place at the beginning of the Cold War - the nuclear explosives were supposed to survive the re-entry. Highly accelerating missiles such as the American anti-missile missile Sprint and hypersonic missiles also require a heat shield.
Basics
When the atmosphere enters , the spacecraft is strongly decelerated by the surrounding atmosphere. The atmosphere in the shock front in front of the vehicle heats up very strongly due to compression, and with it the spacecraft. It was discovered in the 1950s that a blunt body and the wave it generated dissipate the heat generated better than an aerodynamically shaped body; however, such a spacecraft would burn up without a heat shield . As a rule, the landing capsules of manned spaceships , space shuttles and space probes that land on a planet or moon with a dense atmosphere are equipped with a heat shield for protection.
The material of the heat shield is subject to enormous demands, as it has to withstand temperatures of up to several thousand degrees Celsius . The heat shield on the one hand possible effectively dissipate the heat absorbed from the shock front to the surroundings and on the other hand due to low thermal conductivity of the spacecraft and its payload, z. B. Spacemen and equipment, protect from the heat.
Reusable heat shields
Reusable heat shields such as the heat protection tiles of the space shuttles usually consist of highly porous glass fiber materials bonded by sintering with a dense, brittle , temperature-resistant thin cover layer ( borosilicate ).
Parts of the space shuttle heat shield that are subject to particular stress, such as the leading edge of the wing, are made of carbon fiber reinforced carbon (CFC).
Non-reusable heat shields
The ablative heat shields or ablative TPS ( English Thermal Protection System ) belong to this category . Ablative heat shields (from Latin ablatus , 'taken away') have two tasks: On the one hand, they should protect the systems from the high temperatures when the atmosphere enters, and on the other hand, the melted and vaporized material carried away by the flow should achieve cooling. With this type of (ablative) cooling, the physical effect of the phase transition is used , in which energy is absorbed by the phase change of the material. The ablative heat shield was initially developed for the re-entry heads of ICBMs .
Ablative heat shields were used in spacecraft such as the American Apollo and are used in the Russian Soyuz spacecraft , on landers , in hypersonic aircraft such as the X-15, and also in ICBMs .
Such a heat shield consists of cork or fiberglass composite materials and / or plastic foam ( polystyrene ) on a support structure (usually an aluminum alloy ). There are also ablative heat shields made from a synthetic resin that is difficult to melt and vaporize. This jacket pyrolyzes and sublimates when the atmosphere enters the surrounding plasma . The soot- laden boundary layer hinders the radiation transport of heat from the plasma of the impact front to the surface of the heat shield. Its porous, charred crust represents a further barrier. In addition, penetrating heat is partly consumed by endothermic reactions (cleavage of chemical bonds, evaporation), and partly transported back to the outside with the gas ( ablative cooling ). Heat, which nevertheless penetrates through the heat shield, is distributed by the highly thermally conductive structural material in such a way that no harmful high temperatures occur.
The idea for the ablative heat shield came about when developing control surfaces in the jet of rocket engines . Even today, ablative cooling is used in the nozzles of inexpensive or smaller rocket engines. For this purpose, the inner surface of the combustion chamber or nozzle of the engine is lined with a layer of a material (e.g. graphite , tungsten , molybdenum or niobium ) that only evaporates at high temperatures . This passive cooling method was used, for example, in the Merlin engine of the Falcon 1 rocket and in the AJ-118 of the Delta II upper stage; It is still in use on the RS-68 first stage engine of the Delta IV Heavy and on the RD-58 of the Block D / DM of the Proton .
The ablative heat shield of the atmospheric capsule, a discarded daughter probe of the Galileo space probe , had to survive the entry into the atmosphere with the greatest load to date when it entered the Jupiter atmosphere on December 7, 1995 at around 170,000 km / h (47 km / s). The gas in the shock front heated up to 16,000 K (approx. 15,700 ° C ) and the heat shield had to withstand a heat flow density of 43 kW / cm². The heat shield therefore made up about 43% of the weight of the immersion capsule and about two thirds of it burned and evaporated when it entered Jupiter's atmosphere.
literature
- TA Heppenheimer: Facing the Heat Barrier: A History of Hypersonics. Part 1 (PDF; 1 MB). and Part 2. (PDF; 496 kB). NASA History Series, 2006.
- Frank S. Milos et al .: Updated Ablation and Thermal Response Program for Spacecraft Heatshield Analysis. (PDF; 4.2 MB). 17th Thermal and Fluids Analysis Workshop, Maryland 2006.
Web links
- Shuttles Thermal Protection System (TPS )
- SHEFEX: Heat shield with corners and edges.
- Buran: heat shield. (English)
Individual evidence
- ↑ Free Science Experiment - Combustion - Air Pressure, Friction, Speed and Heat (English).
- ^ J. Hansen: Engineer in Charge: A History of the Langley Aeronautical Laboratory, 1917-1958. The NASA History Series, volume sp-4305, Chapter 12: Hypersonics and the Transition to Space; United States Government Printing Office, 1987, ISBN 0-318-23455-6 .
- ↑ Ryan Grabow: Ablative Heat Shielding for Spacecraft Re-Entry. (PDF) (No longer available online.) December 7, 2006, formerly in the original ; accessed on November 6, 2011 . ( Page no longer available , search in web archives )
- ^ Bernd Leitenberger: rocket engines.
- ↑ Bernd Leitenberger: Galileo's atmosphere probe. Retrieved April 27, 2011 .