Now with the benefit of two more days and one decent night’s sleep, I can add a few more notes about SPIDER’s launch.
What Does it Take to Launch?
Balloons like ours only launch on perfect days. The balloon has a total volume of 34 million cubic feet; only a fraction is filled with helium on the ground, but the helium expands to fill the volume at the lower pressures of float altitude. The balloon is massive and pushed around by the slightest wind, so the launch crew needs near-zero surface winds in order to handle it safely. This needs to last for a few hours, enough time to lay out and inflate. This is doubly true for SPIDER, which bucks in even a light breeze due to the massive sail formed by its sun shield. When the balloon is released it swings up above the payload, temporarily extending about 900 feet above the ground before final release. This means we also need vanishingly-small low-level winds at these altitudes so that the balloon does not pull away before the payload is released from the launch vehicle, which would endanger the launch crew (and the payload). Antarctic weather is fickle, and we really need special weather conditions to launch.
Furthermore, even if you can launch the balloon, you don’t really want to until the stratospheric winds are in the right configuration. This is a big part of what makes Antarctic ballooning special: circa December a polar vortex sets up that will take balloons on a circular path around the Pole, staying over solid ground (land or ice) for weeks on end. Launch too early or too late in the season and you won’t have this wind pattern, and can drift out to sea.
Worse yet, showing up for a launch attempt is an exhausting and time-consuming operation occupying more than three dozen people. For a launch at 0900, the launch crew might show up at LDB circa midnight, and the science team must show up even earlier to prepare. The payload must go through many steps:
- system checkout and head out to the deck
- attach NASA flight electronics
- transferred from our crane hook to that of the launch vehicle
- move away from the building for further checks
- move out to the pad if winds are holding
- lay out and attach flight train and parachute
- Attach balloon and start inflation
(only if we’re definitely going, because the balloon can’t be folded back up without damage)
Even an aborted launch attempt involves many hours of work outside in the cold, and is tough on the sleep schedule. Special personnel density restrictions this season (imposed on station in November) mean that the launch crew down here is stretched especially thin, even after 2 of the 3 science payloads were canceled and their teams kept home.
Worst of all, we found out in mid-December that these personnel restrictions would force our entire LDB team to leave continent by the end of the year, weeks earlier than planned. That would force us to give up and start packing well before Christmas, ending the season almost before it began. We were on the clock, and at the mercy of the weather.
Groundhog Day: Launch Attempts
Our early weather briefs indicated two possible days (Saturday 17Dec and Wednesday 21Dec), but with a great deal of uncertainty due to a chaotic weather pattern around McMurdo.
Saturday 17Dec: We showed for a first nominal launch attempt even before we were quite ready, because this was forecast as a great launch day and time was short. We had the long-shot goal of breezing through compatibility testing and heading to the pad if conditions allowed. In practice the winds never died down enough to even go outside for compatibility testing, so this was a bust. Furthermore, the stratospheric winds were setting up slower than expected
Monday 19Dec: Compatibility testing, which ended up being a very long day. But conditions looked good for Tuesday, so we took a nap after work and headed back that night.
Tuesday 20Dec: First “real” launch attempt. Got pulled back from the deck but the winds never died down enough to head to the pad.
Wednesday 21Dec: Second launch attempt, but the weather was even worse: cloudy and breeze, with no expectations of improvement.
Thursday looked poor but Friday good, and the launch crew was required to take a rest day soon. So we planned to take a day off from launch attempts and do a liquid helium refill and some electronics work. But we can’t afford to miss a launch day, so we on the science team requested a weather evaluation Wednesday evening to see if we should show Thursday. We found improving but still risky conditions, but we had almost no days left on the calendar to try. So at about 1900 we decided to go again, even on very short sleep. And I’m very glad we did.
Launch Day – Thursday 22Dec
Launch day went quickly and smoothly, as such things go. Scientist teams headed in at 2200 and midnight, with a couple of sleepers hitching a ride at 0330 with the galley crew. Roughly speaking: payload pickup at 0200, road closed at 0500, headed to the pad at 0600, inflation began at 0800, launch at 10:28 (just minutes shy of the solstice). This was thanks to an unbroken string of great piball readings of the wind profile (most released by our camp manager, Rose McAdoo, but a few by students Elle and Jared!). It was a beautiful sunny day, and a pleasure to be outside. And I even managed to squeeze in a long-postponed call during inflation to talk about Antarctica with my daughter’s 5th grade class (yay, South Side Elementary!).
My launch day post had some of the key pictures, but here are a few more!
Line of Sight
After launch there is a period of celebration, and then work to do. The team checks pointing control and performance during ascent. In the upper atmosphere the wind dies down, and we get enough control of the payload to point it (notably away from the Sun!). Then tuning of the attitude control system, detector checkouts, and commencement of science scans.
This initial tuning is enabled by our Line-of-Sight (LOS) link: a relatively high bandwidth radio connection available for the first day or so, as long as the payload is visible above the horizon. After LOS dies out SPIDER goes into over-the-horizon (OTH) mode, with much lower data rates. We still have some decent satellite transmitters on board (Iridium Pilot, TDRSS, etc.), but these cause radio interference in our detectors, so we turn them off as much as possible. For most of the flight, SPIDER essentially communicates via text messages full of highly-compressed status bits.
LOS is a busy time. Everyone sleeps in staggered schedules, which is hard after so many days of launch attempts. I don’t think I slept longer than a 1-3 hour nap for several days. But we’re excited for SPIDER-2 to do its work!