Thursday, 17 May 2012

Curing at space temperatures

Well, go to next problem. Temperature.
The temperature in space is tricky. Usual imagination, what we have on Earth, does not work there. Imagine: there are no walls, furniture, ground, neighbours, friends and wind (air) surrounded your body. Temperature depends on the irradiation from the Sun and on internal sources. All bodies irradiate following Stefan–Boltzmann law. 
If no irradiations from neighbours, your temperature goes to absolute zero, well, not zero, because of space irradiations. But anyway, it comes to very low temperature; the space flight measurements give temperatures about -150 C. If you are irradiated by the Sun, your sun-side will be heated up in dependence on reflectivity of your surface, thermocapacity, thermoconductivity and geometry of your body. Very quickly you start to feel, that one side of you is burning (can be +150 C and more), while the other side is still frozen. You will want to turn. If you turn quickly enough, your temperature will be more or less steady. So, the rotation of the body is very important.
Now let’s remember, how the chemical reaction depends on temperature. At first approximation, the rate of reaction follows Arrhenius law: the rate increases with temperature. Usually, the curing system is selected to be non-reacting at storage temperatures so, that uncured material can be kept in container at transportation without the reaction. Therefore, the temperature at curing should be higher than on Earth and at transportation.
There are some ways to get it. First of all, rotation of the construction side by side to the Sun should be so, that each part of the construction will be heated enough to be cured completely. It does not need massive efforts, because no friction there, and if the construction is accelerated it will rotate forever. You have to be smart enough to calculate the rotation speed and direction. It can be calculated, measured, compared with experiments on orbit. The regime of rotation can be optimised to get complete curing in all sides and parts of the construction.
But what can you do on the Moon or on an asteroid? You cannot rotate the Moon of asteroid as we want. If you are settled down on equator of the Moon, you can expect heating enough with the turning of the Moon. But if your construction has to be placed on polar, what is more likely because of found water there, there is no way to get a heat enough. You have to heat it with internal sources, for example, with internal electrical heaters. And you have to be ready to spend a lot of energy during curing reaction. It is not a huge amount of energy: ISS astronauts spend a comparable amount of energy to support life there. The construction can be heated partially: sector by sector, that can decrease an amount of power, you have to apply.
Another way is to use photocuring reaction. There are compositions that can be cured under UV light. That’s nice way if you need to cure quickly, on command after storing long time in the container. In such case, the curing reaction is not so sensitive to the storage temperature, while the rate of curing anyway depends on temperature. Such photocuring systems can be suitable for repairmen set. But the photocured materials have usually lower mechanical strength, lower radiation stability, shorter life-time and narrower diapason of exploitation conditions, than thermocured materials. These two kinds of materials (photocured and thermocured) are specialised for different constructions. You can choose one of them for particular construction and exploitation conditions.
However, there is a serious problem with the temperature in space: thermostresses. This problem needs attention. Usually, on Earth the prepreg (uncured material) is placed into curing oven, heated slowly with optimised rate of the temperature increase, cured at uniformly distributed temperature, and cooled slowly. The heating/cooling process is optimised to avoid the thermostresses in the construction. In space, when the construction is irradiated from one side, the Sun irradiation creates a temperature gradient. The curing reaction follows to the temperature gradient in the construction. Therefore, the different parts of the construction will be cured at different temperature and will keep memory of the temperature gradient. When the cured construction changes an orientation, the temperature gradient changes and it generates the stresses. As higher temperature gradient is at curing, as higher stresses appear. The stresses deform your construction and decrease the mechanical strength of the construction.
Because no one large curing oven with temperature stabilisation is installed in Earth orbit, where you can put your construction for precise curing, the temperature regime of the construction should be precisely calculated and the flight regime should be optimised to get completely cured material of the construction without significant stresses.

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