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A challenge to the highest balloon altitude
Available online at www.sciencedirect.com
Advances in Space Research 49 (2012) 613–620 www.elsevier.com/locate/asr
A challenge to the highest balloon altitude Y. Saito a,⇑, D. Akita a,1, H. Fuke a, I. Iijima a, N. Izutsu a, Y. Kato a, J. Kawada a,2, Y. Matsuzaka a, E. Mizuta a, M. Namiki a, N. Nonaka a,3, S. Ohta a, T. Sato a, M. Seo a, A. Takada a,4, K. Tamura a, M. Toriumi a, T. Yamagami a,3, K. Yamada a, T. Yoshida a, K. Matsushima b, S. Tanaka b a
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo, Sagamihara, Kanagawa 252-5210, Japan b Fujikura Parachute Company, 115-25 Udagasaku, Funehiki, Tamura, Fukushima 963-4312, Japan Received 26 July 2011; received in revised form 25 October 2011; accepted 9 November 2011 Available online 25 November 2011
Abstract Development of a balloon to fly at higher altitudes is one of the most attractive challenges for scientific balloon technologies. After reaching the highest balloon altitude of 53.0 km using the 3.4 lm film in 2002, a thinner balloon film with a thickness of 2.8 lm was developed. A 5000 m3 balloon made with this film was launched successfully in 2004. However, three 60,000 m3 balloons with the same film launched in 2005, 2006, and 2007, failed during ascent. The mechanical properties of the 2.8 lm film were investigated intensively to look for degradation of the ultimate strength and its elongation as compared to the other thicker balloon films. The requirement of the balloon film was also studied using an empirical and a physical model assuming an axis-symmetrical balloon shape and the static pressure. It was found that the film was strong enough. A stress due to the dynamic pressure by the wind shear is considered as the possible reason for the unsuccessful flights. A 80,000 m3 balloon with cap films covering 9 m from the balloon top will be launch in 2011 to test the appropriateness of this reinforcement. Ó 2011 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Scientific balloons; High altitude balloons; Polyethylene film
1. Introduction One of the most fundamental challenges for balloon technology is to fly balloons higher. The ISAS balloon group has been engaged in the development of the high altitude balloon since 1991 by decreasing the weight of the balloon and on-board housekeeping equipment (Yama⇑ Corresponding author.
E-mail address: [email protected] (Y. Saito). Current address: Dept. of International Development Engineering, Tokyo Institute of Technology, Graduate School of Science and Engineering, Japan. 2 Current address: Dept. of High Energy Physics, University of Bern, Switzerland. 3 Current address: Mino Industry Co. Ltd., Japan. 4 Current address: Research Institute for Sustainable Humanosphere, Kyoto University, Japan. 1
gami et al., 1998; Matsuzaka et al., 2000). In 1997, a new polyethylene film with a thickness of 3.4 lm formed by using a metallocene as a catalyst was developed (Saito et al., 2002). In 2002, the world highest altitude record of 53.0 km with a 60,000 m3 balloon using the 3.4 lm film was established (Yamagami et al., 2004) . Then, the development of a thinner film was started again. In 2003, a 2.8 lm film was developed (Saito et al., 2006). In 2004, a test flight of a 5000 m3 balloon using the 2.8 lm film was carried out successfully. After that, three 60,000 m3 balloons using the 2.8 lm film were launched, however, they were not successful (Yoshida et al., 2008). 2. Unsuccessful launching of 60,000 m3 balloons The first balloon was launched on May 25, 2005. After reaching the altitude of 3.8 km, the ascending speed
0273-1177/$36.00 Ó 2011 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2011.11.013
Y. Saito et al. / Advances in Space Research 49 (2012) 613–620
decreased. All ballast was dropped, however, it descended after reaching the altitude of 4.8 km. A manufacturing problem is considered to be the cause of the failure. After this, the shapes of the patched films for fish-eye defects were changed from the rectangular to ellipses so that the edges of the patched films may not damage the balloon films. The second balloon was launched on September 13, 2006. This balloon ascended without problem up to the altitude of 16 km, however, it then suddenly descended. At that time, a handling problem was considered to be its cause again. The third balloon was carefully manufactured and launched on May 18, 2007. This time the balloon came down just after the launch. The highest altitude was 0.5 km. Then, at last, the existence of the another fundamental cause besides a manufacturing cause was recognized. Fortunately, this balloon was recovered from the ground without serious damage. Candidates for the break point were searched in the top portion of the balloon and a tear point with a length of 50 cm, at a 20° inclination from the machine direction of the film, and located 2.2 m from the top of the balloon around the center of the panel width was found. It was evident that the tore point was different from the seal lines. 3. Investigation of mechanical properties 3.1. Possible reasons and their studies The following possible reasons for the unsuccessful flights were speculated from the film mechanical properties point of view: 1. Whether the mechanical properties are measured without bias.The ultimate strength and its elongation strongly depends on how well the test pieces are prepared. Data with low values tend to not be used, since these test pieces may not have been prepared well. 2. Whether the film is well uniform. The test area of a few 10 cm2 is usually used for the validation. On the other hand, film area of 8000 m2 is used for the 60,000 m3 balloon. The test is not sufficient to check the uniformity of the film. 3. Whether the film is strong against the bi-axial elongation. The one dimensional tensile test is usually performed, however, the film is elongated bi-axially at the time of inflation. The one dimensional tensile test may not reflect the strength at the time of inflation. 4. Whether the film is strong against the continuous stress. It is known that the polyethylene films show creep properties, which causes weaker strength against continuous force. However, the film is evaluated only by the strength of the elongation speed at 100 mm/min. 5. Whether the film has the high brittleness temperature.To certify that the brittleness temperature is below 80 °C, the ultimate strengths and the elongations with temperatures of 80 °C and 40 °C are used and compared. The direct measurements of the brittleness temperature are better.
