Simulated Low Earth Orbit Environment Research Articles

Material erosion measurements and expected operational lifetime of a deployable photon sieve payload

Spacecraft operating in low-Earth orbit are subjected to a number of hazardous environmental constituents that can lead to decreased system performance and reduced operational lifetimes. Due to their thermal, optical, and mechanical properties, polymers are used extensively in space systems; however they are particularly susceptible to material erosion and degradation as a result of exposure to the LEO environment. The focus of this research is to examine the material erosion and mass loss experienced by a custom Kapton-like polyimide due to exposure in a simulated low-earth orbit environment. The deployable membrane telescope design discussed in this research is named Peregrine and is the scientific payload for FalconSat-7, a 3U cubesat designed and developed at the United States Air Force Academy (USAFA) for the purpose of demonstrating the capability of deployable membrane telescope technology on a nano-satellite platform. In addition to the polymer samples, chrome, silver and gold specimens will be examined to measure the oxidation rate and act as a control specimen, respectively. A magnetically filtered atomic oxygen plasma source has previously been developed and characterized for the purpose of simulating the low-Earth orbit environment. The plasma source can be operated at a variety of discharge currents and gas flow rates, of which the plasma parameters downstream of the source are dependent. The characteristics of the generated plasma were examined as a function of these operating parameters to optimize the production of O+ ions with energy relevant to LEO applications. The erosion yield of the Kapton-like polyimide was experimentally determined to be 2.84 × 10−24 cm3 per atom which is in close agreement when compared to the on-orbit measurement for reaction efficiency of Kapton HN. The experimentally determined reaction rate for the Kapton-like polyimide was used to estimate the operational lifetime of the photon sieve during the solar conditions expected beginning in May 2019. The effective lifetime is estimated between 126 and 153 days.

Development of Cycloaliphatic Epoxy-POSS Nanocomposite Matrices with Lambas Enhanced Resistance to Atomic Oxygen.

The preparation of ultra-thin CFRP laminates, which incorporate a cycloaliphatic epoxy resin reinforced with polyhedral oligomeric silsesquioxane (POSS) reagent nanofiller, using out-of-autoclave procedure is reported. The influence of the amount of POSS within the laminate on the mechanical properties and surface roughness of the laminates is analysed before and after exposure to atomic oxygen (AO) to simulate the effects of low Earth orbit (LEO). The addition of 5 wt% POSS to the base epoxy leads to an increase in both flexural strength and modulus, but these values begin to fall as the POSS content rises, possibly due to issues with agglomeration. The addition of POSS offers improved resistance against AO degradation with the laminates containing 20 wt% POSS demonstrating the lowest erosion yield (1.67 × 10−24 cm2/atom) after the equivalent of a period of 12 months in a simulated LEO environment. Exposure to AO promotes the formation of a silicon-rich coating layer on the surface of the laminate, which in turn reduces roughness and increases stiffness, as evidenced by measurements of flexural properties and spectral data after exposure.

Effects of low-temperature thermal cycling treatment on the microstructures, mechanical properties and oxidation resistance of C/C-ZrC-SiC composites

Carbon/Carbon (C/C) composites modified with ZrC and SiC particles were prepared by precursor infiltration and pyrolysis (PIP) process. Effects of the simulated low earth orbit environment including the vacuum environment (under the high-vacuum state of 1.3 × 10−3 Pa) and low-temperature thermal cycling (the temperature is ranged from 153 K to 393 K for 200 times) on C/C-ZrC-SiC composites were investigated. These results show that the significantly generated microcracks, interspaces and interface debondings contributed to the increase of open porosity. The interfacial damages of C/C-ZrC-SiC composites caused by low-temperature thermal cycling in short-cut webs and non-woven webs were generated at fiber/matrix interface and fiber bundle/matrix interface, respectively. The flexural strength of C/C-ZrC-SiC composites increased by 9.35% after 100-time thermal cycles and then decreased dramatically to 83.99% of pristine strength after 200-time thermal cycles, accompanied by the transformation of fracture behavior from the brittle fracture mode into the pseudo-plastic fracture mode. In addition, the defects induced by the low-temperature thermal cycling accelerated the oxidative damage of C/C-ZrC-SiC composites during thermal shock between 1773 K and room temperature.

