NASA TECHNICAL MEMORANDUM

NASA T M X-64549

TMOSPHER I C ELECTR I C ITY CR ITER IA

& % GUIDELINES FOR USE IN SPACE VEHICLE DEVELOPMENT By Glenn E . Daniels Aero-Astrodynamics Laboratory

August 25, 1970

NASA

George C. Ma~shaZZ Space FZebt Center ManbaZZ Space Flight Center, AZubama

https://ntrs.nasa.gov/search.jsp?R=19710002321 2018-07-31T15:38:39+00:00Z

TECHNICAL REPORT STANDARD TITLE PAGE

4. Title and Subtitle

ATMOSPHERIC ELECTRICITY CRITERIA GUIDELINES FOR USE I N SPACE VEHICLE DEVELOPMENT

9. Performing Orgmization Name and Address

Aero-As trodynamics Laboratory

Marshall Space F l i g h t Center, Alabama 35812

Ja t iona l Aeronautics and Space Adminis t ra t ion

George C. Marshall Space F l i g h t Center

2. Spansoring Agency Name and Address

11. Contract or Grant No.

13. Typeof Report and Period co Vere

Technic a1 Me morandum

14. Sponsoring Agency Code

UNCLASSIFIED

I

5. Supplemenhary Notes

UNCLASSIFIED 21 $3.00 I

Work performed by Aerospace Environment Div is ion

6. Abstract

The a c c i d e n t a l t r i g g e r i n g of l i gh tn ing discharges by the Apollo 12 space veh ic l e s h o r t l y a f t e r launch i n i t i a t e d a d e t a i l e d s tudy of t hese discharges as they r e l a t e t o space veh ic l e missions. Aboah. thdae time, two new books were published on atmospheric e l e c t r i c i t y [1 ,2] . From t h e information r e s u l t i n g from the con- fe rences , t echn ica l meetings, d i scuss ions wi th the p ro fes s iona l s i n the f i e l d of atmospheric e l e c t r i c i t y , and pub l i ca t ions eva lua t ing the Apollo 1 2 d ischarge inc iden t [ 3 , 4 ] , Chapter I X , e n t i t l e d "Atmos- pheric E l e c t r i c i t y , " of NASA TM X-53872 [5] has been rev ised . Since the r e v i s i o n t o NASA TM X-53872 w i l l not be completed f o r publ ica t io i u n t i l l a t e s p r i n g 1971, information on atmospheric e l e c t r i c i t y is presented i n t h i s r e p o r t so t h a t it w i l l be a v a i l a b l e f o r use i n cur - r e n t space v e h i c l e design and ope ra t iona l s t u d i e s .

17. Key Words 18. Distribution Statement

Space v e h i c l e s , a i r c r a f t , s a f e t y , l i gh tn ing hazards , electrical networks E. D. Ge i s s l e r

Di rec tor , Aero-As trodynamics Lab. 19. Security Classif. (of this report) 10. Security Closrif. (of this pago) 21. No. of Pages 22. Prico

Figure 2A. Technical Report Standard Title Page. This page provides the data elements required by DoD Form DD-1473. H E W Form OE-6000 (ERIC). and similar forma.

TABLE OF CONTENTS

I . INTRODUCTION .............................................. 11 . THUNDERSTORM ELECTRIC1 TY ..................................

1 . P o t e n t i a l Gradient .................................... 2 . C h a r a c t e r i s t i c s of Lightning Discharges ............... 3 . Frequency of Occurrence of Thunderstorms 4 . Frequency of Lightning Strokes t o Ear th

.............. ............... 111 . STATIC ELECTRICITY ........................................ IV . E L E C T R m L BREAKDOWN OF THE ATMOSPHERE ....................

REFERENCES ................................................

Page

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7

10

1 2

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TECHNICAL MEMORANDUM X- 64549

ATMOSPHERIC ELECTRICITY CRITERIA GUIDELINES

FOR USE I N SPACE VEHICLE DEVELOPIG3NT Y

SUMMARY

.'

The a c c i d e n t a l t r i g g e r i n g of l i g h t n i n g d ischarges by the Apollo 1 2 space v e h i c l e s h o r t l y a f t e r launch i n i t i a t e d a d e t a i l e d s tudy of t h e s e discharges as they re la te t o space veh, ic le missions. About th i s , t i m e , two new books were published on atmospheric e l e c t r i c i t y [1 ,2 ] . From the information r e s u l t i n g from the conferences , t echn ica l meetings, d i scuss ions wi th the p ro fes s iona l s i n the f i e l d of atmospheric e lec- t r i c i t y , and publ ica t ions eva lua t ing t h e Apollo 1 2 d i scharge i n c i d e n t f3 ,4] , Chapter IX, e n t i t l e d "Atmospheric E l e c t r i c i t y , " of NASA TM X-53872 [5] has been rev ised . Since t h e r e v i s i o n t o NASA TM X-53872 w i l l n o t be completed f o r pub l i ca t ion u n t i l l a t e sp r ing 1971, information on atmos- pher ic e l e c t r i c i t y is presented i n t h i s r e p o r t s o t h a t i t w i l l be avail- a b l e f o r use i n c u r r e n t space v e h i c l e des ign and ope ra t iona l s t u d i e s .