To answer these questions, following tests were performed, respectively. 1. The systematic tensile tests were performed by a research center. 2. The burst tests of cylinder balloons with large volumes were performed. 3. The burst pressures of cylinder balloons with the other balloon films with different thickness were measured and compared. 4. The 3-h holding pressure of cylinder balloons using various balloon films with different thickness were measured and compared. 5. The viscoelasticity tests were performed by a research center. 3.2. Systematic tensile tests The systematic, non-biased tensile tests were carried out by a research center, Mitsui Chemical Analysis and Consulting Service Inc. The test was based on the JIS K7113 and dumbbell shaped pieces (JIS Z1702) were used. The elongation speed was set to 100 mm/min. The strain–stress curves of the machine direction (MD) and the transverse direction (TD) were obtained at the temperatures of 23 °C and 40 °C. From the curves, the yield strengths and their elongations, and the ultimate strengths and their elongations were obtained. The yield point is determined as a point where the measured stain corresponds to 75% of the strain obtained by the extrapolation of two points, one of which elongation is 0.2 and 0.5 times that of the yield point, as illustrated in Fig. 1. Table 1 summarizes the results of the tensile tests. Figs. 2–5 show the yield strengths and their elongations at temperatures of 23 °C and 40 °C as functions of the thickness of the balloon film. No degradation is found for the thin films. Figs. 6–9 show the ultimate strengths and their elongations as functions of the thickness of the balloon film at
Ys/0.75
Ys
Strength
614
0.2Ye
0.5Ye
Ye
Elongation
Fig. 1. Definition of the yield point. The yield point (Ye, Ys) is determined as a point where the measured stain corresponds to 75% of the strain obtained by the extrapolation of two points, one of which elongation is 0.2 and 0.5 times that of the yield point.
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Table 1 Summary of the tensile test. Thickness (lm)
23 °C 2.8 3.4 6.0 10 20 40 °C 2.8 3.4 6.0 10 20
Yield point
Ultimate point
MD
TD
MD
TD
Strength (MPa)
Elongation (%)
Strength (MPa)
Elongation (%)
Strength (MPa)
Elongation (%)
Strength (MPa)
Elongation (%)
9.9 ± 1.0 8.6 ± 0.4 8.6 ± 0.4 8.9 ± 0.2 9.22 ± 0.04
5.6 ± 0.3 6.1 ± 0.2 6.3 ± 0.3 6.7 ± 0.2 8.03 ± 0.05
10.5 ± 0.1 7.2 ± 0.8 7.4 ± 1.3 9.2 ± 0.6 9.0 ± 0.1
7.0 ± 1.2 4.7 ± 0.5 6.6 ± 2.2 7.0 ± 0.3 7.2 ± 0.2
35 ± 7 44 ± 6 41 ± 6 59 ± 9 63 ± 3
340 ± 20 300 ± 20 410 ± 20 560 ± 40 663 ± 9
28 ± 7 31 ± 4 37 ± 6 56 ± 6 59 ± 3
390 ± 100 500 ± 50 490 ± 50 640 ± 20 752 ± 9
26.9 ± 3.0 27.3 ± 0.8 29.3 ± 1.4 24.8 ± 0.9 24.1 ± 0.7
3.4 ± 0.5 2.3 ± 0.1 3.0 ± 0.1 2.8 ± 0.4 2.53 ± 0.05
24.7 ± 3.4 29.1 ± 0.1 24.5 ± 0.4 26.1 ± 0.8 24.5 ± 0.5
2.7 ± 0.4 3.2 ± 0.5 2.4 ± 0.2 2.6 ± 0.2 2.5 ± 0.4
51 ± 6 57 ± 8 73 ± 11 72 ± 8 87 ± 7
250 ± 20 200 ± 20 330 ± 50 440 ± 10 550 ± 20
36 ± 5 32 ± 1 62 ± 0.5 79 ± 13 88 ± 4
160 ± 60 160 ± 30 350 ± 40 500 ± 30 599 ± 5
Fig. 2. Comparison of the yield strengths in the room temperature (23 °C) of balloon films.
Fig. 4. Comparison of the yield elongations in the room temperature (23 °C) of balloon films.
Fig. 3. Comparison of the yield strengths at 40 °C of balloon films.
Fig. 5. Comparison of the yield elongations at 40 °C of balloon films.
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Fig. 6. Comparison of the ultimate strengths in the room temperature (23 °C) of balloon films.
Fig. 8. Comparison of the ultimate elongations in the room temperature (23 °C) of balloon films.
Fig. 7. Comparison of the ultimate strengths at 40 °C of balloon films.
Fig. 9. Comparison of the ultimate elongations at 40 °C of balloon films.
temperatures of 23 °C and 40 °C. Degradation for thin films can be found for the ultimate strengths and their elongations, however, those of the 2.8 lm film are also on the same trend. This indicates that the properties of the 2.8 lm film obtained by the tensile test are not especially degraded.
balloon and comparable to the inflated area on the ground. The area size is sufficient to check the rare defects which could be a problem for 60,000 m3 balloons. The average burst pressure was 51.8 Pa and the minimum was 48.7 Pa. Fig. 11 shows the distribution of the burst pressures. It shows the mean of 51.8 Pa and the standard deviation of 1.4 Pa, which corresponds to 2.7% of the mean value. This distribution showing that the minimum value is deviated by 6% from the average and strongly indicates that the film is very uniform.
