David J. Bents

Glenn Research Center, Cleveland, Ohio

Long-Duration Low- to Medium-Altitude

Solar Electric Airship Concept

NASA/TM—2011-216815

October 2011
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David J. Bents

Glenn Research Center, Cleveland, Ohio

Long-Duration Low- to Medium-Altitude

Solar Electric Airship Concept

NASA/TM—2011-216815

October 2011

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
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7115 Standard Drive

Hanover, MD 21076–1320

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2011-216815 1 

Long-Duration Low- to Medium-Altitude 

Solar Electric Airship Concept 

David J. Bents 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Summary 

This report presents the conceptual design for a solar electric lighter-than-air, unmanned aerial 

vehicle, based on existing technology already reduced to practice, that could carry a 600-kg (1322-lbm) 

payload to altitudes up to 30 kft (9000 m), continuously maintain an airspeed up to 40 kt (21 m/sec), and 

remain in flight for up to 100 days. The design is based on modern nonrigid airship technology, highstrength polymer fabrics and barrier films, and previously demonstrated aerospace electrical power technology, including lightweight photovoltaics and hydrogen-air polymer electrolyte membrane (PEM) fuel 

cells. The vehicle concept exploits the inherent synergy between the use of hydrogen as a lifting gas and 

the use of hydrogen-air PEM fuel-cell technology for onboard solar energy storage. In this report, the air 

vehicle concept is physically characterized and its estimated performance envelope is defined. 

Airship Description 

The air vehicle’s overall configuration is illustrated in Figure 1. This vehicle is of conventional 

nonrigid airship construction, with an outer envelope of semiellipsoidal/cylindrical shape and an external 

keel framework slung below the envelope. The hull has a 15-m diameter and a 110-m overall length. The 

external keel carries all of the onboard systems and hardware, including four swiveling electric-motor 

propeller-drive propulsion pods that are cantilevered outboard on each side fore and aft. Because of the 

directed differential thrust available from the propulsion pods, the hull has no empennage. The vehicle is 

stabilized by the control of its center of gravity and the controlled application of differential thrust to 

damp out oscillations (roll and pitch) as required. Yaw damping is achieved by an approximately 7.33 

length to diameter ratio. The envelope is slightly pressurized over ambient according to conventional 

nonrigid airship practice, and there are seven internal air-filled ballonet chambers fore and aft that ballast 

the ship and control pitch attitude. The envelope is designed to accommodate the pressure rise associated 

with a maximum temperature change of 40 F (22 C) within a 2-hr period plus 24-hr sustained operation at flank speed. The airship hull shape comprises three sections: 

(1) Nose section—half of a prolate spheroid (ellipsoid of revolution) of 15-m minor diameter and 

24-m major diameter, or 12-m leading length 

(2) Midships—a right circular cylinder of 15-m diameter and 80-m axial length 

(3) Tail cone—half of a prolate spheroid (ellipsoid of revolution) of 15-m minor diameter and 36-m 

major diameter, or 18-m trailing length 

The nose and tail sections of the hull are fully exposed, but the center section is covered by a solar 

array blanket. Although slightly less aerodynamically efficient than a fully ellipsoidal shape, the right 

circular cylinder section amidships allows a multiple-series-string photovoltaic array blanket to be draped 

over its length such that all cells in each string (which is oriented lengthwise) will remain uniformly 

illuminated independent of the ship’s orientation. The array blanket provides the ship’s electrical power in 

daylight and also serves as an ultraviolet light barrier over much of the envelope surface, allowing some 

weight reduction in this section. The blanket also performs a structural function, serving as the catenary 

curtain that suspends the external gondola and keel. 
NASA/TM—2011-216815 2 

Envelope 

The outer envelope is the major structural component of the hull, serving as a fixed-volume container 

(17 671 m3

) for the hydrogen lifting gas when it is inflated. The envelope is a laminated assembly of four 

layers: two plies of Kevlar 49 cloth with a 1-mil bonded layer of Kynar (Arkema Inc.) polyvinylidene 

difluoride (PVDF) film between the plies (which are oriented at 45 to each other) plus an additional 

1-mil layer of Kynar film bonded to the interior ply. This produces a flexible laminate of approximately 

230 g/m2

 that can withstand up to 1360-N/cm tensile stress. The envelope midsection, which is shielded 

by the array blanket, has no additional barrier coating; however, there is an outer layer of aluminized 

Mylar (DuPont biaxially-oriented polyethylene terephthalate) applied over the exposed nose and tail cone 

sections for ultraviolet light resistance. This adds 30 g/m2

 to these sections. Over its 15-m diameter, the 

envelope can withstand internal pressurization up to 300-mm Hg (40 kPa), which exceeds the maximum 

operating pressure differential of 90-mm Hg (12 kPa) by more than 3 times. 

