Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 68
5
Approaches Toward Achieving Advances in
Critical Power Technologies
In the discussion of space-based power requirements (Chapter 2), the
committee pointed out the advantage of pursuing high-leverage areas;
sirn~lar approaches can yield some very useful results in advancing
critical power technologies. In this chapter, the following subjects
are discussed: advancing thermal-management techniques, advanc-
ing power-conditioning components and technologies, and materials
advances required for developing power component technologies.
ADVANCING THERMAL-MANAG1:MENT TECHNOLOGIES
The thermal-management problem is that all heat generated on a
space platform must be (a) converted into another form of energy
(with the associated thermodynamic constraints); (b) absorbed as
temperature rise in components or thermal storage elements; (c)
absorbed by a coolant that Is vented; or (~) radiated to space either
directly from the component or by use of a higher-temperature, more
efficient radiator.
The last option requires a heat-pump (refrigeration) cycle, in
which heat is absorbed at a low temperature and rejected by the
radiator at a higher temperature. Only the first three options are
available for heat rejection from the space power system itself.
The space power system defined for this purpose to include
68
OCR for page 69
A CHIEVING AD VOICES IN P O HER TECHNOL O GINS
69
a heat source, power conversion devices, and its loads is the pri-
mary source of spacecraft-generated thermal energy that must be
disposed of. Thus the efficiencies and losses of the overall power
system including those of its subsystems and components—are ma-
jor factors determining how much heat is generated and thus must
subsequently be absorbed or rejected. Availability of survivable,
cost-effective technology to store, pump to higher temperatures, and
radiate thermal energy effectively with low mass penalties is an im-
portant ingredient of space power system design.
The problem of thermal management is very important for space-
craft of any size, to say nothing of spacecraft power systems ranging
from hundreds of kilowatts to multimegawatts. The primary means
for heat rejection currently employed is to use heat radiators. This
method is basically the only Tong-term means of rejecting heat in
space without spacecraft mass alteration. Obviously, heat can be
stored in a mass that is then ejected from the spacecraft. The practi-
caTity of this method is limited (to about 30 min) by the rapid increase
of the mass required with increasing duration of operation. Further,
heat storage (in a heat sink) IS a very useful method of point cooling
and has considerable potential for SDI utilization. These methods
will be discussed separately.
Heat-Rejection Considerations
As is well known, the amount of heat radiated from a surface is
proportional to the fourth power of the surface temperature (mea-
sured in °K) and to the emissivity of the surface material. For these
reasons, reductions in radiator size and mass can be realized if the
operating temperatures and emissivities of space power radiators can
be increased. Because of the high sensitivity to temperature, dra-
matic mass reductions can be achieved, as discussed in Chapter 4,
whereas there is less sensitivity to ern~ssivity improvements.
Significant innovation in this area has the potential to alter
conventional views of power-system design trade-offs and should be
examined in connection with the preliminary vehicle design proposed
in Recommendation 1 of this report. Innovative radiating systems
based on liquid-droplet radiators, moving-belt radiators, heat pipes,
or on radiators that are deployed on power demand have been pro-
posed. Although there is no assurance that any of these concepts will
prove feasible, such approaches might produce significant reductions
in radiator size and specific mass, and hence warrant exploratory
OCR for page 70
70
AD VANCED PO WER SO URGES FOR SPA CE MISSIONS
research. An unarmored deployable radiator would be less massive
than an armored radiator, yet still be survivable to attrition attacks
by ground-based or space-based lasers.
For high-power systems, needs for heat rejection and mass mini-
mization cause system designers to favor power systems that operate
at high temperatures, thereby reducing the size of power-conversion
equipment (through higher conversion efficiencies), the amount of
heat that needs to be rejected, and the size of the radiators. For low-
temperature power systems, low-density materials (e.g., aluminum,
beryllium, or titanium) can be employed as radiators, thereby pro-
viding a means for reducing mass. Unfortunately, most highly devel-
oped heat-rejection technology was optimized based on cost factors,
rather than on considerations such as survivability, efficiency, or
high-temperature capabilities. Consequently there is only a limited
available technology base applicable to the problem at hand.