3.3. Burst test of large size cylinder balloons The burst test of cylinder balloons with lengths of 10 m were performed using a setup as shown in Fig. 10. The cylindrical tubes used for these tests were made from the originally extruded film diameter and attached to the top and bottom fittings as shown. The tubes are inflated with air with a flow rate of 400 l/min and the burst pressures were measured. The number of the test balloons were 30. The total surface area of the balloons was 800 m2, which corresponds to the 10% of the surface area of the 60,000 m3
3.4. Comparison of the burst pressures of cylinder balloons The bust pressure of cylinder balloons using the other balloon films were measure to check if the value of the 2.8 lm film shows any degradation. This time, 5 balloons of each film with lengths of 3 m were tested. Since the radii
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Fig. 10. The setup for the burst tests of cylinder balloons. To test the large area size, balloons with lengths of 10 m were used. For the other tests, 3 m balloons were used.
films as a function of the thicknesses of films. This figure indicates that the effective strengths of the films do not depends on the thickness and the 2.8 lm film shows the similar strength to the other films. 3.5. Comparison of the 3-h holding pressure of cylinder balloons
Fig. 11. The distribution of the burst pressure, showing the standard deviation of 1.4 Pa, which corresponds to the 2.7% of the mean burst pressure. This indicates that the balloon film is well uniform.
Using the same setup and cylinder balloons with the same size, the pressure with which balloons can withstand more than 3 h were investigated to check the creep properties of films. The number of the test balloons were set to 5 again. Fig. 13 shows effective strengths of films as a function of the thicknesses of films. The effective strengths are again derived by dividing the withstandable pressures by the radii of balloons. This figure indicates that the effective strengths of the films are constant and the 2.8 lm film shows no degradation. 3.6. Viscoelasticity tests The viscoelasticity tests ranging from 23 °C to 130 °C were performed for balloon films by Mitsui Chemical Analysis and Consulting Service Inc. based on JIS K7244-1 and K7244-5. Under the periodic stress r with a frequency of x as shown in the equation, r ¼ r0 sinðxt þ dÞ
ð1Þ
the strain in a viscoelastic material can be represented as follows: ¼ 0 sinðxtÞ
Fig. 12. Effective strengths dividing the burst pressures by the radii of cylinder balloons as a function of the thicknesses of films.
of the original tubes are different, effective strengths dividing the burst pressures by the initial radii were used for the comparison. Fig. 12 shows effective strengths of
ð2Þ
Here, 0 and r0 are the initial stress and the initial strain, t is the time, d is the phase lag between the stress and the strain. The storage modulus of E0 representing the elastic portion of the stored energy and the loss modulus E00 representing its energy dissipated portion are defined as follows: r0 E0 ¼ cos d ð3Þ 0 r0 sin d ð4Þ E00 ¼ 0
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Fig. 13. Effective strengths dividing the pressure, with which cylinder balloons can withstand more than 3 h, by the radii of cylinder balloons as a function of the thicknesses of films.
Fig. 14 shows the storage and loss modulus. Among these curves there are no discontinuities. It indicates that the brittleness temperatures of these films including the 2.8 lm film are below 130 °C. 4. Investigation of the required strength 4.1. Current stress model In addition to the investigation from the mechanical property point of view, the requirements for the balloon film were also investigated. The current stress model is based on the arguments in Izutsu et al. (2006). It is to estimate the stress of rmax by the equation, minðrðsÞ; RmðsÞÞDp ð5Þ rmax ¼ tðsÞ max assuming an axis symmetric shape considering the weight distribution of films, tapes and fittings. Here, r is the radius of the cross section of the balloon against the symmetry axis, s is the distance from the bottom of the balloon measured along the gore, Rm is the meridian radius of the curvature, Dp is the differential pressure and t is the thickness of the balloon film. The balloon is designed to have the safety factor larger than 2.25 comparing with the ultimate stress at the room temperature for the inflation time and larger than 7 comparing with that at 40 °C for the level flight time. Though this is an empirical relation based on the flight data, there was no problem including the application to the thin film balloons except for the 60,000 m3 balloons with the 2.8 lm film. 4.2. New stress estimate Here, a new physical model (bulge model) is proposed as described in the Appendix A to estimate the maximum
Fig. 14. Young’s modulus of balloon films. Upper panel shows the storage modulus and the lower panel shows the loss modulus.
cross meridian tension. In this model, there is no elongation of the load tape, no meridian tension on the film is assumed and the cross meridian tension contains all of the differential pressure. In the real situation, load tapes are not used for the thin film balloons, but heat protection tapes made of weak polyethylene films are used. The thickness of the protection tape is 14 lm and its width is 25 mm for balloons launched in 2005 and 2006, and the thickness of 10 lm and the width of 15 mm for 2007. The heat protection tapes are elongated, and the part of the pressure is sustained by the meridian tension of the film and the cross meridian tension must be smaller than the predicted value from the bulge model. The meridian tension must be smaller than that of the cross meridian, since the tear line of the recovered 60,000 m3 balloon was almost along the meridian direction. Thus, the cross meridian tension derived from the bulge model should be considered as the upper limit. Fig. 15 shows the estimated tensions from the two models as functions of the length from the top of the balloon at the ground. The meridian tension estimated from
Y. Saito et al. / Advances in Space Research 49 (2012) 613–620
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of 30 Pa at the top of the balloon. If this ram pressure is sustained by two layers of films with thicknesses t of 2.8 lm and local curvatures R of 3 m, the films should withstand the tension T of 1 T ¼ RP =t 2 ¼ 0:5 3 60=ð2:8 106 2Þ ¼ 16 ½MPa
ð9Þ ð10Þ ð11Þ
which is beyond the yield strength of 10 MPa of a single layer of the 2.8 lm film. 5. Summary and future plan
Fig. 15. Tensions as functions of the length from the top of the balloon estimated from the two models. rDp and RmDp indicate the cross meridian, and the meridian tension from Izutsu et al. (2006). The bulge model indicates the cross meridian tension from the model in Appendix A.
