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Adv. Space Res. Vol. 30, No. 5, pp. 1215-1220, 2002 © 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0273-1177/02 $22.00 + 0.00

PII: S0273-1177(02)00534-3

DESIGN AND TESTING OF THE ULDB VEHICLE

H. M. Cathey, Jr. 1, M. Smith 2, and R. Stephens 2

tNew Mexico State University, NASA/GSFC, Wallops Flight Facility, Wallops Island, Virginia 23337, USA 2Raven Industries, Rt. 9, Box 1233, Sulphur Springs, Texas 75482, USA

ABSTRACT

The development and testing of the next generation of superpressure balloons that are part of the Ultra Long Duration Balloon project are presented. General descriptions of the subsystems that compose the balloon and the designs are shown and testing is also presented. Results of two test flights of scaled versions of the Ultra Long Duration Balloon Vehicle are highlighted along with future plans. © 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

INTRODUCTION

The U.S. Balloon Program continues the development of a new class of balloons intended for long duration flights approaching 100 days (Cathey 1998, Smith 1998, 2000). The development of the balloon vehicle for the Ultra Long Duration Balloon (ULDB) project is a key to this project's success. The proposed ULDB vehicle is a superpressure pumpkin-shaped balloon that assigns most of the pressure containing function to the load tendons (Cathey 1999).

Extensive effort has gone into this balloon development. All aspects of the balloon have been addressed from materials development, fabrication approaches, assembly, flight operational considerations and more. A brief description of the design approach is given along with the balloon design and fabrication. The analyses and testing that were part of the development are presented. Major emphasis is given to the flight tests performed. This will include the goals of the tests, balloon descriptions, flight results, conclusions, and lessons learned. This paper is intended to give the reader a clear picture of the ULDB Vehicle design, results of the testing, and test flights to date.

PUMPKIN BALLOON DESIGN AND DEVELOPMENT

The pumpkin I~dloen requires longitudinal tendons that carry both the pressure load and payload weight. The balloon shell material is required to ordy carry a minimal pressure load. The pumpkin design has been attempted previously, but failed to achieve its potential due to unavailability of strong, stiff, light-weight tendons (Smalley). Schur (1999, 2000a, 2000b) has presented the general design considerations of the pumpkin balloon.

The ULDB Vehicle development has been approached as a phased progression in the materials development, design implementation, and fabrication approaches. Cathey (1999) has previously presented the phased development approach for ULDB. The test flights documented in this paper represent the Phase II and Phase Ill test flights.

DEVELOPMENT TESTING, TRAINING, AND SPECIFICATIONS

There has been extensive development testing performed as part of this balloon development effort. Hundreds of tests have been performed that touch on all aspects of the design, production, and fabrication process. These tests have included material testing under different temperature and loading conditions (Said), subsystem functionality tests, sealing tests, handling tests, and much more. Table 1 presents some of the structure tests that have been part of the development. Production persotmel were extensively trained and qualified for all aspects of the balloon production. Detailed specifications were also developed for each operation.

BALLOON TEST FLIGHTS

There are two test flights covered in this paper. As part of the phased development of the ULDB Vehicle, the Phase II flight was numbered 474NT and the Phase HI flight was numbered 485NT. Table 2 presents a description of each balloon.

1215

1216 H. M. Cathey et al.

Table 1. Sphere and Pumpkin Test Balloons

Size Shape Material Predicted Burst Pressure at Burst Date Tested Pressure 9815 Pa (1.4 psil 14m 3 Sphere DP6611.25.25 5998 Pa (0.87 psil February, 1997

14m 3 Sphere DP6611.25.25 9815 Pa (1.4 psi: Approx. 6895 Pa (lpsi) July, 1995 14m 3 Sphere DP6611.25.25 9815 Pa (1.4 psi] Approx. 6895 Pa (lpsil August, 1995