Heat rejection is essential, and for closed-cycle (noneffluent)
space power systems at the multimegawatt level, the heat rejec-
tion subsystem (see Chapter 4, Figure 4-1) can easily account for
half the mass of the overall power system. The SP-100 power system
has large, massive radiators because of the low conversion efficiency
of its thermoelectric converters. These radiators are made even more
massive by the imposed survivability requirements. Two other heat
rejection options are discussed below which avoid using radiators but
are mass-intensive, hence they become impractical as the duration of
power usage increases beyond about 1,000 s.
One of these options is to use heat storage aboard the spacecraft
for thermal management of multimegawatt systems that are operated
for only short periods of time (i.e., in the burst mode). Most of
the heat storage needed can be accomplished through endothermic
chemical reactions, use of specific heat capacity, and phase changes.
The other option that avoids radiators is gross heat rejection
from a thermal engine, where the waste heat is simply thrown over-
board with the effluent. This is a viable concept if the effluent does
not unduly interfere with friendly weapon, sensor, spacecraft, or
power systems. On the other hand, liberating effluents may hinder
hostile action.
From a strategic standpoint, duration constraints on the use of
the above mass-intensive options make them ineffective against a
counter-strategy of prolonging the period of combat to an hour or
longer.
Power system working fluid can typically be used for weapon
OCR for page 71
ACHIEVING AD DANCES IN POWER TECHNOLOGIES
71
cooling prior to entering the power-generation system. The ejected
effluent will thus have served the dual function of disposing of waste
heat from both weapon and power-generation systems. Resolution
of the question of whether or not the release of effluents is tolerable
is addressed in Recommendation 2.
Survivability Consideration
The survivability of space radiators is a major design problem, ow-
ing to the ease of detection of such localized thermal sources. This
problem is especially serious at the high rejection temperatures that
might be used for nuclear reactor systems. Radiators at those tem-
peratures could act as infrared homing beacons for hostile detection
anti action. Early in the process of advancing candidate space power
systems, thermal rejection techniques need to be identified that mit-
igate the risk of detection and attack but do not impose excessive
mass penalties for hardening. Candidate heat-rejection techniques
showing promise should then be subjected to feasibility studies and
scalability validation.
ADVANCING POWE:~-CONDITIONING COMPONENTS
AND TECHNOLOGIES
Advances in power system components, materials, and technology
are necessary to meet envisioned SDI requirements, as discussed
below.
Advancing the Design of Conductors
Conductors usually make up a significant fraction of the overall mass
of a power device and also determine its characteristics. Conductor
mass is generally traded off against electrical losses, device efficiency,
conductor temperature rise, complexity of cooling, and the amount
of coolant required.
Normal Conductors
Practical conductors at ambient temperatures generally consist of
copper or aluminum or their alloys. The materials are selected to give
the appropriate combination of low resistivity, mechanical strength,
and ease of fabrication required for specific applications.
OCR for page 72
72
ADVANCED POWER SOURCES FOR SPACE MISSIONS
High-strength conductors are important in applications (gener-
ally circular or solenoidal windings) where the conductor is also the
only part (or a major part) of the necessary structure. In applica-
tions where the structure is separate or is not of major concern, the
resistivity of the conductor becomes the major factor.
It is desirable to have conductors that can operate at high current
density consistent with achieving structural, dielectric, and thermal
requirements of the winding.
Essentially all pure metallic elements of interest as conductors
exhibit decreasing resistance with decreasing operating temperature.
This decrease is limited at the low-temperature extreme by impu-
rities, magneto-resistance, mechanical stress level, work hardening.
size, and so on.
High-conductivity, high-strength, wide-temperature-range me-
tallic conductors are distinctly possible, but do not appear to have
been examined over the temperature ranges of interest. The two best
metallic conductors (commercial, at practical cost levels, formable,
ductile, and tough) are copper and aluminum and oxide-dispersion-
stabilized (ODS) alloys of Cu and Al.