the bulge model is well below the yield strength of the film. This physical model is not able to explain the cause of the unsuccessful flights of the 60,000 m3 balloons made of 2.8 lm film also. This indicates that the assumptions of the models using the axis symmetry shape and the static pressure environment are not suitable to estimate the realistic environment of the balloons. 4.3. The wind shear It is also known that the balloon shape during the ascension is not the axis symmetric shape due to the wind shear. Since the free lifts of thin film balloons are much smaller than those of normal film balloons, the dynamic pressures to thin film balloons due to the wind shear could be larger than those of the static pressures and their balloon shapes could be distorted much more than the normal film balloons. It is suspected that it is possible for empirical relations being valid for normal film balloons and small thin film balloons to ignore the effect of the dynamic pressure and the distortion, since these effects are within the tolerances for these balloons. Although it is quite difficult to establish qualitative model including the wind shear, it is possible to present an example showing that the dynamic pressure requires larger stress beyond the yield strength of the 2.8 lm film as follows. If there is a wind shear v of 10 m/s, the ram pressure P becomes, 1 P ¼ qv2 2 ¼ 0:5 1:2 102 ¼ 60 ½Pa
ð6Þ ð7Þ ð8Þ
Here, a value q as the air density of 1.2 kg/m3 is used. The ram pressure of 60 Pa is twice as large as the static pressure
Following the three unsuccessful flights of the 60,000 m3 made of the 2.8 lm film, their causes were investigated intensively from both mechanical properties of the film and the required strength point of view. The mechanical properties are tested through the tensile tests, the uniformity tests, the bi-axial tensile tests, the 3-h withstandable pressure tests, and the viscoelasticity tests. Properties of the 2.8 lm film were compared with the other balloon films and found there were no apparent degradation except for the ultimate strengths and their elongations. The required strength is estimated using models assuming the axis symmetric shape and the static pressure and it was found that the 2.8 lm film has sufficient strength. Since more stress on the film is estimated under a dynamic pressure and a distorted shape due to the wind shear, the effect is considered as the possible cause of the unsuccessful flights, though no qualitative model has yet to be established. It is commonly recognized that the top part of the balloon is able to be protected by covering with additional layers (cap) of films, though the technique has not yet applied for thin film balloons. From the investigation above, the large thin film balloons also have to be covered and protected with the cap apparently, however, the suitable length is difficult to be determined, since the length of the cap film has been determined from the empirical stress index relation, which is not adequate for large thin film balloons. The other method is to determine the length experimentally from the balloon flight. A 80,000 m3 balloon with a cap is going to be launched in 2011. Its cap length is determined to be 9 m, which corresponds to the collar position where the balloon launcher may damage the film and will be effective to soften the maximum stress to the balloon film until the altitude of 27 km is reached. The altitude is well beyond the highest altitude of 16 km ever reached by the 60,000 m3 balloon made of single 2.8 lm film. The appropriateness of the length will be checked through the flight test. Acknowledgements We would like to thank the launching staffs at the Sanriku Balloon Center and at the Taiki Aerospace Research Field. Dr. K. Goto, Dr. E. Sato and Dr. R. Yokota in JAXA and
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T t ¼ 2 cos a T rv α
using Tt as the tension on the single load tape, and rv as the local curvature of the load tape being perpendicular to the Fig. 16 plain. Since a and / has the relation of p a ¼ 2/ ðA:4Þ 2 they also have the relation of
l
2φ
R
cos a ¼ sin 2/
Fig. 16. A model with an elongated film making a bulge among load tapes.
Dr. K. Ichimura in Ube-Maruzen Polyethylene gave us useful suggestion for the investigation of the film properties. We also would like to thank Mr. Henry Cathey Jr. at WFF/ NASA for help in polishing the English. Comments from anonymous referees are quite helpful to clarify this paper. This work was performed under the foundation from the Japan Aerospace Exploration Agency. Appendix A. A physical model of the film stress Assume a model that the balloon film is elongated among load tapes to make a bulge with a curvature radius of R as shown in Fig. 16. In this model, the following are also assumed: The load tapes have no elongation. Tapes sustain all tension along the meridian direction and there is no film tension along the direction. The only tension working on the film is to the cross meridian direction. From the geometrical condition, one can find R sin 2/ ¼ l
ðA:1Þ
Setting the differential pressure on the film to be Dp, and the tension to be T, there is a balance of the forces on the plain shown in Fig. 16 as, T ¼ DpR
ðA:3Þ
ðA:2Þ
As a balance of the force on the plain being perpendicular to the Fig. 16 plain, one can find
ðA:5Þ
Substituting this relation to Eq. (A.1), one finds the relation of cos a ¼
l R
ðA:6Þ
Then, substituting the relation to Eq. (A.3) and using Eq. (A.2), one finds the relation of T t ¼ 2l Dp rv
ðA:7Þ
This equation indicates that the differential pressure on the film among the load tapes is sustained by the tensions of the load tapes. The curvature of the film and the tension working on the film is independent of Eq. (A.7), and they are determined by the relation between the tension and the elongation of the film as follows: T ¼k
2R/ l l
ðA:8Þ
Here, k is used as the spring constant of the balloon film. References Izutsu, N. et al. Optimum design for improvement of performances in scientific, 2006. Matsuzaka, Y. et al. Thin-film balloon for high altitude observations. Adv. Space Res. 26 (9), 1365–1368, 2000. Saito, Y. et al. High altitude balloons with ultra thin polyethylene films. Adv. Space Res. 30 (5), 1159–1165, 2002. Saito, Y. et al. Development of a 2.8 lm film for scientific balloons. Adv. Space Res. 37 (11), 2026–2032, 2006. Yamagami, T. et al. Plastic balloons with thin polyethylene films for high altitude observations. Adv. Space Res. 21, 983–986, 1998. Yamagami, T. et al. Development of the highest altitude balloon. Adv. Space Res. 33 (10), 1653–1659, 2004. Yoshida, T. et al. Ballooning activities in Japan. Adv. Space Res. 42 (10), 1619–1623, 2008.