2352m 3 Sphere DP6611.25.25 N/.4 N/A, Fabrication Leak Check August, 1998 14m 3 Sphere DP6611.25.25 9815 Pa (1.4 psf~ Approx. 6895 Pa ( lps i ) February, 199~ 14m 3 Pumpkin DP6611.25.25 19303 Pa (2.8 psi] Tendon attachment failed - test halted January, 30, 199~ 14m 3 Pumpkin DP6611.25.25 19303 Pa (2.8 psi'~ 16582 Pa (2.405 psi) June 9, 199~ 14m 3 Pumpkin 2 mil Stratofilm 1792 Pa (0.26 psi~ 2206 Pa (0.32 psi June 22, 199g 14m 3 pumpkin 1.5 mil Stratofil~ 1344 Pa (0.20 psi] 1551 Pa (0.225 psi} January, 200( 141133 pumpkin 0.8 mil Stratofilm 717 Pa (0.104 psi'~ 793 Pa (0.115 psi) January, 200C 60m 3 Pumpkin Co-extruded test film A 1098 Pa (0.1592 psi: 2206 Pa (0.32 psil March 20-21,200( 60m 3 Pumpkin Co-extruded flight film 2068 Pa (0.3 psi'~ 2606 Pa (0.378 psi) April 8, 20~

Table 2. Test Flight Balloon Descriptions

Fl igh t 4 7 4 N T

51,442 m 3 (1,187,000 ft 3)

-62 g/m 2 (-1.83 oz/yd 2)

Balloon Volume

31.8 m (104.5 ft) heisht

Material Wei~ht

F l igh t 4 8 5 N T

68,554 m 3 (2,421,000 ft 3

-37.7 g/m 2 (-1.11 oz/yd 2)

134 N u m b ~ of Gores 150

1.31 m (51.64 in) Maximum Gore Width 1.33 m (52.18 in)

Inflated Dimensions

53.2 m (174.4 ft) diameter

7,716 m 2 (83,060 ft 2) Surface Area 9,218 m 2 (99,216 f12)

69.74 m (228.8 ft)

739 k~ (1630 lb)

Bi-Tape Seam

Braided Z~,lon Tendon

5%

Wei~t Seam

Tendon

Foreshortenin~

Sus -nded Float Altitude

Flight Time

743 k~ (1639 lbs)

26,200 m (86,000 ft)

2h r 3 min

35 m (115 ft) heif~ht

58.5 m (192 ft) diameter

78.34 m (256.9 ft)

1637 k~ (1404 lb)

Heat seal with sleeve

Braided Z~,lon Tendon

variable -2%

753 k~ (1660 lbs)

28,400 m (93,200 ft)

> 3 0 h r

Phase H Test Flight - Flight 474NT

Flight 474NT was a test flight of a pumpkin balloon built for the Phase II Balloon Vehicle development as part of the Ultra Long Duration Balloon project. Phase II of the ULDB Vehicle development was the Technology Demonstration phase. This phase included the teaming of NASA, Physical Science Laboratory (PSL), and Raven Industries, the design of subsystems for balloon, and the scale up of flight structure. The test flight took place on October 23, 1999. This was the first test flight of a pumpkin shaped balloon constructed of a composite material for the ULDB project.

Flight 474NT Test Flight Objectives

The Phase II pumpkin balloon test flight was designed to be a measured step in the design, fabrication and materials development process for long duration balloon vehicles. The purpose of the ULDB Phase II flight test was to gather data that

Design and Testing of the ULDB Vehicle 1217

would 1) aid in the assessment of the ULDB balloon material and design, and 2) contribute to the design and development of the ULDB vehicle. The data collected included balloon flight performance data, visual inspection data of the balloon before, during, and after launch, and post flight inspection/testing.

