In December 2017 a Museo Aero Solar sculpture was launched near Newcastle in Australia. The flight was very successful. Too successful, as the sculpture broke free from its tether and escaped. Here is a summary of the launch and whilst this is well after the event, hopefully the aerocene community can still gain something from our experience.
Over the previous 6 months the sculpture had been constructed and decorated by the students of local artist Sandii Walker during her home-based art classes and at a local infants school. Sandii and I collaborated on the construction, technical details and the launch. Waste single-use shopping bags were used and a supply of large pieces of waste plastic film greatly expanded the size of the sculpture. The theme of ‘creatures that fly’ was used to inspire the children’s decorations and some fantastic images were created. The Aerocene “MAS How to build” instructions were followed and the final sculpture weighed around 7 kg and was in the form of a Tetro based on a rectangle of 8 m x 24 m.
On the launch day the temperature was expected to be in the mid 30’s with a light breeze. Inflation started about 1.5 hour after sunrise and took 30 minutes. For most of this time the sun was behind light clouds and the ground temperature was around 25C. The sculpture had a 10 m long, 3 mm diameter main rope tied to stakes in the ground. A backup lightweight cord was attached to a corner of the sculpture. There were about 30 excited parents and students gathered for the launch. As the sculpture reached full inflation it became buoyant, was released and rose very quickly. When it reached the limit of the main rope it jerked to a stop and within a minute had broken free at the tether attachment. A photograph taken just before it broke free shows that of the three main rope attachment points one had torn out completely and one appeared to be torn but still attached. What remains of the tether rope shows tears through the plastic film rather than the tether reinforcements. The backup cord also broke, but in the cord itself, not at the attachment.
The sculpture rose quickly and within 10 minutes was out of sight. A chase car lost sight of the sculpture after 60 minutes and 15 km. Media, police and aviation authorities were notified and an alert was broadcast to domestic flights in the expected flight path. There were no reported sightings.
Our first prediction was that the sculpture would come down about 120 km away and a post was made on facebook targeting that area. The post reached over 4,000 people and gained 175 reactions. No sightings were reported.
Over the following days and with the help of @joaquin and data from the bureau of meteorology and ventusky.com we determined that the sculpture most likely rose to 16,000 m (above commercial aviation) and landed at least 600 km north of the launch site. Given the wind direction at all altitudes, there was very little chance of a ground landing. Various local fishing and coastal groups in the predicted landing area were contacted but no sightings have been reported.
For our own benefit and for the school that sponsored the launch we conducted an investigation into what occurred.
A number of factors seem to have contributed to the failure of the tether attachment:
- the sculpture was probably larger than anticipated by the design document. Earlier contacts with aerocene had indicated that a sculpture of about 6m was needed and ours was 8m, or about twice the volume and therefore twice the lift. The “MAS How to build” document gives no indication of appropriate sculpture size.
- the much greater lift than anticipated. We had all expected that it would be touch and go whether the balloon would fly and were surprised and unprepared when it rose so quickly and with such a strong lift.
- the shock loading on the tether attachment due to the uncontrolled release of the sculpture
- three participants had separately looked at the instructions for the tether attachment and thought that the design seemed weak. We did not discuss this together and at the time of construction only one of us worked on the attachment. The attachment withstood a 10 kg static load at the time it was made.
It appears that the design of the tether attachment was inadequate for the combination of sculpture size, the lift experienced on the day and the shock loading as the sculpture reached the end of the tether.
- Minimise the dynamic loads on the tether attachment by manually controlling the rise of the sculpture. This is obvious in hindsight, but was overlooked during the excitement of the launch.
- Review all elements of the process. While a comprehensive risk assessment was conducted to ensure the safety of all participants this did not cover the design of the sculpture. A team design review would have highlighted the individual concerns about the strength of the tether attachment.
- Strengthen the tether attachment. The “MAS How to build” guide gives no information as to sculpture size. For small sculptures (perhaps half this size) the design may be sufficient under most circumstances. For larger sculptures the three tether ropes should be extended, at least 3 metres or perhaps to the top of the sculpture, and attached at multiple points or continuously across the faces of the sculpture. This way a failure would be unlikely and if it occurred should tear the sculpture and allow the release of air.
We were overwhelmed by the success of the project and the power of the sun, and certainly had no intention of polluting the environment. It has been extremely disappointing to us that we could not recover the sculpture but most of the responses to the project across the media and social platforms have been positive. The team and the school involved are keen to be part of other launches.