Dissolved impurities have major negative effects, especially on
electrical conductivity, but also on thermal conductivity. The purer
the Cu and Al, the higher the conductivity values. Fortunately it is
possible (commercially) to produce Cu at purity levels of 99.9 percent
or better, and Al at 99.99 percent or better. Each dissolved impurity
element has a different effect (percent conductivity loss per unit of
impurity content) on conductivity.
The potential availability of liquid hydrogen as a conductor
coolant can have a major effect on system operating temperatures.
Conductors capable of operating with liquid hydrogen should be de-
veloped either separately or as part of the component development.
Of existing conductors, high-purity aluminum in a composite
with an aluminum alloy is a promising candidate for use in a liquid-
hydrogen high-power alternator. If estimates of conductor perfor-
mance hold up for this option, it could be important in many direct-
current and alternating-current applications.
Superconductors
Superconductors exhibit zero reset ance only below a certain
critical temperature. They are poor electrical conductors above this
value. The metallic superconductors NbTi and Nb3Sn, which require
OCR for page 73
A CHIEVING AD VANCES IN P O WER TECHNOL O GINS
73
liquid helium for their operation, are capable of achieving operational
winding current densities of 50,000 A/cm2, but are not suitable for
operation at temperatures other than in the liquid helium range
(about 4°K). Because of their potentially high current density, these
alloys will continue to be major technological contenders for SDI
applications.
For radiofrequency (RF) operation at low magnetic fields, su-
perconductors exhibit low surface resistivities the lower the tem-
perature, the lower the Tosses. Nb cavities are being used at 2°K for
accelerators in the gigahertz range. Since operation at radio frequen-
cies is a surface phenomenon, a layer of superconducting material
of the appropriate thickness must be carefully applied to a suitable
substrate.
The use of superconductors in power systems generally leads to
high-efficiency, compact components and subsystems. High efficien-
cies of power generation, power transmission, and power condition-
ing have direct beneficial effects on the ratings and masses of prime
power sources and, in addition, a low-Ioss RF cavity reduces mass
requirements for the entire system.
At present, most superconductor applications are either direct
current (DC) or quasi-DC, where the currents change slowly. It is
only during DC operation that superconductors have zero resistance
to the flow of electrical current. The limits of this region of zero
resistance are a function of operating temperature, current density,
and magnetic field.
For alternating current (AC) operation at 6~Hz power frequen-
cies, superconductors exhibit Tosses above a threshold magnetic field.
These losses decrease with decreasing filament size one of the rea-
sons for the multifilamentary configuration currently being used in
Nb-based superconductors. Until now, because of the penalty of hav-
ing to operate at liquid-helium temperature, applications at power
system frequencies have been limited by the unavailability of con-
ductors having micron-size filaments. Achieving higher operating
temperatures for these materials means reduced refrigeration re-
quirements, and the prospect of doing so suggests that it is timely
to review potential space power applications (at frequencies much
greater than 60 Hz), such as transmission, transformers, inductors,
armature windings, and so on.
Superconductor transition temperatures up to 125°K have now
been reported by several institutions. There is no reason to be-
lieve that the optimum materials have been discovered, and further
OCR for page 74
74
ADVANCED POWER SOURCES FOR SPACE MISSIONS
progress is expected. Reports of achieving critical temperatures of
room temperature or above have been erratic, unconfirmed, or have
used inadequate measurement techniques.
To summarize the present status of superconducting materials:
1. High-critical-temperature superconducting materials have
the potential of carrying high critical currents.
2. The superconducting materials are ceramics and, at this
stage of their development, they have poor mechanical properties.
3. Indications of transition temperatures above room tempera-
ture have been reported (Materials Research Society, 1987), but have
not been confirmed and may not be reproducible.
4. Applications depend on the availability of superconductors
or superconductor sections with consistent properties that can be
fabricated into reliable windings for magnetic components or struc-
tures resembling permanent magnets.
This committee concludes that high-critical-temperature super-
conductors may well play a major role in SDI power applications
someday. Nevertheless, because of their early stage of development,
such superconductors are not currently available nor will they likely
be available for many years to come- to replace present or improved
power technology. Accordingly, the development of other SD! power
technology should not be curtailed until these superconductors begin
to become a viable option.
