The Sky Anchor System was a two balloon system developed and tested by the National Scientific Balloon Facility (NSBF), during the 70's and 80's decade as part of a long lasting effort of the agency to develope a long duration balloon system. The concept -originally tested in a small scale in 1963- was boosted by the needs of the scientific ballooning community which could benefit in many areas of interest from longer exposure times at altitude. The final goal of the developmental effort was to built a vehicle capable of transport a 500 lb scientific payload at 130,000 ft for a minimum of 100 days duration.
The Sky Anchor system used a two balloon system on which a conventional zero pressure balloon (open) carried below a super pressure balloon (closed) which acted as ballast and anchor. In the image at left is a basic scheme of the system (a more detailed description can be seen clicking over the drawing).
The theoretical operation of the system was as follows. Once the system was airborne, on the way to the main balloon's operational altitude, the super pressure became filled and pressurized. As the "super pressure" continued to be carried upward it losed more and more lift. When sunset occured the entire system descended to a new equilibrium altitude where the increase in lift on the super pressure balloon just equaled the sunset effect on the main balloon. Although its volume was decreased, no gas was lost from the main balloon. At sunrise, the main balloon expanded and the system once again rose. In so doing the super pressure balloon losed the lift that it gained at sunset and the system stabilized at the same altitude as the preceding day. Since there was no change in suspended weight on the main balloon, it should not overshoot and again there will be no loss of gas. As in the pure super pressure system, assuming there were no leaks, the flight duration was limited only by creep, gas diffusion and ultraviolet degradation of the balloon fabric.
Throughout the development of the program, the actual flight experience turned out to be much more complex than planned, so after mixed results on 14 flights over six years, the program was abandoned in 1982. However, the experience gained would pave the way for other groups to develop more successful designs in the future. This would allow the original goal behind Sky Anchor to be achieved in the early decades of the 21st century.
Balloon launched on: 8/17/1979
Launch site: Donaldson Center, Greenville, South Carolina, US
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen 116.948 m3 (12.70 microns Stratofilm) Raven 35.396 m3 (19.00 microns Mylar)
Flight identification number: 159N
End of flight (L for landing time, W for last contact, otherwise termination time): 8/19/1979
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): F 46 h
Landing site: Near Las Vegas, Nevada, US
Payload weight: 249 kgs.
This was the tenth test flight of the system. Sky Anchor X mission was launched from Greenville, South Carolina on the afternoon of 17 August 1979. The balloon system consisted of a 4.130.000 cubic feet zero pressure balloon, a 564 lb payload and a 1.250.000 cubic feet super pressure anchor balloon. This mission was launched from the east coast to facilitate a trans-continental flight. This would allow the system to go through at least two sunrises.
The launch and ascent went as planned with an average ascent rate of approximately 183 M/min (600 ft/min). All ballast was dropped the first night bringing the system to a float altitude of 36.0 Km (118 K ft) where it stayed the next day. The following night it descended to a float altitude of 34.7 Km (114 K ft) as expected. Later during the night, however, it descended to 30.8 Km (101 K ft) and then rose back to the expected float altitude of 34.7 Km (114 K ft) where it stayed until sunrise. The system again floated at 36.0 Km (118 K ft) the following day as expected. Termination and recovery were normal with the package being recovered near Las Vegas, Nevada.
The unexpected 3 hour descent to 30.8 Km (101 K ft) was later explained by extra cooling of the balloon gas caused by the system passing over an overcast area (cirrus and alto cumulus with tops to 32,000 ft) during the night. This was verified by the correspondence of the overcast condition with a sharp drop in down radiometer temperature and the extra descent of the system. The probability of weather conditions such as this will be allowed for in the design of future Sky Anchor systems by raising the required ballast effect. The success of Sky Anchor X was a milestone in the long duration development program. It was the largest system of its kind ever to have flown successfully. A 256 Kg (565 lb) payload was floated through two sunrises (46 hrs) with a day float altitude of 36 Km (118 Kft).
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