Material Properties Analysis of Structural Members in Pumpkin Balloons

Abstract

The efficient design, service-life qualification, and reliability predictions for lightweight aerospace structures require careful mechanical properties analysis of candidate structural materials. The demand for highquality laboratory data is particularly acute when the candidate material or the structural design has little history. The pumpkin-shaped super-pressure balloon presents both challenges. Its design utilizes load members (tendons) extending &om apex to base around the gas envelope to achieve a lightweight structure. The candidate tendon material is highly weight-efficient braided HM cord. Previous mechanical properties studies of Zylon@ have focused on fiber and yam, and &dustrial use of the material in tensile applications is limited. For high-performance polymers, a carefully plamed and executed properties analysis scheme is required to ensure the data are relevant to the desired application. Because no directly-applicable testing standard was available, a protocol was developed based on guidelines fiom professional and industry organizations. Due to the liquid-crystalline nature of the polymer, the cord is very stiff, creeps very little, and does not yield. Therefore, the key material property for this application is the breaking strength. The pretension load and gauge length were found to have negligible effect on the measured breaking strength over the ranges investigated. Strain rate was found to have no effect on breaking strength, within the range of rates suggested by the standards organizations. However, at the lower rate more similar to ULDB operations, the strength was reduced. The breaking strength increased when the experiment temperature was decreased &om ambient to 1 8 3 q which is the lowest temperature ULDB is expected to experience. The measured strength under all test conditions was well below that resulting fiom direct scale-up of fiber strength based on the manufacturer's data. This expected result is due to the effects of the braiding process and material ageing.

Improving high-altitude UV–Vis resistance of PBO braided tendons of NASA’s super pressure balloons

“…It has the highest specific strength and specific modulus among the commercially available high-performance fibers (Toyobo Co Ltd., 2005;Yang, 1989). The superior mechanical properties including high creep resistance (Said, Seyam, Vallabh, & Bolin, 2013) makes the PBO fibers especially suitable for the braided tendons used as the primary load-bearing structural component of giant scientific balloons, such as super pressure balloons (SPBs) used in ultra-long duration ballooning missions (Sterling, 2003).…”