Superconducting Magnetic Energy Storage
Storage of energy in a magnetic field occurs when electricity flows
through one or more coils. Since any electrical resistance in the circuit
causes energy loss, the use of superconducting coils" which have
no DC resistance is a very efficient approach to storing electrical
energy for any length of time.
A major application of energy storage is to allow energy sources
to be sized for average or low power. In the case of superconducting
magnetic energy storage (SMES), the coin are energized at tow power
levels and then discharged at a higher power level.
LJow-critical-temperature superconductor technology has been
demonstrated on several large-scale projects primarily in magnetic
fusion, high-energy physics, and magnetic resonance imaging appli-
cations. Technology for these applications usually operates reliably,
and even larger-scale applications of superconductivity in these areas
are planned.
OCR for page 75
ACHIEVING ADVANCES IN POWER TECHNOLOGIES
75
Studies aimed at providing ground-based power of limited du-
ration at gigawatt power levels with rapid rise-times have indicated
that SMES is a very attractive approach for SDI applications, es-
peciaIly in view of the possibility of time-sharing the facility with a
utility during peacetime. Two large contracts for independent con-
ceptual design studies of SMES systems were awarded by SDIO late
in 1987.
ADVANCEMENT POTENTIAL OF TECHNOLOGY FOR
DYNAMIC POWE:R-CONVERSION CYCLES*
The existing gas-turbine industry builds gas turbines for propelling
aircraft and builds both gas and stemn turbines for terrestrial power
generation. The largest gas turbines generate 100 to 200 MWe per
module, and specialized gas turbines operating on stored compressed
air produce up to 290 MWe from a single machine (Gas Turbine
World, 1987~. About 20 separate models of gas turbine power plaints
currently marketed have power ratings exceeding 100 MWe.
Advances in the gas turbine for solar-dynamic power generation
aboard the Space Station and for use with nuclear reactors such as
SP-100 could occur (English, 1987) in the following ways:
. Using a taut alum-based refractory metal alloy (ASTAR-811C)
for the hot components of the power plant would permit operation at
peak temperatures up to 1500°K. That alloy has been creep-tested for
over 300,000 hours at temperatures from Il44°K to 1972°K (Klopp et
al., 1980~. Refractory alloys based on molybdenum and niobium-
having considerably lower density may in the future prove to be
applicable at these temperatures; specifically, the M>HfC alloy has
been tested for only a few hundred hours at temperatures up to
1800°K in ~ inert gas atmosphere, much less than the testing (over
22,000 hours) to which ASTAR-811C was subjected at temperatures
above 1800°K (Klopp et al., 1980~. However, both molybdenum and
niobium alloys must still undergo a very considerable testing program
before final conclusions can be drawn.
*The committee has discussed the Brayton cycle in considerable detail.
Many of the advances described in this section are also applicable to the
Rankine cycle. The committee believes additional study of both cycles is
warranted in view of unexplained or inconsistent SPAS analysis results, which
were unavailable in published form during the course of this study.
OCR for page 76
76
AD VANCED PO WER SO URGES FOR SPA CE MISSIONS
. By using the Brayton cycle combined with molten-lithium
heat storage, since the sensible heat capacity of the molten lithium
is higher by a factor of two or more than the latent heat capacity
of the fusible salts now contemplated for the Space Station. Use of
lithium, because of its extremely favorable heat-transfer properties,
would also permit a significant reduction in the size and mass of the
solar heat receiver.
. Inasmuch as molten lithium is not tied to any given working
temperature (as Is the melting and freezing of a salt), using lithium
in a Brayton cycle would peanut the gradual evolution of a given
power plant by first operating it at, say, 1200°K and then gradually
raising the operating temperature toward the potential of the power
plant, 1500°K in this case.
. This rise in peak temperature would increase not only the
power generated but also the efficiency of power generation; the sizes
of the solar collector and waste-heat radiator could therefore remain
constant with up to a 50 percent increase in generated power.
. Finally, for application to nuclear power, the solar mirror and
solar heat receiver of the solar Brayton power plant could be replaced
by a lithium-cooled nuclear reactor, such as the SP-100.