I. INTRODUCTION

Atmospheric e l e c t r i c i t y must be considered i n the des ign , t r anspor t a - t i o n , and opera t ion of space veh ic l e s . The e f f e c t of t h e atmosphere as an in su laco r and conductor of high vo l t age e l e c t r i c i t y , a t var ious atmos- p h e r i c pressures , must also be considered. Space vehic les no t adequate ly pro tec ted can be damaged by (1) a d i r e c t l i g h t n i n g s t r o k e t o t h e v e h i c l e whi le on t h e ground o r a f t e r launch, (2) c u r r e n t induced i n t h e v e h i c l e from the electromagnet ic f i e l d generated i n a nearby object: s t r u c k by l i g h t n i n g , and (3 ) a l a r g e bui ldup of t h e atmospheric p o t e n t i a l g r a d i e n t near t he ground as a r e s u l t of charged clouds nearby. Also high vo l t age systems n o t proper ly designed could arc o r break down a t low atmospheric pressures .

The v e h i c l e can be pro tec ted by (1) ensuring t h a t a l l meca l l i c sec- t i o n s are connected e l e c t r i c a l l y (bonded) so t h a t the c u r r e n t flow from a l i g h t n i n g s t r o k e is conducted over t he s k i n wi thout any gaps where sparking would occur o r c u r r e n t would be c a r r i e d i n s i d e (MIL-B-5087B (ASC)), 15 October 1964, and la ter amendments [6] g i v e requirements f o r e l e c t r i c a l bonding); (2) p ro t ec t ing ob jec t s on t h e ground, such as bu i ld ings , by a system of l i gh tn ing rods and wires over t h e o u t s i d e t o c a r r y t h e l i gh tn ing s t r o k e t o t h e ground; (3) providing a cone of pro tec- t i o n (as shown i n r e fe rence 7) f o r t h e l i g h t n i n g p ro tec t ion p l an f o r

Saturn Launch Complex 39; ( 4 ) providing p ro tec t ion devices i n c r i t i c a l c i r c u i t s [8 ] ; (5) using systems which have no s i n g l e f a i l u r e mode; i . e . , t h e Sa turn V launch veh ic l e uses t r i p l e redundant c i r c u i t r y on the au to-abor t system, which r equ i r e s two ou t of t h r e e of t he s i g n a l s t o be c o r r e c t before a b o r t i s i n i t i a t e d [3 ] ; and ( 6 ) appropr i a t e s h i e l d - ing of u n i t s s e n s i t i v e t o e lectromagnet ic r a d i a t i o n .

I f l i gh tn ing should s t r i k e a space v e h i c l e ready f o r t e s t o r f l i g h t , o r a l a r g e m e t a l l i c o b j e c t nearby such as the t e s t s tand o r gan t ry , su f - f i c i e n t system checks should be made t o in su re t h a t a l l e l e c t r o n i c com- ponents and subsystems of t he v e h i c l e a r e func t iona l .

11. THUNDERSTORM ELECTRICITY

On a normal day wi thout c louds , t he p o t e n t i a l g r a d i e n t i n the atmos- phere near t he s u r f a c e of t he e a r t h is r e l a t i v e l y low (- 300 v/m), bu t when clouds bu i ld up, t h e p o t e n t i a l g r a d i e n t near t h e s u r f a c e of t h e e a r t h w i l l increase . I f t h e clouds become l a r g e enough t o have water d r o p l e t s of s u f f i c i e n t s i z e t o cause r a i n , t h e p o t e n t i a l g r a d i e n t between t h e cloud and ground may be s u f f i c i e n t t o r e s u l t i n a l i g h t n i n g d ischarge (- 1,000,000 v / m ) .

