electrical power technologies for spacecraft: options and issues

8/10/2019 Electrical Power Technologies for Spacecraft: Options and Issues

1/18

Copyright 1997, American Institute

of

Aeronautics

and

Astronautics, Inc.

Electrical Power TechnologiesforSpacecraft Op tionsandIssues

GaryL. Bennett*

Metaspace Enterprises

5000 ButteRoad

Etnm ett, Idaho 83617-9500

Abstract

Some of the principal new electrical power

technologies

for

spacecraft will

be

discussed.

Specifically,

new

solar array designs, photovoltaic

cells,batteries, and po wer m anagem ent and distribution

(PMAD) options will be discussed. Developments in

radioisotope power sources for outer planet missions

will be covered. For each of the principal technologies

for power sources (solar, nuclear), energy storage

(batteries, fuel cells, capacitors, flywheels) and PMAD

the ma in options will be discussed. Issues, both

technical and political/economic, will be highlighted.

Overall,

the

case will

be

made that there

are

some

exciting options available which can greatly improve

the performance of commercial, science, and military

spacecraft.

Introduction

Electrical power, like onboard propulsion and

structures, is an essential sp acecraft technology and can

be thought of as a

"utility".

Th e spacecraft will no t

function withoutit Intoday's competitive environment

where

spacecraft owners

are

trying

to get the

most

for

their money an d scientists are trying to get the most

payload

possible on the

smallest spacecraft there

is a

strong m otivation to reduce the m asses of these utilities.

Depending upon

the

mission,

the

electrical power

system can

consume

as

much

as 25

percent

of the

mass

of a spacecraft. Fortunately, there are a number of

exciting

new

electrical power technologies which will

enable just such reductions, sometimes

to as

much

as

50% ofstate-of-practice electrical power systems. This

paper, which builds upon an earlier general article

1

,

will discuss some

of

these options

and

some

of the

issues facing

future

electrical power systems.

Types

of

Spacecraft Electric Power Sources

The

classic

spacecraft electrical power system (EPS),as

illustrated

in

Figure

1,

consists

of a

power source

(usually photovoltaic

but

sometimes nuclear),

an

energy

storage device (usually rechargeable batteries bu t

sometimes fuel cellswith afuture possibility of using

Copyright 1998 by Gary L.

Bennett.

Published by the

American Institute

of

Aeronautics

and

Astronautics, Inc., w ith

permission.

*Fellow,

AIAA

capacitors an d flywheels), and the power management

and distribution (PMAD) subsystem (sometimes

referred

to as power conditioning and control

rt

subsystem or PCCS). Figur e 2 shows

diagrammatically

the

power source

an d

energy storage

options

for

spacecraft.

The

choice

of the

subsystems comprising

theelectrical

power system is largely dictated by the mission

requirements. A

qualitative overview

of the

trade space

can be seen in Figure 3 which is discussed below:

Photovoltaic power sources provide a long-term

source of

power

at a

known degradation rate.

Photovoltaic power sources coupled with rechargeable

batteries have been the usual choice for spacecraft

operating

in the

inner Solar System.

For short missions, energy storage systems may be

sufficient

to power the space system.

This

was the

practice in theearly d aysof thespace programan dthis

is how the crewed U.S. missions (e.g., Mercury,

Gemini,Apollo, Space Shuttle)

are

powered.

Nuclear power sources provide

a

long-term source

of

power and are very attractive for missions operating

where there is very little sunlight (e.g., outer Solar

System, polar regions

of

Mars, lunar nights, surface

o f

Venus)

or in hostile environments

(e.g.,

radiation

belts,

veryclose

to the

Sun).

Thechoiceof apower source shouldnot beseenas an

"either-or"choice, as spacecraft h ave been flown using

all three power sources (solar cells, batteries, and

radioisotope therm oelectric generators).

Generally, the parameter of principal interest for solar

power

sources and nuclear pow er sources is the specific

power,

i.e., watts per kilogram (We/kg). Fo r certain

applications

(e.g.,

low-Earth orbit, LEO) involving

solar power sources

the

area power density (watts

per

square meter

of the

solar array

or

We/m

2

)

is

also

important.

The parameters of interest for energy storage devices

(batteries, fuel

cells,

flywh eels, capacitors) include

specificenergy (watt-hoursperkilogram, We-h/kg)and

energyden sity (wa tt-hours per liter, We-h/1).


2/18

Copyright

1997,

American

Institute

ofA eronau t icsand

As t ronau t ics , Inc.

Some General

Issues

in

Spacecraft

Power Systems

As with any technology there areboth technical an d

political issues facing the spacecraft designer when

deciding on the elements of an electrical power

subsystem. The designer

will want

the highest

performance with thelowestmass andcost (including

life-cycle costs) bu tsafety an dre liability mustalsobe

considered.

With the highcostof spacecraft and launch

vehicles,

the

electrical

power system

(EPS)

must

be

highly

reliable.

Itmustbefault tolerant so

that

it does

no tcausealossof spacecraft functions.

Fault

tolerance

is essentialto any mission involvinghumans.

The twin goals of high reliability and

reduced

EP S

mass

pose areal challenge for

designers.

Historically

they

have

opted for conservative, proven

electrical

power subsystems

with

highermassescompared to the

newer, lower-mass technologies

which

often were

untested in

space.

This tensionbetween theso-called

"proven" technologies and the newer, better

technologies shouldbe seen as an opportunity for the

spacecraft

designers

and the

technologists

to work

cooperatively

to

realize

thebenefitsof

these lower

mass

technologies. (As an

aside,

it is

worth noting

that

sometimes

so-called"proven"

technologies

cease

to be

proven,

forcing

spacecraft designers to opt for newer

technologies that can be built. An example of this

situation occurred several years

ago when

problems

surfaced with

the

"NASA-Standard" nickel-cadmium

cells

thereby advancing the

newer

nickel-hydrogen

battery

technology.

3

)

For some applications growth potential is also

important. Can the

electrical

power subsystem grow

with anevolving

space

system

(involving,

fo rexample,

a humanpresenceon the

Moon

or

Mars)?

With these issues in

mind let's

consider

some

of the

power sources, energy storage devices and PMAD

designs. The reader interested in additional

technologies or more technical information is

referred

to the September-October 1996 issue (Volume 12,

Number

5) of theJournal o f

Propulsionand Power

an d

the

annual

Proceedings of the Intersociety Energy

Convers ion

Engineering

Conference

(TECEC).

Solar

Power

Sources

For the purposes of this paper we will consider two

principal

types

of

solarpower sources according

to the

system

used

to convert sunlight

into electricity

(see

Figure2):

Static,

i.e.,photovoltaic

Dynamic

PhotovoltaicSolar

Arrays

Th e

classic solar array

used

on most spacecraft is

usually composed

of a

large number

of

solar cells

mounted

on a

honeycomb

or fiberglass substrate. The

solar

cells are

connected

into the

appropriate

series-parallel electrical circuits

to

produce needed

power

and required

voltages

and curren ts. Toachieve

higher performance such

as a higher specific power

(watts/kilogram)meansusinghigh

efficiency

solar

cells

or

possibly

lower mass

cells

in combination with

low-massarray

materials.

The typical silicon

(Si)

solar

cell

in use today

(including those planned for use on the International

Space Station)

have efficiencies

on the order of 14%

although some promising overseas work reports that

18%-efficient

silicon solar

cells have

been

made.

Newer

cells

suchasgallium

arsenide

(GaAs),whichis

rapidlybecoming the

technology

o fchoice,and

indium

phosphide (InP) offer higher efficiencies (on theorder

of

18%)

an dbetter tolerancetoionizing

radiation

but at

increased

cost

(although as

thesenewercells

are

used

morethecostsare comingdown).

5

'

6

Galliumarsenide

cellswereused onM ars Pathfinder

(cruise

stage,lander

and Sojourner

rover),

an dtheyare in use on the NEAR

(Near Earth

Asteroid Rendezvous) spacecraft, and on

two

of the four panels on

Mars

Global Surveyor.