By virtue of their high efficiency, closed Brayton and Rankine
cycles could generate about 500 kWe using the same reactor from
which the present SP-100 thermoelectric conversion design generates
100 kWe. Similarly, from a 2~MW reactor required for thermoelec-
tric generation of 1 MWe, these power cycles would generate up to
about 5 MWe.
For generating very high power in the burst mode, use of molten
lithium as the heat sink for a high-power closed-cycle system would
provide a low-mass power plant that discharges no effluent during
operating periods of 1,000-2,000 s. This same technology could also
provide the megawatts of power needed for long periods in the alert
mode.
Advancement Potential for Alternator Technology
Alternators are electrical rotating machines that convert shaft en-
ergy into AC electrical power that can then be used as generated
or transformed and/or rectified as required by the load. A field
winding—usually DC-energized is rotated, with the power being
generated in the stationary armature.
The power for a given-size machine generally increases with
OCR for page 77
ACHIEYING AD VANCES IN POWER TECHNOLOGIES
77
increasing speed and current density in the field and armature
windings within the limits of structural integrity—consistent with
the requirements for high rotational speeds and rapid start-up some-
times imposed. The alternator technology relies heavily on the avail-
able prime mover, the available conductors, and thermal management
of the Tosses in the rotor and stator.
For an ambient-temperature application, the U.S. Army is de-
veloping a 3-MWe, gas-turbine-driven, oil-cooled machine. The al-
ternator for that device has a specific mass of about 0.1 kg/kWe
(for the generator alone) and rotates at 10,500 to 15,000 rpm. The
output power, which has a frequency of about 1 kHz, is fed into
transformers.
Because superconductors have losses when subjected to time-
varying currents or magnetic fields, the use of superconducting tech-
nology has been limited up to now to the field windings of the
alternator, where they are exposed essentially to DC operation. An
example of this technology Is a machine using a liquid-helium-cooled
superconducting rotating field and an ambient-temperature arma-
ture, being developed by General Electric for the U.S. Air Force The
machine is undergoing Preliminary testing. It has a rating of 20 MWe
at 6,000 rpm and is capable of starting up in Is from a cooled-standby
condition. The machine has a specific mass of 0.045 kg/kWe, and
is designed with several system-oriented unique features, such as a
rectified 40-kV DC output, potentially eliminating the need for ad-
ditional transformers. It also has an ambient-temperature aluminum
shield that reduces external time-varying magnetic fields, which is an
important design feature for space applications. Because of its Tower
speeds and high-voTtage winding, the 0.045 kg/kWe machine is not
directly comparable to the 3-MWe army machine.
An experimental air-core alternator with a disc rotor is being de-
signed by ARDE~KAMAN with a continuous rating of 0.1 kg/kWe
at about 5,500 rpm. This rating is projected to decrease to 0.03
kg/kWe for a future cryogenic machine with counterrotating discs
and a 2~MWe rating.
An approach using liquid-hydrogen-cooled, high-purity alumi-
num conductors for both field and armature is being undertaken
for SDI by Westinghouse and Alcoa. Recent measurements of the
resistivity of high-purity aluminum samples by these organizations
(Biliman, 1987; Eckels, 1987) are lower than previously attained. If
such resistivity can be maintained in finished windings, these results
indicate that high-purity aluminum may be an even better conductor
OCR for page 78
78
AD DANCED PO WER SO URGES FOR SPA CE MISSIONS
than previously thought for high-current-density operation in the
liquid hydrogen range. Estimated specific masses are of the order of
0.03 kg/kWe for a 3() MWe machine with output in the 50 to 100 kV
range.
In the United States there is essentially no operating experi-
ence with alternators other than at ambient temperatures. Cryo-
genic and superconducting techniques have been successfully demon-
strated in homopolar types of machines and in other stationary ap-
plications, such as magnets for high-energy physics, magnetic fusion
experiments, and magnetic resonance imaging. While experimen-
tal developmental hardware does exist, the successful application of
these techniques to high-power alternators still remains to be demon-
strated.
