A new polyethylene balloon film

Ad~. L ~ c c ~c~. Vol.5, No.], pp.17-20, 1985 Printed in Great Britain. All rights reserved.

0273-1177/85 $0.00 + .50 Copyright © COSPAR

A NEW POLYETHYLENE BALLOON FILM Patrick J. Cannon and Robert M. Enderson Raven Industries Inc. P.O. Box 1007, Sioux Falls, SD 57117, U.S.A.

ABSTRACT A f t e r o v e r t w e n t y y e a r s t h e r e i s a new f i l m i n t r o d u c e d i n t h e U n i t e d S t a t e s f o r f a b r i c a t i n g s c i e n t i f i c b a l l o o n s . The f i l m was d e v e l o p e d by Raven and i s d e s i g n a t e d A s t r o f l l m . The f i l m i s a r e s u l t o f a c r i t i c a l s e l e c t i o n o f b o t h t h e r e s i n and e x t r u d i n g p a r a m e t e r s . The f i l m ' s p h y s i c a l p r o p e r t i e s a n d r e s u l t s o f a t e s t f l i g h t a r e r e p o r t e d . Also reported a r e a p r o p o s e d d e s i g n m o d i f i c a t i o n and r e / l u r e a n a l y s i s . INTRODUCTION Raven Industries was formed in 1956. Scientific ballooning was the basis upon wnioh the company was founded. The seleotlon of Sioux Falls as the site f o r the company was, in fact, based prlmarily on fllght operational considerations. Raven has experienced continued growth and today has four operational divisions vlth total annual sales of approxlmately $40 milllon. Product areas range from electronics to clothlng. Balloon denign and manufacture are acoompllshed within the Applled Technology Division. BACKGROUND Qualified balloon film had been available from one or more sourQes and at an eoonomloally acceptable price until rather recent times. Although Raven had for years a mild interest i n d e v e l o p i n g i t s own f i l m , i t c o u l d n o t he e c o n o m i c a l l y ~ u s t i f i e d a t t h e t h e n - e x i s t i n g price of film. W i t h i n t h e l a s t two t o t h r e e y e a r s t h e s i t u a t i o n h a s changed d r a m a t i c a l l y . The p r i c e o f f i l m h a s e s c a l a t e d t o a l e v e l w h i c h i s u n a c c e p t a b l e . T h i s , among o t h e r f a c t o r s , p r o v i d e d t h e m o t i v a t i o n f o r a c o n c e r t e d p r o s r a m t o d e v e l o p our own f i l m i f Raven was t o r e m a i n i n t h e s c i e n t i f i c b a l l o o n m a r k e t . A c o m p r e h e n s i v e program was i n i t i a t e d w i t h f u l l s u p p o r t o f c o r p o r a t e manaEement. The p r o ~ a m h a s i n v o l v e d b a l l o o n d e s i g n a n a l y s i s , e v a l u a t i o n o f m a n u ~ a o t u r i n g t e c h n i q u e s , t h e i n t r o d u c t i o n o f a l i n e a r low d e n s i t y f i l m and, f i n a l l y , t h e i n t r o d u c t i o n o f a c o n v e n t i o n a l low d e n s i t y f i l m , wnioh we c a l l A s t r o f l l m . The d e v e l o p m e n t o f A a t r o f i l m has p r e s e n t e d a o h a l l e n E e , t o say t h e l e a s t . The two o z - i t i u a l p a r a m e t e r s i n v o l v e d i n p r o d u c i n g a b a l l o o n - ~ a d e p o l y e t h y l e n e a r e t h e r e s i n and t h e e x t r u s i o n process. They are essentially of equal importance. Candidate resins fall into the low or fraotlonal melt index classification. They are highly vlecous and difflcult to extrude into thin film. The number of resins that fall into this classification is rather limited. The number of candidate resins is further reduced by those having an unacceptable gel count. Gels tend to cause holes in the film and, in fact, can make successful extrusion impossible. While a particular resin choloe was made for producing our first test lots of Astrofilm, there are, however, other c~ndidate resins. Over t h e c o u r s e o f s e v e r a l y e a r s , b e f o r e t h e c o m m i t m e n t t o d e v e l o p our own f i l m , Raven had c o n d u c t e d a l i t e r a t u r e s e a r c h and s t u d y on e x t r u d i n 8 f i l m . The c h a l l e n g e h a s b e e n p u t t i n g t h e o r y i n t o p r a c t i c e and d e v e l o p i n g t h e n e c e s s a r y amount o f a r t . The e f f o r t h a s been t i m e consuming and c o s t l y . There a r e o v e r 20 v a r i a b l e s t h a t need t o be m o n i t o r e d and c o n t r o l l e d d u r i n g t h e e x t r u s i o n process. A d d i t i o n a l l y , a number o f f i x e d m a c h i n e p a r a m e t e r s a r e c r i t i c a l . As m e n t i o n e d , t h e p r o c e s s i s p a r t i a l l y an a r t . A s e t o f e s t a b l i s h e d p a r a m e t e r s t h a t works t o d a y m i g h t n o t work tomorrow. I t i s n o t a l w a y s o b v i o u s what t h e r e q u i r e d a d j u s t m e n t i s . The properties of a film can be altered within limits by changlng extruding parameters. If you change one physloal property of the film, another property will usually be affected and will not neaessarily be improved. In the final analysis, the resulting film is an acceptable b a l a n c e o f t h e c r i t i c a l p h y s i c a l p r o p e r t i e s . PROGRAM RESULTS AND CONCLUSIONS