1. P o t e n t i a l Gradient

The e a r t h can be considered as a l a r g e c a p a c i t o r , t h e e a r t h ' s s u r f a c e being one p l a t e , t he ionosphere t h e o t h e r p l a t e , and t h e atmos- phere the d i e l e c t r i c . The e a r t h is nega t ive ly charged.

a. Fine-WeatherYc P o t e n t i a l Gradients

The fine-weather e l e c t r i c a l f i e l d i n t e n s i t y ( p o t e n t i a l g r a d i e n t ) measured near t h e ground is on t h e o rde r of 100 t o 300 v o l t s / meter and is p o s i t i v e ; i . e . , t h e e a r t h i s nega t ive ly charged and t h e atmosphere above the e a r t h is p o s i t i v e l y charged. The fine-weather va lue of 100 t o 300 vol t s /meter w i l l vary some i n t i m e a t a s p e c i f i c l o c a t i o n and w i l l a l s o be somewhat d i f f e r e n t a t var ious l o c a t i o n s , These v a r i a t i o n s i n f i n e weather w i l l be caused by v a r i a t i o n s i n wind speed and d i r e c t i o n , amount of p a r t i c u l a t e matter i n the atmosphere (dus t , s a l t par t ic les , e t c . ) , atmospheric humidity, and instrument l o c a t i o n and exposure [9] . The fine-weather p o t e n t i a l g r a d i e n t decreases w i t h a l t i - XI

tude, reaching a va lue near zero a t 10 k i lometers . This fine-weather

* The term " f a i r weather" is a l s o used w i t h t h e same meaning.

2

p o t e n t i a l on a 100-meter-high v e h i c l e could r e s u l t i n a 10,000 v o l t , o r g r e a t e r , p o t e n t i a l between t h e ground and top, i f the v e h i c l e is no t grounded.

b. P o t e n t i a l Gradients w i t h Clouds

When clouds develop, t h e p o t e n t i a l g rad ien t i nc reases . Because of t h e p o t e n t i a l g r a d i e n t , on days when s c a t t e r e d cumulus clouds occur , severe shocks can r e s u l t from t h e charge induced along a m e t a l c a b l e on c a p t i v e bal loons. S imi l a r ly induced charges on home t e l e v i s i o n antennas have exploded f i n e wire c o i l s i n t e l e v i s i o n sets. Such equiqi- ment damage can be prevented by i n s t a l l i n g l i gh tn ing arresters wi th a i r gaps small enough t o d ischarge the c u r r e n t before i t d ischarges w i t h i n the equipment.

e. P o t e n t i a l Gradients During Thunderstorms

I f t he cloud development reaches the cumulo-nimbus type of c loud, l i g h t n i n g d ischarges r e s u l t when t h e p o t e n t i a l g rad ien t a t some l o c a t i o n between the base and the ground reaches a va lue equal t o t h e c r i t i ca l va lue when e l e c t r i c a l breakdown of t he a i r occurs . Labora- t o r y d a t a i n d i c a t e t h i s va lue t o be as much as l o 6 vol t s /meter a t normal sea - l eve l atmospheric pressure . f a c e of t he e a r t h a r e much l e s s than l o 6 vo l t s /me te r during l i g h t n i n g d ischarges because of s e v e r a l e f f e c t s : bo th p o l a r i t i e s which tend t o n e u t r a l i z e values measured a t the su r face . ( 2 ) Each charge i n the atmosphere and i t s image w i t h i n t h e e a r t h comprise a n e lectr ical d ipo le , and t h e i n t e n s i t y of t he e l e c t r i c a l f i e l d decreases wi th t h e cube of t h e d i s t a n c e t o t h e d ipole . ( 3 ) The atmospheric e lec t r ic f i e l d measured over land a t the s u r f a c e is l imi ted by d ischarge c u r r e n t s a r i s i n g from grounded po in t s , such as g r a s s , t r e e s , and o the r s t r u c t u r e s , which i o n i z e t h e a i r around the po in t s , thus reducing t h e e l e c t r i c s t r e s s . For these reasons , t h e measured e l e c t r i c a l f i e l d a t the s u r f a c e never is more than about 15 x lo3 vol t s /meter . The p o t e n t i a l g r a d i e n t values ind ica t ed by measuring equipment a t the s u r f a c e w i l l show only the h igh values of proper s i g n when t h e charged cloud is d i r e c t l y overhead. As t h e d i s t a n c e of the p r o j e c t i o n of the charged c e n t e r of t h e cloud on t h e ground becomes g r e a t e r , t h e readings of t h e measuring equipment become lower, reaching zero a t some d i s t ance , and then changing t o t h e oppos i te s i g n a t g r e a t e r d i s t ances [1 ,9] .

E l e c t r i c a l f i e l d s measured a t the s u r -

(1) Most c louds have c e n t e r s of

d. Coronal Discharge

As t h e atmospheric p o t e n t i a l g r a d i e n t increases , t h e a i r surrounding exposed sharp poin ts is inc reas ing ly ionized. I f t he i o n i z a t i o n is s u f f i c i e n t , coronal d i scharges may occur. The induced charge from a nearby l i g h t n i n g s t r o k e may a i d such a d ischarge , which may be q u i t e s e v e r e when l i g h t n i n g storms or cumulus development are w i t h i n about 16 km (10 m i l e s ) of t h e launch pad.