The flight test was used to determine manufacturing capability and material strength for a multi-gore structure that requires all of the elements of the balloon's shell manufacture involved for larger flight balloons. The mission objectives of the ULDB Phase II flight test were to 1) gather data related to the specific design, materials, and fabrication techniques used on this superpressure pumpkin balloon to assess performance, 2) gather data which will aid in the assessment of the ULDB superpressure pumpkin balloon design approaches. The data will contribute to the design, development, and fabrication approaches for future ULDB vehicles, and 3) demonstrate that the operations aspects for the superpressure balloon pumpkin design proposed for ULDB can be successfully addressed

The three minimum mission success criteria for this flight that related to the balloon performance were to 1) have a successful inflation and launch operation, 2) pressurize the balloon to greater than 200 Pa, and 3) recover the balloon carcass. The comprehensive mission success criteria for this flight related to the balloon performance also included stabilization of the balloon at a constant pressure altitude, sunrise and/or sunset transition at float, and operation of the balloon destruct device.

Flight 474NT Test Flight Results

Flight 474NT was a qualified success. All of the minimum success criteria were met with a successful inflation and launch operation, balloon pressurization to greater than 277 Pa, and recovery of balloon carcass. Figures 1 and 2 present the flight profile and differential pressure (DP) for this test flight. The balloon burst just after reaching float altitude. The altitude had stabilized and the balloon continued to pressurize due to adiabatic expansion of the gas. The balloon was pressurized to greater than 200 Pa for 168 seconds before balloon failure. The failure was directly attributable to a series of tendons not remaining in their intended "tracks". The balloon shell material pulled away from the one of the tendons in the "in-spool" configuration. This took place on one of the tendons in the subpressure region of the balloon. As the balloon pressurized, this tendon was "shucked" over to the next stable tendon location. As the pressure increased, this lead to a cascading failure as tendons moved and produced a significantly larger lobed section. This was clearly caught on video from an up-looking camera. The balloon burst when the material stresses in the large lobed section were too high.

3OOOO

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¢C I(300(3

5OOO

0 14:24

/ 14:52 15:21 15:50

GMI Time

\ N

16:19 1 6 : 4 8 17:16

3 0 0 ¸

250

2 0 0

1 5 0

1 0 0

5O

0

13:26 13:55 14:24 14:52 15:21 15:50 16:19

GMT Time

Fig. 1. Hight 474NT Flight Profile Fig. 2. Flight 474NT Differential Pressure

Design and Fabrication Issues and Directions for Phase HI

There were a number of issues that became clear after the Phase II test flight 474NT. Tendon retention was obviously a main issue. All of the tendons must be retained in their respective "tracks" to prevent the cascading failure that was experienced on the test flight. There were numerous fabrication issues generally related to the very slow balloon production speeds and significant final product cost. The overall film weight was also much too high. This weight translates into very large balloon weights and limited growth potential for this class of balloons.

All of these issues above set the directions for Phase III balloon development and fabrication. The specific Phase HI goals were to redesign the tendon attachment, reduce the areal density of the balloon shell material, focus on improvements in the fabrication processes, and retain "good balloon film qualities" including strength, low cold brittleness temperature, low creep, gas retention, etc. As can be seen in Table 2, the areal density of the material was reduced by -40%. Over 40 different film combinations, with a concentration in co-extruded films, were fabricated and tested. The Phase III material, "Aeolus" film, was developed and selected. This film is a three layer polyethylene film composite that is two times stronger than

1218 H.M. Cathey et al.

current Stratofilm on a strength to weight basis. It is also significantly lower in cost, -7 times less expensive, than the fabric film composite used on Flight 474NT.

The fabrication speeds for sealing the gores together were increased by five times by switching from a bi-tape seam to a heat sealed seam. The process uses "known" and "comfortable" fabrication techniques currently used in Zero Pressure balloon production. The heat sealing method was modified to attach the tendon and retain the tendon in a single pass. The tendon retention method was analyzed and thoroughly tested to ensure that the tendon would not move from its "designated track" under all worst case loading conditions and for all phases of the flight. The tendon is heat sealed in a sheath along the edge of each gore in a sheath. The sheath also provides UV protection of the tendon. The foresh~tening required is accomplished in much shorter production time. Extensive qualification of production techniques through testing was performed as well as extensive training of all production personnel.