The interior ballonet is made of 1.4-oz (47.6-g/m2

) polyester fabric impregnated with 1-mil Kynar 

PVDF film on both sides. Because of the approximately 90 percent of interior volume that they will 

displace when the vehicle is at low altitude, the ballonet chambers cover the entire bottom of the hull. 

They take the form of a partitioned liner bonded to the inner wall of the envelope around its edges, 

conformal to the interior surface. It can best be thought of as an air/hydrogen separator membrane that 

lies rumpled against the bottom of the envelope when no ballast air is present, but that occupies up to 

90 percent of the interior volume when puffed up with air. The ballonet is partitioned into seven chambers 

of roughly equal volume; these are filled and emptied independently to effect trim and balance. The total 

volume of all seven chambers, when fully inflated, is 15 900 m3

. The ballonet does not have to withstand 

pressure loads, but it must be flexible enough to withstand at least two inflation cycles per day over a 

5- to 10-year lifetime while maintaining its effectiveness as a hydrogen-diffusion barrier. The ballonet 

material is much lighter than the Kevlar (DuPont) matrix-reinforced envelope, only 60 g/m2

. 
NASA/TM—2011-216815 3 

The permeation, or diffusion, of hydrogen through both envelope components (outer envelope and 

ballonet) is less than 4.92 cm3

/m2/hr because of the excellent hydrogen-diffusion resistance of Kynar 

PVDF (Ref. 1). Nevertheless, a hydrogen diffusion loss rate of approximately 1 kg/day is expected from 

the envelope’s interior to the surrounding air and the air-filled ballonet chambers. This loss rate is 

mitigated by replacing the lost hydrogen with fresh hydrogen produced by onboard water electrolysis. 

External Gondola and Keel 

The gondola and keel is a semirigid truss structure that carries all the onboard systems (except for the 

solar array and hydrogen storage) and the payload. The truss is approximately 10 m wide, 2 m high, and 

80 m long. It is the primary mounting location for all the system components (except for the solar array), 

including the propulsion and power. The keel is attached to rest of the airship by means of the solar array 

blanket, which is slung over the cylindrical portion of the hull envelope and clipped to attach points 

(electrical connections) at regular intervals along its length. By itself the keel is not stiff enough to remain 

straight over its entire length (although it is strong enough to support distributed loads from the various 

system components). Instead, the keel depends on the pressurized envelope’s stiffness to maintain 

alignment. Loading is limited to less than 500 kg over any 10-m section of the keel’s length, and no single 

point load attached to any frame member exceeds 100 kg. Lacking further mechanical detail, the gondola 

and keel is estimated to weigh no more than 10 percent of the subtotal of components supported, which 

are distributed along the keel to satisfy balance requirements. 

Solar Power and Propulsion 

The airship utilizes solar energy alone for its propulsion and onboard power. It is the inexhaustible 

solar power source that gives this airship the capability for extended-duration flight. Flight duration is 

extended, but not infinite, owing to the inevitable loss of lifting gas from the envelope due to permeation 

and diffusion. Consequently, the onboard electrolysis water supply has to be replenished periodically. The 

nine major components of the power and propulsion system are the 

(1) Solar array 

(2) Water electrolyzer and hydrogen generator 

(3) Onboard hydrogen storage 

(4) Hydrogen induction and pressurization system 

(5) Air induction and pressurization system 

(6) Hydrogen-air fuel-cell powerplant 

(7) Electric motor drive propulsion system 

(8) Thermal management system 

(9) Water-recovery system 

Note that the onboard hydrogen storage and the air induction and pressurization system are also used 

to provide buoyancy for the vehicle. The thermal management system includes payload cooling up to 

1 kWt at 25 C. 

Solar Array 

Primary electrical power generation comes from the photovoltaic array blanket that is draped around 

the airship’s midsection. The array blanket, similar to the solar array that powered the Aerovironment 

“Helios” fixed-wing solar aircraft under NASA’s Environmental Research Aircraft and Sensor 

Technology project several years ago (Ref. 2), is a laminated assembly of identical solar cells arranged in 

a matrix of series strings that are encapsulated and hermetically sealed between two layers of transparent 

 
NASA/TM—2011-216815 4 

polymer film (such as Kapton (DuPont)) to form a semiflexible flat sheet (blanket) that absorbs solar flux 

and that, when connected to an electrical load, delivers direct current at a voltage depending on the 

number of cells in the series strings and the load current being drawn. 

Maximum power is achieved when all the cells in a series string are at an identical angle of incidence, 

and when the incidence angle is near perpendicular. A series cell string’s performance is always limited 

by the cell with the lowest incidence angle, so series strings in the airship array must be oriented axially, 

not circumferentially. Typical individual cell dimensions are 2 to 8 cm per side, with an output voltage of 

roughly one-half volt per cell. Depending on the dimensions available for the array blanket, the output 

voltages for each series string can be tailored from just a few volts to several hundreds of volts. The 

Helios array blankets, which covered an area of 186 m2

 and weighed 92 kg, delivered 214 We/m2 blanket 

under 1-sun illumination (Ref. 2). The corresponding figures of merit were thus 

 Area specific weight, 0.658 kg/m2

 Area specific power, 214 W/m2

 Mass specific power, 3.07 kg/delivered kW 

Similarly, the airship array blanket, which if laid out horizontally would form a flat-plate collector 

40 by 80 m, weighs approximately 2050 kg and delivers up to 684.8 kWe under 1-sun illumination. 

