Material Selection

Kevlar was selected as the main material for the final design. Kevlar is a highly flexible composite developed by DuPont. It is manufactured as a fiber and woven into sheets. The high strength, low cost, impact resistance, and flexible nature make Kevlar an ideal material for an inflatable design. Past space structures have already utilized Kevlar as a design material. During the shuttle mission STS-77 an inflatable antenna, called the Spartan, was deployed. The inflated struts were made of Kevlar11.

Table 3 shows the properties of Kevlar

Density Young's Modulus Yield Strength Thermal Expansion
1380 kg/m3 76 GPa 1240 MPa -4µmK

Number of Launches

Kevlar possesses a higher yield strength per density than most alloys, such as titanium and aluminum. This yield strength per density for Kevlar (898.5 Mpa*m3/kg) is approximately eight times greater than the yield strength per density of titanium (187.5 Mpa*m3/kg) or aluminum (98.6 Mpa*m3/kg). This difference results in significant mass and volume savings for Kevlar over other materials. The calculated volume of Kevlar required to construct the structure is 54.89 m3. This volume is well within the set goal volume of 70 m3. However, further analysis will be required to determine that adequate support is provided to the array.

Survivability of Space Environment

Radiation effects on the Kevlar structure will be minimal due to the shielding provided by the solar array. Kevlar is a non-magnetic surface; therefore the Earth's magnetic dipole will have no influence on the structure's attitude. The support structure is effectively ground with the solar array. Thus, differential charges caused by the plasma fields will safely discharge through the array. Kevlar has a small negative thermal expansion coefficient (-4 mm/mK) which results in low internal stress in differential temperatures. Kevlar, like most composites, is vulnerable in the vacuum of space. Sublimation is directly proportional to a material's vapor pressure. Due to Kevlar's high vapor pressure, the sublimation rate is large. Kevlar tends to absorb humidity because of its woven texture. Serious outgassing is a result of this moister intake. These effects of near zero atmospheric pressure can be minimized by coating the Kevlar with a sealant such as Neoprene. The inflated structure is subject to possible punctures caused by high velocity space debris. However, the high tensile strength (29*106 N/m2) of Kevlar resists punctures.11

Structural Support

The interwoven design of Kevlar resists ripping, tearing, and cracking. This material has a yield strength of 180 kpsi which is sufficient for inflation pressures capable of supporting the structure.11