Phase HI - Fright 485NT

Flight 485NT was a test flight of a pumpkin balloon built for the Phase HI Balloon Vehicle development, the Production Demonstration phase. The test flight was launched from Ft. Sumner, NM on June 4, 2000. This was the first test flight of a pumpkin shaped balloon constructed of a heat sealable co-extruded material for the ULDB project.

Flight 485NT Test Flight Objectives

The primary mission objectives of the ULDB Phase III flight test were to 1) pressurize a sub-scale pumpkin balloon, 2) collect data that will aid in the assessment of the performance of superpressure pumpkin balloons, 3) demonstrate altitude stability of a superpressure pumpkin balloon, 4) demonstrate that fabrication issues with building a large superpressure pumpkin balloon have been successfully addressed and that the manufacau~ng processes used to fabricate this balloon are acceptable, and 5) demonstrate that the operations aspects for the superpressure balloon pumpkin design proposed for ULDB have been successfully addressed. The data collected included video, differential pressure, temperature, altitude, and tendon load. This data will be used to assess balloon design, materials, and thermal design. It should be noted that the processes used to fabricate this balloon will be the same used for the full-scale ULDB Vehicle.

The secondary mission objectives were to demonstrate that a superpressure balloon will maintain it 's superpressure through one diurnal cycle, and to demonstrate the performance of the Commandable Apex Package (CAP) to qualify it for longer duration flights. The minimum mission success criteria for this flight that related to the balloon performance were to 1) have a successful inflation and launch operation, 2) pressurize the balloon to greater than 200 Pa, and 3) balloon pressure stabilization of 110 Pa or greater and float for at least 2 hours. The comprehensive mission success criteria for this flight, related to the balloon performance, were for 1) stabilization of the balloon at a constant pressure altitude for the flight duration, 2) verification of proper response from CAP autovalve algorithm (ascent and float), 3) command/TM ability from Primary and Backup CAP for 95% of flight, 4) sunset and sunrise transition at float, 4) balloon destruct device operation, 5) data from all (4) differential pressure gages on the Primary & Backup CAP, 6) no anomalous behavior (unexpected reboots, errors, loss of any function) on the CAP, and 7) recovery of balloon carcass.

Flight 485NT Test Flight Balloon Description

A general description of the test flight 485NT balloon is presented in Table 2. The balloon shell was fabricated with a co-extruded polyethylene film. This film was developed specifically for this project. Heat sealed seams, with an optimized process, were used to join the gores together. The tendons were attached at the same time as the shell was heat sealed. This process was about five times faster when compared to the previously used bi-tape seam. The 48,000 denier ZYLON tendon was attached in a UV protective sheath. This was required because the ZYLON material degrades over time due to light exposure. There was -2% foreshortening between tendon and gore edge. The balloon was constructed with a 0.8 rail SF-372 cap, similar to zero pressure balloons. While this cap was not required for this size balloon, it was incorporated to demonstrate that it could be successfully constructed, if it is required for a larger balloon. There were two wiring cables that ran from the base of the balloon to the apex, one power and one signal cable. One tendon had a load cell mounted in line.

The apex and base fittings serve dual purposes, to clamp the balloon shell for a gas seal and for the tendon termination and retention. The fittings were -1.2 m in diameter and were each made from a single stock piece of Aluminum. The apex plate had three mounting holes for the two inflation assemblies and one for the Commandable Apex Package which contains the pressure monitoring electronics (Cathey 1999). The base fitting had one hole for a mounting a plate with a camera. This digital camcorder was used to record the termination of the flight as seen at the base from inside the base of the balloon. The flight 485NT balloon was inflated through the apex fitting. Two inflation assemblies, positioned at right angles, were attached to the apex plate. The assemblies had diffusers built into inflation ports, and were connected to standard zero pressure balloon inflation tubes. Inflation was through modified EV-13 valves, which were commanded closed at the end of the inflation. These valves also serve a dual purpose of being used as the balloon vent valves during the flight. To be used

Design and Testing of the ULDB Vehicle 1219

for this function, the inflation tubes are removed from balloon after completion of inflation. To assist launch operations, inflation tube orientation marks were included down the sides of the balloon. The weight of the apex plate and the use of apex inflation required a tow balloon be used for inflation. Destruct of the balloon was accomplished via two "V" rip panels, one near the apex of the balloon and one near the base of balloon. The balloon also used a standard reefing sleeve. The balloon was packaged and boxed similar to zero pressure balloons. The absorptivity to emissivity (a/e) ratio for the balloon film was assumed to be 0.3 (Cathey 1996) for design purposes since no measured properties were available at the time of the flight.

