Technologies and materials for space applications Flashcards
(5 cards)
What are common problems and challenges faced with space structures, materials and technologies?
Space environment: photonic radiation cycles, ionic radiation, debris and micrometeorites, thermal cycles with really low and high temperatures, ATOX, outgassing, stress corrosion etc. Additionally, surviving launch whilst minimizing volume during launch is extremely complex as a challenge.
What are gossamer structures?
To challenge the complexity of deploying larger structures in space, we use gossamer structures. These are light structures which can be deployed and/or inflated; usually have large dimensions and are made of thin, flexible membranes with sustaining/stiffening structure. They are packed and stowed in a small volume during launch and are deployed in outer space.
Common applications include antennae, reflectors, solar arrays, solar concentrators, solar sails, telescopes, living modules, etc.
What are the main challenges in design of gossamer structures?
Materials must be selected based on mechanical flexibility whilst keeping low weight. As such, we use thin films which may have adequate stiffness yet may be folded easily. The material should, in general, have: low temperature toughness and ductility, dimensional stability and controller CTE, resistance to space environment, no folds or wrinkles, uniform thickness, specific mechanical characteristics and functional characteristics, compatibility with deployment and rigidization systems, and thermal and electromagnetic properties.
See figure in doc.
The deployment monitoring usually requires active control via actuators and transducers, generating a need for smart gossamer structures implementing concepts from smart materials (FO sensors, for instance, could be utilized).
How do rigidization systems of gossamer structures work?
These are the systems that allow the structure to maintain its desired shape after inflation. Due to permeability and micrometeorite punctures, structures may deflate in time, so something to keep them stiff is required. The main requirements include high mech. flexibility (before deployment) and high modulus (after deployment), reversibility of the process, no or low energy request, CTE zero or very low, minimal dimensional variation, resistance to space environment, and no outgassing or contamination. Common techniques include layers of AI stiffened by stretching beyond the yield point, composites with crosslinkable resins activated thermally or by UV radiation, composites with thermoplastic matrixes stiffened by cooling below T_g (glass transition temperature), matrices and hydrogels stiffened by water/solvent evaporation, or shape memory polymers and polymer foams.
Using layers of aluminum to be stretched beyond yielding is convenient as it requires no external energy source, is fast and controllable, and has long storage times with no significant effects on material behavior. However, it is not a reversible process. UV or thermally crosslinkable resins offer very good design flexibility and can be packaged and stowed in a very compact form, but an external energy source is required, the process is irreversible for thermosets, matrix characteristics will vary with time, UV requires transparency (no carbon reinforcement), and has some compatibility issues with MLI. Matrices and hydrogels stiffened by water/solvent evaporation are stores in a umid condition but with a permeable coating that allows slow water permeation. The water then permeates and evaporates, stiffening the structure. This requires no external energy and is reversible, but has some compatibility issues with reinforcements and can cause contamination (condensation of volatiles on cold surfaces). Lastly, composites with thermoplastic to be stiffened by cooling below T_g and shape memory polymers are very modular solutions based on the polymer design.
What are the main causes of degradation in space, its effects, and fixes?
ATOX (particularly during LEO) which is highly reactive/corrosive. At orbital speed the impact velocity is enough to break chemical bonds. In metals this may create a protective outer layer of oxide (this however can compromise thermal and optical properties). On polymers organic material is eroded and volatile oxidation products are generated. Polymers become brittle, thinner, loose strength, get surface cracks, and variations in surface roughness affecting emissivity and absorption. Mitigation includes protective surface films (fluorinated polymers with metal oxides, or silica, alumina, silicon, aluminum or gold sputtering), surface modification, and inorganic fillers within the polymers.