7

Other types of cells include

InP/InGaAs,

AlGaAs/Si,

GaAs/InGaAs, Al/GaAs/GaAs, InGaAs/InP,

AlGaAs/GaAs-InP/InGaAs, AlGaAs/GaAs/InGaAsP,

GaAs/GaSb, GaInP

2

/GaAs, GalnP/GaAs/Ge,

CuInGaSe^andZnO/CdS/uiP.

2

'

4

-

5

-

6

One

way

around

the

cost problem

is to use

less

expensive optical

systemstocon centratethe

sunlight

on

a

smaller

(hence,

less

expensive)cell. Th e

smaller

cells

are

easier

to

shield against ionizing radiation (e.g.,

protons and

electrons)

such as that found in the Van

Alien radiation

belts.

Experiments with combinations

of

cells

stacked

together

(e.g.

gallium arsenide on

gallium

antimonide) and

concentrators have

shown

efficiencies close

to 30% at 40

suns. Pointing

concentrator cells

toward the Sun

requires

more

attention

to

attitude

control

thanplanarsolararrays

do .

Efficiency is not the whole story, however. Under

NASA

and JPL

sponsorship,

TR Wdevelopeda flexible

blanket array

termed

the advanced photovoltaic solar

array (APSA) which canprovide a specific

power

of

130 We/kg using 14%-efficient thin silicon solar

cells

in

geosynchronous

Earth

orbit (GEO)

for

12-kWe

applications at beginning of life/* For comparison,

state-of-practice arrays

have

typically provided about


3/18

Copyright1997,American Instituteof Aeronautics andAstronaut ics, Inc.

10-25

We/kg.

A variation of

APSA

technology using

GaAs/Ge solarcellsinstead of S icellswillfind itsfirst

application on the Earth Observing

System

(EOS)

AM-1 spacecraft.

10

Earlier

flexible

blanket arrays

include

Milstar,

O lympus-1,

ERS-1,

a nd

Hubble Space

Telescope.

Th e International

Space

Station

will also

use

a

flexible blanket

array.

4

To carry

over

the benefits of

flexible

blanketarrays to

lower powers, AEC-Able

Engineering

Company has

developed the Ultraflex array which

uses

a round,

antenna-like

structure to put the blanket in tension.

Ultraflex promises specific powers

of

>100We/kg

for

1- to

2-kilowatt arrays.

11

The Department of Defense (DoD) has sponsored

development by AEC-Able Engineering of a

concentrator

array

known as

SCARLET

(solar

concentrator array with refractive linear element

technology) that is

based

on a

low-mass, low-cost

Fresnel

concentrator

system developed

originally by

Entech

underNASAand DoD

sponsorship.

SCARLET

offers the

potential

for array

specific

powersup to 100

We/kg.

SCARLET

will

be used to power NASA'sfirst

New Millennium spacecraft

(known

asDeep Space 1)

which

is being designed to test a number of new

technologies in an asteroid an d comet flyby mission.

(For

Deep

Space 1,becauseof the

tight schedule,

the

array will have

a

specific power

of 48

We/kg using

24%-efficient

solar cells made of gallium-indium

phosphide and gallium

arsenide

on a germanium

substrate (GaInP

2

/GaAs/Ge) but

this

is

still

about

twice

the

specific power

fo r

state-of-practice

arrays and a

factorof

seven reduction

in the

requiredsolar

cellarea.)

Deep Space

1

will

be the

first spacecraft

to be

powered

solely by multi-bandgap

solar

cells and by a

concentrator array. Deep Space

1

will show

the

synergy of power and propulsion because the2.6-kW e

SCARLET array will be used to power an ion

propulsion

system.

An innovative concentrator

array

that uses thin-film

reflectors toprovideabout1.5 concentration ofsunlight

ha s been developed by Astro

Aerospace

with USAF

and NASA

sponsorship.

The overall

array,

whichhas a

channel-like appearance, was

planned

to be used on

NASA's Clark spacecraft which was part of NASA's

Small Satellite Technology Initiative (SSTT).

This

array, termed Astro-Edge, promises over 50 We/kg.

Th e

Naval

Research

Laboratory

is

developing

a

similar

concept

with a concentration ratio of 2.5 that is

predicted to

produce

about100W e/kg.

4

Another innovative

approach

to array

design

is to use

inflatable cylindrical

tubes to deploy the

array instead

of

a

mast

or set of

rigid

panels. Fo r

certain

applications,

specific

powers>140We/kga re

expected.

Using

this

technology,

L'Garde,

Inc. ha s proposed a

concept,

termed

the Power Antenna,

that

combines

three functions in one structure:

electrical

power,

telecommunications,

and thermal management.

Another way to

reduce

themass of a solar array is to

use shape

memory alloy devices

an d ultra-light

composite materials with

thin-film

copper

indium

diselenide

photovoltaics. This technology,

which

is

being considered for thefirstEarth orbiting

mission

of

the New

Millennium

program, may yield specific

powers >150

W e/kg.

13

In fact, thin-film arrays

offer

the

potential

of

specific powers

in the

range

of

kilowatts

perkilogram .

5

The choice of

solar

array technology

depends

upon a

number of factors including the power

level

(some

innovativedesigns providetheirlargest

specific

powers

at

higher powers rather than lower powers); stowage

volume;

an d

environmental

conditions (including

ionizing radiation,

space

charging

effects,

atomic

oxygen exposure,

etc.).

Regardlessof the

array

design

or

the

cell type

it is

clear that existing advanced

technology can reduce the mass of state-of-practice

arrays by

over

afactorof two (and in

some

cases

more

like

a factor of

ten).

Even more

improvements

ar e

expectedin the

near

future.

Solar

Dynamic

Power

Another way of using the

Sun's power

is to heat a

working

fluid

that ca n

drive

a

turbine-alternator

much

the

same

way that most

U.S. electrical

power is

produced.

The component

technologies

for dynamic

power conversion ar e

fully

mature, having

been

developed

and

tested

for

more

than

20

years.

NASA's

Lewis Research Center has conducted the world's first

full-scale demonstration

of a complete

space-configured 2-kilowatt

solar

dynamic system

basedon the

closed Brayton

cycle in a relevant space

environment

Testing includes simulation of orbital

startups, both transient and steady-state

orbital

operation,

operation of the

heat receiver,

and hot

restarts andshutdowns. The preliminary resultso f this

testing

show

an overall solar dynamic on-orbit

efficiency

(defined as the

total electrical energy

produced over the orbit

divided

by the

solar insolation

energy received over the Sun portion of the orbit) of

14% to 16%. This compares very favorably with an

on-orbit system efficiency for traditional

photovoltaic-battery systemsof 4% to6%.

14

There

are

several operational advantages

for

solar

dynamic power

conversion compared to photovoltaic

systems. Unlike

photovoltaic

systems, solar dynamic


4/18

Copyright1997, American Institute o fAe ronau t ics and Ast ronau t ics , Inc.

system performance is not degraded by environmental

(ionizing) radiation.Ityieldsa stablepowerlevel

over

many

years

ofoperation.

Solar

dynamic powersystems

use a thermal energy storage system rather than

chemical

energy storage

system

toprovidethe heat to

continue running during eclipse periods. Thermal

energy storage,

which

is basically a

means

of storing

heat (usually in a molten

salt)

that is used todrive the

turbine-alternator

when the

solar dynamic

system is

shielded

from

the Sun, is potentially more enduring

than batteries which often have limited lifetimes in

low-Earthorbit(LEO) and

medium-Earthorbit

(MEO).

The

three-

to four-fold

efficiency advantage

of

solar

dynamic power systems over photovoltaic systems

results in a similar three- to four-fold

reduction

in the

solar collection area required for the solar dynamic

system at a

given

power. At LEO altitudes this

translates

to

lowera tmosphericdrag

for the

spacecraft;

thus, less

reboost propellant is required to

maintain

altitude.

Studies

conducted

for

Space

Station Freedom

showed substantially

lower

life

cycle costs for solar

dynamic power compared to

photovoltaic power.

