Anthony Colozza

Analex Corporation, Brook Park, Ohio

Airships for Planetary Exploration

NASA/CR—2004-213345

November 2004
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Anthony Colozza

Analex Corporation, Brook Park, Ohio

Airships for Planetary Exploration

NASA/CR—2004-213345

November 2004

National Aeronautics and

Space Administration

Glenn Research Center

Prepared under Contract NAS3–00145
Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

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Available electronically at http://gltrs.grc.nasa.gov
NASA/CR—2004-213345 iii

TABLE OF CONTENTS

ABSTRACT 1

INTRODUCTION 1

PLANETARY ENVIRONMENTS 2

ATMOSPHERIC ENVIRONMENT OF VENUS 3

ATMOSPHERIC ENVIRONMENT OF MARS 5

ATMOSPHERIC ENVIRONMENT OF TITAN 6

OUTER PLANETS (JUPITER, SATURN, URANUS, NEPTUNE) 6

AIRSHIP EVALUATION 7

AIRSHIP FOR VENUS 13

AIRSHIP FOR MARS 17

AIRSHIP FOR TITAN 18

OUTER PLANETS 19

SUMMARY 19

REFERENCES 20

NASA/CR—2004-213345 1

Abstract

The feasibility of utilizing an airship for planetary atmospheric exploration was assessed.

The environmental conditions of the planets and moons within our solar system were

evaluated to determine their applicability for airship flight. A station-keeping mission of

50 days in length was used as the baseline mission. Airship sizing was performed

utilizing both solar power and isotope power to meet the baseline mission goal at the

selected planetary location. The results show that an isotope-powered airship is feasible

within the lower atmosphere of Venus and Saturn’s moon Titan.

Introduction

Most planets and some moons within out solar system have atmospheres. The presence of

an atmosphere enables the possibility of utilizing vehicles that can fly within the

atmosphere as a potential tool for planetary exploration. Atmospheric flight vehicles offer

a unique science data gathering capability. Planetary air vehicles should be thought of as

another tool in the exploration toolbox. They are not intended to take the place of other

types of exploration vehicles but to complement them. They can be used in conjunction

with land based (rovers and landers) and space based (orbiters) exploration to provide a

complete set of capabilities for planetary exploration. Whereas rovers and landers can

provide very high-resolution data on a local scale and orbiters can provide lower

resolution data on a planetary scale, air vehicles can provide high-resolution data on a

regional scale.

In addition to the unique imaging and remote sensing data collection capabilities of air

vehicles, they also provide a means of direct sampling of a planet's atmosphere. This

sampling can be performed over a region of the atmosphere and potentially over a range

in altitudes. For example, an airship could be used to sample the atmosphere over a

region looking for signs of life. This can be accomplished by searching for concentrations

of biogenic gases (such as CH4, NH3, H2S etc.) If a concentration of these are found, a

future mission can be used to send a rover or lander to the site of interest to complete the

investigation. An airship, if feasible, would provide a stable, slow-moving platform that

could provide data of a large region.

One of the main obstacles to planetary air vehicles is the need for continuous power in

order to remain in controlled flight. This issue is not as severe with airships as it is with

aircraft since an airship’s lift is not dependent on its velocity. Because of the inability to

refuel, the mission duration, with a conventional combustion engine powered airship, will

be limited to the amount of fuel & oxidizer that can be carried on board. This inherently

limits the mission time to a duration of hours or at best days. Although useful and

significant data can be collected within this time frame, the mission duration will be

much less than that of landers, rovers (duration of months), or orbiters (duration of

years). To make the mission duration of airships equivalent, a non-consuming power

source (solar or nuclear) needs to be considered. The type of system that is applicable to a

given mission will depend on the airship’s characteristics and the environment in which it

is to operate.

To achieve long duration flight, not only will innovative power and propulsion systems

be needed but also innovative airship designs that take advantage of the environment in
NASA/CR—2004-213345 2

which they are operating. A long duration airship for planetary exploration will need to

be matched to and take advantage of the environment in which it is to fly.

Planetary Environments

The environment in which the airship will operate will dictate its capabilities and will be

the determining factor in the airship’s feasibility. Items such as atmospheric density,

atmospheric composition, temperature, and solar radiation are the main characteristics

that will influence the airship’s design and mission capabilities.

The initial and most basic environmental requirement for an airship is the presence of an

atmosphere. Of the planets within our solar system 7 of the 9 have atmospheres as well as

5 moons of the giant outer planets. This initial grouping of places with atmospheres is

shown in figure 1.

Figure 1 Planets and Moons within the Solar System with Atmospheres

Of the planets and moons shown in figure 1, not all have atmospheres capable of

supporting the flight of an airship. Of the moons that have atmospheres (Europa, Io,

Ganymede, Titan, and Triton) only Titan can be considered as a location potentially

suited for flying an airship. Europa, Io, Ganymede, and Triton have very tenuous

atmospheres that would not be capable of supporting an airship. The characteristics of the
NASA/CR—2004-213345 3

planets and moons that have atmospheres potentially thick enough to support an airship

are given in table 1.