3

2 , Charac te r i s t i c s of Lightninp Discharges

The l i gh tn ing d ischarge t o ground which appears t o the eye as a s i n g l e f l a s h , i s usua l ly made up of 3 o r 4 s t rokes . These s t r o k e s a r e preceded by a l eade r s t r o k e of l e s s e r i n t e n s i t y . t e r i s t i c s of var ious types of l i g h t n i n g discharges a r e given i n Table 1. (Also, s e e re ferences 4 and 10.)

st

A summary of t he charac-

.I

a . Lightning C h a r a c t e r i s t i c s f o r Design

Based on the l a t e s t information ( see Table l ) , t he follow- ing summary of l i gh tn ing c h a r a c t e r i s t i c s should be considered i n design:

(a) On the Launch Pad o r During Ground Transpor ta t ion

( a ) An average peak c u r r e n t of 20,000 amperes can be expected. The peak c u r r e n t flow is reached 6 microseconds a f t e r s t a r t - of s t r o k e , w i th a f a l l t o one-half the peak va lue i n 24 micro- seconds. A t o t a l f l a s h charge of 20 coulombs i s t ransmi t ted t o the e a r t h wi th 90 percent of t he c u r r e n t flow, a f t e r t he i n i t i a t i o n of t he f i r s t s t roke . Addit ional s t r o k e s w i l l have cu r ren t s a t l e s s than 1,000 amperes , and the peaks of t h e c u r r e n t w i l l be a t 40-millisecond i n t e r v a l s .

(b) The maximum peak c u r r e n t w i l l not be g r e a t e r than 100,000 amperes 98 percent of t h e time. This peak c u r r e n t flow is reached i n 10 microseconds a f t e r s t a r t of t he s t r o k e , and the c u r r e n t then f a l l s t o one-half t he peak value i n 20 microseconds. A t o t a l s t r o k e charge of 100 coulombs is t ransmit ted t o t h e ea r th , wi th 95 per- cen t of t he c u r r e n t f low, a f t e r t he i n i t i a t i o n of t h e f i r s t s t r o k e , a t l e s s than 5000 amperes.

( a ) I n f l i g h t Triggered Lightning

The space veh ic l e while i n f l i g h t should be capable of withstanding an e l e c t r i c a l d i scharge from t r igge red l i gh tn ing . The c h a r a c t e r i s t i c s of such a d ischarge is an average peak c u r r e n t of 20,000 amperes. The peak c u r r e n t flow is reached i n 6 microseconds a f t e r t he s ta r t of t h e s t r o k e , wi th a f a l l t o one-half t he peak value i n 24 micro- seconds. Af t e r t he c u r r e n t reaches 185 amperes, i t w i l l remain a t t h i s l e v e l f o r a t least 175 mi l l i seconds (17,500 microseconds) before f a l l i n g t o zero. There w i l l be only one s t r o k e i n the d ischarge , c a l l e d a long- cont inuing-cur ren t d i scharge ( see re ferences 3 , 4, 6 , 10 and 11). *

b. Surges from Lightning Discharge

I f an e l e c t r i c a l l i n e , antenna, o r b the r m e t a l l i c o b j e c t i s s t r u c k by a l i gh tn ing discharge, t h e r e w i l l be a surge of curr;ent through the objec t . I f t he o b j e c t is grounded and is of s u f f i c i e n t s i z e , then

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c h a r a c t e r i s t i c s c u r r e n t s equal t o t h e c u r r e n t i n t h e l i g h t n i n g d ischarge as given i n paragraph 2a w i l l be conducted through t h e o b j e c t t o ground. I f t h e o b j e c t i s not grounded, then the c u r r e n t flow w i l l be less i n r e l a t i o n t o the r e s i s t a n v e of t h e o b j e c t and t h e ground. Metallic ob jec t s whose c ros s s e c t i o n s a r e too s m a l l t o c a r r y the c u r r e n t from a l i gh tn ing s t r o k e may be melted o r vaporized.

c. Ground Current

When l igh tn ing s t r & k e s an o b j e c t , t h e c u r r e n t w i l l flow through a pa th t o t h e t r u e e a r t h ground. The vo l t age drop along t h i s pa th may be g r e a t enough over s h o r t d i s t ances t o be dangerous t o person- n e l and equipment [8 ] . C a t t l e and humans have been e l ec t rocu ted from t h e c u r r e n t flow through the ground and t h e vo l t age p o t e n t i a l between t h e i r f e e t wh i l e s tanding under a t r e e s t r u c k by l i gh tn ing .

