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'''SSI-91''' (project designation '''CY-5''') was the fifth launch of [[Cycloon|Project Cycloon]] on August 5, 2019. It was the second launch of an in-house manufactured polyethylene envelope and the first SSI zero-pressure balloon to report telemetry throughout the flight. It was also a rare sunset launch and featured an even lighter payload than its predecessor, [[SSI-90]].

'''SSI-91''' (project designation '''CY-5''') was the fifth launch of [[Cycloon|Project Cycloon]] on August 5, 2019. It was the second launch of an in-house manufactured polyethylene envelope and the first SSI zero-pressure balloon to report telemetry throughout the flight since [[SSI-20]]. It was also a rare sunset launch, a weekday launch, and featured an even lighter payload than its predecessor, [[SSI-90]].

==Project context==

==Project context==

[[File:ssi91traj.png|400px|thumb|Mission trajectory]]

[[File:ssi91traj.png|400px|thumb|Mission trajectory]]

[[File:ssi91launch.png|400px|thumb|left|Balloon ascending after release]]

[[File:ssi91launch.png|400px|thumb|left|Balloon ascending after release]]

[[File:Ssi91profile.png|400px|thumb|right|Mission profile]]

[[File:Ssi91profile.png|400px|thumb|right|The altitude profile resembled a standard latex launch due to reasons discussed in the synopsis]]

Inflation was eventful thanks to strong ground winds from due west. The envelope ruptured midway through filling and was patched before refilling. Additionally, a hole approximately 5 mm in diameter was patched.

Inflation was eventful thanks to strong ground winds from due west. The envelope ruptured midway through filling and was patched before refilling. Additionally, a hole approximately 5 mm in diameter was patched.

'''August 5 2059 PDT:''' After almost an hour of steady ascent, balloon reaches a maximum altitude of 10.298 km over (37.1502, -121.4077). Ascent rate had fluctuated above and below the initial rate of 3.8 m/s. In theory, a slight acceleration in ascent was expected. By this time the balloon was in the jet stream layer travelling north at nearly 30 m/s.

'''August 5 2059 PDT:''' After almost an hour of steady ascent, balloon reaches a maximum altitude of 10.298 km over (37.1502, -121.4077). Ascent rate had fluctuated above and below the initial rate of 3.8 m/s. In theory, a slight acceleration in ascent was expected. By this time the balloon was in the jet stream layer travelling north at nearly 30 m/s.

'''August 5 2103 PDT:''' Balloon reported an altitude of 10.291 km, a highly unexpected measurement. In retrospect, it is believed that venting began at this altitude, but the vent hole was far too small to vent the necessary volume, leading to envelope rupture.

'''August 5 2103 PDT:''' Balloon reported an altitude of 10.291 km, a highly unexpected measurement. In retrospect, it is believed that venting began at this altitude, but the vent hole was far too small to vent the necessary volume, leading to envelope rupture. The resulting leakage prevented the balloon from reaching the calculated equilibrium altitude of 14-15km and caused what more resembles a latex profile than a zero-pressure balloon profile.

'''August 5 2055 PDT:''' Balloon reported first definitive descent at an altitude of 9.905 km.

'''August 5 2055 PDT:''' Balloon reported first definitive descent at an altitude of 9.905 km.