Planet Gravitational

Force

Temperature Atmosphere

Pressure /

Density at

the Surface

Atmospheric

Composition

Venus 8.93 m/s2 735 °K near

surface to 175°K

upper

atmosphere

92.1 Bar

~65 kg/m3

Carbon Dioxide

~97%

Nitrogen ~3%

Mars 3.73 m/s2 ~220 °K 6.36 mBar

~0.02 kg/m3

Carbon Dioxide

~95%

Nitrogen ~3%

Argon ~2 %

Jupiter 24.82 m/s2 ~165°K >>1000 Bar Hydrogen ~90%

Helium ~10%

Saturn 10.49 m/s2 ~134°K >>1000 Bar Hydrogen ~96%

Helium ~3%

Titan

(moon of

Saturn)

1.35 m/s2 ~100°K near

surface to

~200°K upper

atmosphere

1.5 Bars

5.8 kg/m3

Nitrogen ~95%

Methane ~3%

Argon ~2%

Uranus 9.03 m/s2 ~76°K >> 1000 Bar Hydrogen ~83%

Helium ~15%

Methane ~2%

Neptune 11.57 m/s2 ~72°K >> 1000 Bar Hydrogen ~80%

Helium ~19%

Methane ~1%

Table 1 Atmospheric Characteristics of Candidate Locations for Airship Flight

Atmospheric Environment of Venus

The atmosphere of Venus is very thick, which is a benefit to airship flight. However the

atmosphere is a place of environmental extremes. Near the surface, the atmospheric

temperature is very hot (over 700°K) and there is very little useable sunlight due to the

extensive cloud cover that covers the whole planet. This cloud cover resides within an

altitude range of 45 km to 64 km above the surface. Near the surface, the atmosphere is

very still with little wind. Towards the upper atmosphere the situation is completely

reversed. It is relatively cold with temperatures under 200°K; there is abundant sunlight

and the wind speeds are very high.

The main characteristics of Venus’ atmosphere that are needed to evaluate an airship’s

feasibility include the temperature in °K (T), density in kg/m3

 (r), viscosity in kg/m s (m),

solar attenuation (I/I0), and wind velocity in m/s (V). These quantities are provided as

functions of altitude (h) in kilometers by equations 1 through 8 [1].
NASA/CR—2004-213345 4

T hh h

h Eh Eh

=- + -

+ - -+ -

738 26 9 1909 0 17429 0 007965

0 0001518 1 2336 6 3 7325 9

2 3

45 6

.. . .

.. .

[1]

r = - + - + --

-- -

64 85 3 3257 0 067373 0 00066981 3 224 6

5 6694 9 1 8971 12

2 34

5 6

.. . . .

. .

h h h Eh

Eh E h

[2]

m = -- - + - - -

+ -- - + - -

-+ - - -

3 5827 5 5 906 9 8 8642 7 5 9485 8

2 1929 9 4 8611 11 6 6513 13 5 5001

15 2 5199 17 4 9099 20

2 3

456

67 9

.. . .

.. . .

. .

e Eh Eh Eh

Eh E h E h E

h Eh Eh

[3]

Above the cloud layer there is an abundant amount of solar energy. The solar flux at the

orbit of Venus is 2600 W/m2

, which is much greater than the 1360 W/m2 available at

Earth orbit. This nearly 100% increase in solar flux can significantly increase the

performance of solar powered vehicles. Even within or below the cloud layer there may

be sufficient solar energy to operate an airship. At the bottom of the cloud layer (45 km

altitude), the solar intensity is between 520 W/m2

 and 1300 W/m2 depending on the

wavelength of the radiation being collected. The solar attenuation within the atmosphere

is given by equations 4 and 5.

From 0 to 50 km altitude:

I

I h Eh Eh

Eh Eh

o

= + - -+ -

- -+ -

0 10306 0 017383 7 99 4 2 752 5

5 2011 7 3 874 9

2 3

4 5

.. . .

. .

[4]

From 50 km to 65 km altitude:

I

I h o

=- + 1 3639 0 036023 . . [5]

Above 65 km there is effectively no attenuation and therefore I/I0 has a value of 1.

The winds within the atmosphere blow fairly consistently in the same direction as the

planetary rotation (East to West) over all latitudes and altitudes up to 100 km. Above

100 km, the winds shift to blow from the day side of the planet to the night side. The

wind speeds decrease as a function of altitude from ~100 m/s at the cloud tops (60 km) to

~0.5 m/s at the surface. These high wind speeds and the slow rotation of the planet

produce a super rotation of the atmosphere (nearly 60 times faster then the surface).

From the surface to 58 km altitude:

V hh h h

Eh eh Eh E h

=- - + +

- -+ -- -+ -

0 89941 0 11201 0 017082 0 0040604 0 0010345

9 96 5 3 28 6 4 7 8 2 495 10

234

5 67 8

.. . . .

. . ..

[6]
NASA/CR—2004-213345 5

From 58 km to 66 km altitude:

V hh h =- + - 21498 1087 9 18 31 0 10214 2 3 .. . [7]

From 66 km to 100 km altitude:

V hh h h

Eh Eh

=- + - + -

+ -- -

3860 1 637 42 32 206 0 76199 0 009357

5 783 5 1 42 7

23 4

5 6

.. . . .