Advances in Space Research 49 (2012) 613–620 www.elsevier.com/locate/asr
A challenge to the highest balloon altitude Y. Saito a,⇑, D. Akita a,1, H. Fuke a, I. Iijima a, N. Izutsu a, Y. Kato a, J. Kawada a,2, Y. Matsuzaka a, E. Mizuta a, M. Namiki a, N. Nonaka a,3, S. Ohta a, T. Sato a, M. Seo a, A. Takada a,4, K. Tamura a, M. Toriumi a, T. Yamagami a,3, K. Yamada a, T. Yoshida a, K. Matsushima b, S. Tanaka b a
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo, Sagamihara, Kanagawa 252-5210, Japan b Fujikura Parachute Company, 115-25 Udagasaku, Funehiki, Tamura, Fukushima 963-4312, Japan Received 26 July 2011; received in revised form 25 October 2011; accepted 9 November 2011 Available online 25 November 2011
Abstract Development of a balloon to fly at higher altitudes is one of the most attractive challenges for scientific balloon technologies. After reaching the highest balloon altitude of 53.0 km using the 3.4 lm film in 2002, a thinner balloon film with a thickness of 2.8 lm was developed. A 5000 m3 balloon made with this film was launched successfully in 2004. However, three 60,000 m3 balloons with the same film launched in 2005, 2006, and 2007, failed during ascent. The mechanical properties of the 2.8 lm film were investigated intensively to look for degradation of the ultimate strength and its elongation as compared to the other thicker balloon films. The requirement of the balloon film was also studied using an empirical and a physical model assuming an axis-symmetrical balloon shape and the static pressure. It was found that the film was strong enough. A stress due to the dynamic pressure by the wind shear is considered as the possible reason for the unsuccessful flights. A 80,000 m3 balloon with cap films covering 9 m from the balloon top will be launch in 2011 to test the appropriateness of this reinforcement. Ó 2011 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Scientific balloons; High altitude balloons; Polyethylene film
1. Introduction One of the most fundamental challenges for balloon technology is to fly balloons higher. The ISAS balloon group has been engaged in the development of the high altitude balloon since 1991 by decreasing the weight of the balloon and on-board housekeeping equipment (Yama⇑ Corresponding author.
E-mail address: [email protected] (Y. Saito). Current address: Dept. of International Development Engineering, Tokyo Institute of Technology, Graduate School of Science and Engineering, Japan. 2 Current address: Dept. of High Energy Physics, University of Bern, Switzerland. 3 Current address: Mino Industry Co. Ltd., Japan. 4 Current address: Research Institute for Sustainable Humanosphere, Kyoto University, Japan. 1
gami et al., 1998; Matsuzaka et al., 2000). In 1997, a new polyethylene film with a thickness of 3.4 lm formed by using a metallocene as a catalyst was developed (Saito et al., 2002). In 2002, the world highest altitude record of 53.0 km with a 60,000 m3 balloon using the 3.4 lm film was established (Yamagami et al., 2004) . Then, the development of a thinner film was started again. In 2003, a 2.8 lm film was developed (Saito et al., 2006). In 2004, a test flight of a 5000 m3 balloon using the 2.8 lm film was carried out successfully. After that, three 60,000 m3 balloons using the 2.8 lm film were launched, however, they were not successful (Yoshida et al., 2008). 2. Unsuccessful launching of 60,000 m3 balloons The first balloon was launched on May 25, 2005. After reaching the altitude of 3.8 km, the ascending speed
0273-1177/$36.00 Ó 2011 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2011.11.013
Y. Saito et al. / Advances in Space Research 49 (2012) 613–620
decreased. All ballast was dropped, however, it descended after reaching the altitude of 4.8 km. A manufacturing problem is considered to be the cause of the failure. After this, the shapes of the patched films for fish-eye defects were changed from the rectangular to ellipses so that the edges of the patched films may not damage the balloon films. The second balloon was launched on September 13, 2006. This balloon ascended without problem up to the altitude of 16 km, however, it then suddenly descended. At that time, a handling problem was considered to be its cause again. The third balloon was carefully manufactured and launched on May 18, 2007. This time the balloon came down just after the launch. The highest altitude was 0.5 km. Then, at last, the existence of the another fundamental cause besides a manufacturing cause was recognized. Fortunately, this balloon was recovered from the ground without serious damage. Candidates for the break point were searched in the top portion of the balloon and a tear point with a length of 50 cm, at a 20° inclination from the machine direction of the film, and located 2.2 m from the top of the balloon around the center of the panel width was found. It was evident that the tore point was different from the seal lines. 3. Investigation of mechanical properties 3.1. Possible reasons and their studies The following possible reasons for the unsuccessful flights were speculated from the film mechanical properties point of view: 1. Whether the mechanical properties are measured without bias.The ultimate strength and its elongation strongly depends on how well the test pieces are prepared. Data with low values tend to not be used, since these test pieces may not have been prepared well. 2. Whether the film is well uniform. The test area of a few 10 cm2 is usually used for the validation. On the other hand, film area of 8000 m2 is used for the 60,000 m3 balloon. The test is not sufficient to check the uniformity of the film. 3. Whether the film is strong against the bi-axial elongation. The one dimensional tensile test is usually performed, however, the film is elongated bi-axially at the time of inflation. The one dimensional tensile test may not reflect the strength at the time of inflation. 4. Whether the film is strong against the continuous stress. It is known that the polyethylene films show creep properties, which causes weaker strength against continuous force. However, the film is evaluated only by the strength of the elongation speed at 100 mm/min. 5. Whether the film has the high brittleness temperature.To certify that the brittleness temperature is below 80 °C, the ultimate strengths and the elongations with temperatures of 80 °C and 40 °C are used and compared. The direct measurements of the brittleness temperature are better.