Finally,

preliminary studies indicate

that

solar

dynamic

power

systems

using

advanced

technology receivers

and

concentrators

can

attain

system

specific

powers of

approximately 10

W e/kg

which is about the same

that

can be

attained

by advanced photovoltaic-battery

systems.

Issuesin SolarPower

With

on e exception (NEAR), the use of photovoltaic

solar arraysha s

been

restricted to spacecraft thathave

no t

traveled beyond

the

orbit

of Mars. A number of

factors have

led to this restriction, the

primary

on e

being that insolation falls off as the square of the

distance from the Sun. Thus, a

spacecraft

at Jupiter,

which is about

five

times as far from the Sun as the

Earth, will

"see"

25 times

less

sunlight For the

spacecraft to have the

same

power at Jupiter as it had at

Earth thesolararrayw ould have to be 25 times aslarge

as at Earth. Concentrators

don't

really solve this

problem

because

the

necessary

sunlight stillhas to be

collected

somehow

which

means the

array

has to be an

appropriate

size.

Studies

have shown that the use of

solar

arrays at the outerplanets(i ftheycan bemadeto

work

a t

all)generally

constrainsth e

spacecraft

to

point

at the Sun which means an

increase

in thepropulsion

subsystem mass to maintain tight attitude control with

three different alignments:

with the Earth for

communication;with

the Sun for

power;

and with the

planet to be studied. Two of these alignment goals

(Sun and Earth) become easierto

meet

as the

spacecraft

moves

farther

into

the outer

SolarSystem

but this is the

realm w here thearraybecomes largerandlarger.

Ionizing radiation is another problem that

must

be

addressed in the use of solar arrays. Jupiter, for

example, is notorious for its

radiation

belts which are

over

10 0

times

as

intense

as the V an Alien

belts.

Solar

cells,

particularly silicon cells,

are vulnerable to

ionizing radiation.

Finally,solarcells(especiallysilicon

cells) that

operate

in regionso fspacewith low solar illuminationand low

temperatures

(usually this

effect makes

itspresencefelt

at

>2.5

astronomical units, AUs)

are

subject

to a

performance

degrading

effect known by the acronym

LILT

which stands for

"low-intensity,

low

temperature". Some of theLILT degradationm ay be

caused by a kind of

corrosion

of theelectrical contacts.

Better cells

an d

fabrication techniques (including

the

use of different materials in the construction of the

contacts) coupledwiththe use of concentrator

lenses

to

raiseilluminationa ndtemperaturewillhelp.

Atthe

other extreme

is a

need

for

solar

cellswhich can

operate

at high temperatures. Anexampleof the need

for such high-temperature solarcells can be

found

in

the proposed Solar Probe mission to fly

within

four

solar radii

of the Sun where the sunlight is

3000 times

the intensity

at

Earth

(400 W/cm

2

) an d where

spacecraft surface temperatures canexceed 2000K .

Since this temperature exceeds thecapabilities of any

known

solar

array,

m ission plannersar e

considering

the

use of a disposable array for the early approach and

then

using

shielded

batteries for the closestapproach.

(The

ideal solution would be a

shielded

radioisotope

power

source

which

would

also

permit

an

extended

mission, but,

unfortunately,

politics ha s

once

again

hobbled

a proposed

space mission.)

Solarcells an d

arrays

capableo foperatingat high temperatures would

atleastalleviate

the power drain on the

batteries.

In

considering photovoltaic systems

for planetary

surfacepower

possible

solarspectrumchanges need

to

be

addressed.

Fortunately, the atmosphere of

Mars

is

quitethinand the Moon has noeffective

atmosphere

so

absorption is no t a problem.

Different insolationlevels

need

to be

considered

(Mars

receivesabout 43% of the

sunlight

Earth

receives;

some

have

saidthatagoodda y

on

M ars islikea bad day in

Philadelphia).

Duststorms

on

Mars can cloud the sky and cover the array.

Researchers at

NASA's

Lewis Research Center

have

examined

some of

theseissues

and identified

possible

solutions.

Fo r

example,

studies at

Lewishave shown

that

a

Martian

dust

storm will

produce a

diffuse light

which would

allow planar

solar arrays to function;

however,

this"white sky"

would

presentdifficulties

for

concentratorarrays.

17

'

18

If

solar

power

is to be used on hard

landersthen

arrays

thatcan

take

decelerationsas

high

a s

tens

of

thousands


5/18


fj

of

G swill

beneeded.

Perhaps the toughest challengeforphotovoltaicpower

sourceswou ld be their use on the Moon

where

thelunar

nightstretchesto atleast 14Earth days. Sucha long

period of

darkness

requires good energy storage,

whether batteries

or

fuel cells.

A

realistic specific

power goal

for

advanced

photovoltaic arrays used for

human lunar bases

is at

least

30 0

We/kg, over twice

that of the existing advanced technology

described

earlier. Thin-film arrays might allow

achieving 1000

We/kg

if such

arrays

can be built to

operate

for long

periods

withoutdegrading.

Solar

dynamic

power

systems can

overcome

some of

the issues such

as radiation

damage

to

solar cells

an d

the

limited life

of chemical

energy

storage. But there

are

issues

in solardynamic

technology

that need to be

solved. One criticism often leveled at

solar

dynamic

power

(or

nuclear dynamic

power for

that matter)

is

that

the conversion systems

depend

on moving parts

that

are

assumed

to beintrinsically

lessreliable

and

less

long-livedthan

static conversion

systems

(suchas solar

cellsor thermoelectrics).

Thus, there is a need to evaluate the

reliability

of an

operational solar dynamic

system

on a spacecraft

Therewas a

plan

to testone on the Mir space station

but thatplan was

canceled.

Thereare studies now to

consider such

a test on the

International

Space

Station.

19

A

successful

space

test shouldremove

an y

lingering

doubts about

the

viability

of solar

dynamic

power.

Nuclear

Power

Nuclear power sources come in two

types

(radioisotopesand nu clearreactors)depending

upon

the

sourceof thethermalpower (seeFigure 2). Likesolar

powersources,nuclear powersources canproducetheir

electrical

power

by static conversion systems (e.g.,

thermoelectrics)

or by

dynamic

conversion

systems

(e.g.,

turbine-alternators).

Since 1961, the U.S. has

flown 44

radioisotope

thermoelectric generators

(RTGs) and one

reactor using

thermoelectric

conversion

to provide

power

for 25 space

systems.

Forty-one of these nuclear power

sources

on 23 space

systemsare

still

in

space

or on

other planetary

bodies.

In

ad dition,

the Mars

Pathfinder Sojourner

rover,

which

was launched on 4

December

1996, carried three

radioisotope heater units to

maintain

proper

temperatures

on the

coldsurface

o fMars.

1

'

20

Nuclear

powersources

provide a number ofattractive

featuresincluding long

life

time; relativeinvulnera bility

to the external

environment(e.g., radiation belts

an d

dust

storms); high

self-sufficiency (e.g., no need to

keepthe spacecraft

oriented

toward theSun);a nd high

reliability. All of theU.S.

nuclear

powersourcesflown

have

met or

exceeded their

specified prelaunch

requirements with most performing so well that

extendedmissionswereconducted.

Radioisotope

power

sources

Radioisotope

power

sources derive their power

from

the natural decay

of a

radioisotope. U.S.

radioisotope

power

sources use

plutonium -238with

a

half-life

on the

order of 87

years

meaning

that

the

decay

in thermal

power (which

is the

biggest

contributor to

electrical

power

decay)

is about 0.8% per year,

much less than

the decay of

moststate-of-practice

solarcells.

All

of the U.S. radioisotope

power

sources flown to

date have

used

static conversion,that

is,

they

haveused

thermoelectric materials to convert the heat generated

by

the

radioisotope directly into

useful

electrical

energy. Thermoelectric

conversion

ha s

worked well;

the

RTGson the

Pioneer

10 and

Pioneer

11 spacecraft

are functioning

after

more than 24 years in space.