d . Radio I n t e r f e r e n c e

When an e l e c t r i c a l charge produces a spark between two po in t s , e lectromagnet ic r a d i a t i o n is emit ted. This d i scharge is no t l i m i t e d t o a narrow band of f requencies , bu t covers most of t h e e l e c t r o - magnetic r a d i a t i o n spectrum wi th var ious i n t e n s i t i e s . Most s t a t i c heard i n r a d i o r ecep t ion is r e l a t e d t o e l e c t r i c a l d i scharges , wi th l i gh tn ing s t rokes con t r ibu t ing a l a r g e percentage of t he in t e r f e rence . i n t e r f e r e n c e from l igh tn ing s t r o k e s i s propagated through t h e atmosphere i n accordance wi th l a w s v a l i d f o r ord inary r a d i o t ransmiss ion and may t r a v e l g r e a t d i s t ances l i g h t n i n g s t r o k e s over g r e a t d i s t ances , c e r t a i n f requencies remain prominent, w i th 30 kc being the major frequency. te lemeter ing and guidance needs t o be considered only when thunderstorms are occurr ing wi th in 100 km (60 mi l e s ) of t he space v e h i c l e launch s i te . Thunderstorm loca t ions can be obtained from suppor t ing meteoro logis t s ,

This

With t h e t ransmission of i n t e r f e r e n c e from

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3. Frequency of Occurrence of Thunderstorms

According t o s tandard United S t a t e s weather observing procedure, a thunderstorm is repor ted whenever thunder is heard a t the s t a t i o n . is repor ted along wi th o t h e r atmospheric phenomena on t h e s tandard weather observer ' s form WBANA10 when t h e time thunder is heard and ends 15 minutes a f t e r thunder is las t heard, Notice t h a t t h i s type of r epor t - ing of thunderstorms may be a r e p o r t of one o r more thunderstorms during a period. For t h i s reason, t hese types of observa t ions w i l l be r e f e r r e d t o as "thunderstorm events"; i.e., a per iod during which one o r more thunderstorms is repor ted (heard). Because of t h e method of r epor t ing thunderstorms, most ana lyses of thunderstorm d a t a are based on t h e number of days pe r year i n which thunder is heard one o r more times on a day, i .e . , "thunderstorm days ." More d e t a i l e d s t u d i e s of f requencies of thunderstorms occurr ing i n t h e Cape Kennedy Area have been made [13].

It

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6

a. Thunderstorm Days p e r Year ( I sokeraunic Level)

The frequency of occurrence of "thunderstorm days" (number of days per year on which thunder is heard) is an approximate guide t o t h e p r o b a b i l i t y of l i gh tn ing s t r o k e s t o e a r t h i n a g iven area, The number of thunderstorm days p e r year i s c a l l e d t h e " isokeraunic level." A d i r e c t l i g h t n i n g s t r o k e is poss ib l e a t a l l loca t ions of i n t e r e s t , b u t t h e frequency of such a n occurrence varies between t h e loca t ions (see Table 2 and r e fe rences 7, 8, and 1 2 ) .

b. Thunderstorm Occurrence p e r Day

I I n a s tudy made using t h e WBAN-10 d a t a (which r e p o r t s a thunderstorm when thunder is heard [13] ) , f requencies were computed of t he number o l days which h a d 0, 1, 2 , ..., thunderstorms r epor t ed ; i . e . , none o r more "thunderstorm events." Tables 3 and 3a taken from r e f e r - ence 13, g ive t h i s information.

c. Thunderstorm " H i t s "

There were s u f f i c i e n t data f o r t h e summer months (June, J u l y , August) a t Cape Kennedy, F lo r ida t o make a n a n a l y s i s of t h e frequency of occurrence of "thunderstorm h i t s " [ 131. Thunders torm h i t s are def ined as :

(1) A thunderstorm a c t u a l l y repor ted overhead, o r

(2) a thunderstorm f i r s t repor ted i n a s e c t o r and las t repor ted i n t h e oppos i te s e c t o r . This is assuming thunderstorms move i n s t r a i g h t l i n e s over s m a l l areas. Tables 4 and 4a from re fe rence 13 l i s t t h i s information.

4 . Frequency of Lightning Strokes t o Ear th

Although r e l i a b l e r e p r e s e n t a t i v e d a t a concerning t h e number of thunderstorms a c t u a l l y passing over Cape Kennedy (or t he launch s i t e ) are a v a i l a b l e , t h e d a t a have n o t been d i r e c t l y r e l a t e d t o t h e number of l i gh tn ing s t r o k e s t o t h e launch pad. But i n another s tudy [7] , i t has been determined that, i f t h e i sokeraunic level is mul t ip l i ed by 0.23, a n estimate of t h e s t r o k e frequency t o t h e e a r t h p e r square mi le can be obtained. For t h e 0.2 square-mile launch area of Saturn Launch Complex 39, t h e r e are a n est imated four s t r o k e s per year o r nea r ly one s t r o k e f o r t he month of August. The probable number of s t r o k e s p e r year t o bu i ld ings of d i f f e r e n t he igh t s w i l l i nc rease wi th he igh t (see Table 5).

