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: 5/16/1978
Launch site: Malden, Missouri, US
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen 116.949 m3 (12.70 microns) + Raven 35.438 m3 (38 Celanar)
Flight identification number: 151NT
End of flight (L for landing time, W for last contact, otherwise termination time): 5/16/1978
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): F 16 h 30 m
Payload weight: 262 kgs
This was the sixth test flight of the system. Sky Anchor VI mission was launched from Malden. Missouri in May 16, 1978. The balloon system consisted of a 4.130.000 cubic feet zero pressure balloon, a 578 lb payload and a 1.250.000 cubic feet super pressure balloon. Instrumentation included altitude monitoring, a differential pressure gauge, air temperature monitoring, down radiometer, transit receiver and GOES satellite platform.
The launch of the system occurred just prior to sundown. Shortly after launch the bottom pressurization ballast had to be dropped to counteract an inflation error. The balloon then continued to altitude. The super pressure anchor balloon started over-pressuring at 111,000 feet as predicted and the system continued to climb to a ceiling of 114,000 feet. The system was allowed to float at this altitude during the night with plans to continue slowly pressurizing the system after sunrise the next day. After sunrise the skin stress in the anchor balloon reached approximately 4,000 pound per square inch (psi) and started to stabilize. The dropping of the top liquid ballast was then initiated to bring the anchor balloon up to a calculated skin stress of 7,500 psi. However, at the start of the third drop, the anchor balloon catastrophically failed at a skin stress of approximately 4,200 psi. The system then climbed to an altitude of 119,000 feet and was later terminated.
Upon inspection of the remnants of the balloon it was found that one of the 36-inch diameter load rings had become shifted at some point in time. It was reasoned therefore, that this would have possibly loaded one side of the ''super pressure" highly, thereby possibly causing failure. Steps were taken to prevent further occurrence of this problem, and another flight was scheduled approximately two weeks later.