Draped over the airship hull, the blanket takes on the shape of a cylinder over the upper three-fourths of 

the envelope but forms tangential planar segments extending inward from the upper 270 of arc to the 

bottom edges where they attach to the keel. The illuminated active array area can thus be approximated by 

the projected shadow of the hull cylinder, which is a cross section 80 m long and never less than 15 m 

wide. If we neglect the Brewster angle cutoff and assume that the airship always maintains a flight path 

that keeps it within 30 of broadside to the Sun during the day, we could expect the array blanket to 

continuously deliver at least 200 kWe between sunrise and sunset. 

The array blanket is also a structural component, performing double duty as catenary curtain to hold 

the gondola and keel closely underneath the hull envelope. To address this function, the blanket’s sandwich encapsulation encloses aramid fiber tensile members laid vertically at regular intervals between the 

individual solar array strings. The tensile members may be directly connected to the gondola structure, 

perhaps in conjunction with the electrical power leads, which also extend from the edges of this blanket 

and connect to the gondola. An additional weight of 30 g/m2

 is attributed to the airship blanket’s tensile 

member reinforcement. The array blanket is by far the heaviest and most expensive component of the 

propulsion and power system. Any improvements made in specific weight or collection and conversion 

efficiency during the last 7 years since “Helios” would directly translate to better payload and altitude 

capability for the airship than what is presented here. 

Water Electrolyzer and Hydrogen Generator 

A low-pressure polymer electrolyte membrane (PEM) water electrolysis unit, fed by onboard water 

supply, generates hydrogen gas for the airship. The electrolyzer makes hydrogen to replace lifting gas that 

is lost from the envelope through permeation and diffusion, and the hydrogen that is consumed by the fuel 

cell for onboard electric power generation. The low-pressure unit should weigh less than the PEM water 

electrolyzers used on submarines and regenerative fuel cells, since the outlet pressure that is required for 

the airship is only about 5 psig (34 kPa) versus the 200 to 400 psig (1.37 to 2.76 MPa) used in a 

regenerative fuel cell system or the 3000 psig (20 MPa) required for a submarine unit. 

 
NASA/TM—2011-216815 5 

The electrolyzer characterization made here originates from the 15-kWe liquid-anode-feed balancedpressure PEM water-electrolysis stack that Giner, Inc., delivered to NASA in the early spring of 2004 

under the Low Emissions Aero Propulsion project, where the first fully closed loop hydrogen/oxygen 

regenerative fuel cell was successfully demonstrated (Refs. 3 and 4). This particular stack is highlighted 

because it was, despite its 400-psi (2.76-MPa) pressure requirement, designed for minimum weight. 

Capable of absorbing up to 15 kWe and delivering up to 17 g hydrogen/kWh at pressures up to 400 psi 

(2.76 MPa), the stack weighed 16 kg including the end plates, which accounted for 20 percent of its 

weight. For low-pressure operation, a weight reduction of at least 50 percent could be applied to the 

endplates; this would reduce the stack’s specific weight to less than 1 kg/applied kWe with no other 

modifications. 

If that figure of merit is used and the accepted rule of thumb that the ancillary weight of a water 

electrolysis unit roughly equals the stack’s weight is applied, a preliminary weight estimate for the 

electrolyzer system excluding its water supply, yields 350 kg for an electrolyzer unit that can deliver up 

to 2.67 kg hydrogen/hr on a power draw of 157 kWe. 

Because of the liquid anode feed, the hydrogen and oxygen exit streams from the electrolyzer stack 

are saturated with water. In the water-recovery system (described later), there are condenser heat 

exchangers (HXs) that drop each exit stream’s dewpoint from the 140 F (60 C) electrolyzer operating 

temperature to approximately 45 F (5 C) and remove the bulk of this water from the streams. The 

remaining water vapor, up to 24 g/kg hydrogen produced and up to 3 g water vapor/kg oxygen, is further 

reduced by the water-recovery system until both streams have a dewpoint that is no higher than 18 F 

(10 C) above the ambient outside temperature. At typical flight altitudes, this amounts to a water release 

through each stream of less than 500 g/day. 

At this point, the oxygen stream is vented overboard, but any remaining vapor in the hydrogen stream 

escapes to the envelope interior, where it preferentially condenses on the envelope inside walls (colder 

than the ballonet surfaces) and freezes overnight. During the day, however, the Sun’s illumination reliably 

heats the envelope wall above freezing, causing the condensate to melt and run down to the bottom of the 

envelope, where it can be collected and returned to the water reservoir. In direct sunlight, heating of the 

envelope wall is almost instantaneous, even though that heat does not penetrate very quickly into the 

interior. 