. .

[8]

Atmospheric Environment of Mars

The atmosphere of Mars is very thin. Near the surface on Mars the atmospheric density is

similar to the density of Earth’s atmosphere at 30 km. The atmosphere is made up almost

entirely of carbon dioxide. The temperature on Mars is on average much colder then on

Earth. Although at certain times of the year and locations the temperature will rise above

freezing, most of the time temperatures are well below the freezing point of water. The

gravitational force on Mars is about 1/3 what it is on Earth. Therefore the lift that the

airship must generate is only 1/3 what is needed on Earth. This reduced gravity (and

hence lower lift requirement) is a large benefit for operating an airship. However the

environmental conditions on Mars are not all beneficial. The low atmospheric density

poses a significant problem for a lighter then the atmosphere vehicle. There are also a

number of aerodynamics concerns with propeller operation and flight at the very low

Reynolds numbers which will be encountered within this atmospheric environment. Most

of these are based on flow separation that can affect the airship’s stability, control, and

propeller operation.

Similar characteristics of the Martian atmosphere, as those provided for Venus, are given

in equations 9 through 16. These characteristics are needed to evaluate an airship’s

feasibility within the atmospheric environment of Mars.

The reference atmosphere used to estimate the characteristics of the Martian atmosphere

was supplied by JPL.[2] This data was generated for a latitude of -20°. It provides data

on temperature, viscosity, and density from just above the surface to nearly 10 km. From

this data, curve fits were generated for temperature, density, and viscosity as a function of

altitude.

T h hh

hh h h

=- + -

+- + -

238 74 34 488 35 133 15 96

3 7315 0 47352 0 030962 0 000817

2 3

45 6 7

.. . .

.. . .

[9]

r = - + -- - 0 014694 0 001145 4 6638 5 9 7737 7 2 3 .. . . h Eh Eh [10]

m = -- - + - - -

+ -- -+ -

1 2024 5 1 30002 6 1 0525 6 3 6507 7

6 0536 8 4 8317 9 1 4911 10

2 3

45 6

.. . .

. ..

E Eh Eh Eh

Eh Eh E h

[11]

Because of the thinness of the Martian atmosphere, the solar attenuation was assumed to

be constant with a value of 0.85 over the complete altitude range. This attenuation was
NASA/CR—2004-213345 6

due mainly to dust particles within the atmosphere and therefore is subject of significant

variations throughout the Martian year. The incident solar energy available at Mars’s

orbital radius is 590 W/m2.

As with the Earth’s environment, the wind speeds on Mars are highly variable with the

time of year and location. At the Viking landing sites, the wind speeds were in the range

of 2 to 7 m/s. These measurements were, however, taken within the boundary layer near

the surface. Above the boundary layer it is estimated that the wind speeds can approach

50 m/s.

Atmospheric Environment of Titan

Titan’s atmosphere is not as well documented as that of Venus or Mars. Little exploration

of Titan has been performed to date. However, based on the images of Titan that have

been taken by various sources (Voyager, Hubble, and Earth based telescopes), some

properties of the Titan atmosphere can be inferred. The temperature, atmospheric density,

and viscosity are given as functions of altitude in equations 12 to 14 [2].

T h h Eh =- + - - 92 873 1 1415 0 016895 5 3723 5 2 3 .. . . [12]

r =- + - - 5 4627 0 21851 0 00294 1 2054 5 2 3 .. . . h h Eh [13]

m = -- - + - - -

+ -- -

6 439 6 1 5296 7 1 0343 8 3 2876 10

4 9672 12 2 8791 14

2 3

4 5

.. . .

. .

E Eh Eh E h

Eh Eh

[14]

There is little information on the winds within Titan’s atmosphere. It is estimated that the

wind velocities are on the order of tens of meters per second above the surface boundary

layer. Winds within the boundary layer are estimated to increase linearly from 0 at the

surface to 5 m/s at approximately 4 km in altitude [3].

The cloud cover and thick layer of smog within Titan’s atmosphere significantly limits

the amount of sunlight that reaches the surface. The smog is caused by ultraviolet light

from the sun interacting with the abundant hydrocarbon molecules within Titan’s

atmosphere. The solar intensity at the orbit of Saturn is only 14.87 W/m2

. This low solar

intensity coupled with the clouds and smog within Titan’s atmosphere keeps the surface

of Titan fairly dark.

Outer Planets (Jupiter, Saturn, Uranus, Neptune)

The atmospheres for the outer, gas giant, planets are fairly similar. They are very cold in

the upper layers and are expected to warm considerably at lower levels where the

pressure increases. However, there are little details known about the atmospheres of these

planets. On Jupiter, the Galileo probe measured the atmospheric temperature as it

descended. This temperature changed from approximately 165 to 424 °K before the probe

stopped transmitting data, at a pressure of approximately 22 bar. On these planets, the

atmospheric pressure range extends from near vacuum at the upper atmosphere to 1000’s

(or millions) of bar at the lower atmosphere, and very high wind speeds probably extend

deep into the atmosphere. The Galileo probe encountered wind speeds exceeding 400
NASA/CR—2004-213345 7

mph well below the visible clouds on Jupiter. For airship applications however, the fact

that they are mainly composed of hydrogen is the main issue. This basically eliminates

the possibility of using a “lighter then the atmosphere” gas for the ship’s buoyancy.

