NCAR Technical Notes

TN-32

Temperature Controlof Balloon Packages

ERNEST W. LICHFIELD

and L

NEIL E. CARLSON / 1m

1 t 7UL 19 7 1August, 1967

NCAR Technical NotesNATIONAL CENTER FOR ATMOSPHERIC RESEARCH

Boulder, Colorado

i o z :

' ., |I NCAR Library

l llll 11111111 I II IIIIIIII< 5 0583 01004524 7

The National Center for Atmospheric Research (NCAR) is dedicatedto the advancement of the atmospheric sciences for the benefit ofmankind. It is operated by the University Corporation for AtmosphericResearch (UCAR), a private, university-controlled, non-profit organiza-tion, and is sponsored and principally funded by the National ScienceFoundation.

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TN-32

Temperature Controlof Balloon Packages

ERNEST W. LICIFIELD

and

NEIL E. CARLSON

August, 1967

NATIONAL CENTER FOR ATMOSPHERIC RESEARCHBoulder, Colorado

i

i

FOREWORD

In 1966 NCAR undertook a program of designing a thermal package for

the Nimbus D-IRLS balloon flight. The IRLS (Interrogation Recording and

LocationSubsystem) flight package will contain electronics that will

respond to interrogation signals received from the Nimbus D satellite

which is scheduled to be launched in 1970. The radio interchange between

the balloon and the satellite will enable the satellite to locate the

balloon's position and record meteorological data from the balloon. The

IRLS balloon will float at an altitude where the ambient air temperature

may go as low as -55°C. Since the electronics and battery power supply

of the IRLS system will not operate properly at this temperature extreme,

NASA issued a contract to NCAR to design a thermal package that would

maintain the IRLS components at a reasonable temperature during both day

and night and for a long period of time (several months).

As we analyzed the problems we found that there was very little

published information on temperature control for balloon flights. It

was therefore decided that a report should be written that could be used

as a general guide for thermal packaging for ballooning.

j

iii

CONTENTS

FOREWORD .......................... i

List of Figures ...................... v

List of Tables ....................... vii

I. INTRODUCTION ........................ 1

II. OUTSIDE SURFACE TEMPERATURE OF A BALLOON FLIGHT PACKAGE . 2

Modes of Heat Loss .................. 2Surface Temperature Equilibrium ............. 2Surface Temperature at Night .............. 3Surface Temperature in the Daytime ........... 5Effect of Non-Conducting Outside Package Surface .... 5Techniques for Increasing Surface Temperature ...... 8Greenhouse Effect .. . . ............. 10

III. INSIDE TEMPERATURE OF A BALLOON FLIGHT PACKAGE ...... 13

Modes of Heat Loss .. ....... .13Computing Heat Loss from an Insulated Package .. . 13Dewar Flask .. ..................... 15Super-Insulation ................... 17

IV. DAY-NIGHT TEMPERATURE CONTROL ............... 18

V. DESIGN OF THERMAL PACKAGE FOR IRLS-NIMBUS D PROGRAM . 21

Package Construction ....... .... 21Pre-Flight Tests ................ 21First Flight Test ................... 29Second Flight Test ............ ....... 33

j

v

FIGURES

1. Deviation of average package from ambient nighttimetemperature ........... .. ............ 6

2. Daytime package temperature ................. 7

3. Dewar flask containing battery pack ............. 21

4. Insulation package for first test flight .......... 22

5. Inside temperature versus time, first ground test . . .... 25

6. Inside temperature versus time, second ground test .. .. 26

7. Inside temperature versus time, third ground test . . .... 27

8. Inside temperature versus time, chamber test ........ 28

9. Electrical schematic of telemetry system .......... 30

10. Inside temperature, first flight test ............ 31

11. Outside temperature, first flight test ........... 32

12. Second flight package .................. . 35

13. Outside temperature, second flight test ........... 36

14. Inside temperature, second flight test ........... 37

i

vii

TABLES

1. Power radiated from a blackbody as a functionof temperature ......................... 4

2. Radiative characteristics of surfaces ............. 9

3. Increase in radiation temperature by use ofMylar greenhouses .................... 11

4. Increase in insulation thickness required asinside dimension increases ................... 16

5. Heat-energy storage capabilities of water andbatteries ........................... 19

6. Characteristics of insulation package for firstflight test .......................... 23

/

I

I. INTRODUCTION

Temperature control for ballooning falls into three categories:

1. Daytime only. The easiest temperature control problem is a

daytime only flight. During daytime hours it is possible to use solar

radiation to warm the electronics package. By choosing the proper pack-

age surface treatment, it is possible to achieve almost any desired tem-

perature. The chapter on Outside Surface Temperature of a Balloon Flight

Package describes techniques useful for daytime only flights.

