Written by Vladimir Tuchkov; Originally appeared at Vpk-news.ru, translated by AlexD exlusively for SouthFront
Materials used to build spacecraft presented a number of special requirements. From the point of view of designers of ground equipment, some of them were quite unexpected. For example, resistance to evaporation. In a deep vacuum, absolutely all materials evaporate at one rate or another.
One of the pioneers of space materials science is the All-Russian Institute of Aviation Materials (VIAM), founded in 1932. Creating materials for the aviation industry at the beginning of its activities, the Institute became involved in space issues in the early 50s. It was here that the aluminum alloy AMg6 was created, which was used to make the first artificial earth satellite, launched in 1957.
The alloy is unique – in addition to aluminum, it uses eight other chemical elements: magnesium, iron, copper, silicon, beryllium, manganese, titanium and zinc. The result was a material that has the lowest specific gravity among aluminum alloys. A significant advantage because designers are fighting for every gram to facilitate the push of the load into space. Furthermore AMg6 is the most durable and solid of all aluminum alloys. This was required of it, because due to lack of experience, the designers sought to make a ball with a transmitter inside so that it would not be destroyed by space micrometeorites. But at the same time, the alloy turned out to be cosmic, because in a chemically active environment, that is, on Earth, it has a low resistance to corrosion.
VIAM scientists also had a hand in creating the R-7 launch vehicle. Its RD-107 and RD-108 engines used steel with increased heat resistance. At the same time, the problem of creating a material for fuel tanks that has a high anti-corrosion resistance, since it must work in a highly aggressive liquid oxygen environment, was solved. Silicon-alloyed austenitic-martensitic steel EI654 was developed. According to the engine developer, general designer academician Valentin Glushko, the creation of missile systems would be impossible without VIAM materials.
VIAM also participated in the creation of the Vostok spacecraft, on which Yuri Gagarin made the first space flight in the history of mankind on April 12, 1961. The creation of the ship was a difficult task not only for designers, but also for materials scientists.
The following materials, most of which were created precisely for the spacecraft, were used in the Vostok spaceship:
- aluminum alloy AMg6 (in the sealed lander housing and instrument compartment);
- aluminum alloys AMg2, AMg3 (in pipelines);
- aluminum alloys D16, D19, AK6, AK8 (in power components);
- magnesium alloy MA8 (in the instrument frame of the instrument compartment);
- steel grades 12Kh18N10E, 07Kh16N6, St20, St45 (for fasteners and power components);
- titanium alloys VT6, VT14 (in high-pressure ball cylinders).
Considerable efforts were made in creating the thermal insulation materials for the lander, the outer surface of which was heated up to one and a half thousand degrees or more when entering the atmosphere and braking.
VIAM’s staff proposed a ship’s thermal protection scheme, which was used not only in Vostok but also in subsequent spaceships. Thermal insulation ATM-1 is a screen-vacuum structure, which includes a honeycomb structure based on fibreglass with the size of fibres of 2-3 micrometres, impregnated with polymer binders. ATM-1 has a low density of 10 kilograms per cubic metre. In addition to ATM-1, the spacecraft uses a thermal protection coating of VSh-4, which is sprayed. It is based on modified phenol formaldehyde oligomers and mineral powder fillers.
Together with the “key” materials in the early stage of space exploration, many adhesives, seals, varnishes, enamels, sealants, filtres were created, without which it would be impossible to fly either to orbit, or to the Moon, Venus or Mars.
The shuttle with Brazilian sand
VIAM actively participated in the Buran project, which created more than 70 new materials and technologies used in the reusable spacecraft.
The development of thermal protection of the shuttle was the most difficult in scientific and technological terms. Two heat-resistant composite materials based on carbon fabric – Gravimol and Gravimol-B, were created for the nose fairing and the leading edge of the wing. They can withstand a temperature of 1800 degrees, which is heated to the front elements of the “shuttle” when it enters the atmosphere at hypersonic speed. By the way, the name of the material is made up of the first syllables of the organisations that developed it – Research Institute “Graphite”, VIAM, R&D Company “Molniya”.
The glider itself is designed to operate in the temperature range from minus 150 to plus 1250 degrees. This is provided by successive applications of EP-0214 epoxy rubber primer, Elastosil 137-175M adhesive-sealant (developed by VIAM together with the State Research Institute of Chemistry and Technology of Organo-Element Compounds), heat-protective tiles of 150×150 mm is size. At the same time, super-thin quartz fibres occupy only 10 percent of the volume of the tiles, and the rest is air. The tiles are covered with silicate enamel on top, which solves three tasks: protects against moisture penetration, prevents material erosion, and re-reflects the heat flow. A damping felt lining was glued to the bottom of the plate.
The thermal protection of the Buran is unique not only in terms of technical characteristics. It eliminated the possibility of a failure, such that caused the burning of the American shuttle Columbia. Then, in 2003, during the launch of the spacecraft, a piece of the thermal insulation broke off from the fuel tank and hit the heat-protective tile on the wing of the Columbia. This defect led to the destruction of a significant part of the thermal protection when the shuttle returned to Earth. As a result of the overheating of the structure, the explosion occurred.