Airship Evaluation

Based on the environmental conditions specified in the previous section, an evaluation of

the feasibility of using an airship for exploration was performed for each of the potential

locations. The requirements for the airship were that it would need to be capable of

station-keeping at a given location as well as operating for an extended period of time of

50 earth days or more. Two types of propulsion / power systems were considered. These

were a solar photovoltaic system and a nuclear radioisotope system. The isotope system

can be considered for all of the potential planetary locations whereas the photovoltaic

system would only be applicable to Venus and Mars. Beyond Mars the solar intensity is

too low for photovoltaics to be a practical source of power for an airship.

The airship configuration chosen for the analysis was a standard cylindrical shaped with

two motor systems located on pods. This configuration is shown in figure 2.

Figure 2 Base Airship Configuration

To determine the fin area, a number of existing airships were used to establish a ratio of

fin area to airship volume. The ratio of fin area to airship volume (Rf) used for this

analysis was 0.0121 m2

/m3. For the solar powered airship, it is assumed that portions of

the solar array would be located on the upper surface of the lower two fins and on both

sides of the vertical fin.

The power required by the airship is given by the power needed to operate the onboard

systems and payload and the power needed to overcome the drag on the airship and

maintain station over a specified location. The airship drag (D), given in equation 15, is

based on a volumetric drag coefficient (Cdv) and the airship volume (Va) [4]. The volume

can be determined from the length (l) and fineness ratio (f) of the airship. This
NASA/CR—2004-213345 8

relationship is given in equation 16. For this analysis, it was assumed that the airship was

to maintain position with the ability to maneuver. Therefore, the velocity (V) at which it

is operating is the wind speed plus 1 meter per second. The drag equation is broken down

into two terms: the first is the drag due to the airship; the second is the drag due to the

heat exchanger. This heat exchanger drag is based on the cross-sectional area of the heat

exchange (Ahe) and the density of the atmospheric gas (ro) as it exits the heat exchanger.

This is given by equation 17 with a heat exchanger efficiency of 0.80. The drag due to the

heat exchanger assumes a radiator thickness of 2.5 cm, free flow area to frontal area of

0.5, a friction factor of 0.03, and a heat transfer area to volume of 520 [5]. The heat

exchanger drag is mostly a concern for the isotope powered airship since a considerable

amount of heat must be removed to the atmosphere. For the solar powered airship it can

be assumed that this term is zero.

D V CV A dv a he

o

= +- r r

r

2 23 ( . ( . . )) 0 5 3 28 1 72 / [15]

V L

f f as = + p 3

2 3

1

4

4

24 ( ) [16]

r r o

e

T

T T = 08 02 . . +

[17]

The drag coefficient is based on the fineness ratio of the airship. The fineness ratio is the

ratio of the length of the airship to its diameter (d) or width as given by equation 18. For

this analysis a fineness ratio of 4 was assumed. The drag coefficient for this fineness ratio

is 0.0266.

f l

d = [18]

The assumed power requirements for the fixed systems are given in table 2. These power

levels were assumed constant and the same for each airship location examined.

Table 2 Assumed System Power Levels

The operational efficiency of each of the components of the power system must also be

taken into account when sizing the airship. The airship power train is shown in figure 3.

The total efficiency of the drive train (hp), given by equation 19, is composed of the

motor controller efficiency (hmc), electric motor efficiency (hem), gearbox efficiency (hg),

and propeller efficiency (hprop).

h h h hh p mc em g prop = [19]

System Continuous Power Level

Communications 50 W

Control and Operations 50 W

Payload 50 W
NASA/CR—2004-213345 9

Figure 3 Propulsion System Drive Train

The operational efficiency associated with each of these components is given in table 3.

They are combined to get the drive-line efficiency which consists of all components up to

the propeller. The propeller efficiency has to be calculated based on a propeller sizing for

the operational altitude and thrust requirement. These efficiencies are representative

approximations for each of the components under optimized operating conditions.

Component Efficiency

Control Electronics hmc 0.98

Motor hem 0.90

Gearbox hg 0.90

Drive Line Efficiency hp 0.794

Table 3 Drive Line Component Efficiencies
NASA/CR—2004-213345 10

Sizing the propeller is an iterative process that is dependent on the airship flight speed

and thrust requirement. To achieve the desired thrust at the needed airspeed, the propeller

diameter, RPM, and pitch angle are iterated upon. The goal is to provide a combination of

these that maximizes efficiency. For this analysis it was assumed that a variable pitch 2

bladed propeller was utilized [1,6]. Using this analysis, an approximation for the thrust

(ct) and power (cp) coefficients were derived as a function of advance ratio (J). The thrust

and power coefficient equations are valid for advance ratios within the range of 0.18 to

3.0.

c JJ J J t =- + - + - 0 012122 0 14577 0 1408 0 05374 0 0068444 23 4 .... . [20]

c JJ J J p =- + - + - 0 012752 0 094954 0 053694 0 017534 0 0007872 23 4 . . . . . [21]

The advance ratio, given in equation 22, can be expressed in terms of the flight velocity

(V) the speed of sound within the atmosphere (a) and the desired tip Mach number (M).