To answer these questions, following tests were performed, respectively. 1. The systematic tensile tests were performed by a research center. 2. The burst tests of cylinder balloons with large volumes were performed. 3. The burst pressures of cylinder balloons with the other balloon films with different thickness were measured and compared. 4. The 3-h holding pressure of cylinder balloons using various balloon films with different thickness were measured and compared. 5. The viscoelasticity tests were performed by a research center. 3.2. Systematic tensile tests The systematic, non-biased tensile tests were carried out by a research center, Mitsui Chemical Analysis and Consulting Service Inc. The test was based on the JIS K7113 and dumbbell shaped pieces (JIS Z1702) were used. The elongation speed was set to 100 mm/min. The strain–stress curves of the machine direction (MD) and the transverse direction (TD) were obtained at the temperatures of 23 °C and 40 °C. From the curves, the yield strengths and their elongations, and the ultimate strengths and their elongations were obtained. The yield point is determined as a point where the measured stain corresponds to 75% of the strain obtained by the extrapolation of two points, one of which elongation is 0.2 and 0.5 times that of the yield point, as illustrated in Fig. 1. Table 1 summarizes the results of the tensile tests. Figs. 2–5 show the yield strengths and their elongations at temperatures of 23 °C and 40 °C as functions of the thickness of the balloon film. No degradation is found for the thin films. Figs. 6–9 show the ultimate strengths and their elongations as functions of the thickness of the balloon film at
Ys/0.75
Ys
Strength
614
0.2Ye
0.5Ye
Ye
Elongation
Fig. 1. Definition of the yield point. The yield point (Ye, Ys) is determined as a point where the measured stain corresponds to 75% of the strain obtained by the extrapolation of two points, one of which elongation is 0.2 and 0.5 times that of the yield point.
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Table 1 Summary of the tensile test. Thickness (lm)
23 °C 2.8 3.4 6.0 10 20 40 °C 2.8 3.4 6.0 10 20
Yield point
Ultimate point
MD
TD
MD
TD
Strength (MPa)
Elongation (%)
Strength (MPa)
Elongation (%)
Strength (MPa)
Elongation (%)
Strength (MPa)
Elongation (%)
9.9 ± 1.0 8.6 ± 0.4 8.6 ± 0.4 8.9 ± 0.2 9.22 ± 0.04
5.6 ± 0.3 6.1 ± 0.2 6.3 ± 0.3 6.7 ± 0.2 8.03 ± 0.05
10.5 ± 0.1 7.2 ± 0.8 7.4 ± 1.3 9.2 ± 0.6 9.0 ± 0.1
7.0 ± 1.2 4.7 ± 0.5 6.6 ± 2.2 7.0 ± 0.3 7.2 ± 0.2
35 ± 7 44 ± 6 41 ± 6 59 ± 9 63 ± 3
340 ± 20 300 ± 20 410 ± 20 560 ± 40 663 ± 9
28 ± 7 31 ± 4 37 ± 6 56 ± 6 59 ± 3
390 ± 100 500 ± 50 490 ± 50 640 ± 20 752 ± 9
26.9 ± 3.0 27.3 ± 0.8 29.3 ± 1.4 24.8 ± 0.9 24.1 ± 0.7
3.4 ± 0.5 2.3 ± 0.1 3.0 ± 0.1 2.8 ± 0.4 2.53 ± 0.05
24.7 ± 3.4 29.1 ± 0.1 24.5 ± 0.4 26.1 ± 0.8 24.5 ± 0.5
2.7 ± 0.4 3.2 ± 0.5 2.4 ± 0.2 2.6 ± 0.2 2.5 ± 0.4
51 ± 6 57 ± 8 73 ± 11 72 ± 8 87 ± 7
250 ± 20 200 ± 20 330 ± 50 440 ± 10 550 ± 20
36 ± 5 32 ± 1 62 ± 0.5 79 ± 13 88 ± 4
160 ± 60 160 ± 30 350 ± 40 500 ± 30 599 ± 5
Fig. 2. Comparison of the yield strengths in the room temperature (23 °C) of balloon films.
Fig. 4. Comparison of the yield elongations in the room temperature (23 °C) of balloon films.
Fig. 3. Comparison of the yield strengths at 40 °C of balloon films.
Fig. 5. Comparison of the yield elongations at 40 °C of balloon films.
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Fig. 6. Comparison of the ultimate strengths in the room temperature (23 °C) of balloon films.
Fig. 8. Comparison of the ultimate elongations in the room temperature (23 °C) of balloon films.
Fig. 7. Comparison of the ultimate strengths at 40 °C of balloon films.
Fig. 9. Comparison of the ultimate elongations at 40 °C of balloon films.
temperatures of 23 °C and 40 °C. Degradation for thin films can be found for the ultimate strengths and their elongations, however, those of the 2.8 lm film are also on the same trend. This indicates that the properties of the 2.8 lm film obtained by the tensile test are not especially degraded.
balloon and comparable to the inflated area on the ground. The area size is sufficient to check the rare defects which could be a problem for 60,000 m3 balloons. The average burst pressure was 51.8 Pa and the minimum was 48.7 Pa. Fig. 11 shows the distribution of the burst pressures. It shows the mean of 51.8 Pa and the standard deviation of 1.4 Pa, which corresponds to 2.7% of the mean value. This distribution showing that the minimum value is deviated by 6% from the average and strongly indicates that the film is very uniform.