Hight 485NT Test Hight Results

The ULDB Phase 111 test flight, Hight 485NT, was an unmitigated success. The balloon was pressurized for the entire flight after it reached a float altitude of 28.3 km. All of the desired data to be collected, including video, differential pressure, temperature, altitude, and tendon load were collected for the entire flight. Some of the data is plotted in Figures 3 and 4. The total flight time was over 30 hours in duration. The initial review of the data shows this superpressure pumpkin balloon was stable to within < - 1% (-272 m) and - +1.7% (+491m) of the float altitude. It appears that this flight has demonstrated that fabrication issues with building a large superpressure pumpkin balloon have been successfully addressed and that the manufacturing processes used to fabricate this balloon are acceptable. Post flight inspection of the balloon will confirm many of these aspects. This very successful flight has demonstrated that the operations aspects for the superpressure balloon pumpkin design proposed for ULDB have be successfully addressed. The secondary mission objectives were also met. This superpressure balloon was pressurized for the entire flight superpressure through one diurnal cycle. The CAP performed with no problems during the flight. All of the minimum and comprehensive success criteria for the CAP were met.

All of the minimum mission success criteria were met with this test flight. The inflation and launch operation were successful as evidenced by the flight performance. The balloon was pressurized to greater than 200 Pa for a portion of the flight. The balloon pressure stabilization of 110 Pa or greater and float for at least 2 hours criteria was verified with differential pressure and altitude measured at numerous times during the flight. This criteria was also met in general for most of the flight. All of the comprehensive mission success criteria were met with this test flight. The balloon was at a constant altitude for the entire flight (-1%, +2% of float altitude). The CAP autovalve algorithm performed without error during ascent and float. Command/TM ability from the Primary and Backup CAP was for greater than 99% of the flight. Continuous data was received during both sunset and sunrise. The video record clearly shows the proper operation of the balloon destruct device. Post flight inspection has confirm this. Data was received from all 4 differential pressure gages on the Primary and Backup CAP for the duration of the flight. There was no anomalous behaviors noted on the CAP. The balloon carcass has been recovered and testing is underway. All of the systems worked as designed. This balloon flew over a very bad thunderstorm at night, a worst case cold condition, and still maintained a stable altitude for the flight duration.

CONCLUSIONS AND PHASE IV - FUTURE DEVELOPMENT

Hight 474NT has demonstrated that the balloon design is robust, the fabrication approaches are sound, and that the operations aspects have been successfully addressed. All of the designs, materials, fabrication methods and techniques, quality controls, and operations aspects used for the sub-scale flight are directly transferable toward the fabrication of larger balloons. The success of balloon flight 474NT has paved the way toward the scaling up of the balloon to the final "full size" required by the ULDB project. The original requirement was for a suspended mass of 1,600 kg to an altitude of 35,000 m (Smith et al. 1998). The "full size" balloon volume required for this would be ~ 500,000 m 3. The current weight estimate for the TIGER science instrument, support instrumentation, and associated flight hardware has significantly surpassed the 1,600 kg level. It is clear that the "fixll scale" balloon will have to be designed toward a higher suspended mass which will lead to a l~dloon volume on the order of 600,000 m 3 to 660,000 m 3.

It was noted that the baUoon experienced lower daytime and nighttime superpressures than predicted. A total of 30 kg of ballast were dropped during this flight. Measured radiative properties of the balloon film became available after the flight. The absorptivity to emissivity (A/E) ratio for the balloon film turned out to be less than half than what was assumed. The measured radiative properties for the co-extruded film were Solar Transmission = 0.910, Solar Reflection = 0.079, Solar Absorptivity = 0.011, Thermal Transmission = 0.845, Thermal Reflection = 0.060, Thermal Emissivity = 0.095 for an A/E = 0.116. The pressure difference seen in the flight is attributable to the difference in assumed and measured A/E. This low A/E ratio is very advantageous for balloon designs.

