Round-the-world solar flight postponed until April 2016

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The beguiling attempt by two Swiss pilots to circumnavigate the globe in a solar-powered aircraft was dealt a blow this week after the team learned that the Solar Impulse 2 (Si2) plane would have to remain grounded until at least April 2016.

Damage to the plane’s four 70 liter lithium polymer batteries – incurred during the first day of the Si2’s recent five-day jaunt across the Pacific – has proved irreversible, and with adverse weather conditions likely the longer the team delays, the decision has been made to postpone the second-leg of the journey until next spring.

The aircraft’s batteries overheated during the previous leg, crossing from Japan to Hawaii in a record-breaking non-stop solo flight of five days. Since landing in Hawaii, the Solar Impulse team has assessed the damage to the batteries and concluded that there is no way to cool the current system down. Hence, any further attempt to continue the mission right now would likely lead in failure.

According to the team, there is “no weakness in the technology”, rather, pilots Andre Borschberg and Bertrand Piccard, as well as the experienced ground crew, had not anticipated the temperature fluctuations associated with navigating rapid altitude changes in tropical conditions.

Between now and April, the Si2 will be housed in a hangar at Kalaeloa airport in Hawaii while maintenance is carried out and the team begins testing cooling methods designed to guard against future overheating.

The previous leg was always considered to be the riskiest and most grueling of the circumnavigation attempt, largely because in crossing such a huge expanse of ocean the pilot had no way of finding land should a fault be detected.

However, this setback was a little unforeseen. "During the first ascent on day one of the flight from Nagoya to Hawaii, the battery temperature increased too much due to over insulation," said a Solar Impulse statement released earlier this week. "And while the mission team was monitoring this very closely… there was no way to decrease the temperature for the remaining duration of the flight."

The Si2 aircraft – powered by 17,248 SunPower solar cells and four batteries optimized to 260 watt hours per kg – was required to ascend to 28,000 feet daily in order to best maximize its energy system. This rapid cycle of ascent and descent affected the batteries in ways that the team was unable to anticipate fully.

After initially penciling in August as the next launch date, the bulk of testing and research required, allied to an imminent changing of the seasons, means that the Si2 will be unable to resume the second-leg of its journey until April 2016.