3.3. Burst test of large size cylinder balloons The burst test of cylinder balloons with lengths of 10 m were performed using a setup as shown in Fig. 10. The cylindrical tubes used for these tests were made from the originally extruded film diameter and attached to the top and bottom fittings as shown. The tubes are inflated with air with a flow rate of 400 l/min and the burst pressures were measured. The number of the test balloons were 30. The total surface area of the balloons was 800 m2, which corresponds to the 10% of the surface area of the 60,000 m3
3.4. Comparison of the burst pressures of cylinder balloons The bust pressure of cylinder balloons using the other balloon films were measure to check if the value of the 2.8 lm film shows any degradation. This time, 5 balloons of each film with lengths of 3 m were tested. Since the radii
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Fig. 10. The setup for the burst tests of cylinder balloons. To test the large area size, balloons with lengths of 10 m were used. For the other tests, 3 m balloons were used.
films as a function of the thicknesses of films. This figure indicates that the effective strengths of the films do not depends on the thickness and the 2.8 lm film shows the similar strength to the other films. 3.5. Comparison of the 3-h holding pressure of cylinder balloons
Fig. 11. The distribution of the burst pressure, showing the standard deviation of 1.4 Pa, which corresponds to the 2.7% of the mean burst pressure. This indicates that the balloon film is well uniform.
Using the same setup and cylinder balloons with the same size, the pressure with which balloons can withstand more than 3 h were investigated to check the creep properties of films. The number of the test balloons were set to 5 again. Fig. 13 shows effective strengths of films as a function of the thicknesses of films. The effective strengths are again derived by dividing the withstandable pressures by the radii of balloons. This figure indicates that the effective strengths of the films are constant and the 2.8 lm film shows no degradation. 3.6. Viscoelasticity tests The viscoelasticity tests ranging from 23 °C to 130 °C were performed for balloon films by Mitsui Chemical Analysis and Consulting Service Inc. based on JIS K7244-1 and K7244-5. Under the periodic stress r with a frequency of x as shown in the equation, r ¼ r0 sinðxt þ dÞ
ð1Þ
the strain in a viscoelastic material can be represented as follows: ¼ 0 sinðxtÞ
Fig. 12. Effective strengths dividing the burst pressures by the radii of cylinder balloons as a function of the thicknesses of films.
of the original tubes are different, effective strengths dividing the burst pressures by the initial radii were used for the comparison. Fig. 12 shows effective strengths of
ð2Þ
Here, 0 and r0 are the initial stress and the initial strain, t is the time, d is the phase lag between the stress and the strain. The storage modulus of E0 representing the elastic portion of the stored energy and the loss modulus E00 representing its energy dissipated portion are defined as follows: r0 E0 ¼ cos d ð3Þ 0 r0 sin d ð4Þ E00 ¼ 0
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Fig. 13. Effective strengths dividing the pressure, with which cylinder balloons can withstand more than 3 h, by the radii of cylinder balloons as a function of the thicknesses of films.
Fig. 14 shows the storage and loss modulus. Among these curves there are no discontinuities. It indicates that the brittleness temperatures of these films including the 2.8 lm film are below 130 °C. 4. Investigation of the required strength 4.1. Current stress model In addition to the investigation from the mechanical property point of view, the requirements for the balloon film were also investigated. The current stress model is based on the arguments in Izutsu et al. (2006). It is to estimate the stress of rmax by the equation, minðrðsÞ; RmðsÞÞDp ð5Þ rmax ¼ tðsÞ max assuming an axis symmetric shape considering the weight distribution of films, tapes and fittings. Here, r is the radius of the cross section of the balloon against the symmetry axis, s is the distance from the bottom of the balloon measured along the gore, Rm is the meridian radius of the curvature, Dp is the differential pressure and t is the thickness of the balloon film. The balloon is designed to have the safety factor larger than 2.25 comparing with the ultimate stress at the room temperature for the inflation time and larger than 7 comparing with that at 40 °C for the level flight time. Though this is an empirical relation based on the flight data, there was no problem including the application to the thin film balloons except for the 60,000 m3 balloons with the 2.8 lm film. 4.2. New stress estimate Here, a new physical model (bulge model) is proposed as described in the Appendix A to estimate the maximum
Fig. 14. Young’s modulus of balloon films. Upper panel shows the storage modulus and the lower panel shows the loss modulus.
cross meridian tension. In this model, there is no elongation of the load tape, no meridian tension on the film is assumed and the cross meridian tension contains all of the differential pressure. In the real situation, load tapes are not used for the thin film balloons, but heat protection tapes made of weak polyethylene films are used. The thickness of the protection tape is 14 lm and its width is 25 mm for balloons launched in 2005 and 2006, and the thickness of 10 lm and the width of 15 mm for 2007. The heat protection tapes are elongated, and the part of the pressure is sustained by the meridian tension of the film and the cross meridian tension must be smaller than the predicted value from the bulge model. The meridian tension must be smaller than that of the cross meridian, since the tear line of the recovered 60,000 m3 balloon was almost along the meridian direction. Thus, the cross meridian tension derived from the bulge model should be considered as the upper limit. Fig. 15 shows the estimated tensions from the two models as functions of the length from the top of the balloon at the ground. The meridian tension estimated from
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of 30 Pa at the top of the balloon. If this ram pressure is sustained by two layers of films with thicknesses t of 2.8 lm and local curvatures R of 3 m, the films should withstand the tension T of 1 T ¼ RP =t 2 ¼ 0:5 3 60=ð2:8 106 2Þ ¼ 16 ½MPa
ð9Þ ð10Þ ð11Þ
which is beyond the yield strength of 10 MPa of a single layer of the 2.8 lm film. 5. Summary and future plan
Fig. 15. Tensions as functions of the length from the top of the balloon estimated from the two models. rDp and RmDp indicate the cross meridian, and the meridian tension from Izutsu et al. (2006). The bulge model indicates the cross meridian tension from the model in Appendix A.