The balloon design is ready to be scaled up to the "full size" balloon. The Phase IV efforts have started including the fabrication of the larger balloons. A new balloon fabrication table has been built at Raven specifically for the production of the ULDB balloons. Materials have been ordered with fabrication starting in August 2000 for a January 2001 launch from Alice Springs, Australia. There are still challenges in meeting the weight, size, and operational issues. The test flights have shown the ULDB Vehicle design to be robust. The balloon is ready to be scaled up to meet these challenges and the ULDB's mission goal of providing a new stable science platform for flight durations approaching 100 days.

1220 H.M. Cathey et al.

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30O00

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10000

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T ime (OMT)

6/5100 14:24

6/5/00 19:12

6/6/00 0:00

Fig 3. Flight 485NT Flight Profile

4 0 0 0 3 5 0 0 3000

~ " 2500 2000

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500 0 614/00

9 : 3 6

.. • .~, ,,,,t..t ., ~ i[" 250 i mF~--,r,m-'v,,~-~ Load Cell ~ ~ ~, L-~f.~ Z~,L~.~..', "~ ~ 200

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614100 6•4•00 6 /5 /00 815100 615/00 615100 615/00 6 /6 /00 14:24 1 9 :12 0:00 4 :48 9:36 14:24 19:12 0 :00

T i m e ( G M T )

Fig 4. Flight 485NT Differential Pressure and Tendon Load

ACKNOWLEDGMENTS

The research upon which this paper was based was generated while working under contract to NASA Wallops Flight Facility's (WFF), Balloon Program Office, Code 820.0. This entire effort was the result of a partnership among NASA, PSL and Raven Industries. The authors would like to thank I. Steve Smith, Jr. and NASA's Balloon Program Office at WFF for providing facilities and technical support for this work, and both PSL and Raven management for unflagging encouragement and support. The development, fabrication, testing, and test flights described in this paper would not have been possible without the dedicated PSL, NSBF, and Raven personnel who turned ideas into reality and then made them work.

REFERENCES

Cathey, H. M., Scientific Balloon Effective Radiative Properties, COSPAR, Birmingham, England, 1996 Cathey, H. M., Development of the NASA Long Duration Balloon Vehicle, COSPAIL Nagoya, Japan, 1998 Cathey, H. M., The NASA Ultra Long Duration Balloon Vehicle, Balloon Technology Meeting, AIAA 99-3867, 1999 Said, Magdi A. M., Recent Progress in Materials Selection and Characterization for Ultra Long Duration Balloon Missions,

COSPAR, Warsaw, Poland, 2000. Schur, Willi, W., Analysis of Load Tape Constrained Pneumatic Envelopes, 40th AIAA/ASME/ASCE/AHS/ASC Structures,

structural Dynamics, and Materials Conference, St. Louis, MO, 1999. Schur, Willi, W., Pneumatic Envelopes: Poisson's Effect and Tendon Stiffness Drive Design Decisions, 41st

AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Atlanta, GA, 2000a. Schur, Will, W., The Design Process for a Pumpkin Balloon: Structural Synthesis, Structural Analysis, and Analytical

Assessment of Some Critical Design Issues, COSPAIL Warsaw, Poland, 2000b. Smalley, J., Flight Results from e-Balloons, Flight data folders for flights 528ST, 555PT, 572PT, and 63N, 1970 Smith, I. S., The Ultra Long Duration Balloon Project, COSPAIL Nagoya, Japan, 1998 Smith, I. S., Overview of the Ultra Long Duration Balloon Project, COSPAR, Warsaw, Poland, 2000 Smith, I. S. Stuchlik, D., The Ultra Long Duration Balloon Project Design-to-Requirements Document, 1998