An alternator configuration for use in space- because of its inter-
face with power conditioning/Ioad, thermal management, the prime
mover/energy source, and the torques, magnetic fields, high voTt-
ages, and currents it generates must be the result of a thorough,
interactive systems approach. The basic advantages of the alternator
in being able to generate high voltages without transformers must
be traded off against the loss of flexibility in initially developing a
general purpose alternator that must then be connected to a power
system with transformers to provide load-specific voltage levels. Note
that all loads may not operate at the same voltage level.
Direct generation of high voltages requires either placement of
the alternator near the load or the transmission of power at high
voltages, and has the attendant problems of high voltages in space
and fault management, as cliscussed elsewhere in this report.
Advancing the State of the Art In Power System Components
Funding that has been available for component development has gen-
erally been used within a program to unprove existing manufacturing
techniques, evaluate new materials developed for other applications,
and to make ~rnprovements in techniques for manufacturing compo-
nents. In view of the limited resources available in the past, that was
the only logical approach. However, this strategy is at best capable
of achieving only modest gains.
A more cost-effective approach is illustrated by the example of
the recent joint SDI/DNA capacitor program. This program has
been very successful, in large measure because it made maximum
use of new theory and computer modeling power. This program
OCR for page 79
ACHIEVING AD VANCES IN POWER TECHNOLOGIES
79
makes use of 1980s technology rather than simply extending the
stanclard approach typical of the 1960s. This approach can be ap-
plied to other component technologies as well. Examples of the
rapid advances achieved to date in representative power technology
components when their development was aggressively funded are il-
lustrated in Table 5-1. In contrast, note the dismal evolutionary
advance rates of 1.5 per decade in surface-voltage withstand-level for
resistors. While the comrn~ttee recognizes that technical progress is
often nonlinear, use is made here of average rates of advance in order
to focus attention where it is needed.
The SDI power program should continue an aggressive, coor-
dinated base technology program to parallel and complement its
weapons platform/systems efforts. To enable the multidecades of
advances needed for SD! power, program focus should be on areas
such as:
. high-temperature materials for nuclear reactors and power
generation
high-temperature radiators;
advanced, high-temperature instrumentation and reactor con-
trol;
tw~phase flow evaporation and condensation in reduced grav-
itational fields;
electrical and thermal insulators;
Tow-mass electrical conductors, including superconductors;
thermal conductors;
ferromagnetic and magnetic materials;
survivable devices for switching, power conditioning, and gen-
eration;
techniques for managing/containing high voltages, currents,
and electrical and magnetic fields; and
improvements In inverters, which are not presently being de-
veloped for weapons power.
The comrn~ttee recognizes a clear need to make progress in ma-
terials for increasing the efficiency and compactness of power compo-
nents. There may also be benefit in coupling industry to university
research groups via the SDIO directorates responsible for basic re-
search (DOD category 6.1) and technology base development in the
power area. As an example, mass reduction in high-power thyra-
trons could be substantial if the ceramic insulators could either be
OCR for page 80
80
o -
C: ~ C'
~ ~ ~ At, ~ ~
a)
. _ _
~ ~ ,~ ~ C1
EVER ~
so
o
~ ° U) — >
o ,0 I,, ~ o
as
o
, ~ ~
m O
US
O ~ ~ ~
~ b s==
~ LIZ ~ ~ C5)
m
o
o
V
03
an
3
o
1
m
En
C: m
_ it
V U]
o
C)
a'
CO ~
to
00 00 0
O
X X =^
X
Cal
~ Cal ~
O O O
Cal e4 Cal
~ ~ so
en ~ en ~ ~ ~
~5 Cal ~ ED ~5
X X X
$— ~ ~
o~ o o
bO
~ O . - ~ - , 3 0
m v ~
:^ bO `
c ~ c
0O
O ~ O
bO~
O
v >s
OCR for page 81
81
to
o
o
c~
:
~ ~ ~ c~
:
~ x x x x x
x
~ ~ u: O ~ e~
O C53 0 0 ~ ~
e~ ~ c~ c~ ~ _I
I-. 0
- ~ ~ -
u' ~ ao ~ ~ ~ ~ ~ ~ _
x x x x x
o o ~ ~ ~ - x o
O _ Ut _I
"o ~ ~ ~ o o ~ ~ ° ~o
- ~ ~ o; c~ ~
~ ~ o ~ > ~ ~ ~ ~ ~ 3 ~ c ~ ~
·_ C°) E~ ~, ~ V
s"
o
~ s"
o o ~7 5 ~: ° C) ·o
U] ·_ o
._ s.= ~ ~.c
~Q
o
a,
:^
V]
o,
._
ba
o
oo
o
Ut
oo
C5)
.—
~s
· - ~
u]
3 ~
.