The thermal protection of Buran is designed in such a way that such defects (the separation of single tiles) can be resolved directly in space. On the cosmonaut’s spacesuit there are two “pockets” with various chemicals. When the button was pressed, they blended. And the cosmonaut with the help of a “spatula” applied a layer of organo-silicon paste to the defective spot. Under the influence of sunlight, the “patch” was polymerised.
Such a story, for example, testifies to the “all union scale” with which Buran was built. When creating heat protection it was necessary to use ultra-pure quartz sand. Sand of this quality was not mined in the Soviet Union and had to be imported from Brazil during the R&D phase. But by the time the shuttle production was being prepared, the Urals branch of the USSR Academy of Sciences discovered the Yuzhno-Kyshtymskoye field. And in the shortest possible time the production of sand necessary for the space was deployed.
Alloys and composites
Spacecraft by mass are 90 per cent made up of various metals. This is why materials scientists pay considerable attention to them. It may seem that everything in the field of metallurgy has long been known and developed. However, it is not. Something with new properties is constantly being created and the quality of metals is gradually increasing.
They, as well as other materials, are subject to increased requirements for temperature and pressure limits, vibration loads, radiation resistance and protection, resistance to micro-particles, low temperatures, deep vacuum, corrosion.
Recently, aluminum alloys, lithium alloy and scandium have received considerable attention instead of the traditional nickel additives. And these are not alloys in the traditional sense, but materials derived from technology of granular metallurgy. As a result, the specific weight of the materials produced is reduced by 10-30 percent in relation to traditional metal alloys. And the maximum operating temperature increases to 850-900 degrees.
The essence of technology is this. First, a melt corresponding to the chemical formula of the selected material is prepared. Then you get pellets. For this purpose, either centrifugal spaying of the work pieces melted by the plasma arc or spraying of the melt with compressed inert gases is used. Then there is sifting and separation of pellets, their degassing. The granules are then poured into airtight metal or ceramic forms. Then comes the final phase of the process – isostatic high-temperature pressing in an inert gas medium, when the pressure is applied uniformly from all sides. In this case, the pressure reaches 200 mega-pascal. After that the parts are machined.
Researches with access to technology implementation in manufacture are spent also in the field of very promising materials, which are called intermetallic. They are compounds of two or more metals produced by powder metallurgy. For example, nickel-aluminum, titanium-aluminum, iron-chrome-aluminum compounds are used in space. Such compounds have a complex crystal structure, which increases their heat resistance and high temperature stability (up to 1200 degrees) and other useful properties. At the same time, the specific age is small – 3.7-6.0 g / cu. cm.
The most rapidly developing materials science industry for outer space is, of course, synthetic composites. The direction to which scientists are just approaching is “intellectual” materials. Their distinguishing feature is the ability to change properties depending on the environment. Moreover, in the future they intend to give such quality as the ability to slow down the growth of the defect, to stabilise their condition in critical areas.
The scientific push in this direction is carried out in the way of creating thermo-stabilising coatings. This is very important to improve the life of spacecraft in high, geostationary orbits where ionising radiation is high. As a result of critical heating of on-board electronics, frequent cycles of sharp temperature changes inside satellites, rapid aging of electronic components takes place. In the beginning, this comes in the form of malfunctions in electronic systems. And in the end, it leads to a loss of performance.
Ideally, thermo-regulation coatings are required to maintain the temperature inside the unit within a fairly narrow range. To do so, they must reflect the entrance of external heat energy and output the internal heat to avoid overheating. If the temperature tends to its lower limit, then heat removal must be prevented. At first, the existing thermal control coatings are quite able to cope with the tasks assigned to them. However, over time, under the influence of ionising radiation from space, the thermal radiation characteristics change. Experts believe that with increasing stability and other characteristics of temperature-regulating coatings, the service life of satellites located even in the most “rigid” orbits can be increased to 15 years.
In fact, almost every material exposed to the space environment, there are some shortcoming that needs to be minimised. This also applied to adhesives that are used to attach solar cells, brackets, and other parts. Epoxy-organo-silicon adhesives have sufficient shock and vibration resistance, good elasticity, and tolerate perfectly cyclic temperature changes. But there is also a drawback, quite significant – high gas release (up to 8%) when exposed to vacuum. The released gaseous products are deposited both on the tiles of solar cells and on optical devices. This leads to deterioration in the performance of satellites and again to a reduction in their service life. Now chemists are trying to get adhesives whose gas release does not exceed one percent.
Lighter and stronger composite materials based on carbon fibre and fibreglass are beginning to displace metals from power structures. They have another advantage – stability of size and volume when the temperature changes over the entire operating range. If gas cylinders were made of titanium alloy for the Gagarin ship Vostok, now fibreglass is used for this purpose.
Cellular three-layer materials made of carbon plastic are also in demand in modern space construction. Having excellent transparency for radio waves, they are widely used for radio engineering purposes.