SKY ANCHOR IV

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: 9/2/1977
Launch site: Columbia Scientific Balloon Facility, Palestine, Texas, US  
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon Raven Tandem 14.160 m3 + 5.131 m3
Flight identification number: 1033PT
End of flight (L for landing time, W for last contact, otherwise termination time): 9/2/1977
Landing site: Ground abort
Payload weight: 157 kgs

This was the fourth test flight of the system. Sky Anchor IV mission was launched from the National Scientific Balloon Facility in Palestine, Texas on September 2, 1977. The system consisted of a 500.000 cubic feet zero pressure balloon, a 346 lb payload, and a 70.2 foot diameter super pressure balloon.

The purpose of the flight was to provide more launch experience for the launch crews and to obtain more system dynamic behavior to verify changes in the Sky Anchor models. Instrumentation included altitude monitoring, two super 8-mm surveillance cameras, one super pressure strain gauge, one television camera, and one Satellite Transit Receiver for position. The layout and inflation of the balloons went very smoothly. A wind direction change occurred approximately 10 minutes prior to launch. Then, when the main balloon was launched, the cross wind carried the payload and tether in a direction different from that the system was laid out for. The tether line then snagged on the corner of the super pressure spool trailer which was protruding several feet out. The flight was then terminated. Steps to remedy the fouling of the tether line resulted in the fabrication of a low profile launch spool and the addition of PIBALS to aid in the lifting of the tether line.

• Advances in long duration, high altitude flights Advances in Space Research, Volume 1, Issue 11, 1981, Pag. 193
• Development of the Sky Anchor Balloon System AFGL-TR-79-0053, Special Reports No. 217, 1978
• National Scientific Balloon Facility Annual Report, FY 1977 National Center for Atmospheric Research, 1978