While

efficiencies are low (typically around 10% for

the thermoelectric

elements

an d less than 7% for the

power source) the specific power can be raised by

operating at higher temperatures. For

example,

the

silicon-germanium-alloy

thermoelectric elements in use

on

Voyagers 1 and 2 ,

U.S.

A ir

Force

satellitesL ES 8

and

LES 9, Galileo, Ulysses, an d Cassini operate at

about

1000

C on the hot

junction

and

reject heat

at

almost 300 C. For the

300-W e-class

RTGs in u se on

Galileo, Ulysses,

and

Cassini,

the specific power is

about5.3 We/kg.

This

compares well

with

the

overall

specific

powers

of

existing

photovoltaic-battery

systems in Earth orbit and it is

much better than

an y

existing photovoltaic-battery system

thatcould

be

sent

tothe

outerSolar System.

NASA has

identified fourteen potential future

representative missions

that may use a radioisotope

power

source

(RPS).

Fo r thesefuture

NASAmissions

there

has been a

strong

drive to improve theconversion

efficiency

and the

specific

power of

radioisotopepower

sources.

This

drive

is

fueled

by a

need

to reduce the

mass of the

power source

for the

new, smaller

spacecraft being planned and to reduce the amount of

radioisotope

(andhencethecost).

21

NASA

and DoD have sponsored studies of

improved

static conversion

systems,

including

advanced

thermoelectric materials, thermophotovoltaics (TPV,

essentially solar

cells that

are sensitive to theinfrared

heat of the

radioisotope

heat

source), andalkali

metal

thermal-to-electric conversion (AMTEC, essentially a


6/18

Copyright1997,American Institute of Aeronautics and Astronautics, Inc.

thermally

regenerated sodium

concentration

cell that

uses beta"-aluminasolid

electrolyte

ineffect toseparate

sodium ions and electrons, the latter traveling

through

an

external circuit

to

produce

the

power).

Both

conversion

systems o f f e r the promise of

radioisotope

power sources with efficiencies

an d

specific powers

abouttwicethose

of

existing

RTGs.

22

'

23

In supportoffutureouterplanetarymissionssuch as the

proposed Pluto Express andEuropa Orbiter missions,

NASA

and the Department of

Energy (DOE)

have

initiated

a

program

to

develop

an

advancedradioisotope

power

source

(ARPS) that can

provide

100 watts after

15 years

within

a mass of


7/18


33 low-power (~l-2-kWe) nuclear reactors from 1967

to 1988 to power its radar

ocean

reconnaissance

satellites

(RORSATs).

31

Both theSNAP-lOAand the

RORSAT reactorsemployed thermoelectricelements to

convert the reactor's thermal

power

to electricity

because that

was a

proven

technology and consistent

with

the

power requirements

of the

respective

spacecraft.

In 1987 the former SovietUnion flew two

low-power (~5-6-kWe)

experimental

reactors

known

a s

TOPAZ (a Russian acronym for thermionic,

experimental, conversion in the active

zone)

to test a

thermionic conversion system. Thermionic conversion

operates

o n

principles similar

to the old

vacuum

tubes:

electrons are "boiled off a hot(-1600 C ) emitter,

captured by a

collector

and circulated to thespacecraft

for power.

One

reactor operated

for

about

six

months

the

other

for about a year which is consistent with the

typically shortlives

o ftherm ionic

reactors.

'

While

the SNAP-lOAflightprogramwas

under

way in

the 1960s,

the

U.S.

also sponsored research on a

30-60-kWe

mercuryRankine

conversion

systemknown

as SNAP 8. Two

SNAP

8 reactors

were

built and

tested;one

operated

for a year and the

second

for

7,000

hours. Both reactors

ha dproblems

with

the

zirconium

hydride

used to

moderate

the neutrons

(the

same

materialused

in SNAP-lOA and

TOPAZ).

In the early

1970s the

zirconium-hydride

reactor program was

reoriented for use

with

a 300-kWe

Brayton

cycle.

Other

concepts, including a liquid-metal-cooled fast

reactor with a potassium Rankine cycle,

boiling metal

reactor,gas-cooled reactor,an dthermionicreactorwere

studied in the

1960s

an d

early 1970s.

Th e

U.S. spent

over

$735 million

in

then-year

dollars on the

SNAP

reactor

program.

In today's

dollars

that

represents at

least$4

billion.

In the late 1970s, DOE's Los

Alamos

National

Laboratory

began

studies of a 100-kWe-class nuclear

reactor called

SPAR. By the

mid-1980s this evolved

into a joint DoD/DOE/NASA program

called

SP-100.

32

'

33

SP-100 was to be ageneric

reactor

power

system

that

could

be

scaled

from 10 kWe to

1000

kW e

to provide power for up to 10 years for a range of

missions includingspace-based

power,nuclear

electric

propulsion, an d

planetary

surface

power.

The

design

mass for the 100-kWe version was 4575 kg for a

specific power o f almost 22We/kg.

33

Th egovernment

program

office wisely decided to put the conversion

system external

to thereactorto

avoid

exposure to high

temperatures, high nuclear

radiation fluxes,

an d

potentially corrosive liquid me tals. In this

proven

design approach the SP-100 nuclear reactor could be

coupled

to a range of conversion systems

such

as

thermoelectric, out-of-core thermionic, Brayton,

Rankine,Stirling, or advanced staticsystems(e.g.,TPV

and AM TEC) for growth andflexibility.

By 1993, most of the nuclear component performance

development

work ha d

been

completed on SP-100.

Through

experiments the

reactor design

and computer

codes

were verified. Enou gh

nuclear fuel

ha d

been

fabricated to allow construction of a 100-kWe space

reactor

thermoelectric

power

system.

34

Budgetary

constraints an dchanging agency

priorities

then led to

the cancellation of the program, although testing

continueson thethermoelectric elementsand anactive

technology transferprogram w asinitiated. About$450

million was

spent

on the SP-100

program. Based

on

the

SNAP an d SP-100 experience

some experts

have

estimated that the cost to design, build,

ground-test,

qualify,

an dproducea

flight

reactor thatmeets all the

safety,

reliability,

performance, and

quality

requirements is probably on the order of $1 billion.

Some trade

pu blications

have

estimated

that the

former

SovietUnion

spent

similaramountsto

develop

its low er

powered

reactors.

' [It is

interesting

to

speculate

what might have

been accomplished in the

1980s

if a

truly focused,

goal-oriented reactor

program

ha d been

conducted

with

th ecombinedresourcesthatwerespent

on competing concepts (including an

estimated

$150

million

or more spent on theill-conceived

Timberwind

nuclearrocket).]

Under DoD initiatives the U.S. ha s looked at

other

reactorconcepts,including various thermionic reactors

and

so-called bimodal reactors

that

will provide both

power

and

limited nuclear propulsion.

To

date none

of

these

concepts

has

reached

th emanufacturing stageno r

are there any approved

missions

for them. DoD

also

led an

effort

to

purchase,

test,and fly

another Russian

thermionic

reactor

known

as

ENISY

in

Russia

bu t

called TOPAZ H

in the

U.S.

Unfortunately, the

Russian thermionic reactors

ar e

rather

low

powered

(~5-6 kWe)

fo r

theirmass (over 1000

kg) and

have

low

efficiencies

(


8/18

Copyright

1997,

American Institute of Aeronautics and Astronautics,

Inc.

IssuesinNuclear Power

Safety

is

often cited

as the

primary issue

associated

with

the use ofnuclear power sources in

outer space.

Th e

U.S.

has had three

accidents

(Transit 5BN-3,

Nimbus-Bl,and Apollo 13) involving

launch

vehicles

or

spacecraft carrying

nuclear

power sources.

However, none of these

accidents

was

caused

by the

nuclear power

sources and in each case the RTGs

performed as theywere

designed

to do, showing

that

it

is practical to

design,

build, and launch safe nuclear

power sources. The Russians havehad at least six

unplanned reentries

of

nuclear power sources: four

involving

reactors

and two involving radioisotopes

(including

N ovember's reentry of Mars

*96).

31

As far

as is

known

the

Russian

accidentswere all the

result

of

launch

vehicle

or upper

stage failures.