Effect of cryogenic thermal cycling on SiC-modified C/C composites

The bonding strength, mechanical properties and high-temperature thermal shock resistance of C/C–SiC composites after undergoing simulated low earth orbit environment including cryogenic thermal cycles (CTCs) between 153 and 393 K for 200 times with high-vacuum state of 1.3 × 10−3 Pa were investigated. Results of the tests showed that, the bonding strength at coating/substrate interface monotonically decreased as the CTCs increased. After 100 CTCs, the flexural strength of C/C–SiC composites enhanced and the percentage of remaining strength was 102.60%, while, the mass loss of specimens decreased from 2.08 to 0.89% after thermal shock resistance test between 1773 K and room temperature for 15 times. Once CTCs exceed 100 times, the mass loss increased, the flexural strength reduced and remained only 87.40% of initial strength after 200 CTCs. The fractural behaviour of CTCs-treated modified composites exhibited typical brittle mode. A model based on the evolution of the residual thermal stress after CTCs treatment was proposed to explain the oxidation failure mechanism of C/C–SiC specimens.

Protective effect of nanocomposite film from the low earth orbit environment

The various hazards in the low earth orbit space cause damage mostly on the exposed surfaces of polymer-based materials. Neutral particles, especially atomic oxygen, erode the exposed surface, and cyclic temperature variation produces microcracks that degrade mechanical performance. To protect substrate materials from the harmful environments, this research investigated the protective effect of nanocomposite film under a simulated low earth orbit space environment involving 4 × 10−4 torr high vacuum, 200 nm ultraviolet radiation, 14 thermal cycles ranging from −70℃ to 100℃, and atomic oxygen flux of 2.07 × 1014 atoms/cm2s. The protective films of multi-walled carbon nanotubes or nano-silica dispersed in epoxy matrix were fabricated using a comma roll casting machine. The physical and mechanical changes of the film-protected specimens under the simulated low earth orbit environment were compared to those of no-film-protected specimens. The amount of specimen erosion during the low earth orbit simulation test was analyzed through mass change. Furthermore, the tensile performance before and after exposure to the simulated low earth orbit environment was evaluated. Since the uniformly dispersed nano-silica blocked the attack of atomic oxygen and ultraviolet radiation, and internal multi-walled carbon nanotubes impeded crack propagation due to the bridging effect, the specimens protected by nanocomposite film showed less damage on the exposed surfaces and smallest in degradation of tensile strength after exposure to the simulated low earth orbit environment.

Simulated Low Earth Orbit Environment Interaction with Different Types of Polyethylene

The present study investigated the interactions of four different types of polyethylene (PE) surfaces with RF oxygen plasma components, simulating the low Earth orbit (LEO) environment. The samples were exposed in the plasma afterglow, with and without the plasma-generated vacuum UV (VUV) radiation. The polymers studied were low-density polyethylene (LDPE), ultra high molecular weight polyethylene (UHMWPE), high-density polyethylene (HDPE), and a composite of highly-oriented fibres of ultra-high molecular weight polyethylene (PolyEitan) having crystalline-to-amorphous phases ratio of 65, 74, 87 and 92%, respectively. Erosion of the LDPE, UHMWPE, and HDPE samples under atomic oxygen (AO) irradiation showed an etching rate dependence on the degree of crystallinity - the higher the crystallinity level the lower the erosion rate. The addition of VUV radiation to the AO flux resulted in a significant increase of the etching rate. An increase of the crystallinity level resulted in a decrease of the total oxygen uptake and an increase of the surface roughness. The composite polyethylene, although highly crystalline, revealed similar erosion rate and chemical composition changes as highly amorphous LDPE. The erosion mechanism of the different PE samples under AO or AO+VUV is discussed in terms of the initial internal structure.

