BALLOON STRESS INDEX TEST #6

This flight was an enginnering test conducted for the National Aeronautics and Space Administration (NASA) by the National Scientific Balloon Facility (NSBF). The test was part of a flight program implemented by NASA between 1989 and 1991 to determine the effect of flying at an increased flight stress index value as well as answering several other balloon performance questions. The NASA flight stress index, was developed in 1986 to provide an index of the relative stress magnitudes along a balloon gore as a function of altitude and to provide a means of predicting flight performance.

FLIGHT AND TESTING PLAN

The primary objectives of the test flights were to increase the flight stress index through the increase in suspended payload to determine the performance capabilities of balloons made of Astrofilm "E2" and Stratofilm-372 as compared to the older Stratofilm in use, as well as to provide further validation of the flight stress index as an analysis/predictive tool.

The entire program included the realization of six flights. The first four flight tests would use 23 million cubic feet (mcf) class balloons. The first series of two were to be conducted with a suspended payload of 6,000 lbs at a flight stress index value of approximately 3,400 psi. If successful, the second series of two flight tests would be conducted with a suspended payload of 7,000 lbs. and a flight stress index value of approximately 3,800 psi. Depending on the success of the first series, the final two flights would be conducted using the 28 mcf EXP class design with a suspended payload of 7,000 lbs. and a flight stress index value of approximately 3800 psi. Both Winzen and Raven balloons were used for the flight tests.

Secondary objectives of the test series were to provide: 1) balloon structural and vertical performance data on the balloon's dynamic response to very large and rapid changes in payload weight; 2) balloon structural performance data for balloons with payloads significantly less than the minimum recommended or "design" payload; 3) evaluation of a newly developed thermistor reel profiler system for use in obtaining balloon and atmospheric temperatures; and 4) balloon gas temperature data for the determination of balloon gas stratification for use in balloon vertical performance model studies.

The planned flight profile called for the balloon to ascend under normal NSBF operational control and achieve float stabilization. After approximately 1 hour at float, a large and rapid ballast drop of 2,000 lbs. was to be initiated with successive large ballast drops made until all ballast was depleted. Completion of all ballast drops would thus achieve a suspended payload several hundred pounds less than the minimum design payload. Prior to termination of the flight, the balloon gas temperature would be vertically profiled for gas stratification data. The test flight would then be terminated and the engineering payload recovered.

ENGINEERING PAYLOAD USED FOR THE TESTS

To accomplish the objectives of the program each balloon carried the same engineering gondola containing:

- Two ambient air thermistors located 500' below the gondola;
- One up looking, pan/tilt video camera;
- Four up looking super 8 cameras working at different frame rates and activated during diffferent phases of the flight;
- One balloon gas temperature profiler;
- One full gondola diagnostics package (GDP) for termination event;
- One GDP lower unit for ballast drop;
- One Stow Away Special (SAS) with peak/hold circuit;
- One SAS with standard accelerometer and timing system for launch accelerations;
- One down radiometer.

In addition to the above instrumentation, meteorological conditions under the balloon for the entire ground track and all operational housekeeping data were recorded.

Balloon launched on: 5/8/1991 at 13:13 utc
Launch site: Scientific Flight Balloon Facility, Fort Sumner, (NM), US  
Balloon launched by: National Scientific Balloon Facility (NSBF)
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen - 29.470.000 cuft - Two capped
Balloon serial number: W29.47-2X Serial Nº 1
Flight identification number: 310NT
End of flight (L for landing time, W for last contact, otherwise termination time): 5/8/1991 at 19:17 utc
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 7 h
Landing site: 23 miles ENE of Roswell, New Mexico, US
Payload weight: 7.000 lbs
Overall weight: 11.747 lbs

This was the sixth and last flight of the program. The balloon was launched from Fort Sumner, New Mexico as mission 310-NT at 13:13 UTC on May 8, 1991. This was the second flight of the series using the 28 mcf EXP class balloon. The flight system consisted of the newly modified two capped Winzen 29.47 mcf balloon, a suspended engineering payload of 7,000 lbs., and a prototype thermistor profiling device for obtaining balloon gas temperatures. The ballast systems were the same as used for the previous test flights: an outer ballast conduit, an inner conduit and a normal NSBF ballast hopper. The gross load of the system was 10,679 lbs. Ten percent free lift was used for a total gross inflation of 11,747 lbs. Approximately 377 lbs. of ballast was dropped during the ascent with the balloon arriving at an initial float altitude of 118,600 ft. in 2 hours and 45 min.

The balloon was allowed to float from 15:58 to approximately 18:55 UTC to allow for profiling of the balloon gas temperatures. At the conclusion of this period, the first ballast drop of 2,000 lbs. occurred which resulted in an ascent rate of approximately 4,184 ft./min. and a new float altitude of 123,500 ft. Approximately 8 min. later, a final ballast drop of 2467 lbs. was successfully accomplished resulting in an ascent rate up to 4,809 ft./min. The balloon stabilized at a new and final float altitude of 130,600 ft.

The flight was successfully terminated at 19:17 UTC. The final suspended payload after all ballast drops was 2,156 lbs., which was 464 lbs. less than the minimum design payload of 2,720 lbs.

• NASA Balloon Flights (1989-1998) in NASA Historical Data Book, Vol. VII: NASA Launch Systems, Space Transportation, Human Spaceflight, and Space Science, 1989-1998
• NASA test flights with increased flight stress indices International Balloon Technology Conference, Balloon Systems Conferences, 1992