Onboard Hydrogen Storage 

The outer envelope is the container for the hydrogen lifting gas and the fuel cell’s hydrogen supply. 

For flight integrity, the envelope must always be pressurized to at least 4-mm Hg to prevent buckling 

(Ref. 5). The additional hydrogen to feed the fuel cell translates to extra hull pressurization and weight. 

This means that the envelope must be stronger and heavier than it would otherwise have to be, but the 

added envelope weight is less than the weight of dedicated pressure tanks, and the overpressurization 

provides additional hull stiffness. The envelope is overpressurized by about 30 mm Hg to provide for a 

minimum of 24 hr of flight at flank speed without solar power input. Since the fuel cell powerplant 

requires an inlet delivery pressure that is approximately 10 to 15 times higher than the available hydrogen 

supply pressure, there is a hydrogen induction compressor and ducting system that draws hydrogen from 

the envelope interior and directs it into the fuel cell powerplant. 

Hydrogen Induction and Pressurization System 

The hydrogen induction system consists of a sealed, electric-motor-driven compressor that draws 

hydrogen from the envelope interior, and a ducting system that directs it into the fuel cell powerplant, at 

rates up to 3 kg/hr at a delivery pressure of 15 psi (101 kPa) over sea level ambient, or approximately 

30 psia. The system is schematically depicted in Figure 2 along with the air-induction system, which is 

described in the next section. 
NASA/TM—2011-216815 6 

Air Induction and Pressurization System 

The air induction and pressurization system is a network of ducting, electrically powered blowers, 

valves, and vents whose function is to ingest ambient air from outside, compress it slightly (to no more 

than 90-mm Hg (120 kPa)), direct the pressurized air into the individual ballonet chambers in a controlled 

fashion, and remove ballast air from the ballonet chambers as required. The system also provides induction air for the fuel cell powerplant, which requires up to 2.5 kg/min at a fairly constant delivery pressure 

of 20 psia (138 kPa absolute). 

Two independently operated compressors are employed. The first unit is a high-volume, low-pressure 

electric-motor-driven blower that produces the 10- to 90-mm Hg overpressure needed to fill the ballonet 

chambers and stiffen the hull. The largest airmass flows, up to 3 kg/min, are associated with filling and 

emptying the ballonet chambers against pressure deltas that normally do not exceed 90-mm Hg (120 kPa). 

This compressor operates intermittently, in conjunction with the ballonet fill and drain valves. The second 

compressor unit, also electric motor driven, ingests ambient air and pressurizes the amount of airflow 

required to normalize the fuel cell powerplant to sea level, at flow rates up to 2.5 kg/min and pressures 

up to 20 psia (138 kPa absolute). This compressor operates more or less continuously at night. The 

2.5-kg/min induction airmass flow ascribed to the fuel cell powerplant may be continuously sustained for 

up to 24 hr. Owing to the mass flows and pressure deltas that the induction system is required to work 

against, the adiabatic compressor power for the ballonet supply is 1.14 kW, whereas up to 5.15 kW is 

required to service the fuel cell powerplant. If a compressor efficiency of 65 percent is assumed, the 

power rating for the high-volume, low-pressure ballonet fill blower is 1.75 kW and that for the 

powerplant induction compressor is 7.9 kW. 
NASA/TM—2011-216815 7 

Hydrogen-Air Fuel Cell Powerplant 

The fuel cell powerplant characterization is adapted from a commercial hydrogen electric transit bus 

unit, in this case the Hydrogenics Corp. HyPM HD Mobility Power Module HD65 hydrogen/air fuel cell 

powerplant (Ref. 6). The hydrogen consumption of this powerplant varies from 30 g/hr/delivered kWe (at 

66 percent of rated power) to 46 g/hr/delivered kWe (at 100 percent of rated power). The fuel cell plant 

accomplishes its internal humidification and exhausts its product water through the exit airstream; the 

product water is returned at the rate of approximately 9 g water/1 g hydrogen consumed. Both the air and 

hydrogen exit streams are routed to the water-recovery system to effect further water removal (lower 

dewpoints) from these streams before they are quit. This powerplant unit is rated to only 5000-ft altitude 

(1200 m) mean sea level (MSL), but at higher altitudes the air-induction system normalizes the unit to 

approximately sea level pressure. 

Electric Motor Drive Propulsion System 

The airship is propeller driven, with four propulsion electric drive motors of 16 kWe each that are 

housed in rotating mast-mounted pods cantilevered outward from the gondola keel at the four corners of 

the ship (shown in Fig. 1). The pods swivel on rotating masts such that propeller thrust can be independently directed over a nearly 360 arc in the vertical plane. The propellers are two-bladed fixed-pitch 

units of composite construction 8 ft (2.44 m) in diameter. The rotating masts allow thruster control of 

pitch attitude and, to a limited degree, lift. Differential thrust is used to control yaw and steer the ship. 