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m mY1

0

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r l . 0

Po

6' H .

In N 0 . N

In N O . N

m N O . N

Y 2 h

00 N . m 4

m r. 4 . N m 3

0 e . 4 N 4

m 9 Q

m m h O

m

O

In N O . N

m N

0 . N

r l . 0

0

0 m

4 .

0 \o

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0

N . 30 4

m

0 O h N .

0 N

0 \o .t .

CII rl N

:: m . 4-

3

0 m - m

0

4

~m

0 U\o

m

2 4 . 0

0

0 40:

E

m N

40

+ m . .

m m m h . . O N

N h N N . . o m rl

o m N 4

0.Y N 4 . .

h 3 N O I

;J N 3

m o Y? m m r l4

".m. mu)

N N o m i d

".9 m o

U O

Uu) m n . . 34

h U h Y I

d d

I

6 4

m m 2

a O . 0

m m ? 0

m m 9 0

ICN

0 9

m m 4 0

U N

*d N "2 m

O h rl d

U m 4 4 . 4

u) u)

40 4 .

m ? 0

m

E

rl F l . 0

:: I n . 0

m 29 0

0

0 3 9

O N 3

0

N m 3

0

J N e . 0

J

0 m m

0

0 2:

m 2:

2 a . 40

s 0 . 0

I

2

4

rl? 0

N

N . 0 m

h ~ - m

N

m m - m

h

U

N . e m

m

m m h 9 . 4 . r.

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YI

h

"? N

v)

N "9

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N ? 0

3

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m h A

e .

0 4J

I B 0 " ___

8

Table 3. Frequencies of the Observed Number of Days t h a t Experience x Thunderstorm Events a t Cape Kennedy, F lo r ida f o r t h e l l - y e a r Period of Record January 1957 through December 1967.

x J a n . Feb. March AprillMay J u n e J u l y Aug.

0 335 295 308 299 266 187 1 7 7 185

1 4 9 20 18 43 77 80 89

2 2 4 9 10 25 40 47 30

3 2 3 3 3 1 7 26 24 4 1 3 6 9 1 0

5 0 2 2 3

6 1 1

n 341 310 341 330 341 330 341 341

Sept,

228

54

33

12

3

330

Table 3a. Re la t ive Frequency of Days t h a t Experienced a t Least One Thunderstorm Event a t Cape Kennedy, F lo r ida .

Jan. Feb. March A p r i l May June J u l y Aug. Sept. Oct. Nov. Dec. Spring Summer F a l l

9

Table 4. Frequencies of t h e Observed Number of Days t h a t Experience x Thunderstorm H i t s a t Cape Kennedy, F lo r ida f o r t he 11-year Period of Record January 1957 through December 1 9 6 7 .

Table 4a. Rela t ive Frequency of Days t h a t Experienced a t Leas t One Thunderstorm H i t a t Cape Kennedy, F lor ida .

Table 5 . Estimate of t h e Number of Lightning Strokes per Year f o r Various Heights (Eas te rn Tes t Range) [ 7 ]

Height Number of Lightning ( f e e t ) Strokes (per year ) (m)

30.5 100 0 . 4 61.0 200 1.1 91.4 300 2.3

121.9 400 3.5 152.4 5€30 4.4 182.9 6 0 0 5 . 3 213.4 700 5 .8

!

1 0

111. STATIC ELECTRICITY

A s t a t i c e l e c t r i c charge can accumulate on a n o b j e c t from i t s motion through t h e atmosphere conta in ing ra indrops , ice par t ic les , o r dus t . from wind-borne d u s t o r s a l t (o f t en as n u c l e i too small t o be v i s i b l e ) o r r a i n o r snow p a r t i c l e s s t r i k i n g t h e ob jec t . This charge can bu i ld up u n t i l t h e l o c a l e lec t r ic f i e l d a t the po in t of s h a r p e s t cu rva tu re exceeds the breakdown f i e l d . The q u a n t i t y of maximum charge w i l l depend on t h e s i z e and shape of t h e o b j e c t ( e s p e c i a l l y i f sha rp poin ts a r e on the ob jec t ) . Methods of c a l c u l a t i n g t h i s charge are g iven i n r e fe rence 4 .