J V

aM V = -

p

( )2 2 [22]

By selecting a tip Mach number, the propeller efficiency (hprop) can be calculated from

the above equations.

hprop

t

p

c J

c = [23]

Determining the total power required is an iterative process between sizing the airship to

lift its mass and the power needed to overcome drag. As the airship mass gets larger, the

amount of power needed will increase thereby increasing the necessary size of the

airship. The total power (P) in watts required by the airship is given by equation 24.

P DV

p

= +

h

150 [24]

To determine the total power required, the airship volume must be known. The volume is

dependent on the total weight that the airship must lift. The lifting force (La) of an airship

is based on a centuries old principle discovered by Archimedes, “A body wholly or partly

immersed in a fluid is buoyed up with a force equal to the weight of the fluid displaced

by the body”. This principle is given by equation 25 where g is the gravitational force of

the planet on which the airship is to operate. The ratio of the molecular weights of the

lifting gas (MWlg) and the atmosphere (MWatm) is used to account for the weight of the

lifting gas itself.

L V

MW

MW g a a

atm

= - r ( ) lg 1 [25]
NASA/CR—2004-213345 11

To estimate the total mass of the airship, it was broken down into a number of

components plus the payload. Some of these components were assumed to be fixed

masses and did not scale with the operating location or airship size. These fixed mass

components are listed in table 4.

Component Mass (kg)

Flight Control Computer 3.8

Communications Equipment 4.6

Flight Control Sensors 3.5

Payload 20.0

Table 4 Aircraft Fixed Component Masses

Because the masses of some of the components will depend on the airship’s total mass

(Mac) and or power requirement, determining the total mass is an iterative process. The

mass for the structure (Ms) is based on the density of the envelope material (re, assumed

to be 0.25 kg/m2

 ) and assumes that the internal structure scales as 1/4 of the total mass of

the airship [7].

M R V M dL s f as e as e = ++ r pr 1 2 0 25 . . [26]

The electric motor mass (Mem) [8], motor controller mass (Mmc) [8], gearbox mass (Mg)

[8], and mass of the power conditioning system (Mpc) [7] are based on a linear scaling

with power output. The lower limit at which the equations are valid are listed after each

equation. If the required power was sufficiently small so that the calculated mass of any

component was below this minimum then the minimum value was used.

M P em

mc = h

1291 (minimum of 0.5 kg) [27]

M P mc = 6233 (minimum of 0.1 kg) [28]

M P

g

em mc = h h

3278 (minimum of 0.3 kg) [29]

M P

pc =

1000 (minimum of 0.2 kg) [30]

The solar array mass (Msa) is given by the specific mass of the array in kilograms per

meter squared (estimated to be 1 kg/m2) multiplied by 80% of the total area available for

the solar array (Sa).

M S sa a = 0 8. [31]

The total output of the solar array is calculated based on the incident solar radiation on

the array [7]. The incident radiation is dependent on the shape of the array and the

attenuation due to the atmosphere. The array output or power available (Pa) is given in
NASA/CR—2004-213345 12

equation 32. It is based on the mean solar intensity at the planet’s orbit (Iom) in watts per

square meter and the attenuation due to the atmosphere (I/Io).

P SI I

I a a om A o

= sc l l

=

= -

0 8 Â

2

2

. ( ) sin( ) h q

b p

b p

[32]

It is assumed that the array is placed on the upper half of the airship envelope (as shown

in figure 2) and on the tail fin surfaces. The local solar elevation angle (q l) is the

elevation to the sun as seen from a specific segment of the solar array. This angle will

depend on the inclination of the solar array segment (b), the angle of the solar array

segment cells to the horizontal. The calculation of the local solar elevation angle is given

in reference 7. The solar cell efficiency (hsc) is based on the efficiencies of thin film solar

cells and was assumed to be 8% above the clouds on Venus and on Mars and 5% below

the clouds on Venus due to the increased operational temperature.

The isotope power system includes the engine, isotope heat source, and radiator. The

required radiator area (Ahe) is given by equation 33 and the total engine system’s mass

(Mie) is given by equation 34.

A P

T T Vc he

ee a p

= he r ( ) - [33]

M P

P ie A M

s

= + fr sr [34]

For the isotope system, a Stirling engine is used as the thermal to electrical conversion

device. The specific power of the engine (Ps) including the heat source was estimated at 6

W/kg. The radiator size for the engine will depend on the temperature difference between

the incoming ambient temperature (Ta) and the engine heat source temperature (Te),

estimated to be 950 °K, and the total power required by the airship (P). The operational

efficiency of the Stirling engine (he) was estimated at 27%. The radiator specific mass

(Mr

) is estimated to be 11.18 kg/m2 based on present day lightweight radiator designs [9]

and its efficiency (e) was assumed to be 0.8 [5].