Because of the directed propeller thrust available, there is no need for hull empennage to control attitude. 

Each propulsion pod with its cantilevered rotating mast and associated mechanisms, electric drive motor, 

and propeller weighs approximately 215 lbm (97 kg). 

Thermal Management System 

The thermal management system removes low-grade waste heat from the fuel cell powerplant, water 

electrolyzer, and water-recovery systems and provides controlled temperatures to maintain their required 

internal process conditions. The waste heat is ultimately rejected to the ambient airstream. Depending on 

the operational phase, the thermal management system removes up to 50-kWt waste heat from the electrolyzer and up to 65-kWt waste heat from the fuel cell powerplant at temperatures not exceeding 140 F 

(60 C). The thermal control system also removes up to 200 W heat from the water electrolysis plant’s 

output streams to maintain an exit dewpoint no greater than 18 F (10 C) above local ambient. 

Thermal control is accomplished by an air-cooled two-stage pumped liquid loop system. Primary 

cooling is accomplished by a pumped liquid primary loop circulating coolant (antifreeze solution) through 

a liquid-to-air HX cooled by forced ambient air (electric fan). The primary loop connects to liquid-to-gas 

condenser HXs exchangers “ice catchers” to aftercool the hydrogen and oxygen and exit airstreams to 

their lowest possible temperatures (i.e., below freezing). Then it connects downstream to liquid-to-liquid 

HXs that service the four intermediate cooling loops: 

(1) Liquid water removal from the fuel cell powerplant exit airstream, the electrolyzer hydrogen exit 

stream, and the electrolyzer oxygen exit stream at temperatures between 0 and 5 C 

(2) Payload cooling at temperatures between 25 and 40 C 

(3) Electrolyzer stack cooling via deionized water anode feed at temperatures between 40 and 60 C. 

(4) Fuel cell powerplant cooling at temperatures between 40 and 60 C 

The intermediate cooling loops circulate liquid coolant and deionized water at constant flow rates 

through intermediate temperature cold plates, the powerplant’s fuel cell stack and the electrolyzer stack, 

then back to the liquid-to-liquid HXs that interface the primary loop. Although the primary loop temperature is not controlled (it tracks ambient air temperature), the intermediate loop temperatures are thermostatically controlled by means of bypass loops. 
NASA/TM—2011-216815 8 

In addition to removing waste heat, the thermal management system heats the fuel cell and electrolyzer stacks when they are not active, keeping them at near operational temperatures to minimize startup 

times. Figure 3 is a block diagram depicting the thermal-management and water-recovery systems and the 

basic interrelationships with the other onboard systems. 

Water-Recovery System 

Ancillary to the thermal-control and air-induction systems, there is a water-circulation and waterstorage system that accumulates product water from the fuel cell powerplant, collects electrolysis 

condensate that escaped to the interior hull and was subsequently melted, and provides an electrolyzer 

feedwater storage reservoir of at least 500 liters capacity (and not to exceed 4000 liters). Product water is 

removed from the powerplant via an exit airflow condenser HX (serviced by the thermal management 

system’s water-removal intermediate loop), which drops the local dewpoint to approximately 40 F (4.88 

C), extracting roughly 140-g product water/kg exit air as liquid. The exit airstream is then dried further 

by means of an “ice catcher” low-temperature HX (serviced by the low-temperature primary loop), which 

drops the airstream’s dewpoint to approximately 10 C above local ambient, thereby removing up to 14 g 

additional product water/kg air in the form of frost, before it is injected into the ballonet inlet. Meanwhile, 
NASA/TM—2011-216815 9 

the frost accumulates in the “ice catcher” HX until the next day when it is melted out by the application of 

the high-temperature intermediate loop during electrolysis and is returned to the reservoir as liquid. Water 

that remains in (i.e., is not recovered from) the exit airstream represents a loss of less than 3 kg/day. 

The water-recovery system also removes and collects water directly from the electrolyzer hydrogen 

and oxygen output streams. The hydrogen gas is vented directly into the envelope after most of the water 

has been extracted. The oxygen gas is similarly dried but ultimately vented overboard, with a loss of less 

than 500 g/day. The system also delivers deionized feedwater to the electrolyzer, incorporating a oncethrough pass through a polishing bed that removes contaminants from the feedwater stream, ensuring 

sufficient water purity (better than 10 M-cm) for continuous electrolyzer operation. 

