Lighter than air stratospheric balloons

Modelling the membranes of NASA super-pressure balloons to further scientific missions.

Super-pressure balloons used by NASA are designed to fly in the stratosphere, approximately 33km from Earth. These balloons are used as vehicles to carry scientific payloads for specific missions. Yet these missions are often disrupted due to failures of the thin membranes of the balloons. 

In response to this, Dr Federico Bosi from UCL Mechanical Engineering conducted a research project to characterise and model the thermo-visco-elasto-plastic response of the membrane material used in these balloons. The goal of the research was to enable the better design of balloon membranes, so NASA’s super-pressure balloon missions are not hindered by membrane failures. Dr Bosi collaborated with Professor Sergio Pellegrino from the California Institute of Technology on the research, which was funded by the NASA Balloon Program Office.

Understanding membrane responses under extreme conditions

“The goal is to have systems that fly as long as possible, with the aim of reaching 100 days of autonomous flight,” Dr Bosi explains about NASA’s super-pressure balloons. But the balloons “are gigantic, as big as a football stadium” with membranes of “38 micrometres, less than the thickness of human hair.” Failures due to issues with the membrane are common, and Dr Bosi wanted to find a way to solve this problem. 

Membranes are 2D materials, used for stratospheric balloons because it is possible to use them to create large structures with minimal weight and material use. Yet membrane structures are inherently prone to failure because they are so thin. “Less material means higher chances to fail”, Dr Bosi says. 

A key reason that better scientific balloons have not been developed to date is that there is a poor understanding of the mechanical behaviour of their membranes. And it is this lack of understanding that leads to suboptimal design. Dr Bosi’s research therefore needed to focus on how the membrane behaved in certain conditions, to aid better design in the future. 

The research focused on finding a method of predicting the response of the linear low density polyethylene (LLDPE) membrane in different conditions. Temperature was a key part of the research, because the team already understood that the membrane was particularly sensitive to temperature. Stratospheric balloons in particular are exposed to an ambient temperature when launched, falling to approximately -50 degrees Celsius in the stratosphere during night. As well as measuring the response of the membrane to temperature, Dr Bosi measured the impact of strain rates and loading conditions. The membrane material has a different response depending on the speed at which it deforms, or at which loads are applied. For example, if the load is applied very slowly, the material is softer. If the load is applied very quickly, the material is stiffer.

Improving stratospheric balloon design

As a result of the research, there is now a comprehensive experimental database of the thermo-mechanical response of these membranes to temperature, strain rates and loading conditions. Dr Bosi also created a mathematical model that accurately describes the thermo-mechanical response of the membrane material. 

The data Dr Bosi has collected is vital to understand and predict the membrane response, and to design membrane structures that are less prone to failure and last longer. Importantly, the information “allows engineers to confidently design and monitor NASA balloons,” Dr Bosi says. The information is currently being used by NASA to design super-pressure balloons for use in weather and space science. The UK engineering consultancy Tensys is also using the findings to design stratospheric balloons for Google’s project Loon, which aims to provide internet access in underdeveloped countries. 

This research led to a further grant from Horizon 2020 (MSCA-ITN-EID-LIGHTEN), for Dr Bosi to research membrane structures for a sustainable environment in partnership with Tensys (UK), University of Trento and CAEmate (Italy), TU Delft (Netherlands), and Sbp (Germany).