2. Short duration nighttime flights. For nighttime flights it

is necessary to provide an internal source of heat energy, which must be

contained within the package for efficient insulation. The chapter on

Inside Temperature of a Balloon Flight Package describes techniques that

are effective in insulating for nighttime flights.

3. Long duration day-night flights. For long duration day-night

flights it is necessary to balance the heat lost at night with the heat

received during the daytime. This involves a combination of surface

treatment, insulation and heat energy storage, as described in the chap-

ter on Day-Night Temperature Control.

2

II. OUTSIDE SURFACE TEMPERATURE OF A BALLOON FLIGHT PACKAGE

MODES OF HEAT LOSS

During the ascent of a balloon, a relative wind blows across the

flight package, cooling or warming the package to essentially the ambient

air temperature. However, when the balloon has reached float altitude

and stabilized, the wind becomes zero. Thus, ventilation -- the princi-

pal factor in determining package temperature during ascent -- becomes

negligible at float altitude.

If the flight package temperature differs from the ambient air tem-

perature, convective currents will tend to bring the temperature of the

package toward the ambient air temperature. However, experimental re-

sults show that convection has a small effect compared to radiation,

particularly at very high float altitudes where the air density is low.

Fortunately, it is easier to compute the radiation temperature of an

object than to predict the effects of convection.

SURFACE TEMPERATURE EQUILIBRIUM

The temperature of a surface is at equilibrium when the incoming

energy equals the outgoing energy. For a free-floating balloon package,

most of the energy is obtained by radiation, and the temperature of the

outside surface is approximately that required to re-radiate the re-

ceived energy. The relationship between radiated power and temperature

is

E = e k T (1)

3

where E = power in w/m2

e = emissivity, a char-acteristic of theradiating surface

k = Stefan-Boltzmannconstant

T = temperature in OK

Since T appears in Eq. (1) as a fourth power and is expressed in OK; it

is usually more convenient to work with a table in which the temperature

is expressed in °C (Table 1).

SURFACE TEMPERATURE AT NIGHT

The surface temperature at night is determined by the infrared

power received by the balloon package, and by the blackbody character-

istics of the package. The shape and orientation of a package must be

considered in computing the infrared power received. For a package with

height, width, and length nearly equal, it is reasonable to assume that

one-quarter of the power is received from above, one-quarter from be-

low, and one-half from the sides. For a nighttime flight at 40,000 ft,

the input to the top of the package would be the deep space radiation2

of about 20 w/m2 . The side input is the radiation from the surrounding

air; a typical air temperature at 40,000 ft is -55°C which gives a radi-2

ation input of 128 w/m . The radiation input to the bottom varies con-

siderably; in clear skies over a warm ocean, the input may be as high as2 2

380 w/m , while with high cold clouds it may be as low as 140 w/m2

Temperature extremes computed from these extremes of power input are -41

and -660C.

Over Warm Ocean Over Cold Clouds

Top: 1/4 x 20 = 5 1/4 x 20 = 5

Side: 1/2 x 128 = 64 1/2 x 128 = 64

Bottom: 1/4 x 380 = 95 1/4 x 140 = 35

164 w/m 104 w/m2

Temperature: 41°C 660 C

4

Table 1

POWER RADIATED FROM A BLACKBODY AS A FUNCTION OF TEMPERATURE

Temperature Power Temperature PoweroC w/m2 oC w/m2

205 2968.72 50 619.21200 2846.46 45 581.87195 2728.02 40 546.04190 2613.32 35 511.99185 2502.27 30 479.55

180 2394.80 25 448.68175 2290.83 20 419.33170 2190.28 15 391.44165 2093.08 10 364.97160 1999.15 5 339.86

155 1908.42 0 316.07150 1820.81 -5 293.55145 1736.26 -10 272.26140 1654.68 -15 252.14135 1576.01 -20 233.16

130 1500.18 -25 215.28125 1427.13 -30 198.44120 1356.77 -35 182.61115 1289.05 -40 167.75110 1223.89 -45 153.81

105 1161.24 -50 140.76100 1101.02 -55 128.5695 1043.17 -60 117.1790 987.64 -65 106.5685 934.35 -70 96.6880 883.25 -75 87.50

75 834.28 -80 79.0070 787.37 -85 71.1365 742.46 -90 63.8660 699.5155 658.44

5

As a general rule average package temperature at night is close to

the ambient temperature; deviations are shown in Fig. 1.

SURFACE TEMPERATURE IN THE DAYTIME

In the daytime the balloon package receives the same energy inputs

that it receives at night, plus an additional input from the sun. The

typical solar input at balloon float altitudes is approximately 1200 w/m2.