Weight and Balance 

Table I summarizes the major components that compose the empty weight of the airship: 

TABLE I.—WEIGHTS OF MAJOR AIRSHIP 

COMPONENTS 

Component Weight, 

kg 

External envelope 

 Nose section 128 

 Center section 848 

 Tail cone 180 

 Internal ballonet 220 

Solar array blanket with tensile members 2147 

Gondola and keel 

 Truss structure 380 

 Water electrolysis units 350 

 Induction air pressurization 220 

 Induction air distribution 50 

 Hydrogen induction 150 

 Fuel cell powerplant 321 

 Electrical power distribution 65 

 Electric drive thrusters (four) 390 

 Thermal management 150 

 Water recovery 100 

 Water storage tanks 100 

 Command, communication, and control 20 

Gondola keel subtotal 2295 

Airship total (empty weight) 5820 

Table I thus summarizes the “empty weight” of the entire airship sans payload, consumables, air 

ballast, and hydrogen lifting gas. 

Table II summarizes the gross takeoff weight of the vehicle for five different missions starting with 

the airship empty weight depicted in Table I. For each mission, the following weights must be added to 

the empty weight: 

 Consumables (water) 

 Payload 

 Hydrogen inventory 

 Air ballast 

 
NASA/TM—2011-216815 10 

TABLE II.—WEIGHT DISTRIBUTIONS FOR FIVE DIFFERENT MISSIONS 

Mission altitude, m 3000 6000 7600 9000 10 000 

Mission duration, day 1 109 26 26 1 

Airship empty weight, kg 5820 5820 5820 5820 5820 

Hydrogen inventory, kg 610 610 612 580 516 

Air ballast, kg 7052 2885 1014 82 50 

 Consumables, kg 288 800 402 400 288 

 Payload, kg 1412 900 1297 900 212 

Consumables plus payload, kg 1700 1700 1700 1300 500 

Consumables and payload is the “useful load,” whereas the weight of ambient air displaced by the 

airship’s fixed volume is basically a function of altitude and temperature. Generally, the removal of air 

ballast before flight would buy more useful load (at the expense of flight altitude) until the point where 

structural limits prevail is reached. Within those limits, there is a payload/endurance tradeoff such that 

removal of water from the reservoir allows more payload to be carried, but at the expense of flight 

endurance. The minimum water inventory required to effect a day/night cycle is approximately 288 kg. 

Operational Scenario 

Two flight operations are illustrated: flight to service altitude and cruise flight at service altitude. 

Flight to Service Altitude 

Table III depicts the airship rising from a launch altitude of 1200 ft (366 m) to service altitudes of 

10 kft MSL (3010 m), 20-kft MSL (6096 m), 25-kft MSL (7600 m), and 29-kft MSL (9000 m), 

respectively, in calm conditions at U.S. Standard Atmosphere. Table III shows how positive buoyancy is 

maintained within the fixed envelope volume by the uptake and discharge of air ballast as the ship rises. 

At liftoff, the onboard air ballast occupies roughly the same envelope volume as the hydrogen inventory. 

By the end of climb, however, the air ballast load is significantly reduced, its volume superceded by the 

expanding hydrogen charge. The 1700-kg useful load (restricted by gondola keel structural limits) that 

could be carried to a 7600-m altitude is diminished to only 500 kg at 10 000 m. 

TABLE III.—SOLAR-POWERED HYDROGEN AIRSHIP, ASCENDING FLIGHT PROFILES 

[At neutral buoyancy.] 

Flight to service 

altitude from 366 m 

Outside air 

temperature, 

K 

Internal 

temperature,

K 

Envelope

P, 

mm Hg 

Hydrogen 

mass, 

kg 

Ballast 

airmass, 

kg 

Consumables 

plus payload, 

kg 

Displaced 

airmass, 

kg 

Start at 366 m 286 286 8.3 610 11 639 7 520 19 769 

Level off at 3000 m 270 284 38.7 610 7 052 7 520 15 183 

Temp. eq. at 3000 ma 270 270 7.8 610 7 052 7 520 15 183 

Start at 366 m 286 286 8.4 610 11 639 7 520 19 769 

Level off at 3000 m 249 270 37 610 2 885 7 520 11 015 

Temp. eq. at 3000 ma 249 249 7.3 610 2 885 7 520 11 015 

Start at 366 m 286 286 9.5 612 11 645 7 520 19 769 

Level off at 7600 m 239 268 43 612 1 019 7 520 9 152 

Temp. eq. at 7600 ma 239 239 7.5 612 1 014 7 520 9 152 

Start at 366 m 286 286 9.1 580 12 070 7 120 19 769 

Level off at 9000 m 229 260 38.9 580 85 7 120 7 785 

Temp. eq. at 9000 ma 229 229 7.2 580 82 7 120 7 785

Start at 366 m 286 286 8.6 516 12 933 6 320 19 769 

Level off at 10 000 m 223 255 35.8 516 55 6 320 6 889 

Temp. eq. at 10 000 ma 223 223 6.6 516 50 6 320 6 889 a

Temp. eq. indicates temperature equilibrium. 
NASA/TM—2011-216815 11 

Cruise Flight at Service Altitude, Through One Complete Day/Night Cycle 

and Associated Air Temperature Variations 

Table IV depicts the airship cruising at a fixed altitude of 29.53 kft MSL (9000 m) while experiencing 

air-density changes associated with diurnal cycle temperature variations (Ref. 7, temperate latitude). In 

flight, ambient air temperature variations are the most stressful to the envelope and air-induction system, 

and require the most rapid ballast airflow rates to maintain neutral buoyancy. 