Th e

first

publicized Russian

accident,

Cosmos 954 which

reentered o ver

Canada in

1978,

led to the

1992

adoption

by the

U J S f.General Assembly

of a set of

principles

on

the use of nuclear power

sources

in outer space. The

U.S.

has stated it

believes

its

program

is consistent

with

the general goalsof theU .N. principlesbu tdiffers

with

them insomeparticulars where theU.S.approach has a

provenrecord.

37

From

the beginning of theU.S.

spacenuclear

program

in the 1950s, safety has

been

the principal

design

requirement.

Extensive tests

involving

high-speed

impacts,

projectiles, propellant fires,

an d

reentry

simulations

are combined

with

detailed probabilistic

risk

analyses

to

assess

the performance of

each

U.S.

nuclear power

source

before it is approved forlaunch.

Final

approval

authority for launchrestswithth eOffice

of

the

President

The Clinton administration in its

national space policy released in

September

1996

affirmed

that the nationmus t

m aintain

a spacenuclear

power capability.

Given the

lack

of

sunlight

in the

outer

solar system

and the

harsh environments

on the

Moonan dMars this isbotha

reasonable

and practical

decision. Now the Adm inistration andCongress must

be willingto

fund

the

long-leaddevelopment

items for

future nuclear power sources

in the

absence

of a

defined

mission because the nuclear power

development and qualification time often

exceeds

that

of

thespacesystem.

Another

area

of

concern

for the

radioisotope

power

source program is the availability of plutonium-238.

W ith the end of the coldwar, the

U.S.

has shut

down

its

production reactors

where plutonium-238

was

occasionally produced as a

sort

of

byproduct.

In

overcoming this deficiency, the U.S. has purchased

some

plutonium-238 from Russia. Other countries,

including Canada, France,

an d

Great Britain,

ca n

produce plutonium-238. (Plutonium-238

is

naturally

produced as a byproductin terrestrial nuclear reactors

bu t chemically separating it from activated

uranium-based fuel isvery

difficult).

Relatedto the availability of plutonium-238 is thecost

of the

fuel. Before

DOE

stopped

producing

plutonium-238

it proposed making NASA pay the

full

cost of

production.

This is akin to

asking

another

agency tosupportthe

already bu dgeted

infrastructure of

DOE;

it's

as if

NASA

askeda naircraft com panyto pay

the full costsof itswindtunnels. Since thisunbu dgeted

movecould

end up

costing

NASA

thousands

o f

dollars

per

gram (with

kilogram quantitiesbeing

needed)

there

wa s

reluctance to

accept that proposal.

In contrast,

DOE often gives

D oDnuclearma terial at nocostso, in

effect,

a double

standard

was in

place.

Historically,

DOE

and its

predecessoragencies

ha d

either

given the

fuel

to

NASA

or sold it at a reduced price

under

the

legislative charter

to

foster nuclear research

an d

development. The

issue

of w ho pays is

blurred

because

DOE is budgeted to run the facilities so the taxpayer

has already paid once. The best settlement would

include both the

legislative

charter to

clearly

establish

responsibility for budgeting and thenecessary

funds.

Asnotedearlier,the

cost

and

ava ilabilityissues

have in

part

driven NASA

and DOE to investigate more

efficient conversion systems

that

will reduce the

quantity

of

plutonium-238required.

However,

neither

NASA

nor DOE appear to be

devoting

the resources

necessary

to make the

advanced radioisotope power

source a

reality

in

time

for a

2001

or

2002

launch of the

proposed PlutoExpress or Europa

Orbiter.

To

show

thatit can operate

predictably

for 10 or 15 years a new

electrical

power

technology needs several years

of

ground testing. Several years of design, engineering

tests,an dma terials studies

mustcome

first. Rightn ow

the U.S.

has

only

two

thermoelectric elements

that

could be counted on for a long-term mission: the

silicon-germanium alloy used

on the

Voyagers,

LE S

8/9, Galileo, Ulysses, and Cassini and the

telluride-based alloys used, for example, on the

Pioneers

an d

Viking

Mars Landers. For a higher

efficiency

conversion

system

only the Brayton cycle

(undertestat LewisResearch

Center

as part of thesolar

dynamic

program) and

possibly

the

Stirling cycle

would

be

viable

candidates

with

the

Stirling

cycle

being

the

lowest mass option at 100

watts

o r

less. Based

on

earlier experiments andcertainterrestrial

applications,

a

Rankine

cycle

withan

organic

workingfluidmightalso

be made ready

for higher

powers.

A

small (5.9-kg)

mercury Rankine

turbine-alternator

system was

built

by

TR W

an d successfully tested in the late

1950s,

demonstrating the

capability

to

produce hundreds

of

watts

in a

small

package.

38

Withtoday'scapabilities

in

micromachinery,such

a

conversion systemshouldalso


9/18

Copyright1997,American Instituteo f Aeron aut ics and

As t ronau t ics ,

Inc.

be considereda

candidate

for

futuresciencespacecraft

Small ARPS for landers an d

penetrators

will have to

survive high decelerations (2,000

to

80,000

Gs) at

Mars.

21

An issue not often considered with space

nuclear

reactors is the capacity factor,

i.e.,

howmuch energy

did it produce over time compared

with

theoutputhad

itoperatedat its

rated

capacity or

power.

The one

U.S.

space reactor flown wa s shut down by a spurious

voltage that triggered thereactor'ssafety system which

had been designed

for an

irreversible shutdown.

30

Oftenterrestrial reactors

are shut

downbecause

certain

trip

pointsare

reached

not for

crucial

safety

reasons.

In

the author's opinion a nuclear reactor is

more

likely to

shut

down

forreasonsother

than

safety-relatedevents.

Given thecom plexityof the

control

systemlogicfor a

fully automatic space reactor,

shutdowns

will certainly

occur. Thisbegs the question of providing the

backup

power when a reactor is temporarily shut down. A

completely

independent power

system

is

critical

to

maintaining human

bases. Fo rhigh-power

applications

this could

mean

having

two or

more independent

reactors. It also means that designers and operators

must

understand

the

reactor

and its

operation

as a

total

system (something notmuchin evidence atThree Mile

Island).

Energy

Storage

Both

mechanical

and chemical energy storage systems

have been considered. Mechanical energy storage

includes flywheels which could

be made

synergistic

with momentum

wheels

on spacecraft. The chemical

energy storage

systems

used in spacecraft typically

have been eitherbatteries

or

fuel cells.

Generally the

design and

systemgoals

can be

sum marized,

in order of

criticality,as: life,

reliability

andsafety,

efficiency

and

mass,

and reduced

unit

orlife-cyclecosts.

atteries

Batteriescome in twotypes: primary(used once) and

secondary (rechargeable).

Primary

batteries

have

been

an d

ar e

being

used on launch

vehicles

and a

lithium-based primary battery was used to power the

Galileo Probe

during its

descent

into the

Jovian

atmosphere a

year

ago and one is onCassini's

Huygens

Probe. Primaryba tteriesgenerallyoffer higherspecific

energies

than secondary

ba tteries

a ndtheyare

ideal

fo r

certain one-of-a-kind applications such as short-lived

planetary operations (e.g.,

penetrators).

Secondary

batteries are more

commonly

used in space and are

critical

to certain

surface

operations.

Spacecraft

operating in geosynchronous Earth orbit

(GEO) usua lly

experience

about 100

solar eclipses

per

year

so the

number

of

batterycharge/discharge

cyclesis

relatively low over a typical 10- to15-year design life.

Such batteriescaneasily operateat 80% or so of rated

capacity. Incontrast,spacecraft operatingin LEO may

experience 15 eclipses per day for

30,000

charge/dischargecyclesovera typicalfive-year design

life. Historically, such batteries

have

operated at less

than

25% ofrated

capacity

to

preserve

their

lifetimes.

Two principal battery

chemistries

are

being

used or

considered today: nickel-based and lithium-based

batteries.

There

are other systems, such as

sodium-sulfurbatterieswhichpromisea specific energy

of 100 We-h/kg for GEOap plications.