:^ ~
.
b~
b. ·"
s" o
~Q ~
.
_
-
-
~ 6
~ o
CO —
~ o
m ~
.. ~
V
=) 3
o o
V) P~
OCR for page 82
~2
ADVANCED POWER SOURCES FOR SPACE MISSIONS
eliminated or made of less dense insulating materials capable of high-
temperature operation.
Progress toward advances in the state of the art of components
used in power-conditioning and puIsed-power systems could be ef-
fectively achieved through initiatives anchored in materials tech-
nologies. In view of the major successes achieved in applying ba-
sic science to materials programs in high-energy-density capacitors,
similar approaches should be applied to other areas of power devel-
opment. This committee recommends a development strategy of this
nature, pursued aggressively and funded adequately, to develop scal-
able power technology, particularly if success would enable selection
of one weapon system over a less desirable one by removing power
considerations as the principal constraint.
The following four areas of development form an integrated pro-
gram ensemble in both prime power and power-conditioning technol-
ogy:
.
Technology feasibility projects to demonstrate that a required
capability is possible.
. Scaled experiments to give high confidence in the ability to
design a full-size system.
.
Limited near-full-scale demonstrations of advanced-develop-
ment models, for technology validation and to clarify integration and
compatibility problems associated with production devices.
. A continuous effort to understand fundamental mechanisms
as applied to component technology feasibility and scalability.
In summary, development of subscale (i.e., at about 10 percent
of full power level), scalable, high-performance power components
and associated technology to provide a broad range of system op-
tions is a prudent investment strategy. Emphasis on component
development for generating, conditioning, and transmitting electri-
cal power is required. The issue of high-temperature superconductivity
as it affects scaling feasibility must be addressed. Furthermore, the
Tonger-term and nearer-term technology base development programs
must be brought into balance. A technology-based-option investment
strategy for the longer-term options in SD] is needed by periodically
targeting superior technologies among existing candidates as a means
of achieving future needs through down-selection. Such an increased
emphasis is needed on the technology base for space power system
components, as the existing base is grossly inadequate to meet the
mission challenge.
OCR for page 83
ACHIEVING ADVANCES IN POWER TECHNOLOGIES
MATERIALS ADVANCES REQUI1lE:D FOR THE EVOLVING
SPACE POWER TECHNOLOGIES
83
There are vast differences in the materials requirements for the range
of space power cycles and power systems examined by this commit-
tee. These systems typically demand high temperatures (with little
else specified) ranging from 1300°K to 2500°K. Although the lower
temperatures in this range can be met In reasonable time and at
reasonable cost, the higher ones will necessitate the development of
totally new or different materials, requiring a dedicated effort in or-
der to achieve success in some "short-term period such as 10 years.
The development of SDI space power component technologies will
require significant advances of materials technology in the follow-
ing areas of magnetic materials, insulators, and the development of
high-temperature structural materials.
Magnetic MateriaLs
Magnetic materials are important for induction accelerators, low-
mass, high-frequency inverters, and so on. However, data are cur-
rently being obtained that indicate that FeNdB magnets can be
fabricated for a variety of magnetic applications with outstanding
results. Metallic glasses of selected compositions are soft magnetic
materials. Being free of grains, grain boundaries, and secondary
phases, these materials can be used for making soft magnetic alloys
that are ent*ely free of orientation effects.
Apparently Metglas~R has not yet approached the potential
desirable properties achievable in magnetic materials by applying
rapid quenching techniques to create new alloys. During the past
year, General Electric- using ADied Signal Company MetglassR
compositions built and tested a large number of commercial AC
power transformers that exhibited the outstanding performance pre-
viously predicted.