TABLE IV.—SOLAR-POWERED HYDROGEN AIRSHIP, FIXED-ALTITUDE PROFILE 

[Cruise at neutral buoyancy at an altitude of 9000 m.] 

Day/night cycle Outside air 

temperature, 

K 

Internal 

temperature,

K 

Hydrogen 

mass, 

kg 

Water 

mass, 

kg 

Ballast 

airmass,

kg 

Airship gross 

weight, 

kg 

Envelope 

P, 

mm Hg 

Mid-afternoon, 1st day 231.33 231.33 674.666 196 319.21 6735 54.612 

Late afternoon, 1st day 231.33 231.33 680 147.9 361.68 6687 58.055 

Just after sunset, 1st day 230.22 231.33 684 111.9 431.824 6651 61.777 

Late evening, 1st day 229.67 230.78 674.666 195 375.38 6735 55.597 

Midnight, begin 2nd day 228.56 229.67 669.332 242.6 369.9 6783 51.897 

Early morning, 2nd day 228.56 228.56 661.332 313.8 306.88 6855 45.451 

Predawn, 2nd day 228.56 228.56 654.666 373.2 253.45 6915 41.19 

Just after sunrise, 2nd day 230.22 228.56 652 397 175.36 6939 37.816 

Early morning, 2nd day 230.78 229.11 654.666 72.8 178.1 6914.9 39.626 

Mid-morning, 2nd day 231.33 229.67 661.332 312.6 213.72 6854.7 44.036 

Noon, 2nd day 231.33 230.78 666.666 264.3 255.916 6806.5 48.789 

Midafternoon, 2nd day 231.33 231.33 674.666 192 320.58 6734.3 54.653 

Late afternoon, 2nd day 231.33 231.33 680 143.8 363.05 6686.2 58.095 

Flying into a cold front can change ambient temperatures by 30 to 40 F (17 to 25 C) in less than 

1 hr, but the airship’s thermal response (within the envelope interior) will take more than 4 hr to equalize. 

The fastest heating occurs immediately after sunrise when direct sunlight warms the envelope walls to 

temperatures well above freezing, inducing a temperature rise of 15 to 20 F/hr (8 to 12 C/hr) in the 

previously equalized predawn interior. 

In addition to the envelope pressure variations, notice how the vehicle’s gross weight constantly 

varies over the day/night cycle as oxygen is taken from the water and thrown overboard during the day, 

but returned at night when the fuel cell ingests atmospheric oxygen, replacing water that was consumed 

the previous day. Because of the steady hydrogen leakage diffusion losses, plus the daily losses of water 

from the fuel cell powerplant air exit and the overboard oxygen stream, the weight of water left in the 

reservoir is slowly reduced until there is not enough left to make a full charge of hydrogen: this is where 

the flight must come to an end. 

Performance 

The airship’s steady-state level flight performance is a balance between the propulsive thrust that can 

be indefinitely maintained from limited solar power, and the drag forces arising from its 4980-m2

 wetted 

area, which with its 7.33 length-to-diameter ratio, is reasonably well streamlined. For this vehicle in its 

anticipated flight regime of sea level to 30 kft, the (length-based) Reynolds number range is approximately 40 to 130 million, which results in wetted-area drag coefficients in the neighborhood of 0.003 to 

0.004 for the ellipsoidal/cylindrical shape (Ref. 8). At 65-kWe maximum propulsive power, the top sea 

level speed achievable is 34 kt (17.5 m/sec), but this cannot be sustained for more than 24 hr. The highest 

sea level speed that can be continuously maintained (12 hr/12-hr day/night cycle) is 30 kt (15.5 m/sec). 

 
NASA/TM—2011-216815 12 

Table V summarizes the steady-state level flight performance at a few different altitudes, including a 

payload versus endurance tradeoff. This summary assumes a 12 hr/12 hr day/night cycle with no clouds. 

Flight endurance is governed by the amount of onboard water and the rate at which it is consumed to 

replace lifting gas that was lost from the envelope and water that was not recovered from the overboard 

gas streams. The last two rows of Table V illustrate the limits of payload and endurance. The term 

“ten-day endurance payload capacity” refers to payload capacity when water sufficient for a 10-day flight 

is included in the useful load. “Zero payload cruise endurance” assumes that the entire available useful 

load was dedicated to onboard water inventory. Within limits, payload can be traded for endurance. For 

example, if a mission to 9000 m requires 100 days endurance, the payload would be 600 kg. 