Nickel-based batteries,specifically nickel-cadmium and

nickel-hydrogen,

are used on

almost

all

operational

satellites today.

Nickel-cadmium

has been the

state-of-practice

battery used and state-of-the-art cells

can

produce on the order of 39

We-h/kg.

39

'

40

The

newer nickel-based battery that is replacing

nickel-cadmium, particularly in GEO applications and

on

spacecraft

requiring

powers inexcessof 1

kilowatt,

is nickel-hydrogen. State-of-the-art nickel hydrogen

cells can produce on the order of 50

We-h/kg.

Nickel-hydrogen

batteries

offer longer lifetimes and

greater depths of

discharge

in GEO

applications

than

nickel-cadmium batteries.

39

Recent improvements

in

the

electrolyte have

allowed

nickel-hydrogen batteries

to be considered for LEO applications with greater

depths of

discharge

and

longer

lifetimes. In fact, the

International Space Station will

use

nickel-hydrogen

batteries for

energy

storage

and will use the improved

electrolyte

nickel-hydrogen

batteries

for the

replacements.

The Bubble Space

Telescope currently

uses

an

early nickel-hydrogen

batterydesign.

A

nickel-hydrogen

battery,

based on a

two-cell,

common pressurevessel design, is in use on the Mars

Global Surveyor,

marking

the first use of

nickel-hydrogen

technology

on a planetary

spacecraft.

This design has also been selected for the first Ne w

Millennium

mission (Deep Space

1), Mars

'98,

and the

Discovery Stardust

mission. Earlier,

the Clementine

mission hadshownthe viability of the nickel-hydrogen

battery in the

single

pressure

vessel

design

for lunar

missions.

7

The

Iridium satcomswill

alsouse the

single

pressure

vessel design.

The

long-term

goal for nickel-hydrogenbatteries is to

achieve at

least

100

We-h/kg

of energy

storage

at the

celllevel. Thiscan be achieved by innovativedesigns

(there

are at least five different concepts

under

consideration,

one ofwhich

promises

76 W e-h/kg at the


10/18

Copyr ight

1997,

American Instituteof

Aeronau t ics

and

A st ronaut ics,Inc.

battery level) and by reducing the masses of the

3041 42

components.

^

For small satellites

which

do not have the

volume

availableorpowerdemands that would leadto the use

of nickel-hydrogen batteries,

nickel-metal hydride

batteries

may be the

answer.

In these batteries the

hydrogen is

stored

as a solid hydride instead of as

hydrogen gas which means theoverall volume of the

batterycan bereduced. Nickel-metalhydrideba tteries

also avoid such toxic

m aterials

as lead,cadmium, an d

mercuryfoundin

some

otherbatteries.

Lithium-based

batteries

are

being

considered for

planetary

spacecraft where

thecycle

lifetimes

required

are not asgreatas LEO or GEOsatellitesan d

where

the

spacecraft

powers may be lower

(typically

under 1

kilowatt)

but the

mass

budgeted for

energy storage

is

very

limited.

?

T h e three

principal

types

(and

theirpossiblespecificenergies)

being considered

a re

Lithium-ion

(-95

W e-h/kg)

Lithium

titanium

disulfide

(-135We-h/kg)

Lithium polymer

(-150

We-h/kg)

Lithium batteriesare environmentally

friendly

because

they

contain

no

caustics,

mercury,

lead

or cadmium.

Lithium-ion

cells

provide three times the

voltage

of

nickel-hydrogen cells which

means that a lithium-ion

battery

will

require

about

one-third

thenumbero fcells

required

by a nickel-hydrogen

battery

to

achieve

the

same voltage

which is

indicative

of the mass savings

with

lithium-ion technology.

>4 1

Both NASA an d

DoD are working with industry on

various

lithium

battery technologies,primarily lithium-ion.

In

summary, it can be confidently stated that

existing

advanced battery technologies can

reduce

the mass

allocated to state-of-practice

batteries

by at

least

25 %

an d

more

like

50%,

with

even further

reductions

envisioned.

FuelCells

Fuel cells have been used to

power

the

Gemini

and

Apollo spacecraft an d

they

are currently used to power

the

Space

Shuttle. Fuel

cells

would

be a good

candidate to power a

space

station during eclipse

periods

or a

lunar base during

the long

lunar night.

There

are

about

200 fuel cell

units (mostly using

phosphoric acid as the

electrolyte)

operating in 15

countries on Earth. Four (typed by electrolyte) are

being developed for terrestrial application in North

America: phosphoric acid (operates atabout200C);

proton exchange

membrane

(operatesat

about

80C);

molten

carbonate

(operates at

about

6 50

C);

an d

solid

oxide

(operates

at

about

1,000 C); Tw o

units

of a

phosphoric

acid

fuel

cell ran for about a

year each

powering a hospital in

Riverside,

California. A

megawatt-class

molten

carbonate fuel cellpower plant

has been

built

for the city of Santa

Clara,

California.

43

It

is possible todesign,

build,

an doperate

long-lived,

high-power

fuelcell

power

plants.

For planetary surface

power

applications,NASA

once

considered a goal of 1000We-h/kg of energy

storage

and a

lifetime

of over 20,000 hours to be desirable.

(For

terrestrial applications an

econom icallydrivengoal

of a

40,000-hour

life ha s

been

cited.) There was a

strong

interest

during the

Space Exploration

Initiative

in

using the

proton exchange membrane technology

which has a solid polymer electrolyte that can be

operated

at low temperatures to permit

fast startups.

Such

fuel

cellshave

been used

onbuses in Vancouver

and around

the Los

Angeles

airport.

43

Th e

proton

exchangemembrane

fuel cell is planned toreplacethe

phosphoric

acid fuel

cell

on

Space

Shuttle because it

meets

or

exceeds

all the requirements of the

phosphoric

acid fuel cella t a

lowercost

and lower

mass.

44

Th eJet PropulsionL aboratory,in

cooperationwith

a nd

sponsorship

by Lewis Research Center, ha s

successfully

assembled a Solar RegenerativeFuel Cell

Test Bed

Facility

at Edwards Air ForceBase. Th e

Naval Air

Warfare

Center has

provided

tw o 25-kWe

separately operatingsolar

arrays both made ofthin-film

cadmium

telluride (CdTe) material. In addition, the

facilityincludes a 25-kWe proton exchange

membrane

(PEM)

electrolysis unit,

four

5-kWe PEM

fuel

cells,

high-pressure

hydrogen

and oxygen storage

vessels,

high-purity water

storage

containers, and computer

monitoring,

control

and data acquisition.

This

facility

will allow JPL to

test

fuel

cells, electrolysis units,

photovoltaic

arrays, system

controls an d

integration

hardware in support of

planetary

and

Earth

surface

operations

(if

fully

funded).

Flywheels

Sparked by recent

advances

in

terrestrial

flywheel

technology,

there

has been a renewed

interest

in using

flywheelsfor energy

storage

in spacecraft.

These

new

terrestrial

flywheel

systems,

which

u secom posite

rotors

and

magnetic bearings toachievewheelspeedsgreater

than 60,000 revolutions per

minute,

can provide

specific energies greater than

66

We-h/kg

(at 75%

depth

ofdischarge),whichis

larger

than

presently

used

nickel-hydrogen

batteries.

In addition to the higher

specific

energy,flywheel

systems

offer

other

potential

advantages over

batteries:

(1)

they

are smaller in

10


11/18

Copyright1997,American InstituteofA eronau t icsandAs t ronau t ics ,

Inc.

volume

than nickel-hydrogen

batteries;

(2 )

manufacturing

lead

times

can be shorter

than

for most

batteries;

(3) there is no requirement for

taper

charging

which

can

consume

5% to 10% of the

solararraypower

for LE O missions; (4) there is no requirement for

reconditioning

or stringent

thermalcontrol

beyond that

of standard

electronic boxes; and (5) precise

measurements of the

available

energy can be

obtained

from

the wheel

speeds.