The battery shown in figure 3 is used only with a solar powered system. For the isotope

system this component would be eliminated. The battery mass (Mb) is based on a silverzinc rechargeable battery that has the capacity to provide 5 minutes of full operational

power to the airship with a depth of discharge of 80%. The silver-zinc battery was chosen

because of its very high discharge rate capability and its operating temperature range of

–20 to +30°C. This type of battery has an estimated energy density of 150 W-Hr/kg.

Using this energy density and the 5-minute full power capacity, the mass of the battery is

given by equation 35.

MP E b r = - 6 94 4 . [35]
NASA/CR—2004-213345 13

The propeller mass (Mprop) is based on the propeller diameter, given in equation 36, and

the number of blades (nb). With the diameter and number of blades known, the mass of

the propeller will depend on the volume of each blade, its material density (rprop), and the

void percentage within the blade (Fb). Using the airfoil cross-sectional area and the chord

length distribution given in reference 8, the volume of the propeller blade (Vprop) can be

calculated. This volume is given by equation 37 and the total propeller mass is given by

equation 38. For this analysis it was assumed that a carbon composite, with a density of

1380 kg/m3

, is used to construct the propeller and the void percentage within the blade is

50% of the volume.

d D

c aM V p

t

= -

p2

2 2 (( ) ) [36]

V Ed prop p = - 9 25739 5 3 . [37]

M n FV prop prop b b prop = - r ( ) 1 [38]

The last mass item is the margin mass. This is used to account for various miscellaneous

items and is assumed to be 10% of the calculated airship mass. Using this mass margin,

total airship mass (Mas) for the solar powered airship and for the isotope powered airship

is given by equations 39 and 40, respectively.

M MM M MM

M MM

as s em mc g pc

sa b prop

= ++ + + +

+++

( .

) .

31 9

1 1 [39]

M MM M MM

M M

as s em mc g pc

ie prop

= ++ + + +

+ +

( .

) .

31 9

1 1 [40]

Airship for Venus

Both a solar and isotope powered airship can be considered for flight on Venus. For a

solar powered airship, since there is a thick atmosphere and cloud cover, it was assumed

that all solar radiation below the clouds was diffuse. Therefore below the cloud layer,

there was no variation in array output based on array or airship position relative to the

location of the sun.

With the solar powered airship, below the cloud layer, the availability of power is the

driving factor for the airship design. Its size is based on the ability to collect enough

power from the solar array to operate the propulsion system and other ship systems. The

airship is therefore sized for this requirement. Using the mass-scaling relationships given

above and assuming helium as the lifting gas, mass estimates of the airship were made.

Due to the high-density environment of the Venus atmosphere, the lift produced by the

envelope volume was more than sufficient to lift the airship and its associated systems. In

fact much of the envelope would need to be filled with atmospheric gas with only a small

volume utilized by the lighter lifting gas. Because of this, the mass scaling of the airship
NASA/CR—2004-213345 14

is not a critical factor in the analysis. The lifting capacity of an airship as a function of

altitude is shown in figure 4 with helium as the lifting gas. This figure shows the total

mass that can be lifted on Venus as a function of altitude and airship size.

Figure 4 The Lifting Capacity of Various Size Solar Airships vs. Altitude on Venus

Using the analysis outlined above, the power required for maintaining station above a

specific location and the power available from the solar array was determined for airship

sizes up to 20 m in length. Airships larger then 20 m in length were deemed not

reasonable for autonomous deployment and operation within the Venus atmosphere. The

power required and available was calculated from near the surface to an altitude of

50 km. These results are shown in figure 5.

1

1 0

100

1000

10000

100000

0 5 10 15 20 25 30 35 40 45 50

Altitude (km)

Total Lifting Mass (kg) 

5 m Airship Length

10 m Airship Length

15 m Airship Length

20 m Airship Length
NASA/CR—2004-213345 15

Figure 5 Power Available & Power Required for Various Size Solar Airships on Venus

From this figure it can be seen that there is a sharp rise in required power that occurs near

10 km in altitude. This rise is due to the increasing wind speeds that occur with

increasing altitude. Because of this significant rise in the power required to maintain

station, it is not feasible to operate a solar powered airship, of the sizes examined, above

approximately 10 km in altitude due to the high wind speeds that occur at and above this

altitude. Below this altitude, the required airship size necessary to operate within the

Venus environment was determined and is given in figure 6. This figure shows required

airship size for altitudes up to 9 km.

Although low altitude operation was feasible based on the assumptions used, there are

significant issues that would need to be addressed with getting a solar powered airship to

operate near the surface of Venus. The main issue is the high atmospheric temperature.

Significant materials development would be needed for the vehicle to operate for

prolonged periods of time within this high temperature environment.