the bulge model is well below the yield strength of the film. This physical model is not able to explain the cause of the unsuccessful flights of the 60,000 m3 balloons made of 2.8 lm film also. This indicates that the assumptions of the models using the axis symmetry shape and the static pressure environment are not suitable to estimate the realistic environment of the balloons. 4.3. The wind shear It is also known that the balloon shape during the ascension is not the axis symmetric shape due to the wind shear. Since the free lifts of thin film balloons are much smaller than those of normal film balloons, the dynamic pressures to thin film balloons due to the wind shear could be larger than those of the static pressures and their balloon shapes could be distorted much more than the normal film balloons. It is suspected that it is possible for empirical relations being valid for normal film balloons and small thin film balloons to ignore the effect of the dynamic pressure and the distortion, since these effects are within the tolerances for these balloons. Although it is quite difficult to establish qualitative model including the wind shear, it is possible to present an example showing that the dynamic pressure requires larger stress beyond the yield strength of the 2.8 lm film as follows. If there is a wind shear v of 10 m/s, the ram pressure P becomes, 1 P ¼ qv2 2 ¼ 0:5 1:2 102 ¼ 60 ½Pa
ð6Þ ð7Þ ð8Þ
Here, a value q as the air density of 1.2 kg/m3 is used. The ram pressure of 60 Pa is twice as large as the static pressure
Following the three unsuccessful flights of the 60,000 m3 made of the 2.8 lm film, their causes were investigated intensively from both mechanical properties of the film and the required strength point of view. The mechanical properties are tested through the tensile tests, the uniformity tests, the bi-axial tensile tests, the 3-h withstandable pressure tests, and the viscoelasticity tests. Properties of the 2.8 lm film were compared with the other balloon films and found there were no apparent degradation except for the ultimate strengths and their elongations. The required strength is estimated using models assuming the axis symmetric shape and the static pressure and it was found that the 2.8 lm film has sufficient strength. Since more stress on the film is estimated under a dynamic pressure and a distorted shape due to the wind shear, the effect is considered as the possible cause of the unsuccessful flights, though no qualitative model has yet to be established. It is commonly recognized that the top part of the balloon is able to be protected by covering with additional layers (cap) of films, though the technique has not yet applied for thin film balloons. From the investigation above, the large thin film balloons also have to be covered and protected with the cap apparently, however, the suitable length is difficult to be determined, since the length of the cap film has been determined from the empirical stress index relation, which is not adequate for large thin film balloons. The other method is to determine the length experimentally from the balloon flight. A 80,000 m3 balloon with a cap is going to be launched in 2011. Its cap length is determined to be 9 m, which corresponds to the collar position where the balloon launcher may damage the film and will be effective to soften the maximum stress to the balloon film until the altitude of 27 km is reached. The altitude is well beyond the highest altitude of 16 km ever reached by the 60,000 m3 balloon made of single 2.8 lm film. The appropriateness of the length will be checked through the flight test. Acknowledgements We would like to thank the launching staffs at the Sanriku Balloon Center and at the Taiki Aerospace Research Field. Dr. K. Goto, Dr. E. Sato and Dr. R. Yokota in JAXA and
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T t ¼ 2 cos a T rv α
using Tt as the tension on the single load tape, and rv as the local curvature of the load tape being perpendicular to the Fig. 16 plain. Since a and / has the relation of p a ¼ 2/ ðA:4Þ 2 they also have the relation of
l
2φ
R
cos a ¼ sin 2/
Fig. 16. A model with an elongated film making a bulge among load tapes.
Dr. K. Ichimura in Ube-Maruzen Polyethylene gave us useful suggestion for the investigation of the film properties. We also would like to thank Mr. Henry Cathey Jr. at WFF/ NASA for help in polishing the English. Comments from anonymous referees are quite helpful to clarify this paper. This work was performed under the foundation from the Japan Aerospace Exploration Agency. Appendix A. A physical model of the film stress Assume a model that the balloon film is elongated among load tapes to make a bulge with a curvature radius of R as shown in Fig. 16. In this model, the following are also assumed: The load tapes have no elongation. Tapes sustain all tension along the meridian direction and there is no film tension along the direction. The only tension working on the film is to the cross meridian direction. From the geometrical condition, one can find R sin 2/ ¼ l
ðA:1Þ
Setting the differential pressure on the film to be Dp, and the tension to be T, there is a balance of the forces on the plain shown in Fig. 16 as, T ¼ DpR
ðA:3Þ
ðA:2Þ
As a balance of the force on the plain being perpendicular to the Fig. 16 plain, one can find
ðA:5Þ
Substituting this relation to Eq. (A.1), one finds the relation of cos a ¼
l R
ðA:6Þ
Then, substituting the relation to Eq. (A.3) and using Eq. (A.2), one finds the relation of T t ¼ 2l Dp rv
ðA:7Þ
This equation indicates that the differential pressure on the film among the load tapes is sustained by the tensions of the load tapes. The curvature of the film and the tension working on the film is independent of Eq. (A.7), and they are determined by the relation between the tension and the elongation of the film as follows: T ¼k
2R/ l l
ðA:8Þ
Here, k is used as the spring constant of the balloon film. References Izutsu, N. et al. Optimum design for improvement of performances in scientific, 2006. Matsuzaka, Y. et al. Thin-film balloon for high altitude observations. Adv. Space Res. 26 (9), 1365–1368, 2000. Saito, Y. et al. High altitude balloons with ultra thin polyethylene films. Adv. Space Res. 30 (5), 1159–1165, 2002. Saito, Y. et al. Development of a 2.8 lm film for scientific balloons. Adv. Space Res. 37 (11), 2026–2032, 2006. Yamagami, T. et al. Plastic balloons with thin polyethylene films for high altitude observations. Adv. Space Res. 21, 983–986, 1998. Yamagami, T. et al. Development of the highest altitude balloon. Adv. Space Res. 33 (10), 1653–1659, 2004. Yoshida, T. et al. Ballooning activities in Japan. Adv. Space Res. 42 (10), 1619–1623, 2008.