Our success with small balloons fabricated from linear low density polyethylene for llght payloads has been v~ry gratifying. Some of the flight results will be reported in the paper to be given by Dr. Hofmann. 17

8

P.J. Cannon and R.M. Enderson

Astrofilm Formulation C was s e l e c t e d by Raven and NSBF/NASA as the initial candidate film for qualification. This film has been evaluated by NSBF/NASA and at Raven. The basic physical properties of the film are shown on Table 1. TABLEt

1,

Astrofilm Average Physical Properties

Tensile Properties at 2R°C

'"

Stress . Dsi

Direction Machine Transverse 2.

S.D.(nsi)

4017 4913

261 406

~

Strain Yield Stress S.D.(g} D s i S.D.(osi~

491 470

40 33

1537 1551

58 29

Tensile Prooertiea at -70°C Direction

csi

Machine Transverse

Stress .. S.D.(nsi~

7541

9281

522 725

Strain

| ,. S.C.{S)

Yield Stress ..... usi S.D.(osl)

263

107

6584

522

382

67

6946

522

Tensile Proper,lea as a ~unation of Strain Rate at -70°C Rate of Strain (S/mln~ 9910 9910 991 991

q,

Direction

(hal)

(S)

Machine Transverse Machine Transverse

7672 8020 7541 9281 7918 8194

363 328 263 382 303 263

99.1

Mach~ns

99.1

Transverse

Cold Brittle Transition Temcsmature TransitlRn Tmperature

Thickness (mils) 0.7 0.8 1.0

-85 -85 -80

A test balloon with a volume of 6.l-roof was constructed. The balloon was fabrloated from 0.7-all material with an 0.8-m~l cap and a 1.0-mll cap. The balloon, which was Flown with

a payload o f 4600 lbs.

f a i l e d on ascent at an a l t i t u d e o f a p p r o x i m a t e l y 60,000 f e e t .

The f a i l u r e o f t h e t e s t b a l l o o n w a s f o l l o w e d by a n i n t e n s i v e a n a l y s i s o f t h e f a i l u r e a n d a n increased effort in an existing program of balloon design analysis. In the program of balloon design analysis, Raven had been concentrating on t h e f u n c t i o n o f t h e c a p s . Caps historically have been l o c a t e d i n s i d e the s h e l l . I t had b e e n e s t a b l i s h e d some y e a r s ago t h a t when t h e h e l i u m l e v e l r e a c h e d t h e b a s e o f t h e c a p s t h e h e l i u m w o u l d f l o w b e t w e e n t h e c a p s and t h e s h e l l . The t o t a l p r e s s u r e b e a d i s t h e n a ~ o s s o n l y t h e s h e l l . In our analys i s a " s t r e s s i n d e x " on t h e s h e l l m a t e r i a l was c o m p u t e d w i t h r e s p e c t t o t h e a l t i t u d e at w h i c h t h e h e l i u m l e v e l w o u l d be e x p e c t e d t o r e a c h t h e b a s e o f t h e c a p s . The r e l a t i o n s h i p used to compute the required total top thickness in a balloon, as specified by t h e NSBF design specification, was u s e d i n c o m p u t i n g t h e s t r e s s i n d e x . T h i s r e l a t i o n s h i p i s shown as follows:

t =

/~I ~ 2/3

.i

SI

(I)

~1O/26 where t = total film thioknass in inches GI = gross inflation in pounds SI = stress index The relationship was rewritten to consider the llft of helium at any altitude and is as follows: sz =

c2~

b 1/3(~\2/3

t

where b = l i f t

V2o)

of helium at altitude

of interest

This relationship is, admittedly, somewhat simplistic. Experience has shown, though, t h a t i t h a s b e e n e f f e c t i v e i n d e t e r m i n i n g t h e r e q u i r e d t o p t h i c k n e s s , when a b a l l o o n i s a t i t s lowest stage of relative volume. Raven believes it also gives a useful comparison when d e t e r m i n i n g a s t r e s s i ~ d e x a t any a l t i t u d e on a s c e n t . F i g u r e 1 i s a graph o f v a r i o u s gauges o f f i l m w i t h r e s p e c t to ~ o s s i n f l a t i o n and a l t i t u d e and a t a s t r e s s i n d e x o f 1800. T h i s s t r e s s i n d e x w a s c h o s e n b e c a u s e m o s t f a i l u r e s a n a l y z e d h a d a f i g u r e above t h i s v a l u e .

