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Designations CY-5
Launch date August 5, 2019, 20:11 PDT
Launch site Brigantino Park, Hollister, CA
Launch coordinates 36.8491, -121.4338
Flight duration 2h:00m
Flight length 94 km
Flight profile Standard zero-pressure
Landing time August 5, 2019, 22:11 PDT
Landing site 5 km NW of Vernalis, San Joaquin County, CA
Landing coordinates 37.6634, -121.3217
←SSI-90 SSI Balloon Launches SSI-92→

SSI-91 (project designation CY-5) was the fifth launch of 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. The payload was recovered on August 31, 2019.

Project context

On July 27, 2019 SSI-90 had demonstrated the viability of an in-house manufactured polyethylene envelope for standard-scale payloads. However, due to the mechanisms of that launch, no data on the ascent characteristics, equilibrium altitude, and descent characteristics of a zero-pressure profiles were obtained, thus failing to fulfill one of the major objectives of that flight. Thus, it was immediately decided that a repeat launch would be attempted, with nearly the exact same configurations, except for the addition of a SPOT Trace and strengthening of RockBlock connections. Since a repeat build would require little additional design work, it was decided that the repeat launch would be attempted on the evening of August 6, 2019 from Pigeon Point.

As simulations from Pigeon Point showed the trajectory heading northeast over the Bay Area, it was decided on July 30 to request permission to launch from Garin Regional Park in Hayward, CA instead. That same day, avionics work began and was concluded on August 1. Due to the anticipated short duration of the flight, it was decided to reduce the power bank from 18 to 9 batteries. On July 31, the launch was moved to August 5, 2019, and on August 2 the new launch site was rejected.

The construction of the ZPB and final payload assembly occurred on August 4. It was generally agreed that the quality of the balloon was inferior to that of the first attempt due to several reasons.


Balloon: 0.3 mil polyethylene, paper bag topology, width 1.8 m, length 7 m, volume approximately 6 cubic meters. Total mass 0.26 kg with fill tube (PVC of length approximately 12 inches and diameter 1/2 inch).

Avionics: TinyGPS, Teensy 3.2 transmitting at adjustable intervals over Rockblock. SPOT Trace. Attempts to interface this Teensy with a BMP 280 were universally unsuccessful. This set of avionics was finished from the incomplete second set made for CY-3.

Power: 9 L91 lithium AA batteries. Resistive heater running on single L91 battery.

Payload: Styrofoam enclosure. Mass 0.62 kg.

Mass budget: 0.88 kg total.

Flight synopsis

Mission trajectory
Balloon ascending after release
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.

August 5 2011 PDT: Balloon released from a running start. Initial motion to the east at an ascent rate of 3.8 m/s --- significantly faster than the target ascent rate of 2.5 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. 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 2213 PDT: After an hour of descent at 2.5 m/s - 3.0 m/s, balloon reported a ground elevation of 29 km. The landing site was less than 100 m from a road in an apple orchard.

August 9 0316 PDT: Last RockBlock transmission received, with no indication of movement since flight termination.

Balloon Launches
2014-15 SSI-19202122
2015-16 SSI-23(a) • 2425262728293031323334353637383940414243
2016-17 444546474849505152
2018-19 8386879091
2019-20 929397