100

1000

10000

100000

1000000

0 5 10 15 20 25 30 35 40 45 50

Altitude (km)

Power (W)

Power Available 5 m Airship 

Power Available 10 m Airship

Power Available 15 m Airship

Power Available 20 m Airship

Power Required 5 m Airship

Power Required 10 m Airship

Power Required 15 m Airship

Power Required 20 m Airship
NASA/CR—2004-213345 16

 Figure 6 Solar Airship Total Power and Size as a Function of Altitude on Venus

The second and equally critical issue is the operation of solar cells within the

environment near the surface on Venus. Solar cell performance generally increases with

lower operational temperatures. Therefore it may be very difficult to produce any

reasonable efficiency out of a solar array operating at such high temperatures. In addition

to the temperature, the spectrum of light reaching the surface is mostly on the red side of

the spectrum due to the very thick atmosphere. Very little blue light reaches the surface.

However it is this blue portion of the spectrum that most present-day solar cells utilize in

producing power. To take advantage of the light that does reach the surface, a new type

of solar cell would need to be developed.

The method described above was also used to evaluate an isotope-powered airship for

flight on Venus. The results of this analysis are shown in figure 7. Above about

11 kilometers in altitude there were no airship solutions that converged for either mass or

power. Therefore, based on this analysis above 11 km, the isotope-powered airship is not

feasible on Venus for a station-keeping mission.

Below 11 km it was possible to maintain station with a reasonable size airship. Even

though the temperatures are very high near the surface the very thick atmosphere enables

sufficient heat transfer from the isotope engine. At around 10 km in altitude the airship

size and power requirements increase significantly. As with the solar powered airship,

this is due to the increase in wind speed that occurs at these altitudes. For a low altitude

0

2

4

6

8

10

12

123456789

Altitude (km)

Airship Length (m)

0

200

400

600

800

1000

1200

1400

1600

Total Power Required (W)

Airship Length

Total Power Required
NASA/CR—2004-213345 17

exploration vehicle, the isotope-powered airship holds promise for use on Venus. It is

better suited to high temperature operation then the solar powered airship and, as an

additional benefit, its required size is smaller.

Figure 7 Mass, Power & Length of an Isotope-Powered Airship on Venus

Airship for Mars

Due to the low atmospheric density on Mars, designing and flying an airship that relies

on buoyancy for lift is a difficult task. This is especially true for a solar powered airship

because of the low solar intensity at Mars. To meet the station-keeping mission, the

airship would need to collect enough energy during the day to provide power throughout

the nighttime period. This also requires an energy storage system such as a regenerative

fuel cell system. The atmospheric density near the surface of Mars is similar to the

density at 30 km altitude on Earth. It was assumed that the airship, if feasible on Mars,

would operate near the surface. Therefore, the wind velocity used was 2 m/s. This is a

representative wind speed near the Martian surface based on data returned from the

various landers and rovers that have operated on the surface.

Since this airship required an energy storage system, the modeling tool described in

reference 7 was utilized. The modeling tool was designed to evaluate the feasibility of a

high altitude long endurance airship for Earth based applications. However, by modifying

the environmental conditions to those of the Mars environment, it was capable of being

utilized to model a solar powered airship on Mars. Using this modeling tool, no solutions

0

1000

2000

3000

4000

5000

6000

0 2 4 6 8 10 12

Altitude (km)

Mass (kg) and Power (W)

0

2

4

6

8

1 0

1 2

Airship Length (m)

Total Mass (kg)

Required Power (W)

Airship Length (m)
NASA/CR—2004-213345 18

were found for a solar powered airship that could station-keep on Mars, carry the desired

payload, and meet the size restriction of a maximum of 20 m in length.

Similar results were found for the isotope-powered airship. Based on the analysis and

component scaling given above, no solutions were found for airship sizes up to 20 m in

length within the Mars environment.

Airship for Titan

Due to its distance from the sun and the thick haze and cloud cover surrounding the

moon, a solar powered airship would not be feasible within the Titan atmosphere. An

isotope-powered airship, however, should operate well within this environment. Titan’s

environmental conditions are ideal for a lighter then the atmosphere vehicle. It has a thick

atmosphere that is one and a half times as dense as Earth’s and can provide significant

lift. Also the atmosphere is composed mainly of nitrogen that is compatible with almost

all materials. The cold atmospheric temperatures also provide an excellent heat sink for

the Stirling engine thereby increasing its efficiency and minimizing radiator size. The

sizing results for an isotope powered airship on Titan are given in figure 8.

Figure 8 Mass, Power & Length of an Isotope-Powered Airship on Titan

Results were obtained for altitudes up to 7.5 km. Beyond this there were no solutions for

a station-keeping airship, due mainly to the increasing wind velocity. The results for the

0

1000

2000

3000

4000

5000

012345678

Altitude (m)

Mass (kg) and Power (W)

0

4

8

1 2

1 6

2 0

Airship Length (m)

Total Mass (kg)

Required Power (W)

Airship Length (m)
NASA/CR—2004-213345 19

isotope-powered airship on Titan show that there is a wide range of operation with

reasonable sized airships.