A study of a simulated low Earth environment on the degradation of FEP polymer

Spacecraft flying in a low Earth orbit environment require thermal blankets to provide protection from direct solar heat from the sun. Fluorinated ethylene propylene copolymer is one of the major components of these thermal blankets. In this study, the effect of a simulated low Earth orbit environment on FEP was investigated. UV and VUV degradation of fluorinated ethylene propylene (FEP) copolymer was studied using ESR and XPS. The ESR study revealed the formation of a terminal polymer chain radical. The stability of this radical has been investigated under different environments. An XPS study of FEP film exposed to VUV and atomic oxygen showed that oxidation takes place on the polymer surface. The study revealed also that the percentage of CF in the polymer surface decreased with exposure time and the percentage of CF, CF and carbon attached to oxygen increased. SEM micrographs of FEP film exposed to VUV and atomic oxygen produced a rough surface with regular undulations similar to sand dunes.

Dielectric surface flashover in a simulated low Earth orbit environment

Surface flashover on insulators under UV irradiation or with a plasma background was investigated with high-speed electrical and optical sensors in order to clarify differences in the breakdown development compared to the pure vacuum case. Results with a plasma background show a more rapid development in the breakdown initiation compared to measurements in vacuum with no plasma. With a magnetic shielding technique using permanent magnets, the duration of an applied voltage pulse can be increased by a factor of 2-3 without causing flashover. UV illumination on the electrodes decreases the flashover voltage (for the dc case) or the voltage pulse duration without breakdown (for the pulsed case), whereas UV illumination on the dielectric surface increases the flashover potential.

Low earth orbit resistant siloxane copolymers

Two classes of siloxane copolymers were evaluated for their resistance to the low earth orbit (LEO) environment. Poly(imide–siloxane) (PISX) copolymers were used as the resin for PISX–carbon fiber composites. These composites were exposed to the LEO environment, for 50 h, as part of the “Effect of Oxygen Interaction with Materials” (EOIM-III) experiment aboard the space shuttle STS-46. XPS analysis showed primarily silicon oxides on the LEO-exposed surfaces and evidence of a thermally accelerated oxidation. The results of simulated LEO exposure of the PISX composites show that they are one to two orders of magnitude more resistant than are homopolyimide-based composites. Furthermore, we found, surprisingly, that these materials erode slower when far-UV radiation is combined with the atomic oxygen. XPS analysis of PISX exposed only to far-UV allowed a partial mechanism to be proposed for the effect of far-UV radiation on the PISX copolymers. Polyhedral oligosilsequioxane (POSS)–siloxane copolymers were evaluated in a simulated LEO environment and results indicate that the POSS–siloxane copolymers are even more resistant to the simulated LEO environment than are the PISX copolymers; POSS–siloxanes actually gained weight during the exposure and healed the microcracks present. © 1996 John Wiley Sons, Inc.

Miscible Blends of Poly(siloxane Imide) Segmented Copolymers and Polybenzimidazole as Potential High Performance Aerospace Materials

Compatible blends of high performance engineering thermoplastics have been investigated for aerospace applications, where stability in an aggressive atomic oxygen environment is of significant concern. The blends were comprised of polyimide homopolymers and poly(siloxane imide) segmented copolymers based upon benzophe-none tetracarboxylic dianhydride (BTDA) and m-diaminodiphenyl sulfone, and polybenzimidazole (PBI). The blends exhibited glass transition temperatures in excess of 300°C, and significantly reduced water sorption relative to PBI. Conversion of the siloxane surface to an inorganic, oxidation resistant silicate in the presence of atomic oxygen accounted for the enhanced stability of siloxane-containing blends to a simulated low earth orbit environment.