For a spherical package (or any package with equal or nearly equal height,

width, and length) one-half of the surface area is exposed to the sun;

however, the cross-sectional area exposed to the sun is only one-quarter

of the surface area. If the surface of the package is black to solar

radiation, the package will absorb 1200/4 or 300 w/m2 from the sun. If

the nighttime temperature of the package were -550 C, the solar input

would increase the surface temperature to +210C.

Temperature (0C) Power Radiated (w/m )

-55 128

+300

+21.5 428

In practice such a temperature increase is seldom achieved, because

the temperature differential between the surface of the package and the

air causes convection currents, which decrease the surface temperature.

Therefore, on actual balloon flights at about 40,000 ft, black-surfaced

packages absorbing solar radiation have an average surface temperature

between +10 and +150C. Figure 2 shows the daytime package temperature

for a day of balloon flight.

EFFECT OF NON-CONDUCTING OUTSIDE PACKAGE SURFACE

The previous calculations have been based on the assumption that

the outside surface of the package is at the same temperature at all

places -- equivalent to assuming that the surface is a perfect thermal

20

50

10 0- 100E-_J

W 200c_ /

500

1000 I I I -20 -10 0 10 20

DEVIATION FROM AMBIENT (°C)

Fig. 1 Deviation of average package from ambientnighttime temperature. (University ofMinnesota Report, (Vol. 13, 1-4.)

3011 1

20

B I BALLOON AT°f |wp BALLOON FLOAT ALTITUDEW /AT LAUNCH

I -r-

a. oES |Temperatures shown are warmer

W than average package temperaturesH- due to thermistor being mounted

near the top of the package.-I0-

-20 1000 1100 1200 1300 1400 1500 1600 1700 1800

TIME (hr)

Fig. 2 Daytime package temperature.

8

conductor. If there is no thermal conduction, each point on the surface

must re-radiate the energy that it receives. Therefore, the side facing

the sun would be at a temperature of +1070C, while surfaces shaded from the

sun would be near the ambient temperature, -550C, and the average surface

temperature would be

3(-55) +107 -14.50C= -14.50c

Comparison of -14.50C for the non-conducting surface with +21.50 C for

the perfect conducting surface shows that the surface skin should be a

good thermal conductor if the highest average temperature is desired.

TECHNIQUES FOR INCREASING SURFACE TEMPERATURE

A surface coating material that has low infrared emissivity ( <1 )

will increase the surface temperature of a package. The radiation tem-4

perature of a surface is defined by E = e k T . If e is reduced in value,

the value of T must be increased to radiate at a given power E. Table 2

lists the emissivity of several materials. It should be noted from the

table that, in general, materials with low emissivities also have low

absorptivities. For example, silver has an infrared emissivity of 0.02,

but its absorptivity is only 0.07. (Putting these numbers into the radi-4 4

ation equation gives 0.07 E = 0.02 k T which reduces to E = 0.28 k T4 .)

Since the effective emissivity is the ratio between the emissivity and

absorptivity, a silver surface has a low effective emissivity. However,

although a silver surface would be very hot in space, in an atmosphere

it would be cool because any convection would be sufficient to remove

the small amount of power absorbed. A better surface material is silver

sulfide, which has a high absorptivity for solar radiation but a low

emissivity. A package with a silver sulfide surface will have an average

surface temperature of +450 C, compared with +15 to +200C for a plain

black surface.

9

Table 2

RADIATIVE CHARACTERISTICS OF SURFACES

IR Solar_Matlver(_ial ieEmissivity Absorption Rat

Silver (polished) 0.02 0.07 0.28

Platinum 0.05 0.10 0.50

Aluminum 0.08 0.15 0.53

Nickel 0.12 0.15 0.80

Stellite 0.18 0.30 0.60

Aluminum paint 0.55 0.55 1.00

White lead paint 0.95 0.25 3.80

Zinc oxide paint 0.95 0.30 3.20

Gray paint 0.95 0.75 1.26

Black paint 0.95 0.95 1.00

Lamp black 0.95 0.97 0.98

Silver sulfide1 0.03 0.60 0.05

Nickel black1 '2 0.10 0.90 0.11

Cupric oxide '2 0.15 0.90 0.16

1 These are special surfaces where a metal is covered with a very thin

layer of absorbing material. The layer is so thin that it is a frac-

tion of a wavelength thick in the infrared and is, therefore, almost

transparent to IR. The result is that the IR emissivity is nearly

that of the underlying metal. However, the thickness is large com-

pared to the wavelength of the maximum solar spectrum so the absorp-

tivity is large for solar radiation.