Outer Planets

The outer planets present an interesting problem for an airship design. Their atmospheres

are very thick and dense, however they are mostly made of hydrogen. Therefore the

conventional approach of using a less dense gas to provide the buoyancy force is not

possible. The wind speeds within the atmosphere of the outer planets are estimated to be

very high. It would not be practicable to try and produce an airship that can fly faster then

the estimated wind speeds. However since there is no planetary surface in the sense of the

inner planets in which to station-keep above, the ability to station-keep is not as

important. There are two potential approaches that can provide a lighter then the

atmosphere vehicle for flight within these hydrogen atmospheres.

The first is to heat the atmospheric gas within the airship envelope to provide buoyancy.

This is similar to a hot air balloon in which the density of the gas decreases as the

temperature increases providing a buoyancy force. The main obstacle to this approach is

the heat needed to maintain an elevated gas temperature within the airship envelope. The

cold thick atmosphere provides a significant heat sink to the warmer gas within the

airship. Maintaining a high enough gas temperature will require a significant amount of

heat as well as a substantially insulated gas envelope.

The second option is to produce a neutrally buoyant vehicle by utilizing a pressure vessel

and evacuating it. This type of vehicle is more akin to a submarine then to an airship but

should be applicable to operation within the atmospheres of the outer planets. The flight

altitude of this type of craft can be changed by pumping gas into and out of a storage or

ballast tank, similar to how a submarine works. This type of vehicle doesn’t require the

energy expenditure of heating up the lifting gas as in the previous option and therefore

may be more practical.

Both of these concepts are sufficiently different in design and operation from the standard

airship model previously described that they are beyond the scope of this analysis. For a

future analysis, both of these concepts should be considered and evaluated as potential

means of flight within the atmospheres of the outer planets.

Summary

The use of an airship for planetary exploration shows potential for certain locations.

Airships can bring a unique approach to planetary exploration. Their ability to operate

within the atmosphere and maintain station or move slowly over a specific location

provides an unmatched capability for science data gathering and communications. With

this capability comes some significant limitations. The airship’s capabilities and

feasibility are highly dependent on the environmental conditions. Atmospheric density

and wind speed play critical rolls in this feasibility. After examining the potential planets

of operation, both Venus and Titan are likely candidates for airship operation. On both

these locations, the airship’s operational range was limited to the lower region of the

atmosphere (below approximately 10 km in altitude). Airships of reasonable size were

capable of performing the station-keeping mission and carrying the desired payload
NASA/CR—2004-213345 20

within these environments and altitude range. For use in these environments, the airship

would need to be isotope-powered using a Stirling heat engine for thermal to electrical

energy conversion.

The outer, gas giant, planets also pose an interesting application for a neutrally buoyant

vehicle. The most applicable type of vehicle for the exploration of the outer planets

would be more like a submarine than an airship, in that it would be a pressure vessel

evacuated to hold out the atmosphere (like a submarine) instead of generating lift at

atmospheric pressure with a less dense gas, like an airship. The analysis of this type of

neutrally buoyant vehicle was beyond the scope of this work but it is recommended that it

be further investigated in any subsequent analysis.

References

1. Colozza, Anthony, “Solar Powered Flight on Venus,” NASA/CR—2004-213052,

April 2004.

2. Kevzhanovich, Victor, V., Personal Correspondence, JPL, January 2000.

3. Hall, J.L., Kerzhanovich, V.V., Jones, J., Cutts, J., Yavrouian, A., Colozza, A. and

Lorenz, R., “Titan Airship Explorer,” IEEE Aerospace Conference March 2002.

4. McCormick, B.W., Aerodynamics, Aeronautics and Flight Mechanics, John Wiley

and Sons, 1979.

5. Incopera, F.P. and DeWitt, D.P., Fundamentals of Heat and Mass Transfer, John

Wiley and Sons, Inc., 1990.

6. Colozza, A.J., “High Altitude Propeller Design and Analysis Overview,”

NASA/CR—1998-208520, October 1998.

7. Colozza, A.J., “Initial Feasibility Assessment of a High Altitude Long Endurance

Airship,” NASA/CR—2003-212724, December 2003.

8. Colozza, A.J., “Comparison of Mars Aircraft Propulsion Systems,”

NASA/CR—2003-212350, May 2003.

9. Fluidyne product specifications, www.fluidyne.com, June 2004.
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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27

Airships for Planetary Exploration

Anthony Colozza

Venus atmosphere; Solar powered aircraft

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Subject Categories: 07, 44, and 05 Distribution: Nonstandard

Analex Corporation

1100 Apollo Drive

Brook Park, Ohio 44142

Project Manager, Lisa Kohout, Power and On-Board Propulsion Technology Division, NASA Glenn Research Center,

organization code 5420, 216–433–8004.

The feasibility of utilizing an airship for planetary atmospheric exploration was assessed. The environmental conditions

of the planets and moons within our solar system were evaluated to determine their applicability for airship flight.

A station-keeping mission of 50 days in length was used as the baseline mission. Airship sizing was performed utilizing

both solar power and isotope power to meet the baseline mission goal at the selected planetary location. The results show

that an isotope-powered airship is feasible within the lower atmosphere of Venus and Saturn’s moon Titan.