TABLE V.—SOLAR HYDROGEN AIRSHIP, STEADY-STATE FLIGHT 

PERFORMANCE SUMMARY 

Cruise altitude, m MSL 3000 6000 7600 9000 10 000 

Sustained true airspeed, m/sec 17 19 20 21.3 22.7 

Flank speed, m/sec 19.3 21.4 22.6 24 25.5 

Diurnal temperature variation,a C 4.4 2.5 2.8 2.8 2.8 

Diurnal envelope P variation, mm Hg 33 26 24.2 23.7 23.2 

Initial payload plus consumables, kg 

Diurnal water consumption rate, liter/day 

1700 

5.1 

1700 

4.5 

1700 

4.2 

1300 

4.1 

500 

4.1 

Ten-day endurance payload capacity, kg 1361 1367 1370 971 171 

Zero payload cruise endurance, days 277 314 336 247 52 

a

U.S. Standard Atmosphere Supplement 1966. 

Concluding Remarks 

The vehicle concept for an unmanned solar-powered hydrogen airship was presented, with its 

physical characteristics and potential mission performance roughly estimated. The concept juxtaposes 

previously demonstrated aerospace electrical power technology, including lightweight photovoltaics and 

hydrogen/air polymer electrolyte membrane (PEM) fuel cells, with modern nonrigid airship technology, 

high-strength polymer fabrics, and barrier films. 

On the basis of analysis and characterization estimates presented, the concept appears to be feasible, 

and the costs for its construction and operation can be estimated straightforwardly with a reasonably high 

degree of confidence. The major barrier to the implementation of this concept is cost of the photovoltaic 

array blanket, which is by far the most expensive component of the propulsion and power system. The 

Helios solar array blanket costs approximately $30 million; the acquisition cost for the airship array 

would be approximately $500 million. 

References 

1. Blake, D.M.; and Kennedy, C.: Hydrogen Reactor Development and Design for Photofermentation and Photolytic Processes. DOE Hydrogen Program 2005 Annual Progress Report, National 

Renewable Energy Laboratory, Golden, CO, 2005, pp. 236–240. 

2. ERAST Helios Design Review. Dryden Flight Research Center, Sept. 15, 1999, unpublished 

internal NASA report. 

3. Bents, David J., et al.: Hydrogen-Oxygen PEM Regenerative Fuel Cell Development at the 

NASA Glenn Research Center. NASA/TM—2005-214032, 2005. http://gltrs.grc.nasa.gov/ 

4. Garcia, Christopher P., et al.: Round Trip Energy Efficiency of NASA Glenn Regenerative Fuel 

Cell System. NASA/TM—2006-214054, 2006. http://gltrs.grc.nasa.gov/ 

5. Freudenrich, Craig: How Blimps Work. HowStuffWorks, Inc., Discovery Communications, New 

York, NY, 2008. http://science.howstuffworks.com/transport/flight/modern/blimp.htm Accessed 

June 2009. 
NASA/TM—2011-216815 13 

6. HyPM Fuel Cell Power Modules. HyPM HD Mobility Power Module. Hydrogenics Corporation, 

5985 McLaughlin Road, Mississauga, Ont, Canada L5R 1B8. 

http://www.hydrogenics.com/assets/pdfs/HyPM%20HD-XR-2009-Brochure.pdf 

7. U.S. Standard Atmosphere Supplements, 1966. Superintendant of Documents, U.S. Government 

Printing Office, 1967. 

8. Hoerner, Sighard F.: Fluid Dynamic Drag, Second ed., ch. XIV, 1965, p. 14–1. 

Bibliography 

Powell, Peter C.: Engineering with Fibre-Polymer Laminates. Chapman & Hall, London, 1994. 

Perry, Robert H.; Chilton, Cecil H.; and Kirkpatrick, Sidney D.: Chemical Engineer’s Handbook. 

McGraw-Hill, New York, NY, 1963. 

Bain, A., et al.: Sourcebook for Hydrogen Applications. National Renewable Energy Laboratory, Golden, 

CO, 1998. 
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Long-Duration Low- to Medium-Altitude Solar Electric Airship Concept 

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Bents, David, J. 

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14. ABSTRACT

This report presents the conceptual design for a solar electric lighter-than-air, unmanned aerial vehicle, based on existing technology already 

reduced to practice, that could carry a 600-kg (1322-lbm) payload to altitudes up to 30 kft (9000 m), continuously maintain an airspeed up to 

40 kt (21 m/sec), and remain in flight for up to 100 days. The design is based on modern nonrigid airship technology, high-strength polymer 

fabrics and barrier films, and previously demonstrated aerospace electrical power technology, including lightweight photovoltaics and 

hydrogen-air polymer electrolyte membrane (PEM) fuel cells. The vehicle concept exploits the inherent synergy between the use of 

hydrogen as a lifting gas and the use of hydrogen-air PEM fuel-cell technology for onboard solar energy storage. In this report, the air 

vehicle concept is physically characterized and its estimated performance envelope is defined.

15. SUBJECT TERMS

Airships; Hydrogen; Photovoltaic conversion; Electrolysis; Fuel cell 

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