Multi-hour stratospheric flights with the heliotrope solar hot air balloon

Standard meteorological balloons can deliver small scientific payloads tothe stratosphere for a few tens of minutes, but achieving multi-hour level flightin this region is more difficult. We have developed a solar-powered hot air balloonnamed the heliotrope that can maintain a nearly constant altitude in theupper troposphere/lower stratosphere as long as the sun is above the horizon.It can accommodate scientific payloads ranging from hundreds of grams toseveral kilograms. The balloon can achieve float altitudes exceeding 24 kmand fly for days in the Arctic summer, although sunset provides a convenientflight termination mechanism at lower latitudes. Two people can build an envelopein about 3.5 hours, and the materials cost about USD $30. The low costand simplicity of the heliotrope enables a class of missions that is generallyout of reach of institutions lacking specialized balloon expertise. Here, wediscuss the design history, construction techniques, trajectory characteristicsand flight prediction of the heliotrope balloon. We conclude with a discussionof the physics of solar hot air balloon flight.