I f no t grounded, an unmoving o b j e c t can a l s o accumulate a charge

I f a charge bu i lds up on an ungrounded space v e h i c l e on a launch pad, any d ischarges which occur could i g n i t e explos ive gases or f u e l s , i n t e r f e r e wi th r a d i o ( te lemeter ing) communications, o r cause seve re shocks t o persons. S t a t i c e l e c t r i c charges occur more frequenekp during per iods of low humidity and can be expected a t a l l geographical a r eas .

I V . ELECTRICAL BREAKDOWN OF THE ATMOSPHERE

The atmosphere of t he e a r t h a t normal sea - l eve l p re s su re (101,325 newtons m-2) is an e x c e l l e n t i n s u l a t o r , having a r e s i s t a n c e g r e a t e r than 10l6 ohms f o r a column one square cent imeter i n c r o s s s e c t i o n and one meter long, When t h e r e is a charge i n the atmosphere, i o n i z a t i o n takes p lace , thus reducing t h e conduc t iv i ty of t h e a i r . This charge can be from e i t h e r cloud bui ldups o r e l e c t r i c a l equipment. I f t he vo l t age is increased s u f f i c i e n t l y , t he i o n i z a t i o n w i l l be high enough f o r a spark d i scha rge t o occur.

The breakdown vo l t age (vol tage requi red f o r a spa rk t o jump a gap) f o r d i r e c t c u r r e n t i s a func t ion of atmospheric pressure . The breakdown vo l t age decreases w i t h a l t i t u d e u n t i l a minimum of 327 v o l t s mm-l a t an atmospheric p re s su re of 760 newtons m-2 (7.6 mb), r ep resen t ing a n a l t i - tude of 33.3 km. Above and below t h i s a l t i t u d e , t h e breakdown vo l t age inc reases r a p i d l y [ 1 4 ] , being s e v e r a l thousand v o l t s p e r mi l l ime te r a t normal atmospheric p re s su re (see f i g u r e 1).

The breakdown vo l t age is a l s o a func t ion of frequency of a n a l t e r n a t - ing c u r r e n t . With an inc rease of frequency, t h e breakdown vo l t age decreases . A more complete d i scuss ion can be found i n NASA SP-208 [15].

Severa l measures can be taken t o prevent a r c ing of high vo l t age i n equipment :

11

(1) Have equipment vol tages o f f a t t h e t i m e t h e space v e h i c l e is going through t h e c r i t i c a l atmospheric pressures . capac i to r s should have bleeding r e s i s t o r s t o prevent h igh v o l t a g e charges remaining i n t h e capac i to r s .

Any h igh vo l t age

(2) El imina te a l l sha rp po in t s and a l low s u f f i c i e n t space between high vo l t age c i r cu i t s .

(3 ) Seal high vo l t age c i r c u i t s i n con ta ine r s a t normal sea - l eve l pressures .

( 4 ) Have m a t e r i a l s a v a i l a b l e t o p r o t e c t , w i t h proper use, a g a i n s t high vo l t age a r c i n g by po t t ing c i r c u i t s .

l Altitude (h)

50

40

30

20

I 0

Breakdown Voltage (Volts/mm)

Figure 1. Breakdown Voltage vs A l t i t u d e

12

REFERENCES

1. Coron i t i , Samuel 6. and James Hughes , "Plane tary Electrodynamics ," 2 volumes, Gordon and Breach Science Pub l i she r s , New York, N. Y., 1969.

2. Urnan; Lightning, M c G r a w - H i l l Book Company, New York, N. Y., 1969.

3. "Analysis of Apollo 1 2 Lightning Inc ident , " MSC-01540, prepared j o i n t l y by Marshal l Space F l i g h t Center , Kennedy S p a c e Center , and Manned Spacecraf t Center , NASA, February 1970.

4. Brook, M.,HBlmP. Holmes, and C. B. Moore, "Lightning and Rockets -- Some Impl ica t ions of the Apollo 12 Lightning Event," Naval Research Reviews, Vol. 23, No. 4, pp. 1-17, A p r i l 1970.

5 , Daniels , Glenn E. , e d i t o r , " T e r r e s t r i a l Environment ( C l i m a t i c ) C r i t e r i a Guidel ines f o r Use i n Space Vehicle Development, 1969 Revision, Second P r in t ing , " NASA TM X-53872, March 15, 1970, Marshal l Space F l i g h t Center , Alabama.

6. "Mi l i ta ry Spec i f i ca t ion , Bonding E l e c t r i c a l ( f o r A i r c r a f t ) ," MIL-B- 5087B(ASG), 1964, and amendment 1, 6 February 1968.