19

A New Polyethylene Balloon Film

//

2624 "i 22-"

/

20-

j

'

14

0

I0 8

v

J

J

6 4

r

p_..-

50 a Fig.

~

1.

0.5 + Balloon stress

70

0.6 analysis.

o

90

THOUSANDS)

AL.TITUDE(~t)

0.8 x 1.0 0.7 " V a r y i n g t h i c k n e s s e s a t a s t r e s s i n d e x o f 1800.

It is obvious from the graph that, as the altitude increases, any g i v e n f i l m t h i c k n e s s could support an increasing level of gross inflation. D e s p i t e t h i s , t h e s t r e s s on t h e s h e l l m a t e r i a l i n c r e a s e s d r a m a t i c a l l y a s i n t e r n a l c a p s become i n e f f e c t i v e . T h i s i s shown o n F i g u r e 2 w h i c h i s a g r a p h o f t h e s t r e s s i n d e x on R a v e n ' s t e s t b a l l o o n . An o b v i o u s s o l u t i o n t o p r e v e n t t h e s t r e s s s p i k e s on t h e s h e l l i s t o p u t t h e c a p s on t h e o u t s i d e o f t h e balloon.

2.1 ?. 1.9 1.8 1.7 1.6 1.5 x U}¢O

u) "~

uJO

m

~

ACTUAl

!THEORETICAL

~

14 3 1.2 I,I 0.9 0.8 0.7 0,6 0.5 0.4 0.5 0.2

OUTSIDE CAP 0

20

40

60

80

I00

ALTITUDE (ft) FAg. 2.

Stress index analysis.

A n a l y s i s o f Raven 6 . 1 - m e t t e s t

balloon.

T h i s same cap a n a l y s i s t h e o r y was c o n s i d e r e d i n t h e f a i l u r e analysis of Raven's test balloon. The a n a l y s i s w a s c o m p o u n d e d , t h o u g h , by t h e f a c t t h a t f a i l u r e o o o u r r e d a t a n a l t i t u d e b e l o w t h a t a t w h i c h t h e c a p s would t h e o r e t i c a l l y become i n e f f e c t i v e . Utilizing p h o t o ~ P a p h i o c o v e r a g e o f t h e f l i g h t , known p h y s i c a l p a r a m e t e r s o f t h e b a l l o o n , and I n c l u d i n g e v e n t h e sun a n k l e a t t h e t i m e o f t h e f a i l u P e , a p r o f i l e was c o n s t r u c t e d o f t h e b a l l o o n at falluPe. This constructed profile very strongly supports the position that the helium l e v e l was below t h e b a s e o f t h e cape a t t h e t i m e o f f a l l u m e . The computed s t r e s s i n d e x on t h e s h e l l a t t h e f a i l u r e a l t i t u d e was 2000. At t h i s s t r e s s l e v e l f a i l u r e can be e x p e c t e d . As s h o w n £n F i g u r e 2, £ f t h e c a p s h a d a c t u a l l y become ineffective expectedv the stress index would have been 8lightly a b o v e 1600.

at ?0,000 feet as A l t h o u g h t h i s was

0

P,J. CaE~non and R.M. Enderson

considered an acceptable stress index, if the caps had been external, the stress index to float altitude would have been as indicated by the lower curve on Figure 2. The dlscrepanoy in altitude between that where the caps were expected to become ineffective and the altitude they actually d i d i s a c c o u n t e d f o r i n p a r t by t h e s h a p e o f t h e b a l l o o n . Theoretically, one would expect a nearly circular horizontal cross-section of the gas bubble. The c r o s s - s e c t i o n , as determined from up-camera coverage, was basically rectangular. Raven proposed an externally capped balloon to NSBF in 1979. The balloon, with a volume of 6.0-mcf, was fabricated and f l o w n in 1980. The fllght was successful. In retrospect, Raven should have pursued this design modification more diligently at the time. Our cap analysls, we belleve, clearly demonstrates that caps should be external. This cap analysis does not explain all ascent failures nor, in fact, a lot of successes. Numerous successful flights have been analyzed in which the stress index exceeded 1800 by a s i g n i f i c a n t deEree. N o n e t h e l e s s , Raven c o n t e n d s t h a t e x t e r n a l c a p s p r o v i d e t h e c o n f l E u r a t i o n f o r a c h i e v i n g an e f f i c i e n t d e s i g n w h i l e m a i n t a i n i n g c o n s e r v a t i v e s t r e s s l e v e l s . In conclusion, Raven remains committed to scientific ballooning. We h a v e d e v e l o p e d a considerable expertise in the art of extruding film. A s t r o f i l m C a p p e a r s t o be a s u i t a b l e b a l l o o n f i l m , a n d we e x p e c t t h a t i t w i l l b e a p p r o v e d . At t h e s a m e t i m e , we h a v e o t h e r formulations under evaluation which are equally promisin~ We e x p e c t t o make a p o s i t i v e r e p o r t on o u r f i l m a t t h e n e x t C o s p a r m e e t i n ~