2 H. Tabor, Solar Energy, September, 1961.

10

GREENHOUSE EFFECT

Another way to increase the temperature of a radiation surface is

to cover it with a sheet of material that is transparent in the solar

spectrum but black in the infrared, such as glass or Mylar. As shown

below, the radiation temperature of a black surface is increased by one

additional sun input if the surface is covered with a Mylar sheet.

D

D A Mylar sheet

\/ ~B ' black surface

If the solar input has the value D, then the power radiated from the

system also must equal D. Therefore D = A. Since B is the radiation

from the back of the Mylar sheet, then B = A = D and C = D + B = 2D.

If two or three layers of Mylar are used, the radiation tempera-

ture increases to three and four times the solar input, respectively.

Two layers of Mylar: D

_ F Mylar

JA . Mylar

v717777 #C black

D =F = E

A =E + F = 2D

B = A = 2D

C= B + D= 3D

11

Three layers of Mylar: D

H Mylar

tF Mylar

Mylar

t C ___black

D = H = G

F =G+ H 2D

E = F = 2D

A E + F - G = 3D

B = A = 3D

C= D B = 4D

Theoretical results are shown in Table 3.

Table 3

INCREASE IN RADIATION TEMPERATURE BY USE OF MYLAR GREENHOUSES

Solar input: D = 1000 w/m

Maximum SurfaceNumber Power Radiated Temperature of Temperatureof from Black Non-Conducting of ConductingCovers Surface (w/m ) Surface (0C) Surface (0C)

0 1000 91 121 2000 160 512 3000 207 79.43 4000 243 100.54 5000 272 121.5

In practice the temperatures are likely to be much less than those indi-

cated in the table, for two reasons: (1) the effect of heat removed by

convection and conduction of the air has been neglected, and (2) it has

12

been assumed that the covers over the black surface are completely trans-

parent to the visible spectrum and behave like a blackbody in the infra-

red. Equation (2) describes the performance of a black surface with one

cover when the covering material is not ideal.

(1- r) (1 - aO ) + (a + 2)C = 2A 2 (2)

where C = power radiated from black surface

A = solar input in watts

r = reflectivity of cover

(O = absorptivity of cover in solar spectrum

1 = absorptivity of cover in infrared

For example, if r = 0.1, = 0.1, and a = 0.8, then C = 2A(0.72); if

r = 0.2, o = 0.15, and a = 0.8, then C = 2A(0.63). Therefore the non-

ideal cover reduces the effect of the greenhouse but is still an improve-

ment over no cover at all.

13

III. INSIDE TEMPERATURE OF A BALLOON FLIGHT PACKAGE

MODES OF HEAT LOSS

Most of the heat loss from the inside of an insulated package is by

conduction through the insulation material. To reduce this conduction,

insulating materials with low thermal conductivity are used, and the

insulation is usually fabricated in the form of foams or fibers to keep

heat conduction paths long and thin. In general, the quality of insula-

tion increases as the density of the insulating material decreases, un-

til the density is so low that heat is lost by radiation through the

insulation. An appreciable amount of heat may also be transferred by

conduction and convection within the air spaces of the insulation; for

lightweight insulating materials, this becomes the major mode of heat

transfer. As a result, a low density polyurethane foam is three to four

times as good an insulator at 40,000 ft as it is at sea level, and at

80,000 ft it may be eight to ten times as good. The improvement is due

to the decrease in air conductivity at higher altitudes. Radiation

losses can be reduced by placing reflecting layers of aluminum foil be-

tween the layers of insulation.

Wires running from the inside to the outside of the package are

often a source of heat loss. This heat loss can be reduced by using as

small a wire size as practical and keeping the wire length through the

insulation as long as possible.

COMPUTING HEAT LOSS FROM AN INSULATED PACKAGE

/ K ^\ heat source

sphere of insulation

14

For a spherical enclosure of insulating material with a heat source

located at the center, the rate of heat loss is given by:

-W =dk A (3)dx

Where W = heat power lost in w

T = temperature in OC

dT = temperature gradient

k = coefficient of thermalconductivity

A = area of surface dis-tance x from the cen-ter of the sphere

A = rx2

k4rx2 dTdx

-W dxdT -w= x

w 1 + cIntegration gives: T W . +C (4)

where C is the constant of integration. Solving for the temperature

difference between the inside and outside gives:

W 1 1 )T. T (5)in Tout k4 ir X. x-

in out

where x = outside radius of theinsulation

X.in = inside radius of the

insulation

The temperature difference equations for cubical and cylindrical

packages are given below.

