  Heliotropes are passive solar hot air balloons that are capable of achieving
nearly level flight within the lower stratosphere for several hours. These
inexpensive flight platforms enable stratospheric sensing with high-cadence
enabled by the low cost to manufacture, but their performance has not yet been
assessed systematically. During July to September of 2021, 29 heliotropes were
successfully launched from Oklahoma and achieved float altitude as part of the
Balloon-based Acoustic Seismology Study (BASS). All of the heliotrope envelopes
were nearly identical with only minor variations to the flight line throughout
the campaign. Flight data collected during this campaign comprise a large
sample to characterize the typical heliotrope flight behavior during launch,
ascent, float, and descent. Each flight stage is characterized, dependence on
various parameters is quantified, and a discussion of nominal and anomalous
flights is provided.