Low energy photonic radiation (UV). Ionizing radiation by charged particles and high energy radiation (xray and gamma rays). A lot of polymers are sensitive to UV radiation, which can cause scission of polymer chains and oxidation, outgassing and cross-link breakage. This generates loss of toughness, breaking of films, surface cracks, change of colour and thermoptical properties. Ceramics also have optical properties affected by UV. X rays and gamma rays, as well as charged particles, dangerous in GEO and HEO, and LEO, respectively, degrade polymers and can cause charge accumulation on electronics, radioactivity, modification of digital memories, reduction of solar cell and instrument efficiency. They are mitigated via materials selection and coatings, in the case of UV and ionizating radiations, and metal shielding and redundant designs for ionizing particles (with higher penetration).
Outgassing (exposed volatile matter and contamination). Outgassing mostly is harmful for hygroscopic materials (eg; nylon, polyesters), plasticized materials (adhesives, sealants, flexible tubes, films) and materials with low degradation and oxidation resistance. Metals and ceramics are therefore generally barely affected. In polymers, the effect depends on the content of volatile matter, but in many cases T_g can increase, causing embrittlement, therefore permitting crack generation and propagation. This volatile matter can also end up settling on optical or electric control components/surfaces. Mitigating measures include material selection, pre-treatments (vacuum baking), design considerations (avoid proximity between contamination sources and sensitive components), escape paths for contaminants.
Contamination (condensation and polymerization of volatile substances)
Micrometeorites and debris. Impacts at particles from 4-5 kms and 9kms (micrometeorites and debris, respectively) causes damage upon impact through projectile vaporization and crater creation. To mitigate impacts mission parameters and path selection must be chosen to avoid probability of impacts, design to reduce the effects of impacts, redundant systems, multiwall protections, fiber reinforced covers, and a beta cloth (fabric made of silica fibers coated with PTFE).
Extremely high and low temperatures, and thermal cycling. Thermal protection systems include thermal protections for extremely high temperatures (such as re-entry surfaces, engine nozzles, etc) and low temperature (cryogenic payloads, guaranteeing operating temperatures, etc). They must be able to dissipate and redistribute stored energy, protect internal structures whilst keeping them within admissible temperatures, and maintain control over payload, crew and instrument temperatures. To do so, resistance to erosion, oxidation, chemical attack, dynamic mechanical loads, is essential, as is easy assembly and inspection. Density must also be kept low to adequate costs of launching extra mass. Generally, we can only operate via radiation, as we will face vacuum, but convection and radiation are also essential internally and within re-entry or atmospheric components of the missions. There exists passive and active TPS (thermal protection systems). Passive includes re-radiative protection, thermal insulators (we will generally want high emissivity maximize radiation towards outside and low conductivity to minimize heat flux crossing for high temp, and conventional materials for low temp), and ablatives (organic resins, usually fiber reinforced, which absord and dissipate heat by decomposition, melting, subliming, erosion, etc). Ablatives are classified as subliming, charring, intumescent. Whereas active TPSs (an external action is required) include fluid circuits, electrical heaters and heat pipes. Cryogenic fuel flow itself can be used as cooling through radiation to the outside and receiving thermal energy from the inside by conduction and convection. Heat pipes have phase change materials with a capillary structure so that vapor is transported into a condenser, which emits heat, and the liquid is transported back to an evaporator, which absorbs heat.
This may lead to: change of mechanical properties (embrittlement, strength reduction, deformability increases, thinning, etc), change of optomechanical properties (emissivity and absorbance change, directly affecting heat transfer capability, efficiency of solar cells or change of spectral response, affecting functionality of optic instruments), change of electric properties (could lead to loss of insulation which may then cause discharges and power losses), and change of surface properties (affecting friction and adhesion, or causing stress concentrators).
Metal is most vulnerable to oxidation, erosion and oxide detachment (due to ATOX), debris and micrometeorites, high energy radiation, and vaporizing/outgassing. Ceramics and glasses are most vulnerable to radiation and thermal cycles. Polymers are varied and thus vulnerable to a series of different degradation mechanisms.