W 1 1 Cube: - T (sa)in out = 24k ( x x

in out

15

where 2x = the length of a side

Cylinder: Ti - T W _ ( _ - 1) (5b)in out 18.84k X. Xout

zn out

where x = radius of cylinder

2x = length of cylinder

Since Xout - x. is the thickness of the insulation, we can rewriteout in

Eq. (5) in terms of thickness of insulation, M:

2K x

M = in (6)1 - Kx.

in

T. - T 2lTkin - Tout

where K = .out for a sphereW

Equation (6) describes the thickness of insulation required to maintain

a constant temperature gradient expressed as a function of the inside

package dimension. Table 4 demonstrates how rapidly insulation thickness

increases as the inside dimension increases. This shows the importance

of keeping the inside dimension as small as possible. It can be seen

from the table that when the inside dimension reaches 10, an infinite

thickness of insulation is required. In this case the only way to main-

tain a constant temperature difference is to decrease the value of K by

increasing the power, W, or using a better insulating material.

DEWAR FLASK

The dewar flask takes advantage of the fact that a vacuum is an

ideal insulating material. The dewar is constructed with one container

inside another. The space between the containers is evacuated. In a

vacuum the only way to transfer heat is by radiation, and the amount of

heat that can be transferred by radiation is greatly reduced by silver-

ing the inside surfaces.

16

Table 4

INCREASE IN INSULATION THICKNESS REQUIREDAS INSIDE DIMENSION INCREASES

Tin - T 2kK = n = 1/10

W

Inside InsulationDimension Thickness

1_ M

1 1/9

2 1/2

3 1-2/7

4 2-2/3

5 5

6 9

7 16-1/3

8 32

9 81

10 00

17

SUPER-INSULATION

The insulation qualities of the dewar flask can be further improved

by adding additional reflective surfaces in the evacuated space. The

reflective surfaces are separated by paper or fiber spacers. This pro-

duces the same effect as multiple dewar enclosures. This technique is

often referred to as "super-insulation." At the present state of the

art, super-insulation is the best insulation. The disadvantages with

dewars and super-insulation packages are that they are fragile, heavy

and expensive, and once sealed it is impractical to reopen them.

18

IV. DAY-NIGHT TEMPERATURE CONTROL

For nighttime balloon flights, energy must be stored to maintain

the internal temperature of the balloon flight package. The most com-

mon way of storing heat energy is in water. The energy that can be

stored in water, and which is released when water freezes, is 92.5 w-hr

per kg or 42 w-hr per lb. In addition, 1.16 w-hr per kg (or 0.53 w-hr

per lb) of energy is released per °C as water is cooled to 0°C. Table 5

compares the energy that can be stored in water with that which can be

obtained from batteries. Both zinc-air and silver-zinc batteries have

greater energy storage than water, but the zinc-air battery requires

oxygen for its chemical reaction and is therefore not usable at balloon

float altitudes. When the additional energy required to warm the water

in its liquid state is considered, water is almost as good as silver-

zinc; furthermore, it is easier to handle and less expensive.

For day-night flights of several days' duration, the package must

be able to absorb as much heat energy during the day as was lost during

the night. This means that the temperature of the surface of the pack-

age must run as hot during the day as it runs cold during the night.

With a good black paint a daytime surface temperature of +10 to +150C

can be attained; since a typical nighttime surface temperature is -550C,

it is necessary to push the daytime surface temperature to +550C. This

requires more sophisticated techniques which generally fall into the

following categories:

A. Increased Surface Temperature by Special Coatings.

1. The use of materials, such as silver sulfide,that have a high absorptivity to solar radia-tion, but have a low emissivity in the infrared.

19

Table 5

HEAT-ENERGY STORAGE CAPABILITIESOF WATER AND BATTERIES

w-hr/lb w-hr/kg w-hr/in.

Water (heat of fusion) 50 110 1.7

Water (per °C) 0.63 1.38 0.0225

Zinc-air 80 176 5

Lead-acid 11 24.2 1.2

Nickel-cadmium 8 17.6 0.4

Silver-cadmium 35 77.0 3.5

Silver-zinc 55 121 4.5

2. The use of one or more greenhouses, which effec-tively multiply the solar input.

B. Thermal Diode Techniques. A thermal diode acts asa one-way valve which allows heat to flow into thepackage easily, but resists the outflow of heat.

1. Electrical input. Solar cells can be used toconvert solar energy to electrical energy. Theelectrical energy is transported into the pack-age through a wire; inside, it is stored in abattery or as heat energy in water.

2. Heat pump. A pump can be used that pumps heatinto the package when the outside is warmer thanthe inside. The pump does not operate when theoutside is colder than the inside.

In some cases it may not be necessary to control the temperature

of the entire package. It may be more practical to control the tempera-

ture of a few small critical parts -- as illustrated in Fig. 3, which

shows a dewar flask containing a battery pack only.