Sweat ore
Newly developed products far superior to classic baked clay ceram-
ics are available for making feedthroughs, standoffs, interfaces, and
other insulators. Numerous new classes of polymers and ceram-
ics, processing techniques, and forming techniques can now offer
OCR for page 84
84
ADVANCED POWER SOURCES FOR SPACE MISSIONS
major improvements in insulators. Such improvements include high-
strength materials that can be used at high and low temperatures
and that can produce intricate shapes.
High-Temperature Structural Materials
Because materials are almost always and properly viewed as de-
sign limiters, support for the development of advanced materials
has received reasonable backing since the early 1950s. Unfortu-
nately, performance specifications all too frequently come fairly late
in systems development programs. furthermore, almost every new
application unfortunately requires new or different combinations of
properties and performance: temperature, time at temperature, per-
missible deformation, structural stability (i.e., changes of properties
under operating conditions), surface degradation, joining problems,
and so on. For new or different applications, these requirements em-
phasize the need to define a proposed system so that materials can
be tailored to such needs. It is rare that the more critical materials
can be obtained "off the shelf."
For SDI power systems, radiation hardening is a requirement
for power semiconductor switches and other electrical components.
There are significant opportunities for exploiting new materials such
as gallium arsenide and silicon carbide for this purpose.
Before the use of ceramic materials or carbon-carbon compos-
ites for rotating blades or the use of filament-reinforced ceramics
for temperatures between 1200°K and 1500°K can be seriously pro-
posed, considerable time will be needed to develop and test such ma-
terials for use in a specific power system. This is because only limited
data are available on long-term performance in highly cyclical tem-
perature and stress systems. A few such systems are making excellent
progress, but results for these applications are emerging slowly, hence
careful development of these materials for meeting specific needs wiD
continue to be required.
The preferred cycles and systems must be selected, and all op-
erating conditions must be integrated. Such integration will permit
selection of the alloy systems, if not of the alloys, for preliminary con-
sideration and planning for alloy modification. Thus the committee
notes the following three partial bases for arriving at its Conclusion
5 and Recommendation 5 stated below.
1. Selection of operating temperatures up to about 1500°C
(1773°K) (National Research Council, 1988) may permit preliminary
OCR for page 85
ACHIEVING ADVANCES IN POWER TECHNOLOGIES
85
selection of materials already in existence for specified life cycles and
environments. Usually, and fairly obviously, the lower the planned
operating temperatures, the greater is the number of available appli-
cable alloys. The refractory metal alloys are reasonably well known
and perform well at the right temperatures and atmospheric pres-
sures, but must be carefully selected for ductility.
2. For temperature applications above about 1100°C-1200°C
(1373°K-1473°K) regardless of alloy type (metallic base, ceramic
base, carbon base), only limited data are available for lifetimes in
excess of 100 h or even for lifetimes in excess of only a few hours—
although there are important exceptions. Obviously, low-mass struc-
tures should be emphasized.
3. Coatings may be required for advanced materials operating
at high temperatures for significant periods. This area has received
very little funding, yet it is critical for the selection of appropriate
materials.
CONCLUSION AND RECOMMENDATION
Based on the discussion in this chapter, the committee arrived at the
following conclusion and recommendation.
Conclusion 5: Major advances in materiab, components, and
power system technology wiB be determining factors In making SDI
space power systems viable. Achieving such advances wig require
skins, time, money, and significant technological innovation. The
development of adequate power supplies may well pace the entire
SDI program.
Recommendation 5: Male additional and effective investments
now in technology and demonstrations leading to advanced compo-
nents, including but not limited to:
thermal management, including radiators;
materials structural, theImal, environmental, and super-
conducting;
electrical generation, conditioning, twitting, transmission,
and storage; and
long-term cryostorage of H2 and O2.
Advances in these areas will reduce power system mass and
environmental impacts, improve power system reliability, and, in
the long term, reduce life-cycle power system cost.
Representative terms from entire chapter:
power technologies