46

Fo r spaceapp lications there

would have

to be a scaling

down of the

terrestrial technology,

which has been

designed to provide tens of

kilowatts

for such

applications as

electric

vehicles an d uninterruptible

powersup plies. CurrentlyNASA, in

cooperation

with

USAF, is

considering

two prototypeflywheel

systems:

a small (250-to

300-We-h) flywheel

for small

spacecraft and a 3.25-kWe-h flywheel that could be

used

on International

Space Station.

To be

really

competitive

with

the

masses

of advanced

chemical

energy storage

systems,flywheel systemswillprobably

have to combine the functions of energy

storage

and

attitudecontrol (which has

beenperformed

by smaller

reaction

w heels).

46

Capacitors

Recently there has been renewed study of capacitors,

specifically chemical double layer (CDL) capacitor

banks to powerelectromechanicalactuators andelectric

thrusters.

47

49

Capacitorshave

the

advantage

of being

power dense

(but

limited by energy density)

while

batteries are energy dense bu t deficient in power

density. Some combination of batteries and capacitors

may

provide

the optimum energy

storage

for

those

applications requiring both

power

dense and

energy

denseoperation. Experiments

have

been run

showing

that

the CDL

system

can power

electric thrusters.

Technologists believe "It is reasonable toexpect

units

with

energydensitiesgreater

than

5Whr/kga nd

power

densities greater

than

a

kW/kg

to emerge from the

laboratories into

the

marketplace

in the near

future".

IssuesinEnergy S torage

Nickel-cadmiumba tteries

have

been limited

in

lifetime

by

degradation

of the

electrodes

and by hydrolysis of

the

separator

material.

Chemicals

(such as oxygen and

potassium hydroxide) in the battery can attack the

separator

ma terial.

Electricalshortingof the battery has

been caused by migration of cadmium

inside

the

battery. Both cadmium and one of thecom ponents of

the separator have

come

under environmental and

occupational health and

safety scrutiny.

In response,

the nickel-cadmium technologists have

been

working

on developing new separator materials and there is a

good

chance that

nickel-cadmium

batteries can be

improved

enough

to

compete

with existing

nickel-hydrogen batteries.

Theoretically, nickel-hydrogen batteries should have

specific energies of more than 40 0

We-h/kg.

In

practice, the

specific

energy ha s been

about

50

We-h/kg.

39

A

number

of

factors

including themassof

the

positive electrode,

th emassof the

pressure

vessel,

and

voltage

drops

within

the

battery

have led to

this

limitation.

To

improve

the performance of

nickel-hydrogen

batteries will require the design

changes and mass reductionsnotedearlier.

Lithium batteries

need to be controlled

carefully

to

prevent

overcharging

or overdischarging. (Some early

lithium

batteries exploded

bu t

this problem appears

to

be solved.) With

lithium

batteries (both

primary

an d

secondary)

being

consideredto

power

planetary

surface

penetrators

there is a requirement to

develop

lithium

batteries

which

can survive decelerations of

tens

of

thousands

of

G s.

7

Fuel cells

can

suffer from undesirable

chemical

reactions

that

interfere with reactions at the electrodes

an d from resistance heating within the electrode.

Ideally,fuel cells

should have high

chem ical reactivity

withno corrosion orsidereactions to

produce

thelarge

current

densities

needed for long lifetimes. If proton

exchange

membrane

fuel cells are to be used,

much

development

work

needs to be done to ensure long

lifetimes

of the

mem brane system.

Chemicalenergystoragesystems intendedfo rplanetary

surfaces must be able to withstand very low

temperatures (such a s

duringlunar

nights or on M ars or

on

the

satellites

of the outer

planets)

and

high

temperatures (such as on the surfaces of Venus or

Mercury).

To be

useful,

flywheel energystoragesystemswill have

to

be improved an d scaled

down

to meet spacecraft

requirements. This means further work on materials

and

magneticbearings.

Lifetesting is

clearly needed

to

determine

if all the

components will

last

Moreover,

flywheels

willhave to bedesignedto

accommodate

the

launch

environment.

A

systems-level approach

involving multiple sets of

flywheels

(and

probably

combining

the

functions

of

flywheels

an d

reaction

wheels) will be required to overcome torques and

possible

failures in a mass-competitive

manner.

Finally,

but noless

important,

flywheelsw ill have to be

fail safe especially against the consequences of a

catastrophic

failure such

as

seizures

or

destruction

of

therotor.

11


12/18

Copyright

1997

American Instituteof

Aeronautics

and

Astronautics Inc.

Power Management and Distribution

Th e electrical tie that

binds

the power

source,

the

energy storage,and therestof the spacecraft

together

is

power management

an d

distribution

(PMAD).

Historically,

the PMAD subsystem has been composed

of

discrete parts mounted on planar printed circuit

boards leading to a mass fraction that ha s

been

increasing toward

10% of the dry

mass

of the

spacecraft.

50

This is unacceptable in today's

environment ofsmallerspacecraft.

A solution to

massive

PMAD subsystems is to

incorporate technologies developed in the commercial

electronicsworld where more and more functions are

being

incorporated

on smaller an d smaller chips. In

short, one can think of the new approach to power

management as

"PMAD

on a chip". JPL has already

begun this

approach in a

phased manner

starting

with

Cassiniwhich will use a new solid-statepower

switch

(sometimes called hybridization

technology).

7

' JPL

is extending this

work

to second-generation

power

switches,

high-performance

power converter modules,

an d

field

programmable

gate

arrays for command an d

telemetry

interfacing.

The overall goal is a complete

reconfiguration of the

basic building

blocks of the

PMAD

subsystem,

going from discrete

components to

hybrid systems tom ulti-chip modules. This approach

has the important effect of reducing the amount of

electrical cabling.

The overall

goal

is to

take PMAD

from state-of-the-art

150-We/kg systems to

>600

We/kg.

7

An

even more

revolutionary approach is being taken

under the New Millennium program.

Electronics

packaging, interconnections, an d data an d power

distribution

are

being integratedwiththoseparts

of the

spacecraft bearing mechanical loads and providing

thermal

control.

This approach,

referred to as

multifunctional structures, involves placing passive

electronic

components

within

the composite

structural

ma terials to

achieve

a 75 percentreductionin

electrical

harness

an d cable mass, a 50 percent

increase

in the

payload fraction, and a 40

percent

increase in the

internal

volume of the spacecraft. A modest

experiment

to

allow

an

in-flight test

of the

concept will

becarriedon the

first

New Millennium(Deep Space 1)

spacecraft. Bycontinuingtoworkon

this

revolutionary

approach it may be

possible

to

have

a completely

cableless

system

for the

third

Deep Space

mission.

7

'

13

-

50

'

51

Other technologies

that have

been developed or are

under

development include

new grounding

techniques

for

the high-voltage power systems (such as

International Space Station)

used

on LEO

missions

where

interactions

with ionospheric

plasma occur;

miniaturized microprocessors to condition and control

the electrical power; an innovative photovoltaic

regulator kit experiment

that

was to have

been

testeda s

part of

NASA's

small spacecraft technology initiative

(the Lewis

spacecraft)

an d

ground-based

test beds to

validate

ne wPMADtechnologies.

7

'

13

'

50

53

IssuesinPower M anagementand

istribution

The drivetoward

sma ller,

improved PMA D subsystems

for

th enew,smallerspacecraftappearsto bewellunder

way. Less attention is paid currently to PMAD for

future higher-powered

human habitats.

Terrestrial

experience shows

that

the use of alternating current

(AC)powerleads to smallerPMAD than does the use

of direct

current

DC). Commercialpassenger

aircraft,

for

example,

operate

with

400-HzA Cpower. Yetlittle

has

been

done

to develop a

space-qualified

AC

PMAD

forlarge power systems.

Anotherareawhichreceivedsome

attention

during the

SP-100 space reactorprogramwas thedevelopmento f

radiation-resistant,

high-temperature PMAD

components. Some

work was

done

developing

high-temperature silicon-carbide switches but more

needs

to be done if large nuclear power sources are

used. This isfirsto f all a m aterialsissue(i.e.,finding

materials that will work well under high

temperature

and

high

radiation) so

early

innovations can be

achieved at relatively low costs.