7. Brewster, H. D. and W. G. Hughes, "Lightning P r o t e c t i o n f o r Sa turn Launch Complex 39," TR-4-28-2-D, 1963, Launch Support Equipment Engineering Div is ion , NASA-Launch Operations Center , Cape Kennedy, F lo r Ida.

8. "Lightning P ro tec t ion Guidel ines f o r STADAN Ground Equipment," pre- pared ljy High Voltage Laboratory, General E l e c t r i c Company, P i t t s f i e l d , Mass., N68-24516, NASA CR 94682, Goddard Space F l i g h t Center , Greenbel t , Md., November 1967.

9. Chalmers , 3. Alan, "Atmospheric E l e c t r i c i t y , " Pergamon Press , New York, 1957.

10. "Electromagnetic I n t e r f e r e n c e Charac ter is t ics Requirements f o r Equipment," MIL-STD-461A7 1 August 1968.

11. "Electromagnetic I n t e r f e r e n c e Charac t e r i s tics , Measurement of ," MIL-STD-462, J u l y 31, 1967.

1 2 . UfilPted S t a t e s Weather Bureau and Corps of Engineers: "Thunderstorm R a i n f a l l . I 1 Hydrometeorological Report No. 5 , Hydrometeorological Sec t ion , 2 p a r t s , Waterways Experiment S t a t i o n , Vicksburg, M i s s . , 1947.

13

REFERENCES (Continued)

13. F a l l s , Lee W . , W. 0. W i l l i f o r d and M. C. Carter, "Probabi l i ty D i s - t r i b u t i o n s f o r Thugderstorm A c t i v i t y a t Cape Kennedy, Flor ida," NASA TM X-53867, NASA-Marshall Space F l i g h t Center, Alabama, 1970.

14. Spink, Bradley R., "A P r a c t i c a l Solu t ion t o t h e Arcing Problem a t High Al t i tudes ," P lane tary and Space Science, v o l . 7, J u l y 1961, pp. 11-18.

15. Paul , Fred W. and Donald Burrowbridge, "The Prevent ion of Electr ical Breakdown i n Spacecraft ," NASA SP-208, NASA, Washington, D. C., 1969.

14

APPROVAL NASA TM X-64549

ATMOSPHERIC ELECTRICITY CRITERIA GUIDELINES

FOR USE I N SPACE VEHICLE DEVELOPMENT

BY Glenn E. Daniels

The information i n t h i s r e p o r t has been reviewed f o r s e c u r i t y clas- s i f i c a t i o n . Review of any information concerning Department of Defense o r Atomic Energy Commision programs has been made by the MSFC Secur i ty C l a s s i f i c a t i o n Of f i ce r . This r e p o r t , i n i t s e n t i r e t y , has been d e t e r - mined t o be unc la s s i f Fed.

This document has a l s o been reviewed and approved f o r t echn ica l accuracy.

Chief, T e r r e s t r i a l Environment Branch

L&k, W. W. Vaughan

Chief , Aerospace E vironment Div is ion v

H - kP p. /g&4/Lc’ E. D. G e i s s l e r D i rec to r , Aero-As trodynamics Laboratory

15

D IS TRIBUTlvN

D I R DEP-T A&TS-PAT PM-PR-My M r , Golds ton A&TS -MS - H A&TS -MS - I P A&TS-MS-IL (8) A&TS-TU, M r . Wiggins ( 6 )

AD- S D r . S t u h l i n g e r

PD-DIR PD-MP

PD-RV M r . Mack M r . F. McCulloch

PD- DO PD-SS PM-MO (2) PM-SAT (2) PM-EP PM-MT A&TS-MS - D

Sa-DIR

Sa-CSE D r . Haeussermann (2) M r . B. May M r . G. McKay

Sa-ASTR M r . F. B. Moore (3) M r . W. Horton M r . H. Hosenthien M r . G. Gassaway (3) M r . W. P a t t e r s o n

Sa-ASTN M r . K. Heimburg (2)

S =-ME D r . S i e b e l (3)

S -Q UAL D r . Grau (3)

s a - S S L M r . G . Hel le r M r . H. S t e i n M r . R. Naumann M r . W. S ieber M r . W. Snoddy

S 6rE -AERO D r . Geissler M r . Horn M r . Sims M r . Dahm M r . L ins ley M r . S t r u c k M r . Baker M r . Lindberg M r . Lovingood M r . W. Vaughan (2) M r . R. Smith M r . Kaufman (2) D r . DeVries M r . Turner M r . Sloan M r . 0. Smith (2) M r . Dantels (200)

Sa-COMP D r . Hoelzer

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16

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17

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Abs tracts

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18

Lockheed Co. Sunnyvale, C a l i f . 94088 At tn : D r . G. Boccius

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ASN PS-40

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