20

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21

V. DESIGN OF THERMAL PACKAGE FOR IRLS-NIMBUS D PROGRAM

NCAR, operating under a contract from NASA, undertook a program

for designing a thermal package for the Nimbus D-IRLS balloon flight

program. The IRLS flight package will be flown on high altitude bal-

loons, and will be interrogated by signals transmitted from the Nimbus D

satellite. The radio interchange between the balloon and the satellite

will enable the satellite to locate the balloon position and record the

meteorological data collected by the balloon. The IRLS electronics

must operate during both daytime and nighttime over a period of several

months. The electronics and power supply of the IRLS system will not

operate properly at the existing ambient temperatures of the balloon

float altitude. NASA's contract to NCAR was to solve this thermal pack-

aging problem.

PACKAGE CONSTRUCTION

An experimental thermal package was constructed with inside dimen-

sions designed to enclose the IRLS package and power supply. The insu-

lating package was cylindrical in shape (8 in. diam by 10 in. long,

inside) and was constructed of 1-in. sheets of polyurethane foam. Alum-

inum foil was placed between the layers of foam to act as a radiation

barrier. The outer surface of the package consisted of a layer of alum-

inum foil which had been coated with a silver sulfide surface. Figure 4

shows the insulation package, and Table 6 lists its characteristics.

PRE-FLIGHT TESTS

The first test was to enclose in the package 1 kg of ice in two

plastic bottles. One bottle was lying on its side, the other standing

upright. The package was allowed to remain at room temperature while

the ice melted. The inside temperature of the package was monitored by

a thermistor and recorded by a strip-chart recorder. After 36 hr the

.... ........ : -: .. .- -iiiiiiiiisiiiiii.... .........i i: _ _-iii-l _ -::-:-:: --:--- :----- ::::---:::::::::::::'

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A-?r~~~~~~~~~~~~~~~~~~~~~~~lsiiii~·

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Fig. 4 Insulation pa:-kage for first test flight.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~il~ii

23

Table 6

CHARACTERISTICS OF INSULATION PACKAGE FOR FIRST FLIGHT TEST

PACKAGE SIZE

Inside dimensions: 8-in. diam by 10 in. longOutside dimensions: 14-in. diam by 16 in. long

CONSTRUCTION

Constructed from 1-in. thick sheets of 10 ft /lb polyurethane foam.

A layer of aluminum foil was placed between each layer of foam.

Sides of the package were covered with silver sulfide coatedaluminum foil.

The silver sulfide coating was prepared by (1) copper platingaluminum foil, (2) silver plating over copper, and (3) convert-ing the silver surface to silver sulfide by using an electricalcurrent in a sodium sulfhydrate bath.

Top and bottom of the package were covered with aluminum foil.

PACKAGE WEIGHT

Insulation package 820 gmPackage skin 85 gmWater 453 gmBattery and electronics 767 gm

Total flight weight 2125 gm

24

strip chart indicated that the ice had melted. When the package was

opened, it was observed that the ice in the standing bottle had com-

pletely melted while a small amount of ice still remained in the bottle

lying on its side. This indicated that there was a significant tem-

perature gradient inside the package between the top and the bottom.

Figure 5 is a plot of the inside temperature versus time. The time re-

quired to melt the ice was approximately 36 hr. Since 1-kg water con-

tains 92.5 w-hr of heat energy, the rate of heat loss was 92.5/36 or

2.57 w per 200C temperature differential.

The second test was performed in a similar manner using one bottle

containing 1 lb of ice. In this test the bottle was mounted at the top

of the package. The results of this test are shown in Fig. 6. It can

be seen that it took approximately 19 hr to melt 1 lb of ice. Although

less than one-half as much ice was used in this test as in the first one,

the ice remained for more than one-half the length of time of the first

test. The reason for this is that, for one bottle, the air inside the

package becomes a larger portion of the insulation.

The third test involved placing a frozen mixture of water and alco-

hol in the package in order to experiment with techniques for lowering

the freezing point of the liquid. The results of the test are shown in

Fig. 7. As a result of this test the concept of using a water-alcohol

mixture was abandoned because there was no apparent stabilization point.

In the fourth test the flight package containing 1 lb of water plus

batteries and electronics was placed inside an environmental chamber.

The chamber was pumped down to 100-mb pressure and the temperature was

decreased to -600C. During the test the electronics were operating.

The results are shown in Fig. 8. Even though the temperature differen-

tial was nearly 60°C, it required 13 hr to freeze 1 lb of water. The

power lost from the package was 42/13 or 3.23 w per 600C temperature dif-

ferential. This indicates more than a 2-to-1 improvement in the apparent

insulation qualities of the package. The improvement is due to increased

insulation effectiveness at low pressure.