Concluding

Remarks

The

technology

exists

today to cut the

mass

of

state-of-practice electric

power

systems in half and in

some

cases

more. Technologists

are

continuing

topush

for more efficient and lighter

power components

an d

they

are developing innovative concepts that combine

power,

structures

and thermal control. However, this

effort needs continuing support, both

managerial

and

financial. This

effort also

must be

given

the time to

properly

develop

an dqualifythe newer

technologies.

As

mentioned before,

the choices for

electrical

power

systems should not be seen as "either-or" choices.

There is

room

for all of the options as requirements

evolve. For

example,

the

early

Transit

satellites

were

solar

an d battery powered but then small RTGs were

added for

extra power

then two

all-RTG

Transits were

flown. NASA

also

used

a

combination

of

nuclear,

solar,

and

ba tteries

on its first

n uclear mission (Nimbus

in). Some early

studies

for pow eringhuman-inhabited

lunar

bases showed the early outpost beginning with

photovoltaic arrays or solar dynamic modules

with

batteries

or regenerative fuel

cells

for energy

storage.

12


13/18

Copyright 1997,American Instituteof Aeronautics andAstronaut ics,Inc.

The

overall specific power

for

such

systems is in the

range

of 1.5 to 3

We/kg.

As

these early studies

projected

the powerdemands to

rise

for

later

outposts,

nuclear reactors

could

be employed to provide

continuous day and

night power without

the

need

fo r

energy storage.

Th e

specific

power for the nuclear

reactor power

system can be in the

range

of 25 to 60

We/kg depending upon the power an d conversion

system.

But even with the nuclearreactor,the outpost

may want to maintain an

emergency solar-chemical

and/or

radioisotope

dynamic sourceofpowerfo rthose

occasionswhen

th e

reactor

isshutdown.

Finally,this

is an

exciting time

to be

working

in

space

power becauseof the

technical challenges

a ndbecause

of the chance to contribute in a measurableway to the

human

explorationofspace.

References

1. G. L. Bennett,"Electrical Power for Spacecraft

Opinions

andIssues",Space

Times,

Vol.36, No. 3 and

No. 4,

M ay-June

1997 and

July-August

1997.

2. H. W . Brandhorst, P. R. K.

Chetty,

M . J.

Doherty,

an d

G. L.

Bennett,

"Technologies for

Spacecraft

Electric Power Systems", Journal

of

Propulsion

and

Power ,

Vol.

12, No. 5, pp.

819-827,September-October

1996.

3. M. A. Manzo and S. W. Hall, "Aerospace

Nickel-Cadmium

Cell

Verification - An Update",

Proceed ings

of the Thirty-Second

Intersociety

Energy

Conversion

Engineering Conference,

Vol.1, pp.

99-107,proceedingsof a conference held inHonolulu,

Hawaii,27

July

- 1

August

1997 an dpublishedby the

American InstituteofChemicalEngineers,New York,

New York.

4. D. M.

Alien,

"A

Survey

of

Next Generation Solar

Arrays", AIAA 97-0086, prepared for the 35th

Aerospace

SciencesMeeting & Exhibit, held in Reno,

Nevada,

6-10January1997.

5. G. A.

Landis,

S. G . Bailey, and M . F. Piszczor, Jr.,

"RecentAdvances

in

Solar Cell Technology",Journal

o f Propuls ion and

Po w e r ,

Vol.

12, No. 5, pp.835-841,

September-October

1996.

6. K. A.Bermess,D. J.Friedman,S. R.Kurtz,A . E.

Kibbler, C.Kramer,and J. M . Olson,"High-Efficiency

GalnP/GaAs

Tandem

Solar Cells , Journal

of

Propu l s ion a n d P o w e r ,

Vol.

12, No. 5, pp. 842-846,

September-October 1996.

7. C. P. Bankston, "Progress in Spacecraft Electric

Power

System

Technologiesfor Deep SpaceM issions",

AIAA 97-0089,

prepared

for the 35th Aerospace

Sciences

Meeting

&

Exhibit, held

in

Reno, Nevada,

6-10

January

1997.

8. L. M. Fraas,G. R.G irard,J. E.Avery,B. A. Arau,

V.

S. Sundaram, A. G. Thompson, and J. M. Gee,

"GaSb

BoosterCellsforover 30%Efficient Solar-Cell

Stacks",

Journal o f

Ap plied

Physics ,Vol.66, No. 8, pp.

3866-3870,1989.

9. R. M. Kurland and P. M.

Stella, "The Advanced

Photovoltaic Solar

Array

Program Update",

Proceed ings

of the 3rd

European Space

P o w e r

Conference (Graz, Austria), European Space Agency

Publication

ESAWPP-054, Paris, France,August1993.

10. M. J. Herriage, R. M.Ku rland,C. D. Faust, E. M.

Gaddy, and D. J.

Keys,"EOS A M-1

G aAs/Ge

Flexible

Blanket

Solar

Array Verification Test

Program

Results ,Proceedings

of

the Thirty-Second

Intersociety

Energy

Convers ion Engineer ing

Conference,

Vol.

1,

pp . 556-562,

proceedings of a

conference

held in

Honolulu,

Hawaii, 27

July

- 1

August

1997and

published by the American Institute of Chemical

Engineers,

New York, New York.

11. P. A.

Jones,

T. J.Harvey,and M. V.

Douglas,

'The

UltraFlex SolarArray Qualification Testing Program",

Proceedings o f the30th

Intersociety

Energy Conversion

Engineer ing Conference, Vol. 1, pp.

347-351,

proceedings of a conference held in Orlando,Florida,

30 July- 4August 1995andpub lishedby the American

Society

of

Mechanical Engineers,

New

York,

New

York.

12. D. M.

Murphy

and D. M. Alien, "SCARLET

Development, Fabrication, an d Testing for the Deep

Space 1

Spacecra ft",

Proceedings of the

Thirty-Second

Intersociety Energy

Conversion

Engineering

Conference, Vol.

4, pp.

2237-2245, proceedings

of a

conference held in Honolulu, Hawaii, 27 July - 1

August1997

an d

published

by the

Am erican Institute

of

ChemicalEngineers,New York, New York.

13. A. B. Chmielewski etal.,"The New Millennium

Program Power Technology", Proceedings of the

31s t

Intersociety Energy Convers ion

Engineer ing

Conference, Vol. 4, pp.2193-2198, proceedings of a

conference held in

Washington,

D.C., 11-16 August

1996

and published by the Institute ofElectrical and

Electronics Engineers, Piscataway ,New

Jersey.

14. R. K.

Shaltens

and L. S.

Mason, "Early Results

from

Solar

Dynamic

Space Power

System Testing ,

Journal

of

Propu lsion

and Power ,

Vol.

12, No. 5, pp.

852-858,Septem ber-Octob er 1996.

13


14/18

Copyright 1997,

American

Inst i tu te o f Ae rona ut ics and Astrona ut ics , Inc .

15. J. E.

Randolph,

et

al., The

Technology

Development

Status

of the

Solar

Probe",Proceedings

o f th eSpace Technology andApplications International

Forum (STAIF-97), Part One,

pp .

123-130,

AIP

Conference

Proceedings 387, proceedings

of a

conference held in Albuquerque, New Mexico, 26-30

January 1997an dpu blished by the American

Institute

of Physics,Woodbury,New York.

16. G. A.

Landis, "Mars

DustRemoval

Technology",

Proceedings of the Thirty-Second

Intersociety

Energy

Convers ion

Engineering

Conference, Vol.

1, pp.

764-767,

proceedings

of a

conference

held in

Honolulu,

Hawaii, 27 July - 1August 1997an dpublishedby the

American

Institute

of

Chemical

Engineers,

N ew

York,

New York.

17. J. Appelbaum and G. A.

Landis,

"Photovoltaic

Arrays for

MartianSurface

Pow er",

Acta Astronau tica,

Vol.30, pp.

127-142,1993.

18. J.

Appelbaum,

electrical power technologies for spacecraft: options and issues

Documents