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: 5/30/1978
Launch site: Columbia Scientific Balloon Facility, Palestine, Texas, US
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen 116.997 m3 (12.70 microns - Stratofilm) - Raven 35.438 (35 microns Celanar)
Flight identification number: 1067PT
End of flight (L for landing time, W for last contact, otherwise termination time): 5/30/1978
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): F 4 h
Payload weight: 250 kgs.
This was the seventh test flight of the system. Sky Anchor VII mission was launched from the National Scientific Balloon Facility in Palestine, Texas on May 30, 1978. The balloon system consisted of a 4.130.000 cubic feet zero pressure balloon, a 550 lb payload and a 1.250.000 cubic feet super pressure anchor balloon. Instrumentation included altitude monitoring, a differential pressure gauge , air temperature monitoring, down radiometer, one video camera, and two super 8 mm surveillance cameras.
The launch of the system was conducted at sunrise and went smoothly despite a 90-degree cross wind to the layout direction of the system. Ascent averaged around 600 feet per minute to a fill altitude of the anchor balloon of approximately 111,000 feet. The system then leveled off at approximately 112,000 feet. The bottom pressurization ballast was then dropped to slowly bring the anchor balloon up to a skin stress of approximately 4,800 pounds per square inch (psi). With this completed, the balloon system was allowed to stabilize and once having done so, the top liquid ballasting was started. However, on the very first drop, the anchor balloon catastrophically failed . The system then climbed to approximately 119,000 feet and was later terminated.
After recovery of remnants of the balloon, the prime cause of failure was suspected to be the liquid ballast. To correct this problem future flights carried no top liquid pressurization ballast. This needed a slight increase in the bottom pressurization ballast located at the base of the super pressure.