20

10

o 0

r -10 -1 / Package opened after 47 hr. o

. / One water bottle (standing) contained |E |^~~~~ / ~500gm ofwater at +3°C.

wU The other bottle (on its side) containedI-H / about 25gm of ice out of atotal of 500gm.

-20Room temperature: + 230 C14 March 1967

-30

5 10 15 20 25 30 35 40TIME (hr)

Fig. 5 Inside temperature versus time, first ground test.

lI , I I I l I l I ' I I I , I

20

10·

L- 0

1) ::

<Fig. 6 Inside rRoom temperature: + 23 ~ -io- /Solution: 453.6-gm water,at -

a. /-43 0 C to start

-20

-30

5 10 15 20 25 30

TIME (hr)

Fig. 6 Inside temperature versus time, second ground test.

20

10 -

0

W 0

:D / Room temperature :+230C- / Solution:341-gm water,

< / 113-gm alcohol,C: iO f at-430 C to startW -10

FI--20

-30

-405 10 15 20 25

TIME (hr)

Fig. 7 Inside temperature versus time, third ground test.

30

20Chamber: -60C, 100-mb pressureSolution: 453-gm water

- 0 -1

a. I o

-10

-20

5 10 15 20

TIME (hr)

Fig. 8 Inside temperature versus time, chamber test.

29

FIRST FLIGHT TEST

The first flight package was wired to telemeter to the ground the

temperature of the inside of the package and the temperature of one

point on the outside surface. Power for the flight system was supplied

by 10 nickel-cadmium batteries. Figure 9 is a schematic diagram of the

telemetry system.

The system was launched from the NCAR Marshall site near Boulder,

Colorado. It was carried aloft by a two-balloon system -- the upper

balloon was a 16-ft zero-pressure balloon; the lower balloon was a 7-ft

superpressure balloon. This configuration is referred to as the sky-

anchor system. The system rises until the superpressure balloon is over-

pressured. As the system continues to rise, the apparent weight increases

until it is equal to the total free lift, after which the system flies

at a constant altitude. The first IRLS balloon system was launched at

1455 MST, 3 April 1967, and rose to an altitude of 66,000 ft. Continuous

telemetry data for a 29-hr period were received by radio operators lo-

cated in Boulder, and in Kearney, Nebraska.

The outside temperature of the package was measured by a thermistor

placed under the silver-sulfide coated aluminum foil. The temperature

at the measurement point was, of course, a function of the orientation

of the package with respect to the sun. A plot of inside temperature is

shown in Fig. 10; and outside temperature in Fig. 11.

At launch the outside temperature of the package ran about 15 to

200 C above ambient temperature. During ascent the skin temperature was

cooled by ventilation to about 0°C. At float altitude the temperature

rapidly increased to between:+32 and +560C. At sundown the temperature

dropped to an average ambient value of -450C and remained there through

the night.

The morning temperature was not as hot as the afternoon temperature,

because the gas in the zero-pressure balloon cooled at night and the

balloon system descended to a lower altitude. In the morning the sun

heated the gas, causing the system to rise, and the resulting air motion

SENSOR NO. I SENSOR NO.2 SENSOR NC3 SENSOH NO. 4 30 SEC TIMER CODE SWITCHING GATES rlP FLOP "E" FLIP FLOP "F"R 1 R?25 R12 R13 RIO R14 RR3 R 4 R35 R33 030 33 3

;K 070 10 8K70S 60 IRK K2 070 Bf1K 270 BOES. 270 00 0600K 5600 5~600f 273P00 ~PuR — -562700R 270K

I _ LI 0l ~ CRI4 CR5 CR13 P —RIG R RcSf R F

ICf I LllE 11 11 == l__ __ _ R 010

Iowd 10. d. 2 _ __t .

CODE ENE IN CIRCUTSI FLIP FLOP FLIP FLOP " FLIP FLOP FLP FLOP ""FLIP FLOP CFL FLOP ING C CUITS

A47 R49 R 4 R~43 0 RA 330 33K<33K 33K0 33K 33K 33K 33K 33 33K R5 096 I 0 0 6 RO4

/000B°p^^- c'-l°O°pt tOO pt'-'- -T-lOO~p j/0 00^-_I I I lOO°pf^- c.^OO°pf E o- -- o^- -I I -- '" CE)

1 ^M363aj\ I

R6—5H^OO)~ I 35-3 CO6L ,. BOTO M ANTENNA 4

CI20 R24 ~ R26 C16 RR29OO R30i n Cis16 5 FEETR TR

0239 51 3 PORN 02 I .. 3p I

#26 GA000 WIRE

Fig 9 Electrical schematic of telemetry systemI

TOP ANTEN NA

Fig. 9 Elect1rclsh26 GA. WIRE

30 I I I I 1 I I I I I

20

Iii

I l F Io 10

-20

-300 5 10 15 20 25 30

TIME (hr)

Fig. 10 Inside temperature, first flight test.

