SSI-31

SSI-31 (ValBal Return to Flight)
Launch date	February 13, 2016, 22:48 PST
Launch site	Laird Park, Modesto, CA
Launch coordinates	37.56158, -121.14993
Flight duration	0 hrs, 8 minutes
Landing coordinates	37.55323, -121.16565
←SSI-30 SSI Balloon Launches SSI-32→

SSI-31 featured the launch of the fourth iteration of SSI's ValBal technology that had been most recently demonstrated on SSI-22. The payload was configured to autonomously equilibrate at 16km altitude, and was projected to cross the east coast of the United States within 36 hours of flight. SSI-31 was very heavily ballasted in order to limit its ascent rate, and was ultimately released with what was later determined to be less than neutral buoyancy. A thermal updraft carried the balloon upwards across a river, after which the updraft subsided and the balloon began to fall, ultimately coming to a rest almost exactly a mile away from the launch site. The payload was recovered the following afternoon and brought back to campus for analysis.

Gen IV Design

SSI-31 contained a new custom avionics suite for controlling and monitoring vehicle altitude and internal temperature, generating power from solar cells, and reporting vehicle health over satellite communications. The vehicle also carried an array of solar panels projected to supply more than 48Wh per day of sunlight. Energy generated that could not be stored electrically in the onboard battery pack was diverted to an ohmic heater adjacent to the battery pack, which would reduce the power consumption of the vehicle’s heaters and thus continue to extend battery life.

The gas-venting valve consisted of a latex sheet sealing against a PTFE O-ring on the neck adapter. The design was a refinement of that used on previous flights and was optimised to avoid icing and gas leakage. Assembly and testing of the mechanism was considerably more straightforward than for previous designs and no issues were observed at any point during assembly, testing, and flight. SSI-31’s ballast system was essentially identical to the one flown on SSI-21 and SSI-22. The major distinctions were in the use of a new but functionally very similar gearmotor, and a new encoder. During testing, a mechanical issue was observed with the encoder but deemed too complicated to fix. Since the encoder’s sole function was to detect a ballast-dispenser jam and reverse the motor in order to unjam, a workaround was implemented by reversing the direction of ballast dropping at the start of every 10 second ballast drop cycle. When tested, the workaround appeared to work as reliably as the encoder based algorithm, so it was adopted for flight.

SSI-31 had a novel configuration of three distinct thermal enclosures: a valve enclosure housing the valve servo, a ballast enclosure housing the ballast motor/gearbox, and the main compartment housing batteries and avionics. The split design was intended to allow the mechanical compartments to run colder (-30C) than the main compartment (0C) thus decreasing the heating power required for the system.

Transmissions from the onboard RockSeven RockBLOCK module were sent to an SSI server for analysis. Data could then be viewed at habmc.stanford.edu. In addition to storing satellite messages and notifying viewers when a new message came in, it also provided an interface to send messages back through the Iridium Satellite Network and then to the payload.

Launch site

The launch was conducted at Laird Park, just west of Modesto, CA, the same site used for ValBal's previous flights. The team met at the launch site after 4 pm, with most members returning from the recovery of SSI-30 to meet a car from campus containing the ValBal team. The payload was not flight-ready upon arrival, requiring the addition of a large volume of insulation and final verification of system functionality. As is standard, fishing line used for the cutdown mechanism was added to the valve, code was re-uploaded and verified, GPS lock was attained, and satellite communications were tested and verified. This step proved particularly difficult, as communications were tested iteratively as insulation was installed and failed after the addition of a major layer of aerogel-fiberglass composite. An hour was spent waiting for communications to be reestablished, before that layer was removed and it was discovered that it contained a layer of metal foil believed to have been acting as a Faraday cage. The foil was removed, and communications were reestablished, enabling final launch prep to move forward. Although the payload was exempt from regulations requiring FAA tracking, the FAA was, as usual, notified of the launch and gave a green light to proceed.

Filling of the 1600g balloon began at around 6:45 pm. Filling was mercifully easy do to the near absence of ground winds, a fact which saved the balloon over the course of the three hours it was kept on standby awaiting payload completion. Once payload avionics integration was completed, the balloon and the neck adapter were locked onto the valve mechanism, the ballast hopper was filled, and the balloon was ready to be launched. The balloon was very heavily ballasted, leading to a very slow ascent rate that was ultimately determined to have resulted not from the balloon's own lift but from small ground updrafts that ultimately failed to lift the balloon into sustained flight.

The Flight

The balloon proceeded nominally for the first four minutes of flight, reporting its vitals reliably over satellite communications. However, the balloon stopped rising after this time, beginning to slowly fall. It ultimately landed a mile away from the launch site, and was recovered beneath a high voltage line with evidence of considerable damage. A number of onboard safety systems successfully prevented serious damage to the landing area; however, the payload was damaged beyond repair and could not be relaunched.

Milestones

• Shortest flight of SSI Balloons (as of February 2016)
• Shortest ground distance traveled by any SSI Balloons mission (as of February 2016)