80 '

70

60

50

400 5

w 30

20

oF 10

-10

-20

-30

-40 -

-05 10 15 20 25 30

TIME (hr)

Fig. 11 Outside temperature, first flight test.

33

caused the package to cool. On a conventional superpressure balloon

flight there would be no morning cooling, and the average temperature

should look like the dotted line in Fig. 11.

The inside temperature (Fig. 10) shows the same general shape as

the outside temperature curve; however, the curve is smoother because

of the long thermal time constant of the package. During the night the

inside temperature dropped to -ll0C where it remained through the night.

At sunrise the temperature increased rapidly to +20C for about 2.5 hr.

the time during which the balloon system was ascending to its daytime

float altitude. After reaching altitude, the inside temperature in-

creased to +100C.

Conclusions from the first test flight were: (1) With l-lb water

and existing insulation the thermal time constant of the package is suf-

ficient for day-night flights. (2) The silver sulfide surface provides

an appreciable increase in daytime surface temperature, giving an average

external surface temperature of +40°C. (Previous flight experience with

plain black surfaces gave average surface temperatures of +150C or less.)

(3) The package approached a day-night temperature balance. The package

lost approximately 35 w-hr of heat during the night and gained approxi-

mately 24 w-hr during the day. An additional 6 w-hr would have been

gained if the outside of the package had not been cooled due to ascent

during the morning. The net deficit of 5 to 10 w-hr could be compen-

sated for by a small solar cell panel (i.e., 200 cm2 of active area),

or the external surface temperature could be increased by the addition

of a greenhouse covering of Mylar.

The electronics used on the first test flight were recovered near

Brewster, Nebraska, and returned to NCAR. The package skin was badly

torn, but the electronics were only slightly damaged.

SECOND TEST FLIGHT

After analyzing the results of the first test flight it was decided

that changes would be made to the insulation package to obtain a higher

34

surface temperature and that a second test flight would be made. The

changes were: (1) A silver sulfide surface would be used on the top of

the package. (2) The sides and top of the package would be covered with

a Mylar greenhouse. Otherwise the second flight package was identical

to the first; in fact, since the original flight package had been re-

covered and returned to Boulder, the same electronic instrumentation was

used on both flights. The package with the Mylar covering is shown in

Fig 12.

The second package was launched at 1908 MST, 22 May 1967, from the

NCAR Marshall site located near Boulder. The launch was programmed for

the late afternoon so that the package would not reach altitude until

after dark. This was to avoid the problem encountered in the first bal-

loon flight where the balloon lost altitude during the night and cooled

the package when it returned to daytime float altitude. The package was

carried aloft by a zero-pressure balloon whose float altitude was 22 mb

or 80,000 ft.

Excellent telemetry data were received through the first night and

for most of the following day. Radio contact was lost on the second

night, but data were again received for about half of The second day.

The results of the flight show that the package modifications sig-

nificantly increased tne outside temperature (Fig. 13). The maximum

surface temperature was +1000C when the measurement point was facing

the sun, dropping to +60C when shaded by the package. The approximate

average external temperature was +55°C. It will be noted when analyz-

ing the external temperature that there was a large decrease occurring

through midday; this was evidently caused by the balloon shadowing the

instrument package. On future balloon flights care should be taken to

place the package further from the balloon.

The internal temperature (Fig. 14) dropped to a minimum nighttime

value of -6°C. After sunrise the temperature increased to + 10°C until

.

35

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U ^ /^VMIN.TEMR / oa 0 0

w -10

-20

-30

-40

-50

-6016 18 20 22 24 2 4 6 8 10 12 14 16 18 20 22 24 2 4 6 8 10 12 14 16

22 MAY 23 MAY 24 MAY

TIME (MDST)

Fig. 13 Outside temperature, second flight test.

20

62.210 LOSS OF RADIOCONTACT

Q::

b JJ

w -10I-

-20

16 18 20 22 24 2 4 6 8 10 12 14 16 182022242 4 6 8 1012141622 MAY 23 MAY 24 MAY

TIME (MDST)

Fig. 14 Inside temperature, second flight test.

38

the ice melted, which occurred at approximately 1600 hr. After this the

temperature gradually increased to +18°C. The fact that all of the ice

melted indicates that less heat energy was lost during the nighttime

than was gained during the day. This demonstrates that the package is

able to provide thermal control for an indefinite period of time. The

resulting average temperature would be above 0 C -- it would take sev-